Publication: Magyar Közlöny
Issue: MK-2009-104 (Year: 2009, Number: 104)
Era: 2004-2010
Section: 
Paragraph Index: 2249

b) at a level lower than 9 dB below the radiated amplitude of P6 within the desired arc of interrogation. 3.1.2.2 REPLY SIGNALS-IN-SPACE CHARACTERISTICS 3.1.2.2.1 Reply carrier frequency. The carrier frequency of all replies (downlink transmissions) from transponders with Mode S capabilities shall be 1 090 plus or minus 1 MHz. 3.1.2.2.2 Reply spectrum. The spectrum of a Mode S reply about the carrier frequency shall not exceed the limits specified in Figure 3-5. 3.1.2.2.3 Polarization. Polarization of the reply transmissions shall be nominally vertical. Chapter 3 Annex 10 — Aeronautical Telecommunications 3-11 22/11/07 3.1.2.2.4 Modulation. The Mode S reply shall consist of a preamble and a data block. The preamble shall be a 4-pulse sequence and the data block shall be binary pulse-position modulated at a 1 megabit per second data rate. 3.1.2.2.4.1 Pulse shapes. Pulse shapes shall be as defined in Table 3-2. All values are in microseconds. 3.1.2.2.5 Mode S reply. The Mode S reply shall be as shown in Figure 3-6. The data block in Mode S replies shall consist of either 56 or 112 information bits. 3.1.2.2.5.1 Pulse intervals. All reply pulses shall start at a defined multiple of 0.5 microsecond from the first transmitted pulse. The tolerance in all cases shall be plus or minus 0.05 microsecond. 3.1.2.2.5.1.1 Reply preamble. The preamble shall consist of four pulses, each with a duration of 0.5 microsecond. The pulse intervals from the first transmitted pulse to the second, third and fourth transmitted pulses shall be 1, 3.5 and 4.5 microseconds, respectively. 3.1.2.2.5.1.2 Reply data pulses. The reply data block shall begin 8 microseconds after the leading edge of the first transmitted pulse. Either 56 or 112 one-microsecond bit intervals shall be assigned to each transmission. A 0.5-microsecond pulse shall be transmitted either in the first or in the second half of each interval. When a pulse transmitted in the second half of one interval is followed by another pulse transmitted in the first half of the next interval, the two pulses merge and a one-microsecond pulse shall be transmitted. 3.1.2.2.5.2 Pulse amplitudes. The pulse amplitude variation between one pulse and any other pulse in a Mode S reply shall not exceed 2 dB. 3.1.2.3 MODE S DATA STRUCTURE 3.1.2.3.1 DATA ENCODING 3.1.2.3.1.1 Interrogation data. The interrogation data block shall consist of the sequence of 56 or 112 data chips positioned after the data phase reversals within P6 (3.1.2.1.5.2.3). A 180-degree carrier phase reversal preceding a chip shall characterize that chip as a binary ONE. The absence of a preceding phase reversal shall denote a binary ZERO. 3.1.2.3.1.2 Reply data. The reply data block shall consist of 56 or 112 data bits formed by binary pulse position modulation encoding of the reply data as described in 3.1.2.2.5.1.2. A pulse transmitted in the first half of the interval shall represent a binary ONE whereas a pulse transmitted in the second half shall represent a binary ZERO. 3.1.2.3.1.3 Bit numbering. The bits shall be numbered in the order of their transmission, beginning with bit 1. Unless otherwise stated, numerical values encoded by groups (fields) of bits shall be encoded using positive binary notation and the first bit transmitted shall be the most significant bit (MSB). Information shall be coded in fields which consist of at least one bit. Note.— In the description of Mode S formats the decimal equivalent of the binary code formed by the bit sequence within a field is used as the designator of the field function or command. 3.1.2.3.2 FORMATS OF MODE S INTERROGATIONS AND REPLIES Note.— A summary of all Mode S interrogation and reply formats is presented in Figures 3-7 and 3-8. A summary of all fields appearing in uplink and downlink formats is given in Table 3-3 and a summary of all subfields is given in Table 3-4. 3.1.2.3.2.1 Essential fields. Every Mode S transmission shall contain two essential fields. One is a descriptor which shall uniquely define the format of the transmission. This shall appear at the beginning of the transmission for all formats. Annex 10 — Aeronautical Telecommunications Volume IV 22/11/07 3-12 The descriptors are designated by the UF (uplink format) or DF (downlink format) fields. The second essential field shall be a 24-bit field appearing at the end of each transmission and shall contain parity information. In all uplink and in currently defined downlink formats parity information shall be overlaid either on the aircraft address (3.1.2.4.1.2.3.1) or on the interrogator identifier according to 3.1.2.3.3.2. The designators are AP (address/parity) or PI (parity/interrogator identifier). Note.— The remaining coding space is used to transmit the mission fields. For specific functions, a specific set of mission fields is prescribed. Mode S mission fields have two-letter designators. Subfields may appear within mission fields. Mode S subfields are labelled with three-letter designators. 3.1.2.3.2.1.1 UF: Uplink format. This uplink format field (5 bits long except in format 24 where it is 2 bits long) shall serve as the uplink format descriptor in all Mode S interrogations and shall be coded according to Figure 3-7. 3.1.2.3.2.1.2 DF: Downlink format. This downlink format field (5 bits long except in format 24 where it is 2 bits long) shall serve as the downlink format descriptor in all Mode S replies and shall be coded according to Figure 3-8. 3.1.2.3.2.1.3 AP: Address/parity. This 24-bit (33-56 or 89-112) field shall appear in all uplink and currently defined downlink formats except the Mode S-only all-call reply, DF = 11. The field shall contain parity overlaid on the aircraft address according to 3.1.2.3.3.2. 3.1.2.3.2.1.4 PI: Parity/interrogator identifier. This 24-bit (33-56) or (89-112) downlink field shall have parity overlaid on the interrogator’s identity code according to 3.1.2.3.3.2 and shall appear in the Mode S all-call reply, DF = 11 and in the extended squitter, DF = 17 or DF = 18. If the reply is made in response to a Mode A/C/S all-call, a Mode S-only all-call with CL field (3.1.2.5.2.1.3) and IC field (3.1.2.5.2.1.2) equal to 0, or is an acquisition or an extended squitter (3.1.2.8.5, 3.1.2.8.6 or 3.1.2.8.7), the II and the SI codes shall be 0. 3.1.2.3.2.2 Unassigned coding space. Unassigned coding space shall contain all ZEROs as transmitted by interrogators and transponders. Note.— Certain coding space indicated as unassigned in this section is reserved for other applications such as ACAS, data link, etc. 3.1.2.3.2.3 Zero and unassigned codes. A zero code assignment in all defined fields shall indicate that no action is required by the field. In addition, codes not assigned within the fields shall indicate that no action is required. Note.— The provisions of 3.1.2.3.2.2 and 3.1.2.3.2.3 ensure that future assignments of previously unassigned coding space will not result in ambiguity. That is, Mode S equipment in which the new coding has not been implemented will clearly indicate that no information is being transmitted in newly assigned coding space. 3.1.2.3.2.4 Formats reserved for military use. States shall ensure that uplink formats are only used for selectively addressed interrogations and that transmissions of uplink or downlink formats do not exceed the RF power, interrogation rate, reply rate and squitter rate requirements of Annex 10. 3.1.2.3.2.4.1 Recommendation.— Through investigation and validation, States should ensure that military applications do not unduly affect the existing 1 030/1 090 MHz civil aviation operations environment. 3.1.2.3.3 ERROR PROTECTION 3.1.2.3.3.1 Technique. Parity check coding shall be used within Mode S interrogations and replies to provide protection against the occurrence of errors. 3.1.2.3.3.1.1 Parity check sequence. A sequence of 24 parity check bits shall be generated by the rule described in 3.1.2.3.3.1.2 and shall be incorporated into the field formed by the last 24 bits of all Mode S transmissions. The 24 parity check Chapter 3 Annex 10 — Aeronautical Telecommunications 3-13 22/11/07 bits shall be combined with either the address coding or the interrogator identifier coding as described in 3.1.2.3.3.2. The resulting combination then forms either the AP (address/parity, 3.1.2.3.2.1.3) field or the PI (parity/interrogator identifier, 3.1.2.3.2.1.4) field. 3.1.2.3.3.1.2 Parity check sequence generation. The sequence of 24 parity bits (p1, p2,..., p24) shall be generated from the sequence of information bits (m1, m2,..., mk) where k is 32 or 88 for short or long transmissions respectively. This shall be done by means of a code generated by the polynomial: G(x) = 1 + x3 + x10 + x12 + x13 + x14 + x15 + x16 + x17 + x18 + x19 + x20 + x21 + x22 + x23 + x24 When by the application of binary polynomial algebra, x24 [M(x)] is divided by G(x) where the information sequence M(x) is: mk + mk-1x + mk-2x2 +... + m1xk-1 the result is a quotient and a remainder R(x) of degree less than 24. The bit sequence formed by this remainder represents the parity check sequence. Parity bit pi, for any i from 1 to 24, is the coefficient of x24-i in R(x). Note.— The effect of multiplying M(x) by x24 is to append 24 ZERO bits to the end of the sequence. 3.1.2.3.3.2 AP and PI field generation. Different address parity sequences shall be used for the uplink and downlink. Note.— The uplink sequence is appropriate for a transponder decoder implementation. The downlink sequence facilitates the use of error correction in downlink decoding. The code used in uplink AP field generation shall be formed as specified below from either the aircraft address (3.1.2.4.1.2.3.1.1), the all-call address (3.1.2.4.1.2.3.1.2) or the broadcast address (3.1.2.4.1.2.3.1.3). The code used in downlink AP field generation shall be formed directly from the sequence of 24 Mode S address bits (a1, a2,..., a24), where ai is the i-th bit transmitted in the aircraft address (AA) field of an all-call reply (3.1.2.5.2.2.2). The code used in downlink PI field generation shall be formed by a sequence of 24 bits (a1, a2,..., a24), where the first 17 bits are ZEROs, the next three bits are a replica of the code label (CL) field (3.1.2.5.2.1.3) and the last four bits are a replica of the interrogator code (IC) field (3.1.2.5.2.1.2). Note.— The PI code is not used in uplink transmissions. A modified sequence (b1, b2,..., b24) shall be used for uplink AP field generation. Bit bi is the coefficient of x48-i in the polynomial G(x)A(x), where: A(x) = a1x23 + a2x22 +... + a24 and G(x) is as defined in 3.1.2.3.3.1.2. In the aircraft address ai shall be the i-th bit transmitted in the AA field of an all-call reply. In the all-call and broadcast addresses ai shall equal 1 for all values of i. 3.1.2.3.3.2.1 Uplink transmission order. The sequence of bits transmitted in the uplink AP field is: tk + 1, tk + 2... tk + 24 Annex 10 — Aeronautical Telecommunications Volume IV 22/11/07 3-14 where the bits are numbered in order of transmission, starting with k + 1. In uplink transmissions: tk + i = bi ⊕ pi where “⊕” prescribes modulo-2 addition: i equals 1 is the first bit transmitted in the AP field. 3.1.2.3.3.2.2 Downlink transmission order. The sequence of bits transmitted in the downlink AP and PI field is: tk + 1, tk + 2... tk + 24 where the bits are numbered in order of transmission, starting with k + 1. In downlink transmissions: tk + i = ai ⊕ pi where “⊕” prescribes modulo-2 addition: i equals 1 is the first bit transmitted in the AP or PI field. 3.1.2.4 GENERAL INTERROGATION-REPLY PROTOCOL 3.1.2.4.1 Transponder transaction cycle. A transponder transaction cycle shall begin when the SSR Mode S transponder has recognized an interrogation. The transponder shall then evaluate the interrogation and determine whether it shall be accepted. If accepted, it shall then process the received interrogation and generate a reply, if appropriate. The transaction cycle shall end when:

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