body break with Hall Johnson and Joanne McCleod
Imagine a TV antenna flying 21kms in the air and staying aloft for months at a time. It
would be able to serve an area of almost 200,000 square kilometers in the most remote regions
of the country. A group of Canadian scientists in the early 1980s thought this was a great
idea and developed SHARP, the stationary high altitude relay platform, Canada's alternative
to communication satellites.
The Communications Research Center of Canada, or CRC, played a large role in the early exploration
and exploitation of space. They launched Alouette and ISIS satellites that vastly improved our
understanding of the ionosphere. The CRC was also involved with the early communication
satellites with their HERMES satellite program. This groundbreaking satellite achieved the
world’s first direct-to-home satellite broadcast, and somewhat fittingly, it was a Stanley cup
finals hockey game, the Habs vs. The Bruins.
However, the tremendous impact of communication satellites was not evenly distributed. Northern
communities in particular had little coverage from existing geostationary satellites. Providing
satellite coverage to these regions would require an expensive constellation of satellites
in lower, more eccentric orbits. A more cost effective, sub orbital solution was needed.
The range of radio towers are limited to line of sight. This means that radio transmissions
can only reach out as far as the horizon. To increase the range, radio towers are built
as tall as possible. Satellites in geostationary orbit can be thought of as being on a very
very tall tower, this allows them to reach the largest area possible, theoretically,
almost 50% of the earth at one time. An intermediate solution was sought and the CRC began looking
into the idea of using an airborne platform to accomplish this task. The challenge was
to create something that could reach high altitudes and remain airborne for long periods
of time. High altitudes meant flying in the more predictable and slower winds found above
the jet stream. At these altitudes the system would have a 600km diameter range. One aircraft
could service a region larger than the width of an entire prairie province, or all of France.
The CRC settled on an ambitious project that called for an aircraft to fly at altitudes
in excess of 20kms and have an endurance of several months. Such a platform would have
many potential uses. On the civilian side it could act as a low cost broadcast television
and radio transmitter, a mobile phone relay, and aid in search and rescue. On the military
side such a persistent high altitude platform would be perfect for coastal monitoring, or
act as an early warning radar with a detection range far outside ground based radar stations.
It would also have the benefit of being able to detect low flying targets due to it's high
vantage point.
The planned 6 month flights would allow the system to be easily maintained and upgraded
as compared with inaccessible orbital satellites. This would create a more dynamic and adaptable
system with similar capabilities.
The economical system was projected to cost 30 million dollars per aircraft and ground
power system. Very competitive as compared to the 130 million dollar cost to build and
launch a satellite. These prices would make it accessible to provincial governments and
entrepreneurs.
The critical piece of technology required for the project was a way to power an aircraft
designed to stay aloft for such a long time. Carrying enough fuel on board the plane was
simply impractical. Solar panels with batteries were heavy and wouldn't produce and store
enough power to run the engine and transmitter at night. The solution was found when researchers,
led by Joe Schlesak and John Martin, began looking into microwave transmission and induction.
Power would come from the ground and be beamed up to the plane to power the engine, flight
systems, and payload. This scheme would necessitate an all electric plane. This was no problem
as brushless motors were just coming into widespread use and the required power output
and efficiency was available.
The system worked by covering the underside of the aircraft with microwave absorbing rectennas.
The term rectenna, or rectifying antenna, comes from a combination of the receiver antenna
and DC rectifier that make it up. T-shaped copper wires and diodes absorb the microwave
light emitted from the ground and convert it into an alternating current. This is subsequently
converted to a DC current to power the on board systems. An additional reflective layer
was placed on the top sides of the surfaces to reflect what is left of the beam back into
the rectennas, further increasing power absorption. An array of transmitters on the ground would
focus the microwave energy onto the aircraft.
The term, stationary, in the SHARP acronym is a misnomer. The aircraft would be moving
in cycles within a 2km cylindrical region of airspace and fighting wind speeds up to
200kph. The beam would have to be adjusted throughout the flight path and so a steerable
system was needed. A single large steerable antenna was not practical and so an array
of smaller dishes was preferred. At the time electronically steered, phased array antennas
were becoming more widespread but the technology was still developing. The team ultimately
decided to use a combination of electrically and mechanically steered antennas.
Beaming power through the atmosphere was not without risks. Microwaves induce motion in
water molecules. This is the effect used in microwave ovens but poses an obvious risk
to wildlife and aircraft travelling through the beam. To mitigate the risk, the power
of the beam was kept below a dangerous level to living organisms. Standing in front of
the beam would result in an uncomfortable warming feeling but would pose no further
risk of injury. This limit on the power of the beam would shape the features of the final
aircraft.
Research into the feasibility of powering an airplane using ground based microwaves
began in 1981 when the CRC was joined by the University of Toronto’s Institute for Aerospace
Studies, and was led by Professor James DeLaurier. Between 1981 and 1985, they conducted paper
studies into airframe design, rectenna design, experiments with microwave power technology,
and studies into the feasibility of such an aircraft to fulfill its intended role. This
included the development and construction of a 1.3m staggered wing model called “Stagger
Sharp” in 1982. The proposal to build a working model and ground powering system was
submitted and work on the next generation of Sharp aircraft began.
Since the amount of power induced by the rectennas is proportional to surface area, a way of
increasing the area was required. This led to the SHARP aircraft's most distinctive feature,
the so-called ‘pizza’. This non-lifting surface acted as a rear stabilizer.
In 1985 the first full scale aircraft proposal was put forward in a report to the CRC. Isis
SHARP, named after the Egyptian god of the sky, was to be 17 m long, have a wingspan
of 36 m, and a disc diameter of 4.5 m. It was intended to fly at 22km altitude and carry
a 100 kg payload. At these altitudes it would need to reach speeds up to 200kph to fight
the stratospheric winds and maintain its position. The ground based microwave transmitter would
consist of a few hundred 15 foot mechanically steered antennas. The flight path of ISIS
SHARP was such that the antennas only needed to be able to turn by at most 20°.
A series of test model aircraft were created to further explore the idea. The first 4 SHARP
models were presumably intended for airframe design refinements, windtunnel testing, and
test flights under battery or gas power.
The world’s first successful flight of an unmanned aircraft powered by ground-based
microwave power occurred on September 17th 1987. The UTIAS SHARP-5 took to the air under
battery power until the transmitter was aligned, it then flew for 20 minutes under full ground-based
power. By October they had extended flights to over an hour. It now resides on display
at the Canadian Air and Space Museum in Toronto, Ontario.
SHARP-6B was an improved version based more closely on the intended final concept. The
wings were made parallel to the ‘pizza’ and were covered in rectenna to increase surface
area. Canards were installed to replace the T-tail stabiliser as it was found to be largely
ineffective in its current position. It first flew in late-1987 and testing continued into
1988.
The final proposal was for an aircraft with a wingspan of 39.6m and with a disc diameter
of 9m. It was similar to the ISIS SHARP proposal but incorporated many of the design refinements
learned during the model testing. It contained more that 10,000 rectennas and could carry
a payload of 90kg
The project ended when field mice broke into the hangar containing the SHARP models and
damaged them beyond repair. The project was all but abandoned by 1989.
A group of Japanese researchers picked up the batton and created their own microwave
power aircraft in 1992 called the MILAX. Unlike the Canadian scheme, the Japanese used a fully
phased array transmitter.
The success of the SHARP-5 and SHARP-6B models earned Professor James DeLaurier, and by extension,
his team many awards including the Fédération Aéronautique Internationale’s ‘Diplôme
d'Honneur’ trophy in 1988 for their pioneering work.
The SHARP concept was a revolutionary idea that had the potential to change the lives
of thousands of people living and working in remote communities all over the world.
Unfortunately it was competing against the ever decreasing cost of satellites and the
abundance of low power repeater towers popping up all over the country. The infrastructure
investment was seen as too expensive for its benefits
and the SHARP concept has been quietly forgotten ever since.