The Automatic Position Reporting System An Overview and Introduction


by Arte Booten, N2ZRC

Many of you have heard about a packet radio program called The Automatic Position Reporting System, (also called APRS.) It's a system which, unlike a BBS, node and DX cluster, 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 keyboard-to-keyboard "chat", has direction-finding capabilities and much more.

How does it work? In its simplest form, you transmit a packet containing your callsign, exact latitude and longitude, information on your transmitter's power, antenna height, gain and pattern as well as a brief comment of your choosing, along with some symbols which make the system work. With this information your station appears graphically - on a map (or one in a series of many maps) on your computer's monitor. It'd also appear on the screens of other stations that are on frequency. Your station would similarly appear on theirs. Since APRS uses an UNCONNECTED protocol, on-air packets can be kept to a minimum.

Consider this: When you connect to your local node, using standard AX.25, you send a connect request to it. It'll acknowledge that packet, then send you a connected packet which you must then acknowledge. This same thing happens with each and every packet you, or the other station, send.

With APRS you need send only ONE packet to convey your information. If it's not received on the first transmission, APRS retransmits it, using a decaying time delay (that is, the second packet is sent eight seconds after the first, the third fifteen seconds later, the fourth half a minute later, the fifth a minute later, the sixth two minutes later etc. until, after two hours, you're only sending three packets an hour!) It makes more efficient use of the frequency.

APRS uses several different kinds of digipeaters in order to propagate beyond their immediate area. They use aliases such as RELAY, WIDE, TRACE, ECHO and GATE. There are also variations of WIDE and TRACE known as WIDEn-n and TRACEn-n. A RELAY station (the default setting) are usually base stations, and are used to digipeat low-power portable and mo-
bile stations. They are an essential part of the APRS system as a whole which is why most versions of APRS default to it.

WIDE digipeaters retransmit packets addressed either to their specific MYCall or the generic WIDE to other local VHF stations and WIDEs. Some have the ability to change that generic WIDE to its own MYCall.

ECHO stations performs a similar function on HF. GATEs retransmit signals from HF to VHF. However, they should NEVER be used to retransmit from VHF to HF. This is because VHF APRS uses 1200-baud signals on 144.39 MHz in most parts of North America. HF APRS uses 10.151 MHz LSB, just inside the upper edge of the 30-meter band, which is limited to using a maximum of 300-baud.

When setting up APRS for your location, you'd set your digipath based on the situation at that QTH and where you want your packets to go. In using keyboard-to-keyboard communication (the only comms that use "ACK's") 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, showing real-time weather data at the touch of a key. The potential for this during SKYWARN situations is obvious. You'll get wind speed and direction, temperature, rainfall amounts by the hour and 24-hour period and, in some cases, barometric readings. Such weather data can also be entered manually if a station has the information but not the hardware.

There's 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 beam headings and/or signal strength. Using the antenna gain figures for these stations, circles are drawn on the map. The "fox" will usually be located at the converge of these circles. If you have one of the many "doppler" antenna systems, they can also be used.

If DX-ing is your thing, there's a "DX-mode" which also uses the UI protocol by simply monitoring the DX cluster frequency. As a new spot is posted, they appear on the map with their callsign, based on their preprefix. Obviously, since you're not connected to the cluster, it's not meant as a replacement to your normal AX.25 program, and you can't SEND messages, but 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 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 a stand-alone tracker (including radios such as Kenwood's TH-D7a HT and TM-D700 mobile rig) you don't even need a computer. All you need is an H-T, TNC and a GPS! Think about the possibilities for such a setup in something like a marathon, walkathon or even for someone shadowing an important official.

The DOS version of APRS was written to be able to run on just about any PC clone from the latest Pentium IV 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 detailed street-level maps. It's lots of fun, has many ARES/RACES/SKYWARN uses and I'm sure you'll enjoy playing with it!


Arte Booten ( AEC for Digital Services, NYC ARES/RACES| |Riverdale, New York [FN30bu] !4052.71N/07354.06WNPHG5370/A=00240| PGP Key Fingerprint: D73E B889 C630 6F4A F31F 3083 56BD 0AAD 9996 3B03


This article resides on the Technical Information and Operations portion of the NYC-ARECS wesbite.