1 The Global Positioning System (GPS). 2 nd USAF Space Operations Squadron.

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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

24

24

244,4,4

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23

233,3,3

22

22

222,2,2

21

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.