GPS Satellite Signal Characteristics - Understanding GPS: Principles and Applications - page 127

Table 4.2 Legacy GPS Signal Structure
Signal Priority

Primary Secondary

Signal designation L1 L2

Carrier frequency (MHz) 1,575.42 1,227.60

P(Y) = 10.23 and P(Y) = 10.23 or

PRN codes (Mchip/s)

C/A = 1.023 C/A = 1.023 (Note 1)

Navigation message data
modulation (bps)

50 50 (Note 2)

1. The code usually selected by the CS on L2 is P(Y) code.
2. The 50-Hz navigation data message is usually modulated on L2 P(Y) code but can be turned
off by the CS. There are three possibilities on L2: P(Y) code with data, P(Y) code with no data,
and C/A code with data.

ity/antispoofing module (SAASM). The use of the AS Y-code denies direct (SPS GPS
receiver) access to the precision code. This significantly reduces the possibility of an
enemy spoofing a PPS receiver (i.e., transmitting a stronger, false precise code that
captures and misleads the receiver). However, AS also denies direct access to the
precision code to all SPS users, friendly or otherwise. Indirect access is still possible
as discussed in [11] and Section 5.14.

4.3.1.1 Direct Sequence PRN Code Generation
Figure 4.9 depicts a high-level block diagram of the direct sequence PRN code gen-
eration used for GPS C/A code and P code generation to implement the CDMA tech-
nique. Each synthesized PRN code is derived from two other code generators. In
each case, the second code generator output is delayed with respect to the first
before their outputs are combined by an exclusive-or circuit. The amount of delay is
different for each SV. In the case of P code, the integer delay in P-chips is identical to
the PRN number. For C/A code, the delay is unique to each SV, so there is only a
table lookup relationship to the PRN number. These delays are summarized in
Table 4.3. The C/A code delay can be implemented by a simple but equivalent tech-
nique that eliminates the need for a delay register. This technique is explained in the
following paragraphs.
The GPS C/A code is a Gold code [12] with a sequence length of 1,023 bits
(chips). Since the chipping rate of the C/A code is 1.023 MHz, the repetition period
of the pseudorandom sequence is 1,023/(1.023 10 6 Hz) or 1 ms. Figure 4.10 illus-

trates the design architecture of the GPS C/A code generator. Not included in this
diagram are the controls necessary to set or read the phase states of the registers or
the counters. There are two 10-bit shift registers, G1 and G2, which generate maxi-
mum length PRN codes with a length of 2 10

×

1,023 bits. (The only state not used
is the all-zero state). It is common to describe the design of linear code generators by
means of polynomials of the form 1

− 1 =

X i , where X i means that the output of the i th
cell of the shift register is used as the input to the modulo-2 adder (exclusive-or), and
the 1 means that the output of the adder is fed to the first cell [8]. The design specifi-
cation for C/A code calls for the feedback taps of the G1 shift register to be con-
nected to stages 3 and 10. These register states are combined with each other by an
exclusive-or circuit and fed back to stage 1. The polynomial that describes this shift
register architecture is: G1

+Σ

= 1 X 3 X 10 . The polynomials and initial states for both