(c)
The transmit signal is applied through the transmit isolator to the transmit antenna. The signal from the antenna is transmitted to the earth and reflected back to the receive isolator to the mixer. The isolators provide protection for the R/T circuits.
(d)
In the mixer, the received signal is beat against a sample of the signal currently being transmitted. The output of the mixer is a difference frequency which is proportional to the distance from the ground (altitude). This mixed frequency output is applied to the low and high altitude IF's.
(e)
The low and high altitude IF's supply altitude and presence information to the monitor and altitude processors. The low altitude IF filters and amplifies the signals from -20 to 450 feet. Altitudes higher than 450 feet are filtered and amplified by the high altitude IF.
(f)
The altitude processor computes altitude from the mixed frequency input. It outputs this data in digital format via the ARINC 429 data buses to the interfacing systems. It also outputs status (valid, invalid, test, and no-computed data) on the same buses to these systems.
(3)
Program Pin Selection (R/T)
(a) Input program pins at each R/T select modes of operation of each R/T as follows: 1) The system select input selects the system modulation rate.
The modulation is set at 145 |2 Hz (L-R/T), 150 |2 Hz
(C-R/T), and 156 |2 (R-R/T).
2) The aircraft installation delay (AID) program pins are jumpered for 57 feet. This calibrates the system so that the altitude readout is zero at touchdown. It compensates for the length of the antenna cables plus fuselage to ground distance.
3) The indicator status pins are used for an external display. They are grounded because the system does not use an indicator other than the primary flight displays (PFD).
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747-400 MAINTENANCE MANUAL
4) The continuous data program pins are jumpered to assure noninterrupted altitude data, regardless of validity, to the associated flight control computer (FCC).
(4)
Inhibit of CDU Initiated Functional Test
(a) The test inhibit line from the FCC to the R/T supplies a functional test inhibit signal. This prevents the CDU initiated radio altitude system self test below 1500 feet after glide slope capture and during an automatic landing.
(5)
Bite Monitoring (Fig. 2, Fig. 3, Fig. 4, Fig. 5)
(a)
The monitor processor computes attitude from the input data. It uses this value to verify the value computed by the altitude processor. A hand shake line ties the two processors together for data exchange.
(b)
The monitor processor also checks analog voltages such as power supply and reference outputs. It also checks the 429 data bus output words from the R/T. It checks these words for label, parity, altitude, and status. It compares the altitude output data against the values it has computed to determine if the output data is correct.
(c)
The altitude processor contains self-test and monitoring features that check for proper operation of the system. This BITE program is conducted as part of the stored software program. During normal transmit/receive operations, the program provides a continuous check of the signal flow through each module of the unit. It also provides a continuous verification of the calibration constants. Complete monitoring is performed at cruise altitude even when there are no ground return signals present.
(d)
When no ground return signals are present, the system automatically performs a system self-test. For this condition, the altitude processor provides a test command to the switching circuit. This circuit blocks the incoming signal from the receive antennas and couples the 300-foot delay signal to the signal mixer. The 300-foot delay signal is then processed as a calibrated test signal throughout the R/T. Unit performance is monitored. Monitored data is compared to data stored in memory to verify proper operation.
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