Features of GPS Signals:
Carriers
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 code.
What is Pseudo Random Code?
The Pseudo Random Code (PRC) is a fundamental part of GPS. Physically it’s just a very complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses.The signal is so complicated that it almost looks like random electrical noise. Hence the name “Pseudo-Random
There are two types of pseudo-random code
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 1 MHz 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 10 MHz 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.
