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PRIMUS
r
880
Digital
W
eather
Radar
System
A28-1146-102-00
Radar Facts
5-18
STABILIZATION
The purpose of the stabilization system is to hold the elevation of the
antenna beam relative to the earth’s surface constant at all azimuths,
regardless of aircraft bank and pitch maneuvers. The stabilization
system uses the aircraft attitude source as a reference.
Several sources of error exist in any stabilization system.
Dynamic Error
Dynamic error is the basis of the stabilization system. Stabilization is
a corrective process. It logically follows that there must first be some
error to correct. In stabilization, this error is called dynamic. An
example of dynamic error occurs when a gust lifts the right wing and the
pilot instinctively raises the right aileron and lowers the left. In this
action, the pilot detects a changing (dynamic) error in aircraft attitude
and corrects it.
As thegust lifts the wing, the aircraftattitude source sends a continuous
stream of attitude change information to stabilization circuits which, in
turn, control the motors that raise and lower the beam. In short, a
dynamic error in aircraft attitude (as seen by the radar) is detected, and
the antenna attitude is corrected for it. Extremely small errors of less
than 1_ can be detected and compensated. However, the point is
ultimately reached where dynamic error is too small to be detected.
Without detection, there is no compensation.
Accelerative Error
One of the most common forms of error seen in a radar-antenna
stabilization system results from forces of acceleration on the aircraft
equipped with a vertical gyroscope. Acceleration forces result from
speeding up, slowing down, or turning. Radar stabilization
accuracy depends upon the aircraft vertical gyroscope. Therefore,
any gyroscopic errors accumulated through acceleration are
automatically imparted to the antenna stabilization system.
A vertical gyroscope contains a gravity-sensitive element, a
heavily dampened pendulous device that enables the gyro to erect
itself toearth gravity at therate of approximately2_/min. The pendulous
device is unable to differentiate between earth gravity and an
acceleration force. It tends to rest at a false-gravity position where the
forces of gravity and acceleration are equal. As long as the
acceleration force persists, the gyroscope precesses toward a
false-gravity position at the rate of approximately 2_/min. The radar
follows the gyroscope into error at the same rate. When the
acceleration force ceases, the gyroscope precesses back to true
gravity erection at the same rate.