User manual

6. Turn the profiler and accurately align the sting with northeast. Make sure the
profiler is vertical and record the compass measurements for an additional ten
seconds or more.
7. Continue in this manner through the remaining six cardinal points of the
compass. Compass calibration quality depends directly on the accuracy with
which the sting is aligned to these eight headings. This portion of the spin test
data will be used to calculate the bias angle.
8. The bias angle data should be reduced to eight elements, one associated with
each of the eight cardinal directions. The operator has the option of selecting a
representative measurement or calculating an average for each heading from the
data collected in the second portion of the spin test. The spin test data used
above to illustrate the calculation of offsets and scale factors are an example of
eight element data.
ACM Compass Calibration Step 5 – Removing the Bias Angle
To remove the bias angle, you first need to calculate five compass correction parameters,
X
O
, Y
O
, X
S
, Y
S
, and B
T
. The latter may have some non-negligible dependence on the heading
angle, but it is the offset and scaled heading angle, θ
OS
, the arctangent term in the equation below,
that is used to make the correction, not θ
H
. Several methods by which the parameters can be
calculated from field and spin test data are described next.
Two sources contribute to the total bias angle. First is the bias angle, B
C
, associated with
the compass module itself (B
C
is an inherent characteristic of each compass) B
C
generally has a
small angular dependence (it may, for example, be larger to the south than to the north).
Interpolation can be used to make a bearing dependent correction or ignore the small differences.
The second source of bias is the mechanical pointing error, B
M
. Mounting misalignments
between the compass module and the ACM electronics housing and between the ACM housing
and the MMP frame and sting both contribute to B
M
. To accurately account for the latter
contribution, the spin test and the deployment must be conducted without removing the ACM
housing from the bracket that secures it to the profiler frame. Removing and remounting the
housing introduces a compass rotation relative to the MMP frame and sting. For the same reason,
refrain from inspecting the ACM electronics in the interval between the spin test and the
deployment. B
M
is a constant for a deployment and can be kept small through careful mechanical
alignment of the ACM housing in the frame of the MMP.
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