APRS
Introduction to APRS
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.
Capabilities
In its simplest implementation, APRS is used to transmit real-time reports of the exact location of a person or object via a data signal sent over amateur radio frequencies. In addition to real-time position reporting capabilities using attached GPS receivers, APRS is also capable of transmitting a wide variety of data, including weather reports, short text messages, radio direction finding bearings, telemetry data, short e-mail messages (send only) and storm forecasts. Once transmitted, these reports can be combined with a computer and mapping software to show the transmitted data superimposed with great precision upon a map display.
Equipment Settings
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.
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-D7A(G).
Reporting stations use a method of routing called a "path" to broadcast the information through a network. In a typical packet network, a station would use a path of known stations such as "via n8xxx,n8yyy." This causes the packet to be repeated through the two stations before it stops. In APRS, generic callsigns are assigned to repeater stations to allow a more automatic operation.
RECOMMENDED PATH: Throughout North America (and in many other regions) the recommended path for Mobiles or portable stations is now WIDE1-1,WIDE2-1. Fixed Stations (Homes, etc.) should normally use plain WIDE2-2 or less. The APRS local settings discusses settings that are recommended for use in the western UP in more detail.
OLD PATH: Early on, the widely accepted method of configuring stations was to enable the short-range stations to repeat packets requesting a path of "RELAY" and long-range stations were configured to repeat both "RELAY" and "WIDE" packets. This was accomplished by setting the station's MYALIAS setting to RELAY or WIDE as needed. This resulted in a path of RELAY,WIDE for reporting stations. However, there was no duplicate packet checking or alias substitution. This sometimes caused beacons to "ping pong" back and forth instead of propagating outwards from the source. This caused lots of interference. With no alias substitution, you couldn't tell which digipeaters a beacon had used.
NEW PATH: With the advent of the new "smart" TNC's, the stations that used to be "WIDE" are now "WIDEn-N." This means a packet with a path of WIDE2-2 would be repeated through the first station as WIDE2-2, but the path will be modified (decremented) to WIDE2-1 for the next station to repeat. The packet stops being repeated when the "-N" portion of the path reaches "-0." This new protocol has caused the RELAY, WIDE path to become obsolete. Users are being asked to configure "RELAY" stations as WIDE1-1. This results in a new, more efficient path of WIDE1-1,WIDE2-1.