Automatic Packet Reporting System (APRS) is an amateur radio-based system for real time digital communications of information of immediate value in the local area. Data can include object Global Positioning System (GPS) coordinates, weather station telemetry, text messages, announcements, queries, and other telemetry. APRS data can be displayed on a map, which can show stations, objects, tracks of moving objects, weather stations, search and rescue data, and direction finding data.
APRS data are typically transmitted on a single shared frequency (depending on country) to be repeated locally by area relay stations (digipeaters) for widespread local consumption. In addition, all such data are typically ingested into the APRS Internet System (APRS-IS) via an Internet-connected receiver (IGate) and distributed globally for ubiquitous and immediate access. Data shared via radio or Internet are collected by all users and can be combined with external map data to build a shared live view.
APRS has been developed since the late 1980s by Bob Bruninga, call sign WB4APR, currently a senior research engineer at the United States Naval Academy. He still maintains the main APRS Web site. The initialism “APRS” was derived from his call sign.
Bob Bruninga, a senior research engineer at the United States Naval Academy, implemented the earliest ancestor of APRS on an Apple II computer in 1982. This early version was used to map high frequency Navy position reports. The first use of APRS was in 1984, when Bruninga developed a more advanced version on a Commodore VIC-20 for reporting the position and status of horses in a 100-mile (160 km) endurance run.
During the next two years, Bruninga continued to develop the system, which he now called the Connectionless Emergency Traffic System (CETS). Following a series of Federal Emergency Management Agency (FEMA) exercises using CETS, the system was ported to the IBM Personal Computer. During the early 1990s, CETS (then known as the Automatic Position Reporting System) continued to evolve into its current form.
As GPS technology became more widely available, “Position” was replaced with “Packet” to better describe the more generic capabilities of the system and to emphasize its uses beyond mere position reporting.
APRS (Automatic Packet Reporting System), is a digital communications protocol for exchanging information among a large number of stations covering a large (local) area, often referred to as “ey-pers”. As a multi-user data network, it is quite different from conventional packet radio. Rather than using connected data streams where stations connect to each other and packets are acknowledged and retransmitted if lost, APRS operates entirely in an unconnected broadcast fashion, using unnumbered AX.25 frames.
APRS packets are transmitted for all other stations to hear and use. Packet repeaters, called digipeaters, form the backbone of the APRS system, and use store and forward technology to retransmit packets. All stations operate on the same radio channel, and packets move through the network from digipeater to digipeater, propagating outward from their point of origin. All stations within radio range of each digipeater receive the packet. At each digipeater, the packet path is changed. The packet will only be repeated through a certain number of digipeaters — or hops — depending upon the all-important “PATH” setting.
Digipeaters keep track of the packets they forward for a period of time, thus preventing duplicate packets from being retransmitted. This keeps packets from circulating in endless loops inside the ad-hoc network. Eventually, most packets are heard by an APRS Internet Gateway, called an IGate, and the packets are routed on to the Internet APRS backbone (where duplicate packets heard by other IGates are discarded) for display or analysis by other users connected to an APRS-IS server, or on a Web site designed for the purpose.
While it would seem that using unconnected and unnumbered packets without acknowledgment and retransmission on a shared and sometimes congested channel would result in poor reliability due to a packet being lost, this is not the case, because the packets are transmitted (broadcast) to everyone and multiplied many times over by each digipeater. This means that all digipeaters and stations in range get a copy, and then proceed to broadcast it to all other digipeaters and stations within their range. The end result is that packets are multiplied more than they are lost. Therefore, packets can sometimes be heard some distance from the originating station. Packets can be digipeated tens of kilometers or even hundreds of kilometers, depending on the height and range of the digipeaters in the area.
When a packet is transmitted, it is duplicated many times as it radiates out, taking all available paths simultaneously, until the number of “hops” allowed by the path setting is consumed.
Positions /objects /items
APRS contains a number of packet types, including position/object/item, status, messages, queries, weather reports and telemetry. The position/object/item packets contain the latitude and longitude, and a symbol to be displayed on the map, and have many optional fields for altitude, course, speed, radiated power, antenna height above average terrain, antenna gain, and voice operating frequency. Positions of fixed stations are configured in the APRS software. Moving stations (portable or mobile) automatically derive their position information from a GPS receiver connected to the APRS equipment.
The map display uses these fields to plot communication range of all participants and facilitate the ability to contact users during both routine and emergency situations. Each position/object/item packet can use any of several hundred different symbols. Position/objects/items can also contain weather information or can be any number of dozens of standardised weather symbols. Each symbol on an APRS map can display many attributes, discriminated either by colour or other technique. These attributes are:
- Moving or fixed
- Dead-Reckoned or old
- Message capable or not
- Station, object or item
- Own object/item or other station object/item
- Emergency, priority, or special
The Status packet is free-field format that lets each station announce his current mission or application or contact information or any other information or data of immediate use to surrounding activities. The message packet can be used for point-to-point messages, bulletins, announcements or even email. Bulletins and Announcements are treated specially and displayed on a single “community Bulletin board”. This community bulletin board is fixed size and all bulletins from all posters are sorted onto this display. The intent of this display is to be consistent and identical for all viewers so that all participants are seeing the same information at the same time. Since lines are sorted onto the display, then individual posters can edit, update, or delete individual lines of their bulletins at any time to keep the bulletin board up-to-date to all viewers.
All APRS messages are delivered live in real-time to online recipients. Messages are not stored and forwarded, but retried until timed out. The delivery of these messages is global, since the APRS-IS distributes all packets to all other IGates in the world and those that are messages will actually go back to RF via any IGate that is near the intended recipient.
A special case message can be sent to EMAIL and these messages are pulled off the real-time APRS-IS by the WU2Z Email engine and wrapped into a standard Internet Email protocol and forwarded into regular Internet email.
In its simplest implementation, APRS is used to transmit real-time data, information and 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.
While the map plotting is the most visible feature of APRS, the text messaging capabilities and local information distribution capabilities, combined with the robust network, should not be overlooked; the New Jersey Office of Emergency Management has an extensive network of APRS stations to allow text messaging between all of the county Emergency Operating Centers in the event of the failure of conventional communications.