Ship-borne Gravity Stabilized Antenna

Abstract

A gimballed-platform-mounted antenna arrangement for semi-stabilizing the orientation of the antenna of said arrangement on a ship, said arrangement being semi-stabilized in use by means of its having a long natural period of oscillation and a high moment of inertia as hereinbefore defined.

Description

This invention relates to an antenna arrangement for a ship and in particular to means for maintaining the orientation of the antenna within predetermined limits.

A ship-borne antenna system for a maritime satellite communications service should have a high gain in order to minimise the satellite power requirements (and hence satellite costs) for the satellite-to-ship communications link. However, as the gain increases, the beamwidth narrows and the allowable limits of the antenna orientation converge. Any ship at sea will, in anything other than a flat calm, roll and pitch to some extent and clearly if the degrees of roll and pitch are greater than the beamwidth of the antenna system the antenna orientation will need to be controlled if an adequate communications capability is to be maintained.

For a single helix antenna, the beam-edge gain relative to isotropic is 6 dB, and the corresponding figures for a 1 m diameter dish and a 21/4 m diameter dish are 18 dB and 26 dB respectively. The beamwidths (to the 3 dB points) of these antennae are 64.degree., 15.degree. and 6.degree. respectively. Trawlers are known to roll to more than .+-. 35.degree. and hence some control on the antenna orientation would be required even when using a modest 6 dB gain single helix antenna.

It is known to stabilize an antenna platform by means of gyroscopes and servo-systems but these are expensive and achieve stabilities of the order of .+-. 1/2.degree., which is much more stable than is required even for a 21/4 m diameter dish antenna, which would be impractically large for many ships. It is also known to provide a low gain antenna without any form of stabilisation, but as mentioned above this leads to higher satellite costs. The present invention seeks a compromise solution whereby a moderately high gain antenna may be controlled, in almost all sea conditions, to within about .+-. 5.degree. of its desired orientation. An antenna thus controlled is referred to hereinafter as being "semi-stabilized"; an antenna controlled, e.g. gyroscopically, to within about .+-. 1/2.degree. is herein said to be staiblized.

Broadly considered, the invention provides a gimballed-platform-mounted antenna arrangement for semi-stabilizing the orientation of the antenna of said arrangement on a ship, said arrangement being semi-stabilized in use by means of its having a long natural period of oscillation and a high moment of inertia as hereinbefore defined.

A "long period of oscillation" is hereby defined as a period of oscillation longer than the longest significant period of the periodic components of motion the ship in the sea. A "high moment of inertia" is hereby defined as a moment of inertia sufficiently large that the amplitude of angular periodic motion induced in the antenna arrangement by translational periodic motion of the ship in the sea is at most of comparable magnitude to the amplitude of angular periodic motion induced in the antenna arrangement by angular periodic motion of the ship in the sea.

The natural period of oscillation may be at least two, and preferably two and a half, times the longest significant period of oscillation of the ship in the sea so that the degree of coupling from the motion of the ship to that of the arrangement will be reduced.

In the preferred form, the invention comprises a structure of mass M kg including a directional receiving antenna, said structure being pendulously mounted upon the ship by bearing means having two axes intersecting at a pivotal centre, the arrangement being such that about each axis the total Coulomb friction torque T Nm of the bearing means is related to the moment of inertia I kgm.sup.2 of the structure by the expression T/I .ltoreq. 7.5 .times. 10.sup..sup.-4 N/kgm and the center of gravity of the structure is displaced below the pivotal center by a distance L, where L .ltoreq. 2 .times. 10.sup..sup.-3 I/M m.

The structure may include means for directing the antenna to a predetermined orientation, the directing means preferably including azimuth directing means for directing the antenna in azimuth and elevation directing means for directing the antenna in elevation. The azimuth directing means can, in use, be operatively associated with a compass of the ship so that the azimuthal setting of the antenna is maintained, thus to compensate for yaw and changes of course of the ship.

The invention will now be described by way of example only with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of the phase relationship between transverse, translational, and angular periodic motions of a ship and angular periodic motion induced thereby in a pendulous structure mounted on the ship;

FIG. 2 is a diagrammatic representation of the relationship between antenna beamwidth and required stability for a pendulously mounted antenna;

FIG. 3 is a schematic representation in front elevation of an antenna arrangement according to the invention;

FIG. 4 is a partial side elevation corresponding to FIG. 3;

FIG. 5 shows a tested gimballed platform assembly for an antenna arrangement according to the invention; and

FIG. 6 is a partial cross-section through the assembly of FIG. 5.

At sea, in anything other than a flat calm, a ship undergoes a fluctuation which can be resolved into periodic components of motion. The significant fluctuations, at least as regards the present invention, are known as pitch, surge, roll and sway. Pitch is an angular fluctuation about a beam-wise axis. Surge is a longitudinal translational fluctuation, that is to say movement along a line ahead/astern. Roll is an angular fluctuation about a longitudinal axis and sway is a beam-wise translational fluctuation.

Referring to FIG. 1, the ship as indicated at 10 is at one extreme of its sway movement, i.e., it is at its maximum displacement from its general line of progress in the plane indicated at 11. At this point, its acceleration is in the direction of arrow 12. Sway, which arises when the ship is in a beam-sea with the waves meeting it side-on, is caused partly by the tendency of the ship to slide down the wave and partly by the circular motion of the water in the wave, and occurs in association with rolling of the ship so that by the time the ship passes through the plane 11, it has taken up an attitude as indicated at 13. By the time the ship has reached its other extremity of sway as indicated at 14, it is again substantially vertical. At this other extremity it is subject to an acceleration in the direction of arrow 15.

When the ship is in the position indicated at 10, its roll is in the direction of arrow 16. In position 14, its roll is in the direction of arrow 17.

Any pendulously mounted structure in or on the ship will be subjected to torques by the fluctuating motions of the ship, and from FIG. 1 it is clear that both roll and sway will simultaneously give torques in the same direction, i.e., with the ship in position 10, the torques due to both sway and roll will be in the direction of arrow 18, and in position 14 the torques will both be in the direction of arrow 19. The roll and sway of the ship will thus combine to excite the pendulous structure into fluctuant motion.

Further, unless the pendulous structure is pivotally mounted about the roll-centre of the ship, that is to say the line about which the ship rolls, the roll of the ship will cause the pendulous structure to have an additional beam-wise translational fluctuation.

The amplitude of fluctuations induced in the pendulous structure by the sway of the ship can be reduced by providing a frictional damping torque at its pivot. This would, however, increase the coupling between angular movement of the pendulous structure and angular movement of the ship so that the amplitude of fluctuation induced in the pendulous structure by the roll of the ship would be increased. It is therefore necessary to seek some means, other than controlling the pivot friction, which will limit the sway-induced fluctuation without simultaneously increasing the roll-induced fluctuation. In the present invention, this is accomplished by arranging the pendulous structure of the gimballed-platform-mounted antenna arrangement to have a high moment of inertia and a long period of oscillation as hereinbefore defined.

The relationship between pitch and surge is similar to that between roll and sway described above and a consideration of the relationship leads to a similar conclusion, namely that the pendulous structure must have a high moment of inertia and a long period of oscillation if the amplitudes of fluctuations induced therein by the motion of the ship are to be limited.

The required limitation on the amplitude of fluctuation of a gimballed-platform-mounted antenna arrangement can be determined by considering FIG. 2. In FIG. 2, the line 20 represents the actual orientation of a communications satellite relative to the ship and the line 21 represents the orientation of the antenna, at one extreme limit of its angular oscillation. The angle A is the allowable angular stability of the antenna, the angle B is the beamwidth of the antenna to the 3 dB points and angle C is an angular tolerance obviating the need for continuous resetting of the antenna pointing direction. A fast ship steaming on a great circle route can travel every hour through a distance large enough for the orientation of a geo-stationary satellite relative to the ship to change by as much as 1.degree. per hour. It is assumed that it is reasonable to require that the direction of the antenna be re-set every 5 hours and thus an angular tolerance of 5.degree. must be included in estimating the required stability of the antenna. In other words, it is assumed that the angle C equals 5.degree..

From the geometry of FIG. 2: A = (B-5)/2.degree..

As stated previously, the beamwidth of a 1 m diameter dish antenna is 15.degree. and it follows from this that the required stability for such an antenna is .+-. 5.degree..

It can be shown by means of a computer simulation that this degree of stability can be achieved with an antenna orientation arrangement in the form of a pendulous structure having the following parameters:

Total Moment of Inertia I = 2,000 kg m.sup.2,

Mass of pendulous structure .times. Distance of center of gravity of the structure from the pivot, ML = 4 kg m,

Total Coulomb friction torque between the ship and the antenna platform system, T = 1.5 Nm.

An antenna arrangement with parameters of these values will achieve the desired stability of .+-. 5.degree. when borne on a cargo ship of 14,000 tons dead weight, beam-on to the prevailing sea in a Beaufort 9 wind.

A system with parameters of these values will also attain the desired stability when mounted on a trawler in similar sea and wind conditions. It is expected that the desired stability could be maintained under the above described adverse conditions, for more than 99 percent of the time.

Because the parameters ML, I and T occur in the equations of motion only as ratios, these parameters can be converted into a further pair of parameters T/I and ML/I for which the values should be equal to the ratios of the base parameters given above.

That is, T/I = 1.5/2000 = 7.5 .times. 10.sup..sup.-4 N/kg m and ML/I = 4/2000 = 2 .times. 10.sup..sup.-3 m.sup..sup.-1

The values of these parameters are the limiting values for a system which will attain the desired stability.

The parameter ML/I is easily arranged to have the correct value since the parameter L, being simply the distance between the center of oscillation and the center of gravity of the system, may be varied at will. The parameter T/I is therefore the critical factor. A needle roller bearing of nominal diameter about 4 cm will give a value of T/I equal to the limiting value above and since the loads imposed on the bearing by a system with parameters of the above values can be adequately supported by a needle roller bearing having a diameter of only about 0.5 centimeters, it follows that one can arrive at a suitable design by using needle roller bearings of diameters between these limits.

Referring now to FIGS. 3 and 4, these show schematically an antenna semi-stabilized orientation arrangement having parameters which satisfy the foregoing requirements. The antenna structure, indicated generally at 23, is in the form of a 1 m diameter dish 24 pivotally mounted at 25 at the upper end of an up-standing arm 26. Elevation directing means in the form of a remotely controlled electric motor indicated at 35a is provided for rotating the dish 24 about the pivot 25 to enable its elevation to be set and re-set. A support 27 is rigidly secured to the lower end of the arm 26.

The antenna assembly comprising the dish 24, arm 26 support 27 and motor 35a is mounted upon a platform 28 and azimuth directing means in the form of a remotely controlled electric motor 35b is provided for rotating the antenna assembly about an axis normal to the interface between the support 27 and the platform 28.

As will be apparent from the following description, the interface between the support 27 and the platform 28 will be substantially horizontal so that the azimuth directing means will be operable to rotate the antenna assembly about a generally vertical axis. The azimuth directing means is coupled to the ship's compass and thereby controls the bearing of the antenna to compensate for yaw and changes of course.

The platform 28 is gimbally mounted by means of a universal joint assembly 29 upon some rigid part of the structure of the ship indicated at 30. The part 30 may be a mast or may be part of the bridge structure. The universal joint assembly 29 has a pair of orthogonal axes arrange to lie respectively parallel to the major horizontal axes of the ship, i.e., the longitudinal (ahead/astern) axis and the beam-wise axis. However it will be appreciated that a gimbal mounting such as a universal joint may be regarded as having axes intersecting at a "pivotal center," by which is meant a point about which a structure mounted on the joint is constrained to rotate.

Extending outwards from the platform 28 are four limbs 31 arranged in two orthogonal planes.

A weight 32 is secured to the outer end of each limb 31. The four weights 32 are of substantially equal mass and serve a two-fold purpose: they are of sufficient mass and suitably disposed to give the arrangement a pendulous structure pivotally mounted on the universal joint assembly 29 and having a long period of oscillation; and their masses and their distances from the pivotal centre of the universal joint assembly 29 are such that the arrangement has a high moment of inertia. The dish 24 is substantially counter-balanced by a counter-balance mass 33 so that changes in the elevation of the dish will not significantly alter the moment of inertia of the arrangement or the centre of gravity of the structure.

Each of the weights 32 is of mass 25 kg and they are arranged so that the centre of gravity of the arrangement lies approximately three-fourths mm below its pivotal center. Such an arrangement has a moment of inertia of approximately 30 kg m.sup.2 and a period of oscillation of approximately 45 s.

The arrangement is covered by a radome indicated at 34 which prevents wind forces disturbing the setting of the antenna and protects the arrangement from the maritime environment. The radome may be constructed of any material which is structurally adequate and transparent to electromagnetic radiation of the working frequency, which in the present case is about 1.6 GHz.

The described arrangement is such that the antenna may be stabilized to within .+-. 5.degree. for ships ranging from trawlers to tankers in virtually in all sea conditions. This degree of stability would enable the ship to use an antenna as large as a 1 m diameter dish, which is considered to be a sensible upper size limit for antenna to be fitted in a suitable position to the majority of ocean-going vessels. Even if the full theoretical stability cannot be realized in practice, stabilization to within about .+-. 71/2.degree. should be achievable, and this would allow the use of a quad helix antenna having a beam-edge gain of 16 dB. Use of a quad helix antenna would have the advantage that the size of the radome, and hence the cost of the assembly, can be reduced. If the ship is subject to periodic motions other than the significant motions referred to hereinbefore, it is considered that the period of such motions will either be so short (e.g., vibration from the ship's engines) as to have an insignificant effect upon the stability of the arrangement or so long (e.g., tidal movements of the sea) as to feed energy into the system at a rate which can be dissipated by the pivot friction. It will also be noted that the moment of inertia of the structure is sufficiently large for the effect of, for instance, variations in the bearing friction and the resilience of the motor leads 36 to be negligible.

FIG. 5 shows a gimballed platform assembly which has been tested. The platform comprises a bearing housing 37 and four weighted arms 38 extending therefrom. The upper face of the housing 37 is adapted to receive and carry a support such as that indicated at 27 in FIGS. 3 and 4. A weight 39 is screwed on to the end of each arm 38 and is locked in a desired position thereon by means of a lock nut 40. Each weight 39 is lead-filled and has a mass of approximately 30 kg. The arms 38 are arranged in two coaxial, mutually orthogonal pairs and the weights 39 of each pair are about 1.4 m apart.

The platform is supported by means of a universal joint within the housing 37 upon a stanchion 11 which is adapted to be rigidly secured to some part of the superstructure of a ship. One link of the joint is secured to the upper end of the stanchion 11 and the other is secured against the underside of a spacer 42 shown in FIG. 6. It will be observed that spacers of different vertical dimension can be employed to vary the position of the platform and antenna assembly vertically relative to the pivotal center of the universal joint, so that the arrangement is adjustable to cater for antennae of differing size and form or, where desired, to balance the platform alone. As tested, a spacer was used which located the center of gravity of the assembly 0.75 mm below the pivotal center of the universal joint. The joint employed was a proprietary item having needle roller bearings of about 11 mm nominal diameter. The moment of inertia of the tested assembly was a little over 29 kg m.sup.2.

In the tests the stanchion 11 was secured on the flying bridge of an unstabilized ship of approximately 1,450 tons deadweight fitted with a gyro-horizon device giving an accurate reference to a true horizontal plane. The platform was instrumented using low-friction angular position transducers and the outputs therefrom were arranged to be combined with the outputs of the gyro-horizon device to yield information on the instantaneous roll and pitch of the platform relative to the true horizontal plane. The instantaneous angles of roll and pitch of the platform were subsequently combined vectorially to give the absolute angle or "tilt" of the platform at each instant. The tilt angles were subjected to statistical analysis and a histogram was plotted.

The histogram shows that the platform tilt angle exceeded .+-. 5.degree. for only 0.15 percent of the time during which results were recorded, although it should be noted that this is, of course, strongly dependent upon the temporal variation of the sea conditions during that time. A truer indication of the stabilizing effect is given by the fact that, in pitch, the ship exceeded .+-. 5.degree. for 0.13 percent of the time while the pitch (that is, fore and aft) angle of the platform exceeded .+-. 5.degree. for only 0.05 percent of the time. Further, whilst the pitch angle of the ship exceeded .+-. 10.degree. for 0.015 percent of the time, the pitch angle of the platform exceeded .+-. 10.degree. for only 0.001 percent of the time. It is apparent that the platform was markedly more stable than the ship.

A subsequent examination of the tested assembly showed that the friction in the bearings of the universal joint was greater than predicted. Better quality bearings would, it is believed, have improved the performance of the assembly. A further improvement is anticipated from the employment of a universal joint of the constant velocity type. The joint used in the tests was of the form known as a Hooke's joint and thus was not of the constant velocity type: that is to say rotational movement, about its axis, of one link of the joint was not continuously equally transformed into rotational movement of the other link about its axis when the axes of the links were mutually inclined. Thus there would be a variation in angular velocity of the upper link (that is, the link secured to the platform) about its axis and consequently a couple about that axis, and the gyroscopic effect would then manifest itself as movement around another axis and hence increased tilt of the platform. It will be appreciated that the employment of a constant velocity joint will overcome this problem. It was also found that the friction about the two bearing axes of the joint differed and the platform assumed a small though substantially constant tilt or list. An antenna on the platform may be accurately set despite this list but it is preferred, of course, that the assembly be made symmetrical with substantially the same total Coulomb friction torque about each of the two bearing axes.

Claims

What is claimed is:

1. A system for semi-stabilizing orientation of a directional receiving antenna of an antenna arrangement of a ship, wherein said system comprises: a structure having a mass of M kg and including said antenna balanced about a point fixed with respect to said structure; bearing means which define axes of rotation for said structure, which axes intersect at a pivotal center; and a radome covering said structure; said structure being, in use, gimbally mounted pendulously upon said ship by said bearing means with the center of gravity of said structure located below said pivotal center and spaced therefrom by a distance which, in meters, does not exceed 0.002 times the minimum value of the quotient I/M, where Ikg m.sup.2 is the moment of inertia of said structure about any of said axes, and said bearing means has a maximum total Coulomb friction torque about any of said axes which, in Newton-meters, is not greater than 0.00075 .times. I.

2. An antenna arrangement according to claim 1 wherein said bearing means comprises a universal joint.

3. An antenna arrangement according to claim 2 wherein said universal joint is of the constant velocity type.

4. An antenna arrangement according to claim 1 including means for adjusting the location of said center of gravity relative to said pivotal centre.

5. An antenna arrangement according to claim 1 wherein said total Coulomb friction torque is of substantially the same magnitude about all of said axes.