GPS Nav problem

My GPS Nav system in my 2006 I-Tech Prius is not working correctly. It seems to have lost contact with the GPS satellites or is not calculating the correct spot where the vehicle is.
I drove about a 100 km along a highway and it tracked the highway for about 10 km then decided that I was no longer on the highway. It showed me passing the same piece of real-estate 13 times during the remainder of the journey.
Anyone any clues what could be wrong?

 

Thanks Tom, nothing on the dash and no tinting.
I suspect a poor antenna connection but it could be the receiver that is the problem.
I am taking it to the dealer tomorrow but I fear that I know more about the Prius than they do.

Orf

 

brad_rules_man, that sounds like a good problem with your nav.

I went to the dealer and I was correct in my assumption that I would know more than the dealer.
Having said that, he brought up the diagnostic display which indicated EXCH for the navi. the code was 58 - 13. This code was not on the service sheet and the tech had to go away and make a phone call to find out what to do.
He returned to say I needed a new MFD. The car is still under warranty so cost should not be a problem.
However, seeing the nav is the only problem I am having, I would have thought the fault would be elsewhere.

 

I had a look at the satellite info and found that I am only receiving data from 2 or 3 satellites at a time.
What would cause this type of problem?

 

Previously when the GPS was not working there were no obstructions either on the dash or windscreen and the car was standing in an open space - no trees, no buildings.
This morning I drove about 1km and stopped in a clear area. The nav was working correctly.
I brought up the GPS display and I was receiving about 8 or 9 satellites. As I was counting them, all but 3 suddenly dropped out at the same time. The car was not moving or the engine running so vibration can be ruled out as can shielding of the antenna.

 

I had another look at the GPS this afternoon and only had 2 sats both in the same quadrant. I drove the car to face N, S, E and W and did not pick up anymore sats or lose the ones I had. I presume that the figures after the elevation and heading is the signal strength. For the sat at 70 elevation the signal strength was 87 and the one at 27 elevation was 60.
I then drove into my garage and the signal strength varied between 0 and 20.
To me this indicates the antenna is working OK.

 

The dealership changed the disk in the GPS/Nav system this morning in an effort to fix my problem.
I had doubts that this would do any good but in fact it has enabled more satellites to be picked up. With the original disk I was only getting 2 or 3, with the new disk I am getting 8 or 9.
I will keep you posted to the result after I have used the system for a few days.
One good thing has come out of it, I now have a mapping disk that is current.

 

I have not experienced any problems with the GPS after running with the new disk for several days.
It looks like a bad disk was the problem.