Can't I just use the altitude off my GPS? Or is that not accurate enough? ;-)<br><br>I did dig out a textbook on Celestial Navigation, but it concentrates mainly on how to read an almanac, something I don't intend to cram into my PSK. However, I understood it far better than I did the last time I put it down. <br><br>It's been far too long since I played around with this stuff. Trying to distil the necessary formulae down to a piece of paper the size of an Altoid's tin is going to be a bit of a challenge :-)