10 Direction Finding Using APRS

APRS Article 10

March - April 2001

Direction Finding Utilizing APRS

By Jeff Robbins

VE3JTR (Voice Call)

VA3JTR (APRS Call)

Happy March APRS hams! It is my hopes that spring will be arriving soon, keeping in mind I am writing this in early January, so I am hip deep in snow in spots! Don’t get me wrong, I like winter, and we haven’t had a great snowy winter like this in years. But I know that by March I am looking forward to getting the motorcycle out and can’t wait for warmer weather. An update from last issue; The first person to take me up on my offer from last time turned out to be a neighbor of mine. VE3CJG successfully added my firehall to the APRS map. Now the January issue has just come out, so I expect a few more to come yet. It will be interesting to see how far away the sources of the firehall objects will be.

This issue, we will look at incorporating APRS into a favorite ham radio activity of many of you, and nicely continues our conversation from last issue. Here in the Quinte area, we have an active group of hams that, weekly, go out and participate in Fox Hunting. Now, many of you probably also enjoy fox hunting, but you may be surprised to learn the APRS can be useful for locating transmitter locations.

First, let’s review the traditional method of fox hunting. Using any one of a variety of directional antennas, the hunter, using the S-meter of his radio, will take a bearing on the radio signal, and proceeding in the direction, checking frequently and modifying their course, will hopefully come across the transmitter. No easy feat, as many fox hunters will attest, but perhaps APRS can make things easier. Perhaps we can even create group foxhunts, teams of hunters in separate cars working together. Let’s find out how.

Simple example: We are at our starting point in the hunt, we have our Tim Horton’s coffee in hand (it is winter, remember). Our hunters take their first bearing on the signal, and discover the signal is to the North East. For ease of example, let’s say the signal is due east. The hunters enter their current location and the beam heading into APRS, and the results are printed on the map. An icon will indicate their location, and a straight line indicting the beam heading.

By studying the map, our hunters can choose a location for their next bearing measurement, and off they go to their next measurement location. They continue taking measurements and entering that information into APRS. After several measurements, the lines on the map will begin converging on a position, which will be the location of our hidden transmitter. Simple eh? The fox can monitor the hunter’s progress, by seeing their vehicles locations and their bearing measurements that have been plotted on the map. This is assuming each fox hunting team is using a different frequency so that they cannot see each other’s progress. Having multiple car teams entering information into the APRS network can speed up the process, and each team car will see their partners’ information as well. Co-operative fox hunting! Who is going to try it first?

Now what about locating an unwanted or bothersome transmission? With hams living all over the area, it should be easy to quickly organize and take bearing information from all over your area, and by entering all of the findings into APRS, the location of the nuisance transmitter will quickly be revealed. Now there is nothing new here, hams have been locating transmitters for years by taking bearings and drawing lines on a map. APRS just makes the job easier and allows you to share the information and work within the APRS network. This may be a good exercise to practice so that when the next rouge transmitter shows up in your area, you can quickly establish an APRS DF session, and locate the offender with ease.

Win/MacAPRS Implementation:

Just like last time, we will be adding an Object to the network. Go to Edit, Add Object. Give your object a name (Reading1, 2, etc.), and Lat/Lon location. (Hint: The Lat/Lon fields will be pre-filled with the last place on the map you clicked with your mouse.) Select a RDF bearing, an RDF Quality (S-meter reading), and your bearing information will be placed on the map. If you also select Active Object, your bearing information will be transmitted out into the network. If you select InActive, the bearing information will only reside on your own map.

DosAPRS Implementation:

Enter I for Inputs, than D for direction finding. You are now prompted for a bearing, so enter the bearing in degrees (use 360º for due north). The next prompt is for a Quality indication; again, use the S-meter reading from your radio. Note: This method assumes your current location is to be the location of the bearing. If you wish to add bearing information for a location other than your own, Use I, than A for Add object, and you will be prompted for location information as well.

Again, as always, be sure to read the documentation of your particular flavor of APRS for complete instructions on how to use these features. I do not know if other versions of APRS are capable of direction finding, but I would be interested in hearing your experiences using these features. APRS is also capable of interfacing with Doppler direction finding equipment, for that completely automated feeling, but that would take the fun out of fix hunting, wouldn’t it?

In upcoming issues, we will be looking at UI-View, a new APRS program that is really catching on in the APRS community. We will also be connecting a weather station to APRS, and as always, I appreciate hearing from you, the readers. Let me know about how you are using APRS in your day to day activities, and in your special events. Until then, I will be watching for your fox hunts, ON THE MAP!