GPS

Patrick Caldwell

Chris Kellar

Overview

Basic Concepts History Structure Applications Communication Typical Sources of Error

Basic Concepts Global Positioning

System (GPS) is satellite radio navigation system.

Constellation of 27 satellites

Satellites orbit at 12,000 miles

Provides positioning, navigation, and timing information.

Artist's conception of GPS Block II-F satellite in Earth orbit [1]

Brief History Development of GPS

started in 1973 Built on the work previous

radio navigation projects (notably TRANSIT)

First satellite launched in 1978

System was completed in 1995

Official logo for NAVSTAR GPS [2]

Space Segment Constellation 27

Satellites Medium Earth orbit

(MEO) at 20200 km Arranged into six

equally-spaced orbital planes surrounding the Earth Simulation of Satellite Position [3]

Space Segment Cont. GPS Block III

Next generation Satellite under development

Advanced anti-jam capabilities

Three times more accurate than current GPS

Three times more power for military signals

Summary of GPS Satellites [4]

Control Segment

GPS control segment consists of a global network of ground facilities that track the GPS satellites [5]

Control Segment Cont. Master Control Station (MCS)

Provides command and control of the GPS constellation.

Calculates satellite position from received navigation information from the monitor stations.

Monitor Stations Collects atmospheric data,

range/carrier measurements, and navigation signals

Ground Antennas Used to communicate with the GPS

satellites for command and control purposes

Console at Monitoring Station [5]

GPS Ground Antenna [5]

User Segment Civilian GPS Applications

AviationMarineRailSurveying & MappingTimingConsumer Electronics

GPS Receiver Module [6]

User Segment Cont. Military GPS Applications

NavigationTarget TrackingMissile GuidanceSearch and RescueReconnaissanceNuclear Detonation

Detection

JDAMs loaded onto a Multiple Ejector Rack [7]

Military vs. Civilian GPS The accuracy of the GPS signal in space is

actually the same for both the civilian GPS service (SPS) and the military GPS service (PPS) [8]. 

PPS broadcast on two frequency allowing for ionospheric correction leading to better accuracy.

SPS provides accuracy of 7.8 meters at a 95% confidence level.

Communication

Transmission – BPSK LOSPublic encoding (CDMA)

○ Course/acquisition (C/A)Military encrypted encoding (CDMA)

○ Precise (P)

Carrier FrequenciesL1 = 1575.42 MHzL2 = 1227.60 MHzL3 = 1381.05 MHz (NUDET)

Transmission

Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder [9]

Pseudorandom Noise (PRN) code

C/A – 1023 bit deterministic sequenceUnique to each satelliteRepeats every 1 ms

P – 6.1871x10^12 bit deterministic sequenceRepeat every weekPart of larger master code 2.35x10^14 bitsMaster segments unique to each satellite

P(Y) – encrypted precise codeMilitary use only

Message Format

Navigation Message Content and Format Overview [10]

Message Breakdown

Single sub-frame data format breakdown [10]

Telemetry and Handover Word

Individual TLM and HOW word format breakdown [10]

Almanac Contains information and status

about the satellite Assists in determining what

satellite to be tracked Corrects for ionospheric errors

UTC (Coordinated Universal Time) Synchronized time

signal among all satellites.

Provided during the HOW

Decoding

Modulo-2 addition with Gold Code 1,025 different Gold codes (1023 bits)Highly mutually orthogonal

○ Same frequency transmissionsUsed often in CDMA encoding schemes

GPS Signal Errors Ionosphere and troposphere delays Signal multipath Clock errors Orbit errors Limited satellites in area Satellite geometry/shading

Sources of common errors [12]

Questions?

References [1] http://en.wikipedia.org/wiki/File:GPS_Satellite_NASA_art-iif.jpg

[2] http://en.wikipedia.org/wiki/File:NAVSTAR_GPS_logo_shield-official.jpg

[3] http://en.wikipedia.org/wiki/File:ConstellationGPS.gif

[4] http://en.wikipedia.org/wiki/Gps

[5] http://www.gps.gov/systems/gps/control/

[6] https://www.sparkfun.com/products/8291

[7] http://en.wikipedia.org/wiki/File:JDAM_GBU30_MER.jpg

[8] http://www.gps.gov/systems/gps/performance/accuracy/

[9] http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

[10] http://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdf

[11] http://wireless.ictp.trieste.it/school_2003/lectures/carlo/GPS/DECODING.HTM

[12] http://www8.garmin.com/aboutGPS/