Automatic Packet Reporting System

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Automatic Packet Reporting System (APRS) is an amateur radio based system for real time tactical digital communications of information of immediate value in the local area. In addition, all such data is ingested into the APRS Internet system (APRS-IS) and distributed globally for instant access. In addition to messages, alerts, announcements and bulletins, the most visible aspect of APRS is its map display. Anyone may place any object or information on their map and it is distributed to all maps of all users in the local RF network or monitoring the area via the internet. Any station, radio or object that has an attached GPS is automatically tracked. Other prominent map features are weather stations, alerts and objects and other map related amateur radio volunteer activities including Search and Rescue and signal direction finding.

APRS was developed since the late 80's by Bob Bruninga, callsign WB4APR, currently a senior research engineer at the United States Naval Academy. The acronym "APRS" was derived from his callsign. In the 1990's as GPS excitement dominated many new applications, the "P" was often referred to as "Position" instead of the original "Packet". But this so skewed the public perception of APRS as only a GPS and Weather Position tracking system, that recently, the emphasis has returned to the broader "Packet" applications.

[edit] Positions/Objects

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. APRS contains only four packet types, Position/objects, Status, Messages and Queries. The Position/object packets contain not only the latitude/longitude, course and speed, but also fields for power, antenna height and gain and voice operating frequency. The map display uses these fields to plot communication range and connectivity of all participants and facilitate the ability to contact users during both routine and emergency situations. Position/objects can also contain Weather information or can be any number of dozens of standardized weather symbols. Each position/object packet can use any of several hundred different 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 or object
  • Own object or other station object
  • Emergency, priority, or special

[edit] Status/Messages

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, bulletings, 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 on-line 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.

[edit] 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 Global Positioning System 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.

[edit] Technical Information

In its most widely used form, APRS is transported over the AX.25 protocol using 1200 baud Bell 202 audio frequency-shift keying(AFSK) on frequencies located within the amateur 2-meter band:

  • North America: 144.390 MHz with 144.990 occasionally used as an alternate input frequency for local low power stations
  • Australia: 145.175 MHz with 144.390 MHz available for as a secondary frequency, primarily for satellite and DX work.
  • New Zealand: 144.575 MHz (National APRS) and 144.650 (digipeaters) supports WIDEn-N -- check with locals for details
  • Argentina: 144.930 MHz
  • Brazil: 145.570 MHz
  • Europe: 144.80 MHz

An extensive digital repeater, or "digipeater" network provides transport for APRS packets on these frequencies. Internet gateway stations (i-Gates) 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 and the International Space Station are capable of relaying APRS data.

[edit] 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. Three common models are the mobile Kenwood TM-D700A, its replacement, the Kenwood TM-D710A and the handheld Kenwood TH-D7A(G).

The HamHUD integrates a display for viewing the position of other statioons and weather reports, and a means of sending and receiving APRS messages, and an interface for a GPS receiver. It started out in 1997 as a homebrew device, but more recently, kits have been available from time to time. It connects to a TNC which is in turn connected to a radio. The Alinco DR-135T is popular as the internally mounted T2-135 can be used with it, reducing the number of items that need to be connected together. SmartBeaconing was developed for the HamHUD by Tony Arnerich and Steve Bragg. It varies the beacon rate based on speed, and adds corner pegging. SmartBeaconing has also been adopted by the TinyTrak and the OpenTracker series.

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 not normally use a path routing if they don't need to be digipeated outside of their local area (and most don't). Otherwise a path of WIDE2-2 or less should be used as requirements dictate. This path actually reflects the routing of packets via the radio component of APRS, and fixed stations should carefully consider their choice of path routing(s) to avoid unncessary RF clutter outside of their local VHF listening area.

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.

[edit] Online Data

Much of the data transmitted over APRS can also be seen on the Internet. For a sample of what's available, click on [1] and enter the callsign of NJ2EM to see APRS activity in New Jersey. You can also zoom out to see activity and stations in other areas.

[edit] History

Bob Bruninga 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. In 1984, 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 now called the Connectionless Emergency Traffic System (CETS). Following a series of FEMA exercises using CETS, the system was ported to the IBM PC. During the early 1990s, CETS, now 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 most common use of the system.

[edit] Related systems

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 PSK31 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.

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