Questions on compass calibrations

Hi,

We're preparing for our deployment this summer and I have some questions about the Aquadopp compasses.  We're intending on calibrating the compasses on site away from their steel deployment pipes (we made that mistake last year and the compass measurements went crazy -- we take independent measurements anyway for the fixed instruments) so that the compasses will be correct when we transfer the instruments to drifters/moving profilers. 

We did a deployment recently for which we recalibrated the compasses on the beach (raised up off the sand) and then compared the readings to independent measurements.  We discovered that the accuracy didn't appear to be as good as the estimated compass error, but more strangely when we flipped the instruments upside-down to take measurements we got different readings (by maybe 5-10 degrees or so).  Any ideas why this may be so (problems with magnetite in the sand perhaps)?   Does this imply that if we need the compass readings when the instrument are downward looking that we should do the calibrations with the instruments in that orientation?  What exactly is the instrument doing when it does a compass calibration?

I should note that once we put the instruments into the bay, the readings (upward looking) agreed well with our independent measurements.

Cheers,

Dear Julia

A compass calibration is very simple - we read out the two horizontal magnetometer componenets (which should form a circle) and then we shift the data so that Mx=My=0 is the origo of the circle.   When you collect data, the compass heading is atan(My/Mx).

The purpose of the calibration is to remove the any effect of "hard iron"  (=magnetic properties are independent of orientation) objects in the vicinity of the instrument.  This means that you should preferrably calibrate the instrument when it is mounted in the frame it will be deployed in.  The purpose of the calibration is not measure the "true" magnetic field, it is to improve the quality of the compass measurements.

I hope this is not too far away from what you are trying to accomplish.

Best regards,

Atle Lohrmann

PS Sorry for the delayed resonse - we were all at the NortekUSA user sympoisum last week.

Hi Atle,

Thanks for the response - but I still don't understand why when the instruments are calibrated away from any frame, the readings are less accurate when the instrument is oriented downwards.  We're hoping to get the compass measurements right for the drifter work as we won't have time to recalibrate between moving from fixed frames (which are steel and send all compasses haywire - even with in-situ calibrations) and the plastic drifters, particularly as we can take independent orientation measurements when the instruments are in frames.

Cheers,

Julia