The Automatic Position Reporting System An Overview and Introduction by Arte Booten N2ZRC mailto:n2zrc@weca.org 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 III 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! SOME REALLY SIMPLE BASIC INSTRUCTIONS ON GETTING APRS ON THE AIR NOW by Arte Booten N2ZRC (packet: N2ZRC@KB2VLX.#BRONX.NY.USA.NA) (E-mail: N2ZRC@weca.org) First things first. In order to run APRS, you first have to get it. If you have access to anonymous FTP simply navigate to the following URL: ftp://ftp.tapr.org/aprssig/dosstuff/APRSdos/ and look for the latest version. In this example, I'll use the nonexistent APRS version 9.99, which would be called "APRS999.zip." Substitute the appropriate file name for this. If you're using PkZip204g, put your APRS disk i nto the floppy drive of your choice (I'll call it drive A) then expand the APRS files by using the following commands on the hard drive of choice: Make an appropriate directory MD APRS Change to this directory CD \APRS Switch to the A drive A: Run PKUNZIP with directories PKUNZIP -d APRS999.zip C: Don't forget to use that "-d" switch, which lets it create the necessary subdirectories. If you use WinZip, PkZip250 (or some other Windows-based zip utility), change to drive A and double-click on APRS999.zip, click expand button, tell it where you want it and follow onscreen directions. Make sure your TNC is in Terminal or Command mode (whatever it's called by the manufacturer) and that parity is set to 8-N-0. At the C:\> prompt, go to your chosen APRS folder and invoke "APRS99.exe". If you're using Windows, simply double-click on that folder. This brings you to the LOGO screen. Enter your callsign and ssid, if any. Tell it what kind of TNC you're using. Answer the other various questions. When you're done, the main map screen will appear. Now press M(aplist) then C(onfigure) and C(hange maplist), tell it you want to use the appropriate maplist for your area, either EAS(t), CEN(tral), WES(t), NE(ast), SE(ast), etc. Use the arrows or mouse to bring the cursor to your approximate location (keep your eye on the up- per left part of the screen which shows the l atitude/longitude of the cursor.) Then press HOME to center your screen on it. Use the PgDN key to zoom in a few screens and tweak the cursor to your EXACT QTH. You may have to zoom in to a very small scale to get the necessary resolu- tion. Once the cursor is at the right spot hit the HOME key again. Press I(nput) M(y) P(osition) and confirm your lat/long, choose a symbol for yourself, type in a brief comment, and verify it. Once you've pres- sed that "Y" you're essentially ready to go on the air. In it's most basic form, you're configured! Tune the radio to 144.39 (in most parts of the country), hook it up and see what you can see. It might take a few minutes for other stations to appear (assuming there are some) but if you get a little impatient, try pressing O(perations) Q(uery) and give it a radius such as 64 to force position reports from others. Look for stations whose symbol is a green star. THESE ARE THE WIDE DIGIPEATERS! Is there one fairly close to you? If there is, press the "D" key. If an asterisk (*) appears next to a callsign (hopefully that nearby WIDE) then you're hearing it directly. Make a note of that near- by WIDE station's digipeater path. Now you're going to set YOUR digipeater path. Press O(perations) D(igipeaters). If you heard that WIDE station directly, enter it's callsign and ssid, if any. Follow this with a comma then type in "WIDE". For example: N2MH-15,WIDE would be how I'd enter it here in The Bronx, but the nearest WIDE to YOUR QTH is what YOU'RE looking for. Next, you want to set your Power-Height-Gain figures. Press I(nput) P(ower) and tell it how many watts you're using, your antenna's height above AVERAGE terrain (look at a topographical map of your area,) the gain in dBd and the antenna's directional pattern in degrees or 0 (zero) for an omnidirectional antenna. Finally, set your Beacon Text by pressing I(nput) M(y) S(tatus) and typ- ing in a short comment, different than the text you used for your Posi- tion Text earlier. At this point, you're about as far as you need to go for now. I hope to see you all on my screen in the VERY near future. Look for me at cruise missile coordinates 4052.71N/07354.06W. 73 _______________________________________________________________________ |Arte Booten AEC for Digital Services, NYC ARES/RACES| |PGP Key Fingerprint: 22E5 3EC7 4E3C 2460 C570 BACA E48B 71D5 0F33 54AD| |Riverdale, New York [FN30bv] !4052.71N/07354.06WNPHG5370/A=00240| |Packet: N2ZRC@KB2VLX.#BRONX.NY.USA.NA HTX-202/RS Brick/KPC-3 6.0/X-510| |______________________________________________________________________|