Global Positioning System (GPS)

Physically, the PRC is just a very complicated digital code, or in other words, a complicated sequence of “1” and “0”. It almost looks like random electrical noise; hence sometimes it is called as pseudo random noise (PRN).

The two type of GPS position determination consist of the Pseudo Random Code (PRC). This code plays a significant role in the position determination technique that will be discussed later on.

If the codes were actually random, 2¹⁰²³ possibilities would exist. However, among these large possibilities codes, only few are suitable for the auto correlation or cross correlation which is necessary for the measurement of the signal propagation time. The 37 suitable codes are referred to as Gold-codes (names after a mathematician).

In fact, there are two type of pseudo random code.

The first type of pseudo random code is called C/A (coarse acquisition code), which is used in this code phase pseudo range processing technique. It modulates the L1 carrier (1575.42 MHz), repeats every 1023 bits and modulates at a 1MHz rate. The C/A code is the basis for civilian GPS use.

The second type of pseudo random code is called the P (precise) code, which is used in the carrier phase pseudo range processing technique. It modulates both the L1 and L2 carriers at a 10MHz rate, repeats every 1023 bits on a seven day cycle. This code is intended for military users and can be encrypted. During the anti-spoofing (AS) mode – for protection against interfering signals transmitted by a possible enemy, the P-code is encrypted into Y-code. Therefore, this code sometime refers as P(Y) code. This P code is much more complicated than the C/A code and more difficult for the receiver to acquire. Thus, the military receivers will start on acquire the C/A code first before move to P code.