GPS: Just the Facts

IARU Region 1 ARDF rules currently contain the following rule regarding the use of satellite-based navigation devices:

Part B, Appendix 1, Section T4.2 The use of satellite positioning devices is allowed provided they do not contain digital map of the terrain (“nonmapping” devices).

The above rule is currently being interpreted by some ARDF equipment manufacturers and competitors to permit the use of GPS modules integrated into ARDF receivers, provided that digital maps are not used.

By integrating a GPS module with a microcontroller and a digital compass module within the circuitry of an ARDF receiver, the following navigational assistance is currently available in some competitors’ receivers used at ARDF Championship competitions.

1. Distance Measuring – The ability to set waypoints and then read the straight-line distance between one’s current position and the recorded waypoint. This is useful for determining when an exclusion area border (e.g., around the Start, or around a located Fox) has been reached.
2. Heading Following – The ability to follow a straight line from one point in a particular direction. This is useful for navigating along a straight path, such as along a particular bearing toward a Fox. Using geometrical formulae in an embedded software program, the receiver is able to calculate cross-track error information and present it audibly through the earphones, allowing precise navigation along the heading direction without any need to refer to a map or the features shown on a map. Note: this feature is more accurate and foolproof than using compass guidance alone because GPS can ensure precise steering to the actual rhumb line with high accuracy.
3. Bearing-Crossing Calculations – The processor can calculate the position where bearing lines converge, and then provide distance measurements and heading-following assistance to the convergence location. Note: since GPS positioning is used to determine the location from which each bearing direction was taken, one significant source of bearing error is reduced. So bearings are significantly more accurate and more useful for following and convergence calculations when GPS is used to establish the precise point at which each bearing was taken.
4. Range and Angle – Range (or distance) is the same as “distance measuring” described above. The angle between a waypoint lat/lon and one’s current position lat/lon can also be easily calculated without using a compass. This allows, for instance, a competitor to read from an alphanumeric display both the distance and the compass angle from the start to his current position. That lets a competitor  quickly locate their precise position relative to the start on a paper map.

A future concern: Short-range radio communication technology (currently banned) could theoretically be used to share all the course data wirelessly receiver-to-receiver between competitors in the field.

The same logic that allows the rules to be interpreted as permitting the above functionality, should also permit simple monochrome graphical displays to be integrated into ARDF receivers. Although to my knowledge this is not yet being done, it is almost certain to happen at some point in the future. With a simple small graphical display, lacking any terrain map whatsoever, the following features will be possible.

1. Waypoint Display – Similar to the distance measuring feature described above, the small graphical display will permit setting and viewing waypoint positions relative to one another.
2. Bearing-Crossing Display – By taking several bearings toward the Finish beacon, an accurate Finish location can be determined and displayed relative to the Start, found foxes, and bearing convergence locations for unfound foxes.
3. Exclusion Area Display – Similar to the distance measuring feature described above, the small graphical display will permit seeing one’s current position relative to the Start and the exclusion area around the Start, estimated Finish, found Foxes, and fox bearing-convergence locations.
4. Track Display – As a competitor traverses a course, this feature will allow viewing one’s current location and the path traversed relative to all the waypoints and exclusion areas recorded during the run.
5. Bearing Display – Bearing lines taken toward Foxes can be shown relative to all of the information listed above.

So a simple graphical display would allow the entire course, and a competitor’s location and track to be displayed relative to one another with overlaid bearing lines. Only a terrain map would be missing.

While a graphical display would increase the advantage provided by integrated GPS modules, it should be kept in mind that all of the capabilities listed for the graphical display scenario could, theoretically, be provided using audio and textual cues. Such an audio-and-text interface would be less intuitive and would require more learning on the part of the competitor. But the fact remains that all of the information listed above can be conveyed with or without a graphical display, and without a terrain map.

Accuracy: GPS position accuracy depends on the antenna, atmospheric conditions, satellite constellation geometry, use of filtering (e.g., Kalman filtering), and other factors. Experimentation has shown that for typical GPS modules +/- 10m accuracy is readily achieved and rarely does the error exceed +/- 30m. Weather and vegetation are rarely a factor. Extremely steep terrain (more extreme than found on most ARDF courses) combined with low elevation can sometimes cause higher position error or even loss of position. GPS tends to be unreliable indoors.

The use of GPS for navigation assistance, even without terrain maps, can provide advantages over using only the traditional personal navigation skills historically used for ARDF. This fact is readily admitted by most competitors who use receivers with built-in GPS modules, and of course, it is the reason that such such receivers are used at all.