example, in the bands allocated for observations of hydroxyl (OH) between 1660 and 1668.4 MHz. In recent years, this difficulty has grown greatly in importance, particularly with the introduction of higher-powered space transmitters and the use of spread spectrum modulation techniques. Because the radio astronomy and remote sensing sensitivities to interference are so great and because terrain shielding (the use of geographical features to block radio signals of certain frequencies) cannot be employed, it is most difficult to avoid interference from the sidebands of some spaceborne transmitters, even though their central transmitting frequencies may lie outside the radio astronomy bands. Furthermore, additional geographic restrictions are going to be difficult or impossible to obtain in the United States.

Radio regulations are formulated at several levels and involve a plethora of acronyms (see Appendix I). At the international level, the Radiocommunication Sector of the International Telecommunication Union (ITU-R) formulates regulations through World Radiocommunication Conferences and recommendations through the work of its various study groups.

Much of the work of the ITU-R takes place through its study groups, which are further organized into working parties and task groups. These deal with specific areas or problems and provide studies of questions concerning technical and procedural aspects of radio communications. Study Group 7 has responsibility for use of the spectrum for scientific research (the science services): remote sensing systems are the concern of Working Party 7C (WP7C), and radio astronomy is the concern of Working Party 7D (WP7D). The other services under Study Group 7 are as follows: WP7A, time and frequency standards; WP7B, space research and Earth exploration-satellite services (mostly communications). The