Air-to-ground propagation at 900 MHz

Air-to-ground propagation at 900 MHz exhibits some fading characteristics similar to land-based mobile radio systems and others similar to terrestrial line-of-sight microwave systems. 900 MHz air-to-ground propagation, over long distances and at low vertical look angles, is governed primarily by obstacles blocking the path. At shorter distances the signal sometimes experiences multipath fading. The level varies slowly as the aircraft passes in and out of fresnel zones. When the aircraft is over a ground station, the signal experiences rapid fading similar to land mobile systems. At 900 MHz, with a limited fade margin, the radio horizon is approximately equal to the optical horizon. The horizon can be documented in a variety of ways simplifying the coverage prediction process. Several procedures are discussed with emphasis on a photographic technique which offers the greatest accuracy.