 sys- Of the bands allocated for satellite links between fixed earth stations items, particularly for service to land vehicles, and the allocation of (ITU, 1990a), the fixed-satellite service, those given in Table 51.1 additional spectrum will be considered at an ITU conference in are the ones which are already used, or are likely to be used soon, 1992. 51/4 Communication satellites and systems Table 51.2 Frequency allocations for mobile satellite Table 51.4 Frequency bands allocated for feeder links services used in broadcast satellite services Up-links Down-links Direction Limitations Region Frequency band (MHz) (MHz) unspecified on use Regions 1 and 3 14.5 - 14.8 GHz and (MHz) 17.3 - 18.1 GHz 806-890 Not aircraft Region 2 17.3 - 17.8 GHz 942 - 960b Not aircraft 1626.5 - Ships only c 1631.5 51.2.2 Constraints on frequency assignments 1631.5 - 1530 - 1533 Not aircraft 1634.5 Satellite systems share the frequency spectrum with other satellite 1634.5 - 1533 - 1544 Ships only c systems and, in most frequency bands, with terrestrial radio systems 1645.5 also. Consequently interference arises. In order to keep the inter- 1645.5 - 1544 - 1545 Distress and ference down to acceptable levels, constraints on the characteristics 1646.5 safety only of systems which determine their liability to cause and suffer inter- ference have been agreed. Also, assignments are registered interna- 1646.5 - 1545 - 1555 Aircraft only tionally; a new user of a frequency defers to a registered established 1656.5 user under the jurisdiction of another country where there is no 1656.5 - 1555 - 1559 Land vehicles international agreement to the contrary. It is assumed that inter- 1660.5 ference problems between systems falling within the jurisdiction of the same country will be resolved by the government of that country. The constraints and agreements vary from service to service and from band to band; the more important provisions are as follows. The frequency bands allocated for down-links from broadcasting In the frequency bands at 12GHz in w