  In this paper, we present a probabilistic two-ray (PTR) path loss model for
air-to-air (AA) propagation channel in built-up areas. Based on the statistical
model of city deployment, the PTR path loss model can be applied to suburban,
urban, dense urban, and high-rise urban. The path loss is optimally fitted as
the Weibull distribution and its fluctuation is fitted as the Normal
distribution in ray-tracing simulations. The good agreements between our model
and ray tracing indicate the proposed model can provide a useful tool for
accurate and quick prediction for aerial platforms. As an extended research of
PTR model, we extract the shadowing factor by numerous simulations and propose
the altitude-dependent shadowing model. The result shows that the proposed
shadowing model has very good consistent with the measurement-based model,
which indicates that our research performs well in the extensibility and
generality.
