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The Lincoln Laboratory Journal. Volume 2. Number 3 (1989)
energy in both halves indicates that an interfer-ing pulse was received at the same time.
PPM was also chosen because it enhances monopulse performance. The data block in Fig. 12 contains either 56 bits or 112 bits, independ-ent ofthe data content. The constant number of pulses makes it possible for a monopulse azi-muth estimate for the PPM reply to be based uponalargenumber(e.g., 16 or32)ofindividual pulse azimuth measurements. This feature contrasts with the pulse-amplitude modulation ofATCRBS in which a message composed of all logical as does not contain any data pulses.
Mutual interference with ATCRBS is man-aged on the downlink with a preamble whose spacing is selected so that it is unlikely to be synthesized by ATCRBS replies. The preamble minimizes the possibility that the Mode-S reply processor will be busy handling ATCRBS fruit when the elicited Mode-S reply is received. A second technique for managing mutual interfer-ence is called burst error correction, which can error-correcttheeffectsofa singleATCRBSfruit reply received at the same time as the Mode-S reply.
Mode-S Data Link
TheselectiveaddressingofModeS providesa natural mechanism for a data link that would supply the capacity and performance required to support air traffic services. The communica-tions-control features of the Mode-S data link have been designed for compatibility with the Open Systems Interconnection Reference Model.
The link design provides for both ground-to-air and air-to-ground message transfers. Air-to-ground messages may be either pilotinitiated or ground initiated. The latter type enables the efficient reading of the technical information available on board an aircraft, e.g., information such as an aircraft's roll angle, which predicts the aircraft's tum.
Messages sent over the Mode-S link benefit from the high degreeoferrorprotection provided by the link's design. For example, the system acknowledges message delivery. (On the uplink, the receipt of a reply to an interrogation that contained a message constitutes the technical acknowledgment ofthat message.) In all critical applications, provision is made so that an air-craft crewcan acknowledge message receipt and acceptance.
Mode-S Data Formats
Mode-S formats (Fig. 13), which are either 56 bits or 112 bits long, all contain a 24-bit address and parity field [11]. The address and parity functions were combined into a single field to minimize the channel overhead. For an interro-gation, a Mode-S sensor generates a 24-bit parity field from the entire message (either 56

1.....4~---Preamble 8.0 Jls----i..~r--Data Siock 56 or 112 JlS 4
Sit ISit 11Sit 21Sit 31Sit 41 IN -1 Isit NI
J1Jl IUl :1:-0: -1-:0-:1-:0:1-:0:--:--r-:-1-:0-:1:-6: 60~51 --3:5 4:5 ---~8io---":""9~io""""""'~I-~1 ~~C2-2~1 ~I~~I ~I-
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Time (J1s) 1 1 a 1 10
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Pulse-Position Modulation (PPM)

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Data Rate: 1 Mb/s

 

Fig. 12-Mode-S reply waveform.
Orlando -The Mode S Beacon Radar System

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Fig. 13-Mode-S data formats. (a) Surveillance interrogation and reply. (b) Surveillance and communication interroga-tion and reply. (c) Communication interrogation and reply.
bits or 112bits long) and overlays the parityfield on the address field to form the address and parity field.
The transponder, when it receives an interro-gation, performs a complementary decoding process. Ifthe message has been received error free, the transponder will recover and process the intended Mode-S address from the address and parity field. On the other hand, one or more errors anywhere in the message will change the decoded Mode-S address. In this case, the trans-ponder will not accept the message, since it appears to be meant for another transponder.
Surveillance formats (Fig. 13[a]) contain sur-veillance and communication-control informa-tion. On the downlink, the formats also convey Mode-C altitude or Mode-A identity codes.
　
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