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Re: Doppler correct and frequencies - from a newbie!



I hope that the osdcillator was designed better than that!!  The
oscillator should show minimal change with temperature changes.  As the
spacecraft goes from AOS to CPA (approaching) the frequenct will be
higher than the actual transmit freq. As the spacecraft goes from CPA to
LOS (departing) the frequency will be lower than the transmitted
frequency.  This is related in the formular:
Doppler (in Hz) = [(2) x (target radial velocity in miles/hour) divided
by (speed of propagation)] x (transmitted frequency in Hz).  This is the
reason for the sift in frequency (doppler shift).  You will note the
same effect when you hear a train whistle as it approaches you the
departs.

73
Glenn
WB4UIV


Chris Jackson wrote:
> 
> For KO-23 this problem is especially bad since the orbit is non sun sync,
> but this also applies for other digital satellites.
> 
> Basically, as temperature changes, the downlink (and uplink) frequencies
> will change.  With an FM satellite, both are completely separate - it's not
> like an SSB transponder.
> 
> So, adjust your tracking software so that the frequency tracks closely to
> the descriminator output.  However, make sure that your keps and time are
> correct as these will also cause the same problem.
> 
> Chris G7UPN / ZL2TPO
> 
> -----Original Message-----
> From: Alan K. Adamson [mailto:ne1h@ne1h.dyndns.org]
> Sent: 01 March 2000 19:23
> To: amsat-bb@AMSAT.Org
> Cc: Mike73@aol.com
> Subject: [amsat-bb] Doppler correct and frequencies - from a newbie!
> 
> All,
> 
> I'm kinda curious.  I've been tracking and copying the pacsats and I've
> noticed an oddity.  I'd just as soon not learn a bad habit, so I figured I'd
> ask.
> 
> With automatic control and doppler correction, I can't seem to center the
> discriminator to keep the satellite decoding on the EVM.  How do others do
> it?  It goes like this.
> 
> Take KO-23, the uplink is suppose to be 145.900 and the downlink is 435.175.
> So I setup the software with those values and when the sat comes over the
> horizon, I start to pick it up.  But the discriminator indication on the 847
> is way off to one side.  I've found that I have to crank in sometimes as
> much as 4K to get it centered and the modem decoding.
> 
> So, my real question is as follows....
> 
> Do you ....
> 
> a) leave the uplink as calculated for doppler based upon the base freqs and
> center the discriminator (for best decode) on the downlink?  (in other
> words, just adjust the downlink)
> 
> b) adjust both the uplink and the downlink to match the offset to center the
> discriminator
> 
> I presume that both the TX and RX at the sat could vary different directions
> as time goes by, perhaps even away from one another.  You know where the TX
> is because you can center for that, but what is the preferred method for the
> RX.
> 
> Do you also setup your software to counteract for this for each sat, so you
> don't have to do it manually?
> 
> I don't want to learn any bad habits, so I figured I'd ask.  Thanks in
> advance,
> 
> Alan
> 
> ----
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-- 
Glenn Little
Engineer
HARK Systems, Inc.
Voice (843) 764-1560
Fax   (843) 764-3692
----
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