constituents such as ozone) makes it possible to study the concen- tion. However the small antenna systems used in new services have tration and spatial distribution of these media. Near the ground, generated a need for time percentages up to 20%, for which diffrac- direct transmission experiments are feasible; for example, to study tion and tropospheric scatter are important. the average water-vapour concentration along a particular path. In In addition, scattering from different types of hydrometers can be addition, it is possible to design radiometers for use on the ground, important at frequencies above about 10GHz. This propagation in an aircraft or in a satellite, which will provide data on the spatial mechanism may couple energy into the receiving antennas of unre- distribution of absorbing atmospheric constituents by measurement lated systems, despite precautions such as site-shielding. Up to now of the emission noise they radiate. This topic of remote probing is there have been only very limited data available to develop predic- one exciting considerable current interest in both radio and meteor- tion models. ology. A third group of studies in response to this general area has reflected that an accurate estimation of diffraction effects due to buildings and terrain is essential in the planning of radio services in 9.12 References the microwave bands. Significant data exist from diffraction over large-scale features of terrain, e.g. hills and mountains, which are of Aarons, J. (1973) Total-electron content and scintillation studies of major concern at VHF and UHF, but little data are available on the the ionosphere, ed. AGARDograph 166, NATO, Neuilly- sur- diffraction losses for microwaves that can be produced by buildings Seine, France. or trees which could be used to shield antennas from interference Anderson, S.J. (1986) Remote sensing with the JINDALEE sky- signals. wave radar, I.E.E.E. Ocean. Eng., OE-11(2), 158. All these aspects have been studied in recent years in a collabor- Appleton, E.V. and Be