  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.
