APRS Article 1
July-August 1999
AN
INTRODUCTION TO APRS
By
Jeff Robbins
VE3JTR
(Voice Call)
VA3JTR
(APRS Call)
Automatic Position Reporting System (APRS) is one of the fastest growing and most exciting facets of the Amateur Radio hobby to come along in a long time. I personally added the APRS functionality to my home station last November, and I haven’t looked back since. Bob Bruniga WB4APR initially conceived APRS in 1984. Taking Amateur Packet Radio to new levels, his system uses unconnected (UI) packets to distribute position reports, weather information and messages between users. Currently Amateurs use APRS to track moving vehicles, keep an eye on propagation, and even to add a new element to fox hunting. Bob introduced APRS to the world at the 1992 ARRL Computer Networking Conference. His paper described his DOS program called APRS that receives the UI packets containing position information and places the objects as icons on a map displayed on the computer screen. Following in Bob’s footsteps, Mark and Keith Sproul (KB2ICI and WU2Z respectively) developed MacAPRS in 1994 for the Macintosh computer and WinAPRS followed in 1995 for the Windows 95 operating system.
One myth needs to be dispelled right away. Contrary to popular belief, you do not need a GPS to participate in APRS. If you are running a packet station on HF or VHF at home, and are using a PC to do so, you already have the necessary equipment and expertise to get started. A GPS is only required for mobile operation. That, for me, was the best part. My APRS station, today, consists of a Pentium 75 PC with all typical accessories, a Kantronics KAM TNC and an HTX-202 radio. The radio is connected to an RS discone antenna located about 40 feet or so above the house. Now don’t panic, you don’t need a Pentium series PC or a KAM to run APRS, it is just what I happened to have. A 386 PC with a KPC TNC will work fine, depending on your choice of software. I downloaded the latest version of WinAPRS (currently 2.2.6) from the Internet and was soon on the map, so to speak.
Let’s take a look at some of the technical details of how APRS works. Before you even try out the APRS software, try tuning your VHF radio to 144.390 MHz. This frequency is becoming standard continent wide, meaning you will always be on the right frequency, no matter where you go in North America. On the HF side, the frequency is 10.151 MHz (LSB). Be sure you turn monitoring on for your TNC (MON ON) so that you can see all packets on the air, and soon you might see a packet that looks like this:
VA3JTR>APRS,RELAY,WIDE,WIDE/V:=4407.41N/07729.18W-PHG2200/WinAPRS
2.2.6
The packet can be broken up into several parts. VA3JTR is the callsign of the transmitting station, in this case, mine. APRS,RELAY,WIDE,WIDE is the path the packet will travel in the network, we will talk more about this later. 4407.41N/07729.18W is the latitude and longitude position of the transmitting station, in this case my house. The final portion, WinAPRS 2.2.6, is the version number of the APRS software that I am running on my PC at home. That is all the information needed to place my station properly on the map. Any station receiving this packet will see my house placed on Hwy 2 between Belleville and Trenton.
The thing that packet experts may notice as missing in this packet is a destination callsign. That is because APRS uses the UI packets mentioned above to distribute information in the network. Packets are sent out into the network for all monitoring stations to receive, with no particular recipient specified. This method of operation can be compared to a voice net where stations with traffic transmit as needed, and all other stations monitor and record the information. The path may also look a little strange to packet diehards who would be expecting callsigns. Especially in the case of mobile units, the transmitting station may not know what digipeater callsigns will be required to send the packet over a wide area. By using generic callsign aliases, such as RELAY or WIDE, packets can be digipeated over great distances without needing to know any of the digipeater’s callsigns!
So what do I see on my screen right now? I keep my Ontario map zoomed out just enough to be able to see all of Lake Ontario and the surrounding terrain. I can usually see stations west through Toronto, down into Hamilton and St. Catherines, and on into New York State. Looking to the east, I can see lots of activity in the Ottawa area. Here in the Quinte area we have several home stations on the air full-time, and many more that are occasionally on the air. There are also two of us running mobile stations. Next time, we will take a closer look at the software available for APRS. If you can’t wait until then, check out my webpage at http://www.lks.net/~ve3jtr. I have plenty of information available with links to download the latest APRS software. In future articles we will discuss how to build a mobile tracker, and other uses for APRS in emergencies and public service events.