timesplitrinex(filename,seconds)

function timesplitrinex(filename,seconds)
% TIMESPLITRINEX splits a RINEX GPS file by time.
%   TIMESPLIT(FILENAME,t) breaks a RINEX file into shorter files of length
%   t in seconds. Useful for breaking a large file into days, or a day file
%   into hours, etc.

%   EXAMPLE: If 'myfile.obs' is a day-long rinex file, 
%   TIMESPLIT('myfile.obs',3600) will make 12 files of 1 hour long segments 
%   with names: myfile0.obs,myfile1.obs,myfile2.obs,etc.
%
% Ian M. Howat, Applied Physics Lab, University of Washington
% [e-mail]
% Version 1: 5/1/2004

%load file
outname = filename(1:findstr(filename,'.')-1);
fid = fopen(filename);

%build header
header1 = [];
header2 = [];
endheader = [];

while fid
    line = fgetl(fid);
    if isempty(findstr(line,'MARKER NAME'));
        header1 = char(header1,line);
    else
        break
    end
end 

while fid
    line = fgetl(fid);
    if isempty(findstr(line,'TIME OF FIRST OBS'));
        header2 = char(header2,line);
    else
        firstob = datenum(str2num(line(1:findstr(line,'TIME')-1)));        
        firstobline = line;
        line = fgetl(fid);
        lastob = datenum(str2num(line(1:findstr(line,'TIME')-1))); 
        break
    end
end

while fid
    line = fgetl(fid);
    endheader = char(endheader,line);
    if ~isempty(findstr(line,'END OF HEADER'))
        break
    end
end

header1(1,:) = [];
header2(1,:) = [];
endheader(1,:) = [];

body = [];
outnum = 1;
while fid
    line = fgetl(fid);
    if line ~= -1
        if ~isempty(str2num(line(1:3)))
            if str2num(line(1:3)) == 5
                tmp = str2num(line);
                newob = datenum([tmp(1)+2000,tmp(2:6)]);
            
                if etime(datevec(newob),datevec(firstob)) > seconds
                    writeit(header1,header2,firstob,lastob,endheader,body,outname,outnum);
                    body = [];
                    firstob = newob;
                    outnum = outnum+1;
                else
                    lastob = newob;
                end
            end
        end
        body = char(body,line);
    else
        writeit(header1,header2,firstob,lastob,endheader,body,outname,outnum);
        break
    end
end

fclose(fid);


function writeit(header1,header2,firstob,lastob,endheader,body,outname,outnum)


if  outnum < 10
fidout = fopen([outname,'0',num2str(outnum),'.obs'],'w');
body(1,:) = [];
A = char(header1,...
    [num2str([outname,'0',num2str(outnum)]),'                                                  MARKER NAME '],...
    header2,...
    ['  ',num2str(round(datevec(firstob))),'.0000000                TIME OF FIRST OBS   '],...
    ['  ',num2str(round(datevec(lastob))),'.0000000                TIME OF LAST OBS     '],...
    endheader,body);
else
fidout = fopen([outname,num2str(outnum),'.obs'],'w');
body(1,:) = [];
A = char(header1,...
    [num2str([outname,num2str(outnum)]),'                                                  MARKER NAME '],...
    header2,...
    ['  ',num2str(round(datevec(firstob))),'.0000000                TIME OF FIRST OBS   '],...
    ['  ',num2str(round(datevec(lastob))),'.0000000                TIME OF LAST OBS     '],...
    endheader,body);
end;

for k = 1:size(A,1)
    fprintf(fidout,'%s\n',A(k,:));
end
fclose(fidout);