GALILEO - Understanding GPS: Principles and Applications - page 576

Superframe k Superfr am e k+1

Frame #1 Frame #(i-1) Frame #i Frame #(i+1) Frame #n Frame #1

Subframe #1 Subframe #j Subframe #j(j+1) Subframe #p

UW CRC

Data field Tail bits

Computed over

FECencoded then blockinterleaved

Figure 10.7 Navigation message structure.

G (133 octal)

2

+ + + +

Output symbols
(SPS)

Data in put
(BPS)

(Alternating G /G )
12

Symbol
clock

+++ +

G (171 octal)

1

Figure 10.8 Convolutional encoder.

where the coefficient on X N is the first transmitted bit of the subframe header and the
coefficient of x 12 is the last transmitted bit of the navigation data ( N is the number of
bits in the frame, including the CRC). This polynomial is divided by the generator
polynomial G ( x ) using modulo-2 arithmetic. The remainder is a polynomial of
degree 11 in X . The first bit of the CRC is the coefficient of X 11 in this polynomial,
and the last bit of the CRC is the coefficient of X 0 .
Finally, block interleaving is applied after convolutional encoding to all
subframes, excluding the synchronization pattern. The block interleaver uses block
sizes of n k symbols and fills a

matrix having k rows and n columns column by column; symbols are then transmit-
ted row by row.

× k bits, where a n × k block interleaver takes n ×

10.3.3 SAR Signal Plan
The SAR distress messages (from distress-emitting beacons to SAR operators) will
be detected by the GALILEO satellites in the 406-406.1-MHz band and then broad-