ALL ABOUT SATELLITES
See also All About GPS
VARIETIES OF SATELLITES
Type Features Examples
LEO Low Earth Orbit
Period about 50 minutes to 2 hours period
You can see 2.2% of the earth from orbit 300 km up.
  • FedSat 1
Geostationary Stays vertically above same spot on Earth's surface
Period is NOT precisely 24 hours as Earth moves in space over a year.
You can see about 42% of the earth from a geostationary satellite.
  • ATS-1     Over Hawai
POLAR In orbit over poles
  • Polar
  • POLAR BEAR
ECHO "Passive" Communication Satellites Large - often > 30 m in diameter
  • Echo I
  • Echo II
GPS GPS = Global Positioning System. GPS Satellite, called a SV for Space vehicle.
GPS receivers identify a satellite via number = PRN decoded from signal data.
Sometimes used generically to include satellites in the Russian Glonass System.
GPS satellites transmit signal packets containing SV location and time. 		In the DoD constellation. there are 24 SVs in 6 orbital planes, each holding 4 satellites. GPS SV's (Space Vehicles) are referred to by their PRNs, or otherwise
e.g, for a Block 11 satellite
PRN14 = B11-01
GLONASS Russian Global Positioning System satellite. Glonass satellites, like GPS SVs transmit signal packets containing location and time data. In the Glonass constellation. there are 24 SVs in 4 orbital planes, each holding 6 satellites. Satellites have numeric lables, e,g.,
Glonass 34
GNSS Global Navigation Satellite Systems (GNSS) GPS and Glonass, and the next GNSS
What's in a Satellite

Cut-away diagram of South African SUNSAT satellite.

SUNSAT is a microsatellite, only 45 cm x 45 cm x45 cm. Satllites, except for those embarking on journeys distant from the sun, are powered by batteries that are kept charged by the output from solar cells. n the cae of SUNSAT, the solar cells are directly mounted on the sides. For this amll area of solar cells, power is limited to about 50 watts. Larger satellites have extensive external panels of solar cells mounted on attached wings, supplementing those directly mounted on the satellite case, to meet power needs.
The star imager is viewing the sky, and inputting to the on-board computer. By reference to a built-in star map in memory, the on-board computer determines the orientation of the satellite in space.
At the top of the satellite, mounted on a boom, is a magnetometer for measuring magnetic fields in space. The boom is required to isolate the coils of the magnetometer from the many other aerials mounted on the satellite.
Note the omnidirection antennae on the top of the satellite, so that this low earth orbitting (LEO) satellite can receive signals from GPS satellites above, and at other orientations. The GPS receiver on-board enables on-board space location to be determined. Before the advent of GPS, very precise tracking was required to enable sensing data to be fixed in space. 		Artists impression of NASA RXTE Satellite Note that the body of this satellite is covered with gold foil as a protective measure. This is usual for larger satellites of longer or critical mission durations.
Julian Day Computation Calculations involving satellites are simplified by using the Jalian Day rather than our standard (gregorian) calendar.
Here is an example:
The GPS Satellite system produces data with week and day starting at 0000 hours January 6, 1980. The signal data packages the week data in 10 bit, so that as 2^10 = 1024, there will a rollover after 1024 weeks. When does the GPS EOW rollover take place?
Computation:
1024 weeks = 1024 * 7 = 7168 days
      6 Jan 1980 == 2444244.5 Julian Day
2444244.5 + 7168 = 2451412.5
      Jaulian Day 2451412.5 == 22 Aug 1999
So the GPS EOW rollover takes place on 22 Auguat, 1999
See GPS Millennium Bug     and     All about GPS
In the above calculation, I've used a Julian/Gregory Converter to convert from a date in the (usual) Gregorian Calendar, to the corresponding Julian Day, and back. A good Julian/Gregory Converter is at:
http://quasar.as.utexas.edu/BillInfo/JulianDateCalc.html
Tasmania:
Black Hole
Above

Australia in Space
a History
TOPEX-POSEIDON

Bouncing radar off the sea


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Australian Satellite Science and Engineering