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MIL-D-70789A (AR)
British National Grid on the Airy spheroid.
6.6.1.5.1  Spheroid  (BCU}.  The BCU Spheroid code number
indicates which spheroid parameters are being used by the BCU.
The BCU spheroid is used to compute Pointing Device Azimuth
output by the DRU. The code number is input to the DRU. When
output is requested, the DRU repeats the last value input.
6.6.1.6  British  National  grid.  The  British  National  Grid
has only one zone in the northern hemisphere. The zone is given
the numerical designation of zero in output messages. It is
assumed that the zone boundaries are never crossed. The British
National Grid is defined in the Ordnance Survey Information
Leaflet, No. 72, March 1983.
6.6.2
Altitude. Altitude is the distance between the
present DRU position and the reference geoid (an equipotential
surface of the earth approximated by mean sea level) measured
along the local gravity vector (normal to the geoid). Altitude
is positive when the DRU is above the geoid and negative when
below. The DRU will accept starting and update altitudes for its
present position. When requested, the DRU will output its present
altitude.
6.6.3
Orientation. DRU orientation is defined by the
relationship of the DRU case coordinate frame to an earth
referenced coordinate frame. An intermediate DRU coordinate
frame relates the case coordinate frame to the DRU mounting
orientation. A suitable DRU coordinate frame and set of
boresight angles should be selected for each installation to
insure proper definition of orientation parameters during
pointing device deployment (DRU rotation).
DRU reference frames.
6.6.3.1
6.6.3.1.1 Case coordinate frames. The DRU case reference
frame is a set of right hand orthogonal axes,
, shown in
Figure 27.
The origin is located where the center axis of
alignment hole R, labeled DATUM axis V in Figure 9 intersects the
precision mounting surface, DATUM plane G-U, Figure 10.  axis x=
is along the intersection of the DATUM plane G-U and Section A-A,
Figure 10, which passes through the centers of alignment holes R
and D.  Axis
coincides with DATUM axis V in Figure 9. The
axis
is along the intersection of DATUM plane G-U and DATUM
plane Z, Figure 9, which passes through the center of the
mounting holes.
6.6.3.1.2  DRU Coordinate frame. The DRU corrdinate  frame
is an intermediate set of right hand-orthogonal axes,
as shown in Figure 28a. The case coordinate frame is related to
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