APRS is a real-time tactical digital communicatons protocol for exchanging information between a large number of stations covering a large (local) area. As a multi-user data network, it is quite different from conventional packet radio.
APRS is different from regular packet in four ways. First by the integration of maps and other data displays to organize and display data, second, by using a one-to-many protocol to update everyone in real time, third, by using generic digipeating so that prior knowledge of the network is not required, AND FORTH, since 1997, a worldwide transparent internet backbone, linking everyone worldwide. APRS turns packet radio into a real-time tactical communications and display system for emergencies and public service applications (and global communications). Normal packet radio has only shown usefulness in passing bulk message traffic (Email) from point to point. It has been difficult to apply conventional packet to real time events where information has a very short life time
and needs to get to everyone.
Although the recent interfaces to the Internet make APRS a global communications system for live real-time traffic, this is not the primary objective. But like all of our other radios, how we use APRS in an emergency of special event is what drives the design of the APRS protocol. Although APRS is used 99% of the time over great distances, and benign conditions, the protocol is designed to be optimized for short distance real-time crisis operations.
APRS provides universal connectivity to all stations by avoiding the complexity and limitations of a connected network. It permits any number of stations to exchange data just like voice users would on a voice net. Any station that has information to contribute simply sends it, and all stations receive it and log it. Secondly, APRS recognizes that one of the greatest real-time needs at any special event or emergency is the tracking of key assets. Where is the Event Leader? Where are the emergency vehicles? Whats the Weather at various points in the County? To answer these questions, APRS is a full featured automatic vehicle location and status reporting system too. It can be used over any 2-way radio system including Amateur Radio (Ham), CB, Marine Band, and Cellular Phone.
APRS is on 144.39 throughout the North American Continent. Other countries often use other frequencies. Check to see what frequency is used in your country for APRS.
WEATHER STATION REPORTING: APRS position reports can also include the wind speed and direction, as well as other important weather conditions. Most APRS software programs support a serial interface option to the ULTIMETER and DAVIS and other home weather stations to do this automatically. All weather stations show up as a blue circle, with a line indicating wind speed and direction. Many APRS programs also have a database of the locations of most NWS sites and can crunch a file of NWS hourly WX conditions for display. Finally, APRS users can set WX alarms and be alerted when WX conditions exceed those values.
DX CLUSTERS: APRS an ideal tool for the DX cluster user. Not only does a user get to see all DX spots on the map, but by operating in the monitor only mode, a user has reduced the overall packet load on the DX cluster. This is a benefit to everyone on the channel. Also the APRS monitoring station will see the SPOT as soon as the first station gets it, rather than later on down the list.
INTERNET: Most all APRS software programs can access the worldwide APRServe system and allow a user to display VHF and HF stations on a map from anywhere in the world allowing you to see who is on the air. Using the normal 144.39 channel, you can send and receive message traffic to ANYONE on APRS anywhere in the world.
FREQUENCY COORDINATION: Every packet asset on every frequency should include a position or at least gridsquare in all routine BEACONS. This alows APRS to be used to monitor network topography on any frequency. Thus, APRS makes an excellent tool for frequency coordination.
PROTOCOL - Although APRS redundantly transmits data, a fundamental precept is that old data is less important than new data. All APRS packets are repeated at an ever decreasing rate. Each new packet is transmitted immediately, then 20 seconds later. After every transmission, the period is doubled. After 20 minutes only six packets have been transmitted. From then on the rate remains at 10 minutes times the number of digipeater hops you are using. This allows the rate to be every 10 minutes for a local event or every 30 minutes for the typical home station running WIDE3-3.