APRS - Automatic Position Reporting System

This is a pretty nifty system using amateur packet radio. A GPS receiver (a Garmin GPS III, in my case) sends a continuous stream of messages out the RS232 port with the position, heading, and speed (as well as a lot of other stuff). A device called a TNC (Terminal Node Controller) which is basically a modem with a microcontroller looks at the messages and parses the ones of interest. Once a minute (or whatever I program it to do), it transmits a message with the information parsed from the GPS messages. The transmission is 1200 baud FSK over the 2 meter amateur band (at 144.39 MHz) using a run of the mill Kenwood TH22 handheld radio. One or more receiving stations will hear the message (hopefully). The message gets retransmitted over the air to anyone who wants to receive it, and, it gets transmitted to a central internet location where a database is kept of the most recent messages. There is also a way to do this on HF frequencies, for longer range applications. On HF, the standard frequency is 10.151 MHz (in the "30 Meter" band).

When you enter the url

http://www.aprs.net:8000/w6rmk-5 - original APRS web server

http://second.aprs.net/w6rmk-5 - the new APRS web server in Southern California

the web server on port 8000 at www.aprs.net looks in its database for the most recent message from my station (w6rmk-5) and generates a dynamic HTML page with links to mapblast.com for the maps. The original port is up and down, and is in Florida, so it doesn't always work. There is now an APRS internet server up in Southern California at second.aprs.net.

Another nifty thing is if someone puts up a carefully surveyed fixed GPS receiver, differential corrections can be transmitted over the APRS frequencies, so that other APRS users can correct their GPS measurements, achieving accuracies of around a meter. Those corrections would also be "served" over the internet.

Larry at Purple Computing makes Garmin compatible connectors as "sharehardware". His web site is http://www.pfranc.com/. It's a much better deal than the $50 cable from Garmin.

More info on APRS at the virtual meeting


aprs.htm - Revised 4 Jan 2000, Jim Lux
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