Summer 2007 Issue

Homing In

Computerized T-Hunting with Doppler RDF

By Joe Moell, KØOV

If there were “Thinking Outside the Box” awards, Bob Bruninga, WB4APR, would clearly be a winner. Bob pioneered remote-location technology in the 1980s and developed many novel uses for it. Here he gives an overview of APRS at the Tampa Bay Hamfest in Florida. (All photos and screen captures by Joe Moell, KØOV)

Technology in the spy movies of the 1960s and 1970s has always given me a chuckle. The hero often had a sophisticated-looking screen on the dashboard of his sports car. It received transmissions from a “bumper beeper” transmitter on the bad guy’s car and displayed his exact location on a moving map as the obligatory chase proceeded through the streets. The real radio-direction-finding (RDF) methods of the time didn’t have nearly enough accuracy to pinpoint cars that way, but it made good fiction.

Twenty years ago, Bob Bruninga WB4APR, took the first step toward making this kind of spy tracking a reality when he wrote the Automatic Position Reporting System (APRS). It was an adaptation of a program he had previously written to map the growing network of packet BBS nodes. APRS displays the position and movement of stations that report their latitude and longitude. When inexpensive Global Positioning System (GPS) receivers with serial data output became available, APRS made instantaneous vehicle location practical for almost every ham.

Early GPS sets were insensitive battery hogs with quadrifilar helix antennas pointed toward the sky and single-channel receivers that took many minutes to acquire the Navstar satellites. Position errors with government-imposed “Selective Availability” often approached 300 feet. Today’s multi-channel GPS receivers are far more sensitive and lock in much more quickly. With SA turned off and with aid from the new Wide Area Augmentation System, typical accuracy is now 25 feet or better.

In this decade, remote tracking using GPS has become so commonplace that young people can’t remember when it didn’t exist. GPS controls the movement of fleets of company vehicles and locates cellular callers to 911.1 Parents can surreptitiously keep track of their teenage drivers almost as easily as the fictional spies of 40 years ago did of the bad guy.2

APRS RDF Networking

During the 1990s, WB4APR constantly devised novel applications for APRS, such as tracking high-altitude balloons, marathoners, emergency vehicles, Olympic torch runners, and even the ceremonial 128-mile game ball relay for the annual Army/Navy football game. Besides their coordinates, APRS stations can beacon their own weather data, DX reports, and RDF bearings for display on the screens of all other users within radio range.

Some transmitter hunters envisioned a time in the future when networked APRS triangulation would provide nearly instantaneous location of any signal of interest, including malicious interference, stuck transmitters, and spurious emissions. At first, all RDF entry was from the keyboard. Stations in strategic locations took accurate bearings relative to true north and typed them in. APRS automatically deleted them after two hours to prevent clutter on the network.

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