March 5, 2003


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The Automatic Position Reporting System
An Overview and Introduction

by Arte Booten N2ZRC

Many of you have heard discussions about a packet radio program called The Automatic Position Reporting System, (also called APRS.) It's a system which, unlike PBBS's, nodes and DX clusters, uses an UNCONNECTED protocol to transmit your exact position, a symbol denoting the type of station you're running and a Brief comment about it. It also uses di- rect keyboard-to-keyboard "chatting," has direction-finding capabilities and much more.

How does it work? In its most simplistic form you transmit a packet which contains your callsign, exact latitude and longitude, information about your station's power, antenna's height, gain and pattern as well as a brief comment of your choosing along with some symbols necessary to make the system work. With this information your station appears gra- phically on a map (actually, one in a series of many maps) on the moni- tor as would other stations that are on frequency. Since this is an UN- CONNECTED protocol, on-air packets can be kept to a minimum.

Consider this: When you connect to a local station using standard AX.25 you send a connect request to that station, they acknowledge that pac- ket, send you a connected packet which you must then acknowledge. The same thing happens to EVERY packet you, or the other station, sends. With APRS you only send ONE packet to convey your information. If it's not received on the first transmission, APRS retransmits this informa- tion using a decaying time delay (that is, the second packet is sent eight seconds after the first, the third fifteen seconds later, the fourth thirty seconds later,, the fifth a minute later etc. until, after an hour, you're only sending six packets an hour!) This makes more ef- ficient use of the frequency.

APRS uses different kinds of digipeaters, which use generic callsigns of RELAY, WIDE, TRACE, GATE and ECHO. RELAY stations (the default setting) are base stations used to digipeat low-power portable and mobile sta- tions. WIDE stations will digipeat packets addressed either to their specific callsign or the generic WIDE to other VHF stations and WIDEs. A packet addressed to TRACE will, with a TNC with appropriate firmware, be digipeated, and the generic callsign will be substituted with the TNC's own callsign. An ECHO performs a function similar to that of a WIDE on HF. A GATE will digipeat from VHF to HF. When setting up APRS for your location you'll set your digipeater path based on the situation at that QTH and where you want your information to go. For keyboard-to- keyboard chats (which are the only comms in which "ACK's" are used) you can also set alternate digipeater paths. Not only does this direct your message via the shortest possible route, but it also reduces QRM.

The program also interfaces with popular weather stations such as those made by Davis and Peet Brothers, thus allowing for real-time weather da- ta which is available at the touch of a key. The potential for this du- ring a SKYWARN situation is obvious. You'll get wind speed and direc- tion, temperature, rainfall amounts by the hour and 24-hour period and, in some cases, barometric readings. Such weather data can also be en- tered manually if a station has the information but not the hardware.

There is also a Direction-Finding mode which can be used by stations with either a beam or omni antenna! When the "fox" transmits, stations can call, by voice (on another frequency!) or keyboard their beam head- ings and/or signal strength. Using the antenna gain figures for these stations, circles are drawn on the map. The "fox" will usually be loca- ted where these circles converge. If you have one of the many "doppler" antenna systems they can also be used.

If DX-ing is your thing, there's also a "DX-mode" which also uses the UI protocol by simply monitoring the DX cluster frequency. As new spots are posted, they appear on the map with their callsign. Their location is based on the callsign prefix of the spot. Obviously, since you're not connected to the cluster, this isn't meant as a replacement to your normal AX.25 program, and you can't SEND messages, you can receive them (the program will flag yours and display them when asked.) It's just another tool for your county- or country-hunting efforts.

If, like me, you have a Global Positioning System (GPS) receiver with NMEA-0183 output this, too, can be utilized with amazing results! Your mobile or portable position can be regularly updated. Using such a "stand-alone tracker" you don't even need a computer. All you'd need is an H-T, TNC and GPS! Think about the possibilities for such a setup in something like a marathon, walkathon or even for someone shadowing an important official.

APRS was written to be able to run on just about any PC compatible com- puter from the latest Pentium Pro down to a lowly 8086. Heck, I know several people that use it with a Hewlett-Packard HP-200 palmtop! Maps are available from a large-scale map of the whole world to extremely de- tailed street-level maps. There's even a mail-reflector about it to which you can subscribe. Send a message to LISTPROC@TAPR.ORG, with a body that reads "subscribe aprssig your_name" without the quotes and substituting your name for "your_name".

APRS is lots of fun, has many potential ARES/RACES/SKYWARN uses. I'm sure you'll enjoy playing with it!

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