Satgen336  Satellite Software Pt3  by GM4IHJ                   2 Sept 95

DOPPLER SHIFT. A satellite approaching your station appears to send you a
radio signal slightly above its nominated transmitter frequency. When
closest to your station the received frequency has decreased to be the
same as the satellite transmitter frequency. Then as the satellite goes
away from your station the received frequency continues to decrease ,
going below the satellite transmitter nominated frequency.

The situation is even more complex if you transmit up to a satellite and
listen for your own signal transponded back to you. If the satellite is
approaching, your signal arrives at the satellite receiver at a higher
frequency than you transmitted, then after, additive frequency conversion
in the sat transponder the signal is returned to you and gets a further
doppler shift still higher as it comes to your receiver.By contrast the
subtractive conversion used in some sats can produce Dopup - Dopdown =
reduced 2 way Doppler. Software must be able to allow for both variations

This situation is manageable at MODE A 145/29 where 2 way doppler can
shift your signal + or - 3Khz. But at mode B 435/145 a two way signal can
be shifted approx + or - 12 Khz, which is difficult to handle without
software assistance.

In its simplest form this software assistance can consist of a print out
of predicted doppler shift, so that you can make manual tuning adjustments
as the doppler changes. There are two methods in general use in amateur
radio software calculation of doppler :-
a. Derived from change of satellite slant range with time.
b. Derived from satellite velocity.
The latter is far more accurate producing much smoother results, but not
all amateur radio software uses this better method.

But any doppler tracking calculation is only as good as the information
you give the computer. Poor out of date Keplerian elements often cause
doppler calculation errors. Less obvious as sources of error are :-
a. the accuracy of your receiver and transmitter tuning
b. the satellite frequency parameters whereby the offset frequency used
   to transpond your received signal to its transmiter signal, may not be
   the same in the cold of space, as it was on the bench, when the
   satellite was tuned before launch.

So anyone using computer doppler calculations must keep several things in
mind. You must, if you want accuracy, find out by trials from your own
station , exactly what frequency offset variation from pre launch figures
each satellite gives you versus the likely small variations in your
station equipment. The higher the satellite operating frequency the bigger
your likely offset correction will be . Tracking NOAA weather sats on 1707
MHz nominal , requires offset on reception alone of 16 Khz on one sat and
8Khz on the another. To get accurate two way frequency tracking results on
Oscar 13 Mode S 435/2401 MHz , needs 22 Khz offset from the printed
frequencies ( with the 2401/145 MHz rx converter the biggest culprit).
Doppler predictions are only a rough guide unless you take practical steps
to fit them to the satellite and your station conditions. Software will
not do this automatically.