ospheric layer, where the refractive index differs from that of temperature inversion promoting tropospheric ducting occurs when the surrounding area. a mass of cold air is overrun by a mass of warm air, and this A 'common scattering volume' as shown in Figure 46.6, is used periphery may extend in excess of 1000km to 2000km along a where a faint signal illumination occurs at the reception end of the stationary weather front. path. The title of 'troposcatter' is also more commonly known in Temperature inversions frequently occur along coastal areas bor- commercial fields as 'forward scatter'. dering large bodies of water, due to the natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than upper air layers. The same action may take place COMMON SCATTERING in the morning when the rising sun heats the upper air layers. VOLUME Ducting over water has produced v.h.f. communication in excess of 4500km. 46.5.5 Limitations to PMR and trunked systems Unlike troposcatter, tropospheric ducting cannot normally be con- trolled, but its presence should be borne in mind when planning PMR services. Disruption to local community repeater and trunking services may occur due to strong received signals on the same 800km channel frequency from users in other areas, and along the South STATION STATION and East coasts of the U.K. this phenomenon is often experienced by PMR users on both v.h.f. and u.h.f.. Careful frequency planning together with suitable digital or analogue selective signalling tech- Figure 46.6 Tropospheric scatter niques may be used to lessen disruption to communication services. 46.5.2 Equipment requirements Transmission standards The propagation medium involving the forward scattering volume Under the Telecommunications Convention, classes of emission are involves a large transmission loss, and it becomes necessary to use designated by groups of a minimum of three characters. Currently high gain, narrow beamwidth aerials for both transmission and the majority of PMR