|GPS Signals in Detail
The GPS satellites transmit signals on two carrier frequencies. The L1 carrier is
1575.42 MHz and carries both the status message and a pseudo-random code for timing.
The L2 carrier is 1227.60 MHz and is used for the more precise military pseudo-random
There are two types of pseudo-random code (see tutorial for explanation of pseudo
random codes in general). The first pseudo-random code is called the C/A (Coarse
Acquisition) code. It modulates the L1 carrier. It repeats every 1023 bits and modulates
at a 1MHz rate. Each satellite has a unique pseudo-random code. The C/A code is the basis
for civilian GPS use.
The second pseudo-random code is called the P (Precise) code. It repeats on a seven day
cycle and modulates both the L1 and L2 carriers at a 10MHz rate. This code is intended for
military users and can be encrypted. When it's encrypted it's called "Y" code.
Since P code is more complicated than C/A it's more difficult for receivers to acquire.
That's why many military receivers start by acquiring the C/A code first and then move on
to P code.
There is a low frequency signal added to the L1 codes that gives information about
the satellite's orbits, their clock corrections and other system status.