Satgen591 Doppler 3 (HF SATS) by GM4IHJ (BID SGEN591) 2000-07-22 In Satgen589, a standard method for calculating the doppler shift in frequency received from a satellite ABOVE your station horizon, was described. It used calculated rate of change of range of satellite to station = velocity towards or away . To calculate doppler shift. Unfortunately this simple procedure does not work accurately when the satellite is far below your horizon. Indeed , on morning passes by RS13 near the south pole in February 2000. The signal received in Scotland was 300 to 500 Hz higher than predicted by the simple formula. Whereas on passes later on the afternoons of the same day, signals over a different portion of the southern ice cap sent back signals which were 200 to 400 Hz lower than expected from the formula. We are of course looking around a near spherical earth, and the aspect of the satellite at its furthest point removed from us on each successive orbit is highly variable. A fact which may be better appreciated if you picture the satellite going around the distant edge of a great circle map centred on your station. So do we need to set up a completely different system for calculating sub horizon doppler? Noting that looking for signal from satellites passing near your antipodeal point is hard enough already, given deep fades, bandwidth spread, southern auroral effects, and very low signal levels. Perhaps not. At IHJ this has not proved necessary if a simple set of rules are followed and, most important if the signal is displayed on an FFT display 1200 Hz wide. The rules are that - South polar satellite passes in the part of the eastern hemisphere seen from the local station longitude, always display a much lower frequency ( = doppler very negative compared with predicted ). Whilst satellites transitting the southern part of the hemisphere west of the local station longitude , always display a higher frequency than the predicted one ( = doppler positive when compared with that predicted ). Maximum differences vary between 100 and 500 Hz high or low, on the prediction. Only satellite tracks which cross the local station longitude when passing 83 south latitude lack this error. Their signal frequency shows little or no variation fronm the simple prediction. On a typical day in June 2000. Morning orbits heard in Scotland as they passed the south pole at about longitude 320W, were subject to doppler 200 Hz low on that predicted by the rate of range change method. By contrast, late afternoon orbits passing near the south pole around longitude 90W , produced a received signal 400 Hz higher than predicted. Please note however , that as RS13s orbit plane precesses slowly month by month. Although the rule east low , west high with respect to your local longitude still applies. The time of day at which passes go east or west of the south pole changes slowly. Such that December 99 afternoon passes gave high readings, whereas April 2000 afternoon passes gave lower values.