![]() Note that “+” symbol is a modulo 2 sum operation. There are a total of 37 types of PRN code to differentiate the satellite number of the GPS signals. Figure 2: The procedure to generate GPS C/A PRN code. Note that “+” symbol in figure 2 is a modulo 2 sum operation. That is, PRN code is also used to identify the satellite number (satellite id).Įach GPS satellite has a unique PRN code that is generated following the Gold procedure.įigure 2 below shows the procedure to generate PRN C/A code for GPS L1. Hence, the PRN code is also known as Gold code.īeside spread spectrum functionality of PRN code, this code also used to identify which satellite a received GPS signal comes from. This PRN code is generated following a specific procedures proposed by Gold. PRN code has essential roles in GPS signals. PRN code generation of GPS L1 C/A signals (More detailed information regarding GPS signal generation and receiver processing can be read from this post). This 2.046 MHz spectrum bandwidth is much higher than the original baseband bandwidth (before modulating with the PRN code) of only 100 Hz (figure 1). The 1.023 MHz is obtained because the number of chips of the PRN code is 1023/ms.Īfter modulating a PRN code with baseband signals, the spectrum (in frequency domain) width of the baseband signals becomes ☑.023 MHz = 2.046 MHz. Meanwhile, chip rate of the PRN of C/A code is 1.023 MHz (= Mega chip per second or Mcps). With this 50 bps rate, the width of the spectrum (in frequency domain) is only ±50 Hz = 100 Hz. The bit rate of GPS L1 signal is 50 bit per second (bps). The PRN code has a special chip (referring to bit that does not represent any transmitted information) structure that has the specific correlation property: high auto-correlation and low cross-correlation. The spread signals are then BPSK modulated before being transmitted with the L1 carrier signal at 1575.42 MHz. The modulation with a C/A PRN code significantly spreads the spectrum of baseband signals. ![]() Figure 1: The GPS signal generation processes. Finally, the BPSK wide spectrum signals are then transmitted with a carrier signal at 1575.42 MHz (figure 1). From figure 1, narrow bandwidth baseband signals (containing the navigation and almanac data) is modulated with a PRN code so that the spectrum of the baseband signal becomes significantly wide.Īfter the spread spectrum process of the baseband signals, the spread signals are modulated using binary phase-shift keying (BPSK) modulation method. The spread spectrum is performed by modulating the narrow bandwidth signal with a pseudo-random noise (PRN) code that has a special correlation property.įigure 1 below shows the steps to generate GPS L1 C/A signals. The spread spectrum modulation increases the bandwidth of the baseband signals. The generation of GPS L1 C/A signals passes several steps including a spread spectrum modulation of the GPS baseband signals. The main benefits of spread spectrum communication, for example, are multiple-access communication, natural and artificial interference resistance, secure communication and others. Spread spectrum communication is a type of communication where the baseband (narrow bandwidth) of transmitted signals are intentionally spread in frequency domain so that the signal has a wider bandwidth than the baseband bandwidth. With the spread spectrum communication, multiple-accesses of the GPS signals can be performed. Global positioning system (GPS), or in general global navigation satellite system (GNSS) is a type of spread spectrum communication. The C/A code implementation in MATLAB and C/C++ are presented as well. ![]() In this post, C/A codes for GPS signals generation will be discussed. ![]()
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