Using a sextant to check GPS accuracy


One of the more pervasive technologies we use every day, whether we’re voyaging or not, is GPS. This omnipresent positioning and timing tool has become the baseline of navigation on land, at sea and in the air. How do we know if it is working properly? Amazingly enough, the old-fashioned, non-electronic sextant comes to the rescue. There is a smartphone app called GPS Anti Spoof from celestial navigation expert Frank Reed that allows you to use a simple sextant sight to determine if your GPS position is accurate.

In the early days of the system, every GPS receiver was a big deal. These first devices were dedicated, stand-alone units like a loran receiver or an HF SSB radio. Over time, though, they became smaller, less expensive and more capable — a radio navigation corollary of the famous Moore’s law that states the number of transistors in a microchip will double every two years. (The “law” is named after Gordon Moore, the co-founder of Fairchild Semiconductor and former CEO of microprocessor maker Intel.)

Then, GPS receivers grew smaller and smaller and began to disappear. Entire GPS units were reduced to a small circuit board that could be installed in other devices, such as an EPIRB or a VHF radio. This miniaturization trend continued, and now GPS receivers fit on a single microchip.

The result, of course, is that GPS is everywhere, embedded in a wide variety of devices from rescue gear to cellphones to wristwatches. Allied with electronic charts, this has resulted in a navigation revolution. Marine navigators have come to wholly rely on GPS as their main, and perhaps only, navigation source.

But how do you know that GPS is accurate? Recent examples of Russian signal “spoofing” show that GPS can be made inaccurate. Spoofing is when fake signals are broadcast from land-based antennas. These signals overwhelm real GPS signals in the area of the ground-based broadcast. Receivers use the spoofed data and calculate their position, but the position solution puts them far away from their true position.

GPS spoofing is not generally a problem for voyagers. However, there are times when GPS signals do go awry, and a way to check on GPS accuracy can be useful.

Enter Frank Reed and his Android smartphone app designed to check on GPS with only a sextant angle (GPS Anti Spoof is available for Android with an iOS version pending). Reed responded to an email with some info on how his app works:

“The operation of the app is quite simple. It uses the actively received GPS position on the navigator’s smartphone to generate an exact predicted value for the raw sextant reading, the ‘Hs’ for the sun or other celestial body. So you shoot the sun, tap pause in the app and compare. If the observed altitude matches (within reasonable limits), then your received GPS position is consistent with the celestial observation, which is of course entirely independent of the electronic position, and therefore any GPS spoofing is minimal. On the other hand, if there’s a consistent or growing discrepancy between the predicted sextant altitude and the observed altitude, then spoofing has to be considered a serious probability, and the difference in altitudes corresponds closely to the magnitude of the spoofing. The reported incident [of Russian spoofing] in the Black Sea off Novorossiysk could easily have been detected, but that was an easy case (assuming the reports were accurate) since the apparent spoofing placed the vessels’ positions on land.

“The app simply takes all the paperwork and does it automatically. The comparison between the displayed value and the sextant reading is instant — no work at all. This allows frequent, quick checks with a sextant that cost almost no time. Furthermore — and this is what makes the app really unique — the calculated altitude includes numerous small factors that are ignored by almost all other apps and software solutions. For example, the system time in smartphones can easily drift when off network. The app gets a corrected time by comparing with the time from the GPS solution itself. Though GPS time is not continuously available, it is the gold standard and allows the determination of system time error. Also, the app includes DOV (gravitational deflection of the vertical) corrections that are greater than a mile near many islands and in tectonically active areas like the Caribbean, but which are almost universally ignored by tools and apps for standard celestial navigation.”

Since spoofing is not a big issue for most voyagers, the GPS Anti Spoof app might not seem all that valuable. It does have another use, however, that can make it a “must-have” for celestial navigators trying to improve their sight taking. Here is Reed’s explanation: “Since the app provides instant comparison between two independent sources of information, it can also be used to test the observer and sextant instead of the GPS receiver. In other words, the app functions secondarily as a slick, efficient ‘sight trainer.’ You take a sight. You compare with the app. That kind of instant feedback helps develop sextant skills in minutes that used to take hours of calculations and days of practice.”

So if you aren’t anywhere near Russian interference with GPS, it turns out this little app has a second value as a way to practice and get better at sextant sight technique. Then once you’ve quickly improved, you’ll realize celestial navigation is more fun, turn off your GPS and navigate the self-reliant way!

By Ocean Navigator