WO1986000998A1

WO1986000998A1 - Antenna tracking system using sequential lobing

Info

Publication number: WO1986000998A1
Application number: PCT/IT1985/000022
Authority: WO
Inventors: Giorgio Perrotta; Giacinto Losquadro
Original assignee: Selenia Spazio
Priority date: 1984-07-27
Filing date: 1985-07-24
Publication date: 1986-02-13
Also published as: IT1199172B; ATE90795T1; IT8448646A0; DE3587408T2; EP0197944B1; US4963890A; EP0197944A1; DE3587408D1

Abstract

System fine tracking R.F. sensor based capable of acquiring ground beacons within an angular field which is substantially larger than the -3 dB antennae beamwidth beams, to be used preferably on board artificial telecommunication satellites equipped with single or multiple beams antennae.

Description

Antenna Tracking system using sequential lobing.

The present invention regards a system, equipped with a RF sensor for the fine control of .tracking satellite antennae which has a wide angular field of acquisition. This invention fits the field of antennae, and more exactly that of highly directive antennae, preferably for satellite applications and is of further use in the field of automatic beacon, or target, tracking systems.

The problem of high directivity antennae fine pointing on board satellites has only in recent years been faced by means of RF sensors incorporated in the antennae illuminators.

The principle on which all RF sensors is based is that of a RF signal transmitted by a ground station and of the receipt on board the satellite of the beacon signal through a device (RF sensor) capable of detecting angular displacements of the beacon signal direction of arrival with respect to the sensors electrical boresight. The most serious problem to solve with these systems is that given by the high pointing accuracy required in presence of fadings in the satellite to earth path, which may reach 35 dB in communication systems which use frequency bands in the 30 GHz range. A further difficult problem to solve is the capabi¬ lity to provide the device with a non ambiguous self acqui¬ sition angle which is wide enough to enable rapid angular acquisition or re-acquisition of the beacon signal direction of arrival upon loss of tracking due to failures in the satel- lite attitude control system, which caters for coarse antenna attitude stabilization; or due to a temporary emission inter¬ ruption of the ground transmitter generating the beacon si¬ gnal; or due to unexpected manoeuvres of the satellite atti¬ tude control, which might cause unlocking of the auto-tracking system.

In fact, conventional systems are designed to enhance fine tracking capabilities, a peculiarity which is clearly incom¬ patible with the capacity to re-acquire the beacon signal over a wide angular field.

the following the problems above will be clarified by ma¬ king reference to the description of the operation of a con¬ ventional type of tracking system.

The radiofrequency sensor is a device providing at its output a measurable signal, such as a voltage, proportional to the instantaneous angular displacement between beacon direction of arrival, expressed in satellite coordinates, and the sensor's radioelectrical boresight.

The angular displacements detected by the sensor are suitably processed and used in a position servo acting on the on board antenna to reestablish the correct alignment between sensor boresight and beacon direction of arrival.

All known R.F. sensors for satellite applications are based on the well known monopulse technique, used for Radars.

A monopulse sensor is capable of providing directly at RF a reference signal, called the sum signal, and difference si¬ gnals having an amplitude which increases with the angular offset between the signal direction of arrival and the sensor boresight, electrical and a phase angle which changes sign on crossing the boresight. As it is necessary to correct antenna position along two orthogonal axes (parallel to the roll & pitch axes of the satellite), the RF sensor provides two difference signals, each related to the axis to be controlled. These two diffe¬ rence signals, RF generated, are used to phase or, preferably, amplitude modulate the sum signal which, after re-modulation, contains information relevant to the instantaneous angular di¬ splacement between, the direction of arrival of the beacon si¬ gnal and the sensor's boresight. The re-modulation technique is essential to achieve weight re- duction, a basic requirement on board satellites: in fact the technique of multiplexing on one communication channel two different information-to be processed at a later stage-requi¬ res that only one modulator - demodulator unit be used. The detection of the two components of the instantaneous angle error along the two orthogonal axes of the sensor, is usually achieved by means of a phase locked loop receiver preceded by a signal amplitude normalizer (consisting of an automatic gain control circuit) which acts on the average value of the sum signal modulated by the difference signals. The sum signal ( "%. ) , amplitude modulated by the difference signals (Δ), is coherently demodulated by mixing with the carrier, regenerated by the phase locked loop detector, and therefore clear of the amplitude modulated component. It will be easily understood how the monopulse sensor system is implicitly limited for what concerns its angle acquisition sector, and is therefore unable to operate outside a restric¬ ted angle where the difference signal provided by the RF sen¬ sor is anyway lesser in amplitude than the sum signal itself. Only within this area in fact, the ratio ι\ / may be linea- rized, in other words it is proportional to the instantaneous angular offset between the direction of arrival of the beacon signal and the sensor's electric boresight. Outside this area there are threshold problems (the sum signal being too low, in particular in the lower part of the beam lobe and in the side- lobes region) and rapid sign inversion of the angular discri¬ mination function .takes place, due to the periodic sign chan¬ ges of the phase, in the sidelobes region for both the sum beam and the difference beams. As a consequence it is impossible to utilize the monopulse sensor for angle acquisition of the beacon starting from angle offset (between beacon direction of arrival and instantaneous direction of the sensor's electrical boresight) greater than the -3 dB "sum" beamwidth. This fact is well known in Radar techniques where the target tracking with monopulse heads, can take place only following target angle designation by means of a surveillance, or acqui¬ sition, radar which in practice performs the coarse angle acquisition function of the target. In satellite techniques, the angle acquisition or reacqui- sition capacity starting from relevant offset of the beacon from the instantaneous direction of arrival of the sensor's boresight, is more important for two reasons:

1) there is nothing on board which is capable of self designation onto the initial beacon direction of arrival; 2) for operational systems which need to keep low the communication system outages, it is important to keep reac- quisition times as low as possible whenever satellite atti¬ tude manouevres, or beacon station malfunctions cause the tracking system to unlock; or have brought the satellite at- titude outside the window for which correct tracking can be assured. Here, although reinitialization of tracking may be conceived by remote control from ground, some operational factors such as: a) the time required to deliver commands to execute an angular sweep of the acquisition field; b) the resulting outages; c) the reliability of the procedure; do not render this solution very attractive.

Therefore it is highly desirable to rely on a fine tracking system, based on a RF sensor, which may acquire the ground beacon within an angle much larger then the -3 dB beamwidth of the antenna beam, and such that: a) it may reduce, or eliminate, the requirement for interven¬ tions at the satellite control station, in particular for te- lecommands; b) may be able to minimize angle acquisition/reacquisition times; c) may minimize the downtimes of the telecommunication system, of which the fine tracking antenna system is an integral part. The system, object of this invention, obviates to the draw- becks due to the limited angle acquisition field typical of radiofrequency sensors usually adopted, and is based upon: a) a lobe switching radiofrequency sensor together with a modulation tracking phase locked receiver/demodulator; b) the use of the characteristic behaviour of the side lobe pattern of the antennae normally adopted on satellites, which are reflected in some peculiar characteristics of the angle discriminator function obtained by video processing of the demodulated signal.

These characteristics are of fundamental importance to achieve angle acquisition even with significant offsets in the. direction of arrival of the beacon with respect to the sensor's electrical boresight; c) the use of on board processor which implements, strategies suitable for the processing of the characteristics of the discriminator function, including those necessary to overcome singularity points or low gradient areas of the angle discri¬ minator function as under point b). The invention well now be described in the preferred forms of its implementation, referred to as illustrations but not li¬ mited to, based of figures, schematics and drawings attached. Fig. 1: Schematic of the fine tracking control system for communication antenna where: ^• 1 is the ground beacon transmitter

2 is the lobe switching RF sensor

3 phase locked loop modulation tracking type of receiver demodulator

4 on board processor for demodulated signal processing; 5 mechanical actuator for the repositioning of the on board antenna reflector 6

6 antenna reflecting surface.

Fig. 2: Schematic of the RF sensor illuminators and of ways of effecting sequential switching of RF signal samples received by the illuminators; a) and b) are two different forms, albeit equivalent, of the same geometric configuration of the illu¬ minators in the focal plane of the antenna along the antenna movement actuation control axes U & V; and c) which shows the utilization of a switch (electronic or electromechanical) for the time multiplexing of the RF signal samples onto one single communication channel.

Fig. 3: Section in U, V plane of the radiation diagrams rela¬ ted to A & B beams received by illuminators A _ B in fig. 2a. Fig. 4: Alternative forms for the implementation of a lobe switching RF sensor using a number of illuminators even greater than four.

Fig. 5: Schematic of the demodulator indicated as 3 in Fig. 1 Here A, B, C, D are the illuminators;

7 is the wideband loop filter; 8 is the VCO

9 & 10 are multipliers;

11 is the level amplifier.

Fig. 6: Effect of the modulation tracking phase locked loop demodulation on the signal corresponding to diagrams in Figure 3.

Fig. 7a, 7b: isolevel maps of the angle discriminator function- in U, V plane-at demodulator 3 output.

Fig. 8: Relationship between angular distances related to antennae, beams associated with the elementary illuminators of the RF sensor, and the crossover point of such beams. Fig. 9: This shows the presence of singularity and stationary points in the U, V plane of the angular discriminator fun¬ ction. Fig. 10: Flow diagram of operations, which may be translated into S/ language and H/ architecture of the on board processor, required to be performed on the demodulated signal at the output of receiver/demodulator 3 during beacon acqui¬ sition. Fig. 11: Flow diagram of operations required, which may be translated into S/W language and H/W architecture, during confirmation and acquisition.

Fig. 12: Flow diagram of operations required, which may be translated into S/W language and H/W architecture, during beacon tracking. Fig. 13: Presentation of the ability of the on board processor 4^' to avoid undesiderable locking on to stationary points shown in fig. 9 and to warrant safe angle acquisition in the beacon direction even when, during acquisition, the singularity points above are encountered. Fig. 14: Map in plane V of paths and possible trajectories for beacon angle acquisition by the system at starting conditions of the acquisition phase with offset angles greater than -3 dB antenna beam width, with respect to the ground beacon di¬ rection of arrival. From what we have said and with reference to the figures, in the following the functioning of the system, object of the present invention, will be described. It must be borne in mind that the lobe switching type of sensor (known in Radar applications) is based upon an array of N elementary illuminators, the number of which is usually between 3 and 6, sequentially swept by a 1 way-N position switch.

With reference to Figure 4, two forms of implementation of the invention are shown: a) 4 + 1 illuminator sensor, where the centre illuminator E is added to each of the other peripherals A, B, C, D; b) 6 + 1 illuminator sensor where the six peripheral illumi¬ nators are added to centre illuminator G following selection made by a 6 position 1 way switch.

At the output of the one way N position switch Z, (Figure 2) or at the output of the summing circuit downstream from switch Y in fig. 4, we have a sequence of beacon RF signal samples picked up by theRF sensor's elementary illuminators. Such sequence shows a discrete amplitude envelope modulation which, approximately, represents conical scan sampling. The resulting amplitude modulation is null only when the sensor's electrical boresight coincides exactly with the beacon signal direction of arrival.

When the beacon signal is not aligned with the sensor's boresight, the N RF signal samples have different amplitudes and exactly, the amplitude of the signal will increase for that, or those, illuminators which in the focal plane of the optical system, are closer to the centre of gravity of the diffraction figure produced by the optical system in the focal plane corresponding to the plane wave front arriving from the ground beacon, while the opposite happens for illuminators which are further away from such centre of gravity. In a nutshell, the variation of the signal received by each elementary illuminator with the variation of the ground beacon direction of arrival referred to the electrical boresight of the sensor, is the same as that of the antenna beam referred to that illuminator (Figure 3).

Quite differently from the monopulse sensor, the lobe switching sensor does not perform any manipulation of received signal samples at RF, these samples being sent directly to receiver 11 (Fig. 5).

The receiver used is of the phase locked type with such locking band characteristics that it works in modulation tracking mode at least up to the frequency at which the one way N position switch performs the time multiplexing of the N RF signals into the single receiver channel 12 (Fig. 5).

Such way of operation is essential for correct functioning of the sensor over an acquisition angle range which is very large. In fact if the phase loop band is sufficiently wide, the loop may reacquire carrier phase correctly at the start of each signal sample coming from the time multiplexer. This implies that the information on the phase related to the lobes of each antenna pattern is deliberately lost and at the output of the coherent demodulator (Fig. 5) demodulation of the absolute value of the signal takes place: or, in other words, the. signal samples at the detector output all have a positive sign (Fig. 6).

Figure 6 shows the equivalent effect of this type of detection over equivalent patterns related to the elementary beams: the resulting effect is th^t of "rectifying" the sidelobes, which lose their sign.

The present invention exploits just-this characteristic behaviour of the single beams sidelobes envelope to achieve those performances of wide angle acquisition field which are claimed by the present invention. To this end, the angle discriminator functions, which are obtained by video processing of the sampled analogue signals obtained at the output of the modulation tracking phase locked loop receiver/demodulator 3, are here here below given making reference to figure 2 which shows one of the preferred implementations of the lobe switching type of sensor:

IA| - |B| (|A| + id) - (|B| + id) f (u) __ 4 or — .4 ₍₁₎

|A|+|B| + |c|+|D| |A| + |B| + I + |D|

Id - |DI _{4 or} del + IDI⁾ - ⁽I^AI * I^BI⁾ ._{4 (2} ^fD ^(v)= |_A| ₊ |B| + |C| + |D| ^' |A| + |c| + |c| + |c|

As for the other implementations of the lobe switching RF sensor, such as those in Figure 4, we may define angle discriminator functions belonging to the family:

with j C (3)

or, in other words, the discriminator function is given by the ratio of the linear combination, with coefficient K , of the j 0 modulus of signals received by illuminators A (some of the K j J coefficients may be null) and the sum, extended to include all illuminators A , of the modulus of signals received by these i illuminators. Under these conditions the angle discriminator 5 function takes on a characteristic form, which in the indi¬ vidual elementary beam sidelobe zone, preserves (in the statistical meaning of the word) in average a gradient which has the same sign of the discriminator peak voltage in the 0 corresponding half plane with respect to the symmetry axis of the function.

Figures 7a & 7b show an example of such characteristic behaviour of f (u) and f (V) discriminator functions in one of the preferred forms of the invention.

These characteristics are essential to provide the system with the capability to acquire angles in a non ambiguous manner even in the case of large offsets in the direction of the beacon with respect to the instantaneous tracking direction (electrical boresight) of the sensor. With reference to Figure 7a in the plane of variables u, v (direction cosines of the generic direction of arrival of the beacon) we may observe the variation of level curves of the discriminator function in direction u. The discriminator function represented is:

- (|B| id ) (4)

DU =

A + B + C . + D

The values taken by such function cause rotations of the pa- raboloid to take place by variable positive null or negative quantities, .depending on weather the beacon is to the left, at the centre or to the right of the eye pattern shown in Figure 7a. By the same taken, Figure 7b shows the variation of the level curves of the discriminator function in direction V. The function represented is:

DV =

A + B + C + D which gives way to paraboloid rotations by variable quantities along an axis which is orthogonal to the one above. To clarify how the concept may be generally applied, we may make reference to the case of a uniformly illuminated aper¬ ture. Assuming that the aperture is circular, having diameter a, and assuming that there are no random illumination amplitude variations, but that there may be phase ones, with a given correlation interval C, we may obtain for the square gain module (normalized to 0 dB for = 0), the following expression:

where : u = Ka sin &

K = 2 - Tf • f / 3 . ^ ^a

= off boresight angle

2 _C_f = variance of the phase error over the aperture (such phase error is considered to follow a Gaussian distribution). In the area close to the main lobe the diffraction term is predominant and has a decreasing envelope; the scattering term is, on the contrary, stationary.

The scattering term does not provide any information on the direction towards which to move to reach the beam crossover point.

Therefore, we may consider using the sidelobes for acquisition purposes, within a field (defined in terms of u) in which the diffraction term is larger than the scattering one (by about 10 dB). The area of use of the diagram of a single beam will extend up to u = 31.4 (N-10), i.e. for the first 10 sidelobes (when c/a

-2 2 = 10 and (6^* ) = 0.1, area in which the scattering term is negligible) .

The diffraction term may be represented, in terms of modulus and phase, for u>:5, as follows:

and therefore the envelope, indB, is 4 - 30 lo (u)

From figure 8 we can see that the squint angle of one beam with respect to the other must be equal to 1.2 Tf (case 1 in Fig. 8), in terms of u, to make possible the traslation of a sidelobe, while it must be equal to 2.2 ";' (case 2 of Fig. 8) 'to make possible the traslation of two sidelobes.

In case (1) we have a gain loss of 4.43 dB at crossover, while in (2) the loss is theoretically infinite.

The case 2 of figure 8 is really of difficult use, even if it theoretically gives the best performance, both in terms of angle acquisition (best characteristic of the discriminator) and in terms of angle error variance during tracking. Unfor¬ tunately the beacon signal detector would have to work with very low S/N ratios (even null) and therefore below threshold. Case 1 of figure 8 is a good compromise in a preferred application of the invention. The gradient at crossover is high and acquisition is obtained on the first 4 + 9 sidelobes. Equally applicable are all those configurations which imply a crossover of the elementary beams falling between - 6 and - 20 dB, so long as the signal at the input of the demodulator makes this work above its threshold. In this field of varia-

--«*« ^■ tion, as crossover point lowers, the angle acquisition cha¬ racteristics better at the expense of performance during tracking. The level curves of the discriminator functions, shown in Figure 7a and 7b, may have local stationary points.

Figure 9 shows the points in plane u, v for which both com¬ ponents, which force the movement, are null (local stationary points) . Such points, which would be hypothetical candidates for steady state conditions, are not in effect very damaging, as it is possible, by dithering the value of measurements A, B, C, D of formulae 1) and 2), to cause antenna movements which give way to a .new set of initial conditions and therefore to the chance of getting close to the correct crossover point. As a matter of fact, through this trick, the acquisition paths become statistically oriented (due to the average gradient in the area of sidelobes) towards the desired crossever point: The measurement dithering must be such not to cause the beacon to leave the central area of the discriminator eye (Figures "7a, 7b) at steady state. After a prefixed time from the start of acquisition phase, this dithering is stopped and the acqui¬ sition confirmation phase starts.

It is worth noting that another essential component of the system, object of the present invention, is the on board pro- cessor which:

Interpreters the demodulator output signals generates the dither time function recognises when acquisition takes place provides for switchover to tracking. To be brief, the task of serch on board processor 4, is to perform comparisons between the voltage values obtained from the discriminator, supposing this is S/W implemented, and the thresholds set. When the processor notes that in the compa¬ rison it passes through a low or null gradient area (which would lead to false angle acquisition) the processor initiates a sequential search procedure for the directions, will refe¬ rence to present position, for which the gradient increases. Then dithering commands are generated on the antenna movement, which is therefore brought out of the stationary area. This control is carried out even when the antenna reaches the desired tracking direction at the end of the angle acquisition procedure: here the angle gradient of the discriminator function is very high and is anyway known in advance. Therefore the processor, while commanding the local procedure for dithering around the new stationary point, is capable of recognizing the high value of the gradient, comparing it with a prefixed threshold an-d therefore confirming the achieved acquisition of the desired ground beacon direction. As shown in flow diagrams 10, 11, 12 the sequence of operations carried out by the on board processor is as follows:

- S represents the initiation of operations

- A, B, C, D indicate the circuits which carry out the reading of the voltages at the demodulator output corre¬ sponding to the sampling of the four illuminators in Figure 2.

- E is the circuit generating pseudorandom voltages which give way to the dithering of measurements A, B, C, D by performing the dithering function described above to get free of the stationary point. - F is the circuit which evaluates, from the four signal sa - pies A, B, C, D, the angle discriminator functions by ope¬ rating on the signal samples according to the algorithm of formulae (1) and (2).

- G is the circuit effecting the filtering of error voltages relevant to the two orthogonal control axes for antenna displacement, derived by evaluating circuit F of the angle discriminator function. The optimum filtering is of the adap¬ tive predictive type, where the actual error estimate is added to a portion of its first derivative in time; and where the filter coefficients may be varied adaptively (either autonomously on board the satellite or by remote control from ground) so as to face different operational and environmental situations which may be encountered on board the satellite.

- H and H represent error voltage transformation circuits, filtered by circuit G, into quantities suitable to drive the electric motors used to actuate movement 5 of the communi¬ cations antenna. In the case, for example, the actuator in¬ cludes step motors (a technique currently adapted for space applications) circuits H , H may include, as an example, circuits which count step motor steps and pulse amplifiers.

- I is the closing of the loop through paraboloid movement

- L is the timing circuit to evaluate whether the time elapsed since procedure initiation is greater or lesser than the a priori estimated time for angle acquisition procedure completion and consequently to decide by means of circuit M, whether acquisition is confirmed or not. In figure 11, the homonym circuits perform the same functions as those indi¬ cated with reference to Figure 10. During the acquisition confirmation time, the on board pro- cessor carries out other operations: Circuit Q effects incremental displacements of the antenna re¬ flector to detect possible stationary points of the angle di¬ scrimination functions; circuit R effects comparisons between the two components of error voltages relevant to reflector displacement induced by Q, and the a priori expected values which would be obtained if the radioelectrical boresight of the sensor were brought to coincide with the ground beacon direction, i.e. when angle acquisition really takes place ba¬ sed upon the result of the comparison: a) in the case of negative result (non confirmed acquisition) circuit V intervenes establishing a return to acquisition phase as per figure 10; b) in the case of positive, result (confirmed acquisition) cir¬ cuit T intervenes effecting a further check on all four sa - pies (iri time sequence) at the angle discriminator output by comparison with preset thresholds. If such comparison is not passed, we go back to circuit Q and another check cycle for acquisition accomplished is restarted. If the comparison is passed, angle tracking is started. Figure 12 shows the flow of operations in the angle tracking mode.

The blocks having the same name perform the same functional tasks as the operational modes described above and the only new device which is put into service is circuit X which car- ries out a comparison between error instantaneous voltage (or any other quantity representative of the instantaneous voltage of the antenna tracking error) and prefixed threshold. If those threshold are not passed, the system maintains its angle tracking mode; while passing these thresholds implies return to the angle acquisition/reacquisition mode (with a possible programmable delay).

Figure 13 shows a typical path and Figure 14 gives the scenario of the different acquisition paths with different initial conditions in plane u, v. The effect of dithering will be noted in Figure 13, which shows up in the areas where the gradient is low. In those areas where the gradient is high, the perturbations on the measurements do not have much of an impact on acquisition trajectories. Dithering therefore makes possible the exploi- tation of the sidelobe area, as shown in figure 13, so that even starting from points far from crossover point by 1.2°, even with elementary beams of 0.3°, acquisi-tion is made possi¬ ble, as in the example referred to in Figures 2, 3, 7a and 7b. Concluding, as should be clear from the description, the in- vention herein solves the following problems:

- Angle acquisition for significant angle offsets between beacon direction of arrival and initial orientation of the RF sensor electrical axes and, with respect to previous solu¬ tions, affords a significant improvement; - Flexibility of use of the system as the lobe switching sen¬ sor may be used with a number of illuminators greater than four, which is a conventional number, making therefore possi¬ ble the use in multibeam antennas with multiple illuminators;

- Use of the invention in different fields of application such as in radar systems, sonar or in any case in radio go- niometry systems and in all those systems where there is the need to acquire a radio frequency signal with the purpose of directing a device for detection of some signal cha¬ racteristic in the. direction of arrival of the signal itself.

Claims

1. System for the fine tracking control of antennae characte¬ rized by being made of a lobeswitching RF sensor (2), by a phase locked demodulator (3) having modulation tracking cha¬ racteristics and by an on board processor (4) which is capa- ble of processing error signals adequately at the demodulator output.

2. System for antennae fine tracking control as under Claim 1, characterized by exploitation of the characteristics of an¬ tenna beam sidelobes associated to n elementary illuminators (2) making up the RF sensor, which shows a decreasing am¬ plitude envelope of such lobes, and n is greater then 3.

3. System for antennae fine tracking control as under claim 1 and 2, characterized by the fact that the RF signals re¬ ceived by each elementary illuminator of the lobe switching RF sensor, are demodulated by a phase locked loop_, demodulator having such loop bandwidth that modulation tracking behaviour results up to at least three times the beam switching frequency, resulting, as a consequence of such characteri¬ stic, that the demodulated video signal has voltage output characteristics which are exclusively unipolar.

4. System for antennae tracking fine control as under claims above, characterized by the fact that angle discrimination is obtained exclusively at video frequency through processing of the unipolar voltages obtained downstream of demodulator 3 and that it belongs to the function family:

or in other words is a ratio between linear combination with weights K. (which may be null) of the modules of the voltages associated to the electrical field received by illuminators Aj and the sum of all voltage modules, associated to electrical fields received by the illuminators, corresponding to the generic angle direction u,v.

5. System for the antennae tracking fine control as under claim 4, characterized by the fact that the sensor is imple¬ mented preferably with 4 elementary illuminators as in figure 3 of the present description and by two angle discrimination functions associated respectively with axes u,v defined as:

f_n.u) - 4 - 1B1 )Al + lcl - (.Bl _^■ ID! ) or 4 ' \c \ ₊ |D| |_AJ ₊ }B| + |CI + |D|

V^v> = ⁴ -^" ' ^Dl _or <r> _* iDl - ⁽lAl + 1B[ ⁾ . 4

I A } + IB I + |CI +JD I , , . , ._, ,

' - ' ' ' |A| + [Bl + |Cl + (Dl

6. System for antennae tracking fine control as under claims 4 and 5, characterized by the fact that the crossover point of the elementary beams' radiation pattern corresponding to each elementary illuminator of the lobeswitching RF sensor is contained within the - 2 to - 20 dB range and preferably within the tighter range - 3 to - 10 dB.

7. System for antennal tracking fine control as under previous claims characterized by the fact that the non ambiguous angle acquisition field is between 3 and 10 elementary beamwidths making up the lobesv.itching RF sensor.

8. Systems for antenna tracking fine control as under previous claims characterized by the fact that the functioning prin¬ ciple is extended to all antennae operating in diffraction limitated regime so long as the sidelobe envelope, beyond the first, shows a decreasing trend.

9. System for antennae tracking fine control as of previous claims, characterized by the fact that its applicability is extended to all radio-goniometry fields and in general to radio signal direction of arrival acquisition, indipendently of the vehicle into which the angle acquisition system is installed and of the radiosignal propagation means.

10.System for the fine tracking control along the lines of what is shown in the description above and in the drawings attacked as also any part of it in isolation or in com¬ bination.