I have the factory nav. 502a high build. But I went up a canyon a few weekends back and the nav went gray and said "no signal". Even when we were out and in an area with poor signal, the nav came back on but placed the truck hundreds of yards off the road.If the truck has "built in navigation" then you will have navigation all of the time via, GPS. If it doesn't have built in navigation then the system is only relying on cellular connectivity.