Imagine having both your own RDF bearings and those of other base and mobile stations appearing in real time on a computer terminal in your home or car. It's possible today using Automatic Packet Reporting System (APRS) software.
In 1995, APRS was augmented by its creator Bob Bruninga WB4APR to include RDF bearing display and triangulation capability. Bearings can be entered manually or automatically. One automatic bearing entry format is 1-out-of-16 direction data from a doppler-to-RS232 interface circuit designed by Robert Swain KA4JSR, who was N7LUE at the time.
APRS averages the incoming bearing stream, which may reduce the effects of multipath somewhat if the doppler-carrying vehicle is in motion. It also calculates a quality factor for the bearing based on deviations within the stream of direction indications.
The photo shows some automatic bearings on a southern California map using a DOS version of APRS. The Macintosh and Windows versions of APRS support maps that are much more detailed than this.
The Swain interface was designed to work with most doppler sets in the ham marketplace. Use it with:
This circuit is not suitable for use with the Agrelo DFjr or the AHHA! Solutions Microfinder. These sets have their own proprietary RS232 interface provisions.
Because of other demands on his time, Robert has placed his interface project into the public domain. It is posted here for the benefit of home builders who wish to duplicate and experiment with it. Complete assemblies or kits are no longer available.
Links to the project details:
See the parts list page for info on an errors in the circuit board.
Important note: For the output of this interface to be recognized, you must use a registered copy of an APRS software version that includes automatic RDF input and have a DF validation code for your callsign. See the WB4APR (DOS) and WU2Z (Windows/Mac) APRS Web sites, especially the README.DF files, for information on latest versions of APRS and details on operation of this interface with APRS-equipped computers.
Although Robert is no longer in the kit business, he has agreed to list his e-mail address here for support to this project. If you have questions/problems that can't be resolved by carefully reading the documentation here and in the APRS files listed above, send e-mail to Robert at rswain@erols.com. He's a busy ham, so be patient.
Disclaimers: I have built and used Robert's circuit successfully, as have others. However, I cannot be certain that it will work with all doppler sets claimed, or that it will work with all versions of APRS, or with the computer you are using, or that all parts are still available. Technical support for this project may not be available in the future. Modifying a commercial doppler set to work with this interface may void its consumer warranty. Build this project at your own risk.
...And hidden T's in the mirror may be closer than they appear.
The original N7LUE circuit board with parts.
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This page updated 14 March 2001
