AU781981B1

AU781981B1 - Aircraft sensor window

Info

Publication number: AU781981B1
Application number: AU78246/01A
Authority: AU
Inventors: Bhadrayu Manherlal Ranat
Original assignee: BAE Systems PLC
Current assignee: BAE Systems PLC
Priority date: 2000-11-20
Filing date: 2001-10-09
Publication date: 2005-06-23
Anticipated expiration: 2021-10-09
Also published as: GB0127243D0; GB0028534D0

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): BAE SYSTEMS PLC Invention Title: AIRCRAFT SENSOR WINDOW The following statement is a full description of this invention, including the best method of performing it known to me/us: AM4 I43DUD -1- AIRCRAFT SENSOR WINDOW This invention relates to a sensor window, for an aircraft-mounted optical sensor, and to an aircraft s equipped.

Optical sensor systems receive radiated or reflected electromagnetic energy from a scene and convert it into an electrical signal; systems typically comprise an optical lens, a photosensitive detector and an electronic processor.

The optical lens receives the radiated energy and focuses it onto the detector, which converts the received energy into an electrical signal. The electronic processor amplifies the electrical signal so that it can be processed so as, for example, to be displayed on a display screen or subjected to pattern recognition processing.

Such optical sensor systems operate over the electromagnetic spectrum, ranging from the ultraviolet through the visible spectrum and into the far infrared, and have a great variety of commercial and military applications. The sensor systems are generally either fixed in orientation, or are movable to allow sensing over a wide field of view; alternatively, a wide field of view is achieved with a fixed sensor system and a scanning device, such as a steering mirror or prism.

Optical sensor systems are generally protected from the environment by a window, the purpose of which is to protect the sensor from damage caused by, for example, dirt, chemicals, erosion due to sun and rain, and weathering. These windows must, of course, be substantially transparent to radiation at the operating wavelength of the sensor. The windows must also be sufficiently large that the sensor system has a wide enough field of view (which may extend through large angles in elevation and azimuth) and sufficiently strong to withstand the expected stresses and strains in use.

-1-lV -2- In applications, such as in aircraft or missiles, where the electrical signals produced by the sensor system must accurately represent the observed scene, it is normally the case that the window is configured so as to minimise optical aberrations and distortion of the received image. In general, optical aberrations and distortion int.roduced by h- indow dengraa the aCCurncy of the sensor system, and often the optical lens of the sensor system is designed to try and correct such aberrations and distortions. Additionally or alternatively electrical circuitry or processing is used to compensate for aberrations and distortions produced by the window. It is desirable that optical aberrations introduced by the window be predictable and regular, so that they may be compensated for by such methods. So, conventional aircraft or missile sensor windows have taken the form of flat, planar plates, or segments of spheres (usually referred to as "domes").

e In the case of a flat parallel plate window only pupil shift occurs for which no compensation is necessary. For domes with concentric surfaces and with the sensor located at the common centre of curvature of the two surfaces of the dome, only spherical aberration arises, which is not field dependent. The spherical aberration can be corrected with relative ease during design of the optical lens.

Where the sensor is not located at the common centre of curvature of the two surfaces, field dependent aberrations such as coma and distortion are introduced as well as spherical aberration. In this case, correction becomes somewhat more difficult to achieve.

Such sensor windows, when applied to aircraft, or the like, also introduce aerodynamic drag which adversely affects the aerodynamic performance of the platform.

In military aircraft applications the contribution made to an aircraft's radar cross-section (RCS) by a sensor window is of particular significance; it is desirable that the sensor window installations do not increase the platform's RCS over a specified angular extent in the azimuth and elevation, however conventional APJ -3aircraft/missile sensor windows are deficient in this respect. The scattering of RF radiation from the window surfaces formed of flat plate windows, or dome windows contribute significantly to the aircraft or missile RF signature. Such sensor windows are therefore highly inclined (or tilted) such that the reflect RF energy away from the major threat direction (which is usually in the frontal and rear sectors of an aircraft).

Integrating such sensor windows into the curved surfaces of an aircraft intended to have a small RCS has been found inevitably to require surface blending. This blending significantly and adversely impacts on the RCS and aerodynamic characteristics of the aircraft. Small blend radii appear as surface features and significantly increase the aircraft RF signature, whereas large blend radii increase the volume of the feature and significantly affect the aerodynamic characteristics of the aircraft. Furthermore, the constraints imposed by the *requirements for small RCS, and low aerodynamic drag usually leads to a restriction in the sensor field of view.

The shapes of sensor windows used on aircraft or missiles with small RCS are usually complex double curvature surfaces. Sensor windows of this form create a considerable amount of optical aberrations which varies greatly as the sensor is gimballed. These optical aberrations are unpredictable and irregular in I d At- I PLeyi tp a it l"erefore uificult toU cUorrect for III LIIe U.JLICII lehi, I JI III.Il L V'y 1

I

aberrations associated with such windows are coma and astigmatism. A focus shift may also occur, as the sensor is gimballed. The coma is the variation of magnification as a function of aperture and astigmatism is the difference in focus location for fans of rays in the sagittal and tangential planes.

There is therefore a need for improved window designs and installation design to be used on aircraft or missiles with small RCS.

-4- Accordingly, the present invention provides an aircraft having an outer su rfa en a nnrtinn of which is shaped such that it has a constant curvature about a longitudinal axis parallel thereto, at least a part of said portion comprising a window for an optical sensor, the arrangement being such that there is no discontinuity of curvature where the circumferential edge of the window adjoins the surrounding aircraft surface.

Such an arrangement provides a conformal sensor window a window which conforms with the configuration of the aircraft outer surface) with no surface blending required at the juncture of window and aircraft, so minimising the adverse affect on RCS. Further because the window has a constant curvature (such that any cross section perpendicular to the longitudinal axis is parallel to any other :.!*:taken at a different point on the axis) the compensation for optical aberrations it produces is significantly less difficult than in windows having complex double curvature surfaces, which create considerable and varying distortion and optical aberrations (such aberrations are unpredictable and irregular and therefor difficult to correct for in the optical lens of the sensor system); in accordance with the invention a window shape (which conforms to the aircraft shape) is chosen that Sgo gives low-order focus, astigmatism and coma aberrations, so that these may be corrected and/or compensated for by the optical lens of the sensor. Such a window is particularly suitable for use with a large aperture sensor apertures greater than the human eye).

Preferably the window is configured such that its curvature is symmetrical about a longitudinal axis parallel to the shaped portion, so as to regularise the optical aberrations produced by the window. Suitably, the shaped portion is cylindrical or cylindroidal having an elliptical cross-section). The window is preferably inclined, relative to the fore and aft axis of the aircraft (or other moving platform), at an obtuse angle with respect to the normal forward direction of flight of the aircraft, so as to produce RF reflections outside the specified threat region.

RAt WLu 4C;_ AJ I4.)UO This is advantageous as such sensor window installations do not increase RF signature, provide an aerodynamically-efficient window installation which does not degrade sensor performance in a way which cannot be corrected for easily in the optical lens of the optical system, and enables a broad field of view. In locations where cylindrical surfaces would give insufficient P signature control then cylindroidal (elliptical) surfaces could be used. An elliptical surface window will give many of the same advantages as a cylindrical window, but at the cost of more complex optics required to compensate for the aberrations created by such a window as compared with a cylindrical one.

When mounted on an aircraft (or indeed any other equivalent platform, such as a missile or helicopter, or ground based or maritime platforms such as a ship or submarine) the window typically has its convex surface outermost. The sensor is o mounted behind the window in a position so as to optimise the sensor field of view and the size of the window. Suitably, the sensor is pivotably or gimbally mounted so as to provide a field of view moveable at least in a plane perpendicular to the longitudinal axis. Preferably the sensor is mounted so as to pivot about an axis perpendicular to the longitudinal axis, as well as about the longitudinal axis, so as to maximise the field of view. Alternatively, as will be apparent to those skilled in the art, a scanning prism or mirror could be used with a static sensor to give a wide field of view, the scanning element in this case being located between the window and the sensor adjacent the axis.

To compensate for optical aberrations such as focus, coma and astigmatism produced by the windows, the optical lens of the sensor system will usually include adjustable correcting optical elements, which may comprise corrector plates, cylindrical lenses and spherical and aspherical lenses, and a system for dynamically adjusting the corrector optical elements.

In another aspect, the invention provides a window for an optical sensor, preferably one which is mounted in an aircraft (or other moving platform) in which

API~JJ

-6the window has an outer surface shaped such that it has a constant curvature about a longitudinal axis parallel thereto.

The window is made of a material selected in conjunction ,with the operating wavelength of the sensor that is to be protected. The sensor may be responsive to, for example, all or part of the ultraviolet, visible and infrared ranges, and the window must be transparent to the range of interest. Materials of construction for windows in specific wavelength transparency ranges are well known in the art.

The invention will now be described by way of example and with reference to the accompanying drawings, in which: *o* Figure 1 is a schematic view of part of a conventional aircraft having a planar sensor window; *Figure 2 is a schematic view of part of an aircraft in accordance with the invention; Figure 3 is a schematic view of the sensor window of Figure 2, which is also in accordance with the invention, and Figure 4 is a schematic view of an aircraft illustrating potential locations on an aircraft where windows in accordance with the invention might be positioned.

In Figure 1 a planar sensor window 2 is mounted between the front of aircraft 4 and the cockpit 6 in a blister 12 in the conventional manner. The airframe surface 8 surrounding the location of the window is smoothly curved to control the RF signature of the aircraft and to be aerodynamic. However, because the sensor window is flat, and mounted proud of the surface 8 (to give the sensor an unobstructed forward looking field of view) there are marked areas of surface -7blending 10 on the surface of the aircraft so as to bring into conformity the adjoining window and aircraft surfaces. These surface blending areas 10 increase the RF signature of the aircraft, and the field of view of the sensor (not shown) mounted behind the window 2 is limited because the window is flat, and a larger fat window would make the blister 'unacceptably large.

In Figure 2 an installation using a conformal sensor window 20 (shown also in Figure 3) is provided in an airframe baseline surface model 22 below the windscreen transparency 24. The window 20 and the surrounding aircraft surface defined by a curve which is constant along a longitudinal axis parallel to the window surface. This baseline surface model 22 is normally of quite a complex shape, in order to control RF signature. To accommodate a conformal window the surface model is modified to conform to an inclined cylindrical surface whilst still ensuring that the specified RF signature and aerodynamic constraints are unaffected. This allows a cylindrical window 20 to be integrated with the airframe without introducing distortion to the baseline surface model 22 of the airframe around the window. There is therefore no need for any RF signature-increasing surface blending areas as in the prior art shown in Figure 1. Instead the window forms a continuous, smooth surface with the surface of the aircraft, therefore conforming to the specified RF signature requirement, whilst ensuring that the aerodynamic characteristics of the aircraft are unaffected. Mounting conformal sensor window in this manner provides improved aircraft sensor field of view, as compared with a conventionally installed flat plate or dome sensor window.

The RF signature from the interface joints between the window and the aircraft is controlled by angling the window joints such that the edges 32 (shown in Figure 3) are aligned outside the major threat region or aligned with the wing leading edge angle and the wind trailing edge angle.

Tight geometrical tolerances between the window and the window-mounting frame and between the window frame and the aircraft skin are applied, such that APIJ'3 -8scattering due to steps and gaps is reduced and there is effectively no physical discontinuity at the juncture of window and airframe. Conductive sealant (not shown) between the window frame and the aircraft skin is used to reduce further scattering by ensuring electrical continuity across the surface.

In Figure 3 a conformal window 20 in accordance with the invention and a pivotal or gimballed sensor (comprising optical lenses 34, 35, 36 and photosensitive detector and electronic processor 40) arrangement are shown, together with adjustable optical correction elements 42 required to correct window aberrations and a system 44 for dynamically adjusting the corrector optics. This is included to show how the combination of conformal window, corrector optics and sensor could operate.

l Figure 4 illustrates other potential locations for conformal windows in accordance r with the invention on aircraft having low RCS, namely at the leading edge wing roots 52, the fuselage sides 54, the upper/lower surfaces of the fuselage 56 and at the air intake bump 58.

.il i•

Claims

2. An aircraft as claimed in Claim 1 wherein the window is configured such that its curvature is symmetrical about a longitudinal axis parallel to the shaped portion.
3. An aircraft as claimed in Claim 1 or 2 wherein the shaped portion is °i o cylindrical or cylindroidal.
4. An aircraft as claimed in Claim 1, 2 or 3 comprising a sensor mounted within the aircraft adjacent the longitudinal axis. o• An aircraft as claimed in any preceding claim comprising a sensor pivotally S"mounted so as to provide a field of view moveable in a plane perpendicular to the longitudinal axis.
6. An aircraft as claimed in Claim 5 wherein the sensor is mounted so as to pivot about an axis perpendicular to the longitudinal axis.
7. An aircraft as claimed in any preceding claim wherein the window has outer and inner surfaces having different curvatures.
8. An aircraft as claimed in any preceding claim wherein the axis is inclined, relative to the fore and aft axis of the aircraft, at an obtuse angle with r% ^f hf nf tho nirrrnft respect to the normal direction of flight of the aircraft. respeCI tU t iIuini, i
9. A window for an optical sensor, the window having an outer surface shaped such that it has a constant curvature about a longitudinal axis parallel thereto. A window as claimed in Claim 9 wherein the curvature is symmetrical about a longitudinal axis parallel to the shaped outer surface.
11. A window as claimed in Claim 9 or 10 wherein the shape is cylindrical or cylindroidal.
12. An aircraft substantially as hereinbefore described and with reference to the accompanying drawings.
13. A window for an optical sensor substantially as hereinbefore described and with reference to the accompanying drawings. Dated this 22nd day of July 2002 RAE SYSTEMS PLC By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia LU ^C-