The Global Positioning System (GPS)

1. The Global Positioning System (GPS) 1

2. 2nd USAF Space Operations Squadron

3. System Description Space Segment • Navigational Signals • Ranging Codes • System Time • Clock Correction • Propagation Delay • Satellite Ephemeris • Satellite Health • Downlink Data • Satellite Ephemeris Data • Clock Data • Uplink Data • Satellite Ephemeris Corrections • Clock Data Corrections User Segment Control Segment

4. SPACE SEGMENT

5. GPS Satellites 24-satellite constellation Six orbital planes, four satellites per plane Semi-synchronous, circular orbits (~11,000 mi) 12-hr ground-repeating orbits

6. Orbital Planes The GPS Constellation utilises the Medium Earth Orbit

7. CONTROL SEGMENT

8. Control Segment GPS Satellite S Band Up/ Downlink Downlink Satellite Links Satellite Links Uplink Station Master Control Station Transmit: - Navigation Data - Commands Collect Telemetry Monitor Stations Collect Range Data Monitor Navigation Services Navigation Estimation Satellite Control Systems Operation

9. USER SEGMENT

10. GPS Services Standard Positioning Service (SPS) Uses Coarse Acquisition Code (C/A Code) only Models Ionospheric errors Think ‘civilian GPS’ Precise Positioning Service (PPS) Uses C/A Code and Precision Code (P-Code) Calculates Ionospheric errors Has encryption capability (Y code) Think ‘Military GPS’ 11

11. GPS Military Missions Navigation • Position, Velocity and Time • Worldwide • Any weather • Any time Time • Users calculate GPS time • GPS time will be within 1000ns of UTC • Time transfer to within 100ns of UTC • Synchronizes digital communications 12

12. GPS Position To determine a GPS position: Distance to satellites Satellite orbit/position Earth’s shape Coordinate reference framework 13

13. GPS Solution 14

14. GPS Solution c = speed of light (3x108m/s) tt,1, tt,2, tt,3, tt,4= times that GPS satellites 1, 2, 3, and 4, transmitted their signals. These times are provided to the receiver as part of the information that is transmitted tr,1, tr,2, tr,3, tr,4= times that the signals from GPS satellites 1, 2, 3, and 4, are received according to the inaccurate GPS receiver’s clock x1, y1, z1 = coordinates of GPS satellite 1. These coordinates are provided to the receiver as part of the information that is transmitted Similar meaning for x2, y2, z2, etc. The receiver solves these equations simultaneously to determine x, y, z, and tc

15. Coordinate Frames 16

16. Position Derivation GPS receivers determine position Cartesian Co-ordinates (X,Y,Z) WGS-84 Ellipsoid Cartesian Co-ordinates are translated Local datum (ie. OSGB-36) Cartesian Co-ordinates are transformed Latitude, Longitude, and Elevation Elevation is determined with reference to: Ellipsoid, Geoid, or Mean Sea Level 17

17. Local Mapping Datum A Map Datum is a coordinate reference system consisting of unique and invariable coordinates which are based on an ellipsoid/geoid model over a portion of the earth. OSGB 36 NAD 27 ED 50 ED 79 Tokyo Indian Bngldsh 18

18. Introduction to SATCOM

19. Satellite Communication Systems Communication Satellites are used to relay information from one point to another. They enable long range communications at high data rates by overcoming The line of sight limitation of traditional communications like VHF and UHF. The low data rate capacity of traditional long range communication i.e. HF. SATCOM is used for both voice and data communications and is extremely important for both the military and commercial world (just think Sky TV), as well as society as a whole (the Global Commons) SATCOM does not require landline point to point connection. Very useful for Military operations Very useful for work in areas of low/no infrastructure, including disaster relief.

20. 1962 – Telstar

21. Categories Frequency Transponder Type Orbit

22. Frequency Applications for frequency allocations are ratified by the International Telecommunications Union (ITU) NATO frequency allocations for Military Communication Satellites are: Uplink Downlink UHF 290-320MHz 240-270MHz SHF 7.9-8.4GHz 7.2-7.75GHz EHF 43.5-45.5GHz 20.2-21.2GHz S Band for Command, Control and Telemetry of satellites

23. Frequency Bands

24. Beamwidth D

25. UHF Characteristics Mature Technology Relatively Cheap Low Data Rates Low Gain Antennas Good Adverse Weather Performance Limited Anti-Jam Capability Poor Performance in Nuclear Environment

26. SHF/X-Band Characteristics Mature Technology Inexpensive Higher Data Rates than UHF Higher Gain Antennas suitable for Spot Beams Adequate Adverse Weather Performance Some Anti-Jam Capability Some Performance in Nuclear Environment

27. EHF & Ku/Ka Characteristics Less Mature Technology than SHF and UHF Expensive Higher Data Rates Very High Gain Antennas for Small Spot Beams Very Poor Adverse Weather Performance Good Anti-Jam Capability *Good Performance in Nuclear Environment * UK – US MoU

28. Categories Frequency Transponder Type Orbit

29. Transparent Transponders RECEIVER & LOW NOISE AMPLIFIER POWER AMPLIFIER FREQUENCY CONVERTER DOWNLINK UPLINK SATELLITE

30. Reconstituting Transponders POWER AMPLIFIER FREQUENCY DOWN- CONVERTER DECODER & LOW NOISE AMPLIFIER RECODER & FREQUENCY UP-CONVERTER RECODED DOWNLINK CODED UPLINK SATELLITE

31. Store Dump Transponders FREQUENCY UP-CONVERTER & POWER AMPLIFIER RECEIVER & LOW NOISE AMPLIFIER DATA STORAGE SYSTEM FREQUENCY DOWN- CONVERTER DOWNLINK UPLINK SATELLITE

32. Categories Frequency Transponder Type Orbit

33. SATCOM Orbits Most SATCOMs are in Geosynchronous Orbit Most of these are in GEO Stationary orbits. Some SATCOM systems reside in Low Earth Orbit (LEO) For example IRIDIUM. Useful for global coverage including the polar regions.