lite transmitters 28MHz and 1GHz. However, these bands are shared with broad- casting and the mobile services. The fixed service has little oppor- Interference from satellite transmitters may enter fixed service tunity to use these bands in areas, like Europe, where there is heavy receivers; this is interference mode 3 in Figure 26.3. It would not be demand for these other services. feasible to control the level of this interference by negotiation, Very wide bands are allocated for the fixed service above 1GHz, because of the very large numbers of stations that may be involved. much of this bandwidth being shared with the mobile service, the Most terrestrial stations are particularly sensitive to interference fixed satellite service or others. The means that have been adopted from satellites which are close to the horizon, that is, from energy to deal with inter-service interference arising from this sharing which the satellite radiates towards the edge of the Earth's disc. See pattern are considered in Sections 26.4.2.3 to 26.4.2.5. These allo- Figure 26.7. More powerful signals can be tolerated from sources cations are heavily used, mostly for radio relay systems. higher in the sky. It has been estimated that interference would not There are non mandatory CCIR radio channelling plans and simi- exceed tolerable limits for terrestrial stations operating at 4GHz if lar national plans for microwave systems but in most other respects the spectral power flux density from any one satellite, reaching the frequencies are chosen for assignment in the fixed service by in- Earth's surface at an angle of elevation below 5°, does not exceed spection, with informal consultation between the administrations of minus 152dB relative to 1 watt/m2 in a sampling bandwidth of neighbouring countries where necessary to ensure that interference 4kHz. At angles of elevation greater than 25°, a flux density 10dB is not likely to occur. After consultation, the responsible authority higher would be acceptable, with a linear dB transition be