Dear PC,
I have tried to be a clear in my description of the potential error and how it can occur, but you seem to miss-interpret my examples as real life situations.
Yes the satellite transmissions will travel at the speed of light, and the system uses that constant, but the speed of the signal is not in its self important, but the time delay between signals from different satellites reaching the GPS receiver is. By knowing the relative position of the satellites the time difference between the signals will allow the GPS to calculate the relative distances from each of the satellites and by triangulation can calculate a fix.
Fact - if a GPS generates positional errors when making a fix, The any speed calculation based on that fix will its self be erroneous.
It is also a fact that some police forces have suggested a zero tolerance should be adopted to speed limits. As has been stated, the EU construction regulations only permit car speedometers to show the accurate speed or greater than. This would enable cameras to be set at precise limits, because no car should indicate less than its real speed.
Until zero tolerance is adopted than the margin or leeway on camera trigger speeds is not prescribed in any law or regulations but is down to the local authorities, and the margin varies across the country.
Each camera is set to trigger at some speed which we will call X. whilst it may not be equal to the speed limit it is still a precise setting which, if a vehicle exceeds that speed the camera is triggered.
The point is that if you have knowledge of what a particular camera's trigger speed (x) is, you may be tempted to use your 'accurate' Sat Nav to match that speed. And again I make the point that if the sat nav is indicating any speed to 2 significant figures (i.e. 0 to 99 mph) what is your actual speed? For any indicated speed the digitisation error, and the truncating of the speed could put you speed anywhere between +/- 1 of the indication. Without the sat Nav Manufacture defining the the way the decimal parts of the speed are truncated you have to assume the worst case.
So if you know a camera is set to for example 35mph, and your sat nav indicates you are travelling at 35mph, with the digitisation error you could be travelling as slow as 34.1 and 35.9 , which is sufficient to trigger the camera. The same principal applies to any speed you care to choose.
These digitisation errors are on top of the positional fix errors that occur - so it is foolhardy to accept as beyond doubt the sat nav display of speed.
Now if you friends find sat nav to be as accurate as their own conventional measurements, ask them why they don't simply use SAT Nav and dispose of all their expensive traceable instrument's . - probably, it is not accepted as authoritative evidence because the instrumentation cannot be verified through national standards. As it is dependant on the motion of the satellites and as a result the number and identity of satellites that are in view continually change, there are too many variables to be able with absolute confidence to rely on the indications of a SAT Nav as a definitive indicator.
Please continue to use Sat Nav as a way to more accurately establish your speed, But understand that there are potential errors in the reading it gives. do allow for the digitisation error not assume it is correct all the time, and also.
This is my last word on this thread.