GPS satellite signal composition and principle analysis
In the GPS antenna unit design using a high-frequency, low-noise amplifier, in order to attenuate the GPS antenna thermal noise and the front several levels of unit circuitry on the performance of the receiver; in the GPS antenna principle of the use of GPS satellites to achieve navigation and positioning, the main task of the user's receiver is to extract the pseudo-random noise code in the satellite signal and the data code, in order to further decode to get the position of the receiver carrier, speed and time (PVT) and other navigation information. Therefore, the GPS receiver is a crucial user device.
The current practical application of GPS receiver circuit is generally composed of antenna unit, radio frequency unit, communication unit and decoding unit and other four parts, this paper analyzes the composition of GPS satellite signals on the basis of the RF front-end GP2010 principle and application is given.
First, the composition of GPS satellite signals
GPS satellite signal using typical code division multiple access (CDMA) modulation technology for the synthesis of the complete signal mainly consists of carrier, pseudo-random code and data code and other three components. The signal carrier is in the l-band, and the center frequencies of the two carriers are noted as L1 and L2. satellite signal reference clock frequency f0 is 10.23MHz, the center frequency of the signal carrier L1 is 154 times the frequency of f0, that is:
fL1=154×f0=1575.42MHz (1)
Its wavelength λ1=19.03cm; the center frequency of the signal carrier L2 is 120 times the frequency of f0, i.e.:
fL2=120&TImes;f0=1227.60MHz (2)
Second, the principle of GPS antenna analysis
Its wavelength λ2=24.42cm. the frequency difference between the two carriers is 347.82MHz, which is about 28.3% of L2, so it is convenient to choose the carrier frequency to measure or eliminate the propagation delay error due to the ionosphere effect when the navigation signal propagates from the GPS satellite to the receiver. Pseudo-random noise code (PRN), i.e. ranging code, mainly has two kinds of fine ranging code (P code) and coarse ranging code (C/A code). The code rate of P code is 10.23 MHz, and the code rate of C/A code is 1.023 MHz. data code is the navigation and positioning data sent from GPS satellites to the user's receiver in binary form, which is also called navigation message or D code, and it mainly includes the satellite calendar, satellite clock correction, ionospheric delay correction, operating status information, C/A code conversion to capture the information of P code, and the approximate ephemeris of all satellites; the total message consists of 1500 bits, and the total message is composed of 1500 bits. The total message consists of 1500 bits, divided into 5 subframes, each subframe transmits 10 words in 6s, 30 bits per word, totaling 300 bits, so the baud rate of the data code is 50bps.
