Direction finding objects

Note

The APRS spec documents a way DF reports to be sent is through a regular station posit with a data extension specifying the DF information and APRSDOS supported just such a thing, including a way of attaching automatic DF devices that reported information in the "Agrelo" format. While it does recognize such reports as they come in, Xastir does not implement transmitting such packets as primary station posits. Xastir only allows you to create OBJECTS that have that data extension.

The APRS spec describes various types of reports used for radio direction finding ("DF" or "RDF" activities). These are "omni" DF reports and those with beam headings, quality, and beam-width data.

They can be used to carry on transmitter hunting activities.

DF reports do not seem to be well supported in APRS clients and parsers

A quick survey of various open-source APRS parsers suggests that many authors have not bothered to implement decoding DF reports. aprs.fi, for example, only displays the symbol for the object, and leaves the data extension as part of the comment.

They remain useful in Xastir, however, which will plot them no matter where they come from.

Beam heading DF reports

These reports are for transmitter hunting activities that use directional antennas to find the bearing from the station in which the signal is heard.

Select the "DF Object" check box and the "Beam Antenna" option to enter this sort of object. Click the appropriate box for your beam width and then enter the bearing at which you hear the signal with that antenna.

Xastir can plot the bearing line from your report and/or two lines representing the beam width. These options are selected from the "Station->Station Display" menu, which allows you to turn them all off, all on, or only one of the two options. The image above displays only the bearing line itself.

Omni DF reports

The other type of DF report is for use with an omnidirectional antenna. Here, you report only that you heard the signal from your location. A circle around the report indicates signal strength (with 0 being "not heard"), height of your antenna above average terrain (HAAT) (NOT the height of your antenna above the ground where you're standing), and gain of your antenna.

With enough omni DF reports (including "not heard") reports you can narrow down the area where the transmitter must be.

In use

The primary value to using Xastir in DF activities is to coordinate the DF activity from reports in the field and to suggest to participants where to go next to refine the location.

Xastir has been used for just this purpose in searches for aircraft emergency location transmitters ("ELT") back before the FAA mandated an update to that sort of equipment, which now transmits a packet burst with the GPS coordinates, drastically reducing the need for RDF activities in that use case.