Issues in RF propagation modeling in an urban environment using the Extended Air Defense Simulation (EADSIM) mission level model.

As military operations in urban environments become more numerous, the ability of combat units to communicate, jam enemy communications, or employ RF weapons within this environment must be evaluated. To perform this evaluation in a mission level model requires a capability to evaluate the contributions of both terrain and man-made structures (interior and exterior) to RF propagation. The present study is an analysis of the adequacy of a mission level model (EADSIM) to perform these RF propagation calculations in an urban environment. Three basic environments must be assessed. The first environment consists entirely of terrain, with no man-made features impacting propagation values. The second environment includes terrain, but also includes the contribution of solid structures with abrupt edges, which may obstruct/influence LOS paths. The third environment includes not only terrain and structures, but also contains structures with interior features which must be evaluated to determine the propagation levels within and around these structures. EADSIM was used as the model for evaluation in view of its suite of propagation tools which can be used for analysis of RF propagation between transmitters and receivers including terrain. To assess EADSIM's capability to perform in these environments, flat terrain maps with an obstruction were created to permit comparison of EADSIM's propagation models with analytical calculations and with measurements. Calculations from the Terrain Integrated Rough Earth Model (TIREM) and the Spherical Earth Knife Edge (SEKE) propagation models included within EADSIM showed that the ability of the models to calculate knife-edge diffraction agreed favorably with analytical values. The representation of multipath effects was less encouraging. SEKE only models multipath when Fresnel clearance exists. TIREM models multipath, but the cyclical characteristics of multipath are not represented, and only subtractive path loss is considered. Multipath is only evaluated along a 2-D path in the vertical orientation. This precludes modeling propagation in the urban canyons of metropolitan areas, where horizontal paths are dominant. It also precludes modeling exterior to interior propagation. In view of the apparent inadequacy of urban propagation within mission level models, as evidenced by EADSIM, the study also attempts to address possible solutions to the problem. Correction of the sparsing techniques in both TIREM and SEKE models is recommended. Both SEKE and TIREM are optimized for DTED level 1 data, sparsed at 3 arc seconds resolution. This led to significant errors when map data was sparsed at higher or lower resolution. TIREM's errors would be significantly reduced if the 999 point array limit was eliminated. This would permit using interval sizes equal to the map resolution for larger areas. This same problem could be fixed in SEKE by changing the interval spacing from a fixed 3 arc second resolution ({approx}93 meters) to an interval which is set at the map resolution. Additionally, the cell elevation interpolation method which TIREM uses is inappropriate for the man-made structures encountered in urban environments. Turning this method of determining height off, or providing a selectable switch is desired. In the near term, it appears that further research into ray-tracing models is appropriate. Codes such as RF-ProTEC, which can be dynamically linked to mission level models such as EADSIM, can provide the higher fidelity propagation calculations required, and still permit the dynamic interactions required of the mission level model. Additional research should also be conducted on the best methods of representing man-made structures to determine whether codes other than ray-trace can be used