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|  MIL-N-81604C(AS)
Para 3.4.4.1f
(cont)
Calibration Data Word Sync (To the IMU - This
(5)
sync pulse shall be provided for use in loading
the IMU memory from the ANCU during bench test
*
or from test equipment designed in accordance
with XAS-1233, or equivalent. The sync pulse
shall be true (high) coincident with the data
label MSB. While in the aircraft, this line
should be set low at all times in order to
guard against inadvertent IMU memory modifica-
tion. The line driver in the ANCU and the line
receiver in the IMU shall be Type A circuits.
Calibration Data (To the IMU) - The gyro biases
(6)
*
and scale factors and the accelerometer biases
*
and scale factors shall be transmitted from
*
the ANCU or test equipment designed in accor-
dance with XAS-1233, or equivalent, to the IMU
*
to load the IMU memory. This data line need
not be connected to the ANCU in the aircraft.
It may be connected to the ANCU while bench
*
Testing. The list of calibration data func-
tions along with the label range, LSB, and
*
number of data bits is shown in table VIII.
*
The gyro and accelerometer scale factor correc-
tions (SFC) shall be used to derive the veloc-
ity (V) or angle ( θ ) represented by a number
of pulses (N) as described in the following
equation: V OR (N)(SF)(l+SFC) where SF i s
the nominal weight/pulse. Scale factor correc-
tions shall be defined as positive when the
weight per pulse is greater than the nominal.
The line driver in the ANCU and the line re-
ceiver in the IMU shall be Type A circuits.
Platform Attitude and Heading - Prior to IMU Ready,
g.
the platform attitude and heading signals may be
used in the IMU with internally supplied excitation.
If this is done, the IMU synchro and resolver out-
puts that are used internally to the IMU shall be
*
disconnected within the IMU and are, therefore, not
available at this time. IMU synchro and resolver
outputs which are not used internally shall be avail-
able externally to the IMU and will be excited by
the IMU with frequencies other than 400 Hz at this
time. Refer to table IX. Following IMU Ready,
47
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