function tracer ( input_file )
% TRACER:  Displays track of massless ball thru ecom system.
%
% USAGE:  tracer ( input_file )
% 
% PARAMETERS:
%   input_file:  Name of ecomsi netcdf file.
%

if ( nargin < 1 )
	[filename, pathname] = uigetfile('*.cdf', 'What ECOMSI File?');
	input_file = sprintf ( '%s%s', pathname, filename );
end


%
% set up the global tracer object
global to;

% this call to tracer_fig is the key 
tracer_fig;
to.figure = gcf;
to.cdf = input_file;
to.title_label = findobj ( to.figure, 'tag', 'title label' );
to.depth_edit = findobj ( to.figure, 'tag', 'depth edit' );
to.axis = findobj ( to.figure, 'tag', 'tracer axis' );


%
% Grab the time
to.jd = ecomtime ( input_file );

set ( to.title_label, 'string', jd2str(to.jd(1)) );

%
% put up depth just for show
[to.depth,x,y] = kslice ( input_file, 'depth' );

%
% convert to kilometers
to.x = x/1000; to.y = y/1000; 

pslice2 ( to.x, to.y, to.depth );



