-------------------------------------------------------------------------------| DESIGNATOR | FIELD | INDICATION | |-------------|----------------|----------------------------------------------| | UM | UTILITY MESSAGE| Transponder status readouts | | VS | VERTICAL STATUS| Aircraft status: 0 = airborne, 1 = ground | -------------------------------------------------------------------------------
**ON A/C 001-049, 101-105, 151-199,
B. Measurement of Intruder Parameters
R (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)
(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 technique used in the TCAS II computer is the interferometer system. The interferometry principle is based on a comparison of signal phases received by four independent elements of the directional antenna, taken in pairs. With two poles, E2 and E4, the phase difference of signals received on the two elements depends on the angle of reception of these signals since the difference in distance to the source of the two reception poles varies with this angle. (Ref. Fig. 016) If the distance separating the two poles, and the source signal wavelength are known, the following ratio is obtained:
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TCAS - Relative Altitude Calculation Principle Figure 015
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TCAS - Determination of Azimuth
Figure 016
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d S2-S4 = -------- x 360 x sin B Lambda where B : source signal reception angle S2-S4 : phase difference of signal received on poles E2 and E4 d : distance between the two poles Lambda : signal wavelength, i.e. for f = 1090 MHz, 3.10expnt8/1.09 x 10expnt9 = 0.2752 m.
To calculate angle B, the TCAS uses 4 elements installed perpendicularly two by two which gives: d S2-S4 = -------- x 360 x sin B Lambda
d S1-S3 = -------- x 360 x cos B Lambda and therefore: S2 - S4 B = Arc tan ---------S1 - S3 If the antenna is aligned on the aircraft centerline, angle B represents the source azimuth and may be used to indicate the intruder's bearing on the ND.
R **ON A/C 051-099,
R B. Measurement of Intruder Parameters
R (1) Principle
R R R R R R R R R (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)
R 1EFF : 001-049, 051-099, 101-105, 151-199, 1 34-43-00Page 57 1 1 May 01/05 1 1 1CES 1
(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 bearing Two basic means exist for performing the direction finding function required by the TCAS: one based on phase and the other one based on amplitude. The amplitude monopulse technique is used. This system is a four element, vertically polarized, monopole array which is capable of transmitting in 4 selectable directions at 1030 MHz and receiving omnidirectionally with bearing at 1090 MHz. The amplitude monopulse system generates four separate cardioid antenna patterns, each with its own receiver channel to view all four 90 degree sectors simultaneously (forward, aft, left and right sectors). The bearing angle of a target is determined by the amplitude ratio of adjacent patterns. The TCAS takes a ratio of the amplitude of the two strongest signals from two 90-degree antenna beams to determine a bearing angle. This technique also prevents a bearing measurement from being reflected into a wrong quadrant. Because an amplitude ratio is used, distortions in the antenna patterns tend to be cancelled out. When transmitting, the TCAS computer interrogates one sector at a time using a whisper-shout sequence of stepped power level interrogations.
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