A method to test HF ray tracing algorithm in the ionosphere by means of   the virtual time delay
  It is well known that a 3D ray tracing algorithm furnishes the ray's coordinates, the three components of the wave vector and the calculated group time delay of the wave along the path. The latter quantity can be compared with the measured group time delay to check the performance of the algorithm. Simulating a perfect reflector at an altitude equal to the virtual height of reflection, the virtual time delay is assumed as a real time delay. For a monotonic electronic density profile we find a very small relative difference between the calculated and the virtual delay for both analytic and numerical 3D electronic density models. 
Cesidio Bianchi, Alessandro Settimi, Carlo Scotto, Adriano Azzarone,   Angelo Lozito
physics.geo-ph