Objects, whether animate or inanimate are desired to be identified, that is: either sensed or designated (illuminated, as by laser), in a wide variety of different industries and environments, including but not restricted to security surveying of the surroundings of a building or of the interior of a room, remote viewing of a dangerous environment such as the enclosure of a nuclear reactor or the interior of a chemical plant and in a variety of military systems including infra-red search and track and targeting/laser designation systems.

While some identifying stations mounting a sensor or designator have in the past been mounted in a fixed position with a fixed orientation, such systems are of limited application and the practice has been to give the identification station an increased freedom of rotational movement so as to increase the field of regard of the sensor or designator. The maximum field of regard is obtained by mounting the identification station on a suitably gimballed head. By"suitably gimballed"we mean rotatable about each of two or more mutually orthogonal axes using, for example, conventional gimbal mounts.

Such systems suffer the drawback that the identification station will usually be visible to or otherwise sensible by the object being sensed or designated which is often undesirable, particularly in security installations or in military installations. To seek to overcome this problem, and also to provide for better protection of the identification station in the case of an identification station for dangerous locations such as the interior of a nuclear reactor or a chemical plant, the identification station, desirably suitably gimballed to give

the maximum available field of regard in the circumstances, is mounted on one side of a window so that it may identify objects on the other side of the window.

Systems of this nature are also suited for aircraft installations where the identification equipment is mounted behind a window for aerodynamic and drag reduction reasons as well as for stealth.

By placing the identification station on the opposite side of a window from the objects to be identified the field of regard is necessarily restricted.

The present invention is concerned to ameliorate this problem.

In accordance with a first aspect of the invention, we provide a method of electromagnetic identification of an object on one side of a window by an identification station using electromagnetic radiation of one or more specified wavelengths or ranges of wavelengths for which the window is transparent, the window being surrounded by a region in which it is mounted and which is generally opaque to said one or more specified wavelengths or ranges of wavelengths, in which method relative motion is provided between the respective centre points of the window and the identification station, the motion including at least a component in a direction transverse to the line connecting the respective centre points, and a rotational component.

By the centre point of the window we mean its centroid. The centre point of the identification station would be, for example, the neutral point of a suitably gimballed arrangement. The sensor or designator itself may be mounted in the gimballed arrangement at a position separated from the centre point.

The window, particularly for use in military installations such as an aircraft is suitably a conforma window to further reduce the likelihood that the

existence of the identifying system can itself be sensed by the target, and also for aerodynamic reasons.

Although the invention is not restricted as to the wavelength or wavelengths of the electromagnetic radiation employed, wavelengths from ultra-violet to far infra-red are of the most interest.

The invention also extends to apparatus suitable for carrying out a method of the kind referred to above.

Thus, in accordance with a second and alternative aspect of the present invention, there is provided an identification apparatus comprising: an enclosure having at least one wall with a window which is transparent to electromagnetic radiation of one or more specified wavelengths or ranges of wavelengths, the window being surrounded by regions of the wall which are generally opaque to said one or more specified wavelengths or ranges of wavelengths; an identification station having a centre point and mounting a sensor or designator operational at said one or more specified wavelengths or ranges of wavelengths and means arranged to move the identification station relative to the window so that its centre point has a component of motion in a direction transverse to the line connecting the centre point of the identification station with the centred of the window, and means to allow relative rotational movement between the identification station and the window.

As will be appreciated, there are innumerable mechanisms readily available which could be used to cause an identification station to move relative to a window. The examples set out hereinbelow are accordingly to be regarded as no more than simple exemplary possibilities within the broad scope of the invention.

In most cases the window itself will be fixed by its wall. However the window may be mounted in a rotating plate with its centroid displaced from

the centre of the plate. Rotation of the window while maintaining the identification station fixed relative to the centre of the plate will produce an element of transverse relative motion between the respective centre points of the window and of the identification station.

In another possibility the wall may be a cylindrical wall a strip of which is enabled to rotate about the axis of the cylindrical wall and the window may be mounted in one section of this cylindrical strip. As the strip rotates there will be an element of motion in a direction transverse to the centre point of the identification station.

In a further embodiment there is a mirror, or other means for redirecting the requisite electromagnetic waves, mounted on the gimbals instead and provided with means such that the mirror or said other redirection means delivers the view through the window to the appropriate sensor, which may be mounted separately and fixed in position.

When mounted on a military vehicle, as for example an aircraft, the wall is suitably canted to increase the forward aspect of the field of regard.

In general, application of the teachings of this invention will enable, as compared with for an otherwise similar installation, either for the field of regard to be increased for a given window size or conversely for the window dimensions to be reduced for a given field of regard.

The invention is hereinafter more particularly described by way of example only with reference to the accompanying drawings in which: Fig. 1 is a generally schematic section view illustrating a prior proposed arrangement with a restricted field of regard; Fig. 2 is a generally similar schematic sectional view through an embodiment of apparatus constructed in accordance with the present

invention illustrating a substantially greater field of regard as compared with the arrangement of Fig. 1; Fig. 3 shows a modified arrangement biased in a particular direction of interest in which that direction is illustrated by an arrow; Fig. 4 shows one simple mechanism for translating the identification station linearly; Fig. 5 shows a simplifie arrangement where there is instead a mirror or other means of redirecting the specified electromagnetic waves mounted in the gimbals, providing the identification sensors with the requisite field of regard; Figures 6 to 9 show further embodiments of the above arrangement incorporating redirection assemblies for the specified electromagnetic radiation, in this case such redirection means taking the form of optical mirrors; Fig. 10 shows another simple arrangement enabling translation of the identification station on its gimbals mount; Fig. 11 is a further schematic view illustrating a modified arrangement in which the central point of the viewing station is essentially fixed and the window is movable; and Figures 12 to 15 show a further embodiment of a sensor installation mounted within a rotatable turret.

In the arrangement schematically illustrated in Fig. 1, a conventional suitably gimballed identification station is indicated schematically at 1.

Identification station 1 is located behind a window 2 within an enclosure, the enclosure including a wall 3 surrounding the window. Desirably the window is

a conformai window so that, from a distance, the existence of the window and of an identification station behind the window is not readily apparent to an observer as would be the case if there were a substantial surface irregularity where the window meets the circumsurrounding wall. Window 2 is transparent to one or more selected wavelengths or groups of wavelengths in respect of which the wall 3 is opaque. The wavelength or wavelengths are preferably between ultraviolet and the far infra-red including all visible wavelengths. In the simples situation the window is transparent to visible light and the wall is generally opaque. Mounted on the identification station 1 is a sensor or designator 5 which can move at least between the positions 5a and 5b shown in the sectional view of Fig. 1 on the gimbal mounts.

The positions 5a and 5b correspond to the edges of the window. It will be seen that an effective field of regard 4 is thereby defined.

In the arrangement of Fig. 2 in accordance with the present invention, the identification station 1 is movable in a plane generally parallel to the plane of the window. Put another way, the identification station 1 is given a component of movement transverse to the line joining the centroid of the window and the centre point of the identification station. The greater the movement, as indicated by two of the possible positions 6a and 6b, the greater will be the resulting field of regard illustrated at 7. As with the arrangement of Fig. 1, the identification station is again suitably-gimballed.

Comparison between Figs. 1 and 2 will show that for the same size of window and for an identification station located the same distance behind the window, a much greater field of regard is achieved.

The invention is applicable to many different situations as explained briefly above, including, but not limited to, security surveillance, viewing of enclosed generally dangerous environments such as chemical plants or the interior of a nuclear reactor, and military vehicles including but not limited to aircraft, ships and submarines. In the case of surveillance of the interior of a

room or viewing of a dangerous environment, the identification station will be located on the exterior and the wall will be the circumsurrounding enclosure or room into which the identification station looks. In the case of surveillance of an exterior or in a military vehicle, the identification station will be located on the inside of an enclosure viewing the outside.

Particularly in the case of a military vehicle such as an aircraft, whether used for sensing or for designation by any of the systems known as Forward Looking Infra-Red (FLIR), Infra-Red Search and Tracking (IRST) or Thermal Imaging And Laser Designation (TIALD), the primary direction of interest is likely to be the forward direction. The field of regard can be extended in the forward direction of motion, as shown in Fig. 3 by canting the window. As can be seen, a wide field of regard 9 is achieved and the window is conforma with the fuselage 8.

In the arrangement of Fig. 4, identification station 1 is supported on its gimbal mount 10 from a block 11 internally threaded and received on a threaded rod 12, thereby forming a worm drive. Rotation of threaded rod 12 in one direction or the other causes translation of identification station 1 along the direction of rod 12.

In Figure 5, on a translatable gimbal mount 13 is located an appropriate means for redirecting the electromagnetic waves, here a mirror 14. The arrangement is such that the redirection means provides the desired field of regard for sensors 15. Other redirection means include internal reflection prisms and fibre optic light guides or other forms of electromagnetic waveguides.

Figures 6 and 7 show a section view of one embodiment according to the above mentioned arrangement whereby a pivoting mirror assembly 16 is utilised to provide the required field of regard to a stationary sensor 17.

The mirror assembly as before could be suitably gimballed as well to provide

image stabilisation. Fig. 6 shows the mirror assembly as when looking to the left of the page, and Fig. 7 shows the mirror assembly looking to the right of the page.

Figures 8 and 9 are isometric views of the same embodiment as in figures 6 and 7, showing the mirror assembly as when looking to the right of the page in Fig. 8 and looking to the bottom of the page in Fig. 9.

Fig. 10 shows another simple arrangement for translation of identification station 1 on its gimbals mount 10. In this case, gimbals mount 10 is mounted on a block 18 fixed to the belt 19 of a belt and pulleys arrangement generally indicated 20. As belt 19 rotates over pulleys 21 in one sense or the other, identification station 1 is translated. Other simple mechanical arrangements which can achieve the same effect will readily occur to those of ordinary skills in this art and doubtless do not need detailed description.

Fig. 11 schematically illustrates an alternative arrangement in which the position of the centre point of the identification station is kept essentially fixed while the window is movable. In the arrangement of Fig. 11 the window is rotatable. The window 22 is mounted with its centroid eccentric of the radial centre of a rotatable disc 23 which is mounted in a wall such as a fuselage wall. The disc is rotatable in one or both senses by means such as a motor 24 at its edge as indicated by the arrows and as a result of which the window traces out an area much greater than the area of the window.

The identification station 1 is again fully-gimballed and the maximum field of regard is defined by the edge of the window 22 furthest from the identification station. As will be appreciated, rotation of disc 23 causes the centroid of the window to move in a direction with a component transverse to the line joining the window centroid to the centre point of the identification station.

Figures 12-15 provide an illustration of embodiments of identification apparatus in accordance with this invention which employ a sensor installation positioned within a rotatable turret. Rotation of the turret further increases the field of regard of the sensors. In Fig. 12 the identification sensors are pivoted about 25 and situated within a housing 26 which is itself pivotable by a rotation motor 27. The above components are installed within a turret fairing 28 which is rotatable by a turret rotation motor 29 in a plane parallel to that of the turret rotation bearing 30. Figure 13 shows the sensor housing of Fig. 12 in a raised position relative to Fig. 12 with the sensor viewing direction in both Figs. 12 and 13 shown by the corresponding arrows.

Figures 14 and 15 are isometric views of the embodiment as shown in Figures 12 and 13 showing the sensor housing 26 in a lowered and raised position in Figures 14 and 15 respectively.

Other arrangements will readily occur to those skilled in this art including arrangements with a cylindrical wall section in which the window is mounted in a section of the wall formed as a cylindrical strip, the identification station being essentially fixed.

Other possibilities include combining features of one embodiment with features of another embodiment. Thus we can contemplate creating turreted sensor installations where the window is rotatable and the sensors are mounted on a translatable gimbal mount.