[amsat-bb] Note ref. doppler w/ UHF iGate listener for ISS Packet

Dani EA4GPZ daniel at destevez.net
Mon Nov 14 15:26:25 UTC 2016


El 13/11/16 a las 20:48, Scott escribió:

> But another issue that several folks have pointed out is that with the more
> substantial doppler shift at 70cm, a receiver set to a single frequency
> will miss much of the traffic sent back down from the ISS.
>
> [...]
> 
> It crossed my mind that, at the point of closest approach, the doppler
> frequency is changing quite rapidly.  So, I thought that I might improve my
> count of iGated packets if I listened on a frequency that would be viable
> for a longer period of time.  Using (WD9EWK) Patrick's doppler adjustment
> guide as a reference, I chose to change my listener to 437.555.

Hi Scott and all,

Another thing that you can do is to correct for Doppler in software
(obvious).

If you're using an FM radio, this may me more complicated because you
need to set up CAT, but if you're using an SDR, it's just a matter of
using the appropriate software.

Since you're asking this specifically for iGates, we can assume that you
have an internet connection on your receiver to fetch TLEs automatically.

Then you can use whatever Doppler correction tool you like to be in tune
during all the ISS pass. For instance, this command line tool:

https://github.com/cubehub/doppler

should be useful, especially for embedded systems such as a Raspberry Pi.

The disadvantage of doing Doppler correction is that you can only do it
for one satellite. In 145.825 there is the problem that there are
several satellites you want to receive, several of them may be in view
simultaneously, and they will have different Doppler offsets. Still, you
could setup the system so that if there's only one satellite in view you
correct for that satellite and if there are several you either
prioritize and correct for one of them, take an average of the
corrections, or just do no correction. This is more complicated, of course.

However, on 437.550 you only have the ISS, so it makes much sense to
track Doppler correction only for the ISS, and this is much easier than
handling several satellites.

So, nothing new here. What I mean is that automatical Doppler correction
for the ISS using an RasberryPi and RTL-SDR is not that hard (and much
better solution than listening on a fixed frequency). Come on guys, we
can do it!


73,

Dani EA4GPZ.



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