GPS Anti Spoof1.40 - App Store

Instantly compare observed celestial navigation altitudes with high precision, live calculated altitudes. Shoot the Sun, tap pause in the app, and get instant feedback.

Standard version includes data for the Sun, Venus, and two stars. The Pro version of the app is for serious navigation use and includes data for the Sun, Moon, Venus, Mars, Jupiter, Saturn, the 57 standard navigational stars, plus Polaris. Both versions function off-network and at sea however the standard version only works for ten days off-network. The Pro version can function for weeks in the middle of the ocean without network access.

This app calculates the exact sextant altitude that should be observed in celestial navigation at sea or on land 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. If the GPS position is wrong, due to Spoofing at sea, observations will not match. With this app and occasional quick sextant sights, mariners can reliably detect GPS spoofing.

No GPS Spoofing? The app also functions as a celestial navigation trainer: take a sextant sight and immediately compare against the app's display. No paperwork and instant feedback will improve celestial sextant skills in hours instead of days or weeks.

The app works off-network, even in the middle of the ocean. If a network connection is available, the app will download temperature and pressure information automatically. Or weather data can be entered manually, if desired.

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 altitudes are accurate to a tenth of a minute of arc.

Version History maintenance updates 1.40 Jun 4 Maintenance update 1.0.39 09/01/2025 maintenance update 1.0.38 04/23/2025 maintenance update 1.0.37 11/27/2024 maintenance update 1.0.36 09/24/2024 maintenance update 1.0.34 05/23/2024 Important maintenance update. 1.0.33 09/24/2023 maintenance update. 1.0.29 07/28/2023 maintenance update. 1.0.28 02/06/2023 Improved almanac / ephemeris data. Better management of net-based meteorological corrections to altitudes. 1.0.27 02/21/2022 Enhanced off-network support. Also standard international units (SI) for height of eye, temperature, and air pressure are now supported in addition to US/imperial units. 1.0.25 03/10/2021 maintenance update. 1.0.23 03/17/2020 Maintenance updates. Extended ephemeris data. 1.0.22 03/12/2020 Maintenance and astronomical data updates. 1.0.21 12/20/2019 1.0.17 06/15/2019