GPS

An Overview of theAn Overview of the Global Positioning Satellite System Global Positioning Satellite System

(GPS)(GPS)Edward J. DelpEdward J. Delp

N9YTEN9YTEOctober 25, 2000October 25, [email protected]@n9yte.net

http://www.n9yte.nethttp://www.n9yte.net

OverviewOverview

The Navigation ProblemThe Navigation Problem Earlier ApproachesEarlier Approaches GPS DescriptionGPS Description How does GPS Work?How does GPS Work?

AcknowledgementAcknowledgement

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

http://www.utexas.edu/depts/grg/gcraft/http://www.utexas.edu/depts/grg/gcraft/ notes/gps/gps.htmlnotes/gps/gps.htmlThe best web site for GPSThe best web site for GPS

The Navigation ProblemThe Navigation Problem

The ancient question: The ancient question: Where am I?Where am I?

Earth coordinates: Earth coordinates: latitude and longitudelatitude and longitude

Lafayette: N40/W86Lafayette: N40/W86 Latitude can be Latitude can be

determined by Sun determined by Sun angleangle

What about longitude?What about longitude?

Latitude and LongitudeLatitude and Longitude

Latitude and LongitudeLatitude and Longitude

Longitude ProblemLongitude Problem

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

established in England to solve the established in England to solve the “longitude problem”“longitude problem”

Two solutions were proposedTwo solutions were proposed -- use of stars and moons-- use of stars and moons -- the “time” solution-- the “time” solution

LongitudeLongitude

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

Dava Sobel

Longitude Problem: ReferencesLongitude Problem: References

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

The “Time” SolutionThe “Time” Solution

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

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

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

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

LongitudeLongitude

How does this work?How does this work? The earth turns 360 degrees in 24 hours:The earth turns 360 degrees in 24 hours: 15 degrees = 1 hour15 degrees = 1 hour If you know the time in Greenwich when it If you know the time in Greenwich when it

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

Must know “datum” reference to use mapsMust know “datum” reference to use maps

Datum ReferenceDatum Reference

Lone Pine Cemetery - N400 13.8’’ E260 17.24”

Satellite NavigationSatellite Navigation

US Department of Defense has need for US Department of Defense has need for very precise navigationvery precise navigation

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

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

NAVSTAR GPS GoalsNAVSTAR GPS Goals

What time is it?What time is it? What is my position (including attitude)?What is my position (including attitude)? What is my velocity?What is my velocity? Other Goals:Other Goals: - What is the local time?- What is the local time? - When is sunrise and sunset?- When is sunrise and sunset? - What is the distance between two points?- What is the distance between two points? - What is my estimated time arrival?- What is my estimated time arrival?

GSP SystemGSP System

Simply stated: The GPS satellites are nothingSimply stated: The GPS satellites are nothing more than a set of clocks in the skymore than a set of clocks in the sky

GPS SegmentsGPS Segments

Space Segment: the constellation of Space Segment: the constellation of satellitessatellites

Control Segment: control the satellitesControl Segment: control the satellites

User Segment: users with receiversUser Segment: users with receivers

Space SegmentSpace Segment

Space SegmentSpace Segment

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

3 other satellites are used for testing3 other satellites are used for testing Altitude: 20,200 Km with periods of 12 hr.Altitude: 20,200 Km with periods of 12 hr. Current Satellites: Block IIR- $25,000,000Current Satellites: Block IIR- $25,000,000 2000 KG2000 KG Hydrogen Maser Atomic ClocksHydrogen Maser Atomic Clocks

Hydrogen Maser ClockHydrogen Maser Clock

These clocks lose one second everyThese clocks lose one second every2,739,000 million years2,739,000 million years

GPS OrbitsGPS Orbits

GPS OrbitsGPS Orbits

Control SegmentControl Segment

Master Control Station is located at the Master Control Station is located at the Consolidated Space Operations Center Consolidated Space Operations Center (CSOC) at Flacon Air Force Station near(CSOC) at Flacon Air Force Station nearColorado SpringsColorado Springs

Control SegmentControl Segment

CSOCCSOC

Track the satellites for orbit and clock Track the satellites for orbit and clock determinationdetermination

Time synchronizationTime synchronization Upload the Navigation MessageUpload the Navigation Message Manage DOAManage DOA

Operational CapabilitiesOperational Capabilities

Initial Operational Capability - December 8,Initial Operational Capability - December 8,19931993

Full Operational Capability declared by theFull Operational Capability declared by theSecretary of Defense at 00:01 hours onSecretary of Defense at 00:01 hours onJuly 17, 1995July 17, 1995

GPS Transmitted SignalGPS Transmitted Signal

Two signals are transmitted on carriers:Two signals are transmitted on carriers: L1 = 1575.42 MHzL1 = 1575.42 MHz L2 = 1227.60 MHzL2 = 1227.60 MHzThese are derived from the system clock of These are derived from the system clock of 10.23 MHz (phase quadrature)10.23 MHz (phase quadrature) Modulation used is Direct Sequence Spread Modulation used is Direct Sequence Spread

SpectrumSpectrum (code division multiple access - CDMA) (code division multiple access - CDMA)

GPS SignalsGPS Signals

GPS Clock SignalsGPS Clock Signals

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

available for civilian use on L1 provides available for civilian use on L1 provides 300 m resolution300 m resolution

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

Spread SpectrumSpread Spectrum

Spread Spectrum is used becauseSpread Spectrum is used because - resistance to jamming- resistance to jamming - masks the transmissions- masks the transmissions - resist multipath effects- resist multipath effects - multiple access- multiple access All 24 GPS satellites transmit on the same two All 24 GPS satellites transmit on the same two

frequencies BUT use a different ID sequencefrequencies BUT use a different ID sequence

GPS SignalsGPS Signals

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

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

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

GPS PositionGPS Position

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

To correct for clock errors one needs to To correct for clock errors one needs to receive four satellitesreceive four satellites

GPS: How does it work?GPS: How does it work?

Typical receiver: one channel C/A code on L1Typical receiver: one channel C/A code on L1 During the “acquisition” time you are receiving During the “acquisition” time you are receiving

the navigation message also on L1the navigation message also on L1 The receiver then reads the timing information The receiver then reads the timing information

and computes the “pseudo-ranges”and computes the “pseudo-ranges” The pseudo-ranges are then correctedThe pseudo-ranges are then corrected

GPS: How does it work?GPS: How does it work?

Corrected ranges are used to compute the Corrected ranges are used to compute the positionposition

This is a very complicated iterative This is a very complicated iterative nonlinear equationnonlinear equation

Navigation MessageNavigation Message

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

Navigation Message - transmitted on both Navigation Message - transmitted on both L1 and L2 at 50 bits/s for 30 sL1 and L2 at 50 bits/s for 30 s

Navigation message consists of two parts:Navigation message consists of two parts: - satellite almanac- satellite almanac - clock bias- clock bias

Why Do I Need Why Do I Need To See 4 Satellites?To See 4 Satellites?

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

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

For a 2D fix <=> 3 satellitesFor a 2D fix <=> 3 satellites

Why Do I Need Why Do I Need To See 4 Satellites?To See 4 Satellites?

Denial of Accuracy (DOA)Denial of Accuracy (DOA)

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

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

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

techniquestechniques

Differential GPSDifferential GPS

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

positionposition Differential signal is not “transmitted” on Differential signal is not “transmitted” on

standard satellite frequenciesstandard satellite frequencies

Uses of GPSUses of GPS

Airplane and Boat NavigationAirplane and Boat Navigation Continental DriftContinental Drift SurveyingSurveying Precise TimingPrecise Timing Iceberg TrackingIceberg Tracking Archaeological ExpeditionsArchaeological Expeditions Mobile MultimediaMobile Multimedia

GPS Clock RolloverGPS 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/http://www.navcen.uscg.mil/gps/geninfo/y2k/gpsweek.htmy2k/gpsweek.htm

ConclusionConclusion

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

systemsystem

ReferencesReferences

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.

ReferencesReferences

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 SitesWeb Sites

GPS Program Office:GPS Program Office:http://www.laafb.af.mil/SMC/CZ/homepage/http://www.laafb.af.mil/SMC/CZ/homepage/ US Coast Guard Navaigation CenterUS Coast Guard Navaigation Centerhttp://www.navcen.uscg.mil/default.htmhttp://www.navcen.uscg.mil/default.htm GPS Precise OrbitsGPS Precise Orbitshttp://www.ngs.noaa.gov/GPS/GPS.htmlhttp://www.ngs.noaa.gov/GPS/GPS.html GPS World MagazineGPS World Magazinehttp://www.gpsworld.com/http://www.gpsworld.com/