User manual

Table Of Contents
The bias angles are simple additive errors to the sting heading angle, θ
H
(it is the total bias
that is of concern, so it is unnecessary to differentiate between the two sources). To measure the
total, measure the difference between the heading calculated using MX and MY, the offset and
scale corrected measurements, and the true heading, as measured using the hand compass.
Consider this expression for the sting heading angle in a true north Cartesian earth frame:
DevPMCH
MBBB
MY
MX
++++
=
1
tan
θ
M
DEV
is the magnetic deviation, the bearing of magnetic north measured from true north in
mathematical compass coordinates. M
DEV
is positive in the North Atlantic and negative in the
North Pacific. B
P
is the error between the actual magnetic bearing of the sting and the target
magnetic bearing during the spin test. M
DEV
for any location on the earth can be looked up in a
reference and used to correct the heading angle. The total bias angle, B
T
= B
C
+ B
M
, can be
measured, but the accuracy of that measurement is limited by the magnitude of B
P
. This is why
good alignment of the sting using the hand compass is a critical part of the spin test. A good
alignment means that B
P
is negligibly small, leaving us with:
DEVTH
MB
MY
MX
++
=
1
tan
θ
Working in the magnetic or compass frame, B
T
can be measured at each of the eight
cardinal points of the compass:
=
MY
MX
B
HT
1
tan
θ
θ
H
is the actual magnetic bearing in mathematical compass coordinates (90° for magnetic north,
45° for magnetic northeast, 0° for magnetic east, etc.). MX and MY are derived from the raw
measurements and the corrections obtained by mapping those measurements to the unit circle.
You can perform this calculation with a hand calculator, or use the Nelder-Mead simplex
minimization provided by McLane. The calculation of B
T
at the eight cardinal points is triggered
whenever the raw data being processed include exactly eight elements in a file named
SPINTEST.TXT. The calculation assumes the data are in north, northeast, east, southeast, south,
southwest, west, northwest order.
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