A high-frequency, low-noise amplifier is used in the design of the GPS antenna unit to reduce the thermal noise of the GPS antenna and the influence of the previous-stage unit circuits on the performance of the receiver; in the GPS antenna principle, the GPS receiver is used to achieve navigation positioning, and the user receiver The main task is to extract the pseudo-random noise code and data code in the satellite signal to further solve navigation information such as position, velocity and time (PVT) of the receiver carrier. Therefore, the GPS receiver is a vital user equipment.
At present, the practical GPS receiver circuit generally consists of four parts: an antenna unit, a radio frequency unit, a communication unit and a solution unit. Based on the analysis of the composition of GPS satellite signals, the principle and application of the radio frequency front-end GP2010 are given.
I. Composition of GPS satellite signals
GPS satellite signals are synthesized using a typical code division multiple access (CDMA) modulation technique. The complete signals mainly include three components: carrier, pseudo-random code, and data code. The signal carrier is in the L band, and the center frequencies of the two carriers are denoted as L1 and L2, respectively. The satellite signal reference clock frequency f0 is 10.23 MHz, and the center frequency of the signal carrier L1 is 154 times the frequency of f0, ie:
The wavelength λ1=19.03cm; the center frequency of the signal carrier L2 is 120 times the frequency of f0, ie:
fL2=120×f0=1227.60 MHz (2)
Second, GPS antenna principle analysis
Its wavelength λ2 = 24.42cm. The frequency difference between the two carriers is 347.82 MHz, which is approximately 28.3% of L2. This selection of the carrier frequency facilitates the measurement or elimination of propagation delay errors due to ionospheric effects when the navigation signal travels from the GPS satellite to the receiver. The pseudo-random noise code (PRN), that is, the ranging code, includes two kinds of precision ranging code (P code) and coarse ranging code (C/A code). The code rate of the P code is 10.23 MHz, and the code rate of the C/A code is 1.023 MHz. The data code is the navigation and positioning data sent by the GPS satellite to the user receiver in binary form. It is also called navigation message or D code. It mainly includes satellite calendar, satellite clock correction, ionospheric delay correction, working status information, and C/A code conversion. To capture the P code information and the summary ephemeris for all satellites; the total message consists of 1500 bits and is divided into 5 sub-frames, each sub-frame emits 10 words in 6 seconds, each word is 30 bits, a total of 300 bits, so the data The baud rate of the code is 50 bps.