By adding GPS positioning to the receiver another cool feature also becomes possible: automatic magnetic declination adjustment.
NOAA provides some pretty cool software called The World Magnetic Model, or WMM. The WMM models the Earth’s geomagnetic field with a high degree of accuracy, and given a Lat/Lon position the WMM will provide the geomagnetic declination for any location on earth.
Given the magnetic declination, the receiver can adjust compass bearings to provide true bearings. Which might not be so desirable in ARDF, since most orienteering maps are rotated to magnetic north, eliminating the need to adjust for declination at all. But there are other advantages.
When calculating direction of travel from GPS-derived location data, some very useful information can be derived from great circle trigonometry equations. For instance, the direction of travel (course), direction between two known points (course between points), and other useful information. But, such calculations result in directions with respect to True North, not Magnetic North. Here’s where the WMM comes in. When you know the declination for your current location, you can adjust the GPS-derived data to obtain magnetic directions.
Suppose you head off in a particular direction of travel from point “A”, say 30° magnetic (M), which is the strongest signal direction toward the fox at point “F”. You run for 250 meters, but trees and other objects force you to divert your path from a straight line, resulting in your current location lying off of the straight line between points A and F. Because you are no longer on that line, the direction that you now need to travel to arrive at point F is no longer 30°M. The processor in your receiver can determine the correct direction to run (desired course) using your current lat/lon position, and the estimated location of point F. But the calculations would be in relation to True North. But with an accurate magnetic declination provided by the WMM, the processor can easily convert and display the calculated direction in degrees magnetic, allowing you to steer by the direction provided by your compass (mechanical or digital).
Floating point math libraries will be needed for the WMM and for great circle calculations. But an AVR processor with 128KB of flash should be sufficient to provide the needed program space, with room to support other receiver functions. With no hardware support for floating point (no floating point processor) the calculations are going to be relatively slow. But even an 8 MHz processor clock speed should allow the calculations to keep up with the GPS position updates, and to appear to the user as being almost instantaneous.