1 The Global Positioning System (GPS)

2 nd USAF Space Operations Squadron

System Description 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 Space Segment Control Segment User Segment

SPACE SEGMENT

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

Orbital Planes The GPS Constellation utilises the Medium Earth Orbit

CONTROL SEGMENT

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

Navigation data MCS Monitor station Ground antenna Telemetry Commands Navigation data uploads Control Segment

USER SEGMENT

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

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

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

GPS Solution 14

GPS Solution c = speed of light (3x10 8 m/s) t t,1, t t,2, t t,3, t t,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 t r,1, t r,2, t r,3, t r,4 = times that the signals from GPS satellites 1, 2, 3, and 4, are received according to the inaccurate GPS receiver’s clock x 1, y 1, z 1 = coordinates of GPS satellite 1. These coordinates are provided to the receiver as part of the information that is transmitted Similar meaning for x 2, y 2, z 2, etc. The receiver solves these equations simultaneously to determine x, y, z, and t c

Coordinate Frames 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

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. 18 NAD 27 ED 50 ED 79 Tokyo Indian Bngldsh OSGB 36

Introduction to SATCOM

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.

1962 – Telstar

Categories Frequency Transponder Type Orbit

Frequency Applications for frequency allocations are ratified by the International Telecommunications Union (ITU) NATO frequency allocations for Military Communication Satellites are: UplinkDownlink UHF MHz MHz SHF GHz GHz EHF GHz GHz S Band for Command, Control and Telemetry of satellites

Frequency Bands DesignationFrequency (Uplink/Downlink) UHF MHz/ MHzMilitary L-Band1.6GHz/1.5GHz S-Band2.5GHz C-Band6GHz/4GHz X-Band (SHF) GHz/ GHzMilitary Ku-Band14GHz/12GHz Ka-Band30GHz/20GHz EHF GHz/ GHzMilitary

Beamwidth Beamwidth for a 1 metre SATCOM antenna BandFrequencyDegrees UHF300MHz60 SHF8.0 GHz2.5 EHF40.0GHz0.5 D

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

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

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

Categories Frequency Transponder Type Orbit

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

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

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

Categories Frequency Transponder Type Orbit

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.