ReedNavigation.com

Instant feedback with no paperwork at all. There's a standard version (free on Android, inexpensive on iPhone/iPad) that works with Sun, Venus, and Polaris sights and can function for three to seven days without network access. If you're serious about your navigation, then get the PRO version which includes the Sun, Moon, Venus, Mars, Jupiter, Saturn, Polaris and all 57 standard "navigational stars" and works offshore and off-network for months at a time. GPS Anti Spoof Pro for iPhone/iPad and Android. GPS Anti Spoof (standard) for iPhone/iPad and Android.

What does it do?
The app calculates a live sextant altitude, exactly as it should appear on the instrument, from the current GPS position. The app has two basic functions.
1: TRUST THE SKY: If you're a reasonably skilled celestial navigator, and your position is mission-critical, then the app is a sensitive, accurate test of the GPS position. If your position is being spoofed by criminals, pirates, military or government intelligence forces, you can detect that with an easy, quick sight of your favorite celestial body. An observed celestial altitude will not match the app's calculated altitude if you're being spoofed. You can detect GPS Spoofing at sea using this app within normal limits.
2: TRUST THE GPS: Turn it around. If you're learning or improving your sextant skills in circumstances where your position is not yet mission-critical, and you have a reasonable expectation that the GPS position is perfect as usual, then the app is a sight-trainer, an instant check on your instrument and your skill using it. Shoot the Sun and compare. No paperwork at all. When learning, this also cleanly separates the two sides of the process of learning celestial navigation: the skill of sextant sights on one side, and clearing and plotting sights on the other. You can quickly ramp up your skills using a sextant with the instant feedback from this app. Save the math for another day.

Some frequently asked questions:
What is T adjust?
The app uses your device's built-in system clock. It periodically checks that clock against the time derived by the GPS/GNSS chip. These will be very close in modern devices on most networks, but differences up to a few seconds are not unusual, and differences may grow substantially when off-network The displayed "UTC" at the top of the app's display running off the system clock adjusted by that amount "T adjust". That is, the UTC is the correct time.
Why is there a delay of 10 seconds?
If you have an assistant to press pause on the app, set the delay to zero and use a "ready... now!" call-out. If you're working alone, call out (aloud or in your head) "zero" and then start counting seconds until you hit 10. Then at that instant, tap pause. The time will be displayed with the notation "-10s". If you're recording sights on paper, you can do that arithmetic easily. The app of course does it automatically. The displayed altitude is correct for the moment when you called out "zero" at the start of your ten-second count.
What is Net Wx?
Wx is a common shorthand for weather (dating from early telegraphy). If you have internet access, and "Net Wx" is ticked, the app will periodically check online for weather data. Otherwise you can manually enter temperature and pressure.
What is SL pressure?
Barometric pressures inland on high mountains, e.g., are normally reported as an equivalent sea level pressure, backing out the decrease in pressure due "purely" to the thinner atmosphere at altitude. It is also possible to measure pressure directly, in which case SL pressure should be de-selected. Usually this should be left selected.
What is DOV?
This is the gravitational deflection of the vertical which is caused by gravitational anomalies, like the attraction from large volcanic peaks, and can amount to one or even two minutes of arc in geologically "young" regions. The app includes a global database of DOV values. In some areas, e.g. the US northeast and nearby coastal waters, the deflection is very nearly zero. When a signifiicant gravitational deflection exists, it is also displayed on the main page of the app. You may see this pop up when sailing in the Caribbean, for example. Normally this should be lefft selected.
Does the app work at sea?
Absolutely. You do not need internet access.The app optionally uses network access for weather data, but otherwise it receives GPS/GNSS signals off-network in the middle of the ocean. One note: when your device is off-network a position fix from GPS may take two to three minutes. Be patient. This is normal. The receiver in your device when it is on-network benefits by downloading initialization data from nearby ground stations which allows nearly instant fixes.
Will it work without built-in GPS?
If you're only using the device for sight training, you may find that you get good results without built-in GPS. The device has alternate means of determining an approximate position fix, for example, triangulating off mapped WiFi networks, and may well provide a position fix accurate enough for the app. Take note of the implied accuracy of the fix as displayed. Note for iPad users: Apple long ago decided that iPads should not normally have GPS chips. Why? We can only speculate. If you want to use an iPad for this app or any other navigation apps that access a GPS/GNSS position, you will need a "cellular-capable" model of iPad (you do not need to purchase service).
Can it detect all GPS Spoofing?
No. This celestial process can only detect spoofing greater than about half a nautical mile in scale, and of course it will not detect spoofing in a direction perpendicular to the observed celestial body. If the Sun is due east of you, and your position is being spoofed ten miles directly to the south, the actual and spoofed positions lie nearly along the same celestial line of position for the Sun. The altitude is unchanged. If you can see two celestial bodies, then you can detect spoofing in any direction. Celestial sights also cannot detect spoofing when skies are overcast (!). Celestial altitudes also have low reliability in the middle of the night.
How often should I check?
If you have any reason to suspect GPS Spoofing, take a sight of the Sun or other celestial body once every 15 minutes. The sight itself and comparison with the app should take 30-60 seconds at most. Look at the app before picking up the sextant to preset to the expected altitude. Sights are quick and easy.
When should I worry?
You should always worry. However, if differences between observations and displayed altitudes in the app are smaller than half a mile to a mile and vary randomly from one observation to the next, then you're probably seeing nothing more than observational "noise". If you see differences above a mile, increasing as time passes, then you should strongly consider the possibility that your GPS position is, in fact, wrong. Your actual position is towards the celestial body's azimuth if the observed altitude is higher than the displayed altitude, bearing in mind that any single celestial sight determines a line of position (perpendicular to the Sun's azimuth) and not a single-point fix.