R **ON A/C 106-149, 201-299, 301-399, 401-499,
 B. Measurement of Intruder Parameters
 (1) Principle
 (a) Determination of relative altitude Upon confirmed transponder reception, the TCAS starts to interrogate the intruder. Its altitude is transmitted directly in the reply (standard barometric altitude) and this information is used to determine the relative altitude of the two aircraft, by calculating the barometric altitude difference. This computation is, however, only possible with respect to Mode C or Mode S transponder-equipped aircraft. (Ref. Fig. 015)
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 (b)
 Range measurement The range is calculated by measuring the elapsed time between transmission of the interrogation signal and return of the reply transmitted by the intruder. Aircraft are detected from a minimum range of 14 NM.

 (c)
 Determination of Azimuth There are several methods for calculating the angle of reception of a radio-electric signal with respect to a reference direction such as the aircraft centerline. The TCAS utilizes a four-element antenna and a form of phase interferometry to derive bearing from the received signal. The interferometry principle is based on a comparison of signal phases received by four independent elements of the directional antenna. For any given direction of reception, three antenna elements and their corresponding receiver channels are used. The elements which are used are the one physically positioned in the center of the quadrant being interrogated and the elements adjacent to it. Two phase measurements are performed: one corresponding to the difference in phase between the center element and the element on its right, and one corresponding to the phase difference between the center element and the element on its left. These two signals are converted from analog to digital information and stored. The bearing is calculated by reading the phase detector outputs, taking the ratio of the phase detector outputs with respect to each other, and performing a table lookup which relates the ratio to the phase difference from beam center.

 **ON A/C ALL
 (d) Tracking Once identified, the intruders are tracked by a series of interrogation-replies in Mode C only all-call for Mode C transponder-equipped aircraft, and in Mode S for Mode S transponder-equipped aircraft. These exchanges permit the TCAS to periodically update the altitude, range and bearing data for each intruder and to compute the range rate and altitude rate variations. These data are then used to determine the time separating the two aircraft from their closest point of approach.
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 (e)
 Broadcast messages Every ten seconds, the TCAS transmits a broadcast message intended to inform nearby aircraft, themselves equipped with a TCAS, of the presence of a TCAS-equipped aircraft in their traffic area. These messages, received by the Mode S transponders, are communicated to the TCAS computer to enable it to know the number of TCAS-equipped aircraft in its detection envelope. This information is then used in the interference limitation formulas whose results modulate the Mode S interrogation output power level in inverse proportion to the number of aircraft. This reduces the number of non-elicited replies received by ground ATC stations. The messages transmitted are of the uplink format type UF = 16 with the Mode S 24 bit address of the interrogating TCAS included in the MID field (bits 65 to 88) with the UDS field (bits 33 to 40) containing the code F50. No response is expected for this type of message.

 (f)
 Communications frequencies Communications between two aircraft are always crossed between transponder and TCAS. The TCAS transmits at a frequency of 1030 MHz to the transponder of the other aircraft, whose reply signals are at a frequency of 1090 MHz to the TCAS receiver. This choice allows system compatibility with ground station-transponder links as the ground stations use the same frequencies as the TCAS.

 (Ref. Fig. 017)
 C. Coordination Two TCAS-equipped aircraft must coordinate their maneuvers to avoid the flight path corrections ordered by each TCAS resulting in a hazardous situation.
 (1) Coordination principle In most cases of encounters between two TCAS-equipped aircraft, mutual identification is almost but not quite simultaneous, with sufficient time lag to establish the priority necessary for the coordination process. The first aircraft to detect a potentially dangerous configuration computes a deviation maneuver sense and communicates it to the other aircraft. This aircraft takes the information into account and in turn computes a correction.
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 TCAS - Communication Principle Figure 017
　
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