(5)
When the MAP pushbutton is selected, the radar system operates in the terrain mapping mode. The STC function is inoperable and noise/clutter appear in the 0-20 mile display area. Also rotating the GAIN control out of PRESET position causes the noise and video in the display to increase/decrease. Varying the GAIN control setting in the MAP mode may increase the definition of certain types of terrain targets.
(6)
When the CYC (cyclic) mode is selected, the system will display normal weather video for a half second alternating with a half second of contoured video, the highest level (red) is blacked out. The GAIN control reverts to the PRESET value when CYC is selected.
C. Functional Description (Fig. 3)
(1) Antenna
(a)
The antenna consists of a base assembly, an azimuth frame with radiator plate and waveguide components, and a stabilization chassis.
(b)
The flat-plate radiator of the antenna is driven by the azimuth motor through a total sweep angle of 160 degrees, or in SEC SCAN, a total sweep angle of 80 degrees. The azimuth motor is a stepper motor driven by two 120 Hz square waves (phase A and phase B) 90 degrees out of phase, at a stepping rate of 240 Hz. In the absence of drive signals, a drive dc voltage provides a high residual holding torque.
(c)
The rotor winding of a sweep-sense resolver that is mechanically linked to the azimuth assembly receives square wave reference signals from the indicator. The stator output is returned to the indicator for coincidence check of antenna radiator scan angle and displayed information.
(d)
The antenna stabilization system is used to automatically maintain the antenna scan parallel to the horizon regardless of airplane attitude. This is accomplished by using pitch and roll attitude inputs from the inertial reference system, which are provided to the weather radar system thru the DAA converter of the flight management system (FMS).
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A 737-300/400/500MAINTENANCE MANUAL
Weather Radar Scan Display and Antenna RF Beam Pattern
Figure 2
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K09201
A
737-300/400/500MAINTENANCE MANUAL
(e)
The antenna stabilization can be turned ON or OFF by the STAB switch on the indicator/control. When the switch is in the ON position, |23v dc is applied to the stabilization circuit in the R/T. This allows pitch and roll signals, from the inertial reference system, to be connected to the isolation amplifiers in the Stabilization Amplifier.
(f)
Outputs from the pitch and roll amplifiers are applied to cosine and sine windings of the azimuth resolver in the antenna. The rotor of the resolver is driven by the azimuth motor. This generates the proper pitch and roll commands as a function of antenna position. The rotor output is applied to the R/T ELEV AMP and from there to the elevator motor in the antenna. A total of 40° of pitch attitude correction (including the |15° tilt) can be applied to the antenna. Pitch and roll amplifier gain pots are located under a pivoted cover plate on the front panel of the R/T.
(g)
TILT control is available with the STAB switch in either position. The TILT control potentiometer receives 10v ac excitation from a transformer in the antenna. Rotation of the TILT knob (on the indicator) causes a change in the amplitude and phase input to the elevation synchro stator windings which determines the angle of the radar beam relative to the horizon.
(h)
The output from the rotor winding goes to the R/T. The signal is amplified by the ELEV AMP and provides the control phase input to the elevation motor. The motor drives the antenna to the desired tilt and is mechanically connected to the tach and elevation synchro. The tach provides rate feedback to the ELEV AMP for smooth operation. When the antenna is tilted the proper number of degrees, the rotor of the elevation synchro is repositioned to a point where the output is no longer unbalanced. The elevation motor then stops.
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