GPS receiver design (1)

Theoretical basis

GPS How the system works

The structure of space constellation is a kind of network structure , By multiple GPS The navigation satellite consists of , They send satellite signals without interruption . The ground monitoring part passes GPS The antenna continuously receives each GPS Signals from satellites , The satellite navigation message is obtained by demodulating the satellite signal , The parameters of satellite orbit can be extracted from satellite navigation messages , Through the relevant algorithm to calculate the trajectory of the satellite , The ground monitoring equipment sends the calculated orbit parameter information to the satellite after modulation , After receiving these parameters, the satellite passes through the modulation circuit , Modulate them to L The band is then sent to the user . Last , The user equipment section receives the currently visible GPS After the signal from the satellite , By calculating the distance from the satellite to the user equipment （ Multiply the time difference between the satellite signal generated from the satellite and the satellite signal received by the user receiver by the speed of light ）, And launched from a satellite L The basic parameters of the current satellite track are demodulated from the band modulation signal , The position of the user receiver in the space coordinate system can be calculated by the correlation algorithm .

GPS Signal structure

The structure and communication mode of satellite communication are analyzed ,GPS The signals transmitted by satellites are divided into three levels ： carrier 、 Pseudo code and data code . The data code is XOR added with the pseudo code to realize spread spectrum , Then their combined codes are controlled by biphasic shift keying （BPSK） To modulate the carrier wave , That is, the modulated signal is attached to the carrier wave with sine wave characteristics , Finally by GPS The satellite transmits the carrier signal .

Clock reference

Atomic clocks are based on electromagnetic waves , Control the clock by calibrating the electronic oscillator , Atomic clocks have very high accuracy . So the satellite's modulation circuit uses an atomic clock to provide an accurate clock , Its reference frequency is 10.23MHz, use f0 Express .GPS The system adopts direct sequence spread spectrum (DSSS) The way , It is the mechanism of realizing spread spectrum by modulating pseudo code .GPS The three carriers used for satellite signals are L1、L2 and L5, They are all composed of fundamental frequencies f0 Produced ,
L1 :154f0 = 1575.42MHz
L2 : 120f0 = 1227.6MHz
L5 : 115*f0 = 1176.45MHz

Pseudo random code

GPS The signal has multiple satellites , Because we have to distinguish between different satellites , So the pseudorandom code of each satellite is not the same ,GPS Using code division multiple access technology （CDMA）,CDMA It is also a way of spread spectrum communication , stay GPS in , What's used is BPSK（ Binary phase shift keying ） and DSSS（ Direct sequence spread spectrum ）. Binary phase shift keying . It is one of the conversion methods to convert analog signals into data values , The information keying phase-shifting mode is represented by the complex wave combination which deviates from the phase .BPSK The reference sine wave and phase reversal wave are used , Cause one party to 0, The other side is 1, Thus, it is possible to transmit and receive at the same time 2 value (1 The bit ) Information about . Pseudorandom code is the key to capture satellite signal , When the receiver receives the satellite signal , The receiver mixes the received signal with the locally generated pseudo-random noise code , If the local pseudo-random code is synchronized with the pseudo-random code generated by the satellite , Then it means that the satellite signal has been received .

m Sequence

Code sequence simulation