Sextant required! The most reliable way to detect GPS Spoofing and other anomalies of electronic navigation is by comparing against the measured altitudes of celestial bodies including the Sun, Moon, bright planets and stars. This app calculates the exact altitude that should be observed with a sextant assuming that the GPS position is correct. No paperwork is required, no tables of corrections. All corrections are included automatically: index error, system time error, dip of the horizon, refraction, temperature and pressure corrections to refraction, altitude above sea level, parallax, and also gravitational deflection of the vertical (a correction of up 2 nautical miles not handled by any other celestial navigation sight-clearing product) are all included automatically. It's also a great sextant trainer: take a sight and immediately compare against the app's display. The app works off-network. If a network connection is available, it can download temperature and pressure information automatically. The ephemeris data is generated from the same JPL numerical integrations of lunar and planetary motions used by the official Nautical Almanac and Astronomical Almanac. All coordinates are accurate to one second of arc, and displayed sextant altitudes are accurate to better than a tenth of a minute of arc (the limiting precision of the best marine sextants).