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Title:
LONG-DURATION, REMOTELY POWERED AIRCRAFT SYSTEM
Document Type and Number:
WIPO Patent Application WO/1995/012237
Kind Code:
A1
Abstract:
An unmanned, remotely powered aircraft for use as a stationary communications platform. The aircraft has an efficient flight profile and an array of rectennas on the underside to convert electromagnetic energy at 35 GHz generated by a ground station utilizing gyrotrons and a 34- meter diameter dual reflector antenna. The aricraft system provides a platform from which a number of previously unavailable or highly expensive missions may be carried out. These missions include the optical relay of energy to an orbiting satellite, a down-link of communication signals from a satellite to a plurality of ground receivers, long-duration surveillance missions, and others. The present aircraft system also includes a phased array antenna incorporated in the surface of tha aircraft.

Inventors:
FOOTE HOWARD A
Application Number:
PCT/US1994/012145
Publication Date:
May 04, 1995
Filing Date:
October 25, 1994
Export Citation:
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Assignee:
SKYSAT COMMUNICATIONS NETWORK (US)
International Classes:
B64D27/24; G05D1/00; H02J17/00; (IPC1-7): H02J17/00; B64D27/24
Foreign References:
US5068669A1991-11-26
US4697761A1987-10-06
FR2623469A11989-05-26
Other References:
JULL ET ALL: "sharp(stationary high altitude relay platform):telecommunications missions and systems", GLOBECOM'85-IEEE GLOBAL TELECOMMUNICATIONS CONFERENCE, 2 December 1985 (1985-12-02), NEW ORLEANS, pages 31.4.1 - 31.4.5
MACHINA ET AL: "power beaming-energy transmission at 35ghz and higher frequencies", PROCEEDINGS OF THE 27TH INTERSOCIETY ENERGY CONVERSION ENGINEERING CONFERENCE, 3 August 1992 (1992-08-03), SAN DIEGO, pages 2285 - 2288
EAST: "a self-steering array for the sharp microwave-powered aircraft", IRE TRANSACTIONS ON ANTENNAS AND PROPAGATION, vol. 40, no. 12, December 1992 (1992-12-01), NEW YORK US, pages 1565 - 1567
MORRIS JR.: "microwave-powered,unmanned,high-altitude airplanes", J.AIRCRAFT, vol. 21, no. 12, September 1984 (1984-09-01), pages 966 - 970
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Claims:
IN THE CLAIMS :
1. A remotely powered aircraft communications system, comprising: a remotely controlled aircraft, having an inner wing and at least two outer wings, capable of flying at constant altitudes for extended periods of time; a first communications transceiver, mounted to a first position on the aircraft, capable of transmitting and receiving a plurality of first communications signals; a second communications transceiver, mounted to a second position on the aircraft, capable of transmitting and receiving a plurality of second communications signals; and a rectenna, mounted to a third position under the aircraft, capable of converting electromagnetic radiation generated by a transmission source into direct current energy, wherein the transmission source is located at a position remote from the aircraft, and wherein the transmission source comprises: a power transmission source capable of generating electromagnetic radiation, a transmission antenna operatively connected to the power transmission source, and a waveguide that guides the electromagnetic radiation generated by the transmission source to the transmission antenna.
2. The remotely powered aircraft communications system as defined in claim 1, wherein the constant altitudes include all of the altitudes between 40,000 and 80,000 feet.
3. The remotely powered aircraft communications system as defined in claim 1, wherein the first position is the underside of the inner wing of the aircraft .
4. The remotely powered aircraft communications system as defined in claim 1, wherein the second position is the top of the inner wing of the aircraft.
5. The remotely powered aircraft communications system as defined in claim 1, wherein the aircraft includes a number of redundant control and power systems for keeping the aircraft aloft for extended periods of time.
6. The remotely powered aircraft communications system as defined in claim 1, wherein the aircraft carries telecommunications transmission and switching equipment capable of transceiving voice, data and video information.
7. The remotely powered aircraft communications system as defined in claim 1, wherein the plurality of first communications signals are transmitted to a terrestrial cellular telephone system.
8. The remotely powered aircraft communications system as defined in claim 1, wherein the plurality of second communications signals are transmitted to an airborne telecommunications system.
9. The remotely powered aircraft communications system as defined in claim 1, wherein the plurality of second communications signals are transmitted to a communications satellite.
10. The remotely powered aircraft communications system as defined in claim 1, wherein the rectenna comprises a plurality of half wavelength dipole antennas, each terminated by a rectifying diode.
11. The remotely powered aircraft communications system as defined in claim 1, wherein the third position is the underside of the outer wings.
12. The remotely powered aircraft communications system as defined in claim 1, wherein the power transmission source is a gyrotron.
13. The remotely powered aircraft communications system as defined in claim 12, wherein the waveguide provides mode conversion of the electromagnetic radiation generated by the gyrotron.
14. The remotely powered aircraft communications system as defined in claim 1, further comprising: a movable subreflector supporting the transmission antenna, capable of controlling the altitude at which the electromagnetic radiation is focused; a direction beacon, mounted to the underside of the inner wing, that generates a tracking signal indicative of the location of the rectenna; a subreflector control unit operatively couple to the subreflector; and a receiver unit, electrically coupled to the sub reflector control unit, which receives the trackin signal from the direction beacon, provides the tracking signal to the subreflector control unit, an thereby causes the electromagnetic radiation to be focused on the rectenna.
15. The remotely powered aircraft communications system as defined in claim 1, wherein the aircraft flies in a figure eight pattern.
16. The remotely powered aircraft communications system as defined in claim 15, wherein pattern has a maximum length of 2 kilometers.
17. A remotely powered aircraft communications system, comprising: an aircraft, having an inner wing and at least one outer wing, wherein the aircraft has a propulsion system capable of maintaining the aircraft at a constant altitude for an extended period of time; a first communications transceiver, mounted to a first position on the aircraft, capable of transmitting and receiving a plurality of first communications signals; a second communications transceiver, mounted to a second position on the aircraft, capable of transmitting and receiving a plurality of second communications signals; and a rectenna, mounted to a third position under the aircraft, capable of converting electromagnetic radiation into direct current energy, wherein the direct current energy is sufficient to power the propulsion system and the communications transceivers; and a power transmission beaming system located at a position remote from the aircraft, wherein the power transmission beaming system generates electromagnetic radiation and focuses the electromagnetic radiation on the rectenna.
18. The remotely powered aircraft communications system as defined in claim 17, wherein the power transmission beaming system comprises: a power transmission source capable of generating electromagnetic radiation having a frequency approximating 35 Gigahertz, a transmission antenna operatively connected to the power transmission source, and a waveguide that guides the electromagnetic radiation generated by the transmission source to the transmission antenna.
19. The remotely powered aircraft communications system as defined in claim 17, wherein the constant altitude ranges between 40,000 and 80,000 feet.
20. The remotely powered aircraft communications system as defined in claim 17, wherein the first position is the underside of the inner wing of the aircraft.
21. The remotely powered aircraft communications system as defined in claim 17, wherein the second position is the top of the inner wing of the aircraft.
22. The remotely powered aircraft communications system as defined in claim 17, wherein the aircraft includes a number of redundant control and power systems for keeping the aircraft aloft for extended periods of time.
23. The remotely powered aircraft communications system as defined in claim 17, wherein the aircraft carries telecommunications transmission and switching equipment capable of transceiving voice, data and video information.
24. The remotely powered aircraft communications system as defined in claim 17, wherein the plurality of first communications signals are transmitted to a plurality of terrestrial cellular telephone systems.
25. The remotely powered aircraft communications system as defined in claim 17, wherein the plurality of second communications signals are transmitted to a plurality of airborne telecommunications systems.
26. The remotely powered aircraft communications system as defined in claim 17, wherein the plurality of second communications signals are transmitted to a plurality of communications satellites.
27. The remotely powered aircraft communications system as defined in claim 17, wherein the rectenna comprises a plurality of half wavelength dipole antennas, each dipole antenna being terminated by a rectifying diode.
28. The remotely powered aircraft communications system as defined in claim 17, wherein the third position is the underside of the outer wing.
29. The remotely powered aircraft communications system as defined in claim 18, wherein the power transmission source is a gyrotron.
30. The remotely powered aircraft communications system as defined in claim 18, wherein the waveguide provides mode conversion of the electromagnetic radiation generated by the gyrotron.
31. The remotely powered aircraft communications system as defined in claim 18, further comprising: a movable subreflector supporting the transmission antenna, capable of controlling the altitude at which the electromagnetic radiation is focused; a direction beacon, mounted to the underside of the inner wing, that generates a tracking signal indicative of the location of the rectenna; a subreflector control unit operatively coupled to the subreflector; and a receiver unit, electrically coupled to the sub reflector control unit, which receives the tracking signal from the direction beacon, provides the tracking signal to the subreflector control unit, and thereby causes the electromagnetic radiation to be focused on the rectenna.
32. The remotely powered aircraft communications system as defined in claim 17, wherein the aircraft flies in a figure eight pattern.
33. The remotely powered aircraft communications system as defined in claim 32, wherein the pattern has a maximum length of 2 kilometers.
34. A remotely powered aircraft system, comprising: an aircraft, having an inner wing and at least one outer wing, wherein the aircraft has a propulsion system capable of maintaining the aircraft at a constant altitude for an extended period of time; a payload, mounted to a first position on the aircraft; a direction beacon, mounted to a second position on the aircraft, that generates a tracking signal indicative of the location of the aircraft; a rectenna, mounted to the second position on the aircraft, capable of converting electromagnetic radiation into direct current energy, wherein the direct current energy is sufficient to power the propulsion system and the communications transceivers; and a power transmission beaming system located at a position remote from the aircraft, wherein the power transmission beaming system generates electromagnetic radiation, receives the tracking signal, and focuses the electromagnetic radiation on the rectenna.
35. The remotely powered aircraft system as defined in claim 34, wherein the constant altitude ranges between 40,000 and 80,000 feet.
36. The remotely powered aircraft system as defined in claim 34, wherein the first position is the top of the inner wing of the aircraft.
37. The remotely powered aircraft system as defined in claim 34, wherein the second position is the underside of the inner wing of the aircraft.
38. The remotely powered aircraft system as defined in claim 34, wherein the aircraft includes a number of redundant control and power systems for keeping the aircraft aloft for extended periods of time.
39. The remotely powered aircraft system as defined in claim 34, wherein the mission payload comprises telecommunications transmission and switching equipment capable of transceiving voice, data and video information.
40. The remotely powered aircraft system as defined in claim 34, wherein the rectenna comprises a plurality of half wavelength dipole antennas, each dipole antenna being terminated by a rectifying diode.
41. The remotely powered aircraft system as defined in claim 34, wherein the power transmission beaming system comprises: a power transmission source capable of generating electromagnetic radiation having a frequency approximating 35 Gigahertz, a transmission antenna operatively connected to the power transmission source, and a waveguide that guides the electromagnetic radiation generated by the transmission source to the transmission antenna.
42. The remotely powered aircraft system as defined in claim 41, wherein the power transmission source is a gyrotron.
43. The remotely powered aircraft system as defined in claim 42, wherein the waveguide provides mode conversion of the electromagnetic radiation generated by the gyrotron.
44. The remotely powered aircraft system as defined in claim 43, further comprising: a movable subreflector supporting the transmission antenna, capable of controlling the altitude at which the electromagnetic radiation is focused; and a subreflector control unit operatively coupled to the subreflector.
45. The remotely powered aircraft system as defined in claim 34, wherein the aircraft flies in a figure eight pattern.
46. The remotely powered aircraft system as defined in claim 45, wherein pattern has a maximum length of 2 kilometers.
47. A method of remotely powering an aircraft system, comprising: towing an aircraft to an initial altitude of approximately 15,000 feet; releasing the aircraft within the near transmission field of a power transmission antenna capable of transmitting a microwave energy beam, wherein the power transmission antenna has a subreflector which moves within the antenna causing the microwave energy beam to focus at varying altitudes; focusing the microwave energy beam into a power spot on the aircraft, wherein the power spot has a diameter of less than 10 meters; increasing the altitude of the aircraft from the initial altitude to a target cruising altitude while maintaining the microwave energy beam focus on the aircraft; and rectifying the microwave energy beam into DC electrical energy onboard the aircraft, wherein the DC energy is sufficient to propel the aircraft and maintain a constant cruising altitude.
48. A remotely powered aircraft system, comprising: an aircraft, having an inner wing and at least one outer wing, wherein the aircraft has a propulsion system capable of maintaining the aircraft at a constant altitude of 70,000 feet for an indefinite period of time; a payload, mounted to a first position on the aircraft; a direction beacon, mounted to a second position on the aircraft, that generates a tracking signal indicative of the location of the aircraft; a rectenna, mounted to the second position on the aircraft, capable of converting electromagnetic radiation into approximately 150 kilowatts of direct current energy, wherein approximately 10 kilowatts is used to power the payload, and approximately 140 kilowatts is used to propel the aircraft; and a power transmission beaming system located at a position remote from the aircraft, wherein the power transmission beaming system generates electromagnetic radiation, receives the tracking signal, and focuses the electromagnetic radiation on the rectenna using less than five power transmission antennas.
Description:
LONG-DURATION, REMOTELY POWERED AIRCRAFT SYSTEM

Field of the Invention The present invention relates in general to a flexible remotely-powered high altitude aircraft system for executing telecommunications, remote sensing, reconnaissance, and other functions using a remotely- powered aircraft and, more particularly, to a system using an unmanned, high altitude electrically-powered aircraft to which power is supplied in the form of microwave energy which is transmitted from a ground station and which is rectified on the aircraft to provide a source of electrical power for the propulsion and on-board systems of the aircraft.

Description of Related Art Attempts have been made for many years to develop a system for transmitting energy to power a remote device with a high degree of efficiency (for a general discussion see "The History of Power Transmission by Radio Waves" by William C. Brown, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 9, September 1984) . In particular, the concept of powering an aircraft with terrestrially radiated electromagnetic energy has received a great deal of attention. The radiated electromagnetic energy incident on the aircraft is converted to direct current (DC) energy which powers the aircraft. One advantage of this technique is that an aircraft can be maintained airborne indefinitely, serving as a telecommunications, remote sensing, or reconnaissance platform. Conversion of the radiated electromagnetic energy to

DC energy at the aircraft is well known in the prior art and typically is performed by a power collecting and conversion device known as a rectifying antenna or "rectenna". In systems of the prior art, the rectenna is typically mounted on the lower surface of the aircraft where it rectifies incident electromagnetic energy of the

â– appropriate wavelength into DC energy. The DC energy is used to power the aircraft's propulsion system and on-board systems. The rectenna typically consists of an array of

half wavelength dipole antennas, each terminated by a rectifying diode. The rectenna converts the maximum power density of incident electromagnetic power into DC electrical energy. By increasing the rectenna surface area exposed to incident electromagnetic radiation, the rectified DC energy may thereby also be increased. Therefore, to increase DC electrical energy output, rectenna surface area has often been increased in the prior art systems. However, increasing the size of the rectennas on remotely-powered aircraft introduces both aerodynamic and communications design problems. Due to the increased size of the rectennas, which are typically not designed to minimize aerodynamic drag, increased aerodynamic drag is introduced into the airflow passing over the aircraft during flight. The increased drag results in increased power required to keep the aircraft aloft, which, in turn, necessitates increased rectenna size.

In addition to the aerodynamic difficulties presented by aircraft systems using larger rectennas, increases in the size of the rectennas also indirectly causes increases in interference to the communications signals transmitted from the aircraft. For example, when larger rectennas are utilized in prior art systems, the diameter of the microwave power beam which is focused on the rectenna becomes correspondingly larger. , Due to the increased diameter of the microwave power beam necessitated by the larger rectennas, the communications systems are exposed to a greater amount of interference created by the microwave power beam. Rectenna designs must exhibit low spurious and harmonic emission levels to prevent potential interference to sensitive communications systems. Therefore, a need exists for a remotely-powered aircraft having a smaller rectenna size requirement which reduces spurious and harmonic emission levels and having communications systems which are not hampered by the reception of a microwave power beam. By minimizing the rectenna size requirement, increasingly smaller rectennas may be utilized, and the communications equipment may be positioned anywhere on the aircraft without undue microwave interference. This

feature allows for much greater flexibility in the design and utilization of the remotely-powered aircraft system.

In 1987, a scale model of a long-endurance, high- altitude aircraft, remotely powered by microwave energy, known as the SHARP (Stationary High Altitude Relay Platform) system, was successfully demonstrated. See Schlesak, et al. , "A Microwave Powered High Altitude Platform," 1988 IEEE MTT-S Digest, pp. 283-286. The SHARP concept employs an array of terrestrial antennas which are used to radiate microwave energy to a remote aircraft. Dual polarization rectennas, positioned underneath the aircraft, formed by two orthogonal linearly polarized rectennas, convert the terrestrially radiated microwave energy to DC energy which is used to power the aircraft. The scale model SHARP aircraft, one-eighth the size of the actual aircraft, was flown for 20 minutes in 1987. The scale model aircraft was powered by energy generated by two 5 kilowatts continuous-wave magnetrons. The scale model aircraft used a ground station having one 4.5 meter diameter parabolic antenna which transmitted 10 kilowatts of the combined microwave energy generated by the magnetrons, at a frequency of 2.45 GHz. The small scale model aircraft had a wingspan of 4.5 meters, an overall length of 2.9 meters, weighed only 4.1 kilograms, and flew at a target altitude of only 150 meters. Due to the small size of the scale aircraft, only one parabolic antenna was needed to generate power which was sufficient to keep the aircraft aloft. However, the full scale 2.1 kilometer altitude microwave powered aircraft contemplated by the SHARP system has much more demanding power requirements, and as a result, disadvantageously demanding, complex, and expensive ground station antenna and aircraft rectenna requirements.

For example, the actual SHARP aircraft requires that the microwave energy be transmitted from a large array of parabolic antennas positioned on the ground in a circle having a diameter of approximately 85 meters. The SHARP microwave power transmission system is comprised of a phased array of approximately 250 5-meter antennas, each

antenna fed by a 5 kilowatt magnetron, each magnetron transmitting microwave energy at 2.45 GHz. The combined energy from the 250 antennas are focussed to a spot area having a half-power diameter of 30 meters. Using this configuration, approximately 500 kilowatts of microwave power is radiated to produce a power flux density of 500 watts per square meter at a 21 kilometer altitude. This requires that the aircraft have at least 100-m 2 of rectenna surface to generate only 35 kilowatts of DC power. Of the 35 kilowatts generated by the rectenna, 25 kilowatts would be used by SHARP'S electric motor for propulsion and the remainder available for the payload electronics.

The requirements imposed by the SHARP system disadvantageously presents difficult ground station and aircraft design problems. For example, the ground station requires the use of hundreds of parabolic antennas, all of which must be steered and combined cooperatively. Each individual antenna must accurately track the rectenna located 21 kilometers above. With 250 individual antennas, this is a difficult and computationally intensive task. In addition, the power generated by each antenna must be cooperatively combined. Furthermore, the use of hundreds of antennas introduces undesirable cost increases and maintenance problems. Each of the 250 antennas contemplated by the SHARP system is fed by a 5 kilowatt magnetron. Each magnetron and antenna introduces increased cost, decreased reliability and availability considerations into the design of the remotely-powered aircraft system. Finally, the SHARP system requires at least 100 square meters of rectenna surface area on the aircraft. This requirement imposes serious constraints upon the design of the aircraft, which, in turn, restricts the size and type of payload which the aircraft may carry. Therefore, a need exists for a remotely-powered aircraft system which requires far fewer ground station antennas, less aircraft rectenna surface area, and facilitates flexible and inexpensive ground station and aircraft design.

A microwave-powered aircraft is also disclosed in U.S. Patent No. 4,955,562, issued to Martin, et al . (hereinafter

"Martin"). Martin teaches a microwave-powered aircraft which flies in a 4 km diameter circle above an array of microwave transmission antennas. The array of transmission antennas are located at ground level within a circular area of approximately 70 meters in diameter. The array of transmission antennas cooperate to transmit a power beam in an upwardly direction toward the aircraft, focussing the power spot on the aircraft's rectenna. At the desired 20 km altitude, the focussed power spot is approximately 30 meters in diameter. The disadvantages introduced by the requirement of having an array of transmission antennas was discussed above with reference to the SHARP system. In addition, the Martin aircraft is flawed because it does not tightly focus the power beam to a desirable diameter. Unless the aircraft rectenna is designed to accommodate 30 meters of power beam transmission, energy will be wasted. Unfortunately, use of a rectenna having a 30 meter diameter introduces aerodynamic drag into the design of the aircraft. Therefore, a need exists for a remotely-powered aircraft system utilizing a minimum number of transmission antennas and, preferably, only one transmission antenna which can tightly focus its power beam to within about 10 meters at the target altitude.

Efforts at remotely-powering an aircraft to date, like the SHARP system, have focused primarily on using the so- called S-band transmission sources due to their ready availability and to reduce power losses due to atmospheric attenuation. S-band power transmission, as demonstrated by the SHARP system, is limited in the amount of power that can be delivered in a practical system. As a result, a large number of ground transmission antennas are needed. The number of antennas used by an S-band transmission system could be reduced, however, the size of each antenna would increase significantly thereby increasing the cost of the ground station while also increasing the difficulty of tracking the airborne aircraft. In addition, as demonstrated by the SHARP system discussed above, S-band power beaming disadvantageously requires a large amount of rectenna surface area on the aircraft to generate

sufficient power to propel the aircraf . Therefore, a need exists for a remotely-powered aircraft system which transmits microwave power at a frequency higher than the S- band. In order to reduce the rectenna size requirements and the number of terrestrial power transmission antennas, yet provide the aircraft with sufficient propulsion and payload power, higher frequency electromagnetic energy has been disclosed in the prior art. For example, U.S. Patent No. 5,068,669, given to Koert, et al. (hereinafter "Koert") teaches a power beaming system using high frequency source transmissions operating at a frequency of at least 10 GHz. Koert discloses the use of gyrotrons to produce the millimeter-wave electromagnetic energy for transmission to a remotely powered aircraft. The Koert reference does not disclose the use of a communications system or other payload systems.

Previous authors have suggested that remotely powered aircraft be used as platforms for telecommunications, surveillance, and remote sensing. See U.S. Patent No. 4,955,562 to Martin, et al. ; see also U.S. Patent No. 4,943,811 to Alden, et al. One author has specifically stated that remotely powered aircraft should be used as telecommunications relay platforms to retransmit television and mobile telephone signals within a metropolitan region. See Arthur Fisher, "Secret of Perpetual Flight? BEAM- POWERED PLANE," Popular Science, January 1988, p. 106. Despite suggesting differing techniques for remotely powering the aircraft, these references fail to sufficiently describe the payload system requirements and design. Therefore, a need exists for a remotely-powered aircraft system which overcomes the disadvantages of the prior art yet provides a novel and nonobvious payload capability of performing telecommunications, surveillance, remote sensing, and other platform f nctions.

Summary of the Invention The present invention teaches a long-duration, remotely powered aircraft system which is highly efficient, flexible, inexpensive, and capable of performing a variety

of functions. The aircraft system is reusable and can be used to replace expensive single-use satellites or other high-altitude platforms having short loitering times. The system comprises the aircraft, the aircraft's payload, and a ground station. In one preferred embodiment, the payload comprises a telecommunications system. In alternative embodiments, the payload performs remote sensing, reconnaissance, and other important functions which may be efficiently performed by a high-altitude remotely-powered aircraft system.

The aircraft is designed to be aerodynamically efficient, with a large-area inner wing and two outwardly extending outer wings. In one preferred embodiment of the present invention, a rectenna is mounted to the underside of the inner wing. The rectenna is used to collect electromagnetic energy and convert the electromagnetic energy into electric power. The electromagnetic energy is radiated upwardly from a ground station which is used to remotely power the aircraft. The electromagnetic energy is converted into DC electric power sufficient to power two motors mounted on the outer wings which drive propellers. The aircraft is of a sufficient size and durable construction to withstand the high winds persisting at altitudes of approximately 70,000 feet. The aircraft includes a number of redundant control and power systems for maintaining long-duration flights.

The aircraft may carry a variety of payloads for performing different functions, including, but not limited to telecommunications, surveillance, and remote sensing. To perform, for example, telecommunication functions, such as relaying mobile telephone and direct broadcast television signals, the aircraft must carry on-board telecommunications transceivers. Airborne telecommunications repeaters, in comparison to their terrestrial counterparts, generally have the advantage of being capable of transceiving signals at increased ranges.

To effect surveillance functions, including drug enforcement and missile detection, the aircraft may carry radar, infrared detectors, and cameras, among other

surveillance equipment. Similarly, the aircraft may carry radiometers, radars, infrared detectors or cameras to conduct remote sensing functions, including environmental monitoring. By implementing these functions with a remotely-powered high-altitude long duration aircraft, a substantial decrease in the costs associated with these functions is realized. In addition, the novel aircraft system of the present invention allows for greater flexibility in systems design. Unlike the remotely powered aircraft systems of the prior art, the aircraft system of the present invention can be networked with one or a plurality of satellites or other communications aircraft. For example, the aircraft can be linked with a satellite to extend the satellite's broadcast transmission range. To facilitate intra-vehicle networking, the aircraft may include multiple electronically steered phased array antennas, positioned on the aircraft, which allow the aircraft to communicate with other systems. Electronically steered phased array antennas may alternatively be used to link the aircraft's payload with terrestrial targets. One phased array antenna, placed on the aircraft's underbelly, may be used in one embodiment to link the aircraft payload with terrestrial targets. Alternatively, other phased array antennas, positioned on the aircraft's upper wing surfaces, may be used to link the aircraft's payloads with other airborne and space vehicles such as communications satellites.

The phased array antennas can be electronically configured to generate function specific antenna radiation patterns. For example, the phased array antenna positioned on the aircraft's underbelly can generate multiple beams having overlapping footprints, forming asymmetrical terrestrial footprints. The asymmetrical footprints can be configured to match the shape of an urban area to provide uniform coverage for an airborne cellular telephone system. Alternatively, the phased array antenna can be electronically configured, for use with a surveillance or remote sensing systems, to scan a single high gain beam

across a terrestrial surface.

As discussed above, the aircraft is remotely powered by electromagnetic energy radiated from an earth or ground station incorporating an electromagnetic power source and a transmission antenna. The power source may be comprised of one or more gyrotrons operating in the millimeter-wave region, such as at a frequency of 35 GHz. The electromagnetic energy generated by the power sources is combined and radiated by the transmission antenna. The transmission antenna is a dual reflector antenna having a diameter of 34 meters.

The aircraft flies in the antenna's near field. The antenna, in conjunction with redundant tracking systems, focus the radiated electromagnetic energy on the aircraft's rectenna. In one preferred embodiment, the focussed electromagnetic energy is focussed at a power spot having a diameter of less than 10 meters.

Brief Description of the Drawings FIGURE 1 is a schematic view of a remotely powered aircraft system of the present invention;

FIGURE 2 is a bottom plan view of the aircraft of FIGURE 1;

FIGURES 3a and 3b together comprise a schematic representation of one embodiment of the ground station used in the present invention;

FIGURE 4 is a schematic view of a phased array antenna placed above the rectenna and the lower surface of the inner wing;

FIGURE 5 is a schematic view of the phased array antenna placed beneath the lower surface of the outer wing; FIGURE 6 is a schematic view of the aircraft system used to perform an environmental monitoring task;

FIGURE 7 is a schematic view of the multiple footprints generated by the phased array antenna; FIGURE 8 is a schematic view of an airborne cellular telephone system using the present invention;

FIGURE 9 is a schematic view of a phased array antenna placed in or on an outer surface of the aircraft's tail;

FIGURE 10 is a schematic view of a phased array

antenna placed in or on the outer surface of the inner wing, opposite the rectenna;

FIGURE 11 is a schematic view of a cellular telephone network employing a plurality aircraft systems contemplated by the present invention;

FIGURE 12 is a schematic view of the aircraft system of the present invention shown repeating communications signals transmitted from a satellite;

FIGURE 13 is a schematic view of the aircraft system of the present invention shown relaying transmissions to a satellite using a laser; and

FIGURE 14 is a schematic view of the aircraft system of the present invention, shown carrying a mirror reflecting light used to provide power to a satellite. FIGURE 15 is a schematic view of the aircraft system of the present invention, shown communicating with marine craft to relay precise navigation information.

Description of Preferred Embodiments System Referring now to FIGURE 1, an unmanned electromagnetically powered aircraft system of the present invention is shown. The system is generally comprised of a ground station 20 having a transmission antenna 22 for transmitting electromagnetic energy indicated by lobe outline 24 to an aircraft 26, which is powered by the electromagnetic energy 24. The transmission antenna 22 focuses the transmitted electromagnetic energy onto the aircraft 26 at a power spot having a diameter less than 10 meters. The electromagnetic energy is converted into DC electric energy by a rectenna (shown in FIGURE 2) positioned on the underbelly of the aircraft's inner wing. A tracking signal 27 is transmitted from the aircraft 26 to the transmission antenna. The tracking signal 27 allows the ground station 20 to focus the electromagnetic energy 24 on the aircraft and particularly on the aircraft's rectenna. The aircraft 26 is propelled by one or more electric motors which are housed in a pair of torpedo- shaped nacelles 28 which drive propellers 30 positioned on the aircraft's wings.

When launched, the aircraft 26 is first towed to a height of approximately 15,000 feet. The aircraft 26 is then released within the near field of the antenna 22, and remotely powered by the antenna to a preferred cruising altitude, such as 70,000 feet. The aircraft 26 typically cruises in a figure-eight loitering pattern having its center located directly overhead the ground station 20. The figure-eight pattern typically has a maximum length of about 2 kilometers. In the absence of remote power, caused, for example, by a failure in the ground station 20, the aircraft may use auxiliary power and its propulsion systems to remain airborne until the ground station is repaired or to land.

Due to the aircraft's high altitude and small loitering area, the antenna 22 must track the aircraft 26 across a small arc. For example, if the aircraft 26 flies at a preferred altitude of 70,000 feet, the antenna 22 must track the aircraft 26 across an arc of about 6 degrees. The antenna 22 may be relatively large. In one embodiment, using a dual reflector antenna, the antenna diameter is preferably 34 meters. The large diameter antenna 22 allows the electromagnetic energy 24 to be tightly focused on the aircraft. For example, the antenna 22, having a diameter of 34 meters, can focus a microwave power beam onto the aircraft 26 at an altitude of 70,000 feet in a power spot having a diameter less than 10 meters. As a result, a large percentage of the weight of the aircraft may be dedicated to the payload. In addition, the aircraft 26 of the present invention includes a number of redundant on- board systems which allow for flights of up to about four months duration. Unmanned Aircraft

Referring now to FIGURE 2, the aircraft 26 has a broad central portion or inner wing 50, a pair of outer wings 52 and a tail assembly indicated generally by reference numeral 54. The unconventionally shaped airframe is •basically a flying wing with the front edge 56 of the wide inner wing 50 forming a leading edge which is generally co- linear with the leading edges of the outwardly diverging

wings 52. The aircraft 26 has a completely aerodynamic shape due to the fact that almost every portion (except the tail 54) , including the inner wing 50, has an airfoil cross-sectional shape. The aircraft 26 is configured with a very high aspect ratio to have extremely efficient flight dynamics with correspondingly low aerodynamic drag in order to reduce the amount of power required to keep the aircraft 26 aloft. Preferably, the overall aspect ratio of the aircraft 26 is between 20 and 36. Of course, the overall aspect ratio does not reflect the aspect ratio at specific points on the aircraft. In particular, the outer wings 52 have extremely high aspect ratios and produce substantially all of the lift required by the aircraft 26. The inner wing 50, having a relatively low aspect ratio, produces less lift but does not add much lift-induced drag. Because the aircraft 26 is extremely aerodynamically efficient, it requires much less terrestrially radiated energy to power its flight than do prior art remotely-powered aircraft. Rectenna Panels

As seen in FIGURE 2, a large portion of the underside of the aircraft 26 is covered with a rectenna. The rectenna converts incident electromagnetic energy into DC energy which is used to power the aircraft 26 propulsion system, control system, and payloads. The region 58 shown cross-hatched in FIGURE 2 represents the maximum area of the underside of the aircraft onto which the rectenna is preferably mounted.

The rectenna in the present invention is preferably substantially similar to the rectenna shown in U.S. Patent No. 5,068,669 to Koert, which is incorporated by reference herein. This preferred rectenna includes layers of etched circuits to maximize the density of energy collecting dipoles on the rectenna' s outer surface. The rectenna may have an area ranging from 16 2 to 196 m 2 . In one embodiment, there are two-thousand 20 cm x 20 cm square rectenna panels on the underside of the inner wing 50. Ground Station

As depicted in FIGURES 3a and 3b, the ground station

20 includes a high power oscillator and the antenna 22. The antenna 22 is preferably a 34-meter diameter parabolic dish such as one provided by Toronto Iron Works (TIW) of Sunnyvale, California. The high power oscillator is formed by two or more gyrotrons 60, having their electromagnetic energy combined by a power combiner 62. Use of multiple gyrotrons is desirable so that energy will continue to be supplied to the aircraft in the unlikely event that one gyrotron fails. The gyrotrons may be model VGA-8003 tubes supplied by Varian Associates of Palo Alto, California. The dual gyrotron outputs are first phase-locked before being added in combiner 62, which is supplied by the Jet Propulsion Laboratory, of Pasadena, California. The electromagnetic mode of the output energy is converted by a mode converter 64 to facilitate transmission to and radiation from the transmission antenna 22. The mode converter 64 is also supplied by TIW. The converted energy passes through a waveguide 66 (FIGURE 3a) prior to being fed to the transmission antenna 22. A number of cooling and antenna control systems within the ground station 20 are provided by Continental Electronics Corp. of Dallas, Texas. The frequency of the radiated electromagnetic energy is preferably in the millimeter-wave region, such as 35 GHz. The energy generated by the oscillator is fed to an antenna which focuses the electromagnetic energy on the aircraft 26. The antenna 22 can be adjusted in two ways to continuously focus approximately one-half of the power, at a given altitude, on the aircraft's rectenna. As the aircraft's altitude changes, a sub-reflector 68 is moved vertically to maintain the focus of the radiated power beam upon the aircraft's rectenna. Preferably, the sub- reflector 68 allows the microwave power beam 24 to be focused from altitudes of between 12,000 and 80,000 feet. The sub-reflector 68 can move several inches to perform the necessary focusing. Furthermore, the main reflector of the antenna 22 can be adjusted by drive motors 70 to focus the radiated energy on the aircraft as the aircraft moves horizontally above the antenna. The position of both

reflectors are controlled by redundant aircraft tracking systems located on the aircraft and ground station. Microwave Power Beam to Rectenna Efficiency

During operation, the antenna 22 focuses approximately one-half of the radiated electromagnetic energy at a given altitude on the aircraft's rectenna in approximately an eight-meter diameter circle referred to as the beam's power spot. Because the rectenna has an approximate diameter of 10 meters, the system will accommodate some error. Preferably, the tracking error is no more than approximately one foot.

The electromagnetic energy incident on the aircraft's rectenna is converted to DC energy by the rectenna. Preferably, at the desired target cruising altitude, which in the preferred embodiment of the present invention is approximately 70,000 feet, the aircraft 26 produces DC power of approximately 150 kW, 10 kW of which is used to provide power to the aircraft's payload and the remaining 140 kW is used to provide power to the aircraft's propulsion system. The propulsion system consists of two motors, each of which may be supplied with a maximum of 75 kW of power, ample for climbing and maintaining a constant altitude. Preferably, the motors require a power input of approximately 54 kW during maximum ascension. Aircraft Functions and Payloads

The aircraft 26 of the present invention is designed to carry a variety of payloads in order to perform many different functions. Typical payloads for the aircraft include telecommunications equipment, communications repeaters, radiometers, radars, high-resolution cameras, infrared detectors, chemical sampling units, search and rescue containers, and other specialized cargo. With these payloads, the aircraft may conduct various functions, including communications, surveillance and remote sensing. Specifically, the aircraft may serve as a mobile telephone repeater, a direct broadcast television repeater, or a platform for conducting anti-submarine activities, drug enforcement functions, missile detection, coastal surveillance, atmospheric studies, geophysical surveys, and

pollution monitoring among other activities.

To facilitate the operation of the various payloads, the aircraft preferably includes electronically steered phased array antennas which link the aircraft payloads with other systems. As shown in FIGURE 4, in one preferred embodiment, the phased array antenna 70 may be positioned above the rectenna 72 and above the lower surface of the inner wing 50. Alternatively, as shown in FIGURE 5, the phased array antenna 70 may be positioned beneath the lower surface of the outer wing 52. This configuration minimizes interference to the communications signals which are transceived by the phased array antenna caused by the electromagnetic energy transmitted to the rectenna.

The phased array antenna can be electronically configured to generate specific antenna radiation patterns to effect desired functions. For example, as shown in FIGURE 6, in an environmental monitoring system, the phased array antenna can sweep a narrow beam over a large area. Alternatively, as shown in FIGURE 7, the antenna can be configured to generate multiple fixed overlapping footprints to emulate a terrestrial cellular telephone network. Each footprint in conjunction with a repeater on the aircraft forms a cell comparable to terrestrial cellular networks. See George Calhoun, Digital Cellular Radio, Artech House, Inc., Norwood, MA, 1988.

Thus, as shown in FIGURE 8, the aircraft can perform as a cellular telephone network. The vehicles shown in FIGURE 8 are shown in communication with the aircraft's payload system. Because the aircraft hovers at high altitudes, the range of communications between cellular vehicles is greatly increased. In addition, because the aircraft is inexpensively manufactured and easily launched, the cellular communications system is much less costly than analogous cellular systems using satellite platforms. Alternatively, the aircraft may carry two additional phased array antennas. Both phased array antennas are shown in FIGURES 9 and 10. One phased array antenna 70' may be placed in or on an outer surface 80 of the aircraft's tail 54. The second phased array antenna 70''

may be placed in or on the outer surface of the inner wing 82, opposite the rectenna 72. These antennas facilitate communications between the aircraft and one or more other aircraft or satellites. For example, the cellular communication system can be territorially expanded by linking two or more aircraft 26 and 26', as shown in FIGURE 11. It is estimated that only six planes 26 would be needed to provide cellular communications service to the entire continental United States. As shown in FIGURE 11, a car 90 may communicate with another car 90', which is located at a remote distance from the car 90, by establishing communication with the aircraft 26. The aircraft 26 communicates with the other aircraft 26' using the phased array antenna discussed above. The communications loop is closed when the aircraft 26' communicates with the car 90' .

Similarly, the aircraft 26 can be used to relay signals transmitted to and from a satellite 100, as shown in FIGURE 12. For example, the aircraft 26 may rebroadcast television signals 102 transmitted from a satellite 100 to a terrestrial receiver 106. In addition, the aircraft 26 may retransmit a terrestrial mobile telephone signal 104 to the satellite 100 for further rebroadcast by the satellite 100. As shown in FIGURE 13, as an alternative to using a phased array antenna, the aircraft can communicate with the satellite 100 using a laser system 110. The laser system 110 can be mounted on the outer surface of the inner wing 82, opposite the rectenna 72. In another embodiment, the aircraft's first phased array antenna, located either on the inner wing 50 above the rectenna 72 or on the lower surface of the outer wing 76, can emit and receive terrestrial signals over a wide range of frequencies. These signals can be demodulated by the aircraft's payload and then re-modulated together on light waves 112 and emitted by the laser system 110.

As shown in FIGURE 14, the aircraft 26 can carry a mirror 120 which is used to reflect the light waves 122 transmitted by a terrestrial laser 124 to a satellite 100

orbiting beyond the line of sight of the terrestrial laser 124 in a lower orbit 126. As shown in FIGURE 14, the light waves 122 may be used by the satellite to provide power to the satellite sufficient to increase the satellite's orbit from a lower orbit 126 to a higher orbit 128.

Finally, as shown in FIGURE 15, the aircraft 26 can function as a precision navigational tool in a similar manner as a global positioning satellite. In contrast to single launch satellites, however, the aircraft 26 can be landed and refitted with updated equipment at little cost. Furthermore, the aircraft 26 replaces a plurality of land- based LORAN transmitters for assisting coastal navigation. Although the present invention has been described in terms of certain preferred embodiments, other embodiments that are apparent to those of ordinary skill in the art are also within the scope of this invention. Accordingly, the scope of the invention is intended to be defined only by the claims.