The Automatic Position Reporting System

by Tom Weeden, WJ9H

#1 - Introduction to APRS

 

You may have seen various articles in amateur radio magazines, ads for radio equipment, or heard people talking about "APRS" and wondered what it was. This series of articles is presented to introduce operators to the Automatic Position Reporting System and present its benefits to the ham radio community.

APRS uses standard packet radio to transmit position information and other brief status messages. It was developed by Bob Bruninga, WB4APR, several years ago and continues to be refined today. WB4APR wrote the APRS software to run under DOS on a PC, even an old 8088. Others have adapted the software to run on Windows and Macintosh machines.

Traditional packet uses a "connected" protocol, where each packet sent is acknowledged by the receiving station, or by an intermediate node. Operators can type to each other in a keyboard-to-keyboard QSO, or connect to a bulletin board (BBS) to read and send messages. In recent years, with the popularity of the internet growing, some have lost interest in packet BBSs. However, the uniqueness and versatility of APRS has renewed interest in packet radio in many areas of the country.

So what's so special about it?

Let's say I wanted to send a beacon packet to let others in my area know that I'm a new station on the air. Using traditional packet, I might program my TNC to send a beacon to say "WJ9H - Tom in Madison, WI" every 5 minutes. It doesn't take a rocket scientist to realize that this type of beacon would quickly annoy everyone on the frequency, and it doesn't convey any new information when it repeats. Multiply this one station by several more and you end up with a clogged frequency.

Suppose I ran a "smart" terminal program that sent the beacon, waited a bit, sent the beacon again, doubled the wait time, sent it again, and continued doubling the wait time. If no information in the beacon changed, soon the transmissions would "decay" to once in several hours.

Now, what if we could standardize how positions were transmitted? Remember the "Tom in Madison, WI" packet? How about adding latitude and longitude to pinpoint my QTH? What if all these position packets could be plotted graphically on a map on the computer's screen? You're starting to get the feel of APRS.

But what good is it? Let's look at the possibilities. You could enter current weather data at your location and send it. Other APRS users would see weather conditions from around their area graphically. If you have a home weather station, APRS could take that data and send it out automatically. Do you have a GPS receiver? Take the data output from a GPS into a newer TNC and transmit your mobile location automatically. APRS also can do direction finding, and even be used in foxhunts by stations using nondirectional antennas! But remember, if you're already on packet, you don't need any extra equipment to get started -- just the APRS software. And if you're just curious, you don't even need that, just your terminal program.

Most APRS activity nationwide is on 144.390 MHz. Fire up your packet station on that frequency and monitor for a while. If you're in range of other APRS stations on VHF, you'll see packets similar to this:

AA9AD>APR827,KB9KEF-10,WIDE*:=4255.40N/08914.11W-NewUser

N9UUR-10>APRS,WIDE3-3:!4302.29NI08755.60W&PHG6340/ Milwaukee APRServe IGate

WB9WOZ>APRSM,KA9VNV-3,AE9A-10*,TRACE3-1:The Northern Illinois APRS Network welcomes you

N9WJV>AA9JW-7:find your antenna?{001

VA3DRV-9>GPS,WIDE:Hdg 2 Chicago, thurs AM, 73!

AE9A-10>BEACON,WIDE3-3:!4258.09NN08811.30W#PHG5370/WIDE

WA9KCU>APRS,AE9A-10*:@110423z4312.34N/08843.50W_315/003g008t035r006p115....h58b10258dDvs

WJ9H-9>GPSLV,WIDE,AE9A-10*,WIDE3-1:$GPGGA,003558,4302.916,N,08930.143,W,1,06,1.1,334.1,M,-34.3,M,,*7E
N9UUR-10 sent the position of his Milwaukee APRS internet gateway along with its power and antenna height and gain. Other stations sent a simple one-line message either to everyone, or to a single station. WA9KCU sent a weather packet with data generated by his Davis home weather station. WJ9H-9 sent a GPS position which APRS decodes and plots on a map.

Let's dissect one of the packets.

WA9KCU>APRS,AE9A-10*: WA9KCU transmits an UNPROTO packet through the AE9A-10 digipeater. The asterisk indicates that you heard the digipeater.

@110423z Date and time stamp. (11th of the month, 0423 UTC)

4312.34N/08843.50W Latitude and longitude in degrees, minutes and tenths of minutes.

_315/003g008 The _ symbol indicates a weather packet. The wind is from 315 degrees (NW) at 3 mph, but a gust of 8 mph was recently measured.

t035r006p115 The temperature is 35 F, the rain in the last hour was 0.06", and precipitation in the last 24 hours was 1.15" (NOTE: automatic rain gauges don't measure snow!)

....h58b10258dDvs The humidity is 58%, the pressure is 1025.8 millibars, and the device generating the data was a Davis weather station.

That's a lot of data compressed into a single packet!

You may notice that APRS stations use digipeaters and not nodes. The reason for this is that APRS by nature is unconnected, sending only UI frames. There are no acknowledgments since the transmissions are meant for many stations to receive. The information that is constantly being updated is transmitted more often, and the messages that don't change decay away.

Next month, we'll discuss how to set up your new 2 meter APRS station. I'll assume you're already on packet with a computer that can run DOS. You'll need to get the latest version of APRS, which can be downloaded from the TAPR (Tuscon Amateur Packet Radio) web site at www.tapr.org. (If you don't have internet access, have someone who does download the software for you. It will fit on one floppy disk.)

In the meantime, you can see APRS in action via the internet by browsing to www.aprs.net. Since the page runs a Java application it may take a minute to load. Select Central USA to see nearly-live activity in southern Wisconsin. Also check out N9UUR's web page at www.execpc.com/~n9uur for Milwaukee-area APRS information, or the Northern Illinois APRS Network's page at nian.aprs.org.

(Tom Weeden received his Novice license in 1971 and today holds Extra class. He operates mostly APRS packet along with some 2- and 10-meter voice and ATV. He has been Chief Engineer at WMTV/Madison since 1991. He runs APRS 24 hours a day on an old 386 so the family can have use of the "good" computer.)
 
 

This page originally appeared in March 1999 Badger State Smoke Signals but was updated 14 June 2000.
Copyright 1999, 2000, Thomas C. Weeden, WJ9H