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plication data sent to the bit scrambler
a248-length, ..., a1, a0 are the virtual fill bits (if any).
b(x) =
i 0 =
Σ b
5
ix + b5x + b4x + b3x + b2x + b1x + b0 = [x m(x)] mod g(x)
3.6.3.3.5.5 The 8-bit R-S check symbols shall be appended to the application data. Each
be transmitted MSB first from b to b , i.e. the first application FEC bit transferred to the bit scram
and the last application FEC bit transferred to the bit scrambler shall be the LSB of b5.
d link (VDL) Mode 2. Moreover, for VDL Mode 2 each R-S chec
Note 3.— Example results of application FEC encoding
23/11/06 APP B-80
Appendix B Annex 10 — Aeronautical Communications
Table B-61. Format of a GBAS message block
Message block Bits
Message block header 48
Message up to 1 696
CRC 32
Table B-62. Format of message block header
Data field Bits
Messag 8
GBAS I 24
e block identifier
D
Message type identifier 8
Message length 8
3.6.3.3.6 Bit scrambling
3.6.3.3.6.1 The output of a pseudo-noise scrambler with a 15-stage generator register shall be exclusive OR’ed with the
burst data starting with the SSID and ending with the application FEC. Bit scrambling of the fill bits is optional and the set
value of the fill bits is optional.
3.6.3.3.6.2 The polynomial for the register taps of the scrambler shall be 1 + x + x15. The register content shall be rotated
1011 00 bit in the first stage of the register. The first output bit of the scrambler shall be sampled prior to
e f r
f the data shall be as shown in the message
bles in 3.6.6. All data fields in the message block shall be transmitted in the order specified in the message tables, with the
.4
identifier ( a message length, as shown in Table B-62.
oding: 1010 1010 = normal GBAS message
Note.— The fill bits are not used by the aircraft receiver and their values have no impact on the system.
at the rate of one shift per bit. The initial status of the register, prior to the first SSID bit of each burst, shall be “1101 0010
1”, with the leftmost
th irst egister shift.
Note.— A diagram of the bit scrambler is given in Attachment D, 7.4.
3.6.3.4 Message block format. The message blocks shall consist of a message block header, a message and a 32-bit
CRC. Table B-61 shows the construction of the message block. All signed parameters shall be two’s complement numbers
and all unsigned parameters shall be unsigned fixed point numbers. The scaling o
ta
LSB of each field transmitted first.
Note.— All binary representations reading left to right are MSB to LSB.
3.6.3 .1 Message block header. The message block header shall consist of a message block identifier, a GBAS
ID), a message type identifier and
Message block identifier: the 8-bit identifier for the operating mode of the GBAS message block.
C
1111 1111 = test GBAS message
All other values are reserved.
APP B-81 23/11/06
Annex 10 — Aeronautical Communications Volume I
GBAS ID: the four-character GBAS identification to differentiate between GBAS ground subsystems.
Coding: Each nal Alphabet No. 5 (IA-5) representation. For each
character, bit b1 is transmitted first and six bits are transmitted for each character. Only upper case letters, numeric
nearest airport. Assignment of GBAS IDs will
e coordinated as appropriate to avoid conflicts.
essage type identifier: the numeric label identifying the content of the message (Table B-63).
essage length: the length of the message in 8-bit bytes including the 6-byte message block header, the message and the
4-byte message CRC code.
3.6.3.4.2 Cyclic redundancy . The GBAS message CRC shall be calculated in accordance with 3.9.
3.6.3.4.2.1 The length of the CRC code shall be k = 32 bits.
3.6.3.4.2.2 The CRC generator po omial
G(x) = x x31 + 1
3.6.3.4.2.3 The CRC information ld, M(x
M = mxn–i = m xn–1 + m xn–2 + . . . + mnx0
3.6.3.4.2.4 M(x) shall be form th der and all bits of the variable-length
message, excluding the CRC. Bits ng e order transmitted, such that m1 corresponds to the first transmitted
bit of the message block header, and m corresponds to the last transmitted bit of the (n-48) message bits.
.2.5 red such that r1 is the first bit transmitted and r32 is the last bit transmitted.
3.6.4 DATA CONTENT
3.6.4.1 Message types. The message types that can be transmitted by GBAS shall be as in Table B-63.
ddresse
the differential correction data for individual GNSS ranging sources
b) low-frequency information (ephemeris decorrelation parameter, satellite ephemeris CRC and satellite availability
information); and
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