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

George Henry ka3hsw at att.net
Mon Nov 14 05:45:14 UTC 2016

If your receiver is equipped with AFC, like the Icom IC-910, it will correct 
for doppler without the need to do it in software...

George, KA3HSW

----- Original Message ----- 
From: "Scott" <scott23192 at gmail.com>
To: <amsat-bb at amsat.org>
Sent: Sunday, November 13, 2016 1:48 PM
Subject: [amsat-bb] Note ref. doppler w/ UHF iGate listener for ISS Packet

> Hello everyone.
> For anyone who has an interest in using the digipeater on the ISS now that
> it has switched to 70cm, I wanted to pass along some non-scientific
> findings.
> Of course it's great to see your packets digipeated from the ISS but part
> of the fun also is showing up on the ariss.net website which lists
> digipeated packets that were received by a ground station setup as an
> "iGate" to forward that traffic over the internet (resulting in a listing
> on the ariss.net website).
> It is my understanding that a couple of factors are contributing to a
> reduction in the number of digipeated packets successfully making it to 
> the
> ariss.net site.
> For one, there are less iGates able to receive on 70cm.  That issue can be
> resolved over time as people naturally add the ability to listen on 70cm
> with an iGate app.
> 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.
> I have never run an iGate and wasn't completely sure what was involved, 
> but
> with a number of SDR-equipped Raspberry Pi's around here I wanted to put
> one more set of ears out there for UHF packet.
> I'm sure there are any number of ways to setup a Pi as an iGate; I used 
> the
> instructions at:
> https://github.com/wb2osz/direwolf/blob/master/doc/Raspberry-Pi-SDR-IGate.pdf
> But how to setup a Pi is not why I wanted to post this.  Of course I
> initially setup my Pi-iGate to listen on the published frequency of
> 437.550.  However, I received very few packets 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.
> Again, this is not scientific, but I am now receiving more packets on the
> Pi-iGate than I was before.  Only time will tell if this continues.  Of
> course the better solution would be to program the Pi to adjust for 
> doppler
> throughout the entire pass and there IS code online to provide that
> compensation.  However, I have not been able to get that working.
> If anyone is interested, here is the script that I run to activate the
> iGate listener on my Pi... it runs in the background so that I can log off
> from the Pi and just leave it running stand-alone.
> -----------------------------------------------------
> #!/bin/bash
> nohup rtl_fm -f 437.555M - | direwolf -l /home/pi/iss-uhf-log/ -c
> /home/pi/sdr.conf -r 24000 -D 1 - > /dev/null 2>&1 &
> #
> -----------------------------------------------------
> Hope someone finds this informative and at least one of you might be
> prompted to add a 70cm iGate to your shack!
> 73,
> -Scott,  K4KDR
> Montpelier, VA  USA
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