A satellite in geostationary orbit (GEO) takes exactly one sidereal day (23 h 56 min) to go round and, seen from the ground, stays at the same point above the equator. Three satellites together see almost the whole Earth, except the polar regions.
- Advantages: a fixed dish is enough; one satellite covers more than a third of the Earth, continuously.
- Disadvantages: great distance — images are coarser and a signal takes more than half a second for the round trip; above ± 75° latitude the satellite is too low on the horizon or below it; the orbit is crowded and scarce, and slots are allocated internationally.
Uses: SATCOM (military and commercial), weather satellites, missile warning with infrared sensors (such as the American SBIRS), and large SIGINT satellites with huge antennas. A satellite that is used up in GEO is parked ± 300 km higher in a graveyard orbit.
Military importance
Because GEO satellites are valuable, scarce and predictable, they are an attractive target. Satellites manoeuvring close to other GEO satellites — to inspect them or worse — are therefore an important point of attention (see 6.3).