Monday, January 11, 2016

Accelerated GPS musings....

GPS L1 is at 1575.42 Mhz.

The normal GPS CA code chip period is  1msec.

Most GPS systems integrate for one or more whole Chip intervals dump the results and repeat.

The hardware typically has a numerical controlled oscillator (NCO) that represents the satellite frequency that is integrated over this time.

So if we integrate a 1575.42 Mhz signal for 1 msec, how far off can our frequency be before our
phase error is 180 degrees at the end of 1 msec?  its about 500hz....
(Which is why the tong or FTP search bins for finding the inital GPS are at 250Hz or so.)

Most tracking implementations have single value of Frequency driving the NCO....
If you want things to work well then your phase error over one period should be less than 45 degrees... (where Tan is linear)  so that is a frequency error of about ~125Hz or a Velocity error of
12 m/sec.   So for 1 msec of Acceleration that is a  1200 G, so it should be

No comments: