1. Patrick Caldwell Chris Kellar

2. Overview  Basic Concepts  History  Structure  Applications  Communication  Typical Sources of Error

3. 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]

4. 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]

5. 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]

6. 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]

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

8. 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]

9. User Segment  Civilian GPS Applications Aviation Marine Rail Surveying & Mapping Timing Consumer Electronics GPS Receiver Module [6]

10. User Segment Cont.  Military GPS Applications Navigation Target Tracking Missile Guidance Search and Rescue Reconnaissance Nuclear Detonation Detection JDAMs loaded onto a Multiple Ejector Rack [7]

11. 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.

12. Communication  Transmission – BPSK LOS Public encoding (CDMA) ○ Course/acquisition (C/A) Military encrypted encoding (CDMA) ○ Precise (P)  Carrier Frequencies L1 = 1575.42 MHz L2 = 1227.60 MHz L3 = 1381.05 MHz (NUDET)

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

14. Pseudorandom Noise (PRN) code  C/A – 1023 bit deterministic sequence Unique to each satellite Repeats every 1 ms  P – 6.1871x10^12 bit deterministic sequence Repeat every week Part of larger master code 2.35x10^14 bits Master segments unique to each satellite  P(Y) – encrypted precise code Military use only

15. Message Format Navigation Message Content and Format Overview [10]

16. Message Breakdown Single sub-frame data format breakdown [10]

17. Telemetry and Handover Word Individual TLM and HOW word format breakdown [10]

18. Almanac  Contains information and status about the satellite  Assists in determining what satellite to be tracked  Corrects for ionospheric errors

19. UTC (Coordinated Universal Time)  Synchronized time signal among all satellites.  Provided during the HOW

20. Decoding  Modulo-2 addition with Gold Code 1,025 different Gold codes (1023 bits) Highly mutually orthogonal ○ Same frequency transmissions Used often in CDMA encoding schemes

21. 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]

22. Questions?

23. References [1] http://en.wikipedia.org/wiki/File:GPS_Satellite_NASA_art-iif.jpghttp://en.wikipedia.org/wiki/File:GPS_Satellite_NASA_art-iif.jpg [2] http://en.wikipedia.org/wiki/File:NAVSTAR_GPS_logo_shield-official.jpghttp://en.wikipedia.org/wiki/File:NAVSTAR_GPS_logo_shield-official.jpg [3] http://en.wikipedia.org/wiki/File:ConstellationGPS.gifhttp://en.wikipedia.org/wiki/File:ConstellationGPS.gif [4] http://en.wikipedia.org/wiki/Gpshttp://en.wikipedia.org/wiki/Gps [5] http://www.gps.gov/systems/gps/control/http://www.gps.gov/systems/gps/control/ [6] https://www.sparkfun.com/products/8291https://www.sparkfun.com/products/8291 [7] http://en.wikipedia.org/wiki/File:JDAM_GBU30_MER.jpghttp://en.wikipedia.org/wiki/File:JDAM_GBU30_MER.jpg [8] http://www.gps.gov/systems/gps/performance/accuracy/http://www.gps.gov/systems/gps/performance/accuracy/ [9] http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html [10] http://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdfhttp://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdf [11] http://wireless.ictp.trieste.it/school_2003/lectures/carlo/GPS/DECODING.HTMhttp://wireless.ictp.trieste.it/school_2003/lectures/carlo/GPS/DECODING.HTM [12] http://www8.garmin.com/aboutGPS/ http://www8.garmin.com/aboutGPS/