GPS软件接收机（3）—

跟踪的主要目的是精确化频率和码相位值，并保持跟踪，然后从跟踪到的卫星中解调出导航数据

Fig1 跟踪流程图

1.初始化输出结构体参数

%%%%tracking.m
trackResults.status         = '-';      % 初始状态为无信号捕获
trackResults.absoluteSample = zeros(1, settings.msToProcess); %共msToProcess输出值，初始值为36000，确保遍历所有子帧%记录每ms的实时码频率
trackResults.codeFreq       = inf(1, settings.msToProcess);% Frequency of the tracked carrier wave:
%记录每ms的实时跟踪载波频率
trackResults.carrFreq       = inf(1, settings.msToProcess);% Outputs from the correlators (In-phase):
% 同相支路的相关器的输出值
trackResults.I_P            = zeros(1, settings.msToProcess);
trackResults.I_E            = zeros(1, settings.msToProcess);
trackResults.I_L            = zeros(1, settings.msToProcess);% Outputs from the correlators (Quadrature-phase):
%正交支路的相关器输出值
trackResults.Q_E            = zeros(1, settings.msToProcess);
trackResults.Q_P            = zeros(1, settings.msToProcess);
trackResults.Q_L            = zeros(1, settings.msToProcess);% Loop discriminators 定义为极大值
trackResults.dllDiscr       = inf(1, settings.msToProcess);
%跟踪环路延迟锁相环鉴相器的输出值
trackResults.dllDiscrFilt   = inf(1, settings.msToProcess);
%跟踪环路延迟锁相环滤波器的输出值
trackResults.pllDiscr       = inf(1, settings.msToProcess);
%跟踪环路载波锁相环鉴相器的输出值
trackResults.pllDiscrFilt   = inf(1, settings.msToProcess);
%跟踪环路载波锁相环滤波器的输出值trackResults = repmat(trackResults, 1, settings.numberOfChannels);
%生成numberOfChannels个结构体%% 初始化跟踪变量 ==========================================codePeriods = settings.msToProcess;     % 跟踪的C/A码周期数%--- DLL variables --------------------------------------------------------
% Define early-late offset (in chips)
%超前-滞后与及时码的间隔 单位为码片
earlyLateSpc = settings.dllCorrelatorSpacing;% Summation interval
% 码环累加间隔1msPDIcode = 0.001;% Calculate filter coefficient values
% 环路采用二阶环
[tau1code, tau2code] = calcLoopCoef(settings.dllNoiseBandwidth, ...settings.dllDampingRatio, ...1);%--- PLL variables --------------------------------------------------------
% Summation intervalPDIcarr = 0.001;
% 载波环环累加间隔1ms 即1个CA码周期
% Calculate filter coefficient values
[tau1carr, tau2carr] = calcLoopCoef(settings.pllNoiseBandwidth, ...settings.pllDampingRatio, ...0.25);
hwb = waitbar(0,'Tracking...');%显示当前跟踪进度，0-1之间取值

这里使用了calcLoopCoef函数，该函数的作用是计算滤波器的系数，calcLoopCoef函数代码如下

function [tau1, tau2] = calcLoopCoef(LBW, zeta, k)%
%   Inputs:
%       LBW           - Loop noise bandwidth
%       zeta          - Damping ratio
%       k             - Loop gain
%
%   Outputs:
%       tau1, tau2   - Loop filter coefficients %--------------------------------------------------------------------------Wn = LBW*8*zeta / (4*zeta.^2 + 1);% solve for t1 & t2
tau1 = k / (Wn * Wn);
tau2 = 2.0 * zeta / Wn;

出处见7.10

2. 初始化载波环参数

for channelNr = 1:settings.numberOfChannelsif (channel(channelNr).PRN ~= 0)%只处理选择出来的通道fseek(fid, ...settings.skipNumberOfBytes + channel(channelNr).codePhase-1, ...'bof');
%将数据句柄移到C/A码相位开始处caCode = generateCAcode(channel(channelNr).PRN);% 生成相应卫星的C/A码caCode = [caCode(1023) caCode caCode(1)];%生成1025个C/A码，便于做超前、滞后处理%--- 载波环初始化s ------------------------------% define initial code frequency basis of NCOcodeFreq      = settings.codeFreqBasis;% define residual code phase (in chips)remCodePhase  = 0.0;% define carrier frequency which is used over whole tracking periodcarrFreq      = channel(channelNr).acquiredFreq;carrFreqBasis = channel(channelNr).acquiredFreq;% define residual carrier phaseremCarrPhase  = 0.0;%code tracking loop parametersoldCodeNco   = 0.0;oldCodeError = 0.0;%carrier/Costas loop parametersoldCarrNco   = 0.0;oldCarrError = 0.0;

3.载波环、码环迭代

     for loopCnt =  1:codePeriods %codePeriods为迭代的总次数，每次迭代积分时间为1ms%% GUI update -------------------------------------------------------------% 每50ms更新一次跟踪进度if (rem(loopCnt, 50) == 0)trywaitbar(loopCnt/codePeriods, ...hwb, ...['Tracking: Ch ', int2str(channelNr), ...' of ', int2str(settings.numberOfChannels), ...'; PRN#', int2str(channel(channelNr).PRN), ...'; Completed ',int2str(loopCnt), ...' of ', int2str(codePeriods), ' msec']);                       catchdisp('Progress bar closed, exiting...');returnendend%确定1ms数据块大小codePhaseStep = codeFreq / settings.samplingFreq;%确定相邻采样点之间的码相位大小%读取的块的大小 若码相位有残余值，则读取残余值剩下的CA码数据 blksize = ceil((settings.codeLength-remCodePhase) / codePhaseStep);
%剩余C/A码采样数值个数% Read in the appropriate number of samples to process this% interation [rawSignal, samplesRead] = fread(fid, ...blksize, settings.dataType);rawSignal = rawSignal';  %转置% 如果没有读取足够多得数据，说明超出数据长度if (samplesRead ~= blksize)disp('Not able to read the specified number of samples  for tracking, exiting!')fclose(fid);returnend

%设置码相位跟踪信息% Define index into early code vector 产生超前码tcode       = (remCodePhase-earlyLateSpc) : ...codePhaseStep : ...((blksize-1)*codePhaseStep+remCodePhase-earlyLateSpc);tcode2      = ceil(tcode) + 1;earlyCode   = caCode(tcode2);%转换到对应的1025个码上% Define index into late code vector 产生滞后码tcode       = (remCodePhase+earlyLateSpc) : ...codePhaseStep : ...((blksize-1)*codePhaseStep+remCodePhase+earlyLateSpc);tcode2      = ceil(tcode) + 1;lateCode    = caCode(tcode2);% Define index into prompt code vector 产生即时码tcode       = remCodePhase : ...codePhaseStep : ...((blksize-1)*codePhaseStep+remCodePhase);tcode2      = ceil(tcode) + 1;promptCode  = caCode(tcode2);%下一个码的初始相位remCodePhase = (tcode(blksize) + codePhaseStep) - 1023.0;%%生成本地正弦余弦信号-----------time    = (0:blksize) ./ settings.samplingFreq;%产生初始相位是remCarrPhase的载波相位序列向量trigarg = ((carrFreq * 2.0 * pi) .* time) + remCarrPhase;remCarrPhase = rem(trigarg(blksize+1), (2 * pi));     %下一个载波初始相位% 生成本地正弦余弦信号carrCos = cos(trigarg(1:blksize));carrSin = sin(trigarg(1:blksize));%%本地正弦余弦信号与接收信号做乘法运算，下变频到基带---------------------------% I&Q支路下变频到基带qBasebandSignal = carrCos .* rawSignal;iBasebandSignal = carrSin .* rawSignal;% I&Q支路基带解扩 即码剥离I_E = sum(earlyCode  .* iBasebandSignal);Q_E = sum(earlyCode  .* qBasebandSignal);I_P = sum(promptCode .* iBasebandSignal);Q_P = sum(promptCode .* qBasebandSignal);I_L = sum(lateCode   .* iBasebandSignal);Q_L = sum(lateCode   .* qBasebandSignal);%% 寻找锁相环偏差与更新本地载波发生器 -----------1ms------------------% 鉴相器的输出 输出为频率值( 单位Hz)carrError = atan(Q_P / I_P) / (2.0 * pi);%实现载波环路滤波器并生成NCO命令 carrNco = oldCarrNco + (tau2carr/tau1carr) * ...(carrError - oldCarrError) + carrError * (PDIcarr/tau1carr);%载波环路滤波器oldCarrNco   = carrNco;%更新本地载波频率误差oldCarrError = carrError;%更新载波相位误差carrFreq = carrFreqBasis + carrNco;%更新NCO载波发生器频率trackResults(channelNr).carrFreq(loopCnt) = carrFreq;%保存第loopCnt载波频率值%% 寻找码跟踪环偏差与更新本地码发生器 -------20ms---------------% 码环鉴相器 超前-滞后 非相干codeError = 0.5*(sqrt(I_E * I_E + Q_E * Q_E) - sqrt(I_L * I_L + Q_L * Q_L)) / ...(sqrt(I_E * I_E + Q_E * Q_E) + sqrt(I_L * I_L + Q_L * Q_L));% 实现码环滤波器并生成NCO命令codeNco = oldCodeNco + (tau2code/tau1code) * ...(codeError - oldCodeError) + codeError * (PDIcode/tau1code);%码环滤波器oldCodeNco   = codeNco;%更新码环频率误差oldCodeError = codeError;%更新码环相位误差% 用NCO控制器更新码频率codeFreq = settings.codeFreqBasis - codeNco;trackResults(channelNr).codeFreq(loopCnt) = codeFreq;%更新码环频率%记录后处理所需要的各种测量值trackResults(channelNr).absoluteSample(loopCnt) = ftell(fid);%记录每一次处理的数据在文件中的具体位置（以字节计算）trackResults(channelNr).dllDiscr(loopCnt)       = codeError;%码环鉴别器的输出trackResults(channelNr).dllDiscrFilt(loopCnt)   = codeFreq;%码环滤波器的NCO输出 codeNcotrackResults(channelNr).pllDiscr(loopCnt)       = carrError;%载波环鉴别器的输出trackResults(channelNr).pllDiscrFilt(loopCnt)   = carrFreq;%载波环滤波器的NCO输出 carrNco% 积分累加器的输出值trackResults(channelNr).I_E(loopCnt) = I_E;trackResults(channelNr).I_P(loopCnt) = I_P;trackResults(channelNr).I_L(loopCnt) = I_L;trackResults(channelNr).Q_E(loopCnt) = Q_E;trackResults(channelNr).Q_P(loopCnt) = Q_P;trackResults(channelNr).Q_L(loopCnt) = Q_L;end % for loopCnt% If we got so far, this means that the tracking was successful% Now we only copy status, but it can be update by a lock detector% if implementedtrackResults(channelNr).status  = channel(channelNr).status;%更新跟踪状态        end % if a PRN is assigned
end % for channelNr % Close the waitbar
close(hwb)

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