Since (1) GPS receivers work our nautical miles by measuring the distance between two coordinates (rather than an INS/IRS-style speed/time integration) - and (2) because the GPS model of the earth is quite an accurate oblate spheroid one, rather than a perfect sphere - one must assume that there is no fixed value that correlates x degrees to z feet (i.e. 1 degree = 60x6076ft or whatever).
It must rather be a variable calculation that takes into account where (i.e. latitude) those coordinates are located. In other words, the same distance travelled in degrees at the equator and at the poles would produce a (slightly) different "distance travelled" value in feet and NM.
Also, the altitude of the aircraft will have an effect, because flying the same angular distance between two coordinates will produce a greater actual distance as altitude increases. For GPS, altitude is a big deal, since the calculated geographic position on earth is merely the result of comparing the various psuedo-ranges from the satellites, which change as the aircraft altitude changes.
Mind you, not a lot of receivers will output feet as a distance unit once a certain threshold has been exceeded.
That's just my assumption. I'll check this with an authority on GPS and report back the definite answer.
This is quite an interesting point.
Last edited by KrazyKraut; 18th December 2005 at 13:42.