capturing and decoding APRS packets with software defined radio

I have a commercial FCC license (general radio operators license) as a requirement for my  current position.  I also have a general class amateur  radio license, KD5UUU, but I’ve never had any equipment.  That is, until I bought a $20 rtl-sdr radio dongle.

It’s pretty amazing what you can do with it.  Recently, I’ve been  capturing and decoding automatic packet reporting system APRS packets.  AX.25 frames are broadcast on 144.39MHz that generally are used to track vehicles by sending their gps coordinates.

There are several high-quality graphical SDR applications out there, my favorite being CubicSDR.

cubicsdr aprs monitoring
CubicSDR monitoring 144.39MHz for APRS packets using RTL-SDR on a VHF Sinclair antenna mounted at 210′

CubicSDR allows for recording live traffic.  I set it up to record a wav in mono at 44.1KHz, and to only record if a signal is breaking the squelch threshold that I set.

Here is a recording of the packets I captured.   Pretty slow day.  Recording for around 10 minutes only captured 8 discernible frames.  I received transmissions from Jackson, TN all the way to Harding University in Searcy, AR (I was located in Wynne, AR).

So what’s in there?  You need to install direwolf to see.  Once you have that installed, go to the directory where the audio is saved and use atest.


atest recording.wav

Here are the decoded frames from the recording above.

You can see that most frames contain gps info.  APRS can also be used for short text messages.  Frame #8 contains one such example ‘Wht RAM 4X4 pickup’.

You can see live and historical  APRS traffic at arps.fi on a map.  It’s pretty cool to see amateur operators travel across the screen.

An example of an amateur operator’s progress along a stretch of I-40 near Memphis, TN from https://aprs.fi

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