News Column

Patent Issued for Active Retrodirective Antenna Array with a Virtual Beacon

July 16, 2014



By a News Reporter-Staff News Editor at Defense & Aerospace Week -- From Alexandria, Virginia, VerticalNews journalists report that a patent by the inventor Crouch, David D. (Corona, CA), filed on June 28, 2011, was published online on July 1, 2014.

The patent's assignee for patent number 8767192 is Raytheon Company (Waltham, MA).

News editors obtained the following quote from the background information supplied by the inventors: "This invention relates to active retrodirective antenna arrays and more particularly to an active retrodirective antenna array (ARAA) that uses an optical carrier beam to reflect an RF pilot signal off a target to create a virtual beacon to individual nodes in the array.

"For many applications, it is necessary or desirable to be able to automatically track a received signal. In a radar or communication application, for example, it might be useful to be able to track a source of a received signal. A satellite or another RF signal, for example, might be tracked and a return signal automatically transmitted back to the satellite or RF signal source in response thereto. An automatic tracking system might allow for more effective and efficient direction of a return beam to complete an RF signal source or communication link.

"Conventionally, beam tracking has involved physically pointing an antenna in the direction of the received beam or processing the received signal with a tracking algorithm to steer a phased array antenna in the direction of the received signal. At any given time, the phase shifts applied to each array element in a phased array determine the direction of the main beam for both reception and transmission. Considerable analog and digital processing is required to track a target and radiate a beam in the direction of said target. Thus, while phased array antennas have been used for some time, automatic beam steering in response to an RF signal with a phased array antenna has been somewhat problematic to date.

"An active retrodirective antenna array (ARAA) is a phased antenna array that automatically steers its transmitted beam towards the apparent source of an incoming pilot signal. The modifier 'active' means that the radiated power is generated by sources associated with the antenna, rather than by reflection of an incident signal as in a passive retrodirective antenna (e.g., corner reflector). Such arrays, which are also known as 'self-focusing' arrays, have been suggested for some time. In such arrays, the transmitted wavefront duplicates the incoming pilot signal wavefront whatever its shape. The self-focusing property is important because it means that the transmitted power is focused back on the pilot source whatever the state of the intervening propagation medium, provided that the state persists for the round-trip propagation time. The retrodirective properties of proposed ARAA's is achieved by 'conjugating' a pilot signal incident at each array node. Though not yet in widespread use, ARAA's are expected to become an important part of phased array technology. They have, for example, been proposed for microwave power-transmission from orbiting solar power stations, communication satellite transmitting arrays, and aircraft transponders.

"When a retrodirective array receives an RF pilot signal from an unspecified direction, each array node receiving the RF pilot signal automatically generates and transmits a phase-conjugated RF pilot signal in the reverse direction without any previous knowledge as to the location of the signal source and without the need for sophisticated digital signal processing. The nodes may be randomly dispersed over a wide area, and knowledge of their positions is not required. The source of the RF pilot signal is a microwave beacon attached to a cooperative target. The beacon broadcasts the RF pilot signal which is received by some or all of the nodes of the array. The phase of the received signal is conjugated with a phase reference derived from the received pilot signal itself, amplified, and retransmitted at a different frequency to preserve isolation. The process of conjugating the phase of the received pilot signal causes the phase accumulated during propagation from the beacon to each node to be cancelled during the return trip. As a result, the signals transmitted by the individual nodes converge at the beacon with nearly identical phases, causing them to add coherently. U.S. Pat. Nos. 4,148,031 and 6,630,905, which are hereby incorporated by reference, describe different techniques for processing the received pilot signal to extract a phase reference for performing the phase-conjugation."

As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventor's summary information for this patent: "The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later.

"The present invention provides for an active retrodirective antenna array for use with uncooperative targets. The active retrodirective antenna array automatically tracks the uncooperative target, friendly or unfriendly, and returns an amplified RF signal to the target. The array may, for example, be used to complete an RF communication link, to transfer microwave power from orbiting solar power stations or as a directed energy weapon to prosecute the target.

"In an embodiment, an optical carrier beam with a first RF pilot signal imposed thereon is directed into a spot on the target. The scattered optical carrier beam forms a virtual beacon. The scattered optical carrier beam is detected by a plurality of the retrodirective array nodes to extract a second RF pilot signal. The second RF pilot-signals are phase-conjugated with a phase reference signal, amplified and retransmitted towards the target where the RF beams add-in phase at the virtual beacon on the target.

"In an embodiment, the RF pilot signal has a frequency of approximately 0.5 to 500 GHz and the optical carrier beam lies in UV, IR or visible spectrum spanning approximately 0.4 to 300 micron wavelength.

"In an embodiment, scattering the optical carrier beam off of the target may create multiple virtual beacons at different points on the target. In another embodiment, the optical carrier beam is focused into a spot whose diameter is less than one-half wavelength of the RF pilot signal to create a single point source for the virtual beacon. The optical carrier beam may be adaptively focused based on range-to-target to maintain the spot size as the target is tracked.

"In an embodiment, the phase reference signal may be derived either from a reference signal used to impose the RF pilot signal on the optical carrier beam or from the received RF pilot signal. The reference signal may be generated by a reference oscillator or extracted from a source of the optical carrier beam. Phase-conjugation may be performed locally at each node or remotely at a common phase-conjugation circuit.

"In an embodiment, the phase-conjugated RF pilot signal is retransmitted at the same frequency as the received RF pilot signal. Because the pilot signal is carried on the optical beam isolating the input (received pilot signal) and output (conjugated pilot signal) is not an issue.

"In an embodiment, a point and track mechanism generates a tracking signal to the target. The point and track mechanism may comprise a manual mechanism, a radar system or an optical detector. An illuminator is responsive to the tracking signal to direct the optical beam onto the target. The point and track mechanism may slew on a gimbal to point a boresight axis at the target. The illuminator may be mounted on the gimbal to follow the target as the mechanism slews.

"In different embodiments, each node may be fixed to retransmit the pilot signal in a predetermined direction, all nodes may be fixed to a single gimbal that slews to nominally point at the target or each node may be individually gimbaled to point its main beam at the target. The single gimbal or individual node gimbals may be slaved to a point and track system used to point the illuminator. The point and track mechanism, illuminator and all of the nodes may be mounted on the same gimbal.

"In different embodiments, each node may retransmit RF energy continuously, only retransmitting the phase-conjugated pilot signal when the optical receiver is illuminated by the virtual beacon, may retransmit RF energy only when the optical receiver has detected a virtual beacon or may retransmit RF energy only when the optical receiver has detected a virtual beacon and the main beam of the retransmitted RF energy overlaps the intended target.

"In an embodiment, the active retrodirective antenna array is configured as a directed energy weapon to deliver a microwave power density onto the target that exceeds a target dependent threshold to incapacitate, damage or destroy the target. The directed energy weapon may be terrestrial (fixed or moving vehicle), air, sea or space based.

"These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

"FIG. 1 is a diagram of an embodiment of an active retrodirective antenna array that creates a virtual beacon for an uncooperative target in accordance with an embodiment of the present invention;

"FIG. 2 is a diagram of an optical carrier beam with an RF pilot signal imposed thereon;

"FIG. 3 is a block diagram of an embodiment of an illuminator that imposes an RF pilot signal onto an optical carrier beam that is directed at the target to create the virtual beacon;

"FIG. 4 is a diagram of an embodiment in which the spot size is less than one-half wavelength of the RF pilot signal to create a point source;

"FIG. 5 is a diagram of an embodiment of a node of the array;

"FIG. 6 is a diagram illustrating multiple nodes retransmitting the phase-conjugated RF pilot signals such that their main beams add in-phase at the target;

"FIG. 7 is a diagram of an embodiment in which the illuminator is mounted on and slaved to the pointing and tracking system;

"FIG. 8 is a diagram of an embodiment in which the nodes of the array and the illuminator are mounted on and slaved to the pointing and tracking system;

"FIG. 9 is a diagram of an embodiment of an active retrodirective antenna array for use as a directed energy weapon in an airport protection system; and

"FIG. 10 is a diagram of an active retrodirective antenna array for use as a directed energy weapon in an aircraft protection system."

For additional information on this patent, see: Crouch, David D.. Active Retrodirective Antenna Array with a Virtual Beacon. U.S. Patent Number 8767192, filed June 28, 2011, and published online on July 1, 2014. Patent URL: http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=8767192.PN.&OS=PN/8767192RS=PN/8767192

Keywords for this news article include: Raytheon Company, Aerospace and Defense Companies.

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Source: Defense & Aerospace Week


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