GPS C/A Code Signal Structure - Fundamentals of Global Positioning System Receivers: A Software Approach - page 73

on the received signal, then one millimeter of data contains all the 1,023 chips.
Different C/A codes are used for different satellites. The C/A code belongs to
the family of Gold codes, ( 5 ) which will be discussed in the next section.
Figure 5.3 shows the GPS data format. The first row shows a C/A code with
1,023 chips; the total length is 1 ms. The second row shows a navigation data bit
that has a data rate of 50 Hz; thus, a data bit is 20 ms long and contains 20 C/A
codes. Thirty data bits make a word that is 600 ms long as shown in the third
row. Ten words make a subframe that is 6 seconds long as shown in row four.
The fifth row shows a page that is 30 seconds long and contains 5 subframes.
Twenty-five pages make a complete data set that is 12.5 minutes long as shown
in the sixth row. The 25 pages of data can be referred to as a superframe.
The parameters mentioned in Section 4.10 are contained in the first three
subframes of a page. If one can receive the information of these three subframes
from four or more satellites, the user location can be found. Theoretically, one
can take a minimum of about 18 seconds of data from four satellites and be able
to calculate the user position. However, the subframes from each satellite will
not reach the receiver at the same time. Besides, one does not know when the
beginning of subframe 1 will be received. A guaranteed way to receive the first
three subframes is to take 30 seconds (or one page) of data. Thus, one can take
a minimum of 30 seconds of data and calculate the user position.

5.6 GENERATION OF C/A CODE ( 1,2,6 )

The GPS C/A signals belong to the family of Pseudorandom noise (PRN) codes
known as the Gold codes. The signals are generated from the product of two
1,023-bit PRN sequence G1 and G2. Both G1 and G2 are generated by a
maximum-length linear shift register of 10 stages and are driven by a 1.023 MHz
clock. Figure 5.4 shows the G1 and G2 generators. Figure 5.4a shows the G1
generator and Figures 5.4b and 5.4c show the G2 generator. Figure 5.4c is a
simplified notation of Figure 5.4b.
The basic operating principles of these two generators are similar; therefore,
only G2 will be discussed in detail. A maximum-length sequence (MLS) gener-
ator can be made from a shift register with proper feedback. If the shift register
has n bits, the length of the sequence generated is 2 n

−

1. Both shift generators
in G1 and G2 have 10 bits, thus, the sequence length is 1,023 (2 10

− 1). The

feedback circuit is accomplished through modulo-2 adders.
The operating rule of the modulo-2 adder is listed in Table 5.2. When the
two inputs are the same the output is 0, otherwise it is 1. The positions of
the feedback circuit determine the output pattern of the sequence. The feedback
of G1 is from bits 3 and 10 as shown in Figure 5.4a and the corresponding
polynomial can be written as G1: 1 + x 10 . The feedback of G2 is from bits

2, 3, 6, 8, 9, 10 as shown in Figure 5.4b and the corresponding polynomial is
G2: 1 +

x 3 +

x 10 .
In general, the output from the last bit of the shift register is the output of
the sequence as shown in Figure 5.4a. Let us refer to this output as the MLS

x 2 x 3 x 6 x 8 x 9 + + + + +