lso planned. But an in the atmosphere) can cause significant interference on co-channel on-going need also still remains for improved predictions for recep- terrestrial and space systems even when the beams from the two tion in low-latitude (equatorial) regions. This has recently been the systems are not directed towards each other on a great-circle path; subject of study in a Working Party of the CCIR. such scattering being, to a first approximation, isotropic. It may also be significant in the case of two Earth stations with beams elevated well above the horizon: for example, with one a feeder link trans- 9.10 Propagation and co-channel mitter to a broadcasting satellite and the other a receiver in the interference fixed-satellite service. This mode of interference may be dominant when hills or other obstacles provide some 'site-shielding' against The increasing need for different microwave terrestrial systems, signals arriving via a ducting mode. satellite communications and broadcast systems to share frequen- Techniques for studying tropospheric structures 9/17 cies has produced a correspondingly greater likelihood of mutual Optical radar (lidar) and acoustic radar have also been used to interference. The development of interference prediction methods probe the troposphere, although the information they provide is only leading to the efficient co-ordination of communication systems indirectly related to radio refractive index. sharing a frequency band requires modelling the transmission loss The millimetre and sub-millimetre spectrum, as yet not exploited due to all possible propagation mechanisms as a function of fre- to any significant degree for communications, is nevertheless a quency, distance and time percentage. Until recently, the primary fruitful region for tropospheric probing. In particular, the presence concern had been for situations occurring less than 1% of time, for of several absorption lines (in water vapour, oxygen and minor which the main clear-air mechanisms are ducting and layer reflec- 