GPS error

[Bob]

I was in visual range of a buoy and my GPS ( Magellan -Meridian XL )indicated a speed several knots less than my knotmeter and a waypoint direction about 15 degrees off my compass direction. It stayed that way for several minutes. I shut the GPS off and turned it back on a few seconds later and both readings were now correct. What could cause this? My GPS is connected to the boat battery which has plenty of juice. I may have turned the engine on while the GPS was on already but don't know if that could make a difference.

 

[Tron Jockey]

Most likely one of two things

It is possible that a voltage sag/surge when starting the engine caused the unit to get itself upset. It is possible that a bit got twiddled someplace but the reset circuit didn't trigger.However, most often when this sort of thing happens it is because the unit is looking at different satellites than it was using before the restart. You may have had poor satellite geometry which was causing a big error. Did the unit indicate a large EPE? After the restart the unit may have found a signal from a different satellite that helped the solution converge and give a much better EPE.BTW, if you have 2 GPS units running and they show different positions, they almost certainly are using different satellites. (Maybe just 1 different satellite out of 6.) There is very little analog processing when it comes to finding the position solution, so they don't get "out of calibration" the way old time circuits did.

 

[R Hebert]

GPS problem (a good link)

I have no personal expertise to offer, but I want to recommend this site:http://joe.mehaffey.com/RegardsReg

 

[BILL SHAKUN]

GPS

Check the date and time on the unit.

 

[Bob]

date & time?

What is the purpose of checking the date and time? When should it be checked? ...Now?? What will it mean??

 

[Ron Koenig]

GPS Problem

A possible reason for the errors is that the GPS was displaying the information based on your original course. The compass direction the GPS was telling you to steer was one to get you back on your original course. This happend to me the first couple of times I used my GPS until I read the book and found out exactly what the numbers ment.