Hello, the sat nav in my 2013 xf sportbrake sometimes doesn't always know where I am , I can be driving along and the sat nav displays my current position about 30 miles away then other times it's accurate, has anyone else experienced this and how can I correct it?

 

The gps aerial is in the "shark fin" on the roof, could be moisture ingress.

 

Thanks for your prompt reply John, I've looked online to see how to remove the "shark fin" but can only find a step by step guide for the saloon, which looks fairly straightforward as opposed to the removal on the sportbrake which is in a different location, why!!?