l stations, the base stations that communicate with land Figure 26.6 With frequency usage as in Figure 26.5, vehicles and the corresponding earth stations, typically after con- interference may also arise when the satellites are very sultation with the administrations of neighbouring countries. These wide apart in orbit, the quasi-antipodal case, as shown here assignments are notified to the IFRB for registration after any necessary frequency co-ordination, as with the fixed and fixed-sat- between radiotelephony and radiotelegraphy, elaborate radio chan- ellite services in corresponding ranges of frequency. The frequency nelling plans have been agreed for the radiotelegraph sub bands and co-ordination process for frequencies assigned to the satellites of there is a frequency allotment plan for the radiotelephone sub bands. mobile satellite systems is as described for the fixed satellite service in Section 26.4.2. Most of the aeronautical bands have been incorporated into fre- quency allotment plans (ITU, 1967; ITU, 1983), suitable for radio- Many kinds of mobile station are authorised to use, not one telephony but available for other kinds of signal also. frequency, but any of many frequencies, the assignment used on any There are important aeronautical mobile (Route) and maritime occasion depending on the circumstances. Assignments to mobile allocations around 125MHz and 160MHz respectively. There is a stations for terrestrial mobile services are not notified to the IFRB. radio channelling plan for the aeronautical mobile band and the use Assignments to mobile earth stations, on ships, aircraft and land of these channels is managed mainly by the national civil aviation vehicles, are not notified individually for registration, but a collec- authorities under the aegis of ICAO. There is also a radio channell- tive notification is made by the responsible administration, indicat- ing plan for the maritime band. ing the area in which the mobile earth stations operate. Wide bands between 28MHz and 470MHz and around 90