A Radio Altimeter

Radio Record, Volume IV, Issue 17, 7 November 1930, Page 13

A Radio Altimeter

For Safe Flying N altimeter that will tell aeroplane pilots exactly how high they are above the ground immediately beneatlr them, rather than their approximate altitude with reference to the point of: departure, has recently been invented by an American radio engineer. The serious drawback with the old or barometric type of altimeter is that it varies with weather conditions. Also, assuming constant air pressure, which is rarely encountered over large. areas, it indicates only the altitude above the starting point, always providing the pilot remembered to set the pointer to zero before taking off. Flying hlind, he might run into a mountain, with his altimeter registering 8000 feet. The perfection of the radio altimeter has removed all such dangers. Its operation depends on a newly-dis-covered principle that a radio receiver can be slightly tuned by the influence of an outside wave as well as by variation of the tuning controls. However, conditions are seldom such that this effect is noticed upon the home broad: cast receiver. . A low-powered transmitter and an oscillating receiver are installed in the ’plane, and so arranged that a beat note or squeal is heard when head-. phones are plugged into the receiver. The waves from the transmitter are reflected by the ground beneath the plane back to the ’plane again, and, as the altitude varies a phase difference can be noticed. The "phase difference" describes the relationship -between the wave returning to the ’plane and the wave generated in the oscillating receiver. At certain altitudes the reflected wave arrives on the *plane at just the right time to be perfectly in phase with the local wave. At other altitudes, the two waves are out of phase. The beat frequency or whistle varies noticeably with the phase changes, definite points being noticed for every half wavelength difference in altitude. Thus, if the transmitter is working on 50 metres, a peak will be noticed every time the altitude changes by 25 metres (approximately 80ft.). These peaks can be made to actuate lights or instruments calibrated in altitude, revealing to the pilot exactly how high he is above the reflecting surface-that is, the ground directly below hi plane. t