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|  MIL-D-70789A (AR)
6.8.3 Data staleness.
The following is a description of the staleness of data
transmitted in a DRU data message.
DRU attitude and angular rate data are updated on a fixed
computational cycle (i.e. 200) Hz). Position and velocity data
are updated at a slower rate (i.e. 12.5 Hz). There is a fixed
computational delay between the time the raw inertial
measurements are made and the time the update is made (i.e. 0.83
msec for attitude and angular rate data).
DRU communications are asynchronous with the computational
cycles. Therefore, there will be a variable delay between the
last parameter update and the start of a message. This delay
will be linearly distributed in the range of O to the computation
cycle interval (i.e. 0 to 5 msec for attitude and O to 80 msec
for position.)
The time to transmit a message varies with the length of a
message. The longest 100 Hz message has 12 data characters for a
total of 136 bits or 3.54 msec of transmission time. The longest
5 Hz message has 26 data characters, totaling 264 bits, or 6.88
msec transmission time.
Data staleness = Fixed Delay + Update Delta + Transmission Time
As an example, the minimum and maximum data staleness for the
longest attitude data message (100 Hz) are:
Delay Type
Minimum
Maximum
Fixed
0.83 msec
0.83
msec
Update
0
5.00
msec
Transmission
3.54 msec
3.54
msec.
Total
4.37 msec
9.37
msec
142
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