Satgen 203   Doppler Pt3   by GM4IHJ                          12 Feb 93
The BID of this msg is SGEN203   Use that BID if you retransmit this msg

In satgen 201 , two readings of station to satellite slant range, taken 2
minutes apart were used to calculate satellite velocity relative to your
station.The following BASIC subroutine converts slant range to doppler 
shift :-
9000 IF A$=T$ THEN PRINT "NA" : LET SR5=R5:GOTO 9040
9010 LET RSHIFT = (R5-SR5)/120 
9020 LET DSHIFT= (-(RSHIFT*DCPA/300000) 
9030 IF A$<>T$ THEN PRINT (DSHIFT + DCPA) :LET SR5=R5
9040 RETURN
9500 REM 9000 allows for the fact that you have made only one slant range
calculation at start time. So it prints "Not applicable " in Doppler
column and sets original Slant range to R5 ( R5 is slant range variable in
most Amsat sware).You then leave the subroutine. Next time say A$+2mins
you can at line 9010 get sat relative velocity in Kms/sec. Then at 9020
you calculate doppler shift, and at 9030 you calculate shift + Tx
frequency ( IHJ puts DCPA Tx frequency as a statement in each set of
keplerian elements) and you reset SR5 ready for the next time around.
Remember , this calculates Doppler at time halfway between your computer
time steps. The Dopplered signal frequency is reported in MHz 

So what sort of changes can we expect on say a near overhead RS10 pass ?
utc 1323    1325    1327    1329    1331    1333    1335    1337    1339
29+ .35952  .35951  .3595  .35945   .35927  .35885  .35847  .35834  .3583
Dop  620 Hz  610     600    550      370     -50     -430    -560    -600
Rate      5Hz/min  5     25       90      210     190     65       20

This data shows a typical near overhead pass. Almost no change of
frequency in the first 4 minutes, but then a sharp drop in frequency with
a high doppler rate of 210Hz/min mean in the centre of the pass as the sat
goes overhead, then a gradual slow down as the doppler shift becomes
negative until at the end there is very little further shift in frequency.
Please note these results do not produce a symetrical curve. Anyone who
produces doppler curves in which the first half is mirror imaged in the
second half  IS CHEATING. The Asymetry is caused by earth rotation.
Equally important, no orbit has perfect circularity. Indeed when we get to
talking about elliptical orbiters like Oscar 13 , the results will be
quite different. But for now, you should instantly recognise a near pass
by a circular orbiter , simply by analysing its doppler. 
Next,  consider RS10 making a pass which hardly comes above our station
horizon we get a much flatter result .
utc 1854    1856    1858    1900    1902
29+ .35914  .35903  .35891  .35878  .35867
Dop  240 Hz  130     10     -120     -230
Rate     55hz/min  60    65       55
Here the doppler shift overall is far smaller but the doppler rate is
nearly steady. Using this "Far orbit " = steady rate but moderate shift ;
" Near Orbit" = slow start but very high rate in centre of pass, plus
large overall shift. An observer hearing only one orbit should be able to
judge quite effectively just how far away the satellite was when it passed
his station. 73 de GM4IHJ @ GB7SAN