Propagation impairment on ka-band SATCOM links in tropical and equatorial regions

Under pressure in the commercial sphere for increased information transfer rates or from the military to further miniaturize equipment to improve its portability, there has been a steady move toward higher carrier frequencies. For more than a decade now, increasing numbers of satellite communications systems have been in operation at millimeter wavelengths. Even where there is a clear view of the satellite from the ground station, a range of meteorological phenomena still combine to make propagation impairment a serious problem. Because of their complexity, difficulties in modeling them, and deficiencies in basic physical understanding, progress in engineering such systems has involved a large dose of empiricism. Ideally, this means a solid database of propagation measurements, made as nearly as possible in the intended geographical area of operations. Because of the time and cost of their accumulation, attempts have been made to use classification into climate zones on a worldwide basis to extend existing data to a wider range of situations. However, most of the measurements in the databases have been made in Northern Hemisphere temperate zones. Much evidence suggests that in tropical and equatorial zones, the factors that make important contributions to propagation impairment are different, putting a strong question mark over the use of existing design methods. This review looks at those deficiencies and their likely impact on system performance. At this time, rather than providing definitive answers - which must wait on further work - the best that can be done is to highlight difficulties and encourage realistic expectations. In a final section, the review examines the question of whether costly, application-specific measurement programs might now be avoided by mining increasingly sophisticated national meteorological records.