GPS off track

[sunquest]

My Cetrek 775 has performed well over the past few years but now shows my position some six miles away. It is receiving nine to eleven satellites and all the checks in the handbook show all is in order. Any ideas. Thanks in advance Geoff.

 

[colingr]

Have you cheked the datum?

 

[jfm]

You don't describe the exact hardware and architecture so this is bit of a guess. Likewise you dont say if the 6miles is a constant offset or if it's fluctuating all over the place. Suggest check-

1. which datum is selected, though 6 miles is a lot
2. whether manual chart offset data has somehow got entered in the plotter - just zero it
3. Take the nmea feed from the GPS and squirt it into some other equipment to see if it's correct. Then you'll know if your GPS engine or your plotter is the culprit. Or, squirt a fresh source of GPS into your plotter to get the same answer

Serious point: you should always (imho) have at least 2 GPS engines on a boat. They generally live up a mast so they're gonna fail before other electronics. And they're cheap as chips now