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Transcript of Patrick Caldwell Chris Kellar. Overview Basic Concepts History Structure Applications ...
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/