Satgen 632  AO40 S Band Doppler by GM4IHJ   (BID SGEN632)  2001-05-05

As the stabilization and attitude of AO40 improves, tracking of the S
Band Middle MB beacon transmitting on 2401.323 MHz, ( 2401.327 via
IHJ coverter), is now a practical proposition, over most of the in
range portions of the orbit. Irrespective of where the satellite is
in its orbit. So, as UHF up to S Band down , and , L Band up to S
Band down, transponding look likely to be the best available
communication channels. A familiarisation program has been initiated
to get the ancient operator up to peak readiness.

A simple Table 1, of satellite mean anomally versus doppler, on a
typical orbit , looks as follows:- satellite transmission 2401.327

           Received    Ma      Received   Ma       Received   Ma
           2401.349   249      2401.319   81       2401.338   176
 perigee   2401.306   0        2401.322   95       2401.341   190
           2401.298   14       2401.325   108      2401.344   203
           2401.306   27       2401.327   122      2401.347   217
           2401.310   41       2401.330   136      2401.351   230
           2401.314   54       2401.332   149      2401.355   244
           2401.317   68       2401.335   163      2401.329 perigee

Actual readings around perigee will differ from station to station,
depending on your location with respect to the satellite, and
readings from one perigee to the next can differ markedly as your
station locations separation from the orbit perigee sub point varies
from one orbit to the next.

Several important points to note about this doppler table are :-
a. There is a very smooth upward climb in received frequency from
about ma 54 to ma 244.
b. Then suddenly the situation reverses as received frequency drops
very quickly from ma 244 through perigee ma 0 and onward to ma 15.
c. After ma 15 the received signal frequency begins to increase again
and soon settles down to the steady climb in frequency noted in a.
above.

The exact received frequency you get will depend on the accuracy of
your receiver. Some old crystal controlled converters (2400 to 144)
can be inexact by 5 to 10 Khz even after being allowed to warm up. So
do not be surprised if your tuning of the beacon differs by a fixed
amount from the above table. This can be easily allowed for.
When receiving SSB or CW communications however, please allow for the
effect of subtractive mixing in the satellites transponder. Where for
example the UHF uplink is subtracted from the satellite receiver
local oscillator to reduce compounding the doppler shift which would
result if you added uplink doppler to downlink doppler. When
communicating via AO40 your centre frequency will be offset from that
of the beacon centre frequency . So this will move your downlink by a
fixed offset from table 1. While the use of subtractive mixing will
reduce the amount of received signal doppler shift about 15% on UHF
up S down. Eg table 1 doppler is about 25 KHz down at ma 14 . With
subtractive mixing this would be about 21 KHz down  when using SSB/CW