% 
%       id2ss
%

  function ss = id2ss(params,data)

  t = data.xdata;
  ydata2 = data.ydata;

  theta1 = params(1);
  theta2 = params(2);

  y2 = (theta1*theta2)*t;

  Res = y2 - ydata2;
  ss = Res*Res';