Automatic Packet Reporting System (APRS) is an amateur radio based system for digital communications, most commonly used to automatically report the position (GPS coordinates) of a person or object, or weather data at a remote location. It was developed by Bob Bruninga, callsign WB4APR, at the United States Naval Academy. The acronym "APRS" was originally derived from his callsign, and subsequently expanded to "Automatic Position Reporting System". As the system was modified and expanded to handle non-position packet data, the meaning of the "P" in the acronym was again changed to "Packet" rather than "Position". Today, it is common for hams to use either expansion of the acronym. APRS is a real-time tactical digital communications 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.
Further Reading
The APRS protocol has been adapted and extended to support projects not directly related to its original purpose. The most notable of these are the FireNet and PropNET projects. APRS FireNet is an Internet-based system using the APRS protocol and much of the same client software to provide fire fighting, earthquake, and weather information in much higher volume and detail than the traditional APRS system is capable of carrying. PropNET uses the APRS protocol over AX.25 and to study radio frequency propagation. PropNET 'probes' transmit position reports, along with information on transmitter power, elevation, and antenna gain, at various frequencies to allow monitoring stations to detect changes in propagation conditions. Open Trac was created to provide an alternative to APRS that was cleaner and more functional than APRS.
Bases Behind the GPS A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time delay between transmission and reception of each GPS microwave signal gives the distance to each satellite, since the signal travels at a known speed. The signals also carry information about the satellites' location. By determining the position of, and distance to, at least three satellites, the receiver can compute its position using trilateration. Receivers typically do not have perfectly accurate clocks and therefore track one or more additional satellites to correct the receiver's clock error.
AX.25 Protocol AX.25 is a data link layer protocol derived from the X.25 protocol suite and designed for use by amateur radio operators. It is used extensively on amateur packet radio networks. AX.25 occupies the first and second layers of the OSI networking model, and is responsible for transferring data (encapsulated in packets) between nodes and detecting errors introduced by the communications channel. It is thus comparable to ethernet in the services it provides. AX.25 supports both connection and connectionless modes of operation, then latter used to great effect by the Automatic Position Reporting System.
What TNC An APRS infrastructure comprises a variety of Terminal Node Controller (TNC) equipment put in place by individual Amateur Radio operators. This includes soundcards interfacing a radio to a computer, simple TNC's, and "smart" TNC's. The "smart" TNC's are capable of determining what has already happened with the packet (unit of information) and can prevent redundant packet repeating within the network.
TNC Radios There are a few radios on the market which include a built-in AX.25 Terminal Node Controller and APRS software, and are capable of working with or without the need for an external GPS device. Two common models are the mobile Kenwood TM-D700A, and the handheld Kenwood TH-D7AG.
