GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 148

Neuman-Hofman (NH) synchronization codes [6] are modulated upon I5 and
Q5 at a 1-kbaud rate. For I5, the 10-symbol NH code 0000110101 is generated over
a 10-ms interval and repeated. For Q5, the 20-symbol NH code
00000100110101001110 is used. Every 1 ms, the current NH code bit is modulo-2
added to the PRN code chip. For example, on I5, the PRN code repeats 10 times
over each 10-ms interval. During this interval, the PRN code is generated normally
(upright) for repetitions 1-4, 7, and 9 (the zero bits in the I5 NH code 0000110101)
and is inverted over repetitions 5, 6, 8, and 10 (corresponding to the set bits in the I5
NH code). The start of the I5 NH code is aligned with the start of each 10-ms data
symbol that results from the FEC encoding. The Q5 NH code is synchronized with
the 20-ms data bits.
The I5 and Q5 PRN codes are generated using the logic circuit shown in Figure
4.25, which is built around three 13-bit linear feedback shift registers. Every 1 ms,
the XA coder is initialized to all 1s. Simultaneously, the XBI and XBQ coders are ini-
tialized to different values, specified in [18], to yield the I5 and Q5 PRN codes.
The minimum specified received L5 power level for signals broadcast from the
Block IIF satellites is -154.9 dBW [18].

4.5.3 M Code
The modernized military signal (M code) is designed exclusively for military use and
is intended to eventually replace the P(Y) code [19]. During the transition period of

Exclusive OR

Reset to all 1 second
on next clock

5

123 4 6 7 891011 12 13

Decode 1111111111101

All 1s

XA(t)

XA coder

XBI State for SV i

XI (t)
i

XBI(t+n T )
ic

Code clock

Initial XBI state

1-ms epoch

Reset

XQ (t)
i

123 4 5 6 7 891011 12 13

Exclusive OR

XBQ(t+n i T c )

XBI coder

XBQ state for SV i

Initial XBQ state

123 4 5 6 7 891011 12 13

Exclusive OR

XBQ coder

Figure 4.25 I5 and Q5 PRN code generation.