function dy = archive_path(t,y)

%*********************************************************************
%  Function to get interpolated (u,v) velocities from a velocity 
%  archive
%*********************************************************************
%  This function is used by trajectories.m to define a system of
%  2*N path equations for use in integrating N particle trajectories 
%  using a discrete velocity archive.  
%*********************************************************************
  
global u v X Y T

extrap_val = 0 ;

ntraj = length(y)/2 ;
dy    = zeros(size(y)) ;
xpts  = (1:ntraj)' ;
ypts  = (ntraj+1:length(y))' ;
time  = t+zeros(size(xpts)) ;

dy(xpts) = interpn(X,Y,T,u,y(xpts),y(ypts),time,'cubic',extrap_val) ;
dy(ypts) = interpn(X,Y,T,v,y(xpts),y(ypts),time,'cubic',extrap_val) ;
