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Relay Aircraft Enable
Cell 'Network in the Sky'
WILLIAM B. SCOTT/MOJAVE, CALIF.
High-altitude drones can provide a round-the-clock communications alternative to satellites
A recent flight demonstration over Southern California validated the concept of using high-altitude aircraft as cellular telephone relay nodes. Consequently, airborne networks offer a relatively low-cost alternative to satellite constellations or hundreds of ground-based towers for wireless communications, particularly in developing nations.
If ongoing negotiations soon produce a firm contract, Platforms International Corp. (PIC) of Redlands, Calif., and its partners will deliver the first functional elements of an Airborne Relay Communications System (ARCS) to TELESP of Sao Paulo, Brazil, in September 1999. Initially, two aircraft orbiting high above the city would provide 24-hr. cellular phone service to approximately 200,000 customers. As now planned, up to 1 million subscribers could be handled by an expanded system, although capacity depends on the level of service required.
PLATFORMS INTERNATIONAL CORP.
Custom-built Stemme S-15 with external equipment pods served as a Platforms International airborne relay for a recent wireless communication system demonstration. An end-to-end flight test proved the feasibility of an airborne relay-based wireless system architecture developed by Jet Propulsion Laboratory under contract to PIC.
Platforms International developed an airborne relay concept that uses off-the-shelf communications technology, then assembled a team of contractors and partners to demonstrate that a cellular network built around orbiting aircraft could provide reliable telephone service. For the flight demonstration in May, a single German-built Stemme S-15 aircraft equipped with standard telephone repeaters provided two "spots" of coverage over San Diego, Calif., enabling test callers within a roughly 8-km. (5-mi.) diameter area to communicate via commercial cell phones.
Since this was a proof-of-concept demonstration, the system was scaled to about 10% of a normal operational ARCS, participants said. The Stemme S-15 was flown 4,000-6,000 ft. above a ground station provided by ADC Telecommunications of San Diego. All communications were conducted within the 900-MHz. band ADC was licensed to use, and cell phone handsets were tuned to test channels.
GROUND STATION PSTN=Public Switched Telephone Network
MSC=Mobile Switching Center
BSC=Base Station Controller
BTS=Base Transceiver Station
Data from three days of testing--one of which was a demonstration witnessed by Brazilian telecommunications officials--confirmed the successful completion and switching of calls:
Between two people using standard cellular handsets. Signals were routed from one handset, through airborne transponders on the orbiting Stemme S-15 aircraft, to a second caller located several miles away.
From a mobile caller to a nearby ground station, which routed the call through a standard public telephone system, then to the aircraft and down to another mobile cell phone user.
Automatically between the two "spots" or cells ("handoff") to maintain adequate signal strength and connectivity throughout the aircraft's orbit.
Although PIC's airborne relay concept is not new, this may have been the first time a functional GSM (Global System Mobile) version was demonstrated in the field, according to test participants. Angel Technologies is developing a similar high-bandwidth system, which will use a Proteus aircraft developed by Scaled Composites (AW&ST Dec. 15,1997, p. 55). Other companies have flight tested a code-division, multiple access (CDMA) type of airborne relay. PIC's GSM system could be immediately applicable in countries that already have this European-standard mobile protocol.
The Platforms demonstration is being viewed by telecommunications officials as a significant confidence-builder that could spur development of such systems. However, getting a cellular "network in the sky" to work reliably is not a trivial task. Consequently, PIC awarded a contract to NASA's Jet Propulsion Laboratory to help Composite Optics Inc., another PIC subcontractor, develop the system architecture and a cost-efficient airborne communications package.
"This is not simple stuff. It's not just like shining a flashlight on the wall," Howard A. Foote, PIC president and CEO, stressed. "That's why I brought JPL into this instead of just hobby-shopping the architecture. There are some major issues here. Even the Big LEO [large low-Earth-orbiting communications] systems are going through the same learning curve--and nobody's sharing data. JPL's already done work [in this area]."
To stay within cost and development-schedule constraints defined by PIC, JPL and Composite Optics chose off-the-shelf cellular technology. "We didn't have a clean sheet to work on, but [the demonstration] was done in record time by using a lot of commercial components," said Steve Morris, task manager in JPL's telecommunications and engineering division. "This isn't hard to do. The technology isn't state-of-the-art; it's just cell phone technology. They've put it together in a smart way," Morris said.
Mark Bonebright, Composite Optics Inc.'s business area manager for antennas and payloads, agreed, noting that the PIC concept is a combination of existing technologies. "This is [basically] a low-flying satellite," he said. Composite Optics fabricated and integrated the telecommunications payload carried on the aircraft, and ran the demonstration. The company is using advanced composite materials to put standard cell phone repeaters in lighter-weight, thermally efficient packages suited for airborne use. The same techniques are being used on satellites.
Bonebright said the biggest challenge will be refining the aircraft to meet PIC's expectations. "If remotely piloted platform issues are resolved, people will want to fly a lot of applications on [this aircraft]," he predicted.
Foote said he spent five years researching the ARCS concept and the aircraft needed for it, eventually choosing a German motorized glider as the basis for PIC's airborne platform. Berlin-based Stemme tailored its S-10 design to the PIC-defined mission, which resulted in an essentially new aircraft--what Foote refers to as a "high-altitude research vehicle." The FAA-certified Stemme S-15 has a 75-ft. wingspan, can operate up to 60,000 ft. and cruise at more than 150 kt. Typically, remotely piloted ARCS aircraft will be flown 36-40 hr. on-station at about 60 kt. and 52,000 ft.
Although two wing pods will carry special-mission packages and ARCS antennas, the telecommunications suite is being installed in the two-place pressurized cockpit. The aircraft is configured for autonomous operation, but a remote pilot can take over and fly it at any time. When necessary, the vehicle can accommodate an on-board pilot.
The avionics suite, which includes a drone flight control system, is being designed by HRP Systems around two flight and two mission computers for overall reliability. On-board radios will allow ground-based remote pilots to communicate with air traffic control centers--changing frequencies and identification codes, for example--as if a person were in the aircraft.
Production S-15s will be powered by a four-cylinder, dual-turbocharged Light Powered Engine Corp. engine driving a special propeller optimized for high-altitude flight. Rated at 290 hp., the powerplant was sized to meet payload electrical power requirements rather than aircraft performance criteria, Foote said.
To satisfy Brazilian airworthiness officials' concerns, the first ARCS system probably will have pilots on board, even though the vehicles will be operated as if they were unmanned. That may limit the payload size initially in order to stay within weight constraints, according to L. Jane Hansen, HRP Systems president. "We don't need the space; it's mainly for weight and [center-of-gravity] control," she explained.
Having a pilot on board in Brazil will alter the ARCS operating plan as well, Foote noted. Flights will be limited to about 5 hr. to mitigate pilot fatigue, rather than the 36 hr. or more planned for unmanned missions. After the system demonstrates a level of maturity, unmanned flights will be allowed.
Operational scenarios were developed to ensure extremely reliable, 24-hr. cell phone service. To guarantee a 0.9999 system availability, each aircraft will carry equipment needed to place 19 "spots" or cells of coverage on the ground. Three aircraft will be dedicated to that area, making up one element of the overall system. Only one S-15 will be on-station at a time, while the other two are prepared to take over after a specified "shift" period. The relief aircraft will take off and climb in about 2 hr. to a holding track, then move into the operating orbit when the telecommunications handoff is commanded.
If the relief aircraft has a problem and must be replaced, an unmanned on-station S-15 can remain in orbit an additional 4.5 hr. before it must start a descent. Climb and descent time margins are built-in to ensure no break in service. Foote said that, in a worst-case situation, service might be lost for no more than 2 hr.
Each ground-based control station will be manned by a crew of nine for a week-long period. One person must be in the primary station at all times.
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