Excerpts from

APRS Puts Doppler Headings On The Map

Homing In, 73 Magazine, August 1995

by Joe Moell KØOV

Retire Your Protractor

Wouldn't it be great if your doppler indications could be displayed directly on a road map? While we're at it, why not have bearings from other base and mobile stations appear on the same map, so you can see instantly where they intersect? Automatic Packet Reporting System (APRS), a shareware computer program, makes all this possible right now.

When Bob Bruninga WB4APR first developed APRS, it was just for mapping packet stations, both fixed and moving, for fun and public service. He envisioned it as a way for officials at events such as marathons and boat regattas to instantly spot the locations of contestants, VIPs, ambulances, and so on. Then he began to add features such as dead reckoning of moving objects, messaging between unconnected stations, and grid-square plotting.

With the latest version, storm tracking nets can pinpoint their weather spotting units, HF contesters can display Packet Cluster DX reports, and transmitter hunters can create multi-station RDF networks to quickly zero in on jammers, stuck transmitters, and stations in distress.

Since APRS DOS version 3.0, the program has included a steadily improving suite of RDF features. The most advanced of these are the doppler inputs, which became fully operational in DOS version 5.03a. Bob added routines to accept and display bearings from Doppler Systems RDF models having 300 bps RS232 output. For Doppler Systems models without serial data output and for other brands of dopplers, WB4APR collaborated with Robert Swain N7LUE to develop a universal interface.

Inside the Interface

Block Diagram of InterfaceIn all of the popular doppler designs in the ham market, the 360-degree azimuth circle is represented by 8, 16, or 32 LEDs. Somewhere in the control/display electronics, 4-bit parallel data representing antenna position and a direction pulse to latch this data for the display are available. The block diagram shows how N7LUE's interface converts this parallel data to a serial output. All signals are tapped from the doppler circuits; normal operation of its LED display is unaffected.

U1 is a 74HC75 4-bit latch. It is disabled when the circuit is used with Doppler Systems, Dick Smith, and other designs that have latched 4-bit direction data available. Other dopplers such as the 16-LED KØOV/WB6UZZ Roanoke Doppler and the WA4BVY DoppleScAnt use a 4514 or 4515 latching 4-to-16 decoder IC, which does not put latched 4-bit direction data onto external pins. Data is latched by U1 for these models.

Latched data from U1 goes to U2, a Microchip AY31015D universal synchronous receiver/transmitter (UART) that performs the parallel-to-serial conversion. The output of U2 is a stream of 8-bit ASCII characters from "@" (01000000) to "O" (01001111), representing the 16 possible states of the 4-bit input data.

Note that no matter how many LEDs or vertical antennas your doppler has, N7LUE's board always outputs 16 output characters, representing 22.5 degree azimuth increments. U4 converts the serial data from 5-volt logic level to RS232 standards.

APRS software can process about eight bearing samples per second. U5 is a 4024 binary countdown, driven by the doppler's antenna rotation clock oscillator. Its 8 Hz output sets the UART character output rate. Q1, Q2, and U7 sense receiver audio, preventing random data output when no signal is being received.

U8 was added to the design for situations where APRS is transmitting packet bearings in the same band that the doppler is monitoring. When the packet TNC is keyed down, the doppler bearing will probably be erroneous. The U8 data mute interrupts serial output during packet transmissions, to prevent these bad bearings from being processed by APRS.

Crunching the Bearings

WB4APR's software accumulates and calculates the average and standard deviation of a series of bearing samples to the nearest degree. It plots the average as a yellow vector on the screen map. The standard deviation calculation gives a measure of the quality of the bearing data. When samples differ greatly in direction over a short time, deviation is large and the displayed line is dotted to indicate a low quality bearing. The more breaks in the line, the larger the deviation is. When doppler indications are steady, deviation is low and the yellow line is solid. A violet rectangle at the top of the APRS map display provides additional bearing quality data.

APRS/Doppler Screen ShotThe photo shows typical APRS doppler data displayed on a base station PC screen. The dashed yellow bearing lines from the KØOV icon were picked up during a 30 second transmission from a mobile station. The cross-bearing from WB6UZZ/M could have been received and plotted by packet or entered manually by the APRS operator from a radio report.

The APRS cursor has been moved manually to the intersection of the KØOV and WB6UZZ bearings. The exact latitude and longitude of this intersection is displayed in a box at the upper left corner of the screen. If the bearings are good, that is where the transmitter is. Even if the APRS map has errors in the location of roads and towns, the coordinates obtained by triangulation are correct if the coordinates of the RDF stations have been correctly entered.

Now that the APRS base station has coordinates of the unknown signal, its operator can notify mobile transmitter hunters. Better yet, the base station can automatically transmit a stream of packet transmissions with bearing data to a group of mobile transmitter hunters running APRS and their own dopplers. The mobiles must input their location and vehicle heading to APRS for bearings to be plotted. Although this can be done manually, the best way is with the NMEA-0183 output of a Global Positioning System (GPS) receiver.

Since most laptop PCs have only one or two serial ports, hooking up three peripherals (TNC, GPS and doppler) poses a major roadblock. WB4APR has created a Hardware Single Port (HSP) mode to permit GPS and TNC to share one port. You will need to build a two-transistor data switch, activated by the Data Terminal Ready (DTR) line on the serial port. Details and circuits are in the README.GPS file in APRS documentation.

To work with the serial RDF interface, your copy of APRS must be registered with a DF validation, which costs an extra nine dollars over the regular APRS registration fee. However, there are other ways to get RDF information into the APRS screen without special registration. Any base or mobile station running APRS can manually put its RDF bearing into its station position report to be transmitted on packet. An APRS operator can also get RDF bearings and positions of other base and mobile stations via voice radio, then enter and transmit them on packet from his station.

Packet stations not running APRS can put their RDF bearings in their beacon texts. If formatted properly, APRS-equipped stations receiving the beacons will automatically receive and display the bearing lines. The APRS README.DF file gives detailed explanations of how to do all this.

The README.DF file also describes how to set up remote unattended RDF stations consisting of a receiver, doppler, serial interface and packet TNC. No computer is needed if the TNC is set up to properly format and beacon the RDF data. Three of these stations at good receiving sites around a city could give any APRS station in the area instant triangulation data whenever an unknown signal comes on the air.

Get In On The Ground Floor

APRS and its interface to dopplers and GPS are not a "plug and play" system yet. The hardware and software are constantly evolving in response to user feedback. Every ham's APRS installation will have unique challenges due to differences in computers, dopplers, TNCs, and GPS receivers. There are important control and local QRM issues involved in setting up remote RDF sites. Your experiences and suggestions are needed to optimize the bearing averaging algorithms.

There is no way that Homing In can cover all the nuances of setting up APRS RDF networking. You will need to read the voluminous APRS documentation and plan your installation carefully. This effort will pay off in your being the "first on your block" to use with a revolutionary RDF technique that may someday become a commonplace way for hams to perform public service and self-policing.

© 1995 Joseph D. Moell. All rights reserved.

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