An Overview of the Global Positioning Satellite System

(GPS)Edward J. Delp

N9YTE

October 25, 2000

n9yte@n9yte.net

http://www.n9yte.net

Overview

The Navigation Problem Earlier Approaches GPS Description How does GPS Work?

Acknowledgement

Many of the graphical images used in this talk are courtesy of Peter H. Dana of the Department of Geography, University of Texas at Austin -

http://www.utexas.edu/depts/grg/gcraft/

notes/gps/gps.html

The best web site for GPS

The Navigation Problem

The ancient question: Where am I?

Earth coordinates: latitude and longitude

Lafayette: N40/W86 Latitude can be

determined by Sun angle

What about longitude?

Latitude and Longitude

Latitude and Longitude

Longitude Problem

No easy way to determine longitude On July 8, 1714 the Longitude Act was

established in England to solve the “longitude problem”

Two solutions were proposed

-- use of stars and moons

-- the “time” solution

Longitude

Longitude : The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time

Dava Sobel

Longitude Problem: References

http://www.pbs.org/wgbh/nova/longitude/

The “Time” Solution

Where am I? <=> What time is it in Greenwich, England?

The ability to find one’s position is based on how well one can tell what time is it?

The development of the chronometer To find longitude to within 0.5 degree

requires a clock that loses or gains no more than 3 seconds/day

Longitude

How does this work? The earth turns 360 degrees in 24 hours:

15 degrees = 1 hour If you know the time in Greenwich when it

is local noon at your location one can find your longitude relative to Greenwich

Must know “datum” reference to use maps

Datum Reference

Lone Pine Cemetery - N400 13.8’’ E260 17.24”

Satellite Navigation

US Department of Defense has need for very precise navigation

In 1973, the US Air Force proposed a new system for navigation using satellites

The system is known as: Navigation System with Timing and Ranging: Global Positioning System or NAVSTAR GPS

NAVSTAR GPS Goals

What time is it? What is my position (including attitude)? What is my velocity? Other Goals:

- What is the local time?

- When is sunrise and sunset?

- What is the distance between two points?

- What is my estimated time arrival?

GSP System

Simply stated: The GPS satellites are nothing

more than a set of clocks in the sky

GPS Segments

Space Segment: the constellation of satellites

Control Segment: control the satellites

User Segment: users with receivers

Space Segment

Space Segment

System consists of 24 satellites in the operational mode: 21 in use and 3 spares

3 other satellites are used for testing Altitude: 20,200 Km with periods of 12 hr. Current Satellites: Block IIR- $25,000,000

2000 KG Hydrogen Maser Atomic Clocks

Hydrogen Maser Clock

These clocks lose one second every

2,739,000 million years

GPS Orbits

GPS Orbits

Control Segment

Master Control Station is located at the

Consolidated Space Operations Center

(CSOC) at Flacon Air Force Station near

Colorado Springs

Control Segment

CSOC

Track the satellites for orbit and clock determination

Time synchronization Upload the Navigation Message Manage DOA

Operational Capabilities

Initial Operational Capability - December 8,

1993

Full Operational Capability declared by the

Secretary of Defense at 00:01 hours on

July 17, 1995

GPS Transmitted Signal

Two signals are transmitted on carriers:

L1 = 1575.42 MHz

L2 = 1227.60 MHz

These are derived from the system clock of

10.23 MHz (phase quadrature) Modulation used is Direct Sequence Spread

Spectrum

(code division multiple access - CDMA)

GPS Signals

GPS Clock Signals

Two types of clock signals are transmitted C/A Code - Coarse/Acquisition Code

available for civilian use on L1 provides 300 m resolution

P Code - Precise Code on L1 and L2 used by the military provides 3m resolution

Spread Spectrum

Spread Spectrum is used because

- resistance to jamming

- masks the transmissions

- resist multipath effects

- multiple access All 24 GPS satellites transmit on the same

two frequencies BUT use a different ID sequence

GPS Signals

The satellites transmit as part of their unique Spread Spectrum signal a clock or timing signal

The range or distance to the satellite is obtained by measuring how long it takes for the transmitted signal to reach the receiver

This is not the “true” range due to clock errors - what is obtained is know as the “pseudo-range”

GPS Position

By knowing how far one is from three satellites one can ideally find their 3D coordinates

To correct for clock errors one needs to receive four satellites

GPS: How does it work?

Typical receiver: one channel C/A code on L1

During the “acquisition” time you are receiving the navigation message also on L1

The receiver then reads the timing information and computes the “pseudo-ranges”

The pseudo-ranges are then corrected

GPS: How does it work?

Corrected ranges are used to compute the position

This is a very complicated iterative nonlinear equation

Navigation Message

To compute your position one must know the position of the satellite

Navigation Message - transmitted on both L1 and L2 at 50 bits/s for 30 s

Navigation message consists of two parts:

- satellite almanac

- clock bias

Why Do I Need To See 4 Satellites?

The problem is that the clock signal from the satellite is corrupted by atmospheric refraction

Another major problem is that the receiver’s clock is not very accurate

For a 2D fix <=> 3 satellites

Why Do I Need To See 4 Satellites?

Denial of Accuracy (DOA)

The US military uses two approaches to prohibit use of the full resolution of the system

Selective Availability (SA) - noise is added to the clock signal and the navigation message has “lies” in it

Anti-Spoofing (AS) - P-code is encrypted The military sometimes turns off both DOA

techniques

Differential GPS

Used to improve accuracy Put a “satellite” on the ground at a precise

position Differential signal is not “transmitted” on

standard satellite frequencies

Uses of GPS

Airplane and Boat Navigation Continental Drift Surveying Precise Timing Iceberg Tracking Archaeological Expeditions Mobile Multimedia

GPS Clock Rollover

GPS System Time rolled over at midnight 21-22 August 1999, 132 days before the Year 2000

On 22 August 1999, unless repaired, many GPS receivers claimed that it is 6 January 1980

http://www.navcen.uscg.mil/gps/geninfo/

y2k/gpsweek.htm

Conclusion

GPS will find more civilian uses DOD has promised to eliminate SA Russia has a system known as GLONASS The EU is discussing deploying its own

system

References

B. Hofmann-Wellenhof, H. Lichtenegger, and J. Collins, GPS: Theory and Practice, Third Edition, Springer-Verlag, 1994.

T. Logsdon, The Navstar Global Positioning System, Van Nostrand, 1992.

A. Leick, GPS Satellite Surveying, Second edition, Wiley, 1995.

References

T. A. Herring, "The Global Positioning System," Scientific American, pp. 44-50, February 1996.

N. J. Hotchkiss, A Comprehensive Guide to Land Navigation with GPS, Alexis, 1994.

Special Edition on the Global Positioning System, Satellite Times, March/April 1996.

D. Sobel, Longitude, Walker, 1995.

Web Sites

GPS Program Office:

http://www.laafb.af.mil/SMC/CZ/homepage/ US Coast Guard Navaigation Center

http://www.navcen.uscg.mil/default.htm GPS Precise Orbits

http://www.ngs.noaa.gov/GPS/GPS.html GPS World Magazine

http://www.gpsworld.com/