AOS, TCA and LOS
AOS (acquisition of signal) is the time the satellite rises above your horizon. LOS (loss of signal) is when it sets again. In between there is the point of closest approach, where elevation is highest. A pass of a satellite in low Earth orbit lasts at most about 10 to 15 minutes; a pass that barely clears the horizon can last two minutes.
Elevation: why higher is better
The maximum elevation is the highest angle the satellite reaches above the horizon, from 0° (grazing) to 90° (straight overhead). High passes are longer, the satellite is closer, and the signal crosses less air and fewer obstacles. For radio, a maximum elevation above 20° is a good target for a first contact; below 10° trees and houses often block the path.
Azimuth: where to point
Azimuth is the compass direction, from 0° (north) clockwise to 360°. A pass table shows the azimuth at rise and at set, for example NNW to SE. On the pass predictor the sky plot draws the same thing as a curve: the centre is straight overhead, the outer ring is the horizon, north is at the top and east on the right, as on a satellite tracking program.
Choosing a pass
Prefer passes with a high maximum elevation, in the hours when you can be outside with a clear view, and for amateur radio with the satellite’s transponder working. Passes of the same satellite repeat in groups: it passes about every 100 minutes, but because the Earth rotates under the orbit you see it only on some of them, usually in two or three groups a day, once in the morning and once in the evening.
