Cooperative RDF Team Preparations

August 14, 1992

by Paul Ternlund, WB3JZV

Each volunteer station that can take a bearing does so on 15 minute marks. Upon request, their location and bearing data is reported. For expediency, the number of bearings input for any sample time is limited to 20.

A transparent grid overlay is used by chase team members. This grid consists of labeled, 5 by 5 mile squares with its origin in the lower left corner. We use our standard Pierson Graphics Colorado State Recreational Map. The location on the map where the overlay origin is to be aligned will be defined after an estimated touch-down point is determined on the day of the launch.

During the flight when bearings are called for by computer net control, (beginning about 2 minutes after each 15 minute sample time), the following standard reporting format shall always be used:

• This is station <ABC>
• Location <X> by (Y>
• Bearing <DDD> degrees true
• <Callsign>
• Clear

Where, X = the number of miles East of the origin; and Y = the number of miles North of the origin. Use negative numbers to represent miles West and South of the origin, respectively. For example:

• This is station Foxtrot
• Location 15 by -3.5
• Bearing 032 degrees true
• WB3JZV
• Clear

Every hunter should add 11 degrees to their magnetic reading to convert to true north. If for any reason a stations does not have a bearing when called upon, "pass" should be reported instead.

Triangulation software and a computer can aid in locating a transmitter. The objective of this program (concept to follow) is to track a transmitter within 40 square miles of its actual location. It assumes that most reported bearings for a given sample time are good, but is tolerant of bearings that are bad due to multipath or other problems.

The program concept is as follows:

A standard map, grid and grid origin are required of all reporting stations. Given bearings from known locations, the point of intersection is calculated for each unique pair of bearings;

Problematical points caused by divergent bearings or bearings taht are equal or opposite are discarded;

A centroid is estimated from all calculated points, and finally, a location point is estimated for the transmitter. A plot is produced showing the location of the beacon location estimate, hunter locations, and prevailing triangulated points.

The greater the number of input bearings, the more calculations will be performed for the final result. E.G., while two bearings will produce one point and three bearings will produce three points, twenty bearings will produce 190 points. The Mac 140 Powerbook computer we use can produce the estimated point in under 2.5 minutes.

The program will note any station's bearings that contribute to discarded points for follow-on analysis. A station Performance Report is produced soon after the flight and can be made available to any participating station. It shows the Bearing Rejection Rate for each station.

After the program calculates the estimated transmitter location, computer net control will relay the coordinates to the field coordinator for any hunter position adjustments.

Finally there is some general hunter location/dispersion guidance that I recommend to each mobile tracking station. If just some of us can apply this guidance some of the time, it will help us all get the best accuracy out of our efforts...

Ideally, mobile RDF stations will locate around the balloon's position at all times.

graphic by Paul Ternlund WB3JZV

Mobile stations should move with the balloon in a cluster surrounding it. This will not work if the balloon's horizontal velocity is faster than mobile stations can move. If this is the case, it would be best to have some stations well ahead of the balloon along its estimated track. The field coordinator might request some trackers report bearings at 15 and 45 minutes marks while the others report at 30 and 60 minute marks. Then a greater distance can be covered by the chasers.

In the special case where the balloon can be spotted a visual bearing should be submitted instead of a DF bearing. Refrain from saying it is overhead if it is really not directly overhead.

If a DF antenna cannot be pointed toward the balloon when it is flying out of sight but close in, the following is most important: the distance between a reporting station and teh ground point directly under the balloon should not be less than the balloon's altitude in miles. I.E., when the balloon is at an altitude of 20 miles DF stations should be positioned further than 20 miles away.

When the balloon is o the way down and within 20 minutes of touchdown, the field coordinator might want to try sustained bearing taking on time marks called out by computer net control. Immediately after an estimated position is given, a request for the next bearing in 2 minutes can be given. Some stations can be mobiling into the drop zone while most that have good positioning could hold their location and support the final bearings.

Thanks for helping and good hunting!