Before 1:25, HMS Devonshire's RN Commander (three rings of gold braid of equal length) asks

Are we absolutely sure of our position?

The Lt Cdr (two medium gold braid stripes with one thin gold stripe running in between) answers

Yes, Sir. An exact satellite fix.

enter image description here

After this screenshot, we see Carver's cyber-terrorist Henry Gupta meaconing the GPS signal using an encoder obtained at a terrorist arms bazaar, and Carver's company's satellite overtopping a US satellite. Gupta's undeniably hornswoggling the HMS Devonshire's satellite fix, but I don't know how.

Was Lewis Collins on YouTube correct? He proclaimed:

in reality they’d absolutely have a fix of their position without using GPS.... but that wouldn’t make for a very good film would it...

  • Yes, ships have been knowing their locations for hundreds of years before the invention (and use) of GPS. Nov 29, 2019 at 8:37

1 Answer 1


GPS is only one of several methods used by the Royal Navy and the British military in general to determine location, and military personnel are trained not to rely solely on GPS.

Ships, in particular, as well as vehicles, also make use of inertial navigation systems. These are not as accurate as GPS, and need to be recalibrated fairly frequently, but if there were a sudden discrepancy as large as that depicted in the movie, this would be a red flag.

The navigation officer would also be charting the movement of the ship over time, including as it changed course and changed speed. The military uses technology, but is always prepared for its failure. Again, this is not entirely accurate as sea currents and wind can affect the motion of travel. However, this is another check/datapoint for the navigation officer, and he can even account for environmental factors.

Synopsis for a course in navigation for Royal Navy officers

So, yes, the ship's navigation officer will have more than just the GPS signal to plot the ship's current position, and probably should have realised the ship was not where the GPS said it was.

  • What if the discrepancy weren't sudden, but had been applied gradually over some time?  Wouldn't the Devonshire have wrongly recalibrated its inertial navigation system, so that by the time shown, it would agree with the (wrong) GPS fix?
    – gidds
    Jan 1 at 16:07
  • @gidds I don't know about INS for ships - but vehicle INS is (or at least at the time the movie was made) calibrated by parking on a well surveyed point (to under a metre accuracy) that has been marked on the ground, not against GPS.
    – HorusKol
    Jan 5 at 10:19
  • Yes, but land is still, so land vehicles can rely on measuring their motion against it.  That's not applicable to vehicles moving through water (or air), which is itself in motion — hence (AIUI) the need for regular recalibration.
    – gidds
    Jan 5 at 14:08
  • @gidds ships are pretty still when they're tied up alongside in harbour, and planes are frequently still at airports in between flights.
    – HorusKol
    Jan 5 at 21:49

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