R 1EFF : 001-049, 101-105, 151-199, 211-220, 1 34-52-00Page 33 1 1 May 01/05 1 1 1CES 1 In addition to its specific transponder functions, it enables communication between the TCAS and a detected aircraft if equipped with the TCAS.
(a) Interrogations (Ref. Fig. 002) Three classes of interrogations are transmitted by the ground interrogator, they are:
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Mode A or C interrogations
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Mode S all call interrogations
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Mode S interrogations.
1_ Mode A or C interrogation
A mode A or C interrogation consists of two pulses labelled P1
and P3. A control pulse P2 is transmitted following the first
interrogation pulse. P1 and P3 are radiated by a narrow
rotating beam antenna.
P2, also called the Side Lobe Suppression (SLS) pulse, is
radiated by an omnidirectional antenna with an amplitude of 6
dB below P1 and P2 within the main lobe.
P1 amplitude is greater than or equal to P2 amplitude from the
greatest side lobe transmission of the antenna radiating P1.
2_ Mode S all call interrogation
(Ref. Fig. 007)
A mode S all call interrogation consists of three transmitted
pulses labelled P1, P3 and P4. A control pulse P2 (Side Lobe
Suppression Pulse) is transmitted following P1. P1 amplitude
is greater than or equal to P2 amplitude from the greatest
side lobe transmission of the antenna radiating P1.
P2 amplitude is at a level lower than 6 dB below P1 amplitude
within the main lobe.
P4 duration is 0.8 or 1.6 microseconds depending on the
interrogation mode.
If P4 is equal to 0.8 microseconds, this means that the
interrogation only concerns conventional ATC mode A or C
transponder, and consequently the Mode S transponder should
not reply.
If P4 is equal to 1.6 microseconds, this means that the mode S
transponder should reply with mode S information.
3_ Mode S interrogation
(Ref. Fig. 008)
A mode S interrogation consists of a two-bit preamble followed
by a data block.
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ATC Transponder - Simplified Block Diagram R Figure 006C
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ATC - Mode S All Call Pulse Characteristics Figure 007
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ATC - Mode S Pulse Characteristics Figure 008
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The preamble consists of two transmitted pulses labelled P1 and P2. The characteristics of P1 and P2 are the same as for the mode S all call interrogation, except that P2 amplitude is equal to P1. The data block consists of a single pulse P6 with a Differential Phase Shift Keying (DPSK) modulation.
Pulse duration : short 15.5 microseconds,
long 29.5 microseconds,
Data block : 56 or 112 bits.
(b) Encoded reply
1_ Mode A or C reply
A mode A or C reply consists of two framing pulses F1 and F2, separated by a 20.3 microseconds time interval. The encoding of the reply is done by means of the presence (1) or absence
(0) of 13 reply pulses (12 pulses plus X pulse).
2 Mode S reply
_ A mode S reply occurs after a mode S interrogation or a mode S all call interrogation (P4 duration 1.6 microseconds). A mode S reply consists of a four-bit preamble followed by a data block. The data block contains 56 or 112 position modulated pulses.
3 Mode S message content
_ The minimum data link transponder supports all surveillance functions, in addition to bidirectional air-to-air data exchange, ground-to-air data uplink (Comm A), air-to-ground data downlink (Comm B), and multisite message protocol. In addition, the transponder is capable of receiving Extended Length Messages (ELMs) from the ground. ELMS are received in the Comm C format. ELM transmittals to the ground use the Comm D format. (Note: Comm C and Comm D are future applications). All discrete Mode S interrogations and replies (except the all-call reply) contain the 24-bit discrete address of the Mode S transponder upon which 24 error detection parity check bits are overlaid. In the all-call reply, the 24 parity check bits are overlaid on the Mode S interrogation address and the transponders' discrete address is included in the text of the reply. The main function of Mode S is surveillance. To accomplish this function, the Mode S transponder uses the 56-bit transmissions (in each direction). In the 56-bit transmissions, the aircraft reports its altitude or ATCRBS 4096 code, and the flight status (airborne, on-ground, alert, Special Position Identification (SPI), etc.
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