GEOG 862
GPS and GNSS for Geospatial Professionals

The C/A Code


The C/A code is also a particular series of ones and zeroes, but the rate at which it is generated is 10 times slower than the P(Y) code. The C/A code rate is 1.023 million bits per second. Here, satellite identification is quite straightforward. Not only does each GPS satellite broadcast its own completely unique 1023 bit C/A code, it repeats its C/A code every millisecond. The legacy C/A code is broadcast on L1 only. It used to be the only civilian GPS code, but no longer; it has been joined by a new civilian signal known as L2C that is carried on L2.

diagram showing C/A Code @ 1.023 MBPS is F0/10 = 10.23 MHz/10, see surrounding text
C/A Code
Source: GPS for Land Surveyors


Still, the C/A code is the vehicle for the Standard Positioning Service, SPS, which is used for most civilian surveying applications. The P(Y) code on the other hand provides the same service for the precise positioning servicer, PPS. The idea of SPS and PPS was developed by the Department of Defense many years ago. SPS was designed to provide a minimum level of positioning capability considered consistent with national security, ±100m, 95% of the time, when intentionally degraded through Selective Availability (SA).

Selective Availability, the intentional dithering of the satellite clocks by the Department of Defense was instituted in 1989, because the accuracy of the C/A point positioning as originally rolled out was too good! As mentioned above, the accuracy was supposed to be ±100 meters horizontally, 95% of the time, with a vertical accuracy of about ±175 meters. But, in fact, it turned out that the C/A-code point positioning gave civilians access to accuracy of about ±20 meters to ±40 meters. That was not according to plan, so the satellite clocks’ accuracy was degraded on the C/A code. The good news is that the intentional error source called SA is gone. It was switched off on May 2, 2000 by presidential order. The intentional degradation of the satellite clocks is a thing of the past. Actually, Selective Availability never did hinder the surveying applications of GPS. We will delve into the reason a bit later. In any case, satellite clock errors, the unintentional kind, still contribute error to GPS positioning.

PPS is designed for higher positioning accuracy and was originally available only to users authorized by the Department of Defense; that has changed somewhat, more about that later in this chapter. It used to be that the P(Y) code was the only military code. That is no longer the case. It has been joined by a new military signal called the M-code.

The C/A Code or Civilian Acquisition or Access Code is generated 10 times slower than the P-Code. The GPS fundamental clock rate is 10.23 megahertz, but C/A Code is generated at 1.023 megabits per second.

The C/A Code is modulated onto the carrier by phase modulation, too.

The image shows a green line, a sine wave that only transitions when it reaches that center line and reverses direction. That is a phase shift, and then the code, it goes from the 1 to the 0 or from the 0 to the 1.