[amsat-bb] Re: Some doppler observations/questions

[Alan Cresswell]

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: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Mark Lunday, WD4ELG
Sent: Sunday, 16 November 2008 04:58
To: amsat-bb at amsat.org
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
wd4elg at arrl.net
http://wd4elg.net

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