A system for transmitting and / or receiving electromagnetic radiation and a method for installing the same on a platform

Abstract

A module comprising a transmitter and / or receiver or Electromagenetic Radiation 2 adapted to be installed on an aircraft, comprising a housing 3 mounted to the internal structure 4 of the host platfor

Description

Electro-optic or radio frequency (RF) systems mounted to platforms, such as aerial platforms, are typically either: secured to a structural framework of the platform and arranged to protrude through skin of the platform, or mounted entirely externally to the skin of the platform, to a structure that protrudes through the skin. Both arrangements compromise the performance of the platform by limiting one or more of its aerodynamic, aerothermal and / or radar cross-sectional (RCS) characteristics. US3662661 describes apparatus for mounting a camera to a spacecraft. The apparatus comprises elongate flexible bellows which form a hermetic seal with a window of the spacecraft. The present invention was conceived as part of a solution to ameliorate these problems. According to a first aspect of the invention there is provided a system for transmitting and / or receiving electromagnetic radiation; the system adapted to be mounted to a platform comprising an external skin and an internal structural frame, the system comprising (e.g. prior to its mounting to the platform): (a) a transmitter and / or receiver for transmitting and / or receiving electromagnetic radiation; (b) a housing for housing the transmitter and / or receiver, the housing adapted to be mounted to an internal structure of a host platform; (c) a window structure comprising: a window that is transmissive to the electromagnetic radiation; and a frame adapted to be rigidly mounted to an external skin of the platform; and (d) a coupling that interconnects the housing to the frame; wherein the coupling is adapted to accommodate a tolerance difference in relative position between the external skin and internal structure and which provides, in combination with the window structure and the housing, a hermetic sealed cavity between the window and housing. This arrangement simplifies installation of the system onto a platform compared with the system of US3662661 as is obviates the requirement to provide a hermetic seal between the system and the platform during installation to provide the cavity. With this arrangement the window structure can be mounted to the skin whilst the housing is mounted within the interior of the platform to the structural framework of the platform, which unlike the skin, is strong enough to support the mass of the system. This allows the window structure to be of a shape to conform with the exterior surface of the skin surrounding the window structure to improve, for example, one or more of the aerodynamic, aesthetic and / or RCS characteristic of the host platform. Mounting the system to the internal structure of a host platform has the additional advantage that it allows the internal structure to be used as a physical alignment reference enabling the system to be integrated with other systems, e.g. sensing systems, of the platform. The coupling accommodates variation in position between the skin and the internal structure that is found between platforms as a result of manufacturing tolerances. This simplifies installation of the system onto a platform. Equally, it accommodates fluctuation in the relative position of the skin and internal structure following installation of the system on the platform, e.g. as a consequence of thermal expansion. In both scenarios accommodating variation through the coupling minimises stresses imparted on the skin which otherwise may cause undesirable deformation of the skin and, as a consequence, reduce the aerodynamic and / or RCS effectiveness of the platform and / or alter the aesthetic external profile of the skin. The coupling may be comprised from multiple pieces that are configured to be movable, e.g. in a telescopic arrangement, relative to one another to accommodate mismatch in relative position between the external skin and internal structure. However, multiple pieces make providing the hermetic seal more difficult as it is necessary to provide a hermetic seal between each piece. Consequently, it is preferred that the coupling comprises a flexible element configured to flex to accommodate a mismatch. The flexibility of the flexible element that accommodates for the tolerance difference in relative position between the external skin and internal structure may be provided primarily by the coupling’s geometry. For example, the coupling may comprise a corrugated bellows element. This allows for selection of a material e.g. a metal such as steel or aluminium for the coupling that is relatively inflexible compared with an conventional elastomer, but is highly impermeable to gases, has a long working lifetime, can withstanding a wide range of environmental conditions and temperatures, and which is sufficiently strong to resist deformation resulting from pressure differentials between the internal hermetic volume and the surrounding atmosphere. A corrugated bellows element has the additional advantage that whilst allowing for relative movement between its two ends in six degrees of freedom, the corrugated geometry provides improved resistance against deformation of the coupling caused by the pressure differentials between that within the cavity and the surrounding atmosphere. The coupling may comprise an electrically conductive material such that the coupling provides, either on its own or in conjunction with the window structure and housing, a continuous electromagnetic shield around the cavity. For example, the corrugated bellows element may comprise a corrugated wall comprised from an electrically conductive material such as a metal. To minimise condensation forming within the cavity e.g. on the window, which may negatively impact the operation of the transmitter and / receiver 2, the cavity is favourably a dry space, which may be achieved, for example, by filling with nitrogen, dry air or equivalent. Alternatively, a desiccant could be placed in the cavity and / or it may be evacuated to form a vacuum within the cavity. Although not to be taken as limiting, the system is particularly suitable for use on aerial platforms, where aerodynamic and / or the RCS characteristics of the platform are often particularly important. In another aspect of the invention there is provided the system in any of the forms variously described is mounted to the host platform wherein the housing is mounted to an internal structure of the host platform and the window structure is mounted within an aperture within the external skin of the host platform. An outer face of the window structure, e.g. an outer face of the window, exposed through the aperture of the external skin to the environment external to the platform, may be shaped to conform to the profile of an external face of the external skin surrounding the window structure, e.g. lie flush with the skin where flat, of the host platform to which the system is intended to be installed. The transmitter and / or receiver may comprise an optical transmitter and / or receiver, e.g. for transmitting and / or receiving one or more of visible, UV or IR light. The transmitter and / or receiver may comprise a radio frequency (RF) transmitter and / or receiver e.g.as part of radar system. The invention will now be described by way of example with reference to the following figures in which: Figure 1 is a perspective of a system for transmitting and / or receiving electromagnetic radiation; Figure 2 is a perspective of the system of Figure 1 mounted to host platform; Figure 3 is a cross sectional perspective view of the system mounted to host platform; and Figure 4 side cross-sectional view of the system mounted to host platform. The figures illustrate a system 1 for transmitting and / or receiving electromagnetic radiation. The system 1 comprises a transmitter and / or receiver 2 retained in a housing 3. The transmitter and / or receiver 2 may comprise, but is not limited to one or more of: a camera, laser and a radio transmitter and / or receiver. As illustrated in Figs 2-4, the housing 3 comprises one or more mounting flanges 3A by which the housing 3 is attached to an internal structural frame 4 of a host platform, by fasteners 5. The housing 3 is comprised from an electrically conductive material. A metal, for example steel or aluminium, would be suitable. The housing 3 has an aperture 6 to allow electromagnetic radiation to pass through the housing 3 towards and / or away from the transmitter and / or receiver 2. In certain applications the aperture 6 may include a transmissive element (not shown) appropriate for the transmitter / receiver 2. For example, where the system 1 is an optical system, there may include one or more optical elements, including for example a lens. In a variant system in which the transmitter and / or receive is a RF transmitter and / or receiver, one or more antenna elements of the system may extend through or he outside of or within the aperture 6 of the housing 3. The system 1 further includes a window structure 7 which, when the system 1 is installed onto the platform, is mounted within an aperture through an external non- structural skin 10 of the platform. The window structure 7 comprises a window frame 8 in which is mounted a window 9. The frame 8 includes a mounting flange 8A about which the window structure 7 is fastened to the external skin 10 by fasteners 11. The window 9 protects the transmitter and / or receiver 2 from the environment external to the skin 10 and platform. The window 9 is comprised from a material that is transmissive to the wavelengths of the electromagnetic energy at which the transmitter / receiver 2 operates. The window structure 7 is configured so that when mounted to the skin 10, the portion of the window structure 7 exposed through aperture of the skin 10 (typically an outer surface of the frame 8 and window 9) lies flush with the exterior face of the skin 10. In the case of a curved skin 10, the window structure 7 would be shaped to conform with the curved outer face of the skin 10 surrounding the frame 8. i.e. to reproduce the intended shape of the missing section of the skin at the location of the aperture, to provide the desired aerodynamic and / or RCS characteristics. In the case of an optical system, the window 9 may be comprised of a material that is electrically conductive, or coated with an electrically conductive material. The window 9 may be fixed into the window frame 8 by any suitable means, including adhesive. The system 1 is mounted on the platform at a location where the external skin 10, which is supported by the structural frame 4 at various locations across the platform, is spaced away from the structural framework 4, to provide, for example, a platform geometry with desired aerodynamic and / or RCS properties. To carry out installation, the housing 3 is mounted to the internal structural frame 4, and the frame 8 to the external skin 10 so that the frame 8 sits within are adjacent an aperture through the skin 10. The separation distance between the skin 10 and the structure 4, as well as the position of the aperture through the skin 10 from the designated mounting point of the housing 3 to the frame 4, will vary from a design specification due to manufacturing tolerances. It will also likely vary dynamically during operation of the platform as a result of thermal expansion and contraction. To accommodate this, the housing 3 and window structure 7 are conjoined by an expansion joint or coupling comprising a bellows 12. The bellows 12 is mounted at one end by fasteners 13A through a first flange 12A to the window frame 8, and at its other end around the aperture 6 through a second flange 12B by fastener 13B. A hermetic seal is provided between the bellows 12 and housing 3 by a first seal element 15, and between the bellows 12 and window structure 7 by second seal element 16, though alternative means, e.g. cold welding between the coupling, housing 3 and window structure 7, could be used. The bellow 12 comprises a square tubular corrugated wall 12C that allows the bellows 12 to be lengthen or shortened to increase or reduce the separation distance between the housing 3 and frame structure 7 to accommodate variation in the spacing between the skin 10 and supporting structure 4. Additionally, the geometry of the bellows 12 inherently provides relative movement between its two ends in six degrees of freedom to accommodate misalignment between the aperture within the skin 10 and a mounting reference point for the housing 3, to minimise stresses placed on the exterior skin 10. A sealed cavity 14 extends through the bellows 12 between the window 9 and housing 6. The cavity 14 provides a pathway for electromagnetic radiation between the window 9 and transmitter and / or receiver 2. To minimise condensation forming within the cavity 14, e.g. on the window 9, which may negatively impact the operation of the transmitter and / receiver 2, the cavity 14 may be fdled with nitrogen, dry air or equivalent. Alternatively, the cavity may be evacuated to form a vacuum within the cavity. The hermetic seals between the bellows 12 and the window structure 7 and housing 3 respectively, isolates the cavity 14 from the environment exterior to the platform, and the environment between the external skin 8 and internal frame 4. Advantageously, the corrugated geometry of the wall 12C inherently increases the wall’s 12C resistance to deformation caused by pressure differentials between that within the sealed cavity 14 and in the space between the exterior skin 10 and structure 4, which will often be at equilibrium with the atmospheric pressure exterior to the platform. The material of the bellows 12 is chosen to provide an electrically conductive shield around the cavity 14, and to have a low permeability to gases to maintain a sealed atmosphere within cavity 14 An example of a suitable construction for the wall 12C is a single-skin aluminium layer, which in combination with the housing 3, and optionally the window structure 7, if an optical system, may provide a continuous electromagnetic enclosure around the cavity 14 and transmitter / receiver 2 to inhibit the system interfering with other systems on the platform or vice-versa. In the case of an optical system, the window 9 may be comprised from an electrically conductive material or may comprise an electrically conductive coating. The housing 3, window structure 7 and bellows 12 are assembled prior to being mounted onto the platform. As the assembly typically needs to be carried out in a clean dry environment in order to ensure the sealed cavity is not contaminated, this avoids the need to assemble the platform as a whole in a similarly controlled environment. In one example, the platform is an aerial platform, such as aeroplane, helicopter or unmanned aerial vehicle. In a variant to the above example, the bellows 12 may be formed as integral to one or both of the housing 3 and / or window structure 7, e.g. through a machining process. In a less preferred variant, the coupling may be comprised from an elastomer or similar material such as synthetic rubber, which may not be corrugated. To provide electromagnetic shielding, if needed, the wall of the coupling may additional comprise a metallic web tube which may be separate to, or formed as a layer on, the elastomer. In the above example the coupling has a square tubular form. This is arbitrary and not to be taken as limiting. Alternative or additional means to fasteners 5, 11, for example adhesive or welding, may be used to mechanically secure the window structure 7 to the skin and housing 3 to internal frame 4 In a variant embodiment a localised controlled environment between sensor and window, hermetically sealed from the external environment, may be provided through a coupling connecting directly between one or more of: the internal structure 4 to the frame 7; the external skin 10 to the housing 3; and the external skin 10 to the internal structure 4. Nevertheless, these options are considered less preferred as the internal environment of the platform would need to be cleaned during installation of the system.

Claims

1. A system for transmitting and / or receiving electromagnetic radiation; the system adapted to be mounted to a platform comprising external skin and an internal structural frame, the system comprising, prior to its installation onto the platform:(a) a transmitter and / or receiver for transmitting and / or receiving electromagnetic radiation;(b) a housing for housing the transmitter and / or receiver, the housing adapted to be mounted to the internal structure of a host platform;(c) a window structure comprising:a frame adapted to be rigidly mounted to the external skin of the platform, and which holds a window transmissive to the electromagnetic radiation;and(d) a coupling that interconnects the housing to the frame; wherein the coupling is adapted to accommodate a tolerance difference in relative position between the external skin and internal structure and which provides, in combination with the window structure and the housing, a hermetically sealed cavity between the window and housing.

2. A system according to claim 1 wherein the coupling comprises a flexible element, to accommodate the tolerance difference in relative position between theexternal skin and internal structure, and in which the flexibility is provided primarily through the coupling’s geometry.

3. A system according to claim 1 or 2 wherein the coupling comprises a corrugated bellows element.

4. A system according to any previous claim wherein the coupling is comprised from an electrical conductive material to electromagnetically shield the cavity.

5. A system according to any previous claim wherein the transmitter and / or receiver is an optical transmitter and / or receiver.

6. A system according to any previous claim wherein the platform is an aerial platform.

7. A system according to any previous claim wherein the housing is mounted to an internal structure of a host platform and the window structure is mounted within an aperture within the external skin of the platform8. A system according to claim 7, wherein an outer face of the window structure exposed through the aperture of the external skin conforms to the profile of an external face of the external skin surrounding the window structure.

9. A method of installing a system for transmitting and / or receiving electromagnetic radiation onto a host platform; the host platform comprising an external skin and an internal structural frame;wherein the system comprises:(a) a transmitter and / or receiver for transmitting and / or receiving electromagnetic radiation;(b) a housing for housing the transmitter and / or receiver, the housing adapted to be mounted to the internal structure of a host platform;(c) a window structure comprising:a frame adapted to be rigidly mounted to the external skin of the platform, and which holds a window transmissive to the electromagnetic radiation;and(d) a coupling that interconnects the housing to the frame; wherein the coupling is adapted to accommodate a tolerance difference in relative position between the external skin and the internal structure and a hermetic sealed cavity between the window and housing provided, in combination with the window structure and the housing, a hermetically sealed cavity between the window and housing;and in which the method comprises:mounting the window structure to the skin of the platform adjacent to and / or in an aperture through the skin; andmounting the housing to the internal structure of the platform.

10. A method according to claim 9 wherein the coupling comprises a flexible element to accommodate the tolerance difference in relative position between the- 15 -external skin and internal structure, and in which the flexibility is provided primarily through the coupling’s geometry.

11. A method according to claim 9 or 10 wherein the coupling comprises a corrugated bellows element.

12. A method according to any 9-11 wherein the coupling is comprised from an electrical conductive material to electromagnetically shield the cavity.

13. A method according to any claim wherein the transmitter and / or receiver is an optical transmitter and / or receiver.

14. A system according to any previous claim wherein the platform is an aerial platform.

15. A method according to claim 14, wherein an outer face of the window structure exposed through the aperture of the external skin conforms to the profile of an external face of the external skin surrounding the window structure.

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