(d)
Airplane ascends so that radio altitude exceeds DH + 75 feet.
(e)
Airplane descends so that radio altitude reaches zero.
(8) The PFDs are located on the captain's and first officer's instrument panels P1 and P3, respectively.
3. Operation_________
A. Functional Description
(1) Radio Altimeter/Integrated Display System Interface Block Diagram (Fig. 2)
(a)
The R/T supplies a FM/CW signal to the transmit antenna. The signal is transmitted, reflected from the terrain, and picked up by the receive antenna. A transmit signal sample and the reflected receive signal are mixed in the receiver to develop a difference frequency. The difference frequency is proportional to the signal travel time to the terrain and back. This difference is converted to absolute altitude (height above terrain) data and output on two 429 data buses from each radio altimeter receiver/transmitter.
(b)
Each R/T provides altitude data in a 32 bit word format via two ARINC 429 digital data buses. These buses supply data to each electronic interface unit (EIU), flight control computers, EICAS, GPWS computer (data bus input from L-R/T only), and to the MAWEA (modularized avionics and warning electonics assembly). DH data from the EFIS control panels (or as an alternate source, from the pilots' CDUs) is supplied to the EIUs. Setting of the left (Capt's) and right (F/O's) EIU source select switches selects which EIU feeds DH and altitude data to the left (Capt's) and right (F/O's) primary flight displays.
(c)
The R/T units receive 115-volt, 400-Hz power from the L, R, and C ac buses via the RAD ALTM L, R, and C circuit breakers on the P7 panel. The R/T power supply provides various regulated dc voltages to all circuits in the R/T. Each R/T is turned on and remains on as long as its associated circuit breaker is closed and the bus supplying power to the circuit breaker is energized.
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H17130
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(2)
Signal Processing (Fig. 3, 4, and 5)
(a)
The R/T transmits a carrier wave signal via the transmitter antenna. This signal consists of a CW carrier sweeping (modulated) linearly from 4.235 to 4.365 GHz at a frequency of 145 |2, 150 |2, and 156 |2 Hz for the left, right, and center systems, respectively. The modulation rate of each R/T is controlled by program pins at rear of each R/T.
(b)
To generate the transmit signal, the R/T transmitter circuit outputs a carrier wave (CW) signal centered at 4.3 GHz. The transmit signal is frequency modulated by the digital closed loop modulator. The calibration circuit shapes the transmit signal such that it is a triangular wave linear with time. It uses the 300-foot delay signal as a reference to control the modulation signal amplitude and rate.
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