function [bclk] = READnav_clock(Filename)
%
% Function reads specified navigation file and returns the 
% clock paramiters for all sats. 
%  [ bclk ] = READnav_clock(Filename)
%  inputs: Filename to read i.e. 'auto0010.01n'
%          
%  Outputs: bclk rows by PRN #
%           Columns:
%           1: Time of ephemeris, in GPS sec of week
%           2: Clock bias (sec) 
%           3: Clock drift (sec/sec)
%           4: Clock drift rate (sec/sec2)
% so if you want the clock values for PRN 3
% 
%   toe = bclk(3, 1)
%   bias = blck(3,2)
%   drift = blck(3,3)
%   drift_rate = blck(3,4)
%
% David Benson
% 7/3/01

fid5 = fopen(Filename);	%load navigation file
% check for valid fid
if fid5 == -1
   bclk = 0;
   disp([' Error, READnav_clock: can not open file'])
   return
end

info = fgets(fid5,60);
header = fgets(fid5);
while strncmp(header,'END OF HEADER',13) == 0;	% find end of header
   info = fgets(fid5,60);
   header = fgets(fid5);
end

bclk = zeros(40,4);
check = zeros(40,1);

while 1
   % get data set
   time_line = fgets(fid5);
   line1 = fgets(fid5);
   line2 = fgets(fid5);
   line3 = fgets(fid5);
   line4 = fgets(fid5);
   line5 = fgets(fid5);
   line6 = fgets(fid5);
   line7 = fgets(fid5);
   
   % check for end of file
   if eval('time_line') == -1
      break
   end
   
   Prn = str2num(time_line(1:2));
   if check(Prn) == 0
      check(Prn) = 1;
      
      CLK_bias = str2num(time_line(23:37)) * 10^(str2num(time_line(39:41)));
      CLK_drift = str2num(time_line(42:56)) * 10^(str2num(time_line(58:60)));
      CLK_drift_rate = str2num(time_line(61:75)) * 10^(str2num(time_line(77:80)));
      
      Toe = str2num(line3(4:18)) * 10^(str2num(line3(20:22)));
      
      % save data in matrix
      bclk(Prn,:) = [Toe, CLK_bias, CLK_drift, CLK_drift_rate];      
   end
end