GEOG 862
GPS and GNSS for Geospatial Professionals

The One-percent Rule of Thumb

 illustration of thumbs up
Source: Big Thumbs Up by 
Charles LeBlanc is licensed under 
CC BY-SA 2.0

Here is convenient approximation. The maximum resolution available in a pseudorange is about 1 percent of the chipping rate of the code used. It offers a basis to evaluate pseudoranging in general and compare the potentials of P(Y) code and C/A code pseudoranging in particular. A P(Y) code chip occurs every 0.0978 of a microsecond. In other words, there is a P(Y) code chip about every tenth of a microsecond. That’s one code chip every 100 nanoseconds. Therefore, a P(Y) code based measurement can have a maximum precision of about 1 percent of 100 nanoseconds, or 1 nanosecond. What is the length of 1 nanosecond? Well, multiplied by the speed of light, it's approximately 30 centimeters, or about a foot. So, just about the very best you can do with a P(Y) pseudorange is a foot or so. Because its chipping rate is 10 times slower, the C/A code based pseudorange is 10 times less precise. Therefore, one percent of the length of a C/A code chip is 10 times 30 centimeters, or 3 meters. Using the rule of thumb, the resolution of a C/A code pseudorange is nearly 10 feet. 

You might ask, at this point, if the pseudorange isn't the full answer, then how do we get the extraordinary accuracies that we depend upon GPS to produce?