By Tom Weeden, WJ9H


In the last installment of this series on APRS, we discussed how to manually input data for plotting on your map display. This month we'll see how to use APRS for direction finding of transmitters. It can be done not just in the traditional "beam heading" method, but also by several stations using omni antennas, and by a single mobile station with an omni!

First, the method that most foxhunters are familiar with, triangulation. Here is a map of Madison showing two stations, mine on the southwest side of the city, and WB9RND on the east side.

Say I have a beam heading on the fox of 20 degrees. I would enter I (Input), D (DF), then my beam heading of 20. (We'll assume that the reading is degrees from true north. This can be done as a relative bearing if mobile, but let's not complicate things for now!). Finally, APRS asks for a signal quality rating from 1 to 8. I'll enter 8 for best visibility on my plot.

The map now shows my beam heading. Now I need the other station's beam heading. If that station is running APRS, he could enter his data as above and his beam heading would be shown on my screen when I received his packet. But for this discussion, I will enter the data of the other station. Place the cursor over the other station's QTH and press I (Input), A (Add Object), D (objects beginning with "D") and the number "1" (to pick "DF" on the list). Next APRS prompts for course and speed. Since the other station is stationary, enter 0 for both. Now enter the beam heading. Let's say WB9RND has a heading of 260 degrees on the fox. Enter 260, then a signal quality of 8. You can then enter optional comments, current time, and finally, "Y" (yes) to accept your entries (or "N" to start over). APRS will now plot WB9RND's beam heading.

We can see that the two vectors intersect just northwest of the lake, the probable location of the transmitter.

Now let's look at how several cooperating stations can narrow down the location of a transmitter based just on signal strengths from omni antennas. Only one station needs to be running APRS. And as mentioned last month, APRS can be used as a stand-alone program if desired, without transmitting any packets.

In the case of omni-DFing, the more reports received, the closer the fox's location can be pinpointed. Even reports of NO signal are valuable. Here's why.

If you receive a very strong "needle-pinning" signal from a transmitter, you can assume that you are near the source of the signal. If you drew a circle with a relatively small radius around your location, the source would be somewhere near the edge of that circle (since you don't know which direction the signal is coming from). If another receiver, some distance away, heard NO signal, you could assume that the transmitter is not near that receiver. You could draw a circle with a relatively LARGE radius around that receiver, and assume that the transmitter is NOT inside that circle. As the number of reports increases, you would plot various-sized circles over a map, and could assume that the transmitter is near the intersection of the edges of all those circles. To illustrate, here is a map of Fond du Lac with three DF stations. Station DF1 has a signal strength of 4, DF2 has an S9+ signal, and station DF3 has a very weak signal which we'll rate as a 1.

Place the APRS cursor over the first DF station's location. Press I (Input), A (Add Object), enter the station's name (DF1), then D (DF), 1 (DF), 0 (Omni DFing), and a signal strength of 4. Next APRS asks for the station's height above average terrain. Take the default of 20 feet by pressing <ENTER>, and do the same with antenna gain (3 dB). The next entry is for direction of best horizon. If you were on the side of a hill with the best view to the east, you would enter 90. This data is used to offset your omni antenna pattern slightly for calculations. In flat terrain, enter 0. Finally enter any optional comments, the time, and "Y" to accept your entry. Go through the same process for station #2, using a signal strength of 9. We'll assume station #3 is a pedestrian with a hand-held radio, so use a height of 5 feet and a gain of 0 dB.

When all three stations' data have been entered, you can use the map plot function of APRS to draw the relative circles around the stations to narrow down the transmitter's location. Press M (Maps), P (Plot), D (DF), and O (Omni). Solid-colored circles will plot over each station. The transmitter will be located near the intersection of the edges of the circles.

Still with us? Here's the third DF method, which takes us back to high school geometry class. How do you find the center of a circle when you know the location of three points on the circle? (Hint: bisect the line segments formed by all the points.) Before you get out your compass and straightedge, remember that your computer can do the computations.

APRS can estimate the center of a circle (fox's location) if you tell it the location of the points where the signal fades out. A single mobile station with a passenger running APRS on a laptop can plot those fade-out points simply by placing the cursor at that location on the map and pressing <F5>. When using the fade-point method, APRS will ask if this <F5> point is the beginning of a new plot. Answer "Y" for the first point, or "N" for the continuation of an existing run.

Here is a map of downtown Madison, showing four points. Each point is named by the computer with the last 3 characters of the station's callsign and then a two-character identifier. Point B1 is where the mobile first acquired the signal. B2 is where the signal faded out. The mobile then turned around and re-acquired the signal at B3. Finally, B4 is where it faded out again.

Now that we have two sets of points, press M (Maps), P (Plot), D (DF), and F (FadePlot). APRS will do the geometry and blot the bisected lines on the map. The fox will likely be located at the intersection of most of the lines. (On this map, I manually overlaid a circle on the fade points for clarity. APRS only assumes the circle and does not show it.)

If you'd like more information on direction finding with APRS, check the <F1>-F help files under DF.TXT. For more information on APRS in Wisconsin, check N9UUR's nice web site at:

...or mine at:

See you on 144.39!

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