Communications, Radar & Electronic Warfare (201.. - Index of

212 Communications, Radar and Electronic WarfareIouQ)en10000 rI1000 :Ii100 jl!o 101--- -- ..cIU)I1 ---_---.":-. : -. -,--10jApproximate short sector length. iI!..' ------- ·---------·-·-:---1- --100 1000 . 10000 . 100900_ _Frequency (MHz)I __ ___ _ J__ __ _ iIiII!iFigure 11.12Approximate ,hart ,ector length ba,ed on frequency, using 50 wavelengths.This is typically held to be about 40-50 wavelengths of the transmission frequency.This is illustrated in Fi gure 11.12 for 50 wavelengths.It can be seen from Figure 1 1.12 that the s.ize of the short sector ranges from over akilometre at the low end of HF, to less than 5 millimetres at 60 GHz. Realistically, athigher frequencies it is highly unlikely that data at the resolution of the short sector willbe available. However, useful modelling can still be performed at these high frequenciesfor the reasons discussed in the next section.11.9.5 Required PrecisionThe resolution discussed in the previous s.ection is important, but there are other factorsthat influence the applicability of lower resolution data to propagation modelling.These include:• The type of prediction being performed. Long range predictions at VHF and abovedepend as much on atmospheric conditions as terrain effects, and for these predictionssmall terrain features are of less importance. Also, so-called 'site-general'predictions do not require terrain models at all, and in other cases, a nominal model 'of tenain vari ability can be used.• How close the terrain is to the two radio terminals. Communications between twoaircraft in flight, between a ground terminal and an aircraft or between two hightowers may not require accurate terrain modelling, with only the highest intrusionsbeing significant.• Where terrai n variations are relatively small over the link ranges, a coarser terrainmodel can be entirely acceptable.