We have had several reports of some vehicles where the GPS (in some cases multiple brands / models of GPS), do not get a satisfactory signal fix.
In some cases, it takes a long time to get a fix, or gets a fix relatively quickly, but then loses the fix intermittently.
Now, obviously, this could be related to a fault with the particular GPS involved, but in many cases when the unit is tested, there is no problem with it.
It seems that there might be another cuplrit.
Some vehicles are showing up which have micro-fine tungsten heating elements in the windscreen as a heating mechanism. We have also heard of some vehicles which have tungsten as some form of strengthening agent in the windscreen.
It appears that these windscreens act as a type of shield for the GPS signals, either blocking them entirely, or making the reception of marginal signals more difficult (when satellites are not in good positions, or the weather is extremely poor).
If you know your vehicle has this type of windscreen, then the only real solution is to use a re-broadcasting or re-radiating antenna in your vehicle.