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a. System Description
The HARS is a doppler aided strapdown inertial
reference system. The HARS provides all attitude,
velocity, and acceleration data for the helicopter. The
HARS aligns itself by adjusting its own vertical axis to
coincide with the earth’s gravity vector and by measuring
the rotational speed of the earth about its inertial
axis.
3-34 Change 4
TM 1-1520-238-10
By aligning the vertical axis with the earth’s gravity,
the HARS is able to determine the UP direction. In
measuring the earth’s rotational speed the HARS is
able to determine which direction is East; North is then
90 degrees counter-clockwise from East. Any change of
heading or movement of the helicopter during
alignment while on the ground will disrupt accurate
measurements and calculations resulting in an
alignment error. The only corrective action is for the
helicopter to be returned to a stationary condition and
the HARS realigned. Rotor vibrations at 100% Nr have
little effect on the accuracy of the HARS alignment.
Inflight, the HARS maintains alignment by
continuously gyrocompassing and estimating system
errors. Inflight alignments or restarts require valid
doppler data. The HARS may be aligned using one of
four methods. The accuracy of the HARS alignment is
dependent on the method chosen. The methods and
accuracies are discussed in Table 3-11.
The HARS uses doppler velocities to damp the drift in
the inertial data. Both the doppler and inertial
velocities are combined by the Kalman filter program
in the HARS to take advantage of the best of each. If
doppler velocities are not valid (memory or
malfunction), the HARS will reject the doppler data
and function in a free inertial mode. In free inertial, the
heading and velocities will drift in a sinusoidal
oscillation called a Schuler period; approximately 84
minutes. The HARS free inertial condition is signaled
to the flight crew by velocity vector flashing. The most
likely cause for the HARS to reject doppler data is that
the doppler is in memory. The doppler normally returns
valid data over flat terrain. Over areas of tall grass or
water, doppler data inconsistencies can develop
resulting in a memory condition and invalid doppler
data.
Change 4 3-34.1/(3-34.2 blank)

TM 1-1520-238-10
Over mountainous terrain, doppler data is better than
over grass or water, but not as good as over flat terrain.
The HARS will reject doppler velocities as long as the
memory or malfunction condition exists. In addition, if
the free inertial condition exists too long, the HARS
inertial velocities will drift enough so that when the
doppler data again becomes valid, the HARS will
continue to reject the doppler velocities because they are
no longer within the capture window of the Kalman filter.
If this occurs, the available corrective actions are limited.
The pilot can slow the helicopter to less than 40 KTAS in
an attempt to let the HARS Kalman filter recapture the
doppler velocities. If this fails, the only remaining
corrective options are either to land the helicopter and
when stationary place the HARS control switch in NORM
to cage the HARS inertial velocities to zero, or to attempt
an in-flight alignment (restart) of the HARS. There is
always the option to continue flight in the free inertial
condition, realizing that everything that uses the HARS
data (HAS, flight symbology, navigation, and fire control)
will be degraded to the extent that the HARS has drifted
and will continue to drift.
The HARS computes error estimates and accelerometer
biases during flight. This data is stored by the HARS as
mission data memory on shutdown. The mission data
memory allows the HARS to maintain a running
calibration of its internal instruments. If, however, the
HARS has experienced more than 12 minutes total of
free inertial since it was turned on it will not update the
mission data memory on shutdown. The mission data
memory can be most easily corrupted by moving the
helicopter during alignment and not realigning before
flight. Extended free inertial and corrupt mission data
memory are the two primary causes for inaccurate
navigation in the non integrated system.
HARS accomplishes internal bit and temperature stabilization
(for approximately 90 seconds) prior to initiating
alignment. The status of the HARS is continuously
monitored by the FD/LS; the on-command FD/LS test
(test 05 HARS) will fault isolate.
The HARS receives 28 vdc from the No. 3 essential dc
bus through the HARS DC circuit breaker and 115 vac
　
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