K77093
Low Range Radio Altimeter System Component Location 5C8
34-48-0 Figure 1 Oct 20/85
Page 2
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
5. ADI Displays
A. A DH (decision height) light is located on each pilot's ADI and is controlled by the height indicator located on the same instrument panel. The brightness of the light is controlled by the master dim control.
B. The receiver-transmitter supplies a dc altitude signal to the ADI rising runway symbol. The rising runway comes into view at the bottom of the ADI at approximately 200 feet and shows relative airplane altitude from 200 feet to touchdown.
6. Operation (Fig. 2)
A. Altitude Measurement Operation
(1)
The LRRA system provides a measurement of aircraft height above terrain, based on accurate timing of the interval required for a radio frequency (rf) pulse to travel from the transmitting antenna to the terrain and back to the receiving antenna. The system converts the time interval to a range signal that is used as the input to the various altitude readout circuits.
(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.
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