GPS FFT algorithm

　　The FFT algorithm in this paper is based on the relationship between FFT and signal convolution. The specific acquisition process is as follows:

　　1) The baseband complex signal is obtained by multiplying the acquired IF signal data with the co directional and orthogonal signals of the locally generated carrier wave, and then through a low-pass filter;

　　2) FFT complex signal;

　　3) FFT the output pseudo code signal of the local pseudo code generator and conjugate it;

　　4) Multiply the results of steps 2) and 3) by complex numbers and perform IFFT (Inverse Fast Fourier Transform);

　　5) Compare the modulus of the result in step 4) with the threshold and determine the capture result. If it is greater than the threshold, the capture is considered successful.

　　By continuously adjusting the local carrier frequency and pseudo code phase until the carrier frequency difference and pseudo code phase difference are zero, the ideal state is achieved. In fact, as long as the difference is greater than the set threshold, the strong autocorrelation peak will be achieved when the signal exists and the strength is sufficient to complete the signal acquisition.

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