There may be one particular sort of application in which the unique capabilities of skyhook could prove extremely important. In another important contrast with the helicopter, Skyhook's jet power makes it capable of sustained operations at any practicable jet-engine altitude - a capability which provides numerous advantages for useful commercial opportunities. Skyhook is particularly suitable for HALO (High Altitude Loiter Operations), which make it ideal as a vehicle for a wide variety of communications applications. Such applications have been widely sought after, and many such applications have been identified. Various HALO schemes already under development. In the discussion that follows, I'm coining the term, "MAritime HALO", and using the acronym, MAHALO - which particularly suits the discussion, because Skyhook HALO flight operations may be limited to maritime locations. If Skyhook MAHALO flight operations are certified for maritime (offshore) applications only, Skyhook MAHALO support for commercial broadcasting is limited to large market coastal venues. In the worst case scenario, MAHALO flight failures could cause temporarily interruption in communications services, as Skyhook drops harmlessly into the ocean. In the best case scenario, these MAHALO flights could provide 24/7 maritime voice and data communications.
If operated at extremes of its altitude capabilities, Skyhook could provide point-to-point communications over great distances. High-frequency communications are desirable for the increased bandwidth they provide, but HF transmissions are limited to "Line-of-sight" transmission distances. The major factor determining these line-of-sight distances is the curvature of the earth, so these distances are given by a formula that takes the Earth's curvature into account: In fact, the LOS propagation range, in miles, is 1.4 times the antenna height (i.e., the altitude), in feet. This formula is widely employed by communications engineers, who work it both forward and backward: If you have a particular range objective in mind, the formula can be worked backwards to compute the antenna height (or the vehicle's flight altitude). If a distance of 420 miles were hoped-for, the engineer must divide 420 by 1.4 (the result is 300), and square the result to get a figure for the required altitude (that is, 90,000 feet).
Theorists have argued the possibilities, and scaled back their expectations. With manned vehicles, of course, those altitude extremes are probably too much to hope for. Skyhook, of course, fills the bill more appropriately, as an unmanned vehicle. With skyhook, such altitude extremes are easier to achieve, as we shed the added weight of systems for crew comfort and/or safety. For purposes of comparison, though, the Nirvana figure is now thought to be 80,000 feet, for a 400 mile reach. It's hard to say whether a balloon tethered to the aircraft could achieve the additional 10,000 feet, but some 500,000 square miles of territory are encompassed within this 400-mile radius, anyway - which opens a number of those highly-attractive telecommunications prospects.
The HALO concept itself is not at all new. In fact, it was born in the early days of television. Television broadcasting requires complex transmitter equipment, and for that reason alone, it was rather expensive. Identifying a business need to spread these costs over expanded service areas gave rise to demands for higher antenna heights, because dramatically-increased bandwidth requirements of television broadcasting required signal transmissions at elevated frequencies - with attendant "line-of-sight" restrictions upon its range of propagation. A recognition of the obvious height advantages of airborne antenna platforms prompted early efforts to find a suitable flight vehicle for persistent, high-altitude (that is, HALO) operations, with their hope, or goal, to loft a TV station to altitudes sufficient to achieve early-day regional "super-stations". Considering loiter time as the primary factor in quest, communications engineers pinned their early hopes on blimps
Skyhook "HALO" operations
The advantages of blimps are obvious, but more than offset by various disadvantages, not least of which is its practical service ceiling - which is nowhere near 80,000 feet. In fact, blimps have difficulty operating at or near their service-ceiling altitude-above-sea- level of approximately 14,000 feet. The greatest need for extreme antenna heights is found in mountainous regions not well-serviced by cable-service providers (in mountainous areas, communications ranges are determined by the effective altitude, above the features of surrounding terrain). Blimp operations are a bit expensive, and it is an expensive proposition to consider the prospect of 8, 12, or 24 hour "loiter" operations on a blimp with a full crew. Still, the "show-stopper" factor was that increased broadcasting range comes at the expense of increased broadcast-power requirements (and larger power-supplies). Blimps are typically capable of relatively small payloads, and a television transmitter power supply is fairly heavy
Still, the HALO proposition has enduring appeal, and efforts to implement the HALO concepts are revived on a periodic basis. Most of the impetus for recent efforts is found in newly-developed communications services, but also in the requirement that all new television sets sold in America are now mandated to be capable of UHF television reception. There is always a demand for specialized television broadcast services, and UHF TV is still "green-fields" in terms of its spectrum utilization. There are still very few UHF stations now operating, and few that do operate at high power levels. If it could be possible to obtain FCC-level "clear-channel" authorization, it could be possible to operate multiple channels (in blocks) among the higher channels of UHF television, to provide multiple-channel "cable-style" television broadcast service.
I feel a great deal of enthusiasm for Skyhook, in looking at the "big picture" presented by the wider scope of its possibilities. At the same time, it may be necessary to temper this enthusiasm by a realistic notions of its limitations. If Skyhook were given certification for offshore operations only, then its broadcast application could be limited to service areas that may be reached from skyhook vehicles in geo-synchronous stationary positioning by GPS-based robotics systems at points above large bodies of water. With that restriction, Skyhooks may be limited to positioning above the ocean, where its land-area coverage would be limited to coastal regions, being circumscribed in something nearly resembling a semi-circle, covering something less than half that 500,000 square-mile territory, so that half the theoretical communications potential could be wasted (not counting, of course, coverage of ships and aircraft offshore).
Whatever "loss" is felt by the technical purist may be more than offset, however, by the value of communications in the remainder. Various examples could be offered for purposes of illustrating the MAHALO Skyhook capability. Pick a pair of coastal cities 800 miles distant from each other, and it may be theoretically possible to reach both cities from an offshore location above the Atlantic, and still reach points nearly 400 miles inland, with television or radio broadcast services, and also with value-added services in point-to-point communications. Some highly-attractive coverage patterns are found in various locations in the Great Lakes region, and there is a particularly interesting spot in the Eastern extremity of Lake Erie, near Buffalo, New York. From Buffalo, it is 400 air miles to Boston, and 450 air miles to Chicago, so it is possible to reach both cities from an aircraft operating at 90,000 feet, at a spot overland, some 20 miles east of Buffalo.
In a pragmatic sense, however, a Boston-to-Chicago connection would require the additional range of antennas installed at heights typical of installations atop tall buildings. A 1600 ft building/antenna height provides an additional 56 miles - or 112 miles, if antenna sites are available on tall buildings in both cities (224 miles, if those antennas were operating in "relay". Such antenna sites would be necessary, in Boston or Chicago, or both, if MAHALO operations were conducted over Lake Erie.
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