GPS units losing signal

I have a Garmin C550 and a TomTom 930T, which I use in my motorhome. The motorhome is also fitted with a Phantom Pro-Active tracker, which uses GPS and GSM. At random times both sat nav units report a loss of GPS signal at identical times. The loss can then be for any duration of time up to possibly 1 hour. When the signal returns it is again seen by both units at the same time. It can then be lost several more time before stabilising.
When the signal is lost, I can remove the sat nav from the vehicle and walk away and the signal will then be detected.
I have tried an external aerial and that has given some relief to the unit mounted on the passenger side of the vehicle.
I have spoken to Phantom, who tell me that it is not possible for their unit to suffer a fault which would cause this phenomenon.
The motorhome is based on the new Fiat Ducato, which uses Canbus wiring.
I am unable to create this situation at will.
Can anybody help?
Gerry

[getsurfers]

If it has a coated UV screen then this is your issue.

Also, locating the GPS in the lower corner nearest the door helps in vans.

It could be that, due to the van's construction, the GPS can only see signals in certain directions rather than the whole sky so sat position could be the problem (unlike sat TV the navigation sats are moving relative to the earth). I have seen this in some trucks and an antenna on the top of the roof helps

[mikealder]

Does it do this loss of signal in the same place each time or can it occur anywhere?
If its always in the same location then something around that position must be sending out RF at 1,575GHz which is the frequency the GPS devices use.
If it can occur at any location but only in the same vehicle I would start to look for something that is switching on and causing RF at 1,575GHz I very much doubt it would be the CAN bus in the vehicle as thats quite a slow data transmission system by modern standards.

You need to try and work out what is switching on when you lose the GPS signal, something like the engine cooling fan?? Whilst DC motors can generate RF I have never heard of one killing the GPS signal - Mike