Satgen 642   Chasing AO40 Doppler by GM4IHJ  (BID SGEN642) 2001-07-14

Apologies to all who hate doppler shift discussions. This return to
the subject is prompted by several queries from readers puzzled by the
frequency changes exhibited by AO40s tlm beacon centred 2401.326 MHz
approx ,+ or - the accuracy of your receiver tuning. Most readers are
clear that AO40s S band signal will not follow the familiar " always
dropping in frequency", as the signal is received from a low earth
circular orbit satellite in range of your station. With a LEO sat the
signal is received at a frequency well above the satellites actual
transmitting frequency as the sat approaches your station. Descending
in frequency to pass through the value of the LEO transmit frequency
as the satellite passes your station. Then continuing to descend in
received frequency until it is lost at your station horizon.

Just after perigee AO40 is moving quickly away from you. So you get a
received signal well below the AO40 Tx frequency. But because the
satellite moves more and more slowly as it climbs towards apogee, this
has the effect of making the doppler less negative. Which is the same
as saying that the frequency you receive is climbing . Albeit not yet
up to the AO40 Tx frequency. Thus for this part of its orbit whilst it
is inrange of your station, you can expect the AO40 received frequency
to move slowly higher. Starting  lower (than the AO40 TX)  frequency
sometime after perigee ,the signal dopplers through the the same
frequency as the AO40 Tx roughly about the time the satellite goes
through its apogee high point. Noting that at apogee the satellite is
hardly moving at all with respect to your station.

After apogee the satellite starts to come towards you , which if this
were a LEO sat would suggest decreasing doppler again going negative.
That this does not happen is the result of the fact that the satellite
is speeding up as it comes towards you, ie its velocity relative to
you is increasing so doppler begins to lift the received signal
frequency well above that recorded at orbit apogee high point. But as
the satellite gets close to earth and perigee orbit low point , there
is a slowing down of the doppler rise to a positive high, suddenly
replaced by a very abrupt speedy drop in frequency received from
perhaps 2401.366 to 2401.295 MHz. Passing through perigee somewhere
near but rarely at the mid point in this decline.
Table 1 reports a set of doppler rate of change results taken at 56N.
Mean anomaly region    Doprate Negative going  Doprate positive going 
233 to 249                                         133Hz/minute
250 to 001                 1Khz/minute
  4 to 11                  300Hz/minute
 13 to 28                                          133Hz/minute
 30 to 42                                           88Hz/minute
 42 to 78     NOTE..These figures vary from          66Hz/minute
 78 to 85     orbit to orbit being dependent         48Hz/minute
 89 to 158    on both satellite velocity             33Hz/minute
166 to 204    towards or away , and, your            48Hz/minute
205 to 212    station location with respect          66Hz/minute
214 to 220    to the orbit plane, which varies       99Hz/minute
223 to 230    from one orbit to the next.           166Hz/minute
232 to 239    73 de GM4IHJ@GB7SAN.#78.GBR.EU        133Hz/minute