conditions arising from sharing have significant impacts on the telecommunications services. There are brief accounts of these around 6, 7, 8, 13, 14 and 30GHz. The corresponding down link allocations are around 4, 5, 7.5, 11, 12 and 20GHz. Three further up allocations for non-telecommunications radio services, as they af- link bands, around 11GHz (Region 1 only), 14.6GHz and 17.7GHz, fect telecommunications systems, in Sections 26.4.6 to 26.4.8. have been allocated for the fixed satellite service but reserved for feeder links to broadcasting satellites. Almost all of these allocation- 26.4.2 Fixed station links S are shared with the fixed service and the mobile service. Some allocations are also shared with other satellite services, the direc- 26.4.2.1 Terrestrial links tions of transmission being opposite to those of the fixed satellite service in some cases. With few exceptions, these allocations have There are extensive allocations for the fixed service, comprising primary status. point to point and point to multipoint radio links, in most parts of The modes of interference arising between fixed satellite stations the radio spectrum. and terrestrial stations are illustrated in Figure 26.3. The corre- The fixed service allocations below 150kHz, at one time very sponding interference modes from one satellite network to another important, are now little used. Those bands are occupied nowadays are illustrated in Figures 26.4 to 26.6. Special methods have to be mainly by radionavigation systems. used to prevent such interference, because it can occur over great About 55% of the bandwidth of the HF spectrum, in about 40 distances and very large numbers of terrestrial stations operating in narrow bands between 4MHz and 28MHz, is allocated exclusively the same frequency band may be in the field of view of a satellite. for the fixed service and is in substantial use, mainly for long distance ionospherically propagated links. Substantial bandwidth is allocated for the fixed service between 26.4.2.3 Constraints on satel