GB2246473A - Cassegrain aerial system - Google Patents

Abstract

A Cassegrain aerial system is provided in which a movable reflector plate (4) is supported on push-rods (7, 8 and 9) which are driven by torque motors (13, 14) so as to maintain the orientation of plate (4) about inertial orthogonal axes Y and Z. The torque motors (13 and 14) are controlled by respective gyroscopes (11, 12) mounted on the reflector plate (4) with their axes perpendicular to the normal of the plate. The arrangement compensates for roll about any axis parallel to the collimated radio beam (3) incident upon the movable reflector plate (4). <IMAGE>

Description

Cassegrain Aerial System The present invention relates to Cassegrain aerial systems. By a Cassegrain aerial system is meant an aerial system utilising dual successive reflection of radio waves. The basic principles of a moving-plate Cassegrain aerial are well-known, and were disclosed in British Patent Specifications Nos. 700868 and 716939 in the name of Elliott Brothers which are hereby incorporated by reference. This invention particularly relates to improvements in the mechanical design and servo control of such Elliott Cassegrain aerial systems using these principles, to enhance performance when used in an agile combat aircraft.The basic elements of one type of the known aerial are shown in Figure 1 and comprise a microwave feed 1 illuminating a wire grid parabolic reflector 2, the collimated (parallel) beam 3 from which is reflected by a second movable plane reflector 4 to direct it in a required direction. The parabolic reflector 2 is electrically designed, by means of the direction of its grid wires, to reflect only radiation in the plane of polarisation of the feed (parallel to the grid wire direction), and to be transparent to radiation in the orthogonal plane. The movable plate reflector 4 (which is usually, but not necessarily planar) is electrically designed to rotate the plane of polarisation of incident radiation through 900 on reflection, so that the radiated beam leaving it may pass through the parabolic reflector without obstruction.In the geometrical arrangement of these elements shown in Fig. 1, the feed 1 passes through a hole in the centre of the moving plate reflector 4 and is therefore referred to as a centre-fed moving plate Cassegrain aerial. Other geometries are possible but the arrangement of Figure 1 is most appropriate for an agile multi-role combat aircraft as it can provide uniform cover over more than the whole forward hemisphere. The present invention is particularly but not exclusively applicable to centre-fed C-assegrain aerials.

Important requirements of an aerial for an agile combat aircraft are wide angular cover, and a high degree of immunity to disturbance by aircraft manoeuvre and perturbation, both in lateral acceleration and in angular rates. The aircraft is most agile and subject to perturbation about its roll axis, but stabilisation of the scanner against this by a roll ring of finite travel is unacceptable because, if discontinuity of operation is to be avoided, it places a limitation on tactical freedom to roll indefinitely. The ability of the aerial to compensate adequately for roll by movements of the azimuth and elevation gimbal is therefore important, and incidentally avoids the weight, space, cost and power consumption penalties of the roll ring mechanism.In most respects the movable flat plate Elliott Cassegrain Aerial is capable of meeting these requirements better than alternative aerials currently in use or practicable.

However, the 2:1 relationship between the reflector plate movement and the movement of the reflected beam produces problems in the control and stabilisation of the beams which have previously placed a limit on the dynamic performance of the system in an agile aircraft. There are two main problems. Firstly, since the direction of the beam in space is not locked to the moving member of the aerial (the plate) it is not in general possible to stabilise the position of the beam in space by reference to gyroscopes mounted on the moving member; this produces difficulty particularly when the aerial is tracking a target in the lock-follow mode.Secondly, since compensation for aircraft rotation requires movement of the plate relative to inertial space as well as relative to the airframe, the inertia of the moving members does not tend to resist disturbance as it does in a more conventional aerial; compensation therefore requires precisely controlled motion of the servo systems, and accurate compensation for aircraft disturbances occurring at frequencies above the servo bandwidth is not possible.

In previous applications of the fl-at plate Elliott Cassegrain Aerial these difficulties have been mitigated by the use of a mechanism known as a "beam-member which resolves the trigonometry of the 2:1 relationship and provides a platform locked to the transmitted beam which may be used to carry gyroscopes or angular pick-offs. The geometrical accuracy of such a device is limited by manufacturing tolerances, backlash and wear, and the bandwidths of the servo loops controlled-by it are limited by its elastic resonances and backlash. Consequently the amplitude and frequency of aircraft perturbation which can be eliminated by the system are correspondingly limited.

With the availability of digital micro processors, a more modern approach to the problems would be to measure the aircraft space rates and attitudes about three axes, compute from them the gimbal rates and positions necessary to compensate for aircraft motion and apply these to high-bandwidth servos working in plate gimbal axes. However, analysis shows that the finite data rates and transport delays associated with the computation, the difficulties of achieving adequate bandwidths in positionfollowing servos with current servo components and conventional mechanical arrangements, and the difficulties of matching the demanded gimbal rates to the achieved servo rates in the presence of the non-linearities and crosscoupling which occur in the system, make it difficult to achieve a satisfactory degree of perturbation rejection.

This is particularly so in respect of the roll axis, about which the aircraft is most agile and experiences the greatest perturbation.

An object of the present invention is to provide a servo system for a moving-plate Cassegrain aerial-which significantly reduces the above difficulties.

According to the present invention a moving plate Cassegrain aerial system for mounting on a dynamic platform subject to rotational disturbance about a platform axis comprises means for defining an electro-magnetic radiation path parallel to said platform axis, a movable reflector mounted in said path and arranged to generate a directional beam, and servo-controlled drive means arranged to control the orientation in space of the reflector about a pair of mutually orthogonal axes and thereby stabilise the orientation in space of said beam against such rotational disturbance.

Preferably, one of said pair of mutually orthogonal axes is a platform axis and the other of said pair of axes is a reflector axis.

Preferably, said movable reflector is connected at three or more spaced pivot points to three or more respective parallel push rods, the free ends of said push rods being linked in such a manner that said movable reflector may be independently tilted about said mutually orthogonal axes, said axes intersecting at a static centre of rotation of said movable plate reflector intermediate said pivot points.

One particular embodiment of the invention will now be described by way of example with reference to Figures 2, 3 and 4 of the accompanying drawings, which drawings comprise: Figure 1 (already referred to), Figure 2, which shows two views in the same direction of a rolling aircraft incorporating a radar system in accordance with the invention, Figure 3, which shows a geometric plan view of the same radar system, and Figure 14, which is a sketch perspective view of the radar system.

Figure 2 shows a movable plate Cassegrain aerial system in accordance with the invention mounted in the radome 5 of an agile combat aircraft 6. Parts (a) and (b) of the Figure are side views of the aircraft in 0 positions relatively rolled through 90 and thus are both views along the Y-axis of a set X, Y, Z of inertial axes.

Xp, Yp, Z are a set of plate axes, Xp (not shown) being the plate normal. Aircraft roll axis XA and pitch axis are coincident with the X and Y axes in part (a) and to the X-and Z axes in part (b), indicating the 900 roll about the roll axis XA between (a) and (b). During the roll, the plane of movable-plate reflector 14 is constrained to maintain a constant orientation with respect to inertial axes X, Y and Z by three parallel push-rods 7, 8 and 9 pivotally connected, at A, B and C to an aircraft-mounted drive linkage L and at A', B' and C' to the movable reflector 4. A collimated beam 3 of microwave radiation is directed along the X-axis onto plate 4 by a parabolic reflector 2.Because beam 3 is directed along the XA (longitudinal) axis, any component of roll about this axis will not affect the orientation of the reflected beam (represented by arrow 10) provided that the absolute orientation of the plane of plate 4 about the X, Y and Z axes remains constant. Platform-mounted drive linkage L maintains this constant orientation by tilting the reflector plate 4 about the YA aircraft axis and Z plate p axis, as will subsequently be described with reference to Figure 3. It should be noted that if beam 3 were inclined at angle Q to any given platform axis, the absolute orientation of beam 10 would swing by + 2C as the aircraft rotated about that axis.The arrangement shown is optimised to compensate for roll about the longitudinal (XA) axis because this is the axis about which the highest rates of rotation are liable to occur. Compensation for pitch and yaw (about the YA and ZA axes) is less demanding and may be provided by altering the orientation of the plane of the plate as will subsequently be described.It will be appreciated that the arrangement shown will be equally effective in compensating for roll about any axis parallel to the XA -axis. Furthermore it will be appreciated that the aerial system shown may be a transmitting or receiving system (or both), transmission having been described for convenience only; provided that an electromagnetic radiation path is provided parallel to the XA, or other axis about which roll compensation is required, which path is reflected at the moving plate reflector 14.

Referring to Figures 3 and 4 a pair of rateintegrating gyroscopes 11 and 12 are mounted directly on the plate 4 in such a manner that they are sensitive to components of absolute rotation about orthogonal plate axes Z and Y but are insensitive to components of plate p p r rotation about its normal Xp (not shown). The plate 4 is carried by three push-rods 7, 8 and 9 connected to two pairs of supporting gimbals. The forward pair of supporting gimbals (not shown) supports a linkage L1 (shown partially in Figure 3) which pivotally supports the push rods at universal joints D and E. The rear pair of supporting gimbals support the push rods at A, B and C and contain azimuth and elevation drive motors 13 and 14 (the azimuth being the inner).The orientations of the plate gyroscopes 11 and 12 are such that the sensitive axis of the azimuth gyroscope 11 remains parallel to the axis of the inner (or azimuth) gimbals and actuator motor 13. The actuator motors 13 and 14 are directly coupled electrical torque motors, driven by respective current controlling amplifiers 15 and 16. The amplifiers 15 and 16 are controlled by the outputs of the respective gyroscopes 11 and 12 fed-back in such a direction that motion of the plate is opposed, the control being through a network which combines a linear proportional term with a suitably chosen time-derivative term to ensure adequte damping and stability.It will be appreciated that the action of the current-controlling amplifiers'and the torque motors is to apply a torque to the plate and gimbal assembly which is substantially independent of gimbal rotation rate, and that the action of the rate-integrating gyroscopes is to produce an output linearly proportional to any movement of the plate from the inertial axes defined by the gyroscopes 11 and 12. It is well known that it is possible to make the response time of both the gyroscopes and the current-controlling amplifiers very short and of the order of one millisecond or less.It will then be appreciated that the rate at which an opposing torque may be applied to the moment of inertia of the plate in response to any disturbance of its orientation will be effectively limited only by the resonance frequency of the plate and gimbal inertias with the elastic compliance of the push-rod system. Consequently it will be seen that it is possible to increase the gain of the feedback, and hence the band-width of the loop, until the band-width approaches the above resonance frequency sufficiently closely for the loop stability to be adversely affected.

By the means which will be described later, the said resonance frequency and hence the said bandwidth may be made high. It will then be appreciated that the control system as so far described will resist any disturbance of the orientation of the plane of the plate caused by airframe perturbations about the x-axis occurring at any frequencies up to the said high bandwidth.

It will be appreciated that because the gyroscopes 11 and 12 are not mounted one on the other as are actuator motors 13 and 114, the gain of the elevation loop will vary with the azimuth gimbal angle. Gain control may be introduced to compensate for this, but such gain control is not essential and has been omitted from Figure 4 for the sake of clarity.

Microwave radiation from waveguide 1 is reflected back in a collimated beam 3 parallel to the x-axis, as described above with reference to Figure 1, and is reflected a second time from plate 4 to generate a directional beam 10.

Now it will be obvious from the mirror-action of the plate that if the direction of the incident radiation is constant with respect to inertial space, and the orientation of the plane of the plate 4 remains constant in space, the direction of the reflected radiation 10 will also remain constant in space, and will be unaffected by any rotation of the plate about its normal, as shown in Figure 2, for example. Consequently it will be appreciated that the control system so far described will stabilise the direction in space of the reflected beam against any angular perturbation of the airframe occurring about the x-axis or any other axis parallel to the incident collimated beam 3, that it will do this for perturbations applied at all frequencies within the bandwidth of the simple servos described, which may be made high, and that it will do this without the need for any measurement of aircraft attitude or body rates, or for any computation based on such measurements. It is therefore a feature of the invention that the aerial system is mounted in the airframe with its collimated beam 3 parallel to that axis of the airframe about which maximum rejection of perturbation is required, which in practice is or is close to the roll axis x.

It is a well-known feature of a rate integrating gyroscope that the orientation in space of its reference axis may be precessed at a rate which is precisely and linearly proportional to a current fed to a torque-input coil. It will therefore be appreciated that application of current to the torque-input coils of the plate gyroscopes will, by the following action of the servo loops, cause the orientation of the plane of the plate 4 to be changed in a similarly precisely controlled manner.

Calculating means 19,20, including microprocessors and digital to analogue converters, are therefore provided for calculating space rates required of the scanning plate 14, and applying these to the gyroscope torque-input coils so as to compensate for components of aircraft rotation about the YA and z A axes. It may be shown by analysis that if angular space rates of the aircraft about these two axes are known, and if the orientation of plate 4 relative to the airframe is known, by combining these in a suitable trigonometrical algorithm, space rates may be calculated which if applied to the reflector plate 14 would cancel any movement of the reflected beam which would otherwise occur due to yaw and pitch motion of the aircraft.Therefore inductosyns 17 and 18 (or other suitable angle sensors) are provided for measurement of gimbal angles relative to the airframe, together with means, (which may be gyroscopes), for measurement of such orthogonal airframe rates if these are not already available in the aircraft system. These data are fed to the calculating means 19, 20 to enable the algorithm to be performed and hence complete the stabilisation of the reflected beam 10 against aircraft angular motion about all three axes.It will be appreciated that after movement of the aircraft in pitch and yaw x A will no longer be parallel to X as shown in Figures 2 to 14. It will be appreciated that because the aircraft angular rates and frequencies about the orthogonal and ZA axes are less than those about the most agile, xA (roll) axis, perhaps by as much as ten times, the data rates necessary for the calculations are significantly less than would be necessary to compensate by means other than the present invention for perturbation about all three axes.It will also be appreciated that by the use of further trigonometrical algorithms in said calculating means, and of further information such as target-tracking data from other sources, additional rate demands may be calculated by calculating means 19 and 20 to move or point the beam 10 in any required direction. It is thus possible to perform scan patterns, acquire targets, track targets in a lock-follow mode, or perform other operational functions.

From the foregoing it will be obvious that the performance of the system in rejecting disturbance due to agile motion is dependent on the achievement of a high mechanical resonance frequency in the transmission path from the torque motors to the scanning plate. The preferred mechanical arrangement for achieving this and of achieving high performance in other respects comprises three parallel push-rods 7, 8 and 9 approximately symetrically spaced about the plate centre at universal joints A', B' and C'. The top two push-rods 7 and 8 are attached to and cantilevered out from two pairs of gimbals namely a forward pair (not shown) on which linkage L' is supported and a rear pair on which motors 13 and 14 are mounted.To avoid redundance in the linkage the third push-rod 9 is attached to the rear gimbal only and to the plate 14. All connections, A,B,C,A',B',C',D and E between pushrods and plate or gimbals are by means of universal joints formed from assemblies of ball or roller bearings suitably pre-loaded to increase their elastic stiffness without introducing an unacceptable level of friction. The rear gimbals contain the azimuth and elevation torque motors 13 and 14 described above, and it will be seen that the upper two push-rods 7 and 8 are attached to the inner or azimuth gimbal, and control the angular motion of the plate in azimuth and, by the cantilever action of the push-rods supported by cross-bar 21 of linkage L, its lateral location.The third push-rod 9 is connected to the lower end of the rear azimuth gimbal shaft 22 at C and completes the control of the angular motion of the plate in elevation. Joints A,B and C are offset behind the plane defined by the intersecting axes of motors 13 and 14 by equal distances.

It is essential for the maintenance of the gain and the polar pattern of the aerial system that the aperture blockage caused by the presence of the feed 1 an-d of the hold in the plate through which it passes should be a minimum. The hole size may be made the minimum which is consistent with accommodating the feed wave-guide if the plate can be made to rotate about a virtual centre in the plane of the plate, or more specifically in the plane of the reflective surface of the plate.It is an additional feature of a preferred embodiment of the present invention that the fixings of the push-rods 7, 8 and 9 to the plate 4 are arranged so that the effective axes of the universal joints A', B' and C1 are offset behind the reflecting plane of the plate by a fixed distance, and the fixings of the push-rods to the respective gimbals are arranged so that the effective axes of their universal joints A, B and C are offset behind the respective planes of the gimbal axes by a similar amount. By the geometry of this arrangement the plate is constrained to rotate about a virtual centre 23 in its reflective plane, as illustrated in Figure 3.The arrangement is effectively supported at centres F and G which are offset from their associated sets of universal joints D,E; A',B',C' from the reflecting surface of plate 14.

It will be appreciated that in the mechanical arrangement described above, the lateral location of the moving plate is determined by the location of the front gimbal bearings (not shown) which support linkage L', through the parallelogram motion of the supper two pushrods 7 and 8 with the rear gimbal and the cantilever action of the upper push-rods. The longitudinal location of the plate is determined by the location of the rear gimbal bearings through the parallelogram action of all three push-rods 7,8 and 9. The rotational motion of the plate 14, which alone determines the direction of the radiated beam, is controlled by the motion of the rear gimbals. It is therefore appropriate -to measure the orientation of the plate by means of sensors mounted inthe rear gimbal bearings. These sensors are preferably inductosyns, but may also be synchros or digital encoders all of known design.

The mechanical arrangement described has a number of important advantages in comparison with previous or alternative arrangements of Cassegrain Aerial.

Without excessive weight, it is possible to choose the sizes and hence the stiffnesses of the push-rods and associated gimbal components to achieve very high first resonance frequencies in the transmission paths from the torque motors to the plate, and hence as earlier described to achieve a very high servo bandwidth and corresponding rejection of aircraft motion disturbance. Loss of aperture by the feed-hole in the plate is kept to a minimum. The front gimbal may without penalty be made large enough to accommodate the waveguide components associated with the feed. The essential symmetry of the moving parts, and hence nearness to static balance, results in little disturbance due to aircraft lateral accelerations and any additional balance weights necessary to improve further this immunity can be small in mass.

The essential symmetry of the push-rod system provides that lateral or longitudinal resonances of the plate mass on the compliance of its mounting, even if lower in frequency than resonances in the angular transmission paths, do not couple into these paths to produce plate rotation or to affect adversely the servo loop stability.

Any lack of perfect symmetry in this respect due to practical constraints on the geometry may be readily compensated by corresponding adjustments to the push-rod stiffnesses. The simplicity of the mechanical arrangement and of the servo system permits ease of initial alignment and the minimum of adjustment and maintenance during life.

It will be appreciated that the aerial system of the invention is suitable for mounting in combat aircraft and also other dynamic platforms where roll compensation is required.

Claims (1)

1. A moving plate Cassegrain aerial system mounted on a dynamic platform subject to rotational disturbance about a platform axis, said system comprising means for defining an electromagnetic radiation path parallel to said platform axis, a movable reflector mounted in said path and arranged to generate a directional beam, and servo-controlled drive means arranged to control the orientation in space of the reflector about a pair of mutually orthogonal axes, and thereby stabilise the orientation in space of said beam against such rotational disturbance.

2. A moving plate Cassegrain aerial system as claimed in Claim 1 wherein one of said pair of axes is a platform axis and the other of said pair of axes is a reflector axis.

3. A system according to Claim 1 or Claim 2 wherein said movable reflector is connected at three or more spaced pivot points to three or more respective parallel push rods, the free ends of said push rods being linked in such a manner that said movable reflector may be independently tilted about said mutually orthogonal axes, said axes intersecting at a static centre of rotation of said movable plate reflector intermediate said pivot points.

14. A system according to Claim 3 wherein said centre of rotation lies in the plane of the reflecting surface of said movable reflector.

5. A system according to Claim 3 or Claim 4 incorporating three said push rods.

6. A system according to any of Claims 3, 4 or 5 wherein the effective longitudinal elastic compliances of the respective push-rods are effectively proportional to the respective distances of said pivot points from said centre of rotation such that longitudinal resonances of the reflector-plate mass are effectively decoupled from rotational resonances of said drive means.

7. A system according to Claim 6 wherein said pivot points are symmetrically disposed about said centre of rotation.

8. A system according to any of Claims 3 to 7 wherein said drive means comprises an inner gymbal-mounted motor supported for rotation about an axis orthogonal to its drive shaft on one or more outer motors, said inner drive motor being connected via at least one cross bar to universal joints form an array which defines a first plane parallel to a second plane defined by said pivot points.

9. A system according to Claim 8 wherein said first and second planes are offset by substantially equal amounts behind the common plane of the drive shafts of said motors and the centre of the reflecting surface of the reflector respectively.

10. A system according to any preceding Claim in which two inertial angular sensors arranged to control said drive means are mounted so as to move with the movable reflector, means being provided to independently maintain their sensitive axes at demanded orientations in space about axes perpendicular to the normal of the movable reflector.

11. A system according to Claim 10 wherein said sensors are mounted directly on the movable plate reflector.

12. A system according to either of Claims 10 and 11 as dependent upon Claim 8 wherein said inner motor is arranged to be controlled by signals from one said angular sensor and the or each said outer motor is arranged to be controlled by signals from the other said angular sensor.

13. A system according to any of Claims 10, 11 and 12 wherein said motors are directly coupled electrical torque motors and are driven by amplifiers which are in turn controlled by the output signals of said angular sensors.

A A system according to any of Claims 10 to 13 wherein said inertial angular sensors are gyroscopes.

15. A system according to Claim 14 wherein said gyroscopes are rate-integrating gyroscopes and are controlled by torque input coils and said amplifiers are current-controlling amplifiers, so as to form a damped Type 2 servoloop.

16. An arrangement comprising a system according to any of Claims 10 to 15 when mounted on said dynamic platform, wherein said sensors are controlled rateintegrating gyroscopes, said arrangement further comprising means for determining the attitude of the movable reflector with respect to the dynamic platform and gyroscope control means for processing said gyroscopes so as to cause them to control said drive means in dependence upon reflector plate angle rate demand signals fed to said gyroscope control means and thereby compensate for components of rotation about platform axes perpendicular to said electromagnetic radiation path.

17. An arrangement according to Claim 12 wherein said reflector plate angle rate demand signals are generated in use in response to a target-tracking algorithm so as to track a target independently of the motion of said dynamic platform.

18. An arrangement according to Claim 12 wherein said reflector plate angle rate demand signals are generated in use in response to a pointing or scanning algorithm so as to point the beam in any desired direction independently of the motion of said dynamic platform.

19. A transmitting system according to any of Claims 1 to 16 or a transmitting arrangement according to Claim 17 or Claim 18 which is centre-fed by a waveguide which extends through a hole at a virtual centre of the movable reflector and defines said electromagnetic radiation path.

20. A radar system according to any of Claims 1 to 16, 21. A radar system according to any of Claims 2 to 16 mounted in a combat aircraft wherein said electromagnetic radiation path is parallel to the longitudinal axis of the aircraft.

22. A radar system according to Claim 21 wherein said movable reflector is a polarisation-twisting reflector, said system further comprising a polarised reflector facing said movable plate reflector.

23. An aerial system substantially as described hereinabove with reference to Figures 2, 3 and 4 of the accompanying drawings.

AM(~NDMENTS TO THE CLAIMS HAVE BEEN D AS FOLLOWS 1. A moving plate Cassegrain aerial system mounted on a dynamic platform subject in use to rotational disturbance about a platform axis, said system comprising means for defining a radiation path parallel to said platform axis, a movable reflector constituting said moving plate, said movable reflector being mounted in said path, movably with respect to said dynamic platform and arranged to generate a directional beam, and servo-controlled drive means arranged to control the orientation of the reflector about a pair of mutually orthogonal axes, wherein two inertial angular movement sensors are fixedly and directly coupled to said reflector with their sensitive axes arranged mutually perpendicular and so as to feed control signals to said drive means tending to stabilise the orientation in space of said beam against such rotational disturbance about said platform axis.

2. A moving plate Cassegrain aerial system as claimed in Claim 1 wherein one of said pair of mutually orthogonal axes is a platform axis and the other of said pair of mutually orthogonal axes is a reflector axis.

3. A system, according to Claim 1 or Claim 2 wherein said movable reflector is connected at three or more spaced pivot points to three or more respective parallel push rods, the free ends of said push rods being linked in such a manner that said movable reflector may be independently tilted about said mutually orthogonal axes, said axes intersecting at a static centre of rotation of said movable plate reflector intermediate said pivot points.

4. A system according to Claim 3 wherein said centre of rotation lies in the plane of the reflecting surface of said movable reflector.

5. A system according to Claim 3 or Claim 4 incorporating three said push rods.

6. A system according to any of Claims 3, 4 or 5 wherein the effective longitudinal elastic compliances of the respective push-rods are proportional to the respective distances of said pivot points from said centre of rotation such that longitudinal resonances of the reflector-plate mass are effectively decoupled from rotational resonances of said drive means.

7. A system according to Claim 6 wherein said pivot points are symmetrically disposed about said centre of rotation.

8. A system according to any of Claims 3 to 7 wherein said drive means comprises an inner gymbal-mounted motor supported for rotation about an axis orthogonal to its drive shaft on one or more outer motors, said inner drive motor being connected via at least one cross bar to universal joints on said push rods, such that said universal joints form an array which defines a first plane parallel to a second plane defined by said pivot points.

9. A system according to Claim 8 wherein said first and second planes are offset by substantially equal amounts behind the common plane of the drive shafts of said motors and the centre of the reflecting surface of the reflector respectively.

10. A system according to any preceding Claim in which means are provided to rotate the reference axes of the said sensors independently in space at controlled rates about axes perpendicular to the normal of the movable reflector.

11. A system according to Claim 10 wherein said sensors are mounted directly on the movable plate reflector.

12. A system according to either of Claims 10 and 11 as dependent upon Claim 8 wherein said inner motor is arranged to be controlled by signals from one said angular sensor and the or each said outer motor is arranged to be controlled by signals from the other said angular sensor.

13. A system according to any of Claims 10, 11 and 12 wherein said motors are directly coupled electrical torque motors and are driven by amplifiers which are in turn controlled by the output signals of said angular sensors.

14. A system according to any of Claims 10 to 13 wherein said inertial angular sensors are gyroscopes.

15. A system according to Claim 14 wherein said gyroscopes are rate-integrating gyroscopes and are controlled by torque input coils and said amplifiers are current-controlling amplifiers, so as to form a damped Type 2 servoloop.

16. An arrangement comprising a system according to any of Claims 10 to 15 when mounted on said dynamic platform, wherein said sensors are controlled rateintegrating gyroscopes, said arrangement further comprising means for determining the attitude of the movable reflector with respect to the dynamic platform and gyroscope control means for precessing said gyroscopes so as to cause them to control said drive means in dependence upon reflector plate angle rate demand signals fed to said gyroscope control means and thereby compensate for components of rotation about platform axes perpendicular to said electromagnetic radiation path.

17. An arrangement according to Claim 16 wherein said reflector plate angle rate demand signals are generated in use in response to a target-tracking algorithm so as to track a target independently of the motion of said dynamic platform.

18. An arrangement according to Claim 16 wherein said reflector plate angle rate demand signals are generated in use in response to a pointing or scanning algorithm so as to point the beam in any desired direction independently of the motion of said dynamic platform.

19. A transmitting system according to any of Claims 1 to 16 or a transmitting arrangement according to Claim 17 or Claim 18 which is centre-fed by a waveguide which extends through a hole at a virtual centre of the movable reflector and defines said electromagnetic radiation path.

20. A radar system according to any of Claims 1 to 16, 21, A radar system according to any of Claims 2 to 16 mounted in a combat aircraft wherein said electromagnetic radiation path is parallel to the longitudinal axis of the aircraft.

22. A radar system according to Claim 21 wherein said movable reflector is a polarisation-twisting reflector, said system further comprising a polarised reflector facing said movable plate reflector.

23. An aerial system substantially as described hereinabove with reference to Figures 2, 3 and 4 of the accompanying drawings.