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

[Robert Bruninga]

Good idea.  Another thing you can do is anticipate where all the other
igates are and choose a fixed doppler for the region you can hear better
than other IGates.   Though without knowing what the other gates are
doing, it is a multivariable problem...  Ill leave that to the
tedious...bob

-----Original Message-----
From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of Scott
Sent: Sunday, November 13, 2016 2:48 PM
To: amsat-bb at amsat.org
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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