I know the Garmin had some advance unit that can take speed signal from the car. The website claimed, with the speed signal and the internal compass inside the unit, it can provide a simple navgation without the true Sat feedings.
I used to use a Garmin all the time, and then a TomTom - but would find that as it only used satellites (the units just clipped onto the dash) you would lose your signal in built up areas and in tunnels. Navigating in Brussels for example was impossible as you are in and out of underpasses all the way into the city centre - by the time it had gotten a signal again, you were lost. :lol:
The speed signal, combined with the compass built into OEM Navigation systems mean that the car will continue to navigate fairly well even in the absence of a GPS signal - something impossible for a TomTom or Garmin.