This chapter discusses the ITU. The five international bodies were meeting separately from time to time to deal with international telecommunications problems, technical, operational, commercial and administrative. Rationalisation was seen to be desirable and in 1932 a rather loose federation of the five organisations was created. The new body was called the International Telecommunication Union (ITU). Codding provides a more detailed account of the ITU and its predecessors.

The assignment of a frequency to a radio station and the issue of the associated licence to the owner of the station gives authority for transmitting or receiving at that frequency for stated purposes using stated emission parameters. The process followed in making a frequency assignment and issuing a licence depends on the service to which the station belongs. For some services the process is quite simple, but for others it is complex and particularly so if circumstances require that spectrum should be used very efficiently, so as to accommodate a large number of assignments in a limited frequency band. The licensing of a private two-way microwave fixed link in a country with a liberal regulatory regime may be taken as an example of the latter and an outline of the important elements of the process follows. Some of these elements are considered in more detail elsewhere in the book.

At the lower end of the spectrum, radio waves from terrestrial transmitters are propagated mainly by the ionospheric wave and the ground wave and they can cause interference at terrestrial receivers at great distances. At higher frequencies propagation between terrestrial stations is mainly by the tropospheric wave and the range at which interference arises is much less. Thus, the control of interference, the primary purpose of spectrum management, is mainly an international problem at the lower frequencies and mainly a national problem at the higher frequencies, and different management measures are used for the two propagation regimes. The transition from one mode to the other is gradual and varies with the state of the ionosphere but 30 MHz is found to be an appropriate changeover point for most purposes. Section 6.1 is concerned with the management of the lower frequencies and Section 6.2 with the higher frequencies. There is a similar division in the chapters concerned with the other major terrestrial services.

This chapter discussed the mobile services includes the maritime mobile service (MMS), the aeronautical mobile service (AMS) and the land mobile service (LMS). Some frequency bands allocated for the AMS are reserved for air traffic control on major civil air routes and related purposes; these allocations are designated AMS(R), 'R' standing for 'Route'; see RR paragraph S43.1. AMS allocations used for other purposes ('Off Route') are designated AMS(OR). RR S43.4 specifically forbids the use of exclusive AMS allocations for public correspondence, that is, for communication facilities for the use of passengers.

The terrestrial broadcasting service, by definition, consists of transmissions that are intended for reception by members of the general public. The definition of the BSS is broader, embracing individual reception, often called direct broadcasting by satellite (DBS), but also community reception. It is assumed that individual reception will involve simple receiving installations with small antennas, requiring a strong signal from the satellite and very effective protection from interference. Community reception, serving the public typically through cable distribution systems, is assumed to involve more complex receiving installations with larger anten nas which do not need such powerful signals.

Spectrum is allocated to the AmS and the AmSS for the use of authorised radio amateurs for self-training and technical investigations; see RR SI.56 and SI.57. Recommendation ITU-R M.1042 refers to the contribution that radio amateurs and their stations can make to the restoration of communications in the event of natural disasters and Recommendation ITU-R M.1043 discusses how Administrations can develop this resource. Recommendation ITU-R M.1041 reviews the directions in which amateur radio could most desirably develop in the future.

It will be seen, in particular from item 4 in Section 13.4.1 and Note (iii) inTable 13.5, that the SR and EES use certain of their allocations for links between spacecraft. However, in addition, a number of millimetre-wave bands have been allocated for the inter-satellite service (ISS) to provide for other links between Earth satellites. All of these ISS allocations are primary and all but one (that is, 25.25-27.5 GHz, see item 3 below) are available for assignments for any satellite-using service.

When a spectrum manager is making a frequency assignment it may be necessary to estimate the minimum transmitter power level that is consistent with the achievement of specified link performance objectives. It is seldom feasible to use direct measure ments for this purpose. Similarly, the impact on the reception of a wanted signal of interference from known sources may have to be estimated. For point-to-area services such as broadcasting or land mobile, the power level required to set up a specified signal strength anywhere within a specified service area may have to be estimated. Accurate estimates enable the best use to be made of the spectrum. Accurate estimates require knowledge of the local topography and of various char acteristics of the radio stations involved, including the gain, polarisation characteris tics and effective height of the antennas, the modulation parameters of the emissions involved, the bandwidth of the receivers and the power of interfering transmitters. In the absence of such knowledge it will be necessary to make assumptions about them, and standards recommended by ITU-R are often used for this purpose. But above all it is necessary to be able to estimate the transmission loss arising from propa gation between the transmitting antennas and the receiving antennas. It may also be necessary to estimate radio noise levels.