This series on APRS has attempted to show how existing packet radio stations can be used in fascinating new ways. We have looked at the operational theory behind APRS, using short broadcast packets to convey position and status information about a station or other object, and displaying that information in a variety of ways, including graphically. From the basic station using only a TNC and 2-meter radio, to the more complex additions of GPS receivers or weather instruments, we have seen how APRS has breathed new life into 1200-baud packet. While strides are being made toward 9600-baud connected node systems, 1200 baud APRS still remains an inexpensive, robust data system, especially in the mobile environment. But where is this mode going?
All over the country, hams are installing APRS stations at National Weather Service offices to give meteorologists access to the nearly-live weather data being transmitted by area amateurs. GPS-equipped mobile Skywarn spotters can have their locations plotted as they track severe storms. The National Hurricane Center also makes use of APRS data when tracking hurricanes.
APRS has become so widespread that some equipment manufacturers are incorporating it into new radios. Kenwood's TH-D7A combines an APRS-compatible TNC and a text display into a handy-talkie to allow tracking and one-line messaging. You can see several of these units in use in the Milwaukee and northern Illinois areas. Stations using the TH-D7A usually identify with an SSID of -7 after their callsign.
A device called a "Mic-Encoder" (MIC-E) can be connected to a standard FM voice rig to insert an APRS position packet at the end of each voice transmission. Some repeaters are set up to accept Mic-E transmissions on the repeater's input frequency and redirecting the packet portion of the transmission to the 144.39 MHz packet frequency and muting the packet on the repeater's output. Using this method, a ham can operate APRS and standard voice on a single radio and not annoy the "regulars" on the repeater with packets!
More and more amateur groups providing communications for public service events are using the mapping capabilities of APRS to show positions of vehicles, runners, bikes, and other "assets." It saves a lot of time if a net control station can see the locations of his participating stations on a map display instead of asking each station for his location over the voice net. See how hams used APRS at the 1999 Pittsburgh Marathon at http://marconi.ece.cmu.edu/aprs/marathon.html.
APRS can even be used to track the Circus Train! And it was used during the July 1999 run from Baraboo to Milwaukee. WB9WRW and others had APRS on board the train's chase vehicle to report the train's position independently from the voice modes. You could see the train's location on your web browser! Try it on me sometime. Point your web browser to http://map.aprs.net/wj9h-9 to find me while I'm mobile.
There are a few stations on 30-meter HF APRS, but the vast majority of activity is on 2 meters. Unfortunately, if you are in a sparsely populated area far from digipeaters and gateways, VHF APRS may be useless. One of the next steps we will see in the near future is the use of satellites and the Internet to make APRS useful world-wide. Work is being done in cooperation with AMSAT to use amateur satellites such as AO-16 which have on-board 1200 baud digipeaters. According to APRS author Bob Bruninga, WB4APR, only a small modification has to be made to a standard TAPR-2 type TNC to output Manchester-encoded packets which are used on satellites. WB4APR believes that 5 watts is sufficient to be able to get an APRS packet to one of the amateur satellites for digipeating. So "wilderness travelers," boaters, and others could report their position and status with simple equipment from anywhere as long as a satellite is overhead.
Receiving satellite packets is more involved and is difficult to automate. The goal several APRS experimenters are working on is an automatic satellite downlink and Internet gateway. Picture this: a ham vacationing out west in a national park would like to let his friends or extended family know his location from time to time. Using his modified TNC and GPS receiver, he transmits his location on 2 meters via satellite. At a ground station possibly hundreds of miles away, but still within the satellite footprint, that position packet is received and relayed to the APRS Internet gateway where it is posted. Anyone running any version of APRS and connecting to the APRS Internet address will then receive a "dump" of, say, the last 24 hours of position packets and then any new live packets as they come in. If our vacationing ham had some sort of input device like a laptop computer or even the keypad of his TH-D7A, he could key in a short status packet such as: "Leaving Yellowstone tomorrow, see you soon."
There are a lot of possibilities for this exciting mode. To the newcomer it may seem overwhelming to learn the setup and the operation of the software. It was for me when I first started, since there is no real "manual" for operations, just a big list of help files. One of the reasons I wrote this series, at the prodding of your editor, was to help make APRS operation understandable. I hope you've taken the opportunity to check out at least one of the versions of APRS software and tried it on the air...or the Internet. A few new stations have popped up in southern/southeast Wisconsin on 2 meters recently, and I've heard about a few more in the works. I hope to see you soon on 144.39!
APRS information on the web at: www.qsl.net/wj9h.
This article originally appeared in the December 1999
issue of Badger State Smoke Signals but
was updated 2000 June 14.
Copyright 1999, 2000, Thomas C. Weeden, WJ9H