Satgen589 Doppler 1 (HF Sats) by GM4IHJ (BID SGEN589) 2000-07-08 In theory, the doppler shift of a satellite signal is proportional to the satellites velocity. When coming towards you , the signal frequency received is higher than that transmitted by the satellite. When going away from you, the received signal frequency is lower than that transmitted by the satellite. With the received frequency being exactly the same as the transmitted frequency , only , at the exact moment that the satellite passes closest to your station. Maximum doppler shifts occur when the satellite is coming directly towards or going directly away from your station. At other times passes to one side or the other produce a range of doppler shift considerably less than the direct approach/withdrawal maximum. Rs13 orbits the earth at a velocity of about 7.5 kms/sec. Which figure placed in the rough equation for calculating doppler shift Eq1 Doppler shift = + or - velocity/speed of light x frequency Hz which gives us 7500 x 29,485000/300,000,000 = + or - 727 Hz , that is a total shift of 1454 Hz. But these maxima are not achieved in practise , because the satellite following a near circular track around its orbit, is never actually coming straight at you or, going straight away from you. Which reduces the biggest shift you can expect down to about 1230 Hz , when going from one horizon to the other on an vertically overhead pass. As an aid to following the dopplering received signal, some tracking software calculates minute to minute doppler shift . Though the accuracy of the process varies from one program to another. In theory you calculate range to the satellite at minute x - 0.5, and again, at minute x + 0.5 , so that you get a reasonably exact rate of change of range = velocity , at minute x. This calculated velocity of the satellite with respect to your station is then inserted in Eq1 above , to get doppler shift, which when added to ,or subtracted from the satellites transmitter frequency, gives you a rough guide to the frequency you can expect to tune to for zero beat on your receiver. Though be warned . A careful study of received frequency with time , on say an FFTDSP display will reveal jumps in signal frequency as the satellites downlink signal path intercepts ionospheric anomalies. At the horizon funny things can happen, both when RS is rising up to your horizon, and when it is dropping below your horizon. All of which , sometimes, but not always results in a signal as acquisition approaches which is about 80 Hz lower than expected but which rapidly shifts up to the anticipated frequency at the horizon. A situation which produces a scimitar like curve on the signal trace of a FFT display climbing up the 80 Hz in about 20 seconds then settling quickly to a relatively smooth trace thereafter. Then later as the satellite crosses below the horizon at normal loss of satellite there is often a scimitar curve on the FFT trace as the signal which has been slowly dopplering down, climbs up about 80 Hz high before fading. Therafter at elevations below the horizon where propagation by the ionosphere gets the signal to you. The doppler picture gets very complicated.