(2)
The system provides altitude data when power is applied to the receiver-transmitter. Low voltage power is then supplied to all circuits in the receiver-transmitter and to the height indicator. The LRRA system is turned on by closing its circuit breaker.
(3)
When the transmitter fires, it generates a time-zero (T-zero) pulse which is used to start the measurement of the transmission-reception time interval. The time measuring circuitry is called the range computer, or tracker. When the ground return (echo) pulse arrives at the receiver, a video return pulse is generated. The video return pulse is applied to the track gate in the range computer and the output of the range computer is the range or altitude signal.
(4)
The transmitter contains circuits that generate and control the 4,300-MHz rf energy pulse applied to the antenna system, and circuits that sample the pulse and supply a corresponding time-zero pulse to the range computer. In normal system tracking mode, the rf energy is pulsed by the pulse rate frequency generator in the range computer at a 7,000 pulse-per-second (pps) rate. A diode detector inside the transmitter cavity generates a positive-going pulse which is then applied to the base of the T-zero amplifier. This causes the T-zero amplifier to conduct, generating a negative voltage pulse to the ramp multivibrator.
(5)
The receiver circuits produce a usable video signal for the range computer and enable control of the IF amplifier. The receiver input is the 4,300-MHz transmitter signal echo attenuated by relative altitude of the aircraft and reflection characteristics of the terrain. The leading edge of the return pulse is essentially unaffected by the terrain or aircraft motion. In the local oscillator/mixer assembly of the receiver, this received signal is combined with a local oscillator frequency which is also 4,300 MHz. Because of the input signal pulse spectrum, doppler frequency shifts, and random phasing at reflection, the output of the local oscillator mixer assembly is a bipolar video pulse having a band of frequencies from zero to 30 MHz, referred to as a zero-IF signal. The mixer signal goes to the IF amplifier before detection. The output is fed to a full-wave detector circuit for a positive-going signal to the video amplifier and delay line. This signal is then fed to the track and tagc gates in the range computer.
590
May 15/81 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 34-48-0 Page 3
T54090 T54134
Low Range Radio Altimeter System Schematic Diagram 566
34-48-0 Figure 2 (Sheet 1) May 20/82
Page 4
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
590 Low Range Radio Altimeter System Simplified Schematic
May 20/82 Figure 2 (Sheet 2) 34-48-0
Page 5
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(6)
The system is built around the closed-loop tracking of the range computer. When the T-zero pulse is generated, it causes the ramp generator to generate a ramp that increases linearly from zero to 25 volts. The internal range circuit is also generating a zero to 25 volts ramp. When these two voltage levels coincide at the comparator, a track gate pulse is generated at the input to the track gate. The internal range amplifier is much slower (comparatively) than the ramp generator, and the level that the ramp reaches at the time the track gate output pulse is generated is proportional to altitude or range. The track gate output is generated when a track gate pulse and a video return pulse occur simultaneously at the track gate. The track gate output controls the instantaneous value of internal range, which represents the radar range (height).
(7)
The length of time that the range ramp is allowed to run is proportional to the aircraft altitude. The track gate pulse is applied to the track gate coincidentally with the video return pulse. The track gate output signal consists of energy proportional to the overlapping portion of the track gate pulse and the video return pulse. When the system is properly tracking the rf echo, the track gate output will consist of pulses at a nominal pulse-repetition frequency (prf) rate of 7,000 pulses per second (pps) with individual values equal to the overlapping portion of the two pulses. The track gate output pulses are then processed so that the interval of the variable delay is retained. If the rf transmission path distance (height) distance should change, the track gate output pulse values would also change.
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