OK, to avoid confusion, this is how the GPS units work:
A true "Vehicle fitted" GPS system will use a gyroscope (compass) to determine which heading it is on, and then will advance in that direction in accordance to the distance increments it has been given by the speed pulses from the vehicle. As it move, the current position will then be plotted on a stored map and the position will then be confirmed annd adjusted by taking a secondary reading from three or more sattellites to give an accurate location and adjust any innacuracies every 30 seconds or so. If it cannot see the sky (or GPS signal) then it will remain fairly accurate as it will constantly receive measured and accuate distance increments in a known given compass heading, and when satellite reception is restored, the position can be adjusted where necessary. Using the GPS as a secondary position also eliminates the errors implemented by the authorities to ensure that the GPS is not totally accurate for positions, and anyone who looked at GPS signals at the start of the Iraqi war will have also seen some dramatic shifts in positions to ensure that the technology was not used against the forces for firing anything.
A Handheld GPS system, the sort that are great for Hikers and Bikers that have appeared now for In-Car use such as Tom Tom / Garmin etc work in a very different way. Some have a gyroscope of sorts (albeit a much inferior compass) to determine which heading it is on, but do not have any form of speed input to determine accurate measurements in a given direction. The current position is determined by receiving data from three or more satellites - this, as always is line of sight from a visable sky above, and can be "confused" like the car ones by tall buildings and tunnels that deflect the signals, however, unlike the car ones using GPS as a secondary position, the hand helds use this as a primary position. As you move the current position will then be again plotted on a stored map, but this is done on a continual basis, which is why "over land" speeds can also be monitored. If it cannot see the sky (or GPS signal) then it will not know its position, but to prevent it from switching the map off, they implement a small amount of intelligence to keep heading in the same direction at the same speed until the GPS signal is restored. If you still beleive that you are receiving a GPS signal in a tunnel, then heres a little test to dispell the myth. Drive into a long tunnel very fast and then stop and reverse back out (do not turn round and activate the compass) and you will probably see that the screen shows you continuing through the tunnel as orginally planned. There is also very little in the form of error correction which is why they can often be unable to distinguish between two parallel roads that are close together - A602 and A1M in Stevenage are a great example of this, and virtually every handheld unit we have tested has failed on this stretch of road.
Hope this now adds to some of the confusion.
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Topic: PHILIPS CARIN SATNAV (Read 2006 times)