[amsat-bb] Re: SDX transponder timing delay measurements?

Robert McGwier rwmcgwier at comcast.net
Tue Sep 12 11:31:17 PDT 2006

Howard Long wrote:
> Hi Ken
>> As a design goal, it would be helpful to the 
>> OD process if the delay could be made consistent 
>> within about +/-5 micro-sec -- i.e., 1500 meters 
>> (smaller is better).  It is also important to
>> quantify the standard deviation to that bias is 
>> since that is also factored into the OD process.
> I may be wrong, but I can't think of a reason why it wouldn't be well under
> this figure. I was thinking more in terms of 100's if not 10's of
> nanoseconds.
> The SDX (a) takes a batch of samples, (b) does its processing, then (c)
> outputs the processed batch of samples.
> Each of the three parts takes the same amount of time and all three are
> executed in parallel like a production line using DMA for (a) and (c), and
> the DSP CPU for (b). This time is dictated by the CPU's crystal oscillator.
> I am guessing any variation would be predominantly from the analog circuitry
> (R's and C's changing value with temperature?), but Bob may have other
> suggestions I hadn't thought of.

Right on the money.  We can learn the variation in these parameters in a 
joint process with determination of the pseudo-ranges.  It is my opinion 
that doing this joint estimation will yield very good results.  I would 
not be surprised in the least if James and/or Stacey had not already 
done this code.  Stacey has worked on a project he calls Ranger (for 
obvious reasons) and I have not yet studied all of the details but I 
know ranging experiments have been done with it.

> 73, Howard G6LVB

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