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RE: Doppler Correction



In the Orbits II and III programs I updated the doppler shift such that the frequency 
  would be set to what it actually would be a half look interval later.  In other 
words, if you doppler correct to what the correct frequency is at any given point in 
time, it is correct for just that point in time and will quickly drift off.  If you 
correct it to what it will be in a short time into the future, the doppler shift will 
cause the signal to drift into the correct frequency and then out.  This will give 
you a bit more time when the signal is close to being "on frequency."  I chose a 
period in the future equal to half the interval between updates.



Date: Mon, 10 Nov 2003 22:40:18 -0800
From: Paul Williamson <kb5mu@AMSAT.Org>
Subject: Re: [amsat-bb] Doppler correction?

At 10:06 PM -0500 11/10/03, Anthony Monteiro wrote:

 >>InstantTune updates the frequency every second.
 >>There is no simple way to change this since it
 >>is driven by InstantTrack/OrbitDRV which updates
 >>the satellite velocity every second.


Nominally every 1.7576 seconds (32 ticks of the PC's 18.2 Hz clock).

It should not be difficult (for me) to provide a patch to OrbitDRV
that makes this happen more frequently, if that's useful. It might be
better architecturally if the tuning program just interpolated,
though.

The original value was chosen to be slow enough that OrbitDRV's
calculations didn't use up too much of the CPU even on the slowest
8088-class PC. On anything approaching a modern computer, that is no
longer a problem. Now the limitation would be how fast InstantTune
can command the radio.

73  -Paul
kb5mu@amsat.org


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