A TERRAIN CLEARANCE INDICATOR 233 



in the same direction. Fortunately, all these are of a percentage nature, 

 so that the error in feet becomes smaller as the ground is approached. 

 An absolute error in the indication is still possible because of the limita- 

 tions of the milliammeter used on the instrument panel. The Weston 

 aircraft meter used is guaranteed to be correct to within one per cent 

 of its full scale reading at any point on its scale, which permits maxi- 

 mum errors of ten feet on the 1000-foot scale and fifty feet on the 

 5000-foot scale. 



When flying over rough water, wooded terrain or cities, reflected 

 signal is received from surfaces at different distances simultaneously, 

 resulting in addition and subtraction interference effects, thus some- 

 times momentarily reducing the echo signal below the minimum re- 

 quired for accurate indication. In such a case, the meter hand may 

 swing down momentarily as much as 10 per cent. For the present 

 limited transmitter power and receiver sensitivity, at altitudes above 

 2500 feet, these momentary signal reduction effects become progres- 

 sively more serious when flying over irregular surfaces so that for a 

 substantial part of the total time the echo signal may be below the 

 minimum required for correct meter reading. This is indicated by a 

 reading fluctuating between 3000 and 5000 feet when flying at 5000 

 feet over a surface dotted by buildings, timber, etc. The meter swings 

 up to the correct reading every time the airplane passes over a smooth 

 field or body of water of any size. Up to 2500 feet the echo signal has 

 proved to be sufficient for steady operation over all kinds of terrain. 



Tests have been made over New York, Raritan, Newark and San 

 Francisco Bays, Great Salt Lake, Lakes Erie and Michigan, the tim- 

 bered mountains of Washington and Oregon, the deserts and mountains 

 of the southwest and the cultivated areas of the midwest during the 

 period of the recent demonstration flights made with the equipment 

 installed in the United Air Lines Flight Research Airplane. 



An indication of the character of the surface over which the air- 

 plane is flying is given by the variations in the meter reading. A 

 city usually causes rapid fluctuations of the order of fifty feet, depend- 

 ing, of course, upon the height and the spacing of the buildings. 

 Cultivated farmland causes fluctuations of lower frequency and 

 amplitude. An isolated high object such as a skyscraper or a chimney 

 is indicated only by a slight meter kick as the airplane passes over it, 

 which may not be noticed by the observer. If the airplane passes 

 over only a few feet above the object and the top is large enough to 

 contribute momentarily most of the echo signal received by the air- 

 plane, the indication is unmistakable and the correct distance to the 

 object is indicated by the meter. For instance, the gas storage tank 



