[amsat-bb] Re: How you do DOPPLER correction in Linear Transponders?
Alan P. Biddle
APBIDDLE at UNITED.NET
Fri Jul 3 03:39:30 PDT 2009
FM reception is much less sensitive to slight frequency errors than SSB.
Just because of the way it works, you can be a 2-3 KHz off frequency, and it
will sound exactly the same.
On the other hand, if you have a good ear, you can hear errors of 20 Hz, and
certainly 50-100 Hz with SSB. Now, consider that there are slight frequency
shifts in the satellite transponders due to temperature changes, aging, etc.
Also, the Keplerians are good approximations, but only approximations. As a
result, you will need to make slight adjustments occasionally. I tune off
to the end of the passband, give a quick test transmission while I talk
myself on to the correct frequency, and that correction is good for the
pass, and probably all the passes that day. SATPC32 and Ham Radio Deluxe
both have ways to do this. The correction day to day is usually less than
100 Hz, and over a long time a few hundred Hz. Because of its lack of power
regulation, AO-7 is particularly bad about this. After you have done it a
couple of times, it only takes a few seconds, and makes operating much more
pleasant for both you and the stations you talk with.
More information about the AMSAT-BB