Fundamentals of Global Positioning System Receivers

Fundamentals of Global Positioning System Receivers

Lecture Notes byHe-Sheng Wang

Lecture One

Sunday, April 9, 2023

2

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 the required information, which in turn is used to calculate the user 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 and navigation scheme, are employed in a GPS receiver. This course covers both areas.

3

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 classified military code, referred to as the P(Y) code. This course concentrates only on the C/A code.

• The material in this course is presented from the software 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.

4

Preface• Aim: To introduce the principles of the

operation of the GPS system and its applications

• There is flexibility in the exact content of the course depending on student interests

• Generic topics include standalone, millimeter accuracy positioning and kinematic GPS

• Emphasis is on fundamental principles and limitations

5

Topics to be Covered• Coordinate and time systems:

– When working at the millimeter level globally, how do you define a coordinate system

– What does latitude, longitude, and height really mean at this accuracy

– Light propagates 30 cm in 1 nano-second, how is time defined

6

Topics• Satellite motions

– How are satellite orbits described and how do the satellites move

– What forces effect the motions of satellites– What do GPS satellite motions look like and

what are the main perturbations to the orbits– Where do you obtain GPS satellite orbits

7

Topics• GPS observables Satellite motions

– GPS signal structure and its uniqueness– Code Phase measurements– Carrier phase measurements– Initial phase ambiguities– Effects of GPS security: Selective availability

(SA) and antispoofing (AS) – Data formats (RINEX)

8

Topics• Estimation Procedure

– Simple weighted-least-squares estimation

– Stochastic descriptions of random variables and parameters

– Kalman filtering– Statistics in estimation procedures – Propagation of variance-covariance

information

9

Topics• Propagation medium

– Neutral atmosphere delay– Hydrostatic and water vapor

contributions– Ionospheric delay (dispersive)– Multipath

10

Topics• Mathematic models in GPS

positioning– Basic theory of contributions that need

be to included for millimeter level global positioning

– Use of differenced data– Combinations of observables for different

purpose

11

Topics• Methods of processing GPS data

– Available software– Available data (International GPS service,

IGS; University consortium– Cycle slip detection and repair– Relationship between satellite based and

conventional geodetic systems (revisit since this is an important topic)

12

Topics• Applications and examples from GPS

– Tectonic motions and continuous time series

– Earth rotation variations; measurement and origin

– Kinematic GPS; aircraft and moving vehicles

– Atmospheric delay studies

13

Text Books and References

• Text– Pratap Misra and Per Enge, Global Positioning System: Signals,

Measurements, and Performance, Ganga-Jamuna Press.– James Bao-Yen Tsui, Fundamentals of Global Positioning System

Receivers – A Software Approach, Wiley-Interscience.• References

– Kayton & Fried, Avionics Navigation Systems, Second Edition, Wiley Interscience.

– E. D. Kaplan, Understanding GPS: Principles and Applications, Artech House.

– Global Positioning System: Theory and Applications, 2 Volumes, edited by B. Parkinson, J. Spilker, P. Axelrad, and P. Enge, AIAA, http://www.aiaa.org, 1996

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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 Phase7. GPS Signals8. Signal-to-Noise Ratio and Ranging Precision9. GPS Receivers

15

Introduction to Radionavigation Systems

Predecessors to GPS

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Global Positioning System• Satellite Navigation

System– Multilateration

based on one-way ranging signals from 24+ satellites in orbit.

– Operated by the United States Air Force

– Nominal Accuracy• 10 m (Stand Alone)• 1-5 m (Code

Differential)• 0.01 m (Carrier

Differential)

17

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

18

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 “-”

19

Definition of Latitude and Longitude

Latitude (Paralles) are formed by the intersection of the surface of the earth with a plane parallel to the equatorial plane

Longitude or Meridians are formedby the intersection of the surface ofthe earth with a plane containing theearths axis.

20

Latitude Determination Using Polaris

Actual location of Polaris is 89o05’

The Sky Above Stanford on Jan 6, 2002

21

Instruments of Navigation

A SextantAn Astrolabe

22

View Through a Sextant

Easier to “align” Sun’s (or othercelestial body’s) limb with thehorizon.

23

Latitude Determination Using the Sun

L= 900

– Sun’s Altitude

± Sun’s Declination

24

The Longitude Problem

• Celestial map changes because of Earth’s 15o/hr (approximately) rotation rate.

25

Longitude Determination• Longitude Determination Methods

–Methods based on time• Compare the time between a clocks at

the current location and some other reference point.

• Requires Stable Clocks–Celestial Methods

• Eclipses of Jupiter’s Moons• Lunar Distance Method

26

Stability of Clocks

• A $20 wrist watch has an oscillator stable enough to meet the accuracy requirements of the longitude prize.

• The size and cost of the “super-stable” clocks makes them unsuitable for use in mass produced device.

27

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 location

28

Classification of Radio Frequencies

Propagation characteristic of RF signals is a function of their frequency

29

Line of Sight Transmission

• VHF (VOR, ILS Localizer) and UHF (ILS Glide Slope, TACAN/DME) are line of sight 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

large geographic area.

30

INS and Radionavigation Systems

* INS is not a radionavigation system but is normally used in conjunction with such systems

31

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 equipped with various navigation systems.

32

VHF Omni-directional Range (VOR)

• Provides Bearing (Y) Information

• Operates 112 – 118 MHz

• Accuracy 1o to 2o.• Principles of

Operation– Transmits 2 Signals

• 1st signal has azimuth dependent phase

• 2nd signal is a reference

• Phases difference between 1st signal and 2nd signal is Y

33

Distance Measuring Equipment (DME)

• Measures Slant Range (r)• 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 ms) ground station resends the pulses back– Airborne unit receives the signal and calculates range by using the following

equation:

34

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)

35

Generic GPS Receiver Functional Block Diagram

36

A Fundamental Software GPS Receiver

37

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 stage in 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 a radio.

• Conceptually, one can tune the radio through software or even change the function of the radio such as from amplitude modulation (AM) to frequency modulation (FM) by changing the software.

38

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 be illustrated with programs.

• Once the software concept is well understood, the readers should be able to introduce new solutions to problem such as various acquisition and tracking methods to improve efficiency and performance.

39

Potential Advantages of the Software Approach

• An important aspect of using the software approach to build a GPS receiver is that the approach can drastically deviate from the conventional hardware approach.

• The software approach is very flexible.• New algorithms can easily be developed

without changing the design of the hardware.

40

OUTLINE1. Introduction2. GPS: An Overview3. GPS Coordinate Frames, Time References, and

Orbits4. GPS Measurements and Error Sources5. PVT Estimation6. Precise Positioning with Carrier Phase7. GPS Signals8. Signal-to-Noise Ratio and Ranging Precision9. GPS Receivers

GPS: An OverviewGPS: An Overview

42

GPS: An Overview• Objectives, Status, and Policies• System Architecture• Signals• Receivers and Measurements• Augmentation System and Differential GPS (DGPS)• Civil Applications• Modernization Plans• Summary

43

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

44

Predecessors• Applied Physics Laboratory’s TRANSIT: Navy Navigation Satellite System

– Doppler Shift– Broadcast Satellite Ephemeris (Satellite prediction algorithm)– Limitation: Velocity Sensitivity, Mutual Interference

• Naval Research Laboratory’s 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

45

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

46

Global Navigation Satellite Systems (GNSS)• GPS is not the only modern satellite-based

navigation system.• GLONASS is a Russian parallel to GPS

– 24 satellite FDMA navigation system• Galileo is expected to be EU offering for

satellite navigation in 2005• Beidou (北斗 ) experimental satellite

navigation system is China’s developing testbed.

47

GPS vs. GLONASS衛星系統 GPS GLONASS

衛星數 24+ 24-

軌道面 6 3

軌道面上衛星數 4 8

軌道傾角 55° 64.8°

軌道半徑 26,560km 25,510km

軌道週期 約 11時 58分 約 11時 15分

信號處理技術 CDMA FDMA

載波頻率 L1:1575.42MHzL2:1227.60MHz

L1:1602.5625~1615.5MHzL2:1246.4375~1256.5MHz

時間標準 UTC(USNO) UTC(SU)

座標系統 WGS84 SGS85

選擇可用性 (SA) 有 (已關閉 ) 無

48

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 Per Orbit

4

Orbital Inclination 550

Orbital Radius 26560km

Period 11h57m57.26s

Ground Track Repeat Sidereal Day

50

GPS Nominal Orbit Planes

51

Control Segment

52

GPS Control Monitor

53

User Segment

54

GPS Positioning Services• Precise Positioning Service (PPS)

– Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System.

• Standard Positioning Service (SPS)– Civil users worldwide use the SPS without

charge or restrictions. Most receivers are capable of receiving and using the SPS signal. The SPS accuracy is intentionally degraded by the DOD by the use of Selective Availability. (SA Turn off on May 1, 2000)

55

Positioning and Timing Accuracy Standard (SPS)

56

Signals• Signal Structure• Anti-Spoofing (AS) and Selective

Availability (SA)• Signal Power

57

Signals• Currently, each GPS satellite transmits

continuously using two frequencies in the L-band referred to as Link 1 (L1) and Link 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 the other for DoD-authorized users.

• The lone signal on L2 is intended for the DoD-authorized users only.

58

Signal Structure• Carrier: RF sinusoidal signal with frequency fL1 or

fL2.• Ranging Code: a unique sequence of 0s and 1s

assigned to each satellite which allows the receiver to determine the signal 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

59

Signal Structure• Navigation Data: a binary-coded message

consisting of data on the satellite health status, ephemeris (satellite position and velocity), clock bias parameters, 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

60

Signal Structure• The three components of a signal are

derived coherently from one of the atomic standard aboard the satellite.– 10.23 MHz– fL1 = 1575.42 MHz = 2×77×10.23 MHz

– fL2 = 1575.42 MHz = 2×60×10.23 MHz

• The specific form of modulation used is called binary phase shift keying (BPSK)

61

Signal Structure

62

Signal Structure

)2cos()()(2

)2cos()()(2)2sin()()(2)(

22)()(

2,

11)()(

1,11)()()(

LLkk

LY

LLkk

LYLLkk

Ck

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 and y(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 and fL2 are the carrier frequencies corresponding to L1 and L2, respectively; qL1 and qL2 are the initial phase offsets.

(1)

Note: In order to express the BPSK signals as (1), we have switched 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.

63

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 MHzL1 1575.42 MHz

GPS signals. Currently, each GPS satellite transmits three signals, two on L1 and one on L2 frequency. The BPSK-modulated signals are shown. The signal carrying C/A-code on L1 was degraded purposely throughout the 1990s, but this practice has now ended. Access to P(Y)-code is limited to the DoD-authorized users via encryption.

64

Spread Spectrum• The modulation of a carrier by a binary code

spreads the signal 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 carrier frequency.

• While the signal power is unchanged, this step reduces the power spectral density below that for the background RF radiation

• Such signals, referred to as spread spectrum signals, have many properties which make them attractive for use in communication and navigation.

65

Spread Spectrum

66

Power Spectra

67

Pseudo-Random Noise (PRN)

• PRN sequences are nearly orthogonal to each other. For satellites k and l, which are assigned unique PRN sequences called C/A-codes x(k) and x(l),

).()1023( where,, allfor ,0)()( )()(1022

0

)()( mxmxlknnixixi

lk

The left hand side of (2) defines the cross-correlation function of the two sequences for shift n.

(2)

• A PRN sequence is nearly uncorrelated with itself, except for zero shift. For a C/A-code

.1 allfor ,0)()(1022

0

)()(

nnixixi

kk (3)

The left hand side of (3) defines the auto-correlation function of a sequence for shift n. The auto-correlation function of a PRN is nearly zero except for zero shift where it has a sharp peak.

68

Anti-Spoofing (AS) and Selective Availability (SA)• Anti-Spoofing: The main mechanism for limiting access to

the full capabilities of GPS has been encryption of the P-code broadcast on both L1 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-frequency measurements 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.

69

Anti-Spoofing (AS) and Selective Availability (SA)• Throughout the 1990s, the signal available for

unrestricted use were purposefully degraded under the policy of Selective Availability (SA) by adding controlled errors in the measurements.– 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

70

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 express power ratios on a logarithmic scale in units of decibel (dB), defined as

compared be tolevelspower are , where,log10 010

110

0

1 PPP

P

P

P

dB

(4)

• Absolute values of power can be expressed similarly in relation to 1 watt or 1 milliwatt in units of dBW or dBm, respectively. Consider a signal with power (P1) of 0.1 watt. This power level can also be represented as -10dBW or 20dBm. A second signal, with a power (P2) of 100 watt is 30dB more powerful than the first signal. A third signal, with 200-watt power (P3), is 3dB stronger than the second signal. We can capture these relationships as follows.

dB3 ,dB30 ,dBW103

2

1

21

P

P

P

PP

71

Signal PowerP C/A

L1 -133 dBm -130 dBm

L2 -136 dBm -136 dBm*

*Presently not in L2 frequency

• The actual signal powers in recent years have been 3-5 dB higher than the specifications. Even so, the powers are still only around 10-16 watt. Interestingly, 10-16 watt 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. It is the knowledge of the signal structure that allows a receiver to extract the signal buried in the background noise and make precise measurements. The signal boost so realized is called processing gain.

• The low signal power is the Achilles’ heel of GPS, especially in military use.

72

Power of Received SignalZenith 5o Elevation

SV Transmit Power 27 W 27 W

SV Antenna Gain 10.5 dB 16.2 dB

Effective Power Radiated Towards Earth 294 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 2.87x10-3m2 2.87x10-3m2

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 dBm

73

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

74

Receivers and Measurements

• Signal Acquisition and Tracking• Estimation of Position, Velocity, and

Time (PVT)• Evolution of Receiver Technology

75

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

76

GPS Collected Data – Time Domain Plot

• This is the digital data that results from the GPS analog front end ASIC.

• Important parameters: sampling frequency=5.0425MHz, IF=1.25MHz

77

GPS Collected Data – Frequency Domain Plot

• This is the frequency domain representation of the digital data that results from the GPS analog front end ASIC.

• Important parameters: sampling frequency=5.0425MHz, IF=1.25MHz

78

GPS Signal Acquisition• No visible signal under any representation – does it exist?• In order to begin processing the signal it is necessary to go

through the acquisition process and find the signal & its parameters.

• This is achieved by referring to the spread spectrum properties of the CDMA system

• The signal can be correlated with the matching replica and achieve a significant gain.– Based on the correlation– But must be done at baseband (no carrier), thus carrier must be

removed.• Acquisition can provides a coarse estimation of the

pseudorange.

79

Q. So How Does GPS Work?

• 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 repeats every 1 millisecond, and we can accumulate many identical signals until the SNR is high enough.

80

Estimation of Position, Velocity, 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 pseudorange measurements.

• Propagation delays in the ionosphere and troposphere, signal distortion due to multipath, and receiver noise also introduce measurement errors.

81

Basic GPS Positioning Concept -- Trilateration

How GPS Works

83

Basic Equations for Finding User Position

2

32

32

33

22

22

222

21

21

211

uuu

uuu

uuu

zzyyxx

zzyyxx

zzyyxx

• Nonlinear Equations: Difficult to Solve• Relatively Easily Solved with Linearization and Iterative

Approach

84

Measurement of Pseudorange

• Every satellite sends a signal at a certain time tsi. The receiver will receive the signal at a later time tu.

riT= c(tu – tsi) ---- true value of pseudorange or geometric range

• From a practical point of view it is difficult to obtain the correct time from the satellite or the user. The actual satellite clock time and actual user clock time are related to the true time as

utuu

isisi

btt

btt

'

'

85

Measurement of Pseudorange – Cont’d

•Besides the clock error, there are other factors affecting the pseudorange measurement. The measured pseudorange ri can be written as

DDi: satellite position error, DTi: tropospheric delay error, DIi: ionospheric delay error, ni: receiver measurement noise error, Dni: relativistic time correction

)()( iiiiutiiiTi ITcbbcD

86

Measurement of Pseudorange – Cont’d

uuuu

uuuu

uuuu

uuuu

bzzyyxx

bzzyyxx

bzzyyxx

bzzyyxx

24

24

244

23

23

233

22

22

222

21

21

211

87

GPS/SPS Performance Specifications for Global

Positioning and Time Dissemination

Error (95%) PPS SPS

SA Active SA OFF*

PositionHorizontal

Vertical22 m28 m

100 m156 m

10 m15 m

Time 200 ns 340 ns 50 ns

*Estimates

88

Evolution of Receiver Technology

• Several generation of GPS receivers came to market between 1980 and 2000. The receivers available today bear the same resemblance to the early receivers as the laptop and palm computers do to the minicomputers of the early 1980s. The advent of very large scale integration (VLSI) has led to powerful microprocessors and memory chips, which have changes the look and feel of all electronic equipment, including the GPS receivers.

89

Complete ReceiversHandheld 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 and turn-by-turn directions

$400 - $2000

Marine navigation. Fixed mount large screens with electronic charts

$400 - $3000

Aviation. FAA certified, panel mounted, with maps $3000 - $15,000

Survey and mapping. Often tripod mounted, exclusively Differential GPS, one meter to centimeter accuracy

$3500 - $30,000

90

ModulesPlug-in modules. Integrated receiver and antenna, used for tracking and monitoring

$100 - $300

OEM boards. Receiver circuitry for customer integration

$60 - $100

Chip sets $10 - $30

91

Augmentation Systems and 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

92

Reducing Measurement Errors and/or Improving Satellite

Geometry• Mitigation of the measurement errors turns out to

be simpler: The errors associated with the worst error sources are similar for users located ‘not far’ from each other, and change ‘slowly’ in time. In other words, the errors are correlated both spatially and temporally.

• We can estimate the error in a measurement if the receiver location is known.

• Such error estimates can be used as differential corrections if made available to the GPS users in the area, allowing them to mitigate errors in their measurements.

• That’s differential GPS, generally abbreviated as DGPS.

93

Reducing Measurement Errors and/or Improving Satellite

Geometry• Satellite geometry can be improved by adding

satellites to the constellation 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, or on a ship.

• A GPS receiver has to be modified to receive and process these signals.

94

Differential GPS

95

Hierarchy of GPS Capability

96

Civil Applications• High-precision (millimeter-to-

centimeter level) positioning• Specialized applications such as

aviation and space navigation• Land transportation and maritime

uses• Consumer products

97

Modernization Plans

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)

98

Summary• 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

99

Summary• 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)

100

Summary• 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

101

Summary• Performance Achievable

– Real time: Typically, absolute positioning error of several meters with a single receiver, decimeters in differential mode

– Batch processing: millimeter-level relative positioning