EP0142397B1 - Antenna stabilisation and aiming device, especially on a ship - Google Patents

Description

The invention relates to the stabilization and pointing of antennas, and in particular telecommunication antennas by means of satellites mounted on ships, to which the sea imposes angular movements of large amplitude compared to the acceptable tolerance on pointing of the antenna and accelerations.

It should be remembered in this connection that the different models of antennas recommended for use by international telecommunication organizations have very varied characteristics, with regard to mass and inertia on the one hand, and on the other hand pointing accuracy required. The antenna stabilization and pointing device must in all cases take account of the characteristics specific to the chosen antenna.

Many solutions have been proposed for a long time to the problem of stabilization and pointing of organs carried by a ship. Among these solutions, some (for example those adopted for telepointers and guns on warships) are very complex and require to have heading and vertical references. They cannot be transposed to merchant ships due to their high cost and the absence of a vertical reference, the gyrocompass of a merchant ship generally only providing a heading reference.

In the recent past, however, antenna stabilization devices have been proposed specifically for use in maritime satellite communications. Among these, we can cite the one described in the communication from MB Johnson entitled "Antenna control for a ship terminal for MARtSAT _" (IEEE Conference Publication N. 160, March 7.9, 1978) which is of the type comprising, on a base, a frame provided with means of orientation in bearing and carrying a gyroscopic assembly with two degrees of freedom whose external gimbal has an axis of rotation (axis X) perpendicular to the axis of bearing and whose internal gimbal has an axis of rotation (Y axis) is orthogonal to the X axis and which is linked in pointing to the antenna.

The device described in this article, the type of which is commonly designated "XY-deposit", uses for stabilization two gyros mounted on the rear of the antenna, intended respectively to stabilize the X and Y axes. But this device requires a vertical reference for the X axis, obtained using an accelerometer or inclinometer mounted on the bearing axis. The voltage from the accelerometer or inclinometer is subtracted from the measurement of the elevation orientation of the X axis. The elevation angle can only be obtained by filtering at great time constant.

We see that these various features make the device unsatisfactory for use on small tonnage merchant ships, whose equipment must remain economical.

Four-axis mounts have also been proposed, comprising a platform stabilized around roll and pitch axes by a pendulum assembly and two steering wheels. The pointing device is then separate. It is carried by the platform and allows the orientation of the antenna around the conventional pointing axes in azimuth and elevation. Such a provision is obviously extremely complex. Yet another arrangement uses a three-axis mount of the "X, Y" type, but with two flywheels each having its own gimbal, which considerably increases the cost and size.

The invention aims to provide a device of the "deposit, X, Y" type which, while remaining simple and economical, makes it possible to provide the pointing and stabilization required for the antennas whose mass and inertia are those commonly used . For this, the invention uses, to ensure stabilization and pointing, a single steering wheel under conditions such as nutation which appears in response to the applied torques and the movements of precision which result therefrom to orient the antenna. by a parasitic movement in the range of acceptable tolerances.

More specifically, the invention provides a stabilization and pointing device of the type defined above, characterized in that the gyroscopic assembly comprises a single flywheel with a significant angular moment relative to the inertia of the antenna, in that each gimbal is provided with a torque orientation motor controlled by a loop, the reaction signal of which is supplied by an orientation sensor of the other gimbal, and in that the orientation means around the bearing axis are provided to approximately ensure the average pointing of the antenna in bearing and, consequently, to maintain the gyroscopic assembly near the canonical position.

In general, and unless the angular momentum of the flywheel is very high compared to the moments of inertia around the cardan axes, each of the servo loops will include means for filtering characteristics determined according to the inertias of the cardan joints, parameters angular movements applied to the base and the required pointing precision. These filtering means may in particular be constituted by phase delay networks having a time constant much greater than the period of the applied stresses, in particular during the swell period.

The orientation means around the bearing axis may include a geared motor for rotating, advantageously by an irreversible link, and a control circuit according to the heading and the displayed value of the azimuth of the satellite, while that the loop associated with the internal gimbal receives a correction signal taking account of variations in bearing, the difference Gis and y being measured by the angular detector 40. The enslavement of y therefore obliges it to follow the direction of bearing and to keep the canonical position.

In practice, the device will generally include a computer for developing an elevation signal, applied to the control loop. ment of the first gimbal, and an azimuth signal, applied to the bearing orientation control circuit, from the heading and the longitude and latitude of the vehicle (ship in general) carrying the antenna. Automatic tracking is then ensured by sending correction signals for Ax and Ay deviations which are superimposed on the calculated information, azimuth and elevation, to cancel all errors, including the heeling error. This allows, in case of loss of the reception signal, for example by mask effect, or fading, to keep the calculated direction, very close to the direction of the satellite. This avoids the panic of the antenna direction which would work in open loop. A more rudimentary solution simply comprises means for displaying the azimuth and the elevation determined using a separate calculator, which can be extremely simple since it only has to perform common trigonometric calculations.

In an alternative embodiment, the antenna, of revolution, is not only linked in pointing to the steering wheel, but also integral with the steering wheel or substituted for it, so that its angular momentum contributes to stabilization or ensures it.

Finally, it should be noted that the device proposed by the invention lends itself to extremely varied configurations, in particular to take account of the type of antenna used (parabolic antenna, antenna with four helices, etc.) and that in particular it does not 'is by no means essential that the X and Y axes are concurrent.

• - la figure 1 est un schéma de principe montrant les composants essentiels d'un dispositif de stabilisation suivant un mode particulier d'exécution, destiné à la stabilisation et au pointage d'une antenne sur navire,
• - la figure 2 est un schéma de principe des circuits d'asservissement du dispositif de la figure 1,
• - la figure 3, similaire à une fraction de la figure 2, montre une variante de réalisation simplifiée,
• - les figures 4 et 5 montrent deux dispositions mécaniques des éléments mécaniques du dispositif suivant l'invention, en coupe suivant un plan de symétrie,
• - la figure 6 montre une autre variante encore de réalisation de l'invention, dans laquelle le volant de stabilisation est constitué par l'antenne entraînée en rotation autour de son axe de visée radioélectrique.

The invention will be better understood on reading the following description of particular embodiments, given by way of nonlimiting examples. The description refers to the accompanying drawings, in which:

• FIG. 1 is a block diagram showing the essential components of a stabilization device according to a particular embodiment, intended for the stabilization and pointing of an antenna on a ship,
• FIG. 2 is a block diagram of the control circuits of the device in FIG. 1,
• FIG. 3, similar to a fraction of FIG. 2, shows a simplified variant embodiment,
• FIGS. 4 and 5 show two mechanical arrangements of the mechanical elements of the device according to the invention, in section along a plane of symmetry,
• - Figure 6 shows yet another alternative embodiment of the invention, in which the stabilization wheel is constituted by the antenna driven in rotation around its radioelectric sighting axis.

The slaving and pointing device of a helical antenna 10 with a viewing axis Z shown diagrammatically in FIG. 1 is intended to equip a ship 12 provided with a gyrocompass 14 providing a heading reference (angle 0 between the line faith of the ship and the geographic North) on an exit 16. The device comprises a mount of the type known as "deposit-XY". This mount comprises a base 18 fixed to the ship and carrying bearings or pivots defining an axis of bearing G around which a crew 22 whose rotation is given by the signal can rotate under the action of a bearing geared motor 20 output of a field detector 24. The crew 22 is integral with the housing of a gyroscopic system and therefore carries in turn, by means of bearings 26 defining an axis X (elevation axis), perpendicular to the bearing axis G, an external universal joint 28 provided with a torque motor 30 and an orientation detector 32. The external universal joint carries in turn, by bearings 34 defining an axis Y orthogonal to the axis X , an internal gimbal 36 provided with a torque motor 38 and an orientation detector 40. The antenna 10 is, in the embodiment shown in FIG. 1, fixed to the internal dial 36.

In this internal gimbal 36 turns a gyroscopic flywheel driven at constant speed ω by a motor not shown around the sighting axis Z so as to present a angular momentum H which we will see later that it must have a minimum value which is a function of antenna inertia and required stabilization accuracy.

The steering wheel 41 and the antenna 10 are arranged so that the gimbals are in static balance.

At this stage of the description, it may be useful to recall some indications on the properties of a free gyroscope with two degrees of freedom, such as that constituted by the steering wheel 41 and the cardan shafts which carry it.

We know that the direction of the angular momentum H can occupy any direction in space and remains in indifferent equilibrium whatever the accelerations undergone, if we disregard the friction couples in the bearings. The sum of the external couples is zero and the direction of the angular momentum H remains fixed in absolute space.

This property remains however only on condition that the displacements do not bring the axis X parallel to e because one then loses a degree of freedom in this configuration known as “prohibited”.

In the case of a direct assembly with two degrees of freedom on a ship which can take any route, we see that, when H is horizontal, one could arrive in the configuration prohibited by gyration of the ship around its yaw axis. This situation is avoided, in the case of the invention, by orientation of the moving assembly around the bearing axis G in order to give approximately to the crew 22 the pointing in elevation for which the universal joints are in canonical position ( X, Y and Z axes defining a triangular trihedron) when the ship is in its normal attitude.

• - de la sortie 16 du gyrocompas 14 indiquant le cap 0 du navire,
• - d'une entrée 44 d'affichage du gisement dans l'attitude normale du navire,
• - du détecteur de gisement 24, qui fournit un signal de réaction.

For this, the gearmotor 20 is controlled by a pointing loop which includes an adder circuit 42 intended to combine the signals received:

• - from the exit 16 of the gyrocompass 14 indicating the heading 0 of the ship,
• an input 44 for displaying the deposit in the normal attitude of the ship,
• - the deposit detector 24, which provides a reaction signal.

The signal produced by the adder 42 is brought by an amplifier 46 to a level sufficient to actuate the gearmotor 20.

It should be noted in passing that the geared motor 20 advantageously has a reduction ratio sufficient to be irreversible. Under these conditions, the torques that the horizontal accelerations imposed on the ship can create have no effect on the orientation around the bearing axis G.

We have seen above that the pointing is carried out using the precession of the steering wheel 41 of the gyroscopic assembly.

donc une variation de l'angle de pointage en élévation ϕ par rapport à l'horizontale, c'est-à-dire une rotation autour de l'axe X.It should be remembered on this subject that the application by the motor 38 of a torque Ci on the internal gimbal of a gyroscope with two degrees of freedom causes a precession of the external gimbal around the axis X at speed w _e :

therefore a variation of the pointing angle in elevation ϕ relative to the horizontal, that is to say a rotation around the axis X.

• - une composante normale au plan du cardan extérieur 28, absorbée par les paliers 26,
• - une composante normale au plan du cardan intérieur, égale à C_e/Sin cp, qui provoque la précession du cardan interne à vitesse ω_i et qui est équilibrée par le couple gyroscopique Pω_i.

The effect of an engine torque This applied by the engine is broken down into two actions:

• a component normal to the plane of the outer gimbal 28, absorbed by the bearings 26,
• - a component normal to the plane of the internal gimbal, equal to C _e / Sin cp, which causes the precession of the internal gimbal at speed ω _i and which is balanced by the gyroscopic couple Pω _i .

We can summarize this reminder by saying that the application of a torque on one of the universal joints changes the direction of the other universal joint by precession, so that we can point the direction of the angular moment H in any given direction by applying a torque to either of the universal joints.

The foregoing explanations, however, assume that the gimbals are perfectly balanced and that the steering wheel remains perfectly fixed in space. In reality, it is not possible to completely cancel unbalances in all positions and avoid the effects of anisoelasticity. This results in a drift or a score difference which must be periodically caught up.

• - une variation de l'angle entre la direction du cardan interne et X, avec une amplitude (β max = Ce l₁/H² (1₁ désignant l'inertie du cardan interne 36 autour de son axe de rotation Y).
• - un mouvement de nutation du cardan interne, avec une pulsation ω₀ et une amplitude maximum C_e/Hco_o.

Furthermore, the above indications assume the established precession movement. But there is a transitional phase between the application of the torque and the appearance of an angular speed of precession. The calculation shows that, when applying a torque, a periodic nutation movement occurs at a pulse puls ₀ . If, for example, an instantaneous torque Ce is applied to the external universal joint, it is superimposed on the precession movement:

• a variation of the angle between the direction of the internal gimbal and X, with an amplitude (β max = Ce l ₁ / H ² (1 ₁ designating the inertia of the internal gimbal 36 around its axis of rotation Y).
• - a nutation movement of the internal gimbal, with a pulsation ω ₀ and a maximum amplitude C _e / Hco _o .

We see that, to limit the amplitude of nutation, it will be necessary to limit the value of the pairs Ce and Ci to a low value, which will imply a low pointing speed (of the order of a few degrees per second in the practical) and give to the angular moment H as high a value as possible.

In general, the telecommunication antenna of a ship is mounted in the superstructures, to have a clear field of view. It is for example at the top of the mast. The frame is therefore subjected not only to the angular movements of roll, pitch and yaw, but also to periodic accelerations of lifting, swerving and horizontal acceleration. In practice, the amplitude in roll and pitch can be up to ^± 30 °.

These conditions of use having been defined, we will now examine how stabilization and pointing are carried out and what are the conditions to be fulfilled to obtain the required precision.

Stabilization:

Stabilization of the antenna is ensured passively by the gyroscopic stiffness of the steering wheel 41. If the gimbals are balanced, that is to say that the center of gravity of each rotating assembly is on its axis, the accelerations and movements angular causes no torque and only remains a residual periodic precession of zero mean value over a sufficiently long time before the period of roll and pitch. This precession, constituting pointing error, retains a very low value if the angular momentum H is large enough. In practice, the requested precession not exceeding a few degrees, this oscillation is not a problem.

However, it should be noted that the angular detectors 32 and 40 measure the movements of the gimbals involved in stabilization while the housing is subjected to a roll and a pitch which can reach ^± 30 °. To avoid the appearance of a periodic parasitic precession caused by the actuation of the motors 30 and 38, it is necessary to filter the output signal of the detectors 32 and 40, unless the time constant of the gyroscopic system is sufficiently large so that the parasitic precession remains lower than the requested precession. In the case shown in FIG. 2, each detector 32 or 40 is followed by a filter constituted by a phase delay network 48 or 50, which can have a time constant of the order of 1 min. Thus, only the component representing the average elevation angle is left in the output signal of the angular detector at X 32, while excluding the components due to the roll and pitch of the detector output signal. However, there may still be a heeling error corrected by the automatic tracking operation.

As for stabilization around the bearing axis in the event of yaw movement or gyration of the ship, it is ensured in response to changes in the signal emitted by the gyrocompass and representing the ship's heading 0.

Score:

The aim of the antenna is to keep it directed towards the satellite and must therefore intervene each time the direction of the satellite changes relative to the ship, which occurs following a change in the position of the vessel and / or course change.

The direction of the satellite is generally defined by its azimuth and its elevation. The azimuth Az is the angle in the horizontal plane between the direction of the satellite and the geographic North. The elevation El is the angle formed in the vertical plane by the direction of the satellite and the horizontal. These two angles are a function of the longitude Lo and the latitude La of the ship. The embodiment of FIG. 2 comprises a computer 52 for developing the azimuth and elevation angles Az and El of the satellite as a function of data stored on the position of the satellite, generally geostationary, and of input data constituted by heading 0 from gyrocompass 14 and by longitude and latitude, entered by display. The elaboration of Az and El requires only classical trigonometric calculations which it is not necessary to describe here.

The output signal Az, constituted for example by a voltage proportional to the azimuth angle, is applied to the adder 42 which also receives the reaction signal from the detector 24. The resulting error signal is sent to the amplifier 46 by means of a phase advance correction network 54 which makes it possible to improve to a certain extent the performances of the servo in field.

In practice, the detector 24 may consist of a multiturn potentiometer coupled, by a reduction gear, to a toothed wheel 56 secured to the crew 22 and meshed by the output pinion of the gearmotor 20.

There is obviously a time difference between two successive displays of the ship's position (longitude and latitude). The progress of the ship therefore makes an increasing error between the displayed position and the actual position. In the embodiment shown in FIG. 2, this error is corrected by automatic tracking means which include a devometer 58 which provides output voltages AX and AY corresponding respectively to the correction of the error in elevation and to the correction of the azimuth error. The control loop of the torque motor 38 of the internal gimbal then includes an analog adder 60 which receives the signals El and ΔX, as well as the filtered feedback signal from the detector 32. The output signal is amplified in a two-quadrant amplifier 62 or applied to a polarized relay to control the motor 38. Similarly, the torque motor control loop 30 comprises, in addition to the detector 40, an adder 64 and an amplifier 66. But the action of the motor 30 will not be aimed always only to give the internal gimbal 36 a slight deviation from the canonical position, the orientation in azimuth being essentially provided by the geared motor 20. During rotation, always slow, in azimuth, the detector 40 provides a signal which causes the intervention of the motor 30 and the maintenance of the pointing of the antenna 60.

The device can be supplemented by means 68 for viewing the actual values of the deposit and the elevation given to the antenna, constituted by voltmeters for displaying the output voltages of the detectors 32 and 40, possibly after filtering.

When the three control loops are thus closed, the steering wheel is fixed relative to space, that is to say to the geostationary satellite.

One can use, instead of the device of FIG. 2, a simplified and very economical version, such as that shown in FIG. 3, which no longer includes a calculator for developing the azimuth and the elevation. These values must be calculated offline, for example using a programmed calculator 70, then displayed on a desk 72 which replaces the calculator 52, the rest of the assembly being unchanged.

The aim of pointing in a field is to avoid coming into a prohibited configuration. The Y axis is almost vertical, at low elevations, that is to say in the conditions where the prohibited configuration can occur, the Y axis is almost vertical and the fixity of the steering wheel subsequently corrects the error in deposit, caused for example by errors due to the kinematics of cardan joints in heavy seas.

We will now describe the material constitutions of the mechanical parts of the device which are particularly suitable for different types of antennas, which differ in their mass, their inertia and the pointing accuracy that they require.

The mass of the antenna is not negligible and, to balance the gimbals, we will have to move the steering wheel relative to the X and Y axes, rather than adding significant additional masses which considerably increase the inertia. However, adjustable weights will generally be provided to achieve fine balancing around the X and Y axes, although a residual balancing is tolerable since all the drifts in position of the gyroscopic system are detected in the angular detectors 32 and 40 when the control loops are closed.

The inertia of the antenna acts on the stability and on the nutation frequency and any increase in this inertia forces, at given stability, to increase the angular momentum il = 1 - m of the steering wheel (1 being the moment of inertia of the steering wheel). This action will bring the antenna as close as possible to the axes of rotation X and Y to reduce the inertia. But, despite this, any increase in the dimensions of the antenna, for example to increase its directivity, must be accompanied by an increase in the angular momentum H.

This increase can be carried out by increasing the speed of the flywheel which has the advantage of bringing no additional inertia. However, in practice, at least if bearings using ball bearings are used, obtaining a satisfactory service life (approximately 50,000 h) prohibits exceeding a speed of approximately 6,000 rpm . We are therefore led to increase the dimensions of the flywheel, but the centrifugal force then constitutes a limiting factor, the circumferential speed should not in practice exceed 120 m / s.

Consequently, at least when conventional bearings are used, the device according to the invention only allows stabilizing medium-sized antennas, the diameter of which does not exceed 1 m in the case of a parabolic antenna. In the case of a flat phased array antenna, larger dimensions can be accepted due to the reduced inertia.

Of course, larger dimensions can be obtained if magnetic bearings with active suspension or hydrodynamic bearings are used which make it possible to adopt high flywheel speeds.

We will now describe, by way of example, two devices intended, one for pointing a four-helix antenna, the other for pointing a parabolic antenna.

Figure 4, where the organs corresponding to those of Figure 1 have the same reference number, shows the orientation device of an antenna 10 with four propellers while the antenna is pointed at the zenith on a ship whose roll and the pitching result in an inclination a of the radioelectric sighting axis Z on the axis G, in the plane GX. We find in Figure 3 the moving element 22, consisting of a bearing ring which rotates in bearings provided in the base 18. The ring 22 carries the dial 28 orientable around the axis X by means of a spindle 74 and bearings 26. The universal joint 36 orientable around the Y axis rotates on the universal joint 28 in bearings not visible in the figure. It can be seen that the "external" universal joint 28 is thus housed inside the "internal" universal joint 36, which simplifies mechanical manufacturing. The torque motor 30 is placed directly around the spindle 74.

To the gimbal 36 are fixed the antenna 10 and the casing 76 containing the flywheel 41 and its drive motor 78 (hysteresis motor for example). The antenna 10 and the steering wheel are placed on either side of the Y axis so as to achieve an approximate balancing, which can be perfect using an adjustable Y balancing weight, 80. A another counterweight 82, the position of which on the universal joint 36 is adjustable, ensures balancing in Y.

In this arrangement, the axes X, Y and G are concurrent, which makes it possible to give the radome 84 for protecting the antenna a value close to its theoretical minimum value.

Such a provision can be adopted for a standard antenna B of the IMMARSAT project or M5 of the PROSAT project intended to provide a gain of approximately 15 dB at 1.5 GHz and which requires a pointing accuracy of 6 °. We manage to maintain an accuracy of ^± 1.3 ° up to roll-pitch angles of: 1: 30 ° for a mount located 30 m from the roll axis, without mounting a corrective network at the exit of the angle detectors 32 and 40, with an antenna weight, with the flywheel, not exceeding 3.8 kg, the flywheel having a moment of inertia of 4.82 kg.m ² / sec. turning at 6000 rpm.

The variant embodiment shown in FIG. 5, where the members corresponding to those of FIG. 4 still bear the same reference number, is intended for pointing and stabilizing a parabolic antenna providing a gain of 20 dB at 1.5 GHz, which requires an accuracy of 2 °. The inertia of this antenna being greater than that of the antenna envisaged in connection with FIG. 3, the flywheel 41 must have 17 kg - m ² / sec. for a weight of 5.5 kg.

The arrangement shown in FIG. 5 differs essentially from that of FIG. 4 by the fact that the axes X and Y are not concurrent, which makes it possible to reduce the inertia of the assembly while maintaining the same maximum roll angle. at. If indeed the X axis had cut the Y axis at point 0 (figure 4), it would have been necessary to extend the distance OS between the Y axis and the bottom of the antenna and, therefore, to increase considerably the inertia, which increases as twice the square of this distance. In return, an additional balancing mass, which can be contained in the equipment box 86, must be placed on the underside of the external gimbal 28 to bring the center of gravity to 0. The required precision can be obtained at using a flywheel rotating at 3000 rpm and having a angular momentum of 18 k · m ² / s rotating in ball bearings under pretension.

Other embodiments of the invention are still possible and, in particular, in the case of a revolution antenna, the latter can be used as a flywheel, to complete the action of the flywheel 41 of FIG. 1 or replace it.

By way of example, FIG. 6 shows a device for stabilizing a disc parabolic antenna 10 in which this antenna, used in rotation by the motor 78 around the axis Z, is used as a stabilization wheel. In this case, it is not necessary to use a rotating contact on the electrical junctions of the antenna with the fixed parts. In FIG. 6, the pointing device is of the type shown in FIG. 3 and the same reference numbers have been used. This solution can be used for small diameter antennas. For example, it can be envisaged for a 0.85 m diameter disk antenna rotating at an angular speed of 200 rpm and having a angular momentum of 15 N.m.s.

Again, to modify the position of the Z axis, gyroscopic precession is used, a torque applied around the X axis causing an exit speed around the Y axis and vice versa. Note that, in the embodiment illustrated, the Z axis is offset from the bearing axis G, instead of being confused with it, when the antenna is aimed at the zenith.

Claims (13)

1. A device for stabilizing and aiming an antenna on a ship, comprising, on a base (18), a mounting having bearing orientational means and supporting a gyroscopic assembly with two degrees of freedom, whose outer cardan transmission (28) has an axis of rotation (axis X) perpendicular to the bearing axis, the inner cardan transmission (36) having an axis of rotation (axis Y) at right- angles to the axis X and being connected to the antenna for having the same aiming, characterized in that the gyroscopic assembly comprises a single flywheel of considerable angular momentum in relation to the inertia of the antenna (10), each cardan transmission has a torque motor (30, 38) controlled by a loop whose feedback signal is provided by sensor (40, 32) detecting the angular position of the other cardan transmission, and the means (24, 42, 46, 28) for orientation around the bearing axis are adapted to substantially provide the mean aiming of the antenna in bearing, and therefore to retain the gyroscopical assembly close to the canonical position.

2. A device according to claim 1, characterized in that each of the servocontrol loops comprises low-pass filter means (48, 50) of characteristics determined as a function of the angular momentum of the flywheel, the parameters of the angular movements applied to the base, and the required aiming accurary.

3. A device according to claim 2, characterized in that the filter means are formed by phase delay networks having a time constant which is much higher than the period of the stresses applied.

4. A device according to claim 1, 2 or 3, characterized in that the means for orientation around the bearing axis comprises a motor-down transmission unit for rotation via an irreversible connection, and a circuit for control as a function of the course and of the displayed value of the azimuth ofthe satellite, while the loop associated with the inner cardan transmission receives a correctional signal taking into account variations in bearing and the difference delivered by the error-measuring device.

5. A device according to any one of claims 1-4, characterized in that it comprises a computer (52) which works out an elevational signal, applied to the servocontrol loop of the first cardan transmission, and an azimuthal signal, applied to the circuit for control of azimuthal orientation, based on the course and the longitude and latitude of the vehicle (typically a ship) carrying the antenna.

6. A device according to claim 5, characterized in that it comprises automatic tracking mean whose error signals betwen the direction of the satellite and the direction of the antenna (Δx and Δy) are determined by an error-measuring device (58) which corrects the position of the programmed tracking given by the processor (azimuth and elevation), the signals (Δx and Δy) being sent to the servocontrol loops of the second and first cardan transmissions respectively.

7. A device according to any one of claims 1-5, characterized in that it comprises means for dispalying the azimuth and elevation determined by means of a separate computer.

8. A device according to any one of claims 1-7, characterized in that the outer cardan transmission (28) is disposed physically inside the inner cardan transmission (36).

9. A device according to any one of claims 1-8, characterized in that the antenna (10) and the flywheel (41) are disposed along the sighting axis, each on one side of the axis Y to produce approximate equilibrium.

10. A device according to any one of claims 1-9, characterized in that the cardan transmissions hace controllable balancing weights (80, 82).

11. A device according to any one of claims 1-8, characterized in that the antenna has a rotational symetry and is rigidly connected to the flywheel or is substituted to the flywheel so that its angular momentum contributes to or ensures stabilisation.

12. A device according to any one of the preceding claims, characterized in that the axes X ans Y are non-concurrent.

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