A New Class of Transmission Systems Splits the Difference Between Terrestrial and Satellite Broadcasting.
Time was that all broadcasting was terrestrial. In fact, broadcasters didn’t even know they were terrestrial, just as no one knew that WWI was WWI until later.
One could argue that AM radio (and other SW/MW/LW service) includes a skywave component, but transmission clearly is originated from the ground. FM and other VHF services are purely line-of-sight, and generally emanate from towers of 1,000 feet or less, or from mountaintop locations about an order of magnitude taller.
Subsequently DBS satellites have emerged, which take the transmission point to a different extreme with their >20,000 mile altitude in geostationary orbit. Some systems, like Sirius Satellite Radio, use an even higher maximum altitude of >50,000 miles in their highly elliptical orbits, or HEO.
More recently, low-earth orbit, or LEO, satellites have also been introduced, with less successful market results (e.g., Iridium).
(click thumbnail)The Helios HAPS prototype appears as a straight wing on the ground. — NASA Dryden Flight Research Center Photo by Tom Tschida
Each of these systems has its respective strengths and weaknesses, of course, essentially traceable to Heisenberg’s Uncertainty Principle: the closer you are to the ground, the more you know about your immediate environs but the less you know about the surrounding areas. For some applications, access to the big picture is an advantage, while in other cases, knowing the grass roots helps.
The distance between transmitter and receiver also has an affect on power requirements and propagation transit times. The two options available generally have quite different parameters in all these respects due to their widely divergent locations.
Now some new variants have been proposed, which would provide transmission points in the heretofore-unoccupied middle zone between terrestrial and satellite broadcasting, and thereby offer some unique advantages.
One such system is the High Altitude Platform Station or HAPS, a NASA-sponsored project that proposes the use of pilot-less, high flight-longevity aircraft as transmitter sites.
The vehicles are designed to fly in a tight circle over a single area for a period of several weeks. They are intended to operate at approximately 65,000 feet, placing them above the disturbances of weather and commercial air traffic once they reach their cruising altitude.
These aircraft, called Unmanned Aerial Vehicles or UAVs, are being developed jointly by NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) program and a commercial partner, California-based AeroVironment, which has recently formed a company called SkyTower Inc. to pursue business applications for the system.
Prototypes for the HAPS design originated in 1977, when the ERAST project began development of its Pathfinder series of aircraft. Using a “flying wing” design with multiple small propeller engines and solar cells on the upper wing surface, these vehicles set new altitude records for propeller-driven aircraft (>80,000 feet).
Second-generation units, dubbed Centurion, extended the wingspan in 1998, while a third generation, called Helios, added more efficient solar cells and further increased wingspan to 247 feet. For comparison, a 747-400’s wingspan is 211 feet.
In 2001, Helios broke the world altitude record for both propeller and jet-powered aircraft, formerly held by the SR-71 Blackbird spy plane, by reaching 96,863 feet. Last year, the Helios platform performed a successful test transmission of HDTV and 3G mobile telephony from 60,000 feet over Kauai, Hawaii.
(click thumbnail)In flight, the flexible design of the Helios wing becomes apparent. The flying wing is shown in 2001 near the Hawaiian islands of Niihau and Lehua during its first test flight on solar power. — NASA Dryden Flight Research Center Photo by Nick Galante/PMRF
This work proved the viability of the pilot-less, high-altitude operation for RF transmission/reception, but maximum mission time remains on the order of a few daylight hours. Next-gen units will include specialized, lightweight fuel cells, now under development, to allow nighttime flight and greatly extended missions.
The target is a vehicle that can remain aloft continuously for up to six months. When maintenance is required, another vehicle would be launched prior to taking the first unit out of service, implying that each area served would require two working vehicles. Alternatively, in-flight spares could be kept aloft as hot standbys and flown where required quickly (assuming the correct complement of transceiver equipment was on board).
A similar approach is proposed by a number of companies that are developing large balloon-based platforms, called “stratellites,” which would hover at approximately 70,000 feet, using small jet thrusters to maintain position within a cube of about 1,000 feet on a side. A Canadian firm, 21st Century Airships, is developing such devices in cooperation with two Atlanta-based companies, Techsphere Communications and Sanswire.
Current models of these “strats” are approximately 1/4-sized prototypes 60 feet in diameter, which have been tested to an altitude of 18,000 feet. By mid-2003, an altitude of 30,000 to 40,000 feet should be reached by models 130 feet in diameter. The final design, expected in 2004, calls for 260-foot diameter Kevlar balloons operating at 60,000 to 70,000 feet, with systems that could remain aloft for as much as a year before requiring ground maintenance.
The operating altitude of 12 to 13 miles occupies a theoretical “sweet spot” for some broadcast and telecom applications. It can offer relatively uniform, line-of-sight coverage to a large circular region about 600 miles across (~300,000 square miles – about the size of Texas) from a single transmission point.
This is impossible by terrestrial means due to the constraints of the radio horizon, and it can be achieved by the HAPS or Strats with less power than DBS due to the much lower altitude used. The latter attribute also increases throughput for two-way services (such as broadband Internet connections) due to the reduced latency provided by shorter path lengths.
The relatively vertical orientation of sightlines will also provide a more unobstructed path in urban areas than geostationary satellite service, and the higher effective power may improve building penetration, perhaps reducing the requirement for external, roof-mounted antennas. Some mobile applications might also be possible.
To date, no specific radio broadcasting applications are envisioned, but there is no technical reason precluding such service to be included. Any broadband digital technology (uni- or bi-directional) intended for regional service is appropriate for these platforms. The use of dynamically tracking spot beams has also been considered.
Raising capital and other market challenges still remain for the operators that propose such services, but all are moving forward. Beyond pure technical viability, ultimate success of these systems will hinge upon operators’ ability to provide adequate, mainstream services at competitive prices. Filling a unique but small niche market is not likely to be sufficient, as recent experience in the LEOsat environment has shown.
Current FCC rules would consider these systems as terrestrial services, but a new class of regulation will likely also have to be developed to address these new transmission sites specifically, should they become widely deployed.
It’s another item to keep on broadcasters’ radar as digital evolution continues.