A TERRAIN CLEARANCE INDICATOR 225 



Even with the most directive systems that can be devised, the beam 

 radiated from the airplane is so spread that echoes can be expected to 

 arrive simultaneously from several surfaces, for instance from both the 

 leaves on the trees and the ground between the trees, or from the top 

 of a building and from the adjacent street. 



Several problems were anticipated in the apparatus itself. The 

 theory is based upon a frequency-modulated signal free from any ampli- 

 tude modulation, and it was questioned whether a transmitter could 

 be built to operate on ultra-high frequencies which would be suffi- 

 ciently free from amplitude modulation, when subjected to the vibra- 

 tion of the airplane, to be satisfactory. Since the receiver utilizes 

 both the direct and reflected signals in making the altitude measure- 

 ment, it is necessary that some signal be picked up directly from the 

 transmitting antenna but not enough to overload the receiver and 

 thus prevent reception of the echo. It was expected that difficulty 

 would be encountered in sufficiently reducing the direct signal. 



After considering all these problems, it was decided that the cheapest 

 and easiest way of determining the answers was to build the apparatus 

 and try it out to see if correct operation could be obtained, first, under 

 the more or less ideal conditions of flying over smooth water and, then, 

 over less favorable surfaces. 



Most of the measuring equipment available for radio frequency test 

 work is useless at ultra-high frequencies. Hence, it was necessary to 

 get the system functioning as a whole before any means were available 

 for determining the best adjustment of the radio-frequency parts of the 

 system. Because of the difficulty of providing, while on the ground, 

 an adequate reflector at distances of from a few feet to thousands of 

 feet from the apparatus, it was necessary to install the equipment in an 

 airplane very early in the development and make most of the tests 

 during flights. Nearly a hundred airplane flights were made in one 

 of the Bell Telephone Laboratories' airplanes during the development 

 period of seven months which preceded the public demonstrations 

 made in the United Air Lines Flight Research Airplane. 



Operation and Theory 

 The fundamental parts of the altimeter in relation to their applica- 

 tion are shown in Fig. L An ultra-high frequency oscillator is pro- 

 vided, whose frequency is varied up and down by a modulator which 

 consists of a small rotating variable condenser driven by a motor. 

 The oscillator is connected through a coaxial transmission line to a 

 transmitting antenna which is located on one of the lower surfaces of 

 the airplane. The signal is radiated downward by this antenna. A 



