Investigation of Low Altitude Air-to-Ground Channel over a Tropical Sea Surface at C Band

Abstract

This paper investigate a low altitude air-to-ground channel over a sea surface in the tropical region at C band. It is found that, compared to a high altitude air-to-ground channel where a free space propagation condition can be assumed, the sea surface re∞ection and other multipath propagation phenomena for lower altitude air-to-ground channel can readily be observed. Therefore, for low altitude ∞ight paths, spatial diversity at the ground station can be used to overcome signal degradation. Due to the rapid increase in aircraft density, the current air tra-c control (ATC) system that operates at VHF band is fast becoming saturated. Therefore, an alternative frequency band is introduced for tra-c control communication systems in order to satisfy this increasing demand. At present, C band is assigned for Microwave Landing System (MLS) in aviation navigation. In order to optimize the MLS performance at C band, thorough air-to-ground channel measurement in order to perform characterization is important. Matolak etal. (1) started an interesting piece of research work on wireless channel characteri- zation for the 5GHz MLS system in 2007. They covered the characterization of both small and large size airports, mainly using wideband characterization. In addition to this work, Tu etal. (2) conducted a comparative study of the conventional VHF system and the MLS C band system. The focus of both studies (1,2) concentrates on the propagation over land environments. However, little research work is done on the study of the air-to-ground link over a sea surface at C band. The evap- oration duct is a well recognized form of propagation mechanism that can result in a substantial increase in signal strength when signals of frequencies above 3GHz propagate over-water paths (3). Therefore, there is interest in the investigation of the air-to-ground link near to a tropical ocean. These results are important to modern military and commercial applications for a seashore country such as Singapore. As a continued work of our previous studies (4,5); C band channel characterization at a small airport (4), and air-to-ground channel investigation over a sea surface at high airborne altitude (20kft to 40kft) (5), in this study, a series of air-to-ground channel measurements at low airborne altitudes (1.2kft to 6kft) are conducted over the sea surface in the tropical region. The main objective of this paper is to report on the preliminary study of the spatial diversity at the ground station for the improvement of signal quality of the communication links. This is helpful for future research work in this area.