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B.  Two complete systems are installed. Each system consists of a receiver-transmitter, height indicator, transmit antenna, receive antenna, and DH light on pilot's ADI and HI. The location of components is shown on (Fig. 1).
C.  The No. 1 system (captain's) receives 115-volt ac power through circuit breaker RADIO ALTM-1 located on panel P18-1 at the left load control center. System No. 2 (first officer's) receives 115-volt ac power through circuit breaker RADIO ALTM-2 located on panel P6 at right load control center.
D.  The LRRA systems supply flag alarm, 500 foot trip signal, and dc altitude signals to the AFCS system; flag alarm, 200-foot altitude trip, and dc altitude signals to the ADI rising runway symbol; and dc altitude and flag alarm signals to the ground proximity warning system.
2.  Receiver-Transmitter (Fig. 1)
A.  The receiver-transmitters are located side-by-side on shelf four of E2 electronic rack. All electrical connections to the receiver-transmitters are made through a connector located on the rear panel. An auxiliary connector located on the front panel is provided for connecting to test equipment.
B.  A single TEST/STATUS switch/indicator on the receiver-transmitter front panel provides self-test functional checks.
3.  Height Indicator (Fig. 1)
A.  Airplane altitude is read from a circular scale which has a range of -20 to 2500 feet. The scale is linear from -20 to 500 feet and is progressively compressed from 500 to 2500 feet.
B.  The DH control knob is used to adjust the position of the DH cursor to any altitude shown on the dial. The DH cursor setting designates the altitude at which the DH lights will be energized during airplane descent or de-energized during airplane ascent.
C.  The warning flag is solenoid operated and will come into view whenever the solenoid is de-energized.
D.  The test button provides self-test functional checks of the LRRA system from the control cabin and is electrically in parallel with the TEST/STATUS switch/indicator on the associated LRRA receiver-transmitter.
Oct 20/84 34-48-03 Page 1
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.

K73391
Low Range Radio Altimeter System Component Location  5C8 
34-48-03  Figure 1  Oct 20/85 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

 

4.  ADI Displays
A.  A DH light is located on each pilot's ADI and HI and is controlled by the decision height selected on the height indicator located on the same instrument panel. The brightness of the light on the ADI is controlled by the master dim control.
B.  The receiver-transmitter supplies a dc altitude signal to the ADI rising runway. 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.
5.  Antenna
A.  The antennas are linearly polarized horns. Two identical antennas are used in the LRRA system, one for transmitting and one for receiving. The antennas are connected to the receiver-transmitter through coaxial cables and are flush mounted on the lower fuselage centerline as follows:
TRANSMIT ANTENNA No. 1 STA 450 RECEIVE ANTENNA No. 2  STA 430
TRANSMIT ANTENNA No. 2 STA 390 RECEIVE ANTENNA No. 2 STA 410

6.  Operation (Fig. 2)
A.  Altitude Measurement Operation
(1)  
The LRRA system receives 115V AC from a circuit breaker on P-18 or P6 load control center, as applicable. The receiver-transmitter unit generates its own low voltage ac and dc power supplies and also provides a 26V AC supply to the height indicator.

(2)  
The receiver-transmitter consists of seven basic subassemblies, namely: modulator, receiver-transmitter, altitude processor, monitor processor, monitor, altitude trips and power supply.

(3)  
The output from a 100-Hz triangle wave generator in the modulator is supplied to a varactor in the transmitter oscillator to produce a 100-Hz FM modulated, output signal. The frequency of the transmit signal, centered at 4300 MHz, is continuously varied  from 4250 to 4350 and back to 4250 MHz at a rate of 100 times per second. From the oscillator, the transmitter signal passes through a directional coupler, is amplified by the power amplifier and then routed through a second directional coupler and an isolator to the transmit antenna.

(4)  
Each directional coupler is connected to a quartz bulk acoustic delay line that is the equivalent of 300 feet of altitude. The delayed signal is heterodyned in a mixer with the nondelayed signal applied from the transmitter. Mixer output is the difference frequency, which is amplified and limited into a square wave. One delay line circuit produces calibration signal 1, which is used for comparison purposes in the altitude processor, the other delay circuit produces calibration signal 2, which is used in the monitor processor.
　
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