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AD921753

Approved for public release; distribution isunlimited. Document partially illegible.

Distribution authorized to U.S. Gov't. agenciesonly; Test and Evaluation; 04 MAR 1973. Otherrequests shall be referred to Space and MissileSystem Organization, Attn: YE, Los Angeles, CA.Document partially illegible.

SAMSO ltr, 17 Jun 1977

/ f I f THIS REPORT HAS BEEN DELIMITED

AND CLEARED FOR PUBLIC RELEASE

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NO RESTRICTIONS ARE IMPOSED UPON

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AD921753

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copy 2. ^ GLOBAL POSITIONING SYSTEM (GPS) FINAL REPORT

S WDL.TR5291

28 February 1974

SAMSO TP 7k-lQ IW 7'+-l63

PARTI

VOLUME B User Segment System Analysis Report

Conuact F 04701-73-C-0796

D D C

Pp* Submitted to: DEPARTMENT OF THE AIR FORCE HEADQUARTERS SPACE AND MISSILE SYSTEMS ORGANIZATION (AFSC) P.O. Box 92960, Worldway Postal Center Los Angeles, California 90009

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PHILCO Philco-Ford Corporation Western Development Laboratories On islon Palo Alto. California 94303

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WDL Technical Report 5291 28 February 1974

GLOBAL POSITIONING SYSTEM (GPS)

FINAL REPORT

PART I - VOLUME B

USER SEGMENT SYSTEM ANALYSIS REPORT

Tcntract F04701-73-C-0296 n D C

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Prepared for

DEPARTMENT OF THE AIR FORCE HEADQUARTERS SPACE AND MISSILE SYSTEMS ORGANIZATION (AFSC)

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WDL-TR5291 PHILCO ^8> Pirt I nulco-Foid Corpofalion _. , _ WMlwn D«««lopnwnt KboralorIM Division Volume B

FOREWORD

This Is Part I, Volume B, of the GPS Definition Study Final Report, submitted by Philco-Ford In accordance with Sequence Number A001 of Exhibit A to Contract F04701-73-C-0296. The period of performance for the report submitted herein Is from 28 June 1973 to 28 Feb- ruary 1974. The following figure Identifies the structure of the

Final Report and the relationship of this volume to other volumes

of this submittal.

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Structure of Global Positioning System Definition Study Final Report

I I I I

I I I

CONTENTS

Page

1. INTRODUCTION 1-1

2. SYSTEMS ENGINEERING '--1

2. 1 Signal Structure 2-1

2.2 Receiver Opoiations 2-4

2.2.1 General 2-4 2.2.2 Classes A, B, and F Operations 2-6 2.2.3 Class C Operations 2-9 2.2.4 Classes D anc. E Operations 2-10

2.3 Receiver Performance 2-11

2. 3. 1 Measurement Accuracy 2-11 2.3.2 Jamming Immunity 2-13

3. USER EQUIPMENT DEFINITION 3-1

3. 1 Equipment Group Design 3-1

3,2 Receiver Types and Description 3-8

3.2.1 Module Definition 3-10 3.2.2 Continuous Tracking Receiver Type I 3-11 3.2. 3 Sequential Tracking Receivers (Type II and 3-20

HI) 3.2.4 Mechanical Design 3-22

3. 3 Computers 3-30

3. 4 Control and Display 3-32

3. 5 Antennae 3-33

3. 5. 1 Discussion 3-33 3. 5, 2 Antenna Types 3-39

3. 6 Auxiliary Sensors 3-41

3. 7 Interface Hardware 3-46

3. 8 Software 3-46

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CONTENTS (Continued)

Page

4. SYSTEM EFFECTIVENESS 4-1

4. 1 Reliability and Maintainability 4-1

4. 2 Life Cycle Cost Analysis 4-2

4. 3 Hardening 4_w

4, 4 Electromagnetic Interference (EMI) 4-7

5. SYSTEM TEST PLANNING 5-1

5. 1 GPS System Test Plan 5-1

5. 2 Test Facilities Requirements Document 5-4

5. 3 Levels of Testing 5-5

5. 3. 1 Generalized Development Model 5-5 5. 3.2 Low Cost (Class C) Prototype 5-9 5. 3. 3 Classes A-F Development Models 5-9

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<i-l Functional Representation of GPS Signal Structure 2-2

2-2 Jamming Immunity 2-16

3-1 Equipment Group Block Diagram Classes C, D, E, 3-4 and F

3-2 F-4 Equipment Block Diagram 3-5

3-3 RF-4C Forward Bay Electronic Equipment Rack 3-6

3-4 Pod Mounted GPS User Equipment Configuration 3-7

3-5 Module Block Diagram 3-9

3-6 Four Channel GPS Receiver, Type I, Class A, B, F 3-12 it

3-7 Sequential GPS Receiver, Type II, Class D, E 3-21

•• 3-8 Four Channel DNSS Receiver 3-2 3

3-9 Typical 4-Substrate Module Package 3-25 I

3-10 Typical PC Card Module 3-26

3-11 Sequential DNSS Receiver Packaging Concept 3-31

3-12 Aircraft Pos-Nav Console 3-35

«• 3-13 Conceptual Phase I Control and Displays 3-36

3-14 Cross-Slot Cavity Backed Antenna 3-40

3-15 Conical Log-Spiral Antenna 3-41

3-16 GPS Phase I Integrated System 3-47

3-17 ASN-91/ROLM 1602 Interface Block Diagram 3-48

3-18 Class A User Navigation Program Functional Block 3-49 Diagram

3-19 Class C User Navigation Program Functional Block 3-51 Diagram

5-1 GPS User Equipment Phase I Test Planning Process 5-2

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5-2 Phase I Test Program Scope

5-3 Generalized Development Model Phase I Test Program Flow

5-4 Class C Low Cost Prototype Phase I Test Program Flow

5-5 Class A-F Development Model Phase I Test Program Flow

Page

5-3

5-7

5-10

5-13

I I I IV

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TABLES

2-1 Signal Frequencies

2-Z Code Lengths and Periods

2-3 Random Measurement Errors

^-4. ...Bias Measurement Errors

3-1 Summary Differentation of GPS User Classes

3-2 GPS Receiver Types (Phase I)

3-3 Type I Receiver Module Listing

3-4 Computer Functions

3-5 User Inputs

3-6 Electrical Performance Specification for GPS Antenna

3-7 Inertial Navigators

3-8 INS Selection Criteria

3-9 Computational Rates

3-10 Low Cost User Software Requirements

4-1 Maintenance Concept Summary (Depot Repair)

4-2 Maintenance Concept Summary (Module Discard at Failure

4-3 Matrix of Test Cases and Results of LCC Calculations

4-4 Summary of L-Band Transmitter Interference to GPS Receivers

5-1 Receiver/Equipment Group Requirements Matrix

5-2 Test Objective Matrix Engineering Development Model Test Equipment Units 001 and 002

5-3 Test Objective, Low Cost (Class C) Prototype Test 5-11 Equipment Units 101-116

5-4 Test Objective Matrix, Classes A-F Test Model Test 5-14 Equipment Units 201-205

Page

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I I

1. INTRODUCTION

A wide variety of tradeoffs and analyses have been conducted to

define the User Segment of the GPS. The purpose of this document is

to provide a summary of the most important considerations that led to the

User Segment design. More complete details of the major system trade-

offs and receiver design are contained in Part II, Volume B, of this docu-

ment.

A User Segment Specification has been prepared along with both CI

and CP Specifications for six user classes. A Computer Specification has

also been prepared, and is being used as a basis for discussion with many

computer vendors.

Details of the GPP Phase I are highly dependent upon the test plan-

ning activities cui rently underway in conjunction with the JPO. Critical

technical decisions are yet to be made which depend upon the results of

the test planning activities as well as a detailed analysis of the Phase I

schedule. For example, the detailed design of interface units depends

upon the computer selection, the test beds, and IMU selection as well as

the approach (interface with an IMU directly or to an IMU processor)

taken. These decisions will be partly judged by the schedule and cost

risk associated with each approach.

Section 2 of this volume provides details on the system level trade-

offs and resultant user operation. Performance of the GPS receivers is

also presented. Section 3 contains a sunamary of the elements of the

User Segment with particular emphasis placed on the electrical and

mechanical aspects of the receiver. Control and displays, computer,

antennae, interface units, auxiliary sensors, and software are also sum-

marized.

Reliability, maintainability, life cycle costing, equipment hardening,

and EMC are discussed in Section 4. Section 5 contains a summary review

of the test planning activities and approach.

1-1

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I s I I I

I I I

2. SYSTEMS ENGINEERING

2. 1 SIGNAL STRTIGTURE

A considerable amount of analytic effort was expended on the GPS

signal structure. This result* d in the structure described in the GPS

System Specification, published by SAMSO, SS-GPS-101A, dated 29

January 1974. This signal is functionally illustrated in Figure 2-1.

All the various compcfhents of the signal, including the carriers,

are coherently derived from the same oscillator, and hence their fre-

quencies are simply related. This is shown in Table 2-1. The multi-

pliers, m and n, which raise the frequency source to the L, and L^

RF frequencies, have not yet been fixed. These multipliers should,

however, be integers to minimize the user equipment cost. For example,

a value of 154 for m would provide an L, frequency of 1. 57 542 GHz and

120 for n would provide an L-, frequency of 1. 2276 GHz.

The System Specification allovs some freedom in selecting the dif-

ference in lengths between the two components of the P-code. In alge-

braic terms, if N, and N-, are the lengths of the XI and X2 components,

then

N, = 15,345,000

and N2 = N + K

where K is an integer between 1 and 33, inclusive, a.

Since N, and N, must be relatively prime for the composite code

length to be equal to N|N2, there are, in fact, only a few allowable values

for K, namely

K = 1, 7, 13, 17, 19, 23, 29, or 31.

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Table 2-1. Signal Frequencies

j Signal Component Frequency Deviation Nominal F requency

Frequency source f 5. 115 MHz

P-code clock 2f 10.23 MHz

C-code clock f/5 1. 023 MHz

L, carrier mf* 1. 57 542 GHz

L, carrier nf** 1.2276 GHz

1 C-code epoch rate f/(5 x 1023) = £c 1 KHz

Data clock f no c 50 Hz

Ambiguity resolution clock fc/5 200 Hz

m equal 154.

n equal 120.

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Since the advantage of K > 1 i8 not significant, our design is pre-

saged on the selection of K = 1. The code lengths and periods are then as

shown in Table 2-2.

In Part II, Volume B of this report ("User Segment Trade Studies"),

an important alternative to the signal structure is presented for consider-

j ation. This alternative employs different data rates for P and C, and

dispenses with the ambiguity resolution code. The two rates are,

C-data fc/6 = 166. 6 Hz, nominally

t ■

P-data fc/(6 x 16) = 10.4 Hz, nominally

where fc = C-code epoch rate (see Table 2-1. )

The trade study analyses also resulted in other alternatives and

extensions of detail to the signal structure, which are of lesser signifi- ! j cance. They are all described in Part II, Volume B. I

2.2 RECEIVER OPERATIONS \

2. 2. 1 General

There are certain basic operational steps which will be common to :

all user equipment groups. They are, in sequential order;

a) Power on and warm-up

b) Input of approximate user position and velocity, and of time

c) Constellation selection

d) Search and acquisition of L. signal

e) Collection of current data

f) Pseudorange and pseudorange rate measurement of L,

g) First navigation fix

h) Pseudorange measurement on L^ (L./L-, receivers only)

i) Continuing accurate navigation fixes

j) Data update

k) Constellation revision

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It is expected that most users will perform these operations auto-

matically, without the need for operator actions. This will allow the

operator to concern himself only with the form of the navigation display,

and not with the mechanism for obtaining the fixes themselves. With

some users, the initial input of user's position and velocity must be

manually effected, too. There is also the potential for designing ex-

tremely simple usor equipment in which more of these steps would be

operator effected, but such devices have not been intensively examined

yet.

In the following, the operational steps are discussed in greater

detail, first for User Classes A, B, and F (which all have similar

sequences). The operational steps for the other classes are then de-

scribed, particularly where they differ from Class A steps.

Z. 2. 2 Classes A, B, and F Operations

These three user classes can be considered together here since

they all employ a 4-channel, continuous tracking, C/P, L,/!^ receiver,

and all include inertial sensors in their equipment groups.

From the time of power-on, all steps are carried out automatically

under computer control. The initial input (to the computer) of the user's

approximate position and velocity, is provided, at a frequent sampling

rate throughout the start-up sequence, from the inertial or other auxiliary

navigation sensors. Similarly, other, non-GPS, clocks provide the time

data.

The user's position and velocity, and the time information, are

needed for constellation selection. This is a process of determining which

four satellites, of all those active in the GPS, should bo used for navigation.

I* In addition to this information, the constellation selection process also

needs to know the approximate positions and velocities of the satellites,

and those are found from the orbital elements of all satellites stored by

1 each user in his nonvolatile memory. The different ways of transmitting

and storing these elements are comparatively discussed in the User Seg-

I ment Trade Studies, Part II, Volume B.

I I 2-6

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The criteria for constellation selection will most probably vary from

user to user. The minimum allowable elevation angle is one of the

criterion; a ship user will probably want to minimize horizontal GDOP,

whereas an airborne user may wish to minimize the 3-dimensional SEP

GDOP. For all cases, the constellation selection will be performed in

a software subroutine. The output will be the identification of the selected

satellites (which includes the feedback tap arrangements for their C-

codes), and the estimates of second mixer injection frequencies needed to

ensure that the received L, signal frequencies lie within the acquisition

passband. These are related to the estimates of the doppler shifts.

This ensures that the subsequent search for the signal need be in

phase only, and not also in frequency. With Classes A and B, it is

possible that the user-induced doppler shift will change so much during

the search process that the initial frequency estimates will not remain

valid. For this reason, the user velocity has to be frequently sent to the

computer during the search, and the computations for second mixer

injection frequencies repeated.

Each of the L, receiver channels will be allocated to one of the

selected satellites, and each will independently go through the following

steps:

a) Set the feedback taps on its replica C-code generator.

b) Preposition its VCO (i. e. , preposition its second mixer injection frequency).

c) Search in phase by stepping the C-code clock pulses the equivalent of half a chip every T msec, where T is about 10 msec, and will be a preset value.

d) When the lock detectors show that the Lj-C signal has been acquired, first the carrier tracking loop, and then the code loop will be enabled, so that the signal will be automatically tracked in both phase and frequency. The VCO prepositioning estimates are no longer needed at this point.

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e) The data bit synchronizer will be enabled, and matched filter data detection can begin. There will be a real time search for the frame sync pattern, followed by acquisition and storage of the navigation message.

f) Following the navigation message is the handover word. This informs the receiver/computer exactly when to replace the replica C-code with the replica P-code at the code demodulator.

g) The channel is now properly tracking the Li-P signal from its assigned satellite. It will make regularly scheduled measurements of pseudorange and pseudorange rate and send them to the computer (which may, however, not make use of them yet).

When all four channels have accomplished the above steps, the

computer will start to make navigation fixes. The first fix will not be

the most accurate one, since L./L^ comparisons are yet to be made,

and the navigation filter needs time before it can obtain very accurate

estimates of the system biases. The time needed to make the first fix

(time-to-first-fix, or TTFF) has been thoroughly investigated, and is

discussed in Part II, Volume B. The time needed to make the first fix

of ultimate accuracy is expected to be just a few seconds longer than

the TTFF.

Once the Li-P signals from all four satellites are being tracked,

the computer will establish a routine in which every 10 sec nominally,

the receiver is reconfigured for the L?-P signals. This changeover has

been examined and it has been found that a new frequency/phase search

will not be necessary, and that the tracking loops will quickly overcome

the switching transient. The pseudorange at L? can then be measured

and sent to the computer. In the computer, these measurements are

processed, and a very accurate ionospheric correction factor is found,

which is then applied to all subsequent L^-P pseudorange measurements.

The correction factor is, generally, updated every 10 sec.

The computer also keeps watch on the data messages. When it is

seen that a new message is being transmitted (this happens every hour).

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the routine reconfiguration to L, will be inhibited, so that the entire new

message (which is carried on L,), can be uninterruptedly acquired.

The computer also keeps watch on the values of the constellation

I selection parameters (elevation angle, GDOP, etc, ) Based on this, it

will, when necessary, command a channel to cease tracking its satellite

and instead search for and acquire a replacement satellite. This pro-

cess does, of course, cause some degradation in the navigation output

accuracy (which is then based odi measurements to only three satellites).

The time for constellation revision (TCR) has been analyzed, and is

reported on elsewhere. It is not so excessive that the degradation be-

comes significant.

Finally, the computer is also keeping watch on the tracking loop

lock status flags. Should a channel lose lock, it will, under computer

control, first try to reacquire the signal by means of a small phase/

frequency search in the P-domain. Should this not be successful within

a specified time, then the computer will command a reacquisition using

the C-signal.

There are other operational steps, automatically effected, whose

need has not yet been firmly established, and hence are considered only

provisional at this stage. These include bandwidth changes in response

to detected jamming levels, and changes in the start-up sequence in the

presence of medium jamming (handover to P before initial navigation data

collection).

As may be seen from the above, all the receiver/computer operations

are automatic. The operator is needed only for control of the display unit,

so that he may select the form of presentation of the navigation data, with-

out concern of how they are generated.

2. 2. 3 Class C Operations

| User Class C equipment groups ^ons' t of a GPS receiver only,

without auxiliary navigation sensors. Further the receiver is a low cost,

1 2-channel sequential tracker, using the L,-C signal only.

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Compared with Classes A, B, and F, the Class C operator has

additional tasks to perform, but these are only needed at start-up. He

must input his approximate position (latitude, longitude, and altitude),

the date, the approximate time, and his speed and course. He must also

fly reasonably straight and level during the search and acquisition regime.

In practice, this will not be a difficult constraint, since he will usually

start navigating before taxiing his aircraft.

The search and acquisition of the L^-C signals is performed auto-

matically, and similarly to that described in the previous section. The

dii'ierence is that one of the two receiver channels (Channel A), is time-

shared among the four satellites but performs this search in an uninter-

rupted manner. As each signal is acquired, in Channel A, then Channel

B is piepositioned to it, and this channel then sequentially tracks the sig-

nal. Channel A then starts searching for the next signal. When all four

signals have been found by A, and hence are being sequentially tracked by

B, then A is used for data collection. It is prepositioned to each signal

by B, and then tracks it, continuously, until it has acquired all the navi-

gation data. While this is going on, Channel B is sequentially tracking

all four signals, and making the pseudorange and pseudorange rate mea-

surements.

In all other respects, the Class C operations are like those just

described for Classes A, B, and F. Channel B is dedicated totally to

sequential tracking and measurement, while Channel A is used for data

collection, the watch for data change, for reacviisition of temporarily

lost signals, and fov acquisition of new satellites at times of constellation

revision.

2. 2. 4 Classes D and E Operatjors

These user classes are similar to Class C, in that they do not

include auxiliary navigation sensors, and in that they use a 2-channel

sequential tracking receiver. However, they differ from Class C opera-

tions in two ways.

2-10

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I I

Firstly, they operate on the L.-P signal, so they do make use of

I the handover word. During the initial start-up as each L.-C signal is

found by Channel A, it stays tracking the signal (uninterruptedly), until

I it has acquired the navigation data and the handover word. Then it pre-

positions Channel B (for the P-signal), so that this channel can keep

sequentially tracking. Thus, in this receiver, Channel B is (normally)

used only with P-signals.

The other difference with Class C, is that the operator does not

input his course and speed at start-up; nor does he have to restrict his

dynamics at this time. This is because the maximum operating velocities

of those users are sufficiently low that frequency prepositioning can always

be made, with sufficient accuracy, by assuming that the user is stationary.

2. 3 RECEIVER PERFORMANCE

The receiver performance parameters of most specific significance

to systems engineering are the measurement accuracies and the jamming

immunity. Both of these parameters are summarized in the following

sections.

2. 3. 1 Measurement Accuracy

The pseudorange and pseudorange rate measurement accuracies

were analyzed and reported in Reference 2-1. The results of this work

are summarized in Tables 2-3 and 2-4. In Table 2-3, an entry is made

for range instrumentation error. This arises from the way in which the

code tracking delay lock loop is implemented. The C-code noise error

is more than 10 times greater than the P-code noise error, since dif-

ferent bandwidths have been assumed for the two processors.

l

I I I

Reference 2-1. DNSDP-AG-230, "Receiver Range and Range Rate Measurement Errors", 7 January 1974

2-11

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Range Acce eration (gs) i

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Range bias (meters)

Range rate bias (meters/sec)

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I I I

The errors in Table 2-3 are shown for various values of the carrier«

to-noise density ratio of either the P- or C-signal, at the receiver input,

and effective after jamming reduction by the spread spectrum code demod-

ulation.

In addition to the random errors, there will also be bias errors if

there is range acceleration, and the carrier tracking loop is of second

order. These errors are shown in Table 2-4. Although the range rate

errors are worst case, and assume that the acceleration is present at the

beginning of the velocity counting interval, but not at the end (or vice

versa), they are sufficiently large to suggest that, for many applications,

the carrier tracking loop will have to be of the third order rather than the

second.

2. 3. 2 Jamming Immunity

There are three ways in which a GPS user can achieve jamming

immunity in his operations:

1) By working with the P-signal, so that the jammer power is spread over a wide spectrum in the replica code demodulator (whereas the desired signal, which is transmitted in a spread spectrum, is concentrated into nearly a line spectrum by the same demodulator).

2) By using a very narrow bandwidth carrier tracking loop after the code demodulator. Since the desired signal has been collapsed into a line spectrum, all of its energy is passed through the narrow band filters; however, the jammer signal is spread over

2-13

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a wide band, and the amount of its energy which is passed through the filter, and hence interferes with proper operation, is proportional to the bandwidth.

3) By using directive receiving antennas so that the gain in the direction of the satellites is considerably greater than the gain in the direction of the jammer.

It is illustrative at this point to derive an expression for the jam-

ming immunity in terms of the parameters which might be selected to

improve it, that is, in terms of the carrier loop tracking bandwidth and

the antenna directivity.

Let J = jammer power, as would be received by an Isotropie antenna

S - P-signal power, as would be received by an Isotropie antenna

G. - actual antenna gain, referenced to receiver input, in J the direction of the jammer

G = actual antenna gain, referenced to receiver input, in the direction of the satellite

N = receiver noise density o '

R = P-code chipping rate

B = two-sided noise bandwidth of carrier tracking loop

SNR = signal-to-noise ratio in carrier tracking loop

Then, it can be readily shown that,

SG SNR = B(N + JG./R )

o J P

One useful measure of jamming immunity is the maximum with-

standable J/S ratio. Note that this is not the ratio of actual received

powers, but the ratio of powers which would be received if an Isotropie

2-14

antenna were used. Another way of looking at this is to recognize that

J/S is the ratio of field strengths in the user's location. The criterion

for withstanding jamming is, of course, that the SNR must be above some

minimum. Thus, we can derive from the previous equation.

(J/S) max

N

B{SNR) . S v 'mm

R

G. 3

Typical valueü for these terms are

S = -163 dBw

N ■ 199.6 dBw/Hz

R = 10.23 MHz P

SNR = 6 dB min

B < 50 Hz

G > -3 dB s

that

With these fixed values, and limits on B and G , it can be shown

(J/S) ,„ ~ 64. 1 + (G /G.) ,„ - 10 log B. v dB, max x s j dB hz

1 1 I I

This expression is plotted in Figure Z-Z. As may be seen, a 50 dB

jamming immunity is reasonably readily achievable (with a nondirective

antenna, and a Z0 Hz loop bandwidth). However, to achieve greater

immunity than 50 dB, one must use either very narrow tracking loop

bandwidths (which will have to be aided), or highly directive antennas

(four of them, which will have to be pointed at the satellites), or both.

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3. USER EQUIPMENT DEFINITION

Using the six classes of users defined by the JPO, the require-

ments of each were examined and reduced to equipment and software

requirements. This process served to demonstrate a high degree

of commonality between the equipments for the different classes.

This commonality can be utilized in reducing the design and test

scope of the Phase I program and thereby, the risk. It also con-

tributes to a large cross class usage of basic hardware to reduce

life cycle costs.

The resulting equipment and software requirements have been

examined to determine the types of design which can satisfy these

requirements and which are practical at low cost. This has led to

the definition of three basic receiver types and three antenna types.

A single computer and control/display answer the Phase I needs. Soft-

ware deltas allow for differing performance needs. Software, written

in a higher order language, facilitates the progression from the Phase

I computer to an operational diversity of computers.

For Phase I testing, equipment has been defined in terms of

equipment groups, the heart of which will become the GPS navigation

set. The equipment group contains all the facility for testing and

equipment group peculiar data collection.

3. 1 EQUIPMENT GROUP DESIGN

Table 3-1 summarizes the driving performance and environ-

mental requirements for each of the six classes. Classes A and B

share the desire for high accuracy under high dynamic conditions

which require continuous tracking of the satellite signals. Classes

C, D, and E require slightly less accuracy and operate in compara-

tively mild environments so the satellite signals may be sequentially

sampled. Class F differs from Classes D and E by requiring a very

short acquisition time so that it is desirable to simultaneously acquire

the satellite signals. Thus, all classes require two basic types of

receiver: a continuous tracker or a sequential tracker. Class C is

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unique in net using the protected code and is therefore a special case

of the sequential tracker. Thus, 3 basic receivers and associated

software modules are required for Phase I.

Classes A and B are also unique in that the high performance

requires the use of auxiliary sensors to provide "carry through" posi-

tion and velocity inputs to the receiver during dropouts caused by

maneuvers or intensive jamming. Two types of a'ding sensors are

I candidates, an inertial measurement system and an attitude heading

( and reference system (AHARS) supplemented by accelerometers. The

outputs of these are input to the filter (in the computer) which com-

bines the auxiliary sensor and GPS data.

In addition to the basic GPS navigation set anH auxiliary sensors,

an equipment group as defined herein provides the supporting equip-

ment and instrumentation for test and demonstration. Figure 3-1

shows the basic equipment group for Classes C, D, E, and F. Con-

figuration change between Class C and D, E, or F is accomplished

by replacing the receiver and entering the proper software. The

equipment mounts in racks on a pallet or in a communications shelter.

This provides for maximum flexibility in moving from vehicle to

vehicle or laboratory and allows for complete verification of the

equipment groups at the factory integration floor.

Figure 3-2 shows the block diagram of the equipment group for

Classes A and B. Note that in providing for the interface between

the aiding sensor and the GPS computer, the existing computer is

retained along with its unique software and interfaces to the IMU.

This allows for little or no change to that software and those inter-

faces and simplifies the GPS Phase I integration task. For opera-

tional systems the possibility of integrating both functions in the same

computer is a system by system consideration.

Figure 3-3 shows a conceptual rack up of a Class A (or B)

equipment group in a test rack for an F4C. Alternatively, it would

be packaged in a pod as represented in Figure 3-4.

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The following sections describe, in more detail, the elements that

make up the equipment groups.

3.2 RECEIVER TYPES AND DESCRIPTION

The receiver design effort supported the system definition so that

items, such as signal structure definition, link budget, time to first navi-

gation for position error allocation, could be optimized. The receiver

design activity stressed feasibility of design concepts, minimization of

life cycle cost and functional operation.

As indicated above, three basic receiver types (see Table 3-2) have

been defined that will meet the GPS system requirements. The baseline

Table 3-2. GPS Receiver Types (Phase I)

User Class Frequency

Type I A, B, and F Ll/L2 Continuous P, C/A

Type II D and E Ll Sequential P, C/A

Type III C Ll Sequential C/A

I I I

receiver design is a continuous four channel tracking type designated as

Type I (see Figure 3-5). This basic type will meet the requirements of

User Classes A, B, and F. Two other types of receivers have evolved

from the system definition study. The Type II receiver serves the needs

of user Classes D and E. This receiver is referred to as a sequential

tracking C/P receiver. The Type III receiver is a low cost model intended

to serve only the needs of Class C users. It is referred to as a sequential

tracking C receiver.

A summary description will be given of the baseline Type I receiver

followed by an explanation of the differences of the Type II and III receivers.

Each receiver design has been divided into a number of basic module parti-

tions for commonality. Care has been taken in the receiver designs such

that all three receiver types make maximum use of these basic common

modules. Therefore, the differences between receiver types are primarily

in the number of modules required to make up the receivers and their operation.

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For a more detailed description of the GPS receivers and

analysis, see Part II, Volume B of the Final Report. The baseline

receiver is a four channel, dual frequency Operation L Band type.

It incorporates digital processing under computer control and employs

phase coherent code and carrier tracking to obtain pseudorange and

range rate tracking making continuous measurements on four GPS

octteliites.

The design is based on TRW experience gained with the receivers

designed and operated at Holloman AFB on the Integrated Navsat

Inertial System (INI) and the Integrated Navsat Hybrid Inertial System

(INHI). The receivers used in these programs have proven very

reliable and many of the techniques and circuits developed for these

receivers can be incorporated directly into the GPS receivers.

The GPS receivers have been partitioned into modular blocks

(see Figure 3-5) and each module utilizes built-in test (BIT) to facili-

tate automatic fault location and to minimize maintenance time.

Definition of the modules was based upon functional allocations and

life cycle cost analysis (see Section 4).

3. 2. 1 Module Definition

The circuitry for the receiver is built on plug-in modules.

Faults in the receiver will be detected, and isolated in the failed

module using BIT. The modules can then be quickly removed and

replaced.

The baseline receiver has been partitioned into functional circuit

modules using the following criteria.

1. Maximum commonality between receiver types.

2. Related functions are grouped together.

3. The number of interface signals between modules is minimized.

4. Digital logic is built exclusively on digital modules, and analogue circuitry build on analogue modules wherever practical.

3-10

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The following Table 3-3 shows the twelve basic modules that

are used in all receiver.

Table 3-3. Type I Receiver Module Listing

QUANTITY PER MODULE NO. DESCRIPTION RECEIVER

1 RF Pre-Amp and First IF Amp 1

2 Frequency Synthesizer 1

3 Reference Oscillator 1

4 BIT Test Generator 1

5 PN Demodulator and 2nd IF 4

6 Code and Carrier Tracking Loops 4

7 AGC and Code Search Detector 2

8 Tracking Measurements 1

9 PN Code Generators 2

10 Bit Sync and Detection 4

11 Computer I/O and Receiver Control 1

12 Receiver Power Supply 1

3. 2. 2 Continuous Tracking Receiver Type I

Figure 3-6 is the detail block diagram of the Type I receiver.

Following is a summary description of its operation.

3.2.2.1 RF Pre-AMP and First IF

The RF Pre-AMP and First IF Module receives L-band signals

from either the antenna or the BIT test generator, filters out-of-

band spurious radiation in the preselector bandpass filter, and then

amplifies the signal in a low noise pre-amp. Overload protection is

provided at the module input to protect against burnout from excess

in-band RF power. A postselector bandpass filter provides image

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rejection. Local oscillator (LO) frequencies are injected into a mixer

to produce a first intermediate frequency (IF) of 55 MHz. The IF is

amplified in the first IF amplifier, and the signal is distributed to

the four PN Demod and 2nd IF modules.

The design of the RF Pre-Amp module provides for receiving

either the L. or L carrier. This is accomplished by designing the

pre-and post-selector bandpass filters as dual response filters having

30 MHz passbands about the L. and L_ center frequencies. The pre-

amp is a wide band device employing low noise figure transistors

which passer both L. and L- signals. The desired carrier is trans-

lated to 55 MHz, while the undesired carrier is rejected by the band-

pass characteristics of the first IF amplifier.

3. 2. 2. 2 Reference Oscillator

The reference oscillator is a crystal oscillator whose frequency

is 5. 115 MHz. This oscillator provides the timing reference for all

locally generated signals in the receiver, and is the source of a time-

of-day interrupt which is used to synchronize computer operations with

the receiver. The 5. 115 MHz frequency drives the frequency synthe-

sizer, which produces and distributes all the locally generated refer-

ence signals.

3.2.2.3 Frequency Synthesizer

The frequency synthesizer module accepts a precision 5. 115 MHz

reference frequency from the Reference Oscillator and generates a

number of local signals used for rf reference frequencies and for

digital timing. A local oscillator multiplier chain generates frequen-

cies at 10.23 MHz, 15.345 MHz, 40.92 MHz, and the first IF injec-

tion frequency (referred to as L-.L. 0. or L.L. 0. ). The 40.92 MHz

clock is sent to the Tracking Measurements module, which produces

divided down versions of the clock. These signals are then mixed

as shown to produce reference frequencies at 10.23 (1-^ )MHz, and

40.92 (1-^ )MHz. Here Fc is the 10.23 MHz code cloÄ frequency.

I and L is tffe L, or L, carrier frequency.

I 3-13

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3.2.2.4 BIT Test Generator

The BIT Test Generator Module is used to produce a tes«-

signal at L-band which is coupled into the receiver in the RF Pre-

Amp Module with a 30 dB directional coupler. The test signal is

used to evaluate the receiver under computer control thus locating

failures to the module level for quick repair of the receiver by module

replacement. Maximum use of computer algorithms, rather than

elaborate test hardware, is used to reliably detect and isolate faults.

The test signal is a replica of a typical satellite signal,

| generated from reference frequencies produced by the frequency syn-

thesizer. Test generators produce PN codes, the manchester code,

and a pseudorandom data word which is phase modulated on a test

carrier which is multiplied up to L-band. A selectable attenuator

provides for strong signal and threshold level checkout of the receiver.

3. 2. 2. 5 PN Demodulator and Second IF

The PN Demod and Second IF module, accepts the amplified

first IF signal, adjusts the receiver gain with the AGC amplifier,

demodulates the selected PN code, and mixes the demodulator

output with the VCO frequency to produce the second IF frequency.

The second IF signal, which is at 15.345 MHz, is amplified by the

second IF amplifier, and sent to the Code and Carrier Tracking

Loops module.

The AGC amplifier is controlled by a control voltage received

from its AGC detector. The AGC output then goes to a PN demodu-

lator which utilizes a locally generated code to reconstruct the carrier

of a desired satellite and generates a code tracking error signal. The

PN demodulator employs sequential balanced mixers and a locally

generated coincidence code to provide 70 dB suppression to a CW

jammer. The demodulator output is applied to a 20 KHz crystal

bandpass filter which suppresses jammer and thermal noise power

by 27 dB. The filter carrier is applied to the second mixer and

multiplied against the 40. 92 MHz VCO output signal from the carrier

3-14

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tracking loop. The mixer output is a signal at 15.345 MHz which,

when the carrier loop is locked, is phase coherent with the 15.345

MHz reference produced by the local frequency generator. The mixer

output is amplified by the second IF amplifier, and sent to the carrier

tracking loop.

3. 2. 2. 6 Code and Carrier Tracking Loops Module

The code and carrier tracking loops modulf, contains the

circuitry to phase lock a Voltage Controlled Oscillator (VCO) to the

incoming signal obtained from the second IF amplifier, and to

generate a code clock whose phase is adjusted such that a reference

PN code generator is kept running in phase with the received PN

code modulation.

The tracking loop is a Costas type, since the carrier is fully

suppressed by the biphase modulation. A 15. 345 MHz reference fre-

quency is used to detect the in-phase (I) and quadrature (Q) compo-

nents of the receiver carrier. Low pass filters follow the multipliers

to establish the optimum noise bandwidth for tracking. The filter

also accepts an AFC control signal which prepositions the VCO and

assures rapid carrier acquisition. The loop filter output drives the

VCO, whose output is used to close the tracking loop at the second

mixer, and to provide a doppler signal to the Tracking Measurement

Module.

An indication of carrier lock is obtained by passing the I- and

Q-signals through square law detectors, and subtracting the Q from

the I component. This signal is filtered and compared against a

reference voltage to obtain lock status.

3. 2. 2. 7 AGC and Code Search Detectors Module

This module contains two AGC detector circuits and two code

search detector circuits.

The AGC detector serves to maintain a constant noise power at

its input when signal is not present, and to maintain a constant signal

to noise power when the signal is present.

3-15

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I The Code Search oetector is a noncoherent envelope detector

I followed by a threshold comparison device. The AGC circuit is de-

signed to keep the noise power constant, even in the presence of

I jamming, so that the false alarm probability of the detector output is

correct. When signal is present, the output of the detector is greater

than the threshold level established for detection. The detector then

I produces an in-lock indication, which stops the code search and

enables the tau dither code tracking loop.

The AGC detector is a noncoherent detector which accepts I-

and Q-input signals from the carrier tracking loop. The signals are

passed through predetection low pass filters to establish the desired

noise bandwidth, and then square law detected. The I and Q components

are summed and post detection low pass filtered to generate a non-

coherent AGC error signal. This signal is added to a reference

voltage and amplified to obtain the AGC control voltage.

The noncoherent code search detector functions in a similar

manner to the AGC detector. The differences are that a narrower

predetection filter bandwidth is used, and the signal out of the post

detection low pass filter is compared against a pre-established

reference voltage. This reference is set up so that when the reference

code is not correlated with the received code, the signal into the

voltage comparitor (which is proportional to noise power) is less

than the reference voltage, and the code lock indicator is out-of-lock.

When the received and reference codes are correlated, the signal into

the voltage comparitor (which is now proportional to signal plus noise

power) is greater than the reference voltage, and the code lock indi-

cator is in-lock.

The lock detector is used to determine the status of the code

search during initial acquisition of a new satellite. The selection of

bandwidths and threshold reference voltage determine the false alarm

and missed detection probabilities which affect the code acquisition

' time.

I I I 3-16

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3. Z.Z. 8 Tracking Mt'£isuremfnt Moduh-

The Tracking Measurements Module, obtains measurements of

course velocity, fine velocity, and course range. It also generates

two reference clocks for use by the Frequency Synthesizer module,

and a receiver time-of-day (TOD) pulse which serves as a read

command, and as an interrupt to the computer.

Velocity measurements are obtained by mixing the VCO signal

from each carrier tracking loop with the locally generated F, reference

signal. (This is done in the code and carrier tracking loop modules).

The output of the mixing process is a signal whose frequency is the 40 92 sum of a bias frequency, -rrk— MHz, and the difference between the

local oscillator 15.345 MHz and the received carrier frequency down-

converted to 15.345 MHz. This difference frequency contains the

doppler shift frequency, and is called biased doppler.

Course velocity is measured by clocking a counter with the

biased doppler signal, and reading the state of the counter at each

TOD read command (each 0. 102-second intervals). Fine velocity is

measured by reading from the time of day (TOD) counter the time

between the read command the end of the current cycle of biased

doppler. Since the TOD counter is clocked at 40. 92 MHz, the fine

velocity count has a resolution of about 25 nanoseconds.

Course range measurements are made by counting cycles of

each code clock in a binary counter called the Course Range counter.

Whenever the TOD read command occurs, the counter contents are

parallel shifted into the course range buffer for readout to the computer.

Since the code clocks are slaved to the received code clock from each

satellite, the counter contents represents a measure of range. The

quantization level of the count is one code chip, which represents

about 100 feet for the P-Code, and 1,000 feet for the C-Code.

3. 2 2. 9 PN Code Generators Module

The PN Code Generators module, accepts a 10. 23 MHz code

clock from the code and Carrier Tracking Loops module, generates

3-17

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the replica C or P code, and produces a PN Code output and a

dithered PN code output which is used to deruoduiate the PN code

from the received satellite signal.

The module provides code search control logic for initial

acquisition of the C code, and provides a C-code epoch detector

which provides C-code epoch pulses to the Bit Sync and Detection

Module. P-code handover logic is used to decommutate P-code

handover information obtained from the C channel data, and start the

P-code PN generators running in synchronism with the received P-

code. A data bit sync detector observes P-code generator epochs

to derive bit sync for extracting P-channel data.

The module provides an interface for a security encryption

device which is supplied by the government. A bypass switch is

included to permit use of the receiver in either a secured or

unsecured mode.

3.2.2.10 Bit Synchronization and Detection Module

The Bit Sync and Detection Module detects data bits in the re-

ceived signal, obtains a bit sync pulse, decodes the differentially

encoded data, and provides data bits and bit sync to the PN Code

Generator module and the computer I/O and Receiver Control module.

The data signal is obtained from the coherent amplitude detector

output of the Costas tracking loop (I-signal). A locally generated

replica of the Manchester code square wave is multipled with the

received data signal to remove the Manchester code. The Manchester

decoding is bypassed using the data seiuct switch if data is extracted

from the P-code.

Bit synchronization is obtained as follows. When data is extracted

from the P-code, bit sync occurs in coincidence with a particular

state of the p-code generator. Thus, a bit sync detector circuit

detects the receiver's P-code generator state which coincides with

bit sync, and generates a bit sync pulse.

3-18

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When data is extracted from the C-code, bit sync occurs in

coincidence with one of the C-code PN generator epochs. However,

there are twenty epochs for every bit sync occurrence, and four

epochs for every Manchester code transition. An epoch detector

generates pulses coincident with the receiver C-code generator epochs.

These pulses are divided by five, and the resultant pulses drive a

local Manchester code generator. The Manchester pulses are further

divided by four to obtain replica bit sync pulses. The bit sync

decision logic slows the divide by five counter until the digitized i

magnitude of the matched filter output is maximized. The sync decision

logic then declares an in-lock condition, and bit sync is automa-

tically obtained by dividing down the C-code epoch pulses.

3.2.2.11 Computer I/O and Control Module

The Computer Input-Output (I/O) and Control Module provide the

necessary logic and control circuitry to transfer control information

from the computer to the receivers, and to transfer satellite data,

tracking measurements, and receiver status from the receiver to the

computer. In addition, a single interrupt (the time-of-day pulse) is

sent to the receiver reference clocks.

The module also contains circuitry necessary to configure the

receiver according to the configuration commanded by the computer,

and to execute carrier and code pre-positioning.

3. 2. 2. 12 Receiver Power Supply Module

The Receiver Power Supply Module accepts primary power from

the user vehicle supply bus, and converts the input power to dc

voltages adequate for operation of the analogue and digital circuitry.

The module includes a primary power on-off switch, fuse, on-off

light, and elapsed running time meter. The power supply itself will

provide AC to DC conversion from aircraft 400 Hz power, and provide

adequate filtering and voltage regulation lor proper operation of the

receiver. A power-on reset flag (POR) is set each time power comes

on. The flag is turned off by the computer, indicating recognition of

3-19

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a POR condition in the receiver. This is important, since a power

glitch in the receiver will introduce timing glitches in the reference

clocks which run off the reference oscillator. When a POR occurs,

the receiver will remain configured in the state it was in prior to

power interruption. It will be up to the computer to decide if the

receiver should change its configuration in response to a power glitch.

A built-in test point (BTP) will provide voltage sensing to insure

that the power supply is providing voltages in the allowable voltage

range.

In the continuous receiver described in this baseline, two sepa-

rate plug-ins constitute the power supply module.

3. 2. 3 Sequential Tracking Receivers (Types II and III)

This section discusses details of sequential tracking receivers.

There are two types of sequential trackers of interest. A Type II

receiver tracks four satellites sequentially in either the C-channel

or the P-channel. The block diagram for this type of receiver is

shown in Figure 3-7. It uses many of the same modules as

the Type I receiver (continuous tracker). The PN code generator

module and the Computer I/O and Control Module are somewhat

different. The Frequency Synthesizer Module does not require L_

carrier capability and the Tracking Measurement Modules need

counters for only one satellite, not four. The PN generator module

must provide flywheel storage for the state of the PN codes of those

satellites not being tracked, but which are in the user's constellation.

Since the user may navigate in either C or P, flywheel storage is

provided for both C and P Code PN generators.

The Sequential receiver contains two processing channels.

Channel A is used for obtaining tracking measurements from each of

the satellites whose code and carrier states have been acquired, and

are therefore in the constellation of sequentially tracked satellites.

Channel B is used to acquire new satellites for constellation revision,

and for receiving satellite data from any satellite in view without inter-

fering with the sequential tracking of the navigation constellation.

3-20

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The computer I/O and Control Modules must accommodate those

features of the Sequential receiver which are unique from the continuous

tracking receiver. Its design is therefore somewhat different, but

its principles of operations are the same.

The Type III receiver is a sequential tracker which tracks only

C-channel signals. Its construction and operation are the same as the

Type II receivers, but all P-code hardware functions are removed.

This receiver is the lowest cost receiver.

3. 2. 4 Mechanical Design

The packaging of the GPS receiver for airborne applications

was based upon modular construction in a Short Air Transportable

Racking (ATR) case per MIL-E-5400 Class 2 and provides for low-

cost economic production.

The basic Type I design is in accordance with the requirements of MIL-C-172 for an MS-91403 case. As depicted in Figure 3-8, the

unit configuration is basically a short ATR, having the parameters of

10. 125 wide x 7.62 high x 12. 56 long.

The construction of the receiver consists primarily of a . 05"

aluminum chassis fabricated in a manner to achieve minimum weight

with< t sacrificing structural strength. A .06" panel is attached as

shown to form the front face. Gussets attached to the chassis and

front panel further strengthen the unit.

The rear mounting interface utilizes DPA rack and panel

connectors that accommodates power, signal, and RF-connections.

Connector alignment is achieved by the rear guide pin receptacles

that interface with the aircraft mounting guide pins. The guide pins

in conjunction with the hold-down hooks attached to the front panel

serve as structural support for the receiver.

The receiver is designed to contain up to 20 plug-in modules of

similar external configuration, for RF/IF processing functions and up

to 10 PC cards for digital functions that are contained in a card cage.

Interconnections between modules have been reduced as much as possible

by sharing related functions in a common module. It is significant to note

i%

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that the 20 modules and 10 PC cards also represent growth possibilities

since higher densitv of packaging is anticipated.

The XTAL oscillator and power supplies are also plug-in modules.

All interconnect wiring and coax-connectors are located and accessible

from the bottom of the chassis.

To provide accessibility to the module back plate interc mnect, the

power supply modules may be removed via a disconnect plug, or pivoted

outward to provide testing without the utilization of an adapter plug and

harness.

To provide BIT capability, fault indicators are included adjacent

to each module that isolates the fault to an SRU.

The front panel contains the following items:

a) Handle

b) Hold-down hooks

c) Fuses and holders (2)

d) Elapsed time indicators

e) Fault indicators (latch-type) isolation to LRU

Currently the concept is based around a common modular diecast

housing having partitions separating the module into one or four com-

partments. RF input/output connections are accomplished via coax

connectors of the plug-in type on the base of each module. Signals that

do not require shielding, such as power, controls, and low data rate

pulses, will use normal open wire connections. See Figures 3-9 and

3-10.

Two open connector concepts are shown: 1) Individual insulated con-

tacts inserted into a defined hole pattern integral with the module, and 2)

conventional connector separately mounted in the module. Guide pins are

used as polarization pins to preclude any mismatch of modules.

Several concepts have been reviewed for the interconnecting of the

modules and the input/output rack and panel interface connector. It is

significant to note that a high degree of maintainability efficiency has

been designed into the unit since all modules, back plate interconnect

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plate, and wiring interconnect harness can be considered as independent

assembles and comply with the plug-in concept.

Interconnect options are:

1) Wire-wrap using automatic assembly

2) Mother-board (multilayered)

3) Flat-cable

4) Hardwire (solder or crimped)

5) Combination

Wire-wrap was selected as the baseline wiring method. The

advantages of wire-wrap are that it provides a programmable automatic

production type of technique with economic advantages, design flexibility,

and ease of repairability.

Cover

The dust cover is fabricated from . 04" aluminum having cutouts in

the back face for the rack and panel connectors and guide pins. The top

and bottom surfaces contain a series of 1/8" diameter punched holes for

the free convection of the internal heat to the outer ambient. Two 1/2"

wide strips of rubber (closed cell) and nylon of suitable thickness are

bonded to the upper surface of the cover. These strips coincide with the

grooves on the top surfaces of the modules to physically contain the mod-

ules, and to assure electrical continuity to their mating connectors on the

back plate under the environmental physical requirements. Quick release

captive fasteners contain the cover to the chassis.

Physical Characteristics

Size

Volume

Weight

Dissipation

10. 125" x 7. 625"H x 12. 563'^

1000 cubic inches/0. 0168 M3

28. 5 lbs (estimated) 13. 0 Kg

60 watts (maximum)

3-27

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Thermal Considerations

Power dissipation in the receiver is less than 60 watts.

An array of 1/8" diameter holes in the top and bottom surfaces of

the dust cover allows cooling air to flow through the electronics. Cool-

ing air entering the bottom of the receiver washes the external surfaces

of all the modules and exits at the top. All modules are enclosed and,

therefore, are not subjected to contamination.

Critical devices are mounted on conductive surfaces that contribute

to maintaining the devices well within their operational rating.

The watts/square inch density relative to the 540 sq in exterior

surface area is approximately 0. 11 w/sq in. This heat dissipation is

considered well within the art of natural cooling, convection, and radiation.

External cooling other than natural convection and radiation is, therefore,

not considered. Temperature rise in the air stream is anticipated not to

exceed Z0oC.

EMI Consideration

A high degree of RF isolation is provided by the inherent design of

the modules. The module containers are pratitioned to minimize local

RF effects. To prevent radiated noise from entering or leaving the en-

closure, the modules are RF sealed. Filters are provided on input/output

power and signal lines to reduce the effects of conducted interference.

Maintainability

To achieve the maintainability goals, the plug-in modular concept is

maximized throughout the design. Virtually all circuitry is packaged on

printed circuit cards and contained on plug-in type modules. The dust

cover utilizes quick-turn fasteners for easy removal and access to the

modules. Fault indicators are provided to quickly assess and isolate to

an SRU. Card extractors are provided to facilitate removal of PC cards.

Guide pins and polarization is provided to preclude mismatch of modules.

Modules are provided with a diagonal white strip that runs in a progres-

sively diagonal fashion to facilitate quick visual location of each module.

3-28

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I I I 1

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The modules plug into a connector backplate that is mounted onto the

chassis. This connector plate is prewired and can be defined as a plug-

in, since by simply removing the I/O connectors via jack-screws and the

coax's connectors will enable quick replacement.

The XTAL oscillator is also modularized and plugs into its recep-

tacles. Quick-turn fasteners are provided on these modules to maintain

physical environmental integrity.

The power supplies are contained beneath the connector backplate

and accessible from below. These units are contained by two quick-turn

release fasteners that interface with a structural support bracket that

also provides good heat-sinking to the chassis. The opposite end of the

power supply is contained on a cradle that is integral with the chassis,

but has the capability of enabling the power supply to pivot outward and

provides complete accessibility to the connector backplate and the coax

connectors thereon. Removal of the mounting screws that maintain the

power supply to the cradle and disconnecting the plug-in hardness con-

nector via jack screws contained on the connector, enables the removal

of each power supply.

The interconnect wiring harness has been designed to provide easy

replaceability, and simply requires removal of the fasteners that contain

the connectors to the chassis.

The 5. 115 MHz crystal oscillator, located at the rear section of the

chassis, contains high quality components in a ruggedized construction

design that is vibration compensated to assure high reliability and optimum

performance.

Mechanical Design Considerations for Sequential Receivers

The basic design is similar to that of the four channel GPS receiver

and was specifically designed for maximizing the plug-in modules utilized in the aforementioned unit.

3-29

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The basic design is «tlso in accordance with the requirements of

MIL-C-172 for a MS-91403 case. The unit configuration is shown in

Figure 3-11, and configures the envelope of a 1.2 short ATR, having the

physical parameters of 4. 875 wide x 7. 624 high x 12. 562 long.

3. 3 COMPUTERS

The computer for an operational navigation set performs the

functions listed in Table 3-4. In addition, for the test phase, it will

log data to printers and recorders to assist in inflight test monitoring

and postflight data reduction. The sum of these functions do not require

any unusual speed, memory size, or special requirements on the com-

puter. Further, although the operand size reaches 29 bits, it is still

possible to use a 16 bit word length computer because the number of

words in excess of 16 is relatively small. The use of 16 rather than

32 bits size will significantly reduce computer hardware costs.

Table 3-4. Computer Functions

!

I I I

Initialization

Executive

Acquisition

Data assembly

Measurement processing

Satellite selection

Orbit propagation

Pseudorange geometry

Filter

Navigation computation

User input/output

Navigation set input/output

Auxiliary sensor processing

Self-test

3-30

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Since the computer is not a technological issue, the use of off-the-

shelf units is desirable. In Phases II and III, it can be expected that

newer computers and perhaps a variety of computers will assume the

tests performed by the Phase I computer. This raisea the question of

effects on GPS of a changing receiver/computer interface. A standard

avionics interface such as envisioned by the DAIS program would satisfy

the differences in hardware, and, to a smaller extent, the software dif-

ferences. Elimination of the majority of software differences is accom-

plished by use of a higher order language such as Fortran.

Examination of the requirements for the user classes reveals that

the same basic functions are required for most classes. The variation

is more one of degree than function. This leads to the conclusion that a

single type of computer can be used across all classes for Phase I. In

later phases, optimization of the computer to specific applications (such

as the manpack) can take place.

A type Cla specification for the user segment computer has been

prepared. The reader is referred to the Philco Ford/TRW Specification

Numbered C237300, dated 30 January 1974, entitled "Prime Item Product

Function Specification for User Segment Computer Global Positioning

System (GPS)".

3. 4 CONTROL AND DISPLAY

The operational user will be interested in the display of the follow-

ing classes of parameters:

a) Position

b) Navigation

c) Time

d) Data being entered

e) GPS/equipment status

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sion and, in some cases, his mode of operation, i. e. , position stated in

terms of latitude and longitude, distance and heading to waypoint, local

grid coordinates, etc. Units and resolution may vary. Methods of pre-

sentation will vary. Operationally, when integrated with existing avionics,

it will be desirable to use existing displays.

Table 3-5 shows user inputs by classep. Again the facility for entry

of these data may already exist onboard a particular user. For cases

where a dedicated unit for control and display is required, a panel, such

as Figure 3-1Z depicts, provides the facilities needed.

For Phase I, the control and display functions can be adequately

provided by use of existing units supplemented by GPS peculiar add-ons.

Figure 3-13 depicts a Class A display and control implementation. The

left panel is a TALONS display and control which is typical of the type

which can be used. The right hand panel contains those GPS peculiar

controls and indicators which are not accommodated by the existing design.

This panel displays the GPS/inertial combined position or the GPS derived

position by operator selection. Inertial derived position is available

separately via the INS control and display panel. All three values are

available for recording for postflight analysis.

Specific details and definition of the control and display units will be

presented when the test vehicles, equipment groups, etc. , are assimilated

with the ongoing test planning activity.

3. 5 ANTENNAE

3, 5. 1 Discussion

Antennae for various users were defined subsequent to preparing

preliminary specification of the antenna characteristics. The preliminary

specifications. Table 3-6, were based upon system considerations such

as link budgets, satellite power, etc. A brief explanation of the specified

parameters is presented below.

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Table 3-6. Electrical Performance Specification for GPS Antenna

Performance The antenna or antenna array is used to receive radio frequency signals from satellites.

Functional Characteristics

Frequency Bands

Polarization

Passband VSWR

Gain and Radiation Patterns

The antenna shall operate in the frequency bands of 1,23 to 1. 25 GHz and 1. 565 to 1, 585 GHz,

The antenna shall be right-hand circularly polarized (RHCP), The definition of "rigl^ hand circular" is in accordance with the " IEEE Standards on Radio and Wave Pro- pagation,

The input impedance of the antenna shall present a voltage standing wave ratio (VSWR) of 1. 3 or 1 or less when referenced to 50 ohm resistive over the specified frequency band.

In response to RHCP, the antenna shall provide a quasi-omnidirectional pattern in the upper hemisphere. The antenna shall have a minimum gain of -1, 0 dBi toward the zenith. The antenna gain shall exceed -2 dBi over 95 percent of the 160-degree radiation cone centered at the zenith. The gain shall exceed -4 dBi over 90 percent of the angular region between 160-degrees and the horizon.

Axial Ratio

Radome

The maximum axial ratio shall be 5 dB over 90 percent of the 160-degree radiation cone centered at the zenith.

The presence of radome (if necessary) should not degrade the above stated requirements.

1 'ft-

37

Gain and Radiation Pattern

The upper hemispherical pattern requirement represents optimum

coverage for the user antenna since it will enable the user to receive

signals from the greatest number of satellites. The additional satellites

visible by the user (beyond the minimum of three or four) will improve

the system accuracy if properly selected.

A cone of 160 coverage is considered the primary viewing direction

because of the larger errors at lower elevation angles. In addition, it was

felt that the user antenna should have a minimum backlobe so that very

little of the noisy earth environment (including multipath) will be illuminated

when the antenna is operating in its normal position pointing skyward.

Polarization

Circular polarization is selected because it possesses desirable

characteristics for rejecting multipath signals and provides the best over-

all system performance in areas of atmospheric and rainfall effects. Multi-

path rejection is accomplished when reflection from ground or water

reverses the sense of circular polarization. The reflected waves with

their reversed polarization sense are then partially rejected by the cir-

cularly polarized user antenna.

VSWR

The antenna bandwidth is usually defined as the frequency band for

a given VSWR which is the ratio of the antenna impedance to the trans-

mission line impedance. The antenna impedance, which is frequency-

dependent, depends on many factors, such as geometry and proximity

to surrounding objects. The specified antenna impedance for a navi-

gation satellite user antenna is established for a VSWR of less than 1. 3:1.

Axial Ratio

The axial ratio of an antenna radiation pattern is an indication of

the antenna's circular quality and should be maintained at a small value.

As the axial ratio becomes lower, smaller polarization losses result.

For example, based on a 2-dB axial ratio for the satellite antenna and a

3-38

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to 0. 68 dB. The range of losses will depend on the orientation of the two

major axes of the satellite transmit antenna and the user receiver antenna.

When the two axes coincide, the polarization loss is at a minimum; when

the major axes are orthogonal to each other, a maximum polarization loss

occurs.

3. 5. 2 Antenna Types

For supersonic aircraft, it is mandatory that the antenna does not

introduce appreciable drag. It is also desired that no elaborate structural

modifications be required. For this application, and possibly for subsonic

A/C as well, a flush mounted crossed slot antenna has been proposed.

The cross slots meet the requirements of Table 3-6. Gains at the horizon

are approximately -2 dB with respect to circular isotropic. A sketch of

a crossed slot antenna is shown in Figure 3-14.

For other users not requiring flush mounting, a conical log spiral

is the recommended configuration. The spiral exceeds the specified

requirements and effectively achieves 0 dBi over the entire upper hemi-

sphere with respect to circular polarization. Also the axial ratio is

generally less than 4 dB over the hemisphere.

The antenna performance of a conical log-spiral antenna can be

properly specified in terms of the following five parameters:

1) Included cone angle, 26

2) Armwidth angle, 5

3) Spiral rate, a

4) Base diameter, D

5) Apex diameter, d

A conical log-spiral antenna with these parameters is shown in

Figure 3-15.

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The apex diameter d. and the base diameter. D. are selected for the highest and lowest frequency of operation.

3. 6 AUXILIARY SENSORS

Auxiliary sensors provide two significant user benefits:

1) Continuous tracking of vehicle dynamics

2) Continuous navigation during satellite signal lo sses.

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Considering number two first, this capability could be used by any class

i as a backup, degraded, or in the submarine case, the prime mode of

navigation. The constitution and degree of integration of the GPS re-

ceiver and other sensors is determined by the mission objectives of the I

I I I

user.

For vehicles having high dynamics and require high accuracy, it is

desirable to supplement the GPS receiver so that rapid reacquisition of

the satellite signal will result in the event the signal was lost due to the

dynamics or masking of the antenna by the vehicle. By including an

auxiliary measurement of attitude and attitude changes on the aircraft,

the computer estimates of range and range rate to the satellites can re-

main within the tracking limits for a significant period, this affording

the rapid reacquisition.

The prime candidate as an auxiliary sensor is the inertial navi-

gation system, many of which are currently in use in high performance

aircraft. They provide continuous incremental measurements which are

then combined with GPS receiver measurements in a recursive filter to

provide the navigation solution. If the GPS signal is lost due to antenna

occulting or jamming, the filter continues to solve for position and

velocity, even if all satellites are lost. When the satellite signal is

again available, the onboard maintenance of position and velocity allows

quick signal reacquisition. Conversely, when satellite signals are avail-

able, the GPS measurements, being of higher precision, drive the total

navigation solution to minimum error states. Table 3-7 shows some of

the inertial sets recently tested or employed in Air Force/Navy aircraft.

Selection among these is narrowed by the criteria of Table 3-8. Recom-

mendations as to which should be used for Phase I will be made subsequent

to more detailed analysis of the testing required, test vehicles, and cost

(risk) of system integration.

Other sensors, such as altimeters, air data systems, attitude and

heading reference systems, accelerometers, etc. , can also be utilized as

aiding devices. As in the case of the IMU, recommendations on the use

and selection of these sensors will be made as the test planning process

continues.

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3. 7 INTERFACE HARDWARE

The interface hardware provides the special interfaces required for

Phase I test flexibility without institutionalizing it as a part of the prime

equipment design. Its functions are:

1) Multiplexing into the computer inputs from the receiver, controls, auxiliary sensors.

2) Demultiplexing computer outputs to the receiver, display, auxiliary sensors, instrumentation, and external systems.

3) Providing operator control to equipment group operation (i. e. , power switching, etc. )

4) Power conversion/isolation.

5) Signal conversion facilities as required for interface compatibility.

It is represented in Figure 3-16 by the I/O block and the power

supplies/inverter block. It will be composed of a combination of off-

the-shelf adapters and, where necessary, special logic and control cir-

cuits. Due to the use of a single computer for Phase I across all classes

and the degree of standardization of other elements of the receiver groups,

the interface hardware also varies little from equipment group to equip-

ment group.

Figure 3-17 shows one method of providing an interface between an

inertial system and GPS. This example uses the ASN-90/91 INS and the

ROLM 1602 computer. The actual implementation will be dependent on the

INS and GPS computer selected. For this example, most of the interface

is accomplished by standard hardware cards, only the time of day clock

being special. Only minor software changes in the ASN-91 would be re-

quired.

3. 8 SOFTWARE

The computer program requirements for the various user classes

were developed. Figure 3-18 depicts the program functions of the Class

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A user software and Figure 3-19 depicts the Class C user software. Again,

the requirements between classes are largely of degree with some functions

(such as auxiliary sensor data processing) deleted for the simpler user.

The program has been organized using a synchronous executive so that

processing proceeds in a predetermined order and rate. Table 3-9 shows

the approximate interaction rates required for the various functions. The

"as-required" functions are those primarily used during acquisition, hand-

over or self-test.

Table 3-10 shows the estimated memory usage for the low cost

user (Class C). These estimates were derived by comparison for similar

functions from the INI/INHI programs; for dissimilar functions (acquisition,

data assembly, satellite selection, and orbit propagation), specific methods

of implementation were identified and estimated. The resulting program

utilizes only 20-40 percent of the duty cycle of candidate avionics computers,

depending on the features and speed of a particular machine. For the class

A and B users, the addition of the axuiliary sensor data and a recursive

filter result in an increase in memory storage of about 4k-16 bit words.

The impact on duty cycle is small since a total filter solution is performed

at a low rate.

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Table 3-9. Computational Rates

Function Computational Rate

Initialization As required

Executive 8/sec {

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Data assembly 60 min 1

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Satellite selection As required

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i Pseudorange geometry 8/sec j

1 Filter (segment) 8/sec

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User input/output 8/sec

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1 Self-test As required

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4. SYSTEM EFFECTIVENESS

Major items considered under the "system effectiveness" title are

reliability and maintainability, life cycle costing, user equipment harden-

ing, and EMI. Each of these is considered in turn,

4. 1 RELIABILITY AND MAINTAINABILITY

A reliability and maintainability analysis was conducted for the

GPS receiver and documented in Reference 4, 1. In addition to a failure

modes and effects analysis (FMEA), this document contains proposed

built-in test concepts, maintenance philosophies, reliability and main-

tainability models and associated calculations, and an Optimum Repair

Level Analysis (ORLA),

The analysis, based upon an early receiver configuration (1 Decem-

ber 1973), is being updated as additional detail on the receiver design is

obtained. The baseline receiver was comprised of eleven types of mod-

ules so defined to provide commonality between the various types of

receivers. Each module was defined in detail so that a piece parts cost-

ing and reliability analysis could be conducted. Parts failure rates (both

military and high reliability) were appropriately modified by K factors

for an airborne application. Utilizing military standard parts, the se-

quential receiver MTBF was calculated to be 1170 hrs, whereas the high

reliability parts provided a receiver with 6400 hrs.

The ORLA analysis considered a "module discard at failure" con-

cept and a "depot repair" concept. Intermediate level repair was not

considered after initial analysis indicated that this was not the optimum

method of support. Again, receivers were considered using both military

and high reliability parts. In the former case, three modules were can-

didates employing the high reliability parts. Since the analyses were

Reference 4. 1. WDL-TR-5293, "User Segment Reliability and Main- tainability Allocations, Assessments, and Analysis Report", dated 30 January 1974

I I 4-1

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preliminary, no firm recommendation was made on the two options.

However, a maintenance concept for each alternative was established.

These concepts are summarized in Tables 4-1 and 4-2.

4. 2 LIFE CYCLE COST ANALYSIS

Subsequent to the Reliability and Maintainability Assessment Report

analyses described in Section 4. 1, additional analysis was performed

utilizing the SAMSO Life Cycle Cost Model. The latter analysis utilized

the current receiver design as well as modified module configurations that

resulted from the earlier R8tM work. Full details of the life cycle cost

analysis are contained in Reference 4. 2.

Based upon the still preliminary nature of the cost and MTBF num-

bers as well as some limitations in the life cycle cost model, the follow-

ing conclusions were reached:

1) Base repair for the receiver is not cost effective.

2) Utilization of high reliability parts in tho receivers is cost effective.

3) Module discard, rather than repair, appears cost effective.

A summary of the LCC analysis test cases is presented in Table

4-3. Full details of the assumptions are contained in Reference 4. 2.

Because of the commonality of modules between the sequential and con-

tinuous tracking receivers, the inputs to the computer program are in

terms of the sequential receiver modules. The continuous tracking

receivers are fully considered in the analysis, and are composed of the

required number of common modules.

4. 3 HARDENING

A brief study was conducted to define the nuclear environment for

hardening effects of the GPS user equipment, indicate the probable effects

I I I

Reference 4. 2. DNSDP-DEW-274, "TRW Life Cycle Cost Analysis as of January 1974", dated 13 February 1974

4-2

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Table 4-1. Maintenance Concept Summary (Depot Repair)

Receiver Maintenance Concept

• Three levels of maintenance:

Organizational

Intermediate

Depot

Organizational Maintenance

• Pre-mission, post-mission, and in-mission end-to-end status testing of the equipment will use BIT equipment.

• No servicing will be required.

• Corrective maintenance will consist of fault isolation to a line replaceable unit (LRU) using BIT, removal and replacement of an LRU and verification of fault correction using BIT.

Intermediate Maintenance

• Intermediate maintenance will be performed either on the vehicle or in the base shop.

• Fault isolation to a shop replaceable unit (SRU) will use BIT.

• Repair will be by replacement of the SRUs.

• Verification of the corrective action will use BIT.

• SRUs will be returned to the depot for repair.

Depot Maintenance

• Depot maintenance will include overhaul, repair, and refur- bishment of LRUs and SRUs.

• Performance of Time Compliance Technical Orders.

• Performance of Analytical Condition Inspections.

4-3

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Table 4-Z. Maintenance Concept Summary (Module Discard at Failure)

Receiver Maintenance Concept

• Two levels of maintenance:

Organizational

Intermediate

Organizational Maintenance

• Pre-mission, post-mission, and in-mission end-to-end status testing of the equipment will use BIT equipment.

• No servicing will be required.

• Corrective maintenance will consist of fault isolation to a line replaceable unit (LRU) using BIT, removal and replacement of an LRU and verification of fault correction using BIT.

Intermediate Maintenance

• Intermediate maintenance will be performed either on the vehicle or in the base shop.

• Fault isolation to a shop replaceable unit (SRU) will use BIT.

• Repair will be by replacement of the SRUs.

• Verification of the corrective action will use BIT.

• SRUs will be discard-at-failurc items.

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of the nuclear environment on the systems and circuits, and to provide

general guidelines to minimize the deleterious effects. Details of that

study are contained in Reference 4. 3.

The environment chosen is the Air Force Specification for the B-l

bomber.

The nuclear radiation and EMP effects include:

Gain degradation

Logic circuit scrambling

Spurious signals

Semiconductor and metalization burnout

Hardening against these effects may be accomplished by using one

or more of the following methods:

Surge arrestors

Circumvention

Hardened registers

Photocurrent compensation

Shielding

Piece part selection and derating

It is envisioned that the user equipment might be hardened to two

levels. Level I hardening would incorporate hardening measures which

could be included in the design with small incremental cost increases.

The equipment produced under the Level I criteria would be suitable for

many applications, but probably would not survive the criteria listed for

the B-l. Level II hardening would include the additional, relatively

expensive, measures necessary to allow the equipment to survive the

environments.

Reference 4. 3. DNSDP-OEA-071, "DNSDP Nuclear Hardening", dated 5 September 1973

4-6

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The development of a hardened equipment design depends upon the

recognition of radiation sensitivities, the assessment of their effects on

system operation, and the application of design techniques to limit unde-

sirable responses to the radiation environment. The circuit functions of

primary concern are those involving semiconductor devices.

For Phase I planning purposes, no major hardening activity is

envisioned. Rather, hardening experts will be utilized as part of the

hardware design review process to ensure that designs are not

employed that mitigate against eventual hardening. This approach, not

only provides the JPO with a minimum cost Phase I Program, but also

allows for the eventual hardening of the equipment in a cost effective

manner.

4. 4 ELECTROMAGNETIC INTERFERENCE (EMI)

The GPS receivers will be subjected to a wide range of electro-

magnetic environments which could degrade performance. These

environments may be the result of unintentional interference or intention 1

such as enemy jamming.

The GPS system utilizes wideband pseudorandom-noise (PRN) modu-

lation techniques to provide significant protection against interference and

jamming. While the receiver RF and IF processors track and reconbtruct

the received spectrum signal, the energy contained within undesired nar-

rowband signals is dispersed over a wide bandwidth where it is rejected

by the narrowband IF following the PRN demodulator. Theoretically, up

to 70 dB of suppression to CW jamming signals can be realized by this

technique. The narrowband IF also affords considerable x-ejection to wide-

band pulse or swept CW signals which are typical of the more common

L-band interferers encountered by the GPS receiver.

Reference 4. 4 contains the results of an EMC survey and analysis for

the GPS receivers. Table 4-4, extracted from that reference, summarizes

the L-band transmitter interference to the GPS receivers.

Reference 4. 4. DNSDP-GEC-108, "EMC Survey", dated 27 September 197 3

4-7

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Table 4-4. Summary of L-Band Transmitter Interference to GPS Receivers

Military System Transmitters

Frequency Band (MHz) Comments

TACAN/DME ATC radar beacon

960-1215 1030-1090

L-band search and air-route surveil- lance radar

Mobile communications

1215-1400

1427-1535

Aeronautical radionavigation

Meteorological aids

Fixed and mobile communications

1540-1660

1670-1700

1710-1850

Low probability sources of interference to co-sited GPS due to sideband splatter. May be troublesome in shipboard environment.

Severe source of interference due to high power broadband pulse spectra.

Potential interference sources due to proximity of GPS receiver band. May also be responsible for intermodulation, cross-modulation and other nonlinear effects.

Particular sources of interference to airborne GPS receivers are co-sited radar altimeters and collision avoid- ance systems. Transmitter spectral sideband energy is again responsible.

No problems envisioned.

Same as above for mobile communications.

1 1 I I I

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Radar systems comprise the primary potential interference sources

to receivers operating in the L-band frequency range. Of particular con-

cern are airborne radar altimeters in the 1600 MHz to 1660 MHz region.

Other potential interferers include high-power ground-based and ship-

board long-range radars. In some instances, co-sited UHF communi-

cations transmitters may provide in-band signals via the mechanisms of

I cross-modulation, intermodulation, and other nonlinear effects. These

will be most evident in shipboard installations where the hull, super-

1 structure, and other topside devices contain numerous nonlinear junctions.

Two situations, involving the GPS receiver and the A.N/APN-133

I radar altimeter were examined as typical interference situations. The

first was an APN-133 equipped aircraft overflying a surface based GPS

I receiver. This analysis indicated that interference only occurred if the

AC altitude was less than 250 meters above the GPS user. Since 250

I meters are less than the normal overflight altitude, and moreover, at

that altitude, the overflight time is insignificant, the AN-] 33 would not

be a source of interference under normal circumstances.

I I I 1 I I I I I I

The second case considered was the potential interference of GPS

equipped aircraft that was also equipped with an AN/APN-133 radar alti-

meter.

For the case considered, no interference problem existed because

of the large (110 dB) path loss between the two antennae.

The above example points out the analysis that will be required for

each type installation of the GPS receivers. Whether on aircraft, ship-

board, or other vehicle, an analysis of the effect of nearby transmitters,

structures, etc. , will be required to ensure nondegraded operation of the

GPS system.

4-9

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5. SYSTEM TEST PLANNING

The system test planning process for the GPS user equipment, as

outlined in Figure 5-1, has been accomplished to the depth necessary to

define:

a) Overall test program scope in the GPS System Test Plan, WDL-TR-5299.

b) Test equipment and facility requirements in a series of Test Facility Requirements Documents, WDL-TR- 5419 A-O.

c) Test concepts that establish levels of testing for each GPS user equipment group.

5. 1 GPS SYSTEM TEST PLAN

A preliminary System Test Plan was submitted to the JPO in accor-

dance with CDRL AOOD in September 1973. This plan is currently under

revision for submittal in June 1974.

The test planning process is an iterative one, in which the testing

baseline is preliminarily established, and then refined as program re-

quirements become more firmly established, testing philosophy is adopted,

and test resources are identified. Recent initiation of bi-weekly test plan-

ning meetings with JPG test personnel has resulted in significant insight

into Army and Navy test philosophy and requirements, and an overall

improvement in our knowledge of test vehicle, equipment, and facility

availability. This new information will be translated into updated GPS

User Equipment Test Plans for the final submittal of the System Test Plan

in June 1974.

The general scope of the GPS user equipment Phase I testing is

depicted in Figure 5-2, which defines the relationship of the initial test

and evaluation of the Engineering Development Models to the Low Cost

Prototype and the Class A-F Development Models. Further definition of

these tests in terms of test objectives, specific assignment of user equip-

ment groups to test vehicles, and test schedules will be presented in the

final System Test Plan.

5-1

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5.2 TEST FACILITIES REQUIREMENTS DOCUMENT

The test requirements analysis accomplished as an integral part of

the test planning process has resulted in the identification of test equip-

ment and facilities needed for successful completion of the GPS test pro-

gram. The Government owned test facilities and equipment required have

been identified in fifteen separate Test Facility Requirements Documents

(TFRDs). The preliminary TFRDs are contained in Part II, Volume E of

this report.

The requirements have been derived from the test objectives defined

in the GPS System Test Plan for test and evaluation of:

a) Generalized Development Models (DT&E)

b) Low Cost (Class C) Prototypes (DT&E, IGT&E)

c) Classes A-F Development Models (DT&E)

Required test facilities include test ranges capable of precision

metric tracking of aircraft, aircraft support facilities, surface vehicle

proving grounds, ocean range areas for sea trails, dockside support

facilities, supersonic wind tunnel, and jamming simulation facility.

Required vehicles to be used as user equipment test beds include

cargo aircraft, high performance bomber, fighter and attack aircraft,

helicopter, transportable communication shelters, surface ships, and

submarine.

Specific requirements for each of these facilities and vehicles are

defined in the following TFRDs:

TFRD No. 1 - Flight Test Range - White Sands Missile Range

TFRD No. 2 - Test Support Facility - Holloman Air Force Base

TFRD No. 3 - Inverted Range Ground Transmitters

TFRD No. 4 - Aircraft Test Bed C-130E

TFRD No. 5 - Aircraft Test Bed C-141

TFRD No. 6 - Aircraft Test Bed RF-4C

TFRD No. 7 - Aircraft Test Bed A-7

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TFRD No. 8 - Helicopter Test Bed H-l

TFRD No. 9 - Communications Shelter Test Bed

TFRD No, 10 - Surface Vehicle Test Range - WSMR

TFRD No. 11 - Surface Ship Test Bed

TFRD No. 12 - Submarine Test Bed

TFRD No. 13 - Ocean Range and Dockside Support Facilities - PMR

TFRD No. 14 - Jamming Simulator Facility - Ft. Huachuca

TFRD No. 15 - Supersonic Wind Tunnel

5. 3 LEVELS OF TESTING

Levels of testing to be accomplished during the GPS User Equipment

Phase I Test Program have been preliminarily outlined so that equipment

group test configurations can be defined, and onboard instrumentation, test

software, and range instrumentation can be sized and cost estimates gene-

rated. In Phase I, three categories of user equipment will be tested:

1) Generalized Development Models

2) Low Cost Prototypes (Cl^is C)

3) Classes A-F Development Models

To accomplish GPS user equipment development objectives, a

minimum of 29 receivers will be needed. These will be utilized in 2 3

test equipment group units configured for specific test beds, as shown

in Table 5-1, Receiver/Equipment Group Requirements Matrix. Several

equipment group types will probably be required by the combination of

receiver types, ancillary equipment, test instrumentation, and test vehicle

space availability.

5. 3. 1 Generalized Development Model

Two Generalized Development Model brassboard units will be fabri-

cated and used for laboratory and field testing that will validate design con-

cepts and identify problem areas (see Figure 5-3). Test results should

strongly influence Low Cost (Class C) Prototype design as well as Classes

A-F test model design. Principal test objectives are related to each of

the two test units in Table 5-2. As with INHI, internal (onboard) test

5-5

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I

I I I I

Receiver Type

(A

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u

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Equip Groups Cont

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Seq. L1L2

P&C

Seq. Ll

P&C

I Seq.

| Ll C Type

A-r General

Dev. Models

TRW Lab C-130 LSvri. Veh. 1

1 1

1 1

Low Cost 1 c Prototypes

TRW Lab C-141 DT&E H-l DT&E Comm. Shltr DT&E Surf. Ship DT&E Landing Cr. DT&E USAF IOT&E USA IOT&E USN IOT&E USMC TOT&E

11

1 1 1 1 1 1

1

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2 2 1 3 ! 3

A Dev.

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h 1 Dev.

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j 1

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Model USA Comm. Shltr. DT&E

V 1 i . ..

1 E Dev.

j Model Manpack DT&E VI 1 l ■ —'

F I Dev.

Model USN Ship DT&E VII 1 1

Spares In-Plant 1 1 2

Totals 23 ' 2 4 5 o 1 18 1

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data will be recorded on magnetic tape. Test data required in near real

time for inflight analysis and test control will be available to the test

operator from C/D displays and an onboard printer. The external (metric

tracking and meteorological) data to be obtained from WSMR is identified

in the TFRD and, in more detail later, in the PRD, Program Require-

ments Document.

5. 3. 2 Low Cost (Class C) Prototype

The 18 Class C Prototype receivers (including spares) will be re-

quired in Phase and will be utilized in 16 equipment groups as shown in the

Phase I Test Program Flow (Figure 5-4). Six of the units will be used for

contractor DT&E testing, ten units allocated to the IOT&E test vehicles,

and two spares. Principal test objectives are related to each of the test

units as shown in Table 5-3. User equipment test units fabricated for con-

tractor testing (DT&E) will incorporate test data measurement, recording,

and display capability for inflight quick-look analysis by the test operators,

as well as sufficient internal data to be combined with external (range) data

in the more detailed post-test accuracy analysis.

IOT&E data requirements will be less stringent, with more emphasis

on R&M, PSTE, and operational mission demonstration data. The equip-

ment groups allocated to IOT&E will, therefore, require less instrumen-

tation than that required during DT&E for problem identification and per-

formance evaluation. The deletion of the magnetic tape recorder and

associated interface hardware and software for IOT&E would result in a

significant cost saving.

External data requirements and test facility requirements are

identified in TFRDs for each of the test areas used during DT&E and IOT&E

testing.

5. 3. 3 Classes A-F Development Models

A minimum of nine Classes A-F Development Model receivers will

be required for Phase I development testing in five equipment groups. All

Classes A-F Development Model Phase I DT&E will be conducted by the

contractor (TRW) with participation by the appropriate JPO test support

agencies and user groups.

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The Class A Development Model will alternately use Type 1 and

Type 2 receivers in a test equipment group installed on a USAF RC-4c

(refer to Figure 5-5).

The Class B Development Model will alternately use Type 1 and

Type 2 receivers in a test equipment group installed on a USAF A-7 air-

craft.

The Class D Development Model will utilize a Type 2 receiver in 0

a test equipment group installed in a US Army standard communications

shelter. Upon completion of prescribed tests, the receiver will be con-

verted to Type 3 and the tests repeated.

The Class E Development Model will similarly utilize a Type 2

receiver for manpack tests, then repeat with the receiver converted to

Type 3.

The Class F Development Model will utilize a Type 1 receiver in

a test equipment group configured for USN ship/submarine testing.

Principal test objectives are related to each of the test units as

shown in Table 5-4. Each of the test equipment groups will include instru-

mentation appropriate to the type of receiver and the extent of testing

required to verify performance of the user equipment class. External

data requirements and test facility requirements are identified in TFRDs

for each of the test areas used during Phase I DT&E.

I I I I 5-12

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Jlobal Positioning System {WS\ lOSSBtfKPm- Part I#^f Vo]/B# User Segment System Analysis Report » \

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12. tPONSOHINO MILITARY ACTIVITY

Space and Missile System Organization, Los Angeles, Calif. ATTN: YE

IS. ABSTRACT

-^Results of signal structure, receiver operations and performance studies axe presented along with general definition of equipment group designs and descriptions including discussions of system effectiveness and test planning issues using satellites.

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DO ,'r..1473 UNCLASSIFIED '-""""' Security Claasiflcation

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RIV WOMQI r«-

- Satellite Radio Navigation

- L Band

- Pseudo Ranging

- Pseudo Random Noise (PRN) Code

- Modulation, Bi Phase Shift Key (PSK)

* User Equipment performance

* Life-cycle-coat

* Test planning

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