Police Scanner and GNURadio
November 9th, 2008Keeping and eye on the police has never been so much fun! Recently, my friend John lent me his USRP, allowing me to play with GNURadio. From wikipedia:
The Universal Software Radio Peripheral (USRP) is a high-speed USB-based board for making software radios.
GNURadio is a free software, software radio. Essentially, the USRP allows you to tune or transmit any radio signal, given the correct tuner/transmitter attachment. Currently it is anything in the 0-5.9Ghz range. The USRP John lent me came with a FM/TV tuner — meaning I can listen to anything between 80Mhz and 800Mhz.
What exists between 80Mhz and 800Mhz? Lots of stuff! For example, normal FM radio is between ~87Mhz - 108Mhz, all of which can be demodulated using GNURadio. Also, analog TV transmits in that range, allowing you to watch or listen to any of that on your computer.
But! It gets even more interesting. For instance, San Jose International Flight Control exists around 124Mhz (although, I have not heard much there…). Or, you can even listen to Police Central at 460.4Mhz. Listening to the police has been pretty fun. So far, Sam and I heard about a stabbing at a local restaurant, and plenty of phone number / license plate checks.
Anyway, so far I have not done much hacking on the device — there are a lot of example applications to do the FM demodulation. The one thing I did figure out was now to remove the FM static when the channel is not in use. It took me awhile to figure out, mainly because I did not know the correct terminology. Turns out, the term is “squelch“, and gnuradio implements a block to do this. All I had to do was create a squelch handling block, put it as part of the rest of the processing chain. Looks something like this (inside the usrp_wfm_rcv.py example program):
squelch = gr.simple_squelch_cc(0, 5e-3)
Then add the squelch to the rest of the connect chain:
self.connect(self.u, squelch, chan_filt...
This thing is super cool — and I’m excited to see what else I can make it do.
