Analysis of GPS antenna principle

　　In the design of GPS antenna unit, high-frequency and low-noise amplifiers are used to reduce the thermal noise of GPS antenna and the influence of the previous unit circuits on the receiver performance; When using GPS satellite to achieve navigation and positioning in the principle of GPS antenna, the main task of the user receiver is to extract pseudo-random noise code and data code from the satellite signal, so as to further calculate the position, speed and time (PVT) and other navigation information of the receiver carrier. Therefore, GPS receiver is a crucial user equipment.

　　At present, the practical GPS receiver circuit is generally composed of antenna unit, RF unit, communication unit and reconciliation unit. Based on the analysis of GPS satellite signal composition, this paper gives the principle and application of RF front-end GP2010.

　　1、 Composition of GPS satellite signal

　　GPS satellite signal is synthesized by typical code division multiple access (CDMA) modulation technology. Its complete signal mainly includes carrier, pseudo-random code and data code. The signal carrier is in the L-band, and the center frequencies of the two carriers are recorded as L1 and L2 respectively. The reference clock frequency f0 of satellite signal is 10.23MHz, and the center frequency of signal carrier L1 is 154 times of f0, namely:

　　fL1=154 × f0=1575.42MHz (1)

　　Its wavelength λ 1=19.03cm; The center frequency of signal carrier L2 is 120 times of f0, namely:

　　fL2=120 × f0=1227.60MHz (2)

　　2、 Analysis of GPS antenna principle

　　Its wavelength λ 2=24.42cm。 The frequency difference between the two carriers is 347.82MHz, which is about 28.3% of L2. In this way, the carrier frequency can be selected to measure or eliminate the propagation delay error caused by the ionosphere effect when the navigation signal propagates from the GPS satellite to the receiver. Pseudo random noise code (PRN), namely ranging code, mainly includes precise ranging code (P code) and coarse ranging code (C/A code). The code rate of P code is 10.23MHz, and that of C/A code is 1.023MHz. Data code is the navigation and positioning data sent by GPS satellite to user receiver in binary form, also called navigation message or D code, which mainly includes satellite calendar, satellite clock correction, ionospheric delay correction, working state information, information of C/A code conversion to captured P code and general ephemeris of all satellites; The total message consists of 1500 bits and is divided into 5 subframes. Each subframe transmits 10 words within 6s, 30 bits for each word, totaling 300 bits. Therefore, the baud rate of the data code is 50bps.

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