GPS接收机设计（4）——帧同步

 function [firstSubFrame, activeChnList] = findPreambles(trackResults, ...

    
                                                     settings)
    
  
    
 %[firstSubFrame, activeChnList] = findPreambles(trackResults, settings)
    
 %
    
 %   Inputs:
    
 %       trackResults    - output from the tracking function
    
 %       settings        - Receiver settings.
    
 %
    
 %   Outputs:
    
 %       firstSubframe   - the array contains positions of the first
    
 %                       preamble in each channel. The position is ms count 
    
 %                       since start of tracking. Corresponding value will
    
 %                       be set to 0 if no valid preambles were detected in
    
 %                       the channel.
    
 %       activeChnList   - list of channels containing valid preambles
    
  
    
 %--------------------------------------------------------------------------
    
  
    
 % CVS record:
    
 % $Id: findPreambles.m,v 1.1.2.10 2006/08/14 11:38:22 dpl Exp $
    
  
    
 % Preamble search can be delayed to a later point in the tracking results
    
 % to avoid noise due to tracking loop transients 
    
 searchStartOffset = 0;
    
  
    
 %--- 初始化子帧序列-----------------------------------
    
 firstSubFrame = zeros(1, settings.numberOfChannels);
    
  
    
 %--- 生成同步码 ----------------------------------------
    
 preamble_bits = [1 -1 -1 -1 1 -1 1 1];
    
  
    
 %重采样子帧
    
 preamble_ms = kron(preamble_bits, ones(1, 20));%每一比特对应20ms（20个）C/A码周期
    
  
    
 activeChnList = find([trackResults.status] ~= '-');%寻找出跟踪到的通道序号
    
  
    
 for channelNr = activeChnList
    
  
    
     bits = trackResults(channelNr).I_P(1 + searchStartOffset : end);%及时码的相关结果是导航电文
    
  
    
     % Now threshold the output and convert it to -1 and +1 ，以获得更好的相关特性
    
     bits(bits > 0)  =  1;
    
     bits(bits <= 0) = -1;
    
  
    
     tlmXcorrResult = xcorr(bits, preamble_ms);%互相关，维度不同则补0
    
  
    
 %% 寻找所有子帧的起始相位 ================
    
     clear index
    
     clear index2
    
  
    
     xcorrLength = (length(tlmXcorrResult) +  1) /2;
    
  
    
     %--- Find at what index/ms the preambles start ------------------------
    
     index = find(...
    
     abs(tlmXcorrResult(xcorrLength : xcorrLength * 2 - 1)) > 153)' + ...
    
     searchStartOffset;%理论互相关最大值为160，考虑了偏差，设定互相关阈值为153
    
   59. %% Analyze detected preamble like patterns ================================
    
     for i = 1:size(index) % For each occurrence
    
   62.         %--- Find distances in time between this occurrence and the rest of
    
     %preambles like patterns. If the distance is 6000 milliseconds (one
    
     %subframe), the do further verifications by validating the parities
    
     %of two GPS words
    
     
    
     index2 = index - index(i); %寻找相邻子帧之间的间隔（以ms为单位）
    
   69.         if (~isempty(find(index2 == 6000)))%判断数组index2中是否存在数值6000
    
   71.             %读取遥测字和交接字以及上1子帧最后两个比特用于奇偶校验
    
         bits = trackResults(channelNr).I_P(index(i)-40 : ...
    
                                            index(i) + 20 * 60 -1)';
    
                                        %读取遥测字和交接字以及上1子帧最后两个比特
    
  
    
         %--- 将每20个值合成一个比特 ------------------------
    
         bits = reshape(bits, 20, (size(bits, 1) / 20));
    
         bits = sum(bits);
    
  
    
         % Now threshold and make it -1 and +1 
    
         bits(bits > 0)  = 1;
    
         bits(bits <= 0) = -1;
    
  
    
         %--- Check the parity of the TLM and HOW words ----------------
    
         if (navPartyChk(bits(1:32)) ~= 0) && ...
    
            (navPartyChk(bits(31:62)) ~= 0)%奇偶校验
    
             % Parity was OK. Record the preamble start position. Skip
    
             % the rest of preamble pattern checking for this channel
    
             % and process next channel. 
    
             
    
             firstSubFrame(channelNr) = index(i);%第一子帧的位置为index(i)
    
             break;    
    
         end % if parity is OK ...
    
         
    
     end % if (~isempty(find(index2 == 6000)))
    
     end % for i = 1:size(index)
    
  
    
     % Exclude channel from the active channel list if no valid preamble was
    
     % detected
    
     if firstSubFrame(channelNr) == 0%如果第一子帧参数恒为初始值，则说明帧同步失败
    
  
    
     activeChnList = setdiff(activeChnList, channelNr);
    
     %返回activeChnList中存在，channelNr不存在的数据；下一次循环排除channelNr通道
    
     
    
     disp(['Could not find valid preambles in channel ', ...
    
                                               num2str(channelNr),'!']);
    
     end
    
     
    
 end % for channelNr = activeChnList