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Automatic Packet Reporting System

Network overview

APRS (Automatic Packet Reporting System) is a 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. 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 website 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 due to the fact that 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 over and radiates out, taking all available paths simultaneously, until its path setting that dictates the number of "hops" is used up.

Positions/objects

APRS contains only four packet types: Position/objects, Status, Messages and Queries. The Position/object 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 packet can use any of several hundred different symbols. Position/objects 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 or object

Own object or other station object

Emergency, priority, or special

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, 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 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.

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.

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:

Bali Island: 144.390 MHz

Indonesia: 144.390 MHz

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-Nheck with locals for details

Argentina: 144.930 MHz

Brazil: 145.570 MHz

Europe: 144.800 MHz

Chile: 144.390 MHz (Santiago), 144.390 MHz (Los andes,Calama & Pta. Arenas), 145.010 MHz (Chilln)

South Africa: 144.800 MHz

Japan: 144.640 MHz

Thailand: 144.525 MHz

An extensive digital repeater, or "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 and the International Space Station are capable of relaying APRS data.

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 TNCs, and "smart" TNCs. The "smart" TNCs are capable of determining what has already happened with the packet (unit of information) and can prevent redundant packet repeating within the network.

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,n8ary." 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 unnecessary 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 old RELAY and WIDE paths to become obsolete. Digi operators are being asked to re-configure fill-in "RELAY" stations to instead respond to WIDE1-1. This results in a new, more efficient path of WIDE1-1,WIDE2-1. While most of the world has adopted the "new WIDEn-N" settings, there is an ongoing debate in the UK about the subject. The goal should be universal settings that will work everywhere.

Available equipment

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). Yaesu has also recently entered the APRS market with their VX-8R handheld.

The HamHUD integrates a display for viewing the position of other stations 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.

The RTrak combined an OpenTracker 1+ and a GPS receiver along with a low power transmitter in one package. The original units produced 500 mW, but the current version puts out 350 mW.

Byonics has introduced an all-in-one device combining a GPS receiver, TinyTrak controller and synthesized 2m radio called the Micro Trak AIO which requires no assembly by the user.

ArgentData now has a 5-Watt VHF Transceiver with Integrated Tracker2 in one compact package. With a non-display GPS receiver connected, the T2-301 can be used as a tracker. With compatible display-type GPS receiver, received position reports can be output as waypoints. With a Garmin Nuvi 350 GPS receiver connected, it can send and receive APRS messages as well as display other stations as waypoints. The Nuvi 350 can be used this way with a stand-alone Tracker 2 (OT2m) or connected to a T2-135 which mounts in an Alinco DR-135 radio. The T2-301 can also be used as a stand-alone digipeater.

The aprs SkyTracker is an APRS Beacon at 144.800Mhz which includes a 8W RF Transmitter, programmable PIC & u-bloc GPS receiver all on one 72x56mm PCB.

The BigRedBee GPS Transmitter is the smallest 2 meter frequency agile completely integrated APRS transmitter available. Output power is 6 watts from a supply voltage of just 7.5 volts. It also features on-board data recording that can be downloaded for later and viewing via Google Earth. A low power 70 cm version is also available.

Online data

Much of the data transmitted over APRS can also be seen on the Internet.

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 Federal Emergency Management Agency (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 more generic capabilities of the system and to emphasize its uses beyond mere position reporting.

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.

Further reading

Stan Horzepa, WA1LOU (1999). APRS Tracks, Maps and Mobiles. ARRL. ISBN 978-0872597747. 

See also

List of APRS nodes

Automatic Identification System - Position reporting system used for marine traffic

TCAS

References

^ a b R. Dean Straw, N6BV, ed (2006). The ARRL handbook for radio communications. Newington, CT. pp. 9.22. ISBN 0-97259-948-5. 

External links

APRS website. The original APRS page with all addendums and updates.

APRS Florida West Central Florida info and the APRSfl.net Tier2 APRS-IS Server

Queensland, Australia APRS Information

APRS Wiki Site More specific APRS setup information

APRS for Windows Mobile Using OpenStreetMaps with PocketPC PDAs

APRS on PocketPC PDA options Using APRS with PocketPC PDAs

APRS for Windows Mobile and OpenStreeMap APRS on WM PDAs.

www.findu.com Web-based access to worldwide APRS real-time data

aprs.fi Automatically updating real-time APRS view using Google Earth and Google Maps, available in 14 languages

OpenAPRS Web-based APRS real-time data using Google Earth Maps

oAPRS Web-based APRS Packet Search Engine for station, weather, telemetry and position lookup and debugging.

DB0ANF Web-based Access to APRS Station and Network Data

HamHUD A message capable APRS "heads up display" with LCD. SmartBeaconing was first developed for the HamHUD.

APRS Server List Publicly available servers hosting the APRS-IS

APRS World Open Source web-based APRS database

APRS Specification Official APRS specification document

KCAPRS Organization Getting started in APRS

PropNET Homepage If the band is open and no one is active, does anybody hear it?

Northwest APRS Homepage Pacific Northwest APRS alternate Wiki

APRS in Australia Australian (VK) APRS National Information Site

APRS Argentina Group Argentinian APRS Group (Spanish)

APRS in Brazil Brazilian APRS Network since 1998 (Portuguese)

Peet Bros. Company, Inc. Homepage Weather Station Hardware with direct support for APRS / TNC Interface

Official UI-View Site Roger Barker G4IDE SK was the author of UI-View

UI-View M0CYP UI-View Web Resource, including registration

Radioactive Networks GPS projects

XastirA popular Open Source APRS client for Linux, Windows, Mac OS X and more.

APRS ON4SAXA free online visual tracker for APRS

USAPhotoMaps A free Windows program

aprs.qrz.ru APRS in Russia

APRS.cz APRS in Czech

APRS Network Analysis New Zealand

Ui-View Webserver New Zealand

General APRS information

ZLhams Wiki

APRS Information in Japanese

Categories: Packet radioHidden categories: Articles that may contain original research from March 2009 | All articles that may contain original research | Wikipedia external links cleanup | Wikipedia spam cleanup
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