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| MIL-D-70789A (AR)
from the time the vehicle is stopped. A zero-velocity update
shall be terminated upon detection of excessive DRU motion.
Early termination of a zero-velocity update shall not degrade DRU
performance. A zero-velocity update is either fortuitous or
mandated. It is fortuitous if it is initiated without a request
from the DRU, i.e., when bit 2 of STATUS character S1 is reset.
It is mandated if it is initiated at the request of the DRU,
i.e., when bit 2 of STATUS character S1 is set. When a mandated
zero-velocity update is initiated, bit 2 of STATUS character S1
shall be reset. If excessive vehicle motion is detected, during
the mandated zero-velocity update, bit 2 of STATUS character S1
and bit 1 of ALERT DATA character D2 shall be set. Bit 1 of
ALERT DATA character D2 shall not be reset until a zero-velocity
update is completed. Whenever a mandated or fortuitous
zero-velocity update is initiated, bit 7 of STATUS character S2
shall be set and reset when the zero-velocity update is completed
or interrupted. When either a mandated or fortuitous
zero-velocity update has been completed, but the DRU remains
stationary, the DRU shall initiate and attempt to complete
additional zero-velocity updates. These additional zero-velocity
updates shall be considered fortuitous.
In the Odometer Mode, the DRU may mandate ZUPTs, no more
frequently than the Exclusive ZUPT Mode Interval (3.5.8.5), when
severe road/terrain conditions, such as soft sand, mud, snow, or
ice, result in excessive erroneous odometer data. The DRU may
mandate a ZUPT when gyro parameters must be adjusted because of
severe thermal rates. Neither of these exceptional conditions
shall be interpreted as relief from the requirements that the DRU
be interchangeable and operate as specified herein without
external calibration, vehicle driving constraints, or unscheduled
ZUPTs (3.5.1 and 3.5.3.2.1.4).
In the Odometer Mode, the DRU shall not require zero-velocity
updates more frequently than once per hour of travel time to meet
the accuracies specified in 3.6.1.1.1 and 3.6.1.1.2. In the
Exclusive ZUPT Mode, the DRU shall not require zero-velocity
updates more frequently than once per every 4 minutes of travel
time to meet the accuracies specified in 3.6.1.1.3 and 3.6.1.1.4.
In either the Odometer Mode or Exclusive ZUPT Mode, the DRU shall
inhibit zero-velocity updates on receipt of the INHIBIT
ZERO-VELOCITY UPDATE command (30.3.12) and shall reinstate
zero-velocity updates on receipt of the ENABLE ZERO-VELOCITY
UPDATE command (30.3.13). These commands may be used during
laboratory evaluation.
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