Digital GPS battery replacement
 

I've seen several threads about replacing batteries in the receiver/antenna used with the digital GPS/Speedo. My Nordskog unit stopped reading last summer and I just got around to replacing the battery. The gauge is functioning again and provides volts and rpm. However it does not display
direction or speed. I'm actually not sure about speed since I haven't put the boat in the water or had someone watch it while going down the road (probably not a good idea). I know one of the threads which I have been unable to find mentioned calibration. Can any one shed some light on the need/procedure to cabibrate?

Thanks,

Ray