[aprssig] Satellite Ranging..

[Jim Lux]

>Date: Mon, 10 Oct 2005 09:35:54 +0100
>From: "Dave Baxter" <dave at emv.co.uk>
>Subject: RE: [aprssig] Satellite ranging
>
>I read somewhere that it has been done, but with a voice repeater on the
>ground.  However, as I seem to recall, it's not just the path delay that
>you see, it's also the delay in all the audio filtering in the box
>(bird) and your own radio's too.  I think the article I read, they were
>trying to confirm measurements from some other experiment, and had to
>calculate the path delay, to determine the "real" group delay through
>the repeater, or was it the other way round, I forget...
>
>No doubt you could do that with a satelite too, but you'd need to know
>in advance the delay through the bird's circuits, and your radio's (at
>least that is easy to do) come to that!.  If that would be significant
>over the extended distance/path delay to/from a satelite that is...
>Try it...
>
>Cheers...
>
>Dave G0WBX
>
> > Howdy,
> >
> > Has anyone done ranging experients, using a two channel CRO
> > and sending a signal via a satelite and working out the slant
> > range from delay ?
> > Andrew Rich - VK4TEC

As Dave points out, you need to know the delay through the 
system.  However, there are some tricks you can use, making use of the fact 
that you KNOW the satellite is in an elliptical orbit (or, at least, so 
close you can't tell the difference easily).

You need to combine the doppler information AND the change in the round 
trip phase shift/time delay, and from that you can extract the (presumably) 
fixed delay through the bird (and your electronics).

The doppler alone will tell you a lot about the orbital position. For 
instance, the inflection point, where the second derivative of the doppler 
goes through zero, tells you the point of closest approach, and the rest of 
the curve essentially can define what the orbit must be.  Once you've 
calculated the geometry, there will be an fixed offset, and that's the 
delay through the bird and your equipment.


We used to do this to measure clock rates within the Seawinds radar on the 
QuikScat satellite (http://windsby looking at the frequency of the radar 
pulses on the ground. There it was complicated by the fact that the radar 
was putting its own doppler correction on the pulses as it scans (when the 
beam is pointed forward it shifts the transmitter frequency down, so that 
the returns always come back at the same frequency).