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TCAS Computer Figure 005
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R TCAS Computer
R Figure 005A
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TCAS Computer
Figure 005B
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R TCAS Computer
R Figure 005C
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R TCAS Computer Figure 005D
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TCAS Computer - Block Diagram
Figure 006
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(a)
General The RF section consists of receiver, transmitter and RF switch portions. The receiver processes the raw ATCRBS or Mode S intruder reply data received from either the top or bottom antenna. The transmitter portion of the RF section interrogates intruders once per second, also using either antenna. The RF switch portion selects the antenna (top or bottom) for the transmission and reception of the TCAS signals. For ATCRBS intruders, interrrogations are accomplished by controlling a "whisper shout" attenuation system which provides successive responses, starting with the nearest aircraft and continuing progressively to those farther away so as to avoid saturating the radio-electric space and resulting interference.
(b)
RF switch
The RF switch is dual-purpose:
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it switches the transmission/reception circuits between the top and bottom antennas
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it directs antenna links either to the transmit circuits or to the receive circuits as the antenna links and elements are used indifferently for transmission and reception.
The four elements of each antenna are connected to hyperfrequency multiplexers controlled by the video controller.
(c)
Transmitter The transmitter generates the 1030 MHz carrier frequency required for TCAS transmission. For a Mode S interrogation, this frequency is modulated in DPSK (Data Phase Shift Keyed) format by the video controller module. For Mode C interrogations the frequency is applied to the whisper shout attenuator which attenuates the signal from 0 to 32 dB in 1 dB increments. The modulated frequency is then split into four parts by a programmable circuit controlled by the video controller so as to divide transmitter output into four separate antenna-element drive channels.
(d)
Receiver The receive circuitry consists of three RF channels. The signals are received through the antenna elements and processed as pairs. The paths of the signals between antenna element pairs and receiver pairs are exchanged by swap switches to eliminate the phase error in the different circuits. In each receiver, the RF signal is mixed with the local oscillator signal to produce a 60 MHz intermediate frequency signal.
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The processed signals are applied to phase detectors that determine the sine and cosine relationship between antenna element pairs. After amplification, these signals are transmitted to the video processor where relative bearing is determined. Logarithmic outputs from the three receivers are also applied to a log summer which passes the log sum signal to the video processor where it is decoded to determine range and altitude.
(3) Not Applicable
**ON A/C 051-099,
A. TCAS Computer
(1) Introduction
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