A single line-of-sight path between two mobile nodes is seldom the only means of propation. The two-ray ground reflection model considers both the direct path and a ground reflection path. It is shown  that this model gives more accurate prediction at a long distance than the free space model. The received power at distance is predicted by
where and are the heights of the transmit and receive antennas respectively. Note that the original equation in  assumes . To be consistent with the free space model, is added here.
The above equation shows a faster power loss than Eqn. (18.1) as distance increases. However, The two-ray model does not give a good result for a short distance due to the oscillation caused by the constructive and destructive combination of the two rays. Instead, the free space model is still used when is small.
Therefore, a cross-over distance is calculated in this model. When , Eqn. (18.1) is used. When , Eqn. (18.2) is used. At the cross-over distance, Eqns. (18.1) and (18.2) give the same result. So can be calculated as
Similarly, the OTcl interface for utilizing the two-ray ground reflection model is as follows.
$ns_ node-config -propType Propagation/TwoRayGround
Alternatively, the user can use
set prop [new Propagation/TwoRayGround] $ns_ node-config -propInstance $prop
Tom Henderson 2011-11-05