GEOG 862
GPS and GNSS for Geospatial Professionals

More About Code Chips

Code Modulation of the L1 Carrier, see text surrounding image
Code Modulation of the L1 Carrier
Source: GPS For Land Surveyors

Up at the top is the C/A Code in the green and the P-Code is in the blue below it. There are several pieces of information here. One of the things you might notice right away is the red square wave from 1 to 0 and 0 to 1 indicated down at the bottom.

For each 180-degree phase shift, there's this shift from the 1 to the 0 and back to 1, and this is represented by this red square wave. See that the C/A Code is also represented by the dashed black square wave that is turned 90 degrees or in quadrature to the P-Code.

If there were no shift in the phase of the carriers, they would not be modulated and would not be carrying the codes.

The codes chipping rates are shown on the right-hand side. Please notice that the C/A Code chipping rate is 10 times slower than the P-Code.

L1 is broadcast at 1575.42 megahertz. Its rate is a multiple of the fundamental clock rate of 10.23 megahertz.

The length of a C/A Code is 960 feet, whereas, the length of a P-Code, 10 times shorter, because the P-Code is 10 times faster, is 96 feet, and also notice the repetition period. You see here, also, the 10 P-Codes per each C/A Code chip, which is exactly as you would expect.

The C/A Code is repeated very quickly; whereas the P-Code does not repeat for seven days, making it more secure as would be expected with a classified precise code.