Longley-Rice model prediction inaccuracies in the UHF and VHF TV bands in mountainous terrain

Coverage prediction is of prime importance for TV broadcasting. A classic model used for TV coverage
prediction is the Longley-Rice ITM (Irregular Terrain
Model). Other well-known multiple knife-edge diffraction models are the Epstein-Peterson, Deygout, and Giovaneli methods. In this paper, comparisons are presented between accurate field-strength measurements, taken by a Rohde & Schwarz FSH-3 portable spectrum analyzer using precision calibrated antennas and calculated results from the Longley-Rice model, and the multiple knife-edge models in conjunction with the 3-arc-second SRTM (Satellite Radar Topography Mission) terrain data.
Calculations are limited to the main 2 knife-edges of the propagation path. The Longley-Rice model predicts
received field strength accurately in most cases even in mountainous terrain with multiple diffracting obstacles in the VHF and UHF TV Bands. However, in some long distance fringe reception areas field-strength is underestimated by the Longley-Rice model, while it is accurately calculated by the multiple knife-edge diffraction models.