PNI CompassPoint Prime - Manual (Page 35)

Prime User Manual r08 Page 29
6.2.3 Simultaneous Mag and Accel Calibration
The Prime allows for a simultaneous magnetic sensor and accelerometer field calibration.
This requires a good 18 point calibration pattern, stable measurements, and installation in
the user’s system. The Accelerometer Calibration Pattern discussed in Section 6.2.2
works well for a simultaneous calibration. Optimal performance is obtained when all
rotations of the cube are performed towards magnetic north to achieve the widest possible
magnetic field distribution.
Note that a Mag and Accel Calibration only makes sense if all the host system’s magnetic
distortions (steel structures or batteries, for instance) are present and fixed relative to the
Prime when calibrating. If Accelerometer Only Calibration is performed, the user’s
system distortions are not relevant, which allows for the Prime to be removed from the
system in order to perform the Accelerometer Only Calibration.
6.2.4 Calibration Scores
Once calibration is complete, the Prime provides calibration scores indicating the quality
of the calibration. These are found in “Calibration Results” on the Calibration tab in
StudioPrime or in the kUserCalScore payload. These apply to both magnetic sensor and
accelerometer calibration. For magnetic sensor calibration, a score of ≥85% is desirable
for the X and Y vectors. The only way to get a Z value greater than 50% is to take
calibration points with the unit upside-down as well as right-side-up. For accelerometer
calibration, a score of ≥95% is desirable for the X and Y vectors, and ≥90% for Z vector.
The value for Mag Score should be ≤0.1 and the value for the Accel Score should be ≤2.
6.3 Declination
Declination is the angle between true north and magnetic north, where a positive value means
true north is east of magnetic north. As such, declination depends on geographic location. It
also changes very slowly over time. The Prime’s default is to output magnetic heading. To
output true heading, use the kSetConfig command to set the kTrueNorth parameter to TRUE
and to store the correct declination angle, kDeclination. (See Section 7.3.6.)
For the greatest accuracy, PNI recommends checking the National Geophysical Data Center
website (below) to get the declination angle based on your latitude and longitude:
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp