Satgen 522 Tracking Sats Heard 1. by GM4IHJ (BID SGEN522) 1999-03-27 As explained in Satgen521. With any of several of small fixed homebrew antennas , carefully aligned on useful azimuths. It becomes possible to find most satellites, even the exotic commercials. Which latter can provide essential cost free training practice for working the L and S band amateur radio satellites ,we hopefully expect to orbit above us before too long. It can also give early warning of the problems radio amateurs will face when they try to use the higher frequency bands. Watching the commercials struggle can be very illuminating. But hearing a satellite speed through your antenna beam does not at first sight tell you much. So, must you get antenna rotation to find out where it is going and, hopefully, begin to recognise its orbit? Answer No. Your guide to its motion is already there as it flashes through your beam, giving information by way of signal doppler shift. There by allowing calculation of highly diagnostic , rate of change of frequency = Doppler rate ( Ddot for short ). You can get Ddot simply by repeatedly noting the frequency, as you tuning the signal to zero beat ( lowest audio tuning point at which your ears begin to lose it). Do this at regular intervals of say 10, 20 or 30 seconds, and you get a time sequence of gradually changing Ddot, in Hz/min for the HF or VHF sats, but much faster in kHz/min for the mode L and mode S sats. All Low Earth Orbit (LEO) satellites have distinct Ddot characteristics depending on signal frequency, orbit height , how close the satellite is passing your station , and very important, which part of the orbit pass ( begining, mid passage or end ) you hear. In almost all LEO cases Ddot is negative, dropping in frequency , but the rate at which this frequency shift occurs is highly diagnostic . A pass coming near overhead ( even if your antenna does not cover that part of the orbit) , will start with a very small negative Ddot. Then if you hear it as it gets closer Ddot begins to increase very quickly, being maximum negative rate as it passes your station. Thereafter as is goes towards your horizon the Ddot quickly reduces so that as it nears your horizon Ddot gets very small indeed. By contrast an orbit which is not going to come overhead produces a much steadier decline in frequency . Ddot start with a greater negative rate than on an overhead pass but it never increases steeply, it continues steadily, peaking slightly as the satellite passes closest approach to your station , before continuing on with a steady Ddot which unlike the overhead pass never drops anywhere near zero rate even at your horizon. Though you may hear only 3 minutes of the pass as the satellite crosses your antenna beam. Ddot should tell you whether it is coming towards you , with a gradual small steady Ddot increase negatively for a distant pass, or very slow start to very big increase negatively for close approach by the satellite . Or, going away from you with a gradual slow reduction in doppler rate for a distant pass, or high negative Ddot quickly slowing to almost nothing for a near pass. Commercial sat operators do not always oblige . They often switch the sats on and off in an irregular fashion particularly when testing. But even this can be useful as explained in next Satgen. US readers queries - Iridium uses digital processing of what looks like a very wide band ,frequency and time division multiplexed signal.