The Global Positioning System (GPS). 1. 2 nd USAF Space Operations Squadron. 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.
Six orbital planes, four satellites per plane
Semi-synchronous, circular orbits (~11,000 mi)
12-hr ground-repeating orbits
The GPS Constellation utilises the Medium Earth Orbit
S Band Up/ Downlink
Master Control Station
- Navigation Data
Collect Range Data
Monitor Navigation 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’
To determine a GPS position:
Distance to satellites
Coordinate reference framework
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
GPS receivers determine position
Cartesian Co-ordinates (X,Y,Z)
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
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.
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.
Applications for frequency allocations are ratified by the International Telecommunications Union (ITU)
NATO frequency allocations for Military Communication Satellites are:
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
Low Data Rates
Low Gain Antennas
Good Adverse Weather Performance
Limited Anti-Jam Capability
Poor Performance in Nuclear Environment
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
Less Mature Technology than SHF and UHF
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
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.