GPS Signal - A Software-Defined GPS and Galileo Receiver: A Single-Frequency Approach - page 17

1227.6 MHz

BPSK
modulator

X 120 -6dB L2 Signal

1227.6 MHz

BPSK
modulator

-3dB

1575.42 MHz

X 154

Σ

L1 Signal
1575.2 MHz

BPSK
modulator

90°

P(Y) code ⊕ data

P(Y) code
generator

Limiter

X1

P(Y) code

Switch

f 0 = 10.23 MHz

C/A code
generator

÷ 10

C/A code ⊕ data

1000 Hz

÷ 20

50 Hz

50 bps data

Data
generator

Data
information

FIGURE 2.1. Generation of GPS signals at the satellites.

Spreading sequence Each satellite has two unique spreading sequences or codes.
The first one is the coarse acquisition code (C/A), and the other one is the
encrypted precision code (P(Y)). The C/A code is a sequence of 1023 chips.
(A chip corresponds to a bit. It is simply called a chip to emphasize that
it does not hold any information.) The code is repeated each ms giving a
chipping rate of 1.023 MHz. The P code is a longer code ( 10 4 chips)

with a chipping rate of 10.23 MHz. It repeats itself each week starting at
the beginning of the GPS week which is at Saturday/Sunday midnight. The
C/A code is only modulated onto the L1 carrier while the P(Y) code is
modulated onto both the L1 and the L2 carrier. Section 2.3 describes the
generation and properties of the spreading sequences in detail.

≈ 2 . 35 ·

2.2 GPS Signal Scheme

In the following a detailed description of the signal generation is given. Figure 2.1
is a block diagram describing the signal generation; see Kaplan & Hegarty (2006),
page 124.
The block diagram should be read from left to right. At the far left, the main
clock signal is supplied to the remaining blocks. The clock signal has a frequency
of 10.23 MHz. Actually, the exact frequency is 10.22999999543 MHz to adjust
for relativistic effects giving a frequency of 10.23 MHz seen from the user on
Earth. When multiplied by 154 and 120, it generates the L1 and L2 carrier signals,
respectively. At the bottom left corner a limiter is used to stabilize the clock signal
before supplying it to the P(Y) and C/A code generators. At the very bottom
the data generator generates the navigation data. The code generators and the