function [nav]=readnav(filename)
% -----------------------------------------------------------------------------
% Purpose: Read GPS RINEX Navigation File
%
% Input:   filname     - GPS navigation file name
% Output:  nav(i,1)    - PRN Nr
%          nav(i,2)    - GPS week
%          nav(i,3)    - GPS seconds
%          nav(i,4:34) - nav messages
%
%          size (nav,1): number of message blocks
%
% Remark:  The function buffers the content of the file. If called a second
%          time using the same filename it simply returns the content of
%          the buffer.
%
% Author:  Urs Hugentobler, FESG
% Date  :  19-10-2006
% Changes: 05-10-2007, PS: Handling of empty spare fields
%          06-10-2007, PS: Reading of BRDC files
%          06-10-2007, PS: Bugfix for year < 2000
%          06-10-2007, PS: Use strcmp for check if file was already read
% -----------------------------------------------------------------------------

persistent fileold;
persistent data;

% if file was already read
read = 1;
if ~isempty(fileold)
    if strcmp(filename,fileold);
         read = 0;
    end;
end;

if read == 1
% open file
    fid = fopen (filename, 'r');
    s=' ';

% skip header
    while ~feof(fid);
        s = fgetl (fid);
        if size(findstr(s,'END OF HEADER'),1) == 1, break, end;
    end;

% read data records
    n=0;
    while ~feof(fid);
    
% read first line
        s  = fgetl (fid);
        s  = strrep(s,'D','e');   % replace D in exponent by e
        s  = strrep(s,'E','e');   % replace E in exponent by e       
% formatted read does not work properly for V6 (but for V7)
%       [line,c,e] = sscanf (s,'%2d %2d %2d %2d %2d %2d %4f%19f%19f%19f');

% add space before negative numbers
        s  = strrep(s,'e-','em'); s=strrep(s,'-',' -'); s=strrep(s,'em','e-');
        [line,c,e] = sscanf(s,'%d %d %d %d %d %d %f %f %f %f');
        if (~isempty(e))
            error(e);
        end;
        n=n+1;
    
% PRN number
        data(n,1)=line(1);
    
% convert date to GPS week and second
        y=line(2);m=line(3);d=line(4);h=line(5);min=line(6);sec=line(7);
        if (y < 80);            y=y+2000;
        else;                   y=y+1900;
        end;
        [data(n,2),data(n,3)]=gpstime(y,m,d,h,min,sec);
        data(n,4)=line(8);
        data(n,5)=line(9);
        data(n,6)=line(10);
    
% read remaining data lines
        for i=1:7;
            s  = fgetl (fid);
            s  = strrep(s,'D','e');
            s  = strrep(s,'E','e');
            
% formatted read does not work properly for V6 (but for V7)
%           line = sscanf (s,'   %19f%19f%19f%19f');

            s  = strrep(s,'e-','em'); s=strrep(s,'-',' -'); s=strrep(s,'em','e-');
            line = sscanf (s,'%f %f %f %f');                        
            data(n,i*4+3)=line(1);
            data(n,i*4+4)=line(2);
            
            % Extra handling of files with empty spare fields
            try
              data(n,i*4+5)=line(3);
              data(n,i*4+6)=line(4);
            catch
              data(n,i*4+5)=NaN;
              data(n,i*4+6)=NaN;
            end
        end;
        
    end;

% close file and return
    fclose (fid);

end;

nav = data;
fileold = filename;