PRIMUSr 880 Digital Weather Radar System
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 the gust lifts the wing, the aircraft attitude 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 to earth gravity at the rate of approximately 2_/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.
Radar Facts | A28- 1146- 102- 00 |
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