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%% Correlate signals 关联信号===== 该捕获方法是并行码相位搜索=================================================
%--- Perform DFT of C/A code ------------------------------------------
caCodeFreqDom = conj(fft(caCodesTable(PRN, :))); %conj为求复共轭的函数，fft对信号进行快速Fourier变换
%--- Make the correlation for whole frequency band (for all freq. bins) 对整个频段进行相关（适用于所有频段）
for frqBinIndex = 1:numberOfFrqBins
%--- Generate carrier wave frequency grid (0.5kHz step) -----------生成载波频率网格（0.5kHz步长）
frqBins(frqBinIndex) = settings.IF - ...
(settings.acqSearchBand/2) * 1000 ...
0.5e3 * (frqBinIndex - 1);
%--- Generate local sine and cosine -------------------------------
sinCarr = sin(frqBins(frqBinIndex) * phasePoints);
cosCarr = cos(frqBins(frqBinIndex) * phasePoints);
%！！ %--- "Remove carrier" from the signal ----------------------混频
I1 = sinCarr .* signal1;
Q1 = cosCarr .* signal1;
I2 = sinCarr .* signal2;
Q2 = cosCarr .* signal2;
%--- Convert the baseband signal to frequency domain -------将基带信号转换为频域
IQfreqDom1 = fft(I1 j*Q1);
IQfreqDom2 = fft(I2 j*Q2);
%--- Multiplication in the frequency domain (correlation in time domain) 频域乘法（时域相关性）
convCodeIQ1 = IQfreqDom1 .* caCodeFreqDom;
convCodeIQ2 = IQfreqDom2 .* caCodeFreqDom;
%--- Perform inverse DFT and store correlation results -----执行逆DFT并存储相关结果---- 傅里叶反变换---
acqRes1 = abs(ifft(convCodeIQ1)) .^ 2; %一旦接收机通过傅里叶反变换计算得到相关值，那么接下来的信号检测与线性搜索捕获法一样，找出所有自相关渎职的峰值
acqRes2 = abs(ifft(convCodeIQ2)) .^ 2; %并将该峰值与捕获门限值相比较，若超过，则接收机捕获到了信号，并且也从中获得频率和码相位两个参数值。
%--- Check which msec had the greater power and save that, will
%"blend" 1st and 2nd msec but will correct data bit issues
if (max(acqRes1) > max(acqRes2))
results(frqBinIndex, :) = acqRes1;
else
results(frqBinIndex, :) = acqRes2;
end
% end
frqBinIndex = 1:numberOfFrqBins
% t=toc;
%% Look for correlation peaks in the results =====在结果中查找相关峰值=========================
% Find the highest peak and compare it to the second highest peak
% The second peak is chosen not closer than 1 chip to the highest peak
%--- Find the correlation peak and the carrier frequency -----找到相关峰值和载波频率---------
[peakSize frequencyBinIndex] = max(max(results, [], 2));
%--- Find code phase of the same correlation peak ------------找到相同相关峰的代码相位-------
[peakSize codePhase] = max(max(results));
%--- Find 1 chip wide C/A code phase exclude range around the peak --在峰值周围找到1个芯片宽的C / A码相位排除范围--
samplesPerCodeChip = round(settings.samplingFreq / settings.codeFreqBasis);
excludeRangeIndex1 = codePhase - samplesPerCodeChip;
excludeRangeIndex2 = codePhase samplesPerCodeChip;
%--- Correct C/A code phase exclude range if the range includes array boundaries 如果范围包括数组边界，则纠正C / A代码阶段排除范围
if excludeRangeIndex1 < 2
codePhaseRange = excludeRangeIndex2 : ...
(samplesPerCode excludeRangeIndex1);
elseif excludeRangeIndex2 >= samplesPerCode
codePhaseRange = (excludeRangeIndex2 - samplesPerCode) : ...
excludeRangeIndex1;
else
codePhaseRange = [1:excludeRangeIndex1, ...
excludeRangeIndex2 : samplesPerCode];
end
%--- Find the second highest correlation peak in the same freq.bin ---
secondPeakSize = max(results(frequencyBinIndex, codePhaseRange));
%--- Store result ----peakMetric:峰度测量-------------------------------------------------
acqResults.peakMetric(PRN) = peakSize/secondPeakSize;
% If the result is above threshold, then there is a signal ...
if (peakSize/secondPeakSize) > settings.acqThreshold
%% Fine resolution frequency search ====精细分辨率频率搜索===================================
%--- Indicate PRN number of the detected signal ------指示检测到的信号的prn号-------------
fprintf('%02d ', PRN); %%d就是输出整型，用%02d，打印时显示至少两位，前面补零
%--- Generate 10msec long C/A codes sequence for given PRN ----为给定的PRN生成10毫秒长的C / A码序列----
caCode = generateCAcode(PRN);
codeValueIndex = floor((ts * (1:10*samplesPerCode)) / ...
(1/settings.codeFreqBasis));
longCaCode = caCode((rem(codeValueIndex, 1023) 1));
%！！ %--- Remove C/A code modulation from the original signal --------从原始信号中删除C / A码调制--
% (Using detected C/A code phase)
xCarrier = ...
signal0DC(codePhase:(codePhase 10*samplesPerCode-1)) ...
.* longCaCode;
%--- Find the next highest power of two and increase by 8x --------
fftNumPts = 8*(2^(nextpow2(length(xCarrier))));
%--- Compute the magnitude of the FFT, find maximum and the
%associated carrier frequency 计算FFT的幅度，找到最大值和相关的载波频率
fftxc = abs(fft(xCarrier, fftNumPts));
uniqFftPts = ceil((fftNumPts 1) / 2);
[fftMax, fftMaxIndex] = max(fftxc(5 : uniqFftPts-5));
fftFreqBins = (0 : uniqFftPts-1) * settings.samplingFreq/fftNumPts;
%--- Save properties of the detected satellite signal -------------
acqResults.carrFreq(PRN) = fftFreqBins(fftMaxIndex);
acqResults.codePhase(PRN) = codePhase;
else
%--- No signal with this PRN --------------------------------------
fprintf('. ');
end % if (peakSize/secondPeakSize) > settings.acqThreshold
end % for PRN = satelliteList
%=== Acquisition is over ==================================================