4. frequency allocations are subject to international negotiation, and the USA is under considerable pressure to reduce its use of this frequency band, thus the suggestion that SPS would reduce the available band might well be received rather unfavorably; and 5. the medium for negotiations is the CCIR, and negotiations for the current revision of frequency allocations are at present under way in Switzerland. Three additional comments may be made: (1) Skywave HF Communication is inherently a variable medium because of normal ionospheric variations. However, while a 40% variation in MUF on a day-by-day basis over a given path is possible, and is accepted as being due to natural ionospheric changes, yet a permanent reduction in the total band by 40% would probably not be considered generally acceptable, although it is unlikely that a single event such as the launch of Skylab-I or of HEAO-C would lead to international repercussions. (2) The various higher altitude burns of circularization, de-orbit, POTV, etc., involve small, highly localized, high-altitude depletions in ionization, that generally will not affect HF skywave propagation in a surface-to-surface mode. Thus it seems quite unlikely that their effects would be significant in the present context. (3) Effects on the mesosphere ("Domain A"), i.e., on the ionospheric D- and E-regions, may be significant for LF, VLF, ELF (i.e., less than 100 kHz), which have some military and navigation (LORAN) uses, but these effects are unlikely to be important for HF signals that transit these low-lying ionized regions without significant attenuation. For the AM Broadcast band (MF) skywave propagation is not an important mode, so that the additional attenuation is unlikely to be critical. 3.2.7 Ionospheric Irregularities Associated with the Depleted Regions At the edges of depleted regions there presumably will be a variety of irregularities, such as bubbles of depleted ionization, which will give rise to Spread F types of ionospheric irregularities and also to traveling ionospheric disturbances (TIDs). The overall scale, morphology and hence impact of this major perturbation is being calculated. It is important but difficult to be sure that nothing has been omitted, and yet an adequate atmospheric simulation is hard to achieve because of the different types of phenomena that may be anticipated on different scales for the depleted region. The morphology of the depleted ionospheric regions will be a strong function of the source characteristics of the H2O and H2 resulting from combustion. These sources are twofold, namely, the upward diffusion of H2 molecules from the HLLV second stage main burn, and the injection of combustion products at F-region altitudes due to HLLV circularization and deorbit and due to POTV main engine burns.
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