Hi Mark,
Almost all the linear sats have some offset due to variations in frequency of the internal oscillators etc. In the case of AO-07 the offset is temperature related and varies slowly depending on the amount of sunlight etc. (AO-07 at the moment is about 4.5kHz) The technique I use to determine the offset at any time is to set my receiver to the centre of the satellite downlink passband (+/- Doppler) and then adjust my transmitter frequency to hear myself correctly on the downlink. The difference between my actual uplink (+/- Doppler) and the nominal centre of the uplink passband is the offset. I save this value and use it built into my Doppler correction until any change warrants a re-calculation.
Alan ZL2BX
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Mark Lunday, WD4ELG Sent: Sunday, 16 November 2008 04:58 To: [email protected] Subject: [amsat-bb] Some doppler observations/questions
I am using HRD with Sat tracker module (thanks, Simon!)
I notice that for the non-FM birds I am working (AO7, FO29, VO52) the Doppler is never quite exact. Or I guess I should say there seems to be a constant *offset* that needs to be included in the Doppler calc. Tonight on AO7 mode B pass, my actual downlink frequency seemed to be about 4 kHz lower than what it should be according to the Doppler. As the bird passed by, I followed the "one true rule" and only adjusted the higher freq (uplink in this case). It was consistently about 4 kHz lower.
On FO29 pass, there seemed to be about a 2 kHz offset. When the bird passed overhead, the Doppler "flipped" quickly and I had to find my signal again.
How can I consistently determine this offset so I can include it in with the Doppler correction? Or is it on a case-by-case basis?
PS Thanks, Frank, K9CIS, for the nice CW rag chew on FO29 tonight.
Mark Lunday WD4ELG [email protected] http://wd4elg.net
_______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb