Active retrodirective antenna array with a virtual beacon

• US 8,767,192 B2
• Filed: 06/28/2011
• Issued: 07/01/2014
• Est. Priority Date: 06/28/2011
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Crouch, David D.
• Primary Examiner(s): Ratcliffe, Luke
• Assistant Examiner(s): Diacou, Ari M
• Application Number:
  US13/171,006
• Publication Number:
  US 20130002472A1
• Time in Patent Office: 1,099 Days
• Field of Search: 356/3.1, 356/5.1
• US Class Current: 356/5.1
• CPC Class Codes: G01S 17/32 using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
• CPC Class Codes: G01S 17/66 Tracking systems using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 7/4863 Detector arrays, e.g. charge-transfer gates
• CPC Class Codes: H01Q 3/2647 Retrodirective arrays

Abstract

An active retrodirective antenna array is provisioned with a virtual beacon for use with uncooperative targets. An RF pilot signal is imposed onto an optical carrier beam and directed at a target. The scattered optical carrier beam is optically detected by a plurality of the retrodirective array nodes to extract the RF pilot signal. Each recovered RF pilot-signal is phase-conjugated with a phase reference signal, amplified and retransmitted towards the target where the RF beams add-in phase at the virtual beacon. The array may be used to complete an RF communication link with the target, to transfer microwave power from orbiting solar power stations to a target or as a directed energy weapon to prosecute the target.

Claims

1. A method of directing RF energy from an active retrodirective antenna array onto an uncooperative target, said array comprising a plurality of nodes that each transmit RF energy, comprising:

directing an optical carrier beam with a first RF pilot signal imposed thereon into a spot on the target, said target scattering the optical carrier beam to illuminate a plurality of the nodes of the retrodirective array to form a virtual beacon; and

at each of the plurality of illuminated nodes in the retrodirective array,

optically detecting the scattered optical carrier beam to extract a second RF pilot signal including a phase accumulated during propagation from the target to said node;

processing the second RF pilot signal and a phase reference signal to produce a phase-conjugated RF pilot signal; and

amplifying and retransmitting the phase-conjugated RF pilot signal towards the target.

2. The method of claim 1, wherein the optical carrier beam has a wavelength within a range of approximately 0.4 to 300 microns and the wavelength of the first RF pilot signal is within a range of 0.6 mm to 0.6 m.

3. The method of claim 1, further comprising:

sensing a range to target; and

adaptively focusing the optical carrier beam based on said range to target to maintain the spot size.

4. The method of claim 1, further comprising:

tracking the target; and

slaving the optical carrier beam to the tracking of the target to maintain the optical carrier beam on the target.

5. The method of claim 4, wherein each node retransmits the phase-conjugated RF pilot signal in a beam pattern including a main beam, further comprising:

slaving the retransmission of the phase-conjugated RF pilot signal at each node to the tracking of the target to maintain the main beam on the target.

6. The method of claim 1, wherein the step of optically detecting the scattered optical carrier beam comprises:

averaging the power of the optical carrier beam incident on the node over multiple optical cycles of the beam to generate a power signal, said power signal including a component at the same frequency as the first RF pilot signal and at least one component at a harmonic of the frequency of the first RF pilot signal; and

filtering the power signal to extract one said component as the second RF pilot signal.

7. The method of claim 6, wherein the power signal is filtered to extract the component at the same frequency as the first RF pilot signal as the second RF pilot signal.

8. The method of claim 1, further comprising:

providing a reference signal having a reference frequency and a reference phase, said reference signal or a first derivative thereof corresponding to the first RF pilot signal imposed on the optical carrier beam; and

using the reference signal or a second derivative thereof as the phase reference signal.

9. The method of claim 1, wherein the spot has a diameter less than one-half wavelength of the first RF pilot signal.

10. The method of claim 1, wherein each node retransmits the phase-conjugated RF pilot signal in a beam pattern including a main beam, further comprising:

tracking the target;

slaving the optical carrier beam to the tracking of the target to maintain the optical carrier beam on the target;

adaptively focusing the optical carrier beam to maintain the spot size; and

slaving the retransmission of the phase-conjugated RF pilot signal at each node to the tracking of the target to maintain the main beam on the target so that the main beams add substantially in-phase at the target and deliver a total directed energy greater than a target dependent threshold to incapacitate, damage or destroy the target.

11. A method of directing RF energy from an active retrodirective antenna array onto an uncooperative target, said array comprising a plurality of nodes that each transmit RF energy within beam pattern having a main beam comprising:

tracking the target to provide a range-to-target and angle-to-target;

directing an optical carrier beam with a first RF pilot signal imposed thereon into a spot on the target, said beam direction responsive to said angle-to-target to maintain the spot on the target, said beam adaptively focused based on the range-to-target to maintain the spot size on the target, said target scattering the optical carrier beam to illuminate a plurality of the nodes of the retrodirective array to form a virtual beacon; and

at each of the plurality of illuminated nodes in the retrodirective array,

optically detecting the scattered optical carrier beam to extract a second RF pilot signal including a phase accumulated during propagation from the target to said node;

processing the second RF pilot signal and a phase reference signal to produce a phase-conjugated RF pilot signal;

amplifying the phase-conjugated RF pilot signal towards the target; and

retransmitting the phase-conjugated RF pilot signal in a directed response to said angle-to-target to maintain the main beam on the target so that the main beams add substantially in-phase at the target and deliver a total directed energy greater than a target dependent threshold to incapacitate, damage or destroy the target.

12. The method of claim 11, wherein the step of optically detecting the scattered optical carrier beam comprises:

averaging the power of the optical carrier beam incident on the node over multiple optical cycles of the beam to generate a power signal, said power signal including a component at the same frequency as the first RF pilot signal and at least one component at a harmonic of the frequency of the first RF pilot signal; and

filtering the power signal to extract the component at the same frequency as the second RF pilot signal.

13. A device for directing RF energy onto an uncooperative target, comprising:

an active retrodirective antenna array comprising a plurality of nodes;

an illuminator that directs an optical carrier beam with a first RF pilot signal imposed thereon into a spot on the target, said target scattering the optical carrier beam illuminating a plurality of nodes of the retrodirective array to form a virtual beacon; and

each node of the retrodirective array comprising,

an optical receiver for detecting the scattered optical carrier beam to extract a second RF pilot signal;

a circuit for processing the second RF pilot signal and a phase reference signal to produce a phase-conjugated RF pilot signal;

gain for amplifying the phase conjugated RF pilot signal; and

an antenna for retransmitting the phase-conjugated and amplified RF pilot signal towards the target.

14. The device of claim 13, further comprising:

a point and track device that tracks the target,

said illuminator slaved to the tracking of the target to maintain the optical carrier beam on the target.

15. The device of claim 14, wherein said point and track device slews to point a boresight axis at the target, said illuminator mounted on the point and track device to direct the optical carrier beam substantially along the boresight axis.

16. The device of claim 15, wherein said nodes are arrayed on the point and track device to transmit nominally along the boresight axis.

17. The device of claim 14, wherein said point and track device generates a tracking signal and each node retransmits the phase-conjugated RF pilot signal in a beam pattern including a main beam, each said node being independently responsive to the tracking signal to track the target and retransmit the phase-conjugated RF pilot signal to maintain its main beam on the target.

18. The device of claim 13, wherein the optical receiver detects the scattered optical carrier by averaging the power of the optical carrier beam incident on the node over multiple optical cycles of the beam to generate a power signal, said power signal including a component at the same frequency as the first RF pilot signal and at least one component at a harmonic of the frequency of the first RF pilot signal and filtering the power signal to extract one said component as the second RF pilot signal.

19. The device of claim 18, wherein the optical receiver filters the power signal to extract the component at the same frequency as the first RF pilot signal as the second RF pilot signal.

20. The device of claim 13, further comprising:

a source of a reference signal having a reference frequency and a reference phase, said reference signal or a first derivative thereof corresponding to the first RF pilot signal imposed on the optical carrier beam,

said circuit using the reference signal or a second derivative thereof as the phase reference signal to produce the phase-conjugated RF pilot signal.