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|  MIL-D-70789A (AR)
determined after coarse gyrocompass. The stored azimuth shall be
considered invalid if excessive vehicle motion is detected during
Align. The stored heading also shall be considered invalid if
the DRU orientation (orientation 1 or 2, as specified in
3.5.3.2.2.1) during initialization differs from the orientation
at the last shutdown.
When in the Odometer Mode and having successfully completed
Stored Heading Align and entered the survey mode, the DRU is
allowed to request ZUPTs, at the Exclusive ZUPT Mode interval
(3.5.8.5), until a total of 5 minutes of stopped time has been
accumulated.
3.5.3.2 Survey. During survey, the DRU shall provide
position, orientation and angular rate parameters to the
accuracies specified in 3.6.1.1. The DRU shall have two survey
submodes: Odometer Mode and Exclusive ZUPT Mode. The DRU shall
determine when the vehicle moves and shall immediately set bit 2
of STATUS character S3. The DRU shall determine within 5 seconds
when the vehicle is stopped and shall reset bit Z of STATUS
character S3. The DRU shall provide positions regernced to the
spheriod designation stored/input during initialization. The DRU
shall provide positions and grid azimuths in the British National
Grid i$ the spheroid designation stored/input during
initialization is No. 0. Otherwise the DRU shall provide UTM
positions and grid azimuths in the normal zone or extended zone
as designated by character D6 of the last received and accepted
ACCEPT POSITION command (which may have been received prior to
the last shutdown). An exception is when the Configuration
Definition Flag for Azimuth (3.5.8.8g) is set. In which case the
DRU shall provide positions In the British National Grid or UTM
normal zone or extended zone, as designated, but azimuths shall
be geodetic.
3.5.3.2.1 Positioning. When in the Survey mode, the DRU
shall continuously determine position as it is moved. The DRU
may be moved by movement of the transporting vehicle. While the
vehicle is stationary or the DRU is operating in the Exclusive
ZUPT Mode the DRU mounting surface may move with respect to the
vehicle chassis. The DRU shall perform as specified herein with
no external aiding except odometer signals or zero-velocity
updates. The positions provided by the DRU shall be for the
applicable vehicle (Orientation 1) or pointing device
(Orientation 2) offset point (3.5.3.2.2.1 and 3.5.8.3).
3.5.3.2.1.1 Zero-velocity updates. The DRU shall utilize
zero-velocity updates (ZUPTs) to control system errors.
Zero-velocity updates shall be initiated automatically whenever
the DRU determines that it is stationary. During survey,
zero-velocity updates shall not exceed 30 seconds in duration
15
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