You can try pulling the fuse from the fuse box, but normally if your drive around in an area where there is clear sat coverage (i.e. without tall buildings, flyovers, tunnels etc) it should recalibrate - unless there is a fault on your Sat or antenna.
How many sat is your SatNav receiving in engineering mode?
my sat nav in my ml320 shows me off road more than on - I'm sure I would have noticed!
does anyone have any suggestions. I note below ulling the fuse which i'll do tomorrow morning and will see if its picking up satalites (although not sure what engineering mode is).
in the "retrofit" position - i.e on top of the instrument cluster (under the dashboard though) or under the back parcel shelf (above the metal supports) as GPS goes through glass/plastic/etc but not metal