19. Rain Radar Although the photographic plan is designed to accommodate a consid- erable variation in the transmission of light through the atmosphere, it is clear that satisfactory photography cannot be accomplished if a rain squall moves into the line of sight. As a means of assuring a clear view, a 3- centimeter radar set has been provided at the Control Building; the antenna is mounted in the photo tower, at the 40-ft level. With this equipmentit is possible to observe the locations of rain storms within 50 miles of the radar. Repeated observations determine the speed and direction of the drift of these storms, and enable us to predict their locations up to an hour in advance. On the basis of these predictions it is possible to say that one given time will be unacceptable, or that an alternate will be preferable; these predictions are made far enough ahead of time to choose a suitable firing time and to get the information to participating aircraft and other manned stations. 20. Aiming and Tilting of Cameras Cameras are aimed visi.ally whenever possible. It is sometimes im- possible to see the shot barge from the photo tower twenty miles away, even with the aid of telescopes, because of poor atmospherics. Recourse is usu- ally taken to aiming at night, by means of a distinctive light placed on the barge. Occasionally even this method cannot be used, and aiming is accom- plished by sighting on a known nearby object or island, and swinging the camera through an angle computed from the survey data. In such cases it is good enough to measure the angle to about 0.5 degrees, placing the image within 0.5 mm from the center of the frame. In order to utilize the entire film area, and to record the largest practicable images, it is desirable to elevate the camera through a tilt angle, 7. This angle is usually chosen to place the horizon about 15 per- cent of the distance from the bottom to the top of the film frame. For the Eastman camera, with a vertical frame dimension of 7.4-mm and using a 63-mm lens, the tilt angle is about 2°-10'. The angle is set by tilting the mounting table, and measuring the angle with an inclinometer. 21. Photo Plan and Check Lists man SGC58 76 - 28 - 4 The evolution and execution of the photographic effort is controlled