##########
APRS IGate
##########

For fun, I run a rx-only APRS iGate at home.  My setup consists of:

* A Tram 1154 5/8-wave amateur radio antenna

* A Nooelec RTL-SDR receiver dongle (and UHF-MCX adapter cable)

* A `Tinkerboard
  <https://www.asus.com/us/Single-Board-Computer/Tinker-Board/>`_ running
  `Armbian <https://www.armbian.com/>`_ and `Direwolf
  <https://github.com/wb2osz/direwolf>`_.

All told, it's about $100 in parts (including case and SD card and all that),
which doesn't seem so bad.  The ``rtl_fm`` and ``direwolf`` processes take
about 3-5% of the CPU power of the board, or about 10-20% of one core; a more
modest CPU could certainly function, but my brief attempt at using a 600MHz
Kirkwood found that it could not keep up with the audio stream.

I don't have a great view of the sky in my flat, but I hear the local environs
reasonably well, at least, as well as a local WIDE digi.

Ultimately, though, I hope to have the RTL-SDR able to listen to several
channels as well as pushing its data into direwolf for igate operation.  I have
not figured out how to do that yet, sadly.

RTL-SDR Calibration
###################

I made use of https://github.com/viraptor/fm_tune and
https://github.com/steve-m/kalibrate-rtl to measure the crystal error in my
RTL-SDR (the ``-p`` parameter to ``rtl_fm``, below).  These tools are
delightfully straightforward.

Direwolf Configuration
######################

Since this is an RX-only IGATE and does not use a traditional radio+soundcard setup,
most features are turned off.  Some parameters have been ``[omitted]``::

  # No audio device
  ADEVICE null null
  CHANNEL 0

  IGSERVER euro.aprs2.net
  IGLOGIN KD8GIN [omitted]
  
  # no incoming packets, ideally
  IGFILTER

  # it is not possible to explicitly state that we cannot tx, but the default
  # configuration is to load ->tx_chan with -1, so by omitting igtxvia, we
  # will indicate our rx-only nature
  # IGTXVIA -1

  # Every 120 minutes, send a position report to the IGate connection
  # R& is RX-only IGATE
  PBEACON EVERY=120 SENDTO=IG LAT=[omitted] LONG=[omitted] SYMBOL=R& COMPRESS=1 COMMENT=[omitted]

runit scripts
#############

Direwolf is overseen by ``runit``.  The main ``run`` script is ::

  #!/bin/bash
  
  mkdir -p /var/log/direwolf
  chown direwolf:direwolf /var/log/direwolf
  
  abort() { kill -s TERM -$(jobs -p) ; }
  
  set -m
  trap abort EXIT
  
  # Use 144.8M for UK, 144.38M for USA
  chpst -u direwolf:direwolf:plugdev rtl_fm -o 4 -p 11.060 -f 144.8M |
    chpst -u direwolf:direwolf direwolf -c /etc/direwolf/sdr-igate.conf -r 24000 -D 1 -