GPS principle two
GPS satellites transmit carrier signals of two frequencies, namely L1 carriers with a frequency of 1575.42 MHz and L2 carriers with a frequency of 1227.60 MHz. Their frequencies are 154 and 120 times of the base frequency of 10.23 MHz, respectively. Their wavelengths are respectively 19.03 cm. And 24.42cm. Various signals are modulated on L1 and L2. These signals mainly include:
The C/A code, also known as the coarse acquisition code, is modulated on the L1 carrier and is a 1 MHz pseudo random noise code (PRN code) with a code length of 1023 bits (period of 1 ms). Since each satellite has a different C/A code, we often use their PRN numbers to distinguish them. The C/A code is a main signal used by ordinary users to determine the distance from the station to the satellite.
The P code, also known as fine code, is modulated on the L1 and L2 carriers and is a 10 MHz pseudorandom noise code with a period of seven days. In the implementation of AS, the P code and the W code are modulo two added to generate a secret Y code. At this time, the general user cannot use the P code for navigation and positioning.
See P code.