Check your DOP (dilution of precision) values in the GPS screen in the service menu.
The lower the DOP, the better. HDOP=horizontal, PDOP=postional
Typical good values are <2. A good car will show anything from 1.0 to around 5, subject to your current GPS constellation.
Often I will see 1.3 or 1.6, which is pretty good.

Greater than 5 is getting poor

Also, check your signal strength (S/N), typical is to have 2 or 3 atellites at around 10 to 12. Best i've ever seen is 20. More = stronger signal.
You'll generally receive 7 to 8 satellites.

A bad antenna will show around 2-3 S/N, and have more than 1 satellite with no signal

Jochen