Ask Slashdot: How Would You Build a Microsatellite?

Yeah, you can have omnidirectional antenna coverage for both uplink and downlink.... This is the preferred method if you can close your link and data budgets because it makes the system vastly simpler and inherently fail safe.... A secondary directional downlink may be reasonable if you have very high data requirements (e.g. streaming video or ultra high definition imagery)

Most commercial and military satellites have a low-bandwidth omnidirectional uplink and downlink for control. USAF satellites used to have (and may still have) almost a complete separation between the "bus" and "payload" sides, with the "bus" side on omni antennas. At the ground end, the USAF had big steerable dishes at about six tracking stations around the world. The spacecraft was piloted through those. Command and control of most USAF satellites were run from the Blue Cube in Sunnyvale until that operation was moved to Falcon and Vandenberg AFBs.

Once the spacecraft was in the desired orbit and oriented, directional antennas were used by the payload to communicate with the payload user's control center. With directional antennas, smaller ground-side dishes could be used. The big steerable dishes were a scarce resource needed for multiple satellites, so tying them up for payload data like imagery was avoided.

Back in the early 1980s, one of the amateur radio satellites was incorrectly commanded to transmit on its own control receive frequency. This blocked the receiver from receiving further commands. To recover the satellite, the Stanford Dish [wikipedia.org] was used. That 46 meter steerable radio telescope had, left over from old USAF work, a 3MW transmitter. The combination of a huge dish and a high powered transmitter allowed focusing enough power on the satellite to get through to the receiver and tell the satellite to change its transmit frequency. It took two tries (the first time the codes sent were wrong) but on the second try it worked.

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/Sc8VAdp2KEA/story01.htm

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