(LH SCALE) ANGLE OF ELEVATION Figure 26.8 Constraints on the spectral power flux density at the Earth's surface from satellite transmitters operating in frequency bands shared with terrestrial services, as a function of 8 and the frequency band and for feeder links carrying programme material up to the satellites Several of these services need frequency bands suitably located in in the frequency bands given in Table 26.1. the spectrum, free from man made signals, in which radio frequency In the down link bands, which are shared with primary allocations energy radiated naturally from the earth, from the atmosphere or for the fixed service and terrestrial broadcasting and primary allo- from matter in space can be measured. Several need suitably located cations (secondary in Region 1) for mobile services, these sharing frequency bands in which the earth, including its atmosphere, can allocations may be used only in ways that cause no significant be probed for information from satellites, as by radar, using "active interference to satellite broadcasting which is in accordance with sensors". And most of them need frequency bands for the trans- the plans, regardless of the date when satellite broadcasting starts. mission of acquired data and for satellite management. Similar protection of the planned feeder link assignments is also For passive observations there are a few allocations below provided by the terms of the plans. 50MHz, and about 50 other allocations, wide or narrow, mostly above 1GHz. Some of these allocations are exclusive to the passive 26.4.7 Location by radio services but others are shared with telecommunications services; administrations do what they can, through their frequency assign- The radiodetermination and radiodetermination satellite services ment practices, to protect the exploration and research services from interference. For active sensors there are 11 allocations between include all systems which use radio to determine position, velocity or similar information, using terrestrial and satel