Three Parts of GPS

　　The first Space Segment mainly consists of 24 satellites operating in semi-synchronous orbit. The so-called semi-synchronous orbit means that it takes about 12 hours (11 hours and 58 minutes) for a satellite to operate a week, so the same satellite happens to appear on the head of a stationary user twice in a day (23 hours and 56 minutes). The 24 satellites are evenly distributed on 6 orbital planes with 4 satellites in each orbit, and each orbital plane has an inclination Angle of 55 degrees from the equator. The average orbital altitude of these satellites is about 20200Km

　　The ranging signal from the 24 satellites has two channels (D-Band), one is called L1, the transmission frequency is 1575.42MHz; The second is L2 with a transmission frequency of 1227.6MHz. These two carrier frequencies are responsible for transmitting the Spread Spectrum signal as a high frequency carrier. GPS uses spread-spectrum communication (Spread-spectrum) technology. ranging codes modulated by spread spectrum can be divided into two kinds: The first is 1.023MHz C/A codes (coarse/acquisition codes). As the name implies, the precision is coarse according to the code. Moreover, C/A codes are only modulated with L1 and are specially opened to general commercial users. The other ranging Codes are P codes (Precision Codes) of 10.23MHz, which are modulated on both L1 and L2 carriers. Due to the short Chip of P codes, more accurate positioning can be obtained. Both range codes can be used to measure the distance between the satellite and the receiver. However, usually P codes are also encrypted and only available to authorized users, so P codes are usually only used in military positioning systems, and because of these two different sets of codes, GPS is also divided into so-called Standard Positioning Service (SPS) and Precise Positioning Service (PPS). The standard positioning service uses only C/A codes on the L1 carrier. Besides C/A code on L1 carrier and P code on L1 and L2 carrier, the last and most important thing for precise positioning service is a 50Hz data information code. The data contained in this code includes the satellite timing data, the exact satellite orbit data (ephemeris), which contains the orbit data of the satellite transmitting the signal, and the almanac data, which contains the basic orbit data of all the satellites, as well as the ionospheric propagation correction of the signal.

　　The Control Segment is composed of the various ground control stations located in different locations. Its Master Control Station (MCS) is located in Colorado, USA, and the remaining stations are evenly distributed along the equator. The main task of these ground control stations is to monitor and control these satellites operating in space, its functions include satellite orbit control, satellite orbit data update, satellite itself maintenance and so on. The operation process of this system is that each sub-station receives the orbit data of its locked satellite, then transmits the data to the main control station through the Defense satellite communication system of the United States for the estimation and reprediction of the orbit data, and then uploads the new orbit data to each satellite through the ground antenna station.

　　The last Segment, the User Segment, is closely related to the general user segment. In fact, the so-called User Segment refers to the GPS receiver, which is the protagonist to be discussed in this paper. Its function is RF receiving, decoding and processing satellite orbit data, so as to calculate the position of the receiver.

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