Satellite stuff

(Click to enlarge pic)

When you put up a satellite dish on your house in Indiana you’re required to point it to the South West. The reason for this is television satellites must be in an orbit over the equator and they are roughly centered on North America. Therefore we need to point our dish south and west. The only way an orbit can be synchronized with the rotation of the earth so that the satellite appears to be stationary overhead is with the equatorial orbit at an altitude of about 23,000mi. There is no other orbit which can achieve this apparently stationary antenna effect.

Once a satellite is put into orbit it must be maintained in that orbit by the use of small gas jets on board. Orbit’s can decay over time and the satellite must be steered back into its proper position and attitude (pointed correctly). Interestingly, running out of gas for the jets is frequently the cause of death for the satellite. This can occur perhaps after eight or ten years. Note that the space shuttle can only reach low earth orbit and therefore can not reach Geostationary satellites to repair them.

The global positioning satellite (GPS) system requires three satellites be in view to an earth observer in order to calculate location. Therefore many GPS satellites are used in many orbits. Spy satellites like to fly low to take detailed pictures and frequently use an elliptical orbit so that they are quite low on their nearest approach. But when they are out on the ellipse they are useless and you must wait for them to swing back. By using control jets orbits can be modified so that the satellite can take a different track. If your orbit is north-south (polar orbit) the earth will rotate underneath the satellite and different areas of the earth will be in view on every pass.