WO2013108204A1

WO2013108204A1 - Laser target seeker with photodetector and image sensor

Info

Publication number: WO2013108204A1
Application number: PCT/IB2013/050426
Authority: WO
Inventors: Rami LEBER; Gavriel MAAYANI
Original assignee: Rafael Advanced Defence Systems Ltd
Priority date: 2012-01-18
Filing date: 2013-01-17
Publication date: 2013-07-25
Also published as: IL217621A0; WO2013108204A4

Abstract

A seeker for detecting reflections of light emitted by a laser, including an image sensor, circuitry for receiving, from the image sensor, electronic signals that represent a scene that includes the reflections; and a photodetector. The circuitry is configured to interrogate the image sensor, in order to receive the electronic signals that represent the scene, in response to electronic signals, from the photodetector, that indicate reception of the reflections by the photodetector.

Description

LASER TARGET SEEKER WITH PHOTODETECTOR AND IMAGE SENSOR

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a laser designator seeker and to laser spot detection and observation systems. More particularly, the present invention relates to a laser designator seeker that uses a photodetector for acquisition and an image sensor for steering.

A common way of providing precision controlled weapons is to point at a target by means of a laser beam and to allow a flyable weapon such as a missile, bomb or the like to guide itself towards the laser spot, thereby "homing" on the target. Consequently, a laser target seeker is required that is able to distinguish the reflected laser beam in relation to the background.

Current laser pointing and designator systems generally use a pulsed solid state laser, often a Nd:YAG laser, for illuminating the target. The pulse width typically is on the order of 20 to 50 ns. Various schemes of temporal coding, such as changing the spacing in time between successive pulses, are used, both in order to discriminate between different designators and in order to keep simple countermeasures from being used to defend the target.

The task of the target seeker is to inform the control system of the flyable weapon where in the field of view of the target seeker the reflected laser spot is present. A common way of achieving this is to use a quadrant photodetector. A quadrant photodetector consists of four separate photosensors placed edge to edge in one plane, with each of the photosensors being able to view a quadrant of a certain visual field. By measuring and comparing the signal intensity from the four quadrants, i.e. the amount of optical power ending up on each of the four elements, it is possible to determine where the point of balance of the laser reflection is located and thus in which direction to guide the flyable weapon. When the intensity of the signal is equally strong from all four elements, the laser reflection is in the center of the detector and the weapon will hit in the middle of the laser reflection.

If the reflected laser beam is defocused to be about as wide as the photodetector, the accuracy of the guidance is low and the weapon may miss the target. If the reflected beam is focused to an area smaller than the area of the photodetector, the center point may be found with higher accuracy, but when the reflection is located totally off-center it is not possible to determine how far the reflection is from the center.

A target seeker may be either rigidly attached to the flyable weapon ("strapdown" or "stiff-neck") or gimbaled. A gimbaled seeker keeps its quadrant photodetector pointed at the reflected beam so that the reflected beam always is centered on the photodetector, and sends signals to the control system of the flyable weapon so that the flyable weapon can imitate the movements of the gimbal. The reflected beam generally is off-center of a quadrant photodetector of a stiff-neck seeker during homing.

An image sensor, such as a CCD sensor or a CMOS sensor, that includes many "sensels" (defined, by analogy with the pixels of an image, as the sensor elements of an image sensor), could be used instead of a quadrant photodetector to more accurately locate a focused reflected beam, if not for a quirk in the operation of the image sensor. The operation of an image sensor includes both an integration period, during which the imaging sensor receives light from the scene, and a readout period, during which electronic signals corresponding to the received light are sent by the imaging device to the circuitry that controls the imaging device. The integration period and the readout period may be either overlapping or alternating. The integration and readout periods generally are much longer than the pulse width of the designator laser, so that even if the combined integration and readout time is about the same as the pulse repetition interval, a seeker based on an image sensor may be blind to the laser pulse reflections, for two reasons. The first reason is that the integration period generally is so long, relative to the pulse width, that the electronic signals due to light from the scene overwhelm the electronic signals due to the laser pulse reflections. The second reason is that if the integration and readout periods alternate, the laser pulse reflections may arrive during the readout period.

It would be highly advantageous to have a laser designator seeker that uses an image sensor for locating a focused reflected beam.

SUMMARY OF THE INVENTION

The present invention addresses this need by providing a laser designator seeker equipped with an image sensor whose operation is synchronized with the laser pulses that the laser designator seeker is supposed to detect.

According to the present invention there is provided a seeker for detecting reflections of light emitted by a laser, including: (a) an image sensor; (b) circuitry for receiving, from the image sensor, electronic signals that represent a scene that includes the reflections; and (c) a photodetector; wherein the circuitry is configured to interrogate the image sensor, in order to receive the electronic signals that represent the scene, in response to electronic signals, from the photodetector, that indicate reception of the reflections by the photodetector.

According to the present invention there is provided a method of attacking a target, including the steps of: (a) equipping a guided missile with a seeker that includes an image sensor and a photodetector; (b) illuminating at least a portion of the target with light from a laser; (c) launching the guided missile towards the target; (d) subsequent to the launching, receiving reflections of the light from the target by the photodetector, the photodetector then emitting electronic signals indicative of the reception of the light; (e) in response to the electronic signals from the photodetector, interrogating the image sensor to obtain electronic signals that represent a scene that includes the reflections; and (f) steering the guided missile toward the target in response to the electronic signals that represent the scene.

According to the present invention there is provided a method of eavesdropping on a laser designator, including the steps of: (a) using a photodetector to receive light from the laser designator, the photodetector emitting electronic signals indicative of reception of the light; and (b) in response to the electronic signals from the photodetector, interrogating an image sensor to obtain electronic signals that represent a scene that includes the light.

A basic seeker of the present invention, for detecting reflections of light emitted by a laser, includes an image sensor (for example a CCD image sensor or a CMOS image sensor), a photodetector, and associated circuitry. The circuitry receives, from the image sensor, electronic signals that represent a scene that includes the reflections. The circuitry also is configured to interrogate the image sensor, in order to receive the electronic signals that represent the scene, in response to electrical signals from the photodetector that indicate reception of the reflections by the photodetector.

Preferably, the circuitry receives the electronic signals from the image sensor during a readout period subsequent to an integration period during which the image sensor receives light from the scene, and the duration of the readout period exceeds a pulse length of the light emitted by the laser.

Preferably, the image sensor receives light from the scene during an integration period whose duration is sufficiently short that the electronic signals from the image sensor that represent the reflections are distinct from the electronic signals from the image sensor that represent the rest of the scene. In other words, the duration of the integration period is sufficiently short that the circuitry can tell which electronic signals from the image sensor are from sensels of the image sensor that have received reflected laser light. Most preferably, the circuitry synchronizes the integration period with the receipt of the reflections by the photodetector.

In some embodiments, the photodetector includes a single photosensor. In other embodiments, the photodetector includes a plurality of photosensors. For example, the photodetector could be a quadrant photodetector. Other exemplary multi-sensor photodetectors are known in the art, for example the pyramid photodetector of Lizotte et al., US Patent No. 7,321,114 and the detector matrix of Lindgren, US Patent No. 7,659,494. If the photodetector includes a plurality of photosensors then, preferably, the image sensor includes a two-dimensional array of sensels and the circuitry is operative, in response to the electronic signals from the photodetector, to select a subset of the sensels to interrogate.

Preferably, the seeker also includes a filter for passing, to the image sensor and to the photodetector, substantially only light from the scene that is in the wavelength band of the light emitted by the laser.

Preferably, the circuitry receives the electronic signals from the image sensor during a readout period subsequent to an integration period during which the image sensor receives light from the scene, and the readout period is shorter than the pulse repetition interval of the light emitted by the laser.

Preferably, the seeker is a stiff neck seeker, rather than a gimbaled seeker.

Preferably, the image sensor and the photodetector have substantially identical fields of view. Most preferably, the fields of view are between about 15 degrees and about 30 degrees.

The scope of the present invention also includes a guided missile that includes the seeker of the present invention.

A first basic method of the present invention is a method of attacking a target. The method starts with the equipping of a guided missile with a seeker that includes an image sensor and a photodetector. At least a portion of the target is illuminated with light from a laser. The guided missile is launched towards the target. Subsequent to launching the guided missile, the photodetector receives reflections of the light from the target and. emits electronic signals that indicate reception of the light. In response to the electronic signals from the photodetector, the image sensor is interrogated to obtain electronic signals that represent a scene that includes the reflections. The guided missile is steered toward the target in response to the electronic signals that represent the scene.

Preferably, the at least portion of the target is illuminated by pulsing the laser according to a pre-determined pulse time interval code. In response to the electronic signals from the photodetector, an integration period of the image sensor is synchronized with the pulse time interval code.

Preferably, the photodetector includes a plurality of photosensors. The guided missile initially is steered toward the target in response to the electronic signals from the photodetector, and afterwards is steered toward the target in response to the electronic signals from the image detector that represent the scene. Most preferably, the at least portion of the target is illuminated by pulsing the laser according to a pre-determined pulse time interval code, and while the guided missile is being steered toward the target in response to the electronic signals from the photodetector, an integration period of the image sensor is synchronized with the pulse time interval code in response to the electronic signals from the photodetector.

Preferably, the at least portion of the target is illuminated by pulsing the laser, and an integration period of the image sensor is adjusted so that the electronic signals from the image sensor that represent the reflections are distinct from the electronic signals from the image sensor that represent the rest of the scene. A second basic method of the present invention is a method of eavesdropping on an enemy laser designator. A photodetector is used to receive light from the laser designator. The photodetector emits electronic signals that are indicative of reception of the light from the enemy laser designator. In response to the electronic signals from the photodetector., an image sensor is interrogated to obtain electronic signals that represent a scene that includes the light from the enemy laser designator. Preferably, the method also includes inferring, from the photodetector' s electronic signals, the enemy laser designator's pulse time interval code. BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments are herein described, by way of example only, with reference to the accompanying drawings, wherein:

FIG. 1 is a conceptual block diagram of a seeker of the present invention;

FIG. 2 is a schematic illustration of a laser-guided missile of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles and operation of a laser target seeker according to the present invention may be better understood with reference to the drawings and the accompanying description.

Referring now to the drawings, Figure 1 is a conceptual block diagram of a seeker 10 of the present invention. Seeker 10 includes optics 20 for focusing light from a scene onto a photodetector 12 and an image sensor 14. Photodetector 12 and image sensor 16 are controlled by optical sensing circuitry 16. The manner of such control is well-known to those skilled in the art and so does not need to be described here. In this specific preferred embodiment of seeker 10, the integration and readout periods of image sensor 16 alternate. It will be clear to those skilled in the art how to apply the principles of the present invention to an image sensor whose integration and readout periods overlap.

Optics 20 is illustrated schematically as including a filter 22, lenses 24, a beamsplitter

26 and a mirror 28. Other than filter 22, the illustrated components are only illustrative, and any suitable alternative could be substituted for these components. The single arrows in Figure 1 show the path of light from the scene through optics 20 to photodetector 12 and image sensor 14. Filter 22 is a passband filter that passes substantially only light in the wavelength band of the light emitted by the laser whose reflected pulses are to be received by seeker 10.

The double arrows in Figure 1 represent electronic signals. Optical sensing circuitry 16 uses photodetector 12 to acquire the reflected laser pulses and infers the pulse time interval code that the laser designator that emits the pulses is using. Optical circuitry 16 then synchronizes the integration period of image sensor 14 with the laser pulse reflections by synchronizing the integration period of image sensor 14 with the inferred pulse time interval code, so that the readout period of image sensor 14 does not coincide with the laser pulse reflections. This is why the double arrow in Figure 1 between image sensor 14 and optical sensing circuitry 16 is double headed: optical sensing circuitry 16 sends control signals to image sensor 14, and image sensor 14 sends to optical sensing circuitry electronic signals representative of the pixels of the scene that is imaged by image sensor 14. By contrast, the double arrow between photodetector 12 and optical sensing circuitry 16 is single headed because optical sensing circuitry 16 only receives electronic signals from photodetector 12. From the fact that the integration period of image sensor 14 is synchronized with the laser pulse reflections it follows that the readout period of image sensor 14 is shorter than the pulse repetition interval of the designation laser.

Seeker 10 also includes guidance circuitry 18. Guidance circuitry 18 receives from optical sensing circuitry 16 electronic signals that identify which pixel or pixels in the scene that is imaged by image sensor 14 represent a reflected laser pulse, and, in response to those signals, sends control signals to the relevant components of the guided missile of which seeker 10 is a part to steer the guided missile towards the designated target.

In order to enhance the contrast between the reflected laser pulses and the rest of the light that image sensor 14 receives from the scene, optical sensing circuitry 16 keeps the integration period of image sensor 14 short, on the order of several microseconds. That the reflected laser pulses usually are much brighter than the ambient light then makes it easy for optical sensing circuitry 16 to distinguish between scene pixels that represent reflected laser pulses and scene pixels that represent the rest of the imaged scene.

Figure 2 is a schematic illustration of a laser-guided missile 40 of the present invention. Missile 40 includes seeker 10 and control surfaces 42 that are manipulated by guidance circuitry 18 to steer missile 40 towards the designated target. Missile 40 could be either unpowered (a laser-guided bomb) or powered.

In one class of embodiments of the present invention, photodetector 12 includes a single photosensor, and photodetector 12 is used only to synchronize the integration period of image sensor 14 with the reflected laser pulses. In another class of embodiments of the present invention, photodetector 12 is a quadrant photodetector. In this class of embodiments, the relative signals from each of the four photosensors of photodetector 12 are used by optical sensing circuitry 16 to estimate which sensels of image sensor 14 are receiving reflected laser light, and only these sensels plus a guard region around these sensels are interrogated by optical sensing circuitry 16 during the readout period of image sensor 14.

In the class of embodiments that uses a quadrant photodetector 12, when missile 40 is far from its designated target, the sensitivity of image sensor 14 may be insufficient to acquire the reflected laser spot. In that case, optical sensing circuitry 16 relies only on the signals from photodetector 12 to guide missile 40 towards its designated target until the reflected laser spot is sufficiently bright for the signals from image sensor 14 to also be used for guiding missile 40 towards its designated target.

The field of view of photodetector 12 preferably is between 15 degrees and 30 degrees. Although in principle image sensor 14 could have a wider field of view than photodetector 12, that would degrade the angular resolution of image sensor 14 relative to the angular resolution that image sensor 14 would have if image sensor 14 and photodetector 12 had the same field of view. Therefore, preferably, image sensor 14 and photodetector 12 have the same field of view.

Seeker 10 has been described in the context of its use for guiding missile 40 towards the designated target of missile 40. Seeker 10 also could be used in a stand-alone mode, for example to receive enemy laser designation signals in order to locate the enemy laser designator and to determine the enemy's pulse time interval code, a process that in the appended claims is called "eavesdropping" on the enemy laser designator.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. Therefore, the claimed invention as recited in the claims that follow is not limited to the embodiments described herein.

Claims

WHAT IS CLAIMED IS:

1. A seeker for detecting reflections of light emitted by a laser, comprising:

(a) an image sensor;

(b) circuitry for receiving, from said image sensor, electronic signals that represent a scene that includes the reflections; and

(c) a photodetector;

wherein said circuitry is configured to interrogate said image sensor, in order to receive said electronic signals that represent said scene, in response to electronic signals, from said photodetector, that indicate reception of the reflections by said photodetector.

2. The seeker of claim 1 , wherein said circuitry receives said electronic signals from said image sensor during a readout period subsequent to an integration period during which said image sensor receives light from said scene, and wherein a duration of said readout period exceeds a pulse length of the light emitted by the laser.

3. The seeker of claim 1 , wherein said image sensor receives light from said scene during an integration period, and wherein a duration of said integration period is sufficiently short that said electronic signals, from said image sensor, that represent the reflections, are distinct from said electronic signals, from said image sensor, that represent a remainder of said scene.

4. The seeker of claim 3, wherein said circutry is operative to syncronize said integration period with said reception of the reflections by said photodetector.

5. The seeker of claim 1, wherein said photodetector includes a single photosensor.

6. The seeker of claim 1, wherein said photodetector includes a plurality of photosensors.

7. The seeker of claim 6, wherein said photodetector is a quadrant photodetector.

8. The seeker of claim 6, wherein said image sensor includes a two- dimensional array of sensels, and wherein said circuitry is operative, in response to said electronic signals from said photodetector, to select a subset of said sensels to interrogate.

9. The seeker of claim 1, further comprising:

(d) a filter for passing, to said image sensor and to said photodetector, substantially only light from said scene that is in a wavelength band of the light emitted by the laser.

10. The seeker of claim 1, wherein said circuitry receives said electronic signals from said image sensor during a readout period subsequent to an integration period during which said image sensor receives light from said scene, and wherein said readout period is shorter than a pulse repetition interval of the light emitted by the laser.

1 1. The seeker of claim 1, wherein the seeker is a stiff-neck seeker.

12. The seeker of claim 1, wherein said image sensor is a CCD image sensor.

13. The seeker of claim 1, wherein said image sensor is a CMOS image sensor.

14. The seeker of claim 1, wherein said image sensor and said photodetector have substantially identical fields of view.

15. The seeker of claim 14, wherein said fields of veiw are between about 15 degrees and about 30 degrees.

16. A guided missile comprising the seeker of claim 1.

17. A method of attacking a target, comprising the steps of:

(a) equipping a guided missile with a seeker that includes an image sensor and a photodetector;

(b) illuminating at least a portion of the target with light from a laser;

(c) launching said guided missile towards the target; (d) subsequent to said launching, receiving reflections of said light from the target by said photodetector, the photodetector then emitting electronic signals indicative of said reception of said light;

(e) in response to said electronic signals from said photodetector, interrogating said image sensor to obtain electronic signals that represent a scene that includes said reflections; and

(f) steering said guided missile toward the target in response to said electronic signals that represent said scene.

18. The method of claim 17, wherein said illuminating is effected by pulsing said laser according to a pre-determined pulse time interval code, the method further comprising the step of:

(g) in response to said electronic signals from said photodetector, synchronizing an integration period of said image sensor with said pulse time interval code.

19. The method of claim 17, wherein said photodetector includes a plurality of photosensors, the method further comprising the step of:

(g) prior to said steering of said guided missile toward the target in response to said electronic signals that represent said scene, steering said guided missile toward the target in response to said electronic signals from said photodetector.

20. The method of claim 19, wherein said illuminating is effected by pulsing said laser according to a pre-determined pulse time interval code, the method further comprising the steps of;

(h) during said steering of said guided missile toward the target in response to said electronic signals from said photodetector, and in response to said electronic signals from said photodetector, synchronizing an integration period of said image sensor with said pulse time interval, code.

21. The method of claim 17, wherein said illuminating is effected by pulsing said laser, the method further comprising the step of:

(g) adjusting an integration period of said image sensor so that said electronic signals, from said image sensor, that represent said reflections, are distinct from said electronic signals, from said image sensor, that represent a remainder of said scene.

22. A method of eavesdropping on a laser designator, comprising the steps of:

(a) using a photodetector to receive light from the laser designator, said photodetector emitting electronic signals indicative of reception of said light; and

(b) in response to said electronic signals from said photodetector, interrogating an image sensor to obtain electronic signals that represent a scene that includes said light.

23. The method of claim 21, fiirther comprising the step of:

(c) inferring, from said electronic signals from said photodetector, a pulse time interval code of the laser designator.