function [x,y,z] = READ_header(Filename)

% Function reads the header for the specified RINEX file 
%  and returns the approx x,y,z position
%  [x,y,z] = READ_header(Filename)
%  opens file to be read by READ_rinex_sat_all, or READ_rinex_epoch
%
% Example: [x,y,z] = READ_header('tmgo0010.01o');
%  Opens the file tmgo0010.01o and returns the approx x,y,z position
% from the header
% 
% uses these global variables: 
%global fid_rinex Obs_line C1_loc P1_loc P2_loc L1_loc L2_loc num_obs

% C1_loc, P1_loc, P2_loc tell you where to find the pseudoranges.  
% L1_loc and L2_loc where to find the phases.  These locations
% only make sense in the context of the next function you need, called
% READ_rinex_sat_all.

% David Benson
% 5/18/01
% update 7/1/01

%Filename = 'tmgo0010.01o';
% set global variables for use by READ_rinex files
global fid_rinex Obs_line C1_loc P1_loc P2_loc L1_loc L2_loc num_obs

fid_rinex = fopen(Filename);
if fid_rinex == -1
   x = 0;
   y = 0;
   z = 0;
   disp([' Error can not open file'])
   return
end

info=fgets(fid_rinex,60);
header=fgets(fid_rinex);
while strncmp(header,'APPROX POSITION XYZ',19)==0;	% find xyz line
   info=fgets(fid_rinex,60);
   header=fgets(fid_rinex);
end
A=sscanf(info,'%g');		% load xyz position
x=A(1,1);
y=A(2,1);
z=A(3,1);

% search for types of OBS
info=fgets(fid_rinex,60);
header=fgets(fid_rinex);
while strncmp(header,'# / TYPES OF OBSERV',19)==0;
   info=fgets(fid_rinex,60);
   header=fgets(fid_rinex);
   if eval('info') == -1
      x = 0;
      y = 0;
      z = 0;
   	disp([' Error, READ_header: can not read OBS types'])
      return
   end
end

Obs_line = info;
C1_loc = 0;
L1_loc = 0;
L2_loc = 0;
P1_loc = 0;
P2_loc = 0;
num_obs = str2num(info(1:6));
for i = 1:num_obs
   obs = info((6*i+1):(6*i+6));
   k = findstr(obs, 'C1');
   if length(k) ~= 0
      C1_loc = i; %position if File where desired info is
   end
   k2 = findstr(obs, 'P1');
   if length(k2) ~= 0
      P1_loc = i; %position if File where desired info is
   end
   k3 = findstr(obs, 'P2');
   if length(k3) ~= 0
      P2_loc = i; %position if File where desired info is
   end
   k4 = findstr(obs, 'L1');
   if length(k4) ~= 0
      L1_loc = i; %position if File where desired info is
   end
   k5 = findstr(obs, 'L2');
   if length(k5) ~= 0
      L2_loc = i; %position if File where desired info is
   end
end

% search for end of header
while strncmp(header,'END OF HEADER',13)==0;	% find end of header
   info=fgets(fid_rinex,60);
   header=fgets(fid_rinex);
   if eval('info') == -1
      x = 0;
      y = 0;
      z = 0;
   	disp([' Error, READ_header: can not read end of Header'])
      return
   end
end