NavList:

My "GPS Anti Spoof" app is out in a new version on the Android app store (Google Play Store). Function is simple. The app does everything. All you do is shoot the Sun (or Moon, planet, star) with your sextant and directly compare with the displayed altitude in the app. No calculations / no paperwork. It's great for sextant training --you get instant feedback. And it easily detects GPS spoofing, if you're worried about pirates and that sort of thing! Naturally it can only detect spoofing or other errors in your GPS position (e.g. from operator error) within the limits of celestial sights, which means no better than about one nautical mile and a single sight can only detect a shift along the same axis as the azimuth of the body you're shooting. Unlike nearly all other available apps and software, this app also incorporates the deflection of the vertical, which caan throw off your sights by more than one minute of arc near many islands. The app is dead-on accurate. Here's the link in the store: https://play.google.com/store/apps/details?id=com.clockwk.GPSAntiSpoof. The basic version is free. There's also a paid "pro" version which you can find on your own. 

And, yes, the iOS version is on the way. The Apple approval process is notoriously slow, but that's how they keep the world safe from security threats, right?

Here's the pdf of my Navigation Symposium presentation on this concept:

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA