3. System Description
Navigational Signals
RangingCodes
SystemTime
Clock Correction
Propagation Delay
Satellite Ephemeris
Satellite Health
Downlink Data
Satellite Ephemeris Data
Clock Data
Uplink Data
Satellite Ephemeris Corrections
Clock DataCorrections
Space Segment
Control Segment
User Segment
8. 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
Satellite Links
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
14. GPS 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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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.
18
NAD 27 ED 50
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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.
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
Designation 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
24. Beamwidth
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
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
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.
