Page 20 - NGTU_paper_withoutVideo

Modern Geomatics Technologies and Applications

are combined in a very special manner. The G register always supplies its output, but the G register supplies two of its states
to a modulo-2 adder to generate its output. The selection of states for the modulo-2 adder is called the phase selection. Moreover,
a shift register is a set of one bit storage or memory cells. When a clock pulse is applied to the register, the content of each cell
shifts one bit to the right. The content of the last cell is “read out” as output. The special properties of such shift registers depend
on how information is “read in” to cell 1. For a tapped linear feedback shift register, the input to cell 1 is determined by the state
of the other cells. For example, the binary sum from cells 3 and 10 in a 10-cell register could be the input. If cells 3 and 10 have
different states (one is 1 and the other 0), a 1 will be read into cell 1 on the next clock pulse. If cells 3 and 10 have the same state,
0 will be read into cell 1. If we start with 1 in every cell, 12 clock pulses later the contents will be 0010001110. The next clock
pulse will take the 1 in cell 3 and the 0 in cell 10 and place their sum (1) in cell 1. Meanwhile, all other bits have shifted cell to
the right, and the 0 in cell 10 becomes the next bit in the output.
The other G polynomial has cells 2, 3, 6, 8, 9, and 10 are tapped and binary-added to get the new input to cell 1. In this
case, the output comes not from cell 10 but from a second set of taps. Various pairs of these second taps are binary-added. The
different pairs yield the same sequence with different delays or shifts. The delayed version of the G sequence is binary-added
to the output of G . There are actually 37 PRN C/A codes, but two of them (34 and 37) are identical. A subset of the first 32
codes are assigned to (nominally 24) satellites and recycled when old satellites die and new satellites are launched. Codes 33
through 37 are reserved for other uses, including ground transmitters. Each of the first 32 segments is associated with a different
satellite. The P code generation follows the same principles as the C/A code, except that 4 shift registers with 12 cells are used.
The code is combined with the binary navigation data using modulo-2 addition: If the code chip and the data bite are the same
(both are 0s or both are 1s), the result is 0; and if both are different, the result is 1. The composite binary signal is then impressed
upon the carrier in a process called modulation. The combination of two polynomials and the generation of C/A code are shown
together in Figure 8.

Figure 8. C/A code generation [17].

It is necessary to mention PRN sequences or C/A codes 33 through 37 are reversed for other users (e.g. ground
transmitters). Let us take a quick look at two special properties of PRN sequences which values ∓1. Frist. PRN sequences are
nearly orthogonal to each other: The sum to the term-by-term products of the two sequences, shifted arbitrarily relative to each
other, is nearly zero. For two GNSS satellites k and l , which are assigned unique PRN sequences C/A codes x ( ) and x ( ) as
Equation 8.

7