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Hi, GPS already has a back-up built in.  If the problem is loss of signal, or spoofed signal, don’t switch it off. Do CN.  Enter best position as start point.  Enter almanac GHA/Dec as destination, and get miles.  Divide by 60, get degrees, divide remainer by 60 to get minutes.  Take away from 90°:  there’s your Hc.  Brian Walton

That’s very cunning Brian. It’s a solution deservous of the Chair in Cunning at a major university.  I bet a lot of people hadn’t noticed that; I certainly hadn’t. What Brian is pointing out is that the front end hardware, software, and ‘firmware’ of almost every GNSS receiver probably use the same formulae to solve the track and distance to waypoint problem as is needed to solve PZX triangle problems. So do many on-line great circle solving apps.  Only probably, because it’s always possible that the ‘firmware’ includes an even more cunning fix to allow for the fact that the Earth is not a perfect sphere like the celestial one

When I get a chance, I’m going to test a couple of Garmin hand-helds v the Hc given by Frank’s GPS Spoof app.  If that works, I might try it with my Garmin Nuvi car receiver in co-ordinate mode and maybe even with Google maps, but it’s mainly a winter fun thing.  There’re too many other things to work on during the summer.  I gotta mend the roof while the Sun shines. DaveP