Saturday, 21 September 2013

$1 GPS Receiver

Looking through the parts box I came across the Airlink Communications  CDMA C3310 vehicle tracker bought for $1 which was used to provide position updates derived from GPS over the CDMA Phone network and had supposedly a GPS receiver. Time to investigate

 In true ham spirit the covers were  removed first.After I opened the box I found a Verizon CDMA phone module and a GPS. On the back panel was a 9 pin RS232 serial connector a connector for GPS antenna an antenna connector for a CDMA antenna and a 12V power connector. I hooked up a computer to the serial port an active GPS antenna to the GPS port and 12V to the power conenctor and waited. Nothing came out of the serial port, however after a while the GPS lock LED on the front panel did illuminate. Time for a web search.

Luckily I came across a document on the unit which I have put on my web site.  raven-cdma-airlink_document.pdf  From this I learnt that by connecting a terminal emulator to the serial port at 1152000 baud and issuing an ATGPS1 command I could get GGA RMC and VTC NMEA strings out of the unit every second. Unfortunately I could not find a way of making the unit remember that setting when it was power cycled. I searched with a logic analyser but could not see a 1PPS signal which was not surprising as the application does not require accurate timing

 I think with a tweak to my GPS LED clock code, originally designed for Motorola GPS boards I could turn it into a GPS clock, so the $1 is not wasted.

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