Fine pointing system of a reflector type focussing antenna

Abstract: System for fine pointing of reflector antennae, particularly suitable for space applications where such reflector (Figure 1) is moved within a spherical surface secor (5) with focus in the centre (F). Thus the beam can be pointed in the required direction and it ensures permanence in the focus, There are no defocussing losses even with a fixed feed system. The system proposed achieves the required scan with low losses. The feed beam at any rate covers a large part of the reflector surface (1), cosidering the usual edge taper values of the reflector of the order of 5 - 15 dB, i.e. the amplitude tapering due to the feed primary diagram, to the reflector edge and bearing in mind the range attenuation. The feed (2) and related feed lines either in waveguide (10) or coaxial cable, are fixed and firm. This solves the feasibility problems which are often unsurmountable, required to reduce possible RF losses of articulated lines, also avoiding undesired modulation effects induced onto the signals. The system is essentially made up of the following (Figure 2): A cardanic joint (3) (4); pressure spring(s); two actuation motors (7), equipped with position stops; two capstans (13) to widn and unwind piloting wires; two piloting wires (6).

Description: The invention relates to a fine pointing system of a reflector type focussing antenna as set forth in the preamble of claim 1. Such a system is known by GB - A-2 114 376. This known antenna apparatus has a support body connected with the reflector be means of a ball joint. A plurality of wires is braced around pairs of pulleys such that the position of the reflector relative to a given axis may be manually changed by push-pull movement of the wires. In such a way the reflector may be adjusted. Once the apparatus has been installed and the direction adjusted, the wires are fixed so that the reflector cannot move. JP-A-59-112703 707 relates to an antenna driver in which the reflector is hinged with a lateral edge and by means of two supporting arms to two magnetic float bodies controlled by actuators, constituting two motors having the same axis of rotation. The resulting force of rotation is opposed to by a spring. In the article of Kawakami et al. "On - Board Antenna Pointing Control System for Multi - Beam Communications Satellite" (Review of the Electrical Communications Laboratories, Vol. 35, No. 2, 1987, pages 169 - 175), an antenna drive control mechanism is described from which it is known to compensate the control torque of the mechanism, which is too small to drive the reflector under earth gravity conditions, through a series of wires and springs taking the weight of the reflector. US - A-4 862 185 refers to a variable wide angle conical scanning antenna, the reflector of which is rotated about one axis by a motor via a gear mechanism. It is an object of the invention to provide a fine pointing system for a focussing antenna which allows a very exact scanning of the reflector in any direction by means of a very simple mechanism. According to the invention, this problem is resolved by a system having the features of claim 1. Further characteristics are set forth in the dependent claims. The invention finds application in: systems for the pointing of the beam or beams of satellites antennae for acquisition and angle tracking in systems adopting the monopulse, the conical scan and step track techniques; antennae which are not in permanent movement, for which a capability of re-pointing of the beam or beams is required; focussed reflector antenna/e systems, with single or multiple reflector where the rotation of the reflector is around the focus The invention has its preferred application in satellite borne antennae, but it can find useful applications also in ground applications. The invention presented can find a number of applications on board a satellite for which there is a requirement to re-point the antenna beam or to track changing directions over a very wide field with low scan losses compared to alternative methods. For the present invention this is achieved by keeping the feed position fixed. Some of the problems solved by this invention: very small losses due to scan, lower than 0.3 to 0.5 dB within a very wide scan field, of the order of +40 times the antenna beamwidth according to conventional antenna design criteria adopting the usual edge taper values in the range between 5 and 15 dBs. For even wider scan fields, the invention is still applicable by increasing the dimension of the reflector and leaving unchanged the feed; adoption of a fixed feed system (not jointed) which eliminates the need for rotary joints (multiple way waveguide type, therefore very complex, when an angle track of the RF sensing closed loop type is adopted) and therefore also the relevant RF losses; less waveguide paths which would otherwise be necessary to connect the transponder to the antenna feed, when the antenna is hinged at a point which is different of the focus of the paraboloid; possibility to adopt RF sensing systems on a shaped coverage antenna (with the restriction that the shaping of the beam is obtained with a shaped feed radiation diagram) and also on multibeam antennae for which the hingeing of many feed lines would be difficult to implement. We must here recall the possibility to reduce the specification of the attitude control system of the satellite by adopting a RF sensing system, where such attitude control cannot be implemented with conventional systems. This can be considered a further advantage; possibility to optimize the configuration of the RF sensor for the detection of the angle error in a given direction of arrival of a beacon signal, freeing from the need to make recourse to minimum waveguide connection RF sensors, which are usually limited in terms of performance; simplification of the antenna arm structural design required to point only the reflector and not the entire antenna with the feed system; great simplification(considering satellite applications) of the positioning of the antenna on its base on board the satellite or in the launcher; use of the mechanism also to unfold the antenna arm following positioning in orbit; use of an actuator which applies tangential forces to the arm and to the reflector edge which do not cause any binding of the same as in alternative solutions, so that less complex,lighter and thinner arms can be adopted; possibility to use bands wider than that of the control loop, considering the lower inertia of the moving structure and the possibility to reach higher resonating frequencies. Till now the mechanical pointing of reflector type antennae was obtained through: pointing mechanisms positioned under the reflector which could tilt the reflector hinged in a given point; this solution, which generates distortions which increase in magnitude as a function of angle scanned (with consequent large reductions to antenna gain, sidelobe increase, asymmetric antenna diagrammes), is suitable only for desired very limited scan angles and otherwise it requires an antenna design where the F/D ratio is very large and impractical due to the considerable dimensions of the antenna; with complex multi-degrees of freedom systems which can move the entire antenna with its feed system, using jointed waveguides or coax cables for the feed, and at any rate adopting high cost rotating joints which are difficult to manufacture and imply further RF losses. The invention will now be described with reference to one of its presently preferred forms of implementation, which is reported for illustrative but non limitimg purposes, with reference to the drawings attached: Figure 1 is a schematic diagram of the parabolic reflector shown in two of its n positions. Here we can see: F Focus; S Sphere; P Paraboloid. Figure 2 is an elevated view of the pointing mechanism. Here we can see: 1 reflector; 2 illuminator; 3 support arm of the reflector rotating around the Y axis; 4 rotating support on X axis; 5 pushing spring; 6 pilot, holding and positioning wires d1 and d2; 7 actuator motors with grooved capstan to wind and unwind the pilot wires; 8 control electronics; 9 possible angle detectors; 10 RF connections; 11 fixed structure (satellite body); 12 parabola focus (universal joint axes). Figure 2 is to be considered the most significant. It shows the structure of the mechanism. A cardanic joint, which has its rotation centre which coincides with the focus F of the reflector 1, acts as a spherical hinge. It therefore enables rotation in space of the arm 3 plus reflector assembly. The circular pressure spring 5 which acts between the reflector arm 3 and the axis of the joint, imposes an angular displacement to the reflector 1 opposite to the one applied to wires 6 which are continuously under tension. The length of the two pilot wires 6 is changed by motors 7 upon command, so that the position of the reflector 1 depends upon the length of the pilot wires 6, which are therefore the status variables of the mechanism. If not opposed by wires 6, the spring 5 would impose a rotation in opposite direction to that imposed by the pilot wires 6 themselves, moving the reflector 1 away from the current position required for pointing. Control electronics 8 send the two actuation signals to the two motors 7 through which it is possible to vary the free length of the two pilot wires 6 through the grooved capstans by winding or unwinding them on the capstans themselves. The free lengths d1 and d2 of the pilot wires sets the position of the reflector compared to the fixed structure, as arm 3 and support 4 are subject to the action of spring 5. Such spring 5 keeps the wires under tension, so as to set the position of the reflector against the fixed reference (satellite body) in a univocally determined manner. Commands sent sequentially to the motors can make the reflector follow the required trajectories. The forces are applied to the point of connection of the two wires to the reflector arm, resulting in a static balance as shown in figure 3. Force F3 is perpendicular to the Y axis and is set by the elastic constant of spring 5. The values of forces F1 and F2 are determined by the breakdown of F3 force into the two component directions, set by the position of the capstans with which the length of the pilot wires with respect to the connection point to the reflector arm is controlled. Figure 3 is a schematic representation of the forces applied to the point of connection to the reflector arm. Figure 4 a): schematic representation of the antenna scan geometry; b): schematic diagram of scan losses (negligible). Figure 5 shows an example of a redunded mechanism. Figure 6 are examples of implementation of the system regarding solutions for alternative actuator devices (such as linear actuators and spherical joints). Some of the most determining aspects of the invention can be summarized as follows: The system proposed can point antennae of large dimensions on angles several times wider than the elementary beam width even for F/D ratioes of the antenna design between zero and one. Moreover, the linear movements to be impressed on the reflector become lesser the shorter the focal length, an attractive feature especially for satellite applications; the scan losses due to the proposed scan method are entirely acceptable and are reported in Figure 4b for a typical example of antenna geometry shown in Figure 4a. The system presented by this invention in its preferred form of implementation, can be applied advantageously to a wide range of antenna type, diameter and geometry by varying only: a)- the length of the pilot wires; b)- the dimensions of the universal joint for correct allocation of the focus feed system; c)- the torque impressed by the push spring; d)- the power of the pilot motors and the maximum traction/release speed of the wires; the mechanism may be easily redunded to achieve high reliability levels: each of the two motors can be redunded by adding the redundancy on the same motor shaft; each wire can be redunded; the push spring can be redunded. The redunded configuration is shown in Figure 5. The proposed mechanism does not make use of levers or complex jointed parallelograms or curved rails or linear actuators as could be imagined as an alternative, all to the advantage of a simple assembly, of reliability and of actuation accuracy. The mechanism also allows for pointing of multibeam antennae with a fixed feed system without any hinge, avoiding rotary joints and their RF losses and avoiding any consequential induced modulations on the signal. The mechanism also allows pointing of single or multi beams for which repointing of the beam is required and for all cases of focussed reflector antennae, with single or multiple reflector, for which the reflector rotation takes place around the focus indipendently of the type of antenna configuration considered. The system and the mechanism proposed are the only viable solution in the case the feed system is of the phase array type or of the matrix beam forming type, where the phase relationship on each single channel must be kept in scan conditions. The system proposed is the only viable solution standing the scan limitations over wide fields with relative low losses, in the case the RF sensor adopts multiple beams, for which the phase relationships between signals received on single beams must be kept during scan conditions.

Claims: Fine pointing system of a reflector type focussing antenna, comprising a support body (11) on which the reflector (1) is mounted by means of a universal joint (3,4), and at least two pilot wires (6) which are connected to the reflector (1) for changing its angular position relative to the support body (11), characterised in that each pilot wire (6) is connected to an actuator motor (7) for commanding, via an electronic control device (8), a push - pull movement of the wire (6), and that a push spring (5) is continuously acting on said universal joint (3,4) in the sense of maintaing the wires (6) under tension. Fine pointing system according to claim 1, wherein the wires (6) are fixed to a supporting arm (3) of the reflector (1), said supporting arm (3) being a part of the universal joint (3,4). Fine pointing system according to claim 2, wherein said push spring (5) is a coil spring inserted into the universal joint (3,4) and acting against said supporting arm (3). Fine pointing system according to anyone of the preceding claims, wherein the centre of rotation of the reflector (1) coincides with the reflector focus (12) connected to the universal joint (3,4).

Inventors: FALCONI MARIO, LOSQUADRO GIACINTO, FALCONI, MARIO, LOSQUADRO, GIACINTO
Applicants: SELENIA SPAZIO SPA, SELENIA SPAZIO S.P.A.
Application year: 1991
Main classification: H01Q1
IPC codes: H01Q1, H01Q3