WO2001075388A1 - Missile having a beam steering optical arrangement using risley prisms - Google Patents

Abstract

A missile seeker. The inventive missile seeker includes a dome within which a novel optical arrangement is retained. The optical arrangement includes a first prism mounted for rotation about an optical axis and a second prism mounted for rotation about the optical axis. In the illustrative embodiment, the first and second prisms are Risley prisms. In addition, the illustrative implementation includes a first motor arrangement for rotating the first prism about the optical axis and a second motor arrangement for rotating the second prism about the optical axis. A controller is provided for activating the first and second motors to steer the beam at an angle ζ and nod the beam at an angle υ. In a specific implementation, the first prism and/or the second prism have at least two surfaces contoured to correct for optical aberration. A teaching is provided to contour the surfaces to correct for astigmatism, coma, trefoil and other non-rotationally symmetric aberration. The contour may be effected by laser etching, micro-machining, optical thin-film coating or other such technique.

Description

MISSLLE HAVING A BEAM STEERING OPTICAL ARRANGEMENT USING RISLEY PRISMS

BACKGROUND OF THE INVENTION

Field of the nvention

The present invention relates to missile systems More specifically, the present invention relates to optically guided missile systems

Description of the Related Art

For certain applications, optical systems are preferred for missile guidance Current optically-guided missiles are often constrained by the need to balance aerodynamic considerations and optical considerations with respect to the missile dome That is, from an optical perspective, a flat dome is preferred However, from an aerodynamic perspective, an elongate aerodynamically shaped dome is preferred to reduce drag Hence, using conventional teachings, an optically-guided missile can not be optimized with respect to either aerodynamic or optical considerations and consequently the designer is forced to accept certain performance compromises to meet design objectives Accordingly, there is a need in the art for a missile having a conformal dome

That is, there is a need in the art for a missile having an optical guidance system which allows for the dome to be 'conformed' for aerodynamic considerations SUMMARY OF THE INVENTION

The need in the art is addressed by the missile of the present invention The inventive missile includes a dome within which a novel optical arrangement is retained The optical arrangement includes a first pπsm mounted for rotation about an optical axis and a second prism mounted for rotation about the optical axis

In the illustrative embodiment, the first and second prisms are Risley prisms In addition, the illustrative implementation includes a first motor arrangement for rotating the first prism about the optical axis and a second motor arrangement for rotating the second pπsm about the optical axis A controller is provided for activating the first and second motors to steer the beam at an angle φ and nod the beam at an angle θ

In a specific implementation, the first pπsm and/or the second prism have at least one surface contoured to correct for optical aberration A teaching is provided to contour the surface to correct for astigmatism, coma, trefoil, oblique and/or focus The contour is effected by laser etching, micro-machining, optical thin-film coating or other such technique known in the art

BRIEF DESCRIPTION OF THE DRAWINGS

Fig 1 is a sectional side view of a missile constructed in accordance with the teachings of the present invention

Fig 2 is a simplified diagram of the optical arrangement utilized in the missile shown in Fig 1 with the front and back prisms being rotated to steer the beam at a nod angle of zero degrees Fig 3 is a simplified diagram of the optical arrangement utilized in the missile shown in Fig 1 with the front and back prisms being rotated to steer the beam at a nod angle of φ degrees

Fig 4 is a side view of a single wedge utilized in the optical arrangement of the missile of the present invention

Fig 5 is a diagram using vector notation to describe the ne-of-sight of the optical arrangement utilized in the missile of the present invention

Fig 6 is a diagram showing side views of the first and second prisms of the optical arrangement utilized in the missile of the present invention Fig 7 is a diagram showing side views of the first and second prisms of Fig 1 with surface contours in accordance with the present teachings

Figs 8, 9, 10a and 10b are diagrams which illustrate conventions used to facilitate an understanding of the teachings provided herein

Fig 1 1 is a graph which illustrates that aberrations can be generated with counter- rotating phase plates

DESCRIPTION OF THE LNVENTION

Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention

While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility Fig 1 is a sectional side view of a missile seeker constructed in accordance with the teachings of the present invention The seeker 10 has a dome 1 1 which is secured to a missile body 12 As is common in the art, the frame 13 is disposed within the missile body 12 along with a guidance and propulsion svstem and a warhead or payload, none of which are shown In addition, a set of control surfaces (not shown) are also typically attached to the missile body 12 as is common in the art Within the frame 13, a set of wedge-shaped prisms 14 and 16 are disposed Those skilled in the an will appreciate that the prisms are Risley prisms The prisms 14 and 16 are independently rotated by first and second motors 18 and 20 respectively First and second conic-shaped cylinders 22 and 24 connect the first and second prisms 14 and 16 to the first and second motors 18 and 20, respectively

As discussed more fully below, the rotation of the pπsms is effective to steer a seeker line of sight (LOS) 26 and thereby change the pointing or roll angle θ (not shown in Fig 1) and the nod angle φ measured with respect to an optical axis 30 The roll angle θ and the nod angle φ are set by the user via an input device 42 and a control circuit 40 The input device 42 may be a microprocessor, ASIC, or other suitable device The control circuit 40 drives the motors 18 and 20 to rotate the first and second prisms 14 and 16 to steer the beam 26 to the desired roll and nod angles The LOS is directed by the prisms 14 and 16 to a spherical lens 32 The spheπcal lens 32 is part of an objective doublet along with an aspherical lens 34 The spheπcal lens 32 and the asphencal lens 34 are of conventional design and construction

The aspherical lens 34 directs the LOS 26 along the optical path to an eyepiece consisting of first and second lenses 36 and 38 The eyepiece subsequently directs the field of view to an integrated detector assembly 39 and ultimately to an output device 44 Fig 2 is a simplified diagram of the optical arrangement utilized in the missile shown in Fig 1 with the front and back prisms being rotated to steer the LOS 26 at a nod angle of zero degrees

Fig 3 is a simplified diagram of the optical arrangement utilized in the missile shown in Fig 1 with the front and back prisms being rotated to steer the LOS 26 at a nod angle of φ degrees As illustrated in Figs 2 and 3, and in accordance with the present teachings, the ne-of-sight of the optical system is stabilized and steered by counter- rotating the wedges 14 and 16 The roll and nod angles α and φ are related to relative angles of the wedges as follows

Fig 4 is a side view of a single wedge 14 or 16 The following equations relate the wedge angle A, the deviation angle D and index of refraction 'n' as follows Equation [1] relates deviation angle D and index of refraction (n) to wedge angle A for a single wedge for the case where the angle of incidence at the first surface is zero as illustrated in Fig 4

A = arcsιn(sιn(φ)/(n-l)) [ 1] hence sιn A = (sιn(φ)/(n-l)) [2] and

therefore φ = arcsin ((n-l)sιn A) [4]

where D is the deviation angle, A is the wedge angle, and n is the index of refraction Thus, a wedge made of germanium having an index of refraction of 4 and a wedge angle of 6 5° yields a deviation angle D of 20° and a high dispersion (Abbe Number of 100) Likewise, a wedge made of silicon having an index of refraction of 3 4 and a wedge angle of 8 2° yields a deviation angle D of 20° and a low dispersion (Abbe Number of 225)

Fig 5 is a diagram using vector notation to descnbe the hne-of-sight of the optical anangement utilized in the missile of the present invention As illustrated in Figs 4 and 5, to achieve a nod motion, the wedges are counter-rotated equal and opposite directions With two wedges 14 and 16, the deviation angle of each wedge Di and D₂ are I elated to the nod angle φ by equation [5] below φ = 2 sin θ sin^"' ((n- l ) sin A) [5]

where θ is the rational angle of each wedge (in opposite direction)

The relationship between the wedge angles, indices of refraction, wedge roll angles and the nod and roll angles φ and are disclosed in U S Patent Nos 1,735,108 issued November 10, 1929 to H. N. Cox entitled OPTICAL ADJUSTING DEVICE, 3,253,525 issued May 31, 1966 to F J. Merkel entitled VARIABLE OPTICAL WEDGE, 3,507,565 issued April 21, 1970 to L W Alvarez et al ENTITLED VARIABLE- POWER LENS AND SYSTEM, 3,378,326 to Alvarez and 3,884,548 to issued May 20, 1975 to S L Linder entitled VARIABLE OPTICAL WEDGE FOR IMAGE STABLILIZATION AND OTHER PURPOSES the teachings of which are incorporated herein by reference.

Fig. 6 is a diagram showing side views of the first and second prisms of the optical arrangement utilized in the missile of the present invention. Fig. 7 is a diagram showing side views of the first and second prisms of Fig. 1 with surface contours in accordance with the present teachings. As illustrated in Figs. 6 and 7, each prism has a wedge or trapezoidal geometry from a side view. The first prism 14 has a first surface 13 and a second surface 15. The second prism 16 has a first surface 17 and a second surface 19 The second surface 15 of the first prism faces the first surface 17 of the second prism 16. In accordance with the present teachings, the second surface 15 of the first prism and the first surface 17 of the second prism 16 (hereinafter the "correction surfaces') are contoured to conect for optical aberrations such as astigmatism, coma, trefoil, oblique and focus Those skilled in the art will appreciate that the conection surfaces may be etched in accordance with the surface descriptions provided below using photolithography, laser etching, micro-machining, optical thin- films or other any suitable technique known in the art

Figs. 8, 9, 10a and 10b are diagrams which illustrate conventions used to facilitate an understanding of the teachings provided herein Table I below is a surface deformation table with provides surface descriptions which may be used to conect for the optical abenations mentioned above along with tilt or nod

Table I

The Zernike terms Z4, Z5, Z7 and Zl l can be generated along with Z3, Z6, Z8 and Z10, respectively, by rotating both plates together

Fig 1 1 is a graph which illustrates that abenations can be generated with counter- rotating phase plates

Thus, the present invention has been described herein with reference to a particular embodiment for a particular application Those having ordinary skill in the art and access to the present teachings will recognize additional modifications applications and embodiments within the scope thereof It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention

Accordingly,

WHAT IS CLAIMED IS

Claims

1 A missile seeker including a dome and a steerable optical arrangement, the optical anangement comprising a first pπsm mounted for rotation about an optical axis and a second prism mounted for rotation about said optical axis

2 The invention of Claim 1 including first means for rotating said first pnsm about said optical axis

3 The invention of Claim 2 further including second means for rotating said second prism about said optical axis

4 The invention of Claim 3 further including means for controlling said first means and said second means to steer a beam at an angle φ

5 The invention of Claim 4 further including means for controlling said first means and said second means to nod said beam at an angle θ

6 The invention of Claim 1 wherein said first pnsm is a Risley prism

7 The invention of Claim 6 wherein said second prism is a Risley pπsm

8 The invention of Claim 1 wherein said first pπsm and said second prism have at least one surface contoured to conect for optical abenation

9 The invention of Claim 8 wherein said optical abenation is astigmatism

10 The invention of Claim 8 wherein said optical abenation is coma

1 1 The invention of Claim 8 wherein said optical abenation is trefoil

12 The invention of Claim 8 wherein said optical aberration is any non- rotationally symmetric abenation

13 The invention of Claim 8 wherein said optical abenation is focus

14 The invention of Claim 13 wherein said optical abenation is astigmatism

15 The invention of Claim 14 wherein said optical aberration is coma

16 The invention of Claim 15 wherein said optical abenation is trefoil

17 The invention of Claim 1 including means for controlling the rotation of said first prism relative to said second pπsm

18 The invention of Claim 17 wherein said means for controlling includes means for rotating said first pπsm

19 The invention of Claim 18 wherein said means for controlling includes means for rotating said second prism

20. A method for steering a missile having a dome including the steps of: mounting a first prism within said dome for rotation about an optical axis; mounting a second prism within said dome for rotation about said optical axis; and rotating said first prism and/or said second prism to direct an optical line of sight therethrough.

21. The invention of Claim 20 including the step of rotating said first prism about said optical axis and rotating said second prism about said optical axis to steer said line of sight at an angle φ and to nod said line of sight at an angle θ.

22. The invention of Claim 20 wherein said first prism is a Risley prism.

23. The invention of Claim 22 wherein said second prism is a Risley prism.

24. The invention of Claim 20 wherein said first prism and/or said second prism have at least one surface contoured to conect for optical abenation.

25. The invention of Claim 24 wherein said optical abenation is astigmatism.

26. The invention of Claim 24 wherein said optical abenation is coma.

27. The invention of Claim 24 wherein said optical abenation is trefoil.

28. The invention of Claim 24 wherein said optical abenation is focus.