NavList:

Ed Popko, to David Pike (expert bubble sextant user!), you wrote:
"Thanks for the advice. I think I have to back out Franks corrections for standard pressure etc.  But yes, that's a simple way to start."

It's a simple way to start and end. Really there's nothing more to it. And no, you don't need to "back out" any of the corrections. That's just odd! The app shows you what your sextant should read. Set it for the right object (probably Sun, Center) and the proper horizon type (bubble), and you're all set. Any difference between what you observe and what you see in the app tells you that your GPS is wrong (not likely... GPS does occur in the continental US where we're located but it's highly localized) or it tells you that you or your sextant are wrong. And "wrong" here is just "index error". Of course it might be a more exotic "arc error" that varies with the measured altitude, but that's not likely to be detectable within the usual rather low accuracy limits of bubble sextants. The error you see is your index error. Of course you should make sure the instrument is stable with the bubble centered, and you should average over a series of trials. Other than that it couldn't be easier.

Link for those unfamiliar: ReedNavigation.com/GPSantiSpoof/

Frank Reed

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