As has been inferred throughout this monograph, military requirements have dominated the development of operational systems that incorporate space weather information, either actively or passively. In Table 4-9 is a listing of telecommunication disciplines that are influenced or even controlled by the ionosphere and its variations. The ionospheric variability is, of course, a manifestation of space weather. Other military mission areas include missile warning, and the dissemination of weather information and imagery, but these may be generally incorporated within the list in Table 4-9.
Table 4-9: Requirements for Command, Control, Communications, and Intelligence (C3I)
□ Global and long-haul communications (HF and Satcom)
□ Tactical communication (LOS, NVIS, Satcom)
□ Navigation services (legacy, GPS, Glonass)
□ Search, tracking, and fire-control radar
□ Signal intelligence and target geolocation
One of the more important services provided the military is navigation, and the lineage of the two-frequency Global Positioning System is traced to military experiments in the 1960s. The original U.S Navy design used L-band and UHF, with the latter being used for removal of ionospheric group-path delay. Ionospheric compensation is achieved in the current Air Force system by using dual L-band signals, a design that reduces, but does not eliminate, scintillation effects on system performance. The GPS constellation of satellites with its ground segment and the array of user systems, likely represents the most significant technological achievement in the field of telecommunications in the last several decades. Figure 4-25 depicts the GPS space segment. The GPS system is critical for a range of military requirements besides navigation. The effect that is most significant for the dual-frequency GPS system and its users is scintillation, with phase scintillation probably being the most critical since it may lead to receivers losing lock.
Space target tracking is a mission of the military, with aspects handled by the U.S. Navy and U.S. Air Force. The radar cross sections of targets may appear to vary as the result of scintillation, and ionospheric inhomogeneities may also introduce angular jitter and group-path delay variations. These effects are not always a major concern, but radar tacking through auroral clutter can be problematic.
Long-haul connectivity, using both HF assets and satellite links, is important to maintain a global communication capability. There are also tactical requirements within theater, and a variety of bands and possible platforms are used to satisfy this mission. Ionospheric effects do not seriously impact all of these possibilities, but many are at least influenced. Near-Vertical-Incidence-Skywave (NVIS) at HF is a peculiar method of connectivity used by tactical commanders to overcome limitations associated with the exploitation of Line-of-Sight (LOS) methods in rugged terrain or jungles. Ionospheric variations can have a serious impact on the utility of this mode, which is restricted to frequencies below the overhead MUF (i.e., foF2 + '/2 fg, where fg is the electron gyrofrequency).
Any system that uses ionospheric bounce, such as HF radar or HF communication, is strongly influenced by the ionosphere and related space weather effects. While Over-The-Horizon-Radar, OTHR, has a rather limited role in the military at this time, there is a continuing interest in Re-locatable OTH radar by the U.S. Navy (i.e., ROTHR). The OTHR methods are used in civilian agencies of government, such as the "Drug War" and for remote sea state monitoring. Target registration is an important function in the detection and surveillance of targets, and HF ray trajectories can be significantly distorted by ionospheric profile changes. Hence, space weather factors can play havoc with the performance of HF radar systems, where the problems are generated by large MUF excursions, TIDs, PCAs, and SWFs.
To the extent that hostile forces use HF communications, the discipline of HF direction-finding (i.e., HFDF) will remain important. There are obvious ionospheric influences for these systems.
In short, the military has a requirement for surveillance as well as the conveyance of voice and data using a variety of platforms (e.g., fixed-station, space-based, air-mobile, land-mobile). Large telecommunication "pipes" are necessary to support transmission of images, and for use in data fusion and assimilation applications. Good information support is a force multiplier for military operations.
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