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Instrument Landing
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Two antennas are used, one under each wing. One is connected to the transmitter, the other to the receiver. The 420-460MHz band was abandoned in 1963, when radio altimeters went to the 4200 – 4600 MHz band
The APN-1 had sinusoidal FM, using a voice coil driven membrane as variable capacitor in the transmitter. The AM210, introduced in 1952, had the same frequency range, but triangular FM, made by a dc motor-driven butterfly capacitor. The advantage is a constant beat frequency, proportional with altitude, and less sensitivity for Doppler shift.
In this graph, the solid line is the transmitted frequency versus time, whereas the dotted line is the frequency of the received echo.
BW = peak-to-peak frequency deviation T = time to deviate from min to max, dF = frequency difference between received and actual transmitted signal c = speed of light ( 3.10 ^ 8 m/s = 983 feet / ms)
The altitude H is then found from : 2 x H = c x dF x T / BW
The dF amplifier in the AM210 has a pass band from 500Hz to 50kHz, the gain rises with the square of frequency to compensate for the weaker echo at higher altitude. A dc motor produces the FM with two, centrifugal contact stabilised speeds: 4000 rpm (T/BW = 3.75ms/40MHz) 20ft— 2000 ft 800 rpm (T/BW = 18.75ms/40MHz) 100ft—10000 ft The combined length of the coax cables to the antennas, and the height of the antennas above the tarmac are over 20 feet, so the altimeter measures straight from 0 altitude although its amplifier is ac-coupled .
The circuit diagram of the AM210 with some description is here Much more background ( in Dutch) is here |
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VINTAGE AVIONICS |
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Instrument landing systems |
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Radio Altimeters based on CW FM were developed during WWII, like the AN/APN-1 in 1943 . A small trans- mitter inside was frequency modulated on about 100Hz, shifting the carrier frequency continuously between 420 and 460MHz. The receiver measured the frequency difference between the transmitted signal and the signal returned from the ground, having the transmit frequency from a few us earlier. |
AM210 radio altimeter |
