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|  MIL-D-70789A (AR)
Coordinate and reference frames shall be as defined in 6.6.
The DRU shall accept the following types of damping to improve
inertial survey accuracy:
a.
Odometer.
b.
Zero-velocity updates (ZUPTS) when vehicle is
stopped.
The DRU will be used in configurations in which both types of
damping will be available, i.e., continuous odometer pulses when
the vehicle is moving and ZUPTS whenever the vehicle is stopped.
When a Vehicle Motion Sensor (VMS) is not used in the
configuration, or odometer data is unavailable because of a
malfunction, or when the DRU is commanded, periodic zero-velocity
vehicle stops at regular intervals will be used to provide
damping. This mode of operating shall be invoked automatically
if odometer data becomes unavailable or erroneous because of a
malfunction.
The DRU shall be interchangeable from prime system and vehicle to
vehicle without requiring hardware changes, calibration
procedures, vehicle driving constraints or unscheduled ZUPTS.
3.5.2 Configurations in which the DRU will be used. Several
possible system applications for the DRU are depicted in Figures
1, 2, and 3. Figure 1 is an odometer aided inertial survey
system with a Control and Display Unit (CDU) used for system
control, data entry; and data display. Figure 2 shows a similar
survey system with control, entry and display functions shared
between the CDU and prime system equipment. In this configura-
tion, the prime system can limit the ability of the CDU to
control the DRU and enter data. Figure 3 is a zero-velocity
aided inertial survey system without odometer input.
3.5.3 Operating modes. The DRU shall have two main
operating modes: Align and Survey, with performance to survey
accuracies (3.6.1.1). In addition, the DRU shall have two
transport modes: Air Transport and Marine Transport. The
transport modes allow the DRU to be transported, while operating
with degraded performance, for extended periods without having to
stop enroute or align the system at the destination. A position
update at the destination is required to restore position
accuracy after transport. To maintain survey accuracies while
transporting the DRU by air or water, the Survey mode should be
used and ZUPT requirements observed. Requirements for the
operating modes are given in the following subsections.
7
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