One of the first modes I tried upon receiving my amateur license was APRS. APRS uses radio to transmit position reports, weather reports and messages between users. To get started I made up a cable for my Kenwood TH-D7 and installed the UI-View software. The Kenwood radio provided a means to receive and transmit APRS data packets via its TNC. Typically the data is transmitted on a 2 metre frequency. In the United Kingdom the APRS frequency is 144.800MHz. Once I had tuned the radio and started UI-View I was able receive position beacons and weather reports from stations across the country. Many APRS maps can be found on the Internet. See the links at the bottom of this article.
APRS basically works like TCP. In my mobile setup the radio is connected to a GPS which provides positioning data. The radio forms this into a packet, assigns it a hop count and transmits on the set frequency. The packet can be received directly or via a local repeater. If a repeater receives the signal, it will decrement the hop count and retransmit. Most APRS repeaters are set to send packets for a maximum of seven hops in the UK. In well networked areas the packet will travel several hundred miles or more. If an area is not so well connected, a repeater can be connected to the Internet. This is how I display my position reports on the Google map to the right.
As well as being fun, APRS also has some serious applications. It can be quickly deployed in an emergency or for public events, such as those supported by RAYNET. The local RAYNET group has made up a compact box containing a TH-D7, a GPS and a battery. This can quicky be positioned on the back of a motorbike or on the seat of a car.
The links below will be useful for anyone wanting to learn a bit more about this mode:
APRS Origin
MB7USK - Good information and exellent UK APRS maps provided by G3OJZ
FindU - Web gateway, find position/weather reports of people using APRS
UI-View - Most useful APRS software for Windows
Xastir - Most useful APRS software for Linux
TinyTrak - Tiny GPS encoder - cost effective way to join in!
TinyTrak Summary - Good summary of the TinyTrak