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The Global Positioning System The Global Positioning System (GPS)(GPS)
2nd USAF Space Operations Squadron
System DescriptionSystem Description
Navigational Signals
Ranging CodesSystem TimeClock CorrectionPropagation DelaySatellite Ephemeris Satellite Health
Downlink DataSatellite Ephemeris DataClock Data
Uplink DataSatellite Ephemeris Corrections Clock Data Corrections
Space Segment
Control Segment
User Segment
SPACE SEGMENTSPACE SEGMENT
GPS SatellitesGPS Satellites
• 24-satellite constellation• Six orbital planes, four satellites per plane• Semi-synchronous, circular orbits (~11,000 mi)• 12-hr ground-repeating orbits
Orbital PlanesOrbital Planes
The GPS Constellation utilises the Medium Earth OrbitThe GPS Constellation utilises the Medium Earth Orbit
CONTROL SEGMENTCONTROL SEGMENT
Control SegmentControl Segment
Monitor Stations
Uplink Station
Master Control Station
Downlink S Band Up/ Downlink
Collect Range DataMonitor Navigation Services
Navigation EstimationSatellite ControlSystems Operation
Transmit: - Navigation Data - CommandsCollect Telemetry
GPS Satellite
Satellite Links Satellite Links
USER SEGMENTUSER SEGMENT
GPS ServicesGPS 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’
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GPS Military MissionsGPS Military MissionsNavigation• 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
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GPS PositionGPS Position
•To determine a GPS position:•Distance to satellites•Satellite orbit/position•Earth’s shape•Coordinate reference framework
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GPS SolutionGPS Solution
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GPS SolutionGPS Solution•c = speed of light (3x108 m/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
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24
244,4,4
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23
233,3,3
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22
222,2,2
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21
211,1,1
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Coordinate FramesCoordinate Frames
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Position DerivationPosition 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
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Local Mapping DatumLocal Mapping DatumA 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.
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NAD 27 ED 50ED 79 Tokyo
Indian Bngldsh
OSGB 36
Introduction to SATCOMIntroduction to SATCOM
Satellite Communication SystemsSatellite 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 – Telstar1962 – Telstar
CategoriesCategories
• Frequency• Transponder Type• Orbit
FrequencyFrequency• 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
Frequency BandsFrequency BandsDesignation Frequency
(Uplink/Downlink)
UHF 290-320MHz/240-270MHz Military
L-Band 1.6GHz/1.5GHz
S-Band 2.5GHz
C-Band 6GHz/4GHz
X-Band (SHF) 7.9-8.4GHz/7.25-7.75GHz Military
Ku-Band 14GHz/12GHz
Ka-Band 30GHz/20GHz
EHF 43.5-45.5GHz/20.2-21.2GHz Military
BeamwidthBeamwidth
Beamwidth for a 1 metre SATCOM antenna
Band Frequency Degrees
UHF 300MHz 60
SHF 8.0 GHz 2.5
EHF 40.0GHz 0.5
D
UHF CharacteristicsUHF 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 CharacteristicsSHF/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 CharacteristicsEHF & 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
CategoriesCategories
• Frequency• Transponder Type• Orbit
Transparent TranspondersTransparent Transponders
DOWNLINKUPLINK
SATELLITE
POWERAMPLIFIER
FREQUENCYCONVERTER
RECEIVER & LOW NOISEAMPLIFIER
Reconstituting TranspondersReconstituting Transponders
RECODEDDOWNLINK
CODEDUPLINK
SATELLITE
POWERAMPLIFIER
DECODER &LOW NOISEAMPLIFIER
RECODER &FREQUENCYUP-CONVERTER
FREQUENCYDOWN-CONVERTER
Store Dump TranspondersStore Dump Transponders
DOWNLINKUPLINK
SATELLITE
FREQUENCYUP-CONVERTER & POWERAMPLIFIER
FREQUENCYDOWN-CONVERTER
RECEIVER &LOW NOISEAMPLIFIER
DATASTORAGESYSTEM
CategoriesCategories
• Frequency• Transponder Type• Orbit
SATCOM OrbitsSATCOM 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.