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

Scott scott23192 at gmail.com
Sun Nov 13 19:48:23 UTC 2016


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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