APRS

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 is typically transmitted on a single shared frequency to be repeated locally by area relay stations (digipeaters) for widespread local consumption. In the United States, the frequency is 144.390 MHz. In addition, all packets 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.

History

Bob Bruninga WB4APR (SK), 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 HF Navy position reports. The first use of the system now called 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 endurance run.

During the next two years, Bruninga continued to develop the system, which he then 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 (by 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. Bruninga has also stated that APRS was not meant to be a vehicle position tracking system, and can be interpreted rather as “Automatic Presence Reporting System."

Network overview

APRS is a digital communications protocol for exchanging information among a large number of stations covering a large (local) area. 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 be repeated through only a certain number of digipeaters — or hops — depending upon the "PATH" setting in the packet data.

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 digitally repeated 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 standardized weather symbols. Each symbol on an APRS map can display many attributes, discriminated either by color or other technique. These attributes are:

• Moving or fixed
• Dead-reckoned or old
• Message capable or not
• Station, object or item
• Own object or other station object/item
• Emergency, priority, or special

An online map of APRS stations is available at aprs.fi.

Status

The Status packet is free-field format that lets each station announce its current mission, application, contact information or any other information or data of immediate use to surrounding activities.

Messages

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.

Messages can be sent directly between two operators, enabling functionality quite similar to SMS texting without relying on a cellular connection or Internet access. They can also be sent and received through the Internet. Either party or both may be using the Internet; an operator who does not have access to the Internet but can reach an iGate can use it to communicate with another operator who is using the Internet.

Not all messages are used for direct communication between two people, however. Many messages instead go to automated services that provide a variety of functions. These services operate using "call signs" that are not actually government-issued call signs, but are handled the same way by the APRS system.

Email

A message sent to "EMAIL" will be wrapped into a standard email and forwarded into the regular Internet email system. To use it, send the message to the "call sign" EMAIL, with the first "word" of the message being the recipient's call sign.

Call sign lookup

The "WHO-IS" service can be used to look up amateur call signs using QRZ.com without Internet access. To get only the operator's name, country, state (if in the US), and license class, simply send the call sign. To get full data, including the mailing address, send the letter "F" followed by the call sign: "F W7BKG".

Some devices, digipeaters, and iGates may not handle the WHO-IS call sign properly. If you are not receiving responses, try using WHO-15 ("WHO" dash fifteen) instead. Note that unlike WHO-IS, WHO-15 often gives outdated information for the license class; however, the other data seem to be accurate.

Weather

The "WXBOT" service can be used to access weather forecasts over APRS. The message format can be broken into three parts, referred to as "where," "when," and "what." All are optional, but you must choose at least one to avoid an empty message.

If you do not receive a forecast in a minute or two, or if you seem to have only received part of it, send your message again.

"Where"

Locations can be specified in several different formats.

• "[city], [state]": City and state must be separated by a comma and optionally a space. State must be abbreviated using standard postal abbreviation. Example: "Brookings, OR"
• "[zipcode]": Five-digit ZIP code. Example: "97415"
• "[grid]": Four- or six-character grid square. Example: "CN72VA"
• "[airport]": ICAO (four-letter) or IATA (three-letter) airport code. Example: "KBOK"
• "[latitude]/[longitude]": GPS coordinates in decimal degree format, separated by a slash. Example: "42.0515/-124.2809"
• "[cwop]": Citizen Weather Observer Program station ID. Example: "FW8609"
• "[call sign]", "[call sign]-[SSID]": Last reported APRS location of call sign, or address from FCC database if no location reports found.

The default is to use the last APRS location report of the station.

"When"

"When" may be specified as "today," "tonight," "tomorrow," "tomorrow night," "Sunday," "Sunday night," and so on. Days of the week may be abbreviated (e.g. "sun night" is the same as "Sunday night").

"What"

• "brief": A short weather forecast. Example: "Mostly Sunny High 67"
• "full": A longer forecast. Example: "This Afternoon, Mostly sunny, with a high near 65. West northwest wind around 7 mph."
• "current": Current conditions. Example: "Temp:55 wind:232@4,na"
• "metar": Current conditions in METAR format used by pilots. (Not for aeronautical decision-making.) Example: "KBOK 061856 AUTO 22004KT 10SM OCV004 13/12 A2998 RMK AO2 SLP152 TO"
• "cwop": Current conditions from nearest CWOP station.

Default is "brief" unless the location is given as an airport code, in which case "metar" is the default.

POTA spotting

"APSPOT" can be used to spot Parks on the Air (as well as World Wide Flora and Fauna, Summits on the Air, and Silos on the Air) activations. To use it, send a message in the format "! POTA [park reference] [frequency in kilohertz] [mode]" (example: "! POTA K-2811 14074 FT8"; note the exclamation mark at the beginning). For World Wide Flora and Fauna, Summits on the Air, or Silos on the Air, replace "POTA" with "WWFF", "SOTA," or "SIOTA," respectively. (Silos on the Air is apparently only in Australia, but the other programs, like POTA, are available worldwide.)

Capabilities

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.

Technical information

In its most widely used form, APRS is transported over the AX.25 protocol using 1200-bit/s Bell 202 frequency-shift keying on frequencies located within the 2 meter VHF band. The standard frequency in North America is 144.390 MHz.

An extensive digipeater network provides transport for APRS packets on these frequencies. Internet gateway stations (IGates) connect the on-air APRS network to the APRS Internet System (APRS-IS), which serves as a worldwide, high-bandwidth backbone for APRS data. Stations can tap into this stream directly, and a number of databases connected to the APRS-IS allow Web-based access to the data as well as more advanced data-mining capabilities. A number of low-Earth orbiting satellites, including the International Space Station, are capable of relaying APRS data.

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