So here I am hanging out at the local hacker space in Richmond. We, the group that meet up every Tuesday evening are small, yet perfectly formed. This evening Paul (MØTZO), a friendly radio ham, and co-founder of our group brought along an interesting device to encode GPS location data in APRS (Automatic Packet Reporting System) packets. These packets are fed into to a VHF radio transmitter which transmits the signal at 144.8 MHz. A network of receivers, run by amateur radio operators receives the packets, and streams them over the internet. An interesting device indeed.
Tonight we were just playing with Paul's GPS board, which contains a U-Blox Neo 6 GPS receiver. As with most GPS receivers, simply powering it on is enough to make it emit coordinates encoded in NMEA 0183 sentences transmitted over UART at 9600bps.
These are trivial to receive with an fx2-based logic analyzer and sigrok-cli:
$ sigrok-cli --driver=fx2lafw --config samplerate=50k \
--continuous -P uart:baudrate=9600:tx=0 -B uart=tx
With the UART protocol decoder and the ASCII binary output, we can see the NMEA sentences:
Here we can see that we're locked on and receiving coordinates. "cat -v" is used to protect the terminal state from being clobbered by any errant binary data being emitted.
Yawn. Let's make this more interesting.
There is a very interesting package called gpsd. This is a GPS location server than can connect to various types of GPS devices, and can serve the position information over the network. And best of all, thanks to the power of UNIX pipes, we can feed the data from sigrok directly into it:
$ gpsd -N <(sigrok-cli --driver=fx2lafw --config samplerate=50k \
--continuous -P uart:baudrate=9600:tx=0 -B uart=tx)
There are a variety of gpsd clients available. There's a nice ncurses based client, gpsmon:
And best of all, KDE's Marble integrates with gpsd. Here we see Marble, showing a live stream of location data captured by the fx2lafw firmware, decoded by sigrok, handled by gpsd, plotted by Marble on OpenStreetMap map data - a complete free-software stack! Pretty freetarded: