GPS navigation signal

　　For C/A code, the navigation signal is composed of 25 frames of data, and each frame contains 1500 bits [2]. In addition, each frame can be divided into five groups of 300 bit sub frames. When the receiver captures the C/A code, it will take 6 seconds to capture a subframe, that is, it must take 30 seconds for a frame. Please note that only some more in-depth measurement operations can actually take 30 seconds to capture the complete framework; We will discuss it later. In fact, 30 seconds is only the average shortest time to capture a complete frame; The first positioning time (TTFF) of the system usually exceeds 30 seconds.

　　In order to perform positioning operations, most receivers must update the information of the satellite ephemeris (Almanac) and ephemeris (Ephemeris). This information is contained in the signal data transmitted by the satellite, and each subframe also contains a dedicated information set. In general, we can identify the information contained in the subframe through its categories [2] [7]:

　　Sub frame 1: Including clock correction, accuracy, and satellite operation

　　Sub frame 2-3: contains accurate orbital parameters, which can calculate the exact position of the satellite

　　Sub frames 4-5: contains rough satellite orbit data, timing correction, and operation information.

　　The receiver must use the information of satellite ephemeris and ephemeris to perform positioning operation. Once the exact distance of each group of satellites is obtained, the high-order GPS receiver will return the position information through a simple triangulation algorithm. In fact, if the information of Pseudorange and satellite position can be integrated, the receiver can accurately identify its position.

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GPS system composition

Composition of GPS system