TM 1-1520-238-10
Change 4
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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 gyrocom-
passing 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 re-
ject the doppler data and function in a free inertial mode.
In free inertial, the heading and velocities will drift in a si-
nusoidal 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 dop-
pler 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.
