AEROGRAPHER'S MATE 3 & 2 



Components 



Cloud Height Set AN/GMQ-13( ), or the 

 rotating beam ceilometer, is composed of a 

 detector, a projector, a recorder, and in some 

 instances, an indicator. It is powered by 

 115-volt, 60-hertz alternating current. Figure 

 5-8 is a diagram of a typical installation and 

 shows that the basic principle involved in making 

 a cloud height measurement with a cloud height 

 set is one of tri angulation. 



The DETECTOR is located at a known 

 fixed distance (usually 400 feet) away from 

 the projector, and the field of view of the 

 detector is directed vertically upward. As 

 the beam of the projector sweeps the cloud 

 base, the light reflected from the cloud base 

 in the detector field of view is received by 

 the detector optical system (a parabolic re- 

 flector and photoelectric cell) and amplified by 

 the detector amplifier. The output of the detector 

 amplifier is fed by means of connecting cables 

 or radio link to the recorder and/or indicator. 



The PROJECTOR comprises two identical 

 optical systems mounted back-to-back on a 

 rotary mount such that modulated light beams 

 which they project are continuously rotated 

 in the plane of the detector's field of view. 

 At some point in the rotation, each portion of 

 the detector field of view from the top of the 

 detector to the zenith is illuminated. Any cloud 

 or other reflective obstruction will cause a light 

 spot to occur as the light beams pass. The 

 detector photocell and amplifier produce a signal 

 voltage corresponding to the intensity of the 

 spot on the clouds. Two light beams are used 

 to increase the rate of measurement and to 

 provide a safety factor in case of failure of 

 one optical system. 



The rotary mount which carries the two 

 back-to-back optical systems rotates at the 

 rate of 5 rpm. This means that the rotary 

 mount makes a complete revolution in 12 

 seconds, and that the optical system projects 

 a beam every 6 seconds. However, since each 

 optical system is blocked off for one-half of 

 the revolution through the upper semicircle, 

 the actual sweep of each optical system is 

 3 seconds in duration. Each measuring sweep, 

 therefore, lasts 3 seconds, and a measuring 

 sweep is provided every 6 seconds. 



The INDICATOR consists of a long persist- 

 ence cathode-ray tube (CRT) with the appropri- 

 ate electronic and mechanical circuits, and is 

 housed in the weather office. The electron beam 

 of the CRT moves up the vertical axis of 

 the tube in synchronism with the rotation 

 of the projector. When an amplified cloud signal 

 from the detector is fed to the indicator 

 (cathode-ray tube), it causes the electron beam 

 to widen momentarily as the beam moves 

 up the face of the CRT. The point at which 

 the electron beam widens corresponds to the 

 angle of the projector at which the light beam 

 strikes the cloud over the detector. The face 

 of the indicator CRT is calibrated in degrees 



209.382 

 Figure 5-9. — Cloud height indicator IP-327B/ 

 GMQ-13. 



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