Bits of SDR

The hackspace is a bit quiet at the moment, we’re waiting for some room to set up our laser cutter.  NSME are kindly giving us some space and have even provided some fire extinguishers, optimistic that they are.  Hopefully by the beginning of April we will have space for it.

In the meantime, I did some tooling around with the RTL SDR dongle, the software support for which has come on marvellously, and after some fun following some guides on the ‘net, we were able to pick up the ADS-B location broadcasts of airplanes overhead – albeit in a very small range, due to the antenna in use that isn’t designed for 1.09GHz.
For the curious, I originally started with the rtl_adsb library that comes with rtlsdr these days (also worth investigating the rtl_tcp server if you want to leave your dongle in an RPi…).  This puts out the decoded broadcasts to an almost human-readable level.  I then installed the VirtualRadar software (which is cross platform) . . . which didn’t decode the messages at all (linked together via netcat).  It confirmed it received messages, and that they were not badly formatted, but couldn’t decode them either.
I then tried using this guide and used the gr-air-modes application instead.  The uhd_modes.py script it recommends has been replaced with rx_modes instead but the syntax is the same.  This wasn’t too painful for me as I already had gnuradio installed.  For those that don’t though, the latest in Ubuntu 12.10 is compatible with the osmocom extra sources.

SDR flowchart: POCSAG

Not sure any of these are entirely working, but the local area doesn’t appear to use POCSAG but has maybe gone onto FLEX as the frequency bands look a bit busy.

Simplest attempt is using a modified version of osmo-pocsag.  It writes out baseband to a file.  Create a fifo, and direct the the cmdline app to its output.
A more complex version is based on this (by bafe), and has some good waterfall pictures to give a better indication of what’s going on.  Confusingly it’s called gr-pocsag, and thus confusable with the POCSAG decoding GRC block by patchvonbraun here, which needs compiling and installing.
I’ll update this once I’ve actually either confirmed that what I’m seeing is not POCSAG or which/either/both/none works.

Playing with the RTL SDR

So the hackerspace will imminently have a number of Realtek-based SDRs as demonstrated last meeting.  An SDR, or Software-Defined Radio is a very flexible device, that performs the equivalent of a “raw recording” of a portion of the RF spectrum and then the raw samples can be processed in any way you like on your computer.

So for example, you can pick up FM radio, the pager network, transmissions from the ISS, or aircraft transponders.  Working GRCs will be posted on the blog
We’re going to try doing some of these things in the space over the next few months, the exact dates will be discussed on the list.  If you’d like to try this, here’s some links:

Another useful resource is the subreddit for it.