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Fundamentals ofGlobal Positioning
System Receivers
Lecture Notes by
He-Sheng Wang
September 19, 2008
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Preface The purpose of this course is to present detailed fundamental
information on a global positioning system (GPS) receiver.
Although GPS receivers are popularly used in every-day life,their operation principles cannot be easily found in one book.
In a GPS receiver, the signal is processed to obtain therequired information, which in turn is used to calculate theuser position. Most other types of receivers process the input signals to obtain the
necessary information easily, such as in amplitude modulation (AM)and frequency modulation (FM) radios.
At least two areas of discipline, receiver technology andnavigation scheme, are employed in a GPS receiver. Thiscourse covers both areas.
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Preface In the case of GPS signals, there are two sets of
information: the civilian code, referred to as the
coarse/acquisition (C/A) code, and the classifiedmilitary code, referred to as the P(Y) code. Thiscourse concentrates only on the C/A code.
The material in this course is presented from thesoftware receiver point of view. It is likely that narrow band receivers, such as the GPS
receiver, will be implemented in software in the future.A software receiver approach may explain the operation
better.
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Preface Aim: To introduce the principles of the
operation of the GPS system and itsapplications There is flexibility in the exact content of the
course depending on student interests Generic topics include standalone, millimeteraccuracy positioning and kinematic GPS
Emphasis is on fundamental principles andlimitations
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Topics to be Covered Coordinate and time systems:
When working at the millimeter level globally, how do youdefine a coordinate system
What does latitude, longitude, and height really mean atthis accuracy
Light propagates 30 cm in 1 nano-second, how is timedefined
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Topics Satellite motions
How are satellite orbits described and how do the satellitesmove
What forces effect the motions of satellites
What do GPS satellite motions look like and what are themain perturbations to the orbits
Where do you obtain GPS satellite orbits
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Topics GPS observables Satellite motions
GPS signal structure and its uniquenessCode Phase measurements
Carrier phase measurements
Initial phase ambiguitiesEffects of GPS security: Selective availability (SA) and
antispoofing (AS)
Data formats (RINEX)
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Topics Estimation Procedure
Simple weighted-least-squares estimation
Stochastic descriptions of random variables andparameters
Kalman filtering
Statistics in estimation procedures Propagation of variance-covariance information
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Topics Propagation medium
Neutral atmosphere delayHydrostatic and water vapor contributions
Ionospheric delay (dispersive)
Multipath
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Topics Mathematic models in GPS positioning
Basic theory of contributions that need be to included formillimeter level global positioning
Use of differenced data
Combinations of observables for different purpose
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Topics Methods of processing GPS data
Available software
Available data (International GPS service, IGS; Universityconsortium
Cycle slip detection and repair
Relationship between satellite based and conventionalgeodetic systems (revisit since this is an important topic)
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Topics Applications and examples from GPS
Tectonic motions and continuous time seriesEarth rotation variations; measurement and origin
Kinematic GPS; aircraft and moving vehicles
Atmospheric delay studies
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Text Books and References Text
Pratap Misra and Per Enge, Global Positioning System:
Signals, Measurements, and Performance, Ganga-JamunaPress.
James Bao-Yen Tsui, Fundamentals of Global PositioningSystem ReceiversA Software Approach, Wiley-Interscience.
References Kayton & Fried,Avionics Navigation Systems, Second Edition,
Wiley Interscience.
E. D. Kaplan, Understanding GPS: Principles andApplications, Artech House.
Global Positioning System: Theory and Applications, 2Volumes, edited by B. Parkinson, J. Spilker, P. Axelrad, and P.
Enge, AIAA, http://www.aiaa.org, 19962008/9/19 13
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Contents1. Introduction2. GPS: An Overview3. GPS Coordinate Frames, Time References, and
Orbits4. GPS Measurements and Error Sources5. PVT Estimation6. Precise Positioning with Carrier Phase
7. GPS Signals8. Signal-to-Noise Ratio and Ranging Precision9. GPS Receivers
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Introduction toRadionavigation SystemsPredecessors to GPS
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Global Positioning System Satellite Navigation
System Multilateration based
on one-way rangingsignals from 24+satellites in orbit.
Operated by the UnitedStates Air Force Nominal Accuracy
10 m (Stand Alone) 1-5 m (Code
Differential) 0.01 m (Carrier
Differential)
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Navigation Terminology Navigation
Answer to the question Where am I?
Implies the use of some agreed upon coordinate system. Coordinates systems will be the subject of future lectures.
Related Terminology
Guidance: Deciding what to do with your navigation information Control: Orienting yourself/vehicle/weapon to follow out the guidance
decision
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Latitude, Longitude and Attitude One of many coordinate systems used to
described a location on the surface of the
earth Lattitude
Range: 90 North latitude are + South latitude are -
Longitude Range: 180 East longitude is + West longitude is -
Altitude Normally Upward is + In a North East Down (NED) coordinate
system up is -
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Definition of Latitude and
Longitude
Latitude (Paralles) are formed by theintersection of the surface of the
earth with a plane parallel to theequatorial plane
Longitude or Meridians are formedby the intersection of the surface of
the earth with a plane containing theearths axis.
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Latitude Determination Using
Polaris
Actual location of Polaris is 89o05
2008/9/19 20The Sky Above Stanford on Jan 6, 2002
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Instruments of Navigation
A SextantAn Astrolabe2008/9/19 21
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View Through a Sextant
Easier to alignSuns (or othercelestial bodys)limb with thehorizon.
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Latitude Determination Using the
Sun
= 900
Suns Altitude Suns Declination
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The Longitude Problem
Celestial map changes because of Earths 15o/hr (approximately)rotation rate.2008/9/19 24
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Longitude Determination Longitude Determination Methods
Methods based on time Compare the time between a clocks at the
current location and some other referencepoint.
Requires Stable Clocks
Celestial Methods Eclipses of Jupiters Moons
Lunar Distance Method
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Stability of Clocks
A $20 wrist watchhas an oscillator
stable enough tomeet theaccuracyrequirements of
the longitudeprize. The size and cost
of the super-
stable clocksmakes themunsuitable for usein mass produced
device.2008/9/19 26
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Fundamentals Radionavigation
Radio Frequency (RF) signals emanating from a source or sources.
The generators of the RF signal are at known locations
RF signals are used to determine range or bearing to the known
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Classification of Radio
Frequencies
Propagation characteristic of RF signals is a function of their frequency2008/9/19 28
Li f Si ht T i i
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Line of Sight Transmission
VHF (VOR, ILS Localizer) and UHF (ILS Glide Slope, TACAN/DME) are line ofsight systems.
Limited Coverage area
LORAN and OMEGA are over the horizon systems Large coverage area
In the case of Omega, coverage was global
Frequency band in which GPS operates makes it a line of sight system.
However, because of the location of the satellites, it is able to cover a largegeographic area.
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INS and Radionavigation
Systems
* INS is not a radionavigation system but is normally used inconjunction with such systems
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Phases of Flight
The required navigation accuracy and reliability (i.e., integrity,continuity and availability) depend on the phase of flight
Currently, as well as in the past, this meant that an aircraft had to be
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VHF Omni-directional Range
(VOR) Provides Bearing ()
Information Operates 112 118
MHz Accuracy 1o to 2o.
Principles of Operation Transmits 2 Signals
1st signal has azimuthdependent phase
2
nd
signal is a reference Phases differencebetween 1st signal and2nd signal is
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Distance Measuring Equipment
(DME) Measures Slant Range () Operates between 962 and 1213 MHz
Based on Radar Principle Airborne unit sends a pair of pulses Ground Station receives pulses After short delay (50 s) ground station resends the pulses back Airborne unit receives the signal and calculates range by using the following equation:
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Instrument Landing System (ILS)
Used extensively during approach and landing to provides vertical and lateral guidance Principle of Operation
Lateral guidance provided by a signal called the Localizer (108-112 MHz) Vertical guidance provided by another signal called the Glide Slope (329-335 MHz)
Distance along the approach path provided by marker beacons (75 MHz)2008/9/19 34
G G S R
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Generic GPS Receiver
Functional Block Diagram
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A F d t l S ft GPS
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A Fundamental Software GPS
Receiver
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Software Approach This course uses the concept of software radio to present the subject. The software radio idea is to use an analog-to-digital converter (ADC)
to change the input signal into digital data at the earliest possible stagein the receiver. The input signal is digitized as close to the antenna as possible.
Once the signal is digitized, digital signal processing will be used to
obtain the necessary information. The primary goal of the software radio is minimum hardware use in aradio.
Conceptually, one can tune the radio through software or even changethe function of the radio such as from amplitude modulation (AM) to
frequency modulation (FM) by changing the software.
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Software Approach The main purpose of using the software radio concept to
present this subject is to illustrate the idea of signal
acquisition and tracking. A software approach should provide a better understanding of
the receiver function because some of the calculations can beillustrated with programs.
Once the software concept is well understood, the readersshould be able to introduce new solutions to problem such asvarious acquisition and tracking methods to improve efficiency
and performance.
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P t ti l Ad t f th
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Potential Advantages of the
Software Approach An important aspect of using the software approach
to build a GPS receiver is that the approach candrastically deviate from the conventional hardwareapproach.
The software approach is very flexible. New algorithms can easily be developed without
changing the design of the hardware.
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OUTLINE1. Introduction
2. GPS: An Overview3. GPS Coordinate Frames, Time References, and Orbits
4. GPS Measurements and Error Sources
5. PVT Estimation6. Precise Positioning with Carrier Phase
7. GPS Signals
8. Signal-to-Noise Ratio and Ranging Precision9. GPS Receivers
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GPS: An Overview
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GPS: An Overview Objectives, Status, and Policies System Architecture
Signals Receivers and Measurements
Augmentation System and Differential GPS (DGPS)
Civil Applications Modernization Plans
Summary
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Objectives, Status, and Policies The principle objective of GPS was to offer the U.S. military accurate estimates of position,velocity, and time (PVT). Position error: 10 m Velocity error: 0.1 m/s
Time error: 10 ns The U.S. DoD decreed that the civil users of GPS would be provided with a reasonable
accuracy consistent with the national security considerations. Standard Position Service (SPS) for peaceful civil use Precise Positioning Service (PPS) for the DoD-authorized users
Access to the full capability of the system (i.e., PPS) is restricted by cryptographic techniques Anti-Spoofing (AS) SPS signals were degraded throughout the 1990s by introducing controlled errors to reduce
their precision Selective Availability Deactivated by a Presidential Order on 2 May 2000
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Predecessors Applied Physics Laboratorys TRANSIT: Navy Navigation Satellite System Doppler Shift Broadcast Satellite Ephemeris (Satellite prediction algorithm) Limitation: Velocity Sensitivity, Mutual Interference
Naval Research Laboratorys Timation Satellites Provide very precise time and time transfer between various points on the Earth Navigation Information: Side-tone ranging
U.S. Air Force Project 621B Satellite-ranging signal based on pseudorandom noise (PRN)
All satellites could broadcast on the same nominal frequency Anti-jamming capability Slow communication link (50bps)
Joint Program Office NAVSTAR (Navigation System with Time and Ranging) GPS
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GPS Design Choices & Enabling
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GPS Design Choices & Enabling
Technology Design Choices
Active or passive
Passive system need only receive transmission Positioning method: Doppler, Hyperbolic, or Trilatertion Trilateration time synchronized signals from satellites
Pulsed or continuous wave (CW) CW signal in the form of code division multiple access spread spectrum
Carrier frequency L-band offering line-of-sight with minimal atmospheric attenuation
Satellite constellation and orbits MEO constellation of 24 satellites
Enabling Technology Stable space platforms in predictable orbits
Ultra-stable clocks Spread spectrum modulation/signaling Integrated circuits
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Global Navigation Satellite
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Global Navigation Satellite
Systems (GNSS) GPS is not the only modern satellite-based
navigation system. GLONASS is a Russian parallel to GPS24 satellite FDMA navigation system
Galileo is expected to be EU offering for satellitenavigation in 2005
Beidou () experimental satellite navigation
system is Chinas developing testbed.
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GPS GLONASS
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GPS vs. GLONASS
GPS GLONASS
24+ 24-
6 3
4 8
55 64.8
26,560km 25,510km
1158 1115
CDMA FDMA
L1:1575.42MHz
L2:1227.60MHz
L1:1602.5625~1615.5MHz
L2:1246.4375~1256.5MHz
UTC(USNO) UTC(SU)
WGS84 SGS85
(SA) ()
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System Architecture Space Segment
Control Segment
User Segment
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Space Segment
Constellation
Number of Satellites 24
Number of Orbital Planes 6
Number of Satellites PerOrbit
4
Orbital Inclination 550
Orbital Radius 26560km
Period 11h57m57.26s
Ground Track Repeat Sidereal Day
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GPS Nominal Orbit Planes
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Control Segment
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GPS Control Monitor
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User Segment
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GPS Positioning Services Precise Positioning Service (PPS)Authorized users with cryptographic equipment and keys
and specially equipped receivers use the PrecisePositioning System.
Standard Positioning Service (SPS)Civil users worldwide use the SPS without charge or
restrictions. Most receivers are capable of receiving andusing the SPS signal. The SPS accuracy is intentionallydegraded by the DOD by the use of Selective Availability.(SA Turn off on May 1, 2000)
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Positioning and Timing Accuracy
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Positioning and Timing Accuracy
Standard (SPS)
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Signals Signal Structure Anti-Spoofing (AS) and Selective Availability (SA)
Signal Power
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Signals Currently, each GPS satellite transmits continuously usingtwo frequencies in the L-band referred to as Link 1 (L1) andLink 2 (L2) L-band covers frequencies between 1GHz and 2 GHz
Subset of the ultra-high frequency (UHF)
L1:fL1 = 1575.42 MHz L2:fL2 = 1227.60 MHz
Two signals are transmitted on L1, one for civil users, and theother for DoD-authorized users.
The lone signal on L2 is intended for the DoD-authorized
users only.
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Signal Structure Carrier: RF sinusoidal signal with frequencyfL1 orfL2. Ranging Code: a unique sequence of 0s and 1s assigned to
each satellite which allows the receiver to determine thesignal transit time instantaneously. PRN (Pseudo-random noise) codes allow all satellites to transmit at
the same frequency without interfering with each other
Each satellite transmit two different codes Coarse/Acquisition (C/A) code
Precision (Encrypted) [P(Y)] code
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Signal Structure Navigation Data: a binary-coded message consistingof data on the satellite health status, ephemeris
(satellite position and velocity), clock biasparameters, and an almanac giving reduced-precision ephemeris data on all satellite in the
constellation data rate: 50 bits per second (bps)
bit duration: 20 ms
12.5 minutes for the entire message to be received
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Signal Structure The three components of a signal are derivedcoherently from one of the atomic standard aboard
the satellite. 10.23 MHz
fL1 = 1575.42 MHz = 27710.23 MHz fL2 = 1575.42 MHz = 26010.23 MHz
The specific form of modulation used is called binary
phase shift keying (BPSK)
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Signal Structure
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Signal Structure
)2cos()()(2
)2cos()()(2)2sin()()(2)(
22)()(
2,
11
)()(
1,11
)()()(
LLkk
LY
LL
kk
LYLL
kk
C
k
tftDtyP
tftDtyPtftDtxPts
++
+++=
where PC is the signal power of C/A-code, PY,L1, and PY,L2 are the signal powers of
P(Y)-code on L1 and L2, respectively;x(k)
(t) =
1 andy(k)
(t) =
1 represent the C/A-code and P(Y)-code sequences, respectively, assigned to satellite number k;D(k)(t) =1 denotes the navigation data bit stream;fL1 andfL2 are the carrier frequenciescorresponding to L1 and L2, respectively; L1 and L2 are the initial phase offsets.
(1)
Note: In order to express the BPSK signals as (1), we haveswitched the binary values of the codes and navigation data to 1.From our old notation, a bit 0 maps into 1; and a bit 1 map into -1.
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L2 1227 6 MHz
L1 1575.42 MHz
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P(Y)-Code
Encrypted
U.S.military use
P(Y)-Code
Encrypted
U.S.military use
C/A-Code
Degraded
Civil use
L2 1227.6 MHz
GPS signals. Currently, each GPS satellite transmits three signals,two on L1 and one on L2 frequency. The BPSK-modulated signals areshown. The signal carrying C/A-code on L1 was degraded purposelythroughout the 1990s, but this practice has now ended. Access to
P(Y)-code is limited to the DoD-authorized users via encryption.2008/9/19 63
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Spread Spectrum The modulation of a carrier by a binary code spreads thesignal energy, initially concentrated at a single frequency,over a wide frequency band: over 2 MHz for the C/A-code and
about 20 MHz for the P(Y)-code, centered at the carrierfrequency.
While the signal power is unchanged, this step reduces thepower spectral density below that for the background RFradiation
Such signals, referred to as spread spectrum signals, havemany properties which make them attractive for use incommunication and navigation.
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Spread Spectrum
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Power Spectra
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Pseudo-Random Noise (PRN) PRN sequences are nearly orthogonal to each other. For satellites kand l,
which are assigned unique PRN sequences called C/A-codesx(k) andx(l),
).()1023(where,,allfor,0)()( )()(1022
0
)()(mxmxlknnixix
i
lk
=
=++
.1allfor,0)()(1022
0
)()( +=
nnixix
i
kk
The left hand side of (2) defines the cross-correlation function of thetwo sequences for shift n.
(2)
A PRN sequence is nearly uncorrelated with itself, except for zeroshift. For a C/A-code
(3)
The left hand side of (3) defines the auto-correlation functionof a sequence for shift n. The auto-correlation function of aPRN is nearly zero except for zero shift where it has a sharp
peak.2008/9/19 67
Anti-Spoofing (AS) and Selective
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Availability (SA) Anti-Spoofing: The main mechanism for limiting access to the fullcapabilities of GPS has been encryption of the P-code broadcast on bothL1 and L2.
Encrypted P-code is referred to as Y-code Access to the Y-code is under cryptographic key
SPS limits civil users to the C/A-coded signal on L1 but dual-frequencymeasurements are essential for precise positioning. Receiver manufacturers have devised proprietary techniques to gain access to
measurements on both L1 and L2. The same P(Y)-code is being transmitted by a satellite on both frequencies. The L2 measurements are more fragile and noisier than they would be if the
codes were known.
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Anti-Spoofing (AS) and Selective
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Availability (SA) Throughout the 1990s, the signal available for unrestricteduse were purposefully degraded under the policy of Selective
Availability (SA) by adding controlled errors in themeasurements.A five-fold increase in positioning error
Dithering the satellite clock
Can be eliminated via differential corrections SA was deactivated on 2 May 2000 in accordance with a Presidential
Decision Perhaps the European plans to develop Galileo accelerated the U.S. move
to drop SA
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Signal Power The GPS signals received on the earth are extremely weak.
RF power at the antenna input port of a satellite is about 50 watts Half is allocated to the C/A-code
In order to deal simply with a wide range of power levels, electrical engineers expresspower ratios on a logarithmic scale in units of decibel (dB), defined as
comparedbetolevelspowerare,where,log10 010
110
0
1 PPP
P
P
P
dB
=
dB3,dB30,dBW103
2
1
21 =+==
P
P
P
PP
(4)
Absolute values of power can be expressed similarly in relation to 1 watt or 1milliwatt in units of dBW or dBm, respectively. Consider a signal with power (P1) of0.1 watt. This power level can also be represented as -10dBW or 20dBm. A secondsignal, with a power (P2) of 100 watt is 30dB more powerful than the first signal. Athird signal, with 200-watt power (P3), is 3dB stronger than the second signal. Wecan capture these relationships as follows.
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Signal Power
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g
P C/A
L1 -133 dBm -130 dBmL2 -136 dBm -136 dBm*
*Presently not in L2 frequency
The actual signal powers in recent years have been 3-5 dB higher than thespecifications. Even so, the powers are still only around 10-16 watt. Interestingly, 10-16watt is enough power to navigate with if we were among friends and people of good will.
The GPS signals are well below the background RF noise level sensed by an antenna. Itis the knowledge of the signal structure that allows a receiver to extract the signalburied in the background noise and make precise measurements. The signal boost sorealized is called processing gain.
The low signal power is the Achilles heel of GPS, especially in military use.
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Power of Received Signal
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g
Zenith 5o Elevation
SV Transmit Power 27 W 27 W
SV Antenna Gain 10.5 dB 16.2 dBEffective Power Radiated Towards
Earth294 W 467 W
Path or Spreading Loss 1.95x10-16m-2 1.20x10-16m-2
Received Power Density5.51x10-14W/m2
5.26x10-14W/m2
Effective Area of Receive Antenna 2.87x10-3
m2
2.87x10-3
m2
Atmospheric Losses 2 dB 0.63 0.63
Effective Received Power 1.00 x 10-16 W 0.95 x 10-16 W
In dBm = 10log10
(Power in mW) -130 dBm -130 dBm2008/9/19 72
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Comparable Power Tracking -130dBm is roughly equivalent to listening to a
500 mW baby monitor a thousand miles away.
1,000 miles
16,000 miles
0.5 W
27 W
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Receivers and Measurements
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Signal Acquisition and Tracking
Estimation of Position, Velocity, and Time (PVT)
Evolution of Receiver Technology
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Signal Acquisition and Tracking The basic functions of a GPS receiver are: to capture the RF signals transmitted by the satellites
spread out in the sky, to separate the signals from satellites in view,
to perform measurements of signal transit time and
Doppler shift, to decode the navigation message to determine the
satellite position, velocity, and clock parameters,
to estimate the user position, velocity, and time
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GPS Collected Data Time
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Domain Plot
This is the digital data that results from the GPS analog front end ASIC.
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Q. So How Does GPS Work?
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Q Answer: By integrating the signal until SNR >> 0 dB
This is the key to everything from here on.
As we will see, the GPS signal has an element that repeatsevery 1 millisecond, and we can accumulate manyidentical signals until the SNR is high enough.
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Estimation of Position, Velocity,
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and Time (PVT) The quality of the PVT estimates obtained by a user from GPS
depends basically upon two factors:
number of the satellites in view and their spatial arrangement in the sky The spatial distribution of the satellites relative to the user is referred to as
satellite geometry
quality of the range and range rate measurements There are several sources of biases and random errors. Errors in the navigation message parameters which specify satellite position
and signal transmission time introduce errors in the pseudorangemeasurements.
Propagation delays in the ionosphere and troposphere, signal distortion dueto multipath, and receiver noise also introduce measurement errors.
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Basic GPS Positioning Concept -- Trilateration
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How GPS Works
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Basic Equations for Finding User
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Position( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( )232
3
2
33
2
2
2
2
2
22
2
1
2
1
2
11
uuu
uuu
uuu
zzyyxx
zzyyxx
zzyyxx
++=
++=
++=
Nonlinear Equations: Difficult to Solve
Relatively Easily Solved with Linearization and Iterative Approach
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Measurement of Pseudorange
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g
utuu
isisi
btt
btt
+=
+='
'
Every satellite sends a signal at a certain time tsi. The receiver will receivethe signal at a later time tu.
iT= c(tutsi) ---- true value of pseudorange or geometric range
From a practical point of view it is difficult to obtain the correct timefrom the satellite or the user. The actual satellite clock time and actualuser clock time are related to the true time as
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Measurement of Pseudorange
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ContdBesides the clock error, there are other factors affecting thepseudorange measurement. The measured pseudorange ican be written as
Di: satellite position error, Ti: tropospheric delay error, Ii:
ionospheric delay error, i: receiver measurement noise error,i: relativistic time correction
)()( iiiiutiiiTi ITcbbcD +++++=
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Measurement of Pseudorange
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Contd
( ) ( ) ( )( ) ( ) ( )
( ) ( ) ( )
( ) ( ) ( ) uuuu
uuuu
uuuu
uuuu
bzzyyxx
bzzyyxx
bzzyyxx
bzzyyxx
+++=
+++=
+++=+++=
2
4
2
4
2
44
2
3
2
3
2
33
2
2
2
2
2
22
2
1
2
1
2
11
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GPS/SPS Performance Specificationsfor Global Positioning and Time
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Dissemination
Error (95%) PPS SPS
SA Active SA OFF*
PositionHorizontal
Vertical
22 m
28 m
100 m
156 m
10 m
15 mTime 200 ns 340 ns 50 ns
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Evolution of Receiver Technology
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Several generation of GPS receivers came to marketbetween 1980 and 2000.
The receivers available today bear the sameresemblance to the early receivers as the laptop andpalm computers do to the minicomputers of the early1980s.
The advent of very large scale integration (VLSI) hasled to powerful microprocessors and memory chips,which have changes the look and feel of all
electronic equipment, including the GPS receivers.
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Complete Receivers
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Handheld receivers for hikers, backpackers and sailors.Small in size with lat-lon displays or simple maps
$100 - $300
In-car navigation systems. Detailed street maps andturn-by-turn directions
$400 - $2000
Marine navigation. Fixed mount large screens withelectronic charts
$400 - $3000
Aviation. FAA certified, panel mounted, with maps $3000 - $15,000
Survey and mapping. Oftentripod mounted, exclusivelyDifferential GPS, one meter tocentimeter accuracy
$3500 - $30,000
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Modules
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Plug-in modules. Integrated receiverand antenna, used for tracking and
monitoring
$100 - $300
OEM boards. Receiver circuitry for
customer integration
$60 - $100
Chip sets $10 - $30
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Augmentation Systems and
Diff ti l GPS (DGPS)
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Differential GPS (DGPS) The accuracy of the different navigation and positioning applications vary widely. Horizontal positioning accuracy of tens of meters is generally more than adequate
for navigation in wide-open spaces: maritime navigation on the open seas;
aircraft navigation in en route, terminal, and non-precision approach phases of flight; recreational use by hikers and backpackers.
Many important applications require greater accuracy: under poor visibility conditions, harbor entry by ships, taxiway guidance on airport
surface, Category I precision approaches by aircraft typically require meter-level
accuracy Automobile navigation over roads and highways has a similar accuracy requirement Category III precision approaches require decimeter-level accuracy vertically
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Reducing Measurement Errors and/or
I i S t llit G t
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Improving Satellite Geometry Satellite geometry can be improved by adding satellites to theconstellation to provide additional ranging signal.
A user can improve the geometry by deploying pseudo-satellites, called pseudolites, which transmit GPS-like signals.
The pseudolites can be deployed on the ground, in the air, oron a ship.
A GPS receiver has to be modified to receive and processthese signals.
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Differential GPS
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Hierarchy of GPS Capability
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Civil Applications
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High-precision (millimeter-to-centimeter level) positioning
Specialized applications such as aviation and space
navigation Land transportation and maritime uses
Consumer products
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Modernization Plans
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P(Y)-Code
Encrypted
M-code (starting 2003)
P(Y)-Code
Encrypted
M-code (starting 2003)
C/A-Code
Degraded(2 May 2000)
L2 1227.6 MHz
L1 1575.42 MHz
C/A-Code
(starting 2003)
L5 1176.45 MHz
Civil signal
(starting 2005)
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Summary
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Basic Description Space-based radionavigation system broadcasting synchronized
timing signals to provide estimates of position, velocity, and time based
on passive, one-way ranging to satellites. Milestones
1973: Architecture approved
1978: First satellite launched 1995: System declared operational
2000: Purposeful degradation of the civil signal stopped
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SummaryS C
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Satellite Constellation
Twenty-four satellites in six orbital planes inclined at 55o;near-circular orbits with radius 26,560 km; orbital period:11h 58m; ground track repeats each sidereal day
Reference Standards
Coordinate frame: WGS 84
Time: UTC (USNO)
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SummarySi l
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Signals Carrier Frequency (Wavelength)
L1: 1575.42 MHz (0.19029 m)
L2: 1227.60 MHz (0.24421 m) Multiple Access Scheme
Code division multiple access (CDMA)
PRN Codes
C/A-code on L1 P(Y)-code on L1 and L2
Code Frequency (Mcps) C/A-code: 1.023 P(Y)-code: 10.23
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Summary
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Performance Achievable
Real time: Typically, absolute positioning error of several
meters with a single receiver, decimeters in differentialmode
Batch processing: millimeter-level relative positioning
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