Low-Cost Doppler-to-APRS Interface

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 KJ4SR, 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.

APRS/Doppler Screen Shot

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 that were available in the ham marketplace at the time.  It works with:

If you have a newer doppler set, see the update at the bottom of this page.

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 you must have a DF validation code for your callsign.

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.

Update:  This circuit is not suitable for use with the current generation of microprocessor-based dopplers such as the Agrelo DFjr, the AHHA! Solutions Microfinder, the WB6EYV Pico-Dopp and the Global TSCM DDF2020T.  All of them have serial (RS-232) output in "Agrelo format."  For bearing display and triangulation with these sets, consider the free GoogleHunt program for Windows by Bob Simmons WB6EYV.

Original Board

The original N7LUE circuit board with parts.

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This page updated 2 November 2015