If by SAT NAV, you mean a GPS (that has the little warnings about the speed cameras), no problem. What is illegal in France is a radar detector.

The automatic camera locations have to be disclosed publicly, and as such are picked up by the GPS companies for download to your maps. The police also set up classic radar traps (basically, a car with a couple officers and a mobile radar gun - and those can be anywhere along the side of the road. Your GPS will do nothing for you in spotting those.

Now, if your GPS is mounted in such a way that it looks like a radar detector, you could be stopped. Or, if there is a GPS out there that includes radar detection capabilities... that WOULD be illegal.
Cheers,
Bev