find_alm.m

此功能包用于各种GPS坐标和时间的转换

function [gps_alm_file, glo_alm_file] = find_alm(GPS_week_start)

% [gps_alm_file, glo_alm_file] = find_alm(GPS_week_start);
%
% Search the Matlab path for the most recent GPS (and GLONASS) almanacs.
% The seach starts with the GPS_week_start and proceeds backward in time.
%
% Inputs:
%   GPS_week_start - GPS week number to begin almanac seach.  (optional)
%                     If no start week is given, the computer clock time will be used.
%                     The files that are searched for have the naming convention of
%                     gps###.alm and glo###.alm (or gps####.alm).  Almanac names
%                     are stored as mod(1024) weeks to conform to YUMA standards.  
%                     This function will search backward across the 1024 rollover
%                     to find the most recent almanac.  Input weeks to begin the
%                     search should not include the rollover effect 
%                     (e.g. Aug 29, 1999 = GPS week 1025).  See UTC2GPS for more 
%                     information on handeling the 1024 week rollover times.  This
%                     function will search back for 250 weeks.  If there are no 
%                     almanacs within the 250 weeks previous to the start date,
%                     blank output file name(s) are returned.
% Outputs:
%   gps_alm_file   - Most recent GPS almanac file name (string).
%   glo_alm_file   - Most recent GLONASS almanac file name (string).
%
% See also READYUMA

% Written by: Jimmy LaMance 8/20/97
% Copyright (c) 1998 by Constell, Inc.

% functions called: ERR_CHK

%%%%% BEGIN VARIABLE CHECKING CODE %%%%%
% declare the global debug mode
global DEBUG_MODE

% Check inputs to determine if a valid week number was provided, if not
% get the time from the computer clock
if nargin < 1  
  now_utc = clock;
  now_gps = utc2gps(now_utc);
  
  GPS_week_start = now_gps(1);
end % if nargin == 1

% Take care of week 1024 rollovers in the GPS start week
max_lookback = 250; 			% maximum number of weeks to look backward for an almanac

if GPS_week_start >= 1024
  rollover_flag = 1;
else 
  rollover_flag = 0;
end

if GPS_week_start < 469
   fprintf('Warning: find_alm is searching for an almanac before 1989.\n');
   fprintf('If this is correct, ignore this message.\n');
   fprintf('If this search is for a date past August 1999, \n')
   fprintf('Use the week number + 1024 (e.g. (week 23) = 1024 + 23 = (linear week 1047)) \n\n')
end % if

GPS_week_start = rem(GPS_week_start,1024);
  
% Get the current Matlab version
% Get the current Matlab version
matlab_version = version;
matlab_version = str2num(matlab_version(1));

% If the Matlab version is 5.x and the DEBUG_MODE flag is not set
% then set up the error checking structure and call the error routine.
if matlab_version >= 5.0                        
  estruct.func_name = 'FIND_ALM';

  % Develop the error checking structure with required dimension, matching
  % dimension flags, and input dimensions.
  estruct.variable(1).name = 'GPS_week_start';
  estruct.variable(1).req_dim = [1 1];
  estruct.variable(1).var = GPS_week_start;
  
  % Call the error checking function
  stop_flag = err_chk(estruct);
  
  if stop_flag == 1           
    fprintf('Invalid inputs to %s.  Returning with empty outputs.\n\n', ...
             estruct.func_name);
    return
  end % if stop_flag == 1
end % if matlab_version >= 5.0 & isempty(DEBUG_MODE) 

%%%%% END VARIABLE CHECKING CODE %%%%%

%%%%% BEGIN ALGORITHM CODE %%%%%

GPS_file_found = 0;
GLONASS_file_found = 0;

% set the output files to the null string
gps_alm_file = blanks(0);
glo_alm_file = blanks(0);

% set the starting week                                          
this_week = GPS_week_start;

% start looping through GPS weeks for the most recent GPS almanac
num_looks = 0;
while GPS_file_found == 0 & this_week >= 0 & num_looks < max_lookback
   % set up the file name for the GPS almanac using the user provided week
   %Fixed handeling of input start GPS weeks < 1024.  Modified file search string to force a 1, 2, 3, and 4 digit search.
   for i=1:4
      format_string = sprintf('gps%%0%dd.alm',i);
      testfile = sprintf(format_string,this_week);

  		% see if the GPS almanac exists
  		if exist(testfile) == 2    % the file exists 
 	   	 GPS_file_found = 1;
          gps_alm_file = testfile;
          break;
       end % if exist(testfile) == 2
   end % for
    
  
   % decrease the week number to check
   if this_week == 0 & rollover_flag == 1
   	this_week = 1024;
   end % if this_week == 0 & rollover_flag == 1
   
   this_week = this_week - 1;
   num_looks = num_looks + 1;
end % while GPS_file_found == 0 & this_week >= 0

% repeat the process for the GLONASS almanac
% set the starting week                                          
this_week = GPS_week_start;                                          

% start looping through GPS weeks for the most recent GLONASS almanac
num_looks = 0;
while GLONASS_file_found == 0 & this_week >= 0 & num_looks < max_lookback
  for i=1:4
    format_string = sprintf('glo%%0%dd.alm',i);
    testfile = sprintf(format_string,this_week);
  
    % see if the GLONASS almanac exists
    if exist(testfile) == 2    % the file exists 
      GLONASS_file_found = 1;
      glo_alm_file = testfile;
      break;
    end % if exist(testfile) == 2
  end % for
  
  % decrease the week number to check
  if this_week == 0 & rollover_flag == 1
    this_week = 1024;
  end % if this_week == 0 & rollover_flag == 1
  
  this_week = this_week - 1;
  num_looks = num_looks + 1;
end % while GPS_file_found == 0 & this_week >= 0

% verify that files were found, if not report a warning to the user
if GLONASS_file_found == 0 & GPS_file_found == 0
  fprintf('No GPS or GLONASS almanacs were found in the Matlab path.\n');
  fprintf('File names will return as a null string.\n');
  return                                                
end % if GLONASS_file_found == 0 & GPS_file_found == 0

if GLONASS_file_found == 0
  fprintf('No GLONASS almanacs were found in the Matlab path.\n');
  fprintf('The GLONASS file name will return as a null string.\n');
  return                                                
end % if GLONASS_file_found == 0

if GPS_file_found == 0
  fprintf('No GPS almanacs were found in the Matlab path.\n');
  fprintf('The GPS file name will return as a null string.\n');
  return                                                
end % if GPS_file_found == 0

%%%%% END ALGORITHM CODE %%%%%

% end of FIND_ALM