Method for controlling the pointing of an antenna

Abstract

The invention relates to a method for controlling the pointing of an antenna located on a platform towards a communication device transmitting a reference signal, comprising the following steps: S1) applying a first pointing mode of the antenna using ephemeris data of the platform and of the communication device so as to obtain an initial pointing position in azimuth and elevation of the line of sight of the antenna; S2) applying a second pointing mode of the antenna, comprising at least one scanning of the antenna around the initial position so as to point the antenna in a direction which maximizes a received power of the reference signal; S3) applying a third pointing mode of the antenna in which at least one triangulation maneuver is implemented to point the antenna from the maximum power. Figure for abstract: Fig. 6L

Description

Title of the invention: Method for controlling the pointing of an antenna Technical field

[0001] The invention relates to a method and a system for controlling the pointing of an antenna provided with two pointing axes located on a platform in the direction of a communication device emitting a reference signal. The invention finds its application in the field of telecommunications, and more particularly in the field of space telecommunications.

[0002] Projects for the development of orbital stations in cislunar orbits pose new challenges in terms of space telecommunications. In this type of orbit, communication distances can be very long. The distance between the gateway of the lunar advanced post and the system in orbit around the moon, or with a fixed or mobile system on the moon, can reach 70,000 km for certain missions currently under development, which represents approximately twice the distance between the Earth and a satellite in geostationary orbit.

[0003] Another problem arises from the fact that, on the orbital stations currently under development, the sensors and other equipment of the attitude and orbit control system (AOCS) are located several meters from the communication system of the lunar advanced post, which can cause problems of coherence of the data frames, accentuated by the vibrations on board as well as by the thermoelastic deformations due to the temperature differences.

[0004] However, the directional antenna must be correctly oriented to allow the communication system to collect sufficient energy from the transmitted signals.

[0005] Open-loop pointing systems based solely on knowledge of the ephemeris of the target system do not provide sufficient accuracy to transmit data at very high speeds with the required energy levels.

[0006] Closed-loop systems exist to optimize this pointing, but they are often based on systems with several RF sources allowing to detect, by gain differential between these sources, the direction towards which the antenna should be pointed. Closed-loop servo-controlled systems use several RF sources in parallel with the nominal communication system. These systems involving several RF sources and several signal detection electronics are well suited for ground stations, on Earth. These systems require, in addition to the nominal communication chain, additional external systems allowing precise servo-control.

[0007] However, in on-board applications (for example for pointing an antenna on board an orbital station towards a rover, towards a station deployed on the moon or towards a lunar orbiter), the constraints in terms of mass, cost and accommodation volume are very high. Thus, multi-source systems are too expensive and their implementation in on-board applications is difficult to envisage.

[0008] It is known to use phased array antennas which allow, by scanning, the rapid detection of a nominal pointing path. Antennas based on phased arrays have very low gains; however, in the intended applications, the systems may have a relative displacement between two measurement times, which would require the phased array antenna to be configured to scan a large area of space. A scanning operation over a large area with a reduced gain would not be satisfactory for the applications mentioned above.

[0009] There is therefore a need for a method for controlling the pointing of an antenna, which offers a sufficiently high throughput, and which does not require an additional antenna system to carry out the pointing. Summary of the invention

[0010] An object of the invention is therefore a method for controlling the pointing of an antenna located on a platform in the direction of a communication device emitting a reference signal, comprising the following steps: SI) applying a first antenna pointing mode using ephemeris data from the platform and the communication device so as to obtain an initial pointing position in azimuth and elevation of the antenna's line of sight; S2) applying a second antenna pointing mode, comprising at least one sweep of the antenna around the initial position so as to point the antenna in a direction which maximizes a received power of the reference signal; S3) applying a third antenna pointing mode in which at least one triangulation maneuver is implemented to point the antenna from the maximum power.

[0011] Advantageously, the second pointing mode of the antenna comprises a first conical scan of the antenna with a first opening angle, and a second conical scan of the antenna with a second opening angle, the first opening angle being determined as a function of the angular range of the secondary lobes of the antenna pattern, the second opening angle being determined as a function of the angular range of the main lobe of the antenna pattern.

[0012] Advantageously, the second mode of pointing the antenna comprises at least an azimuth and elevation movement, in which, for each azimuth and elevation movement, a search for maximum power is carried out.

[0013] Advantageously, the antenna has a monotonic antenna pattern in the angular space in which the at least one scanning of the antenna and the triangulation maneuver take place.

[0014] Advantageously, the monotonic antenna diagram is obtained by a phase shift between the phase center of the antenna source and the focal point of the reflector / sub-reflector combination.

[0015] Advantageously, the reference signal is a pure carrier in the second pointing mode of the antenna.

[0016] Advantageously, the third pointing mode comprises a step of comparing the distance between the antenna and the communication device with respect to a threshold, and - if the distance is greater than the threshold, a new triangulation maneuver is implemented; - if the distance is less than the threshold, the pointing of the antenna (1) is implemented by carrying out a search for maximum power by at least one movement in azimuth and elevation.

[0017] Advantageously, the triangulation maneuver comprises: - an azimuth displacement of half a so-called triangulation increment in one direction, a measurement of the received power, an azimuth displacement of the increment in the opposite direction if the received power is less than a power measured before the azimuth displacement, or an azimuth displacement of half the increment in the same direction if the received power is greater than the power measured before the displacement; - an elevation displacement of half a so-called triangulation increment in one direction, a measurement of the received power, an elevation displacement of the increment in the opposite direction if the received power is less than a power measured before the elevation displacement, or an elevation displacement of half the increment in the same direction if the received power is greater than the power measured before the displacement.

[0018] Advantageously, the triangulation increment results from a compromise between a precision sought for the calculation of the optimal direction to be reached and a minimization of an amplitude of the misalignment associated with the triangulation maneuver.

[0019] Advantageously, the first pointing mode comprises a command to return to the initial position, the command comprising a plurality of angular movements in azimuth and elevation, each angular movement having a predetermined duration.

[0020] Advantageously, the communication device is arranged on a celestial body, and the platform is in orbit around the celestial body.

[0021] Advantageously, the drift linked to the relative movement between the platform and the communication device is compensated, in the third pointing mode, using the ephemeris data of the platform and the communication device.

[0022] The invention also relates to a system for controlling the pointing of an antenna located on a platform and configured to communicate with a communication device emitting a reference signal, the system being configured to: - apply a first antenna pointing mode using ephemeris data from the platform and the communication device so as to obtain an initial position in azimuth and elevation of the antenna's line of sight; - applying a second antenna pointing mode, comprising at least one sweep of the antenna around the initial position so as to point the antenna in a direction which maximizes a received power of the reference signal; - apply a third antenna pointing mode in which at least one triangulation maneuver is implemented to point the antenna from maximum power. Description of the figures

[0023] Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the appended drawings given by way of example.

[0024] [Fig.l] illustrates an example of a space environment comprising an orbital station in communication with lunar systems.

[0025] [Fig.2] illustrates an example of the orientation of the reference frame used for pointing the antenna.

[0026] [Fig.3] illustrates a time-course diagram of the first pointing mode.

[0027] [Fig.4] illustrates an example of a quasi-monotonic antenna pattern in the coverage area.

[0028] [Fig.5] illustrates a time schedule of the second pointing mode.

[0029] Figures 6A-6L illustrate XY plane pointing control diagrams for the second pointing mode.

[0030] [Fig.7] illustrates a time-course diagram of the third pointing mode.

[0031] Figures 8A-8E illustrate XY plane pointing control diagrams for the third pointing mode.

[0032] [Fig.9] illustrates the principle of triangulation.

[0033] [Fig. 10] illustrates the different stages of the method according to the invention.

[0034] In [Fig.l], a ground station 6 can be in communication with a station orbital station 2, for example in X-band for telemetry, tracking and control of orbital station 2 (TTC), and in Ka-band for the transmission of useful data in uplink and downlink. Orbital station 2 may be in communication with satellites 7, for example in S-band, and with communication devices 8 located near orbital station 2 (used for example during sorties to carry out maintenance operations on orbital station 2), using for example a short-distance wireless network. Orbital station 2 is in communication with a communication device 4 located on the moon 5 or in orbit around the moon 5, for example in Ka-band or S-band. The invention is not limited to one of these frequency bands.

[0035] The communication device 4 may be embedded, for example, in a lunar station 9 deployed on the ground, in a mobile rover 10 on the moon 5, in a communication station 11 serving as a relay on the moon, or even a system 12 in orbit around the moon. In the context of the invention, it is essential that the communication device 4 is capable of emitting a reference signal. The reference signal is advantageously a pure carrier, in particular a sinusoidal signal.

[0036] [Fig.2] illustrates a detailed view of the antenna system 1 of the orbital station 2. Each antenna 1 may be of the Cassegrain parabolic antenna type, comprising a main reflector 13 and a secondary reflector 14 located at the focal point of the main reflector 13. The antenna system also comprises a support structure 15, which serves as an interface between the antenna and the satellite platform.

[0037] A box 16 makes it possible to protect the various elements interacting with the antenna, for example:

[0038] - a transmitter / receiver, coupled to the antenna, and capable of measuring the level of received RF signal power,

[0039] - a calculator, connected to the transmitter / receiver, and capable of interpreting the in training received by the transmitter / receiver and to define a control solution based on a locally defined guidance strategy,

[0040] a system for controlling the motors of the antenna axes, capable of receiving the command from the computer and of commanding a movement to the directional antenna.

[0041] The box 16 can also host an external system, providing the computer with a minimum amount of information on the system with which communication must be established, in particular, the fundamental RF parameters (frequency, modulation, coding, flow rate, cryptography key, protocol identifiers, etc.), or even the rough position of the target system in order to restrict the search space.

[0042] The reference used for the description is a reference linked to the antenna reflector, and therefore which is movable depending on the antenna pointing. The references A1, A2 and A3 make it possible to identify the deployment (A1) and pointing motors respectively. (A2 and A3). Since the reference implementation has two antennas, the references A1, A2 and A3 which apply to one antenna correspond respectively to the references A4, A5 and A6 on the second antenna.

[0043] The XYZ reference frame, whose origin corresponds to the center of the parabolic reflector, is defined by the following axes:

[0044] Z: antenna line of sight

[0045] Y: perpendicular to Z, in the same direction and orientation as the axis of rotation of motor A3 for antenna 1 (of motor A6 for the second antenna 1 in [Fig.2]), passing through the origin of the reference frame.

[0046] X: Completes the direct trihedron.

[0047] [Fig. 2] illustrates an antenna system comprising two reflector antennas, but this representation is in no way limiting. The invention could also be implemented with a different number of antennas, without there being any limitation to the type of antenna used. It is however essential that each antenna can perform pointing along two axes.

[0048] The control method according to the invention comprises three modes executed one after the other, which make it possible to acquire the pointing in closed loop after several steps of search and convergence towards the optimal performance, from an extended search space. The pointing is said to be in closed loop because it is based on the emission of a reference signal by the target device.

[0049] The first mode of pointing the antenna 1 is carried out using ephemeris data from the platform 2 and the communication device 4. The ephemeris are translated into relative position and speed commands between the platform 2 and the communication device 4 sent to the computer. These commands can be generated for example in real time at a typical frequency of 1 Hz, or in the form of polynomial profiles interpolated over a time segment. The computer then uses these target directions and an estimate of the direction of the line of sight of the antenna to calculate an angular increment to be carried out by the motors of the antenna pointing mechanism. The first mode thus makes it possible to reach and follow a target direction with a coarse precision corresponding to the precision of the ephemeris supplied to the computer.The rallying of this direction is a prerequisite for the transition to fine pointing modes (so-called closed loop), and therefore constitutes the initial condition.

[0050] The antenna boresight corresponds to the axis of maximum gain (maximum radiated power) of a directional antenna. For most antennas, the boresight is the axis of symmetry of the antenna. For example, for axially fed parabolic antennas, the antenna boresight is the axis of symmetry of the parabolic antenna and the antenna radiation pattern (the main lobe) is sy- metric around the line of sight.

[0051] [Fig.3] illustrates an example of a timing diagram of the CMD_SEAM command pattern of the first pointing mode. The pattern is repeated until the boresight of the antenna points to the target direction determined by the ephemeris data. The CMD_SEAM pattern can be decomposed into a command to move the boresight along the X axis of the antenna frame for a predetermined duration, followed by a command to move the boresight along the Y axis of the antenna frame for a predetermined duration. The predetermined duration can be for example equal to 0.5 seconds, and the homing movement can be performed at a maximum speed of 1.4 degrees / second. These numerical values depend on the characteristics of the antenna.

[0052] The second pointing mode of the antenna 1 consists of performing at least one scan of the antenna 1 around the initial position towards which the antenna points at the end of the first pointing mode, so as to point the antenna 1 in a direction which maximizes a received power of the reference signal.

[0053] In order to unambiguously associate the received energy level with an antenna misalignment angle, the antenna must have a monotonic pattern in the coverage area with a maximum gain in the central position corresponding to the boresight, as illustrated in the antenna diagram of [Fig.4].

[0054] A "monotonic pattern" means a radiation pattern that is strictly increasing or decreasing around the line of sight of the antenna. Thus, over the range of negative angles, the gain is strictly increasing, and over the range of positive angles, the gain is strictly decreasing.

[0055] In [Fig.4], dips appear at approximately -1.8° and 1.8°. Thus, for this antenna, measurements are made in the value range between the dips.

[0056] A completely monotonous antenna pattern makes it possible to optimize the pointing control by precisely associating the setpoint angle with the received energy level, taking into account a set of disturbing elements including: reception level measurement noise, instabilities of the remote communication system, thermo-elastic deformations of the antenna system, its support and motors, flexible modes of the antenna excited by the commands, the dynamics of the remote communication system, and delays in the measurement, calculation and command chain.

[0057] The monotonic pattern is obtained by removing the gain holes between the first and second lobes of the antenna pattern, which can be implemented for example via a phase shift between the phase center of the source and the focal point of the reflector 13 / sub-reflector 14 combination, or for example by optimizing the shape of the reflector for mono-reflector antennas according to known techniques.

[0058] [Fig.5] illustrates an example of a timing diagram of the second pointing mode. In In addition to the first conical scan, it includes a second conical scan, as well as an azimuth and elevation shift. The second conical scan and the azimuth and elevation shift are optional, and allow for fine-tuning the homing operation to the maximum gain position.

[0059] The first conical scan of the antenna 1, illustrated by FIGS. 6A, 6B and 6C, is carried out with a first opening angle which is determined as a function of the angular range of the secondary lobes of the antenna pattern.

[0060] In [Fig.6A], the line of sight is moved along one of the motor axes of the antenna, namely in elevation or in azimuth, relative to the initial position PO provided by the ephemeris data of the first pointing mode towards a position PL In [Fig.6B], a conical movement is carried out from the position PI, and the gain is measured at regular time intervals. It can be noted that the angular range of the conical scan ([-1°, 1°]) in Figures 6A to 6C corresponds substantially to the amplitude of the re-pointing to be carried out to reach the main lobe in [Fig.4].

[0061] For example, the boresight displacement pattern in the first conical scan may include a 0.5 second displacement in azimuth (X-axis), followed by a 0.5 second displacement in elevation (Y-axis), and a gain measurement delay of 0.375 seconds. These values are provided by way of example, and are not limiting. At each measurement point, the gain value is measured and stored in memory.

[0062] In [Fig.6C], the line of sight of the antenna joins the position P2 which corresponds to the maximum value of the gain. In [Fig.6D], the line of sight is moved along one of the motor axes of the antenna, namely in elevation or in azimuth, relative to the position P2 corresponding to the maximum gain of the first conical scan, towards a position P3. In [Fig.6E], a conical movement is carried out from the position P3, and the gain is measured at regular time intervals, in the same way as for the first conical scan. The angular range of the second conical scan is smaller than the angular range of the first conical scan (typically between 0.5° and 1°).

[0063] In [Fig.6F], the line of sight is defocused so as to reach the position P4 which corresponds to the position of the set of measurements of the second scan which maximizes the gain.

[0064] In order to refine the measurement, the second pointing mode can be supplemented by a movement in azimuth and elevation (cross movement), as illustrated in Figures 6G to 6L. In [Fig.6G], the line of sight is moved along one of the motor axes of the antenna (for example elevation in [Fig.6G]), to a position P5. The movement is carried out until the measured gain decreases. The gain measurements are carried out at regular time intervals.

[0065] In [Fig.6H], the line of sight is displaced in the opposite direction relative to the position P4 to a position P6. This movement is commanded when the previous movement has not detected an increase in gain, and until the measured gain decreases again. The gain is measured at each measurement point, and position P7 is retained for the rest as the position which maximizes the measurement of the elevation gain ([Fig.61]).

[0066] The same procedure is performed in azimuth (X axis), as illustrated in [Fig.6J], [Fig.6K], and [Fig.6L]. The azimuth scan starts from the maximum measurement point P7 in elevation. It is finally determined that point P10 maximizes the azimuth gain measurement.

[0067] The use of the two conical scans and the cross search makes it possible to alternate phases of activating the antenna, calming the movement and observing the update of the received power to update the control profile.

[0068] The different stages of the second pointing mode are illustrated by [Fig.5]. The first conical scan RAW SCAN 1 comprises an initialization phase INIT1, a scanning phase CONING1, and a homing phase RALLY1. The second conical scan RAW SCAN 2 comprises an initialization phase INIT2, a scanning phase CONING2, and a homing phase RALLY2. The fine cross scan comprises different sequences of azimuth scanning SCANX and elevation scanning SCANY.

[0069] The method according to the invention comprises a third mode of pointing the antenna 1 in which at least one triangulation maneuver is implemented to repoint the antenna 1 from the maximum power.

[0070] The starting position of the antenna corresponds to the position determined in the second pointing mode, which maximizes a received power of the reference signal (point P10 in [Fig.6L]).

[0071] A first step of the third pointing mode consists of applying a command to move the line of sight in azimuth and elevation, according to a maneuver illustrated by [Fig.7].

[0072] The pointing error is then estimated from knowledge of the antenna pattern, the current measurement of the received power and the estimation of the power emitted by the target obtained from the maximum power measured in the final position P10 of the scan phase which minimizes the mispointing relative to this target.

[0073] When the estimated pointing error exceeds a predetermined threshold corresponding to the control dead zone, the line of sight is displaced in azimuth by half of a triangulation angular increment DX in one direction ([Fig.7], step S1, displacement DX / 2). The triangulation angular increment results from a compromise between the precision sought for the calculation of the optimal direction to be reached (solution of the triangulation problem) and the amplitude of the misalignment associated with the triangulation maneuver that must be minimized. The gain is measured, and if the gain increases compared to the previous measurement made at the end of the second pointing mode, the line of sight is again moved in azimuth by half the triangulation increment in the same direction ([Fig.7], step S2). Conversely, the line of sight is moved in azimuth by the increment in the opposite direction if the received power has decreased compared to the previous measurement made at the end of the second pointing mode ([Fig.7], step S3, -DX displacement). This logic makes it possible to minimize the pointing error during the triangulation maneuver. At the end of steps S2 or S3, the gain GAIN1 is memorized. In [Fig.7], the "deltaGain" value corresponds to the difference between the power measured at the current position and the maximum power estimated in the scan phase.So if the deltaGain increases, the gain decreases.

[0074] The same command is then carried out in elevation, from the point for which the movement procedure ended in azimuth at the end of steps S2 or S3. The line of sight is moved in elevation by half a triangulation increment in one direction ([Fig. 7], step S4, movement DY / 2). The gain is measured, and if the gain increases compared to the previous measurement carried out at the end of the second pointing mode, the line of sight is again moved in elevation by half the predetermined increment in the same direction ([Fig. 7], step S5). Conversely, the line of sight is moved in elevation by the increment in the opposite direction if the received power has decreased compared to the previous measurement carried out at the end of the azimuth movement in the third pointing mode ([Fig. 7], step S6, movement -DY). At the end of steps S5 or S6, the gain GAIN2 is stored.This two-step triangulation procedure minimizes the antenna misalignment relative to the direction of interest (which maximizes the received gain).

[0075] Figures 8A to 8E illustrate an example of the different movement steps for the third antenna pointing mode.

[0076] In [Fig.8A], the point Pli corresponds to the azimuth and elevation references at the instant when the estimated pointing error leaves the control dead zone (threshold from which a pointing correction is commanded). The line of sight is moved in azimuth by a value DX / 2, up to point P12. The gain decreases between points PI 1 and P12, so the line of sight is moved in azimuth by -DX, up to point P13 ([Fig.8B]).

[0077] Similarly, in [Fig.8C], the line of sight is moved in elevation by a value DY / 2, up to point P14. The gain decreases between points P13 and P14, so the line of sight is moved in elevation by -DY, up to point P15 ([Fig.8D]).

[0078] The invention could also be implemented by first performing the elevation movement, then the azimuth movement, for the third pointing mode.

[0079] The calculation of the optimal point to be reached is then implemented from point P15 by calculating the solution to the triangulation problem from the measurements made during the triangulation maneuver described previously. For the example of a triangulation maneuver illustrated in Figures 8A, 8B, 8C, 8D, the measurements used for this calculation are points P12, P13, P14 and P15. In the example illustrated [Fig.8E], the solution to the triangulation problem is point P16, and corresponds to the first order to the correction of the control dead zone represented by the circle in [Fig.8E], and below which no pointing correction is ordered.

[0080] [Fig.9] illustrates the triangulation principle applied to the method according to the invention for a triangulation maneuver simplified compared to that illustrated by [Fig.8E]. This simplified maneuver starts from position P17 ([Fig.9]) and implements a rotation around Y of amplitude x2 (a displacement in azimuth) towards position P18, and a rotation around X of amplitude x2 (a displacement in elevation), towards position P19. The power is measured at points P17, P18 and P19 to calculate the solution to the triangulation problem (P20). The power measurements P17, P18 and P19 make it possible to estimate the angular offsets Dal, Da2, Da3 at these points relative to the optimal point to be reached (P19). The solution to the triangulation problem corresponds to the intersection of the three circles of radius Dal, Da2, Da3.

[0081] The triangulation maneuver advantageously makes it possible to carry out a precise measurement even in the event of relative movement between the antenna and the communication device 4, caused for example by the orbital dynamics of the antenna platform and / or the support of the communication device 4.

[0082] When the distance between the antenna 1 and the communication device 4 is too small, the transmitter / receiver and the computer typically lower the level of the received signal, to avoid damaging the components of the reception chain (a technique called “clamping”). This could disrupt the triangulation operation by distorting the reception levels.

[0083] To avoid this, the third pointing mode is used for distances between the platform 2 and the communication device 4 less than an adjustable threshold, corresponding to the distance below which the communication device 4 reduces its transmitted power (clamping). The distance knowledge is contained in the ephemeris data delivered by the platform 2 to the computer. Thus, if the distance is greater than the threshold, a new triangulation maneuver is implemented. If the distance is less than the threshold, the repointing of the antenna 1 from the maximum power is implemented by performing a search for maximum power by at least one movement in azimuth and in elevation.

[0084] In both pointing modes described, the drift linked to the relative movement between the platform 2 and the communication device 4 is compensated by using the ephemeris data from platform 2 and communication device 4. The commanded angular increment thus corresponds to the angular increment to carry out the maneuver in antenna reference for each of the pointing phases described previously, to which is added an increment to compensate for the drift of the target during this maneuver. This drift is estimated from the relative positions and speeds between the target and the pointing device, contained in the ephemeris data delivered to the computer.

[0085] The method according to the invention makes it possible to implement closed-loop single-source pointing, which makes it possible to go, for the reference implementation, from an open-loop precision of approximately 1.6° to a closed-loop precision of approximately 0.3°. The antenna gain is considerably improved and the possible throughputs go from 100 ksps to 50 Msps.

[0086] Furthermore, the system used to find the optimal line-of-sight pointing is the same as that used for communication.

[0087] Finally, if, in the reference implementation, the communication device 4 is operated in open loop, it is possible to also implement closed loop pointing on this device while taking care to take into account the pointing performance of this system in order to guarantee the stability of the communication.

Claims

Claims

1. A method for controlling the pointing of an antenna (1) located on a platform (2) towards a communication device (4) transmitting a reference signal, comprising the following steps: S1) applying a first pointing mode of the antenna (1) using ephemeris data of the platform (2) and the communication device (4) so as to obtain an initial pointing position in azimuth and elevation of the line of sight of the antenna; S2) applying a second pointing mode of the antenna (1), comprising at least one scanning of the antenna (1) around the initial position so as to point the antenna (1) in a direction which maximizes a received power of the reference signal; S3) applying a third pointing mode of the antenna (1) in which at least one triangulation maneuver is implemented to point the antenna (1) from the maximum power.

2. The method of claim 1, wherein the second pointing mode of the antenna (1) comprises a first conical scan of the antenna (1) with a first opening angle, and a second conical scan of the antenna (1) with a second opening angle, the first opening angle being determined as a function of the angular range of the secondary lobes of the antenna pattern, the second opening angle being determined as a function of the angular range of the main lobe of the antenna pattern.

3. Method according to one of the preceding claims, in which the second mode of pointing the antenna (1) comprises at least one movement in azimuth and elevation, in which, for each movement in azimuth and elevation, a search for maximum power is carried out.

4. Method according to one of the preceding claims, wherein the antenna (1) has a monotonic antenna pattern in the angular space in which the at least one scanning of the antenna (1) and the triangulation maneuver take place.

5. A method according to claim 4, wherein the monotonic antenna pattern is obtained by a phase shift between the phase center of the source of the antenna (1) and the focal point of the reflector (13) / sub-reflector (14) combination.

6. Method according to one of the preceding claims, in which the reference signal is a pure carrier in the second antenna pointing mode.

7. Method according to one of the preceding claims, in which the third pointing mode comprises a step of comparing the distance between the antenna (1) and the communication device (4) with respect to a threshold, and - if the distance is greater than the threshold, a new triangulation maneuver is implemented; - if the distance is less than the threshold, the pointing of the antenna (1) is implemented by performing a search for maximum power by at least one movement in azimuth and in elevation.

8. Method according to one of the preceding claims, in which the triangulation maneuver comprises: - an azimuth displacement of half of a so-called triangulation increment in one direction, a measurement of the received power, an azimuth displacement of the increment in the opposite direction if the received power is less than a power measured before the azimuth displacement, or an azimuth displacement of half of the increment in the same direction if the received power is greater than the power measured before the displacement;- an elevation displacement of half a so-called triangulation increment in one direction, a measurement of the received power, an elevation displacement of the increment in the opposite direction if the received power is less than a power measured before the elevation displacement, or an elevation displacement of half the increment in the same direction if the received power is greater than the power measured before the displacement.;

9. Method according to claim 8, in which the triangulation increment results from a compromise between a precision sought for the calculation of the optimal direction to be reached and a minimization of an amplitude of the misalignment associated with the triangulation maneuver.

10. 10. Method according to one of the preceding claims, in which the first pointing mode comprises a command to return to the initial position, the command comprising a plurality of angular displacements in azimuth and elevation, each angular displacement having a predetermined duration.

11. 11. Method according to one of the preceding claims, in which the communication device (4) is arranged on a celestial body (5), and the platform (2) is in orbit around the celestial body (5).

12. 12. Method according to one of the preceding claims, in which the drift related to the relative displacement between the platform (2) and the communication device (4) is compensated, in the third pointing mode using the ephemeris data of the platform (2) and the communication device (4).

13. 13. System for controlling the pointing of an antenna located on a platform and configured to communicate with a communication device emitting a reference signal, the system being configured to: - apply a first antenna pointing mode using ephemeris data from the platform and the communication device so as to obtain an initial position in azimuth and elevation of the antenna's line of sight; - applying a second pointing mode of the antenna, comprising at least one sweep of the antenna around the initial position so as to point the antenna in a direction which maximizes a received power of the reference signal; - apply a third antenna pointing mode in which at least one triangulation maneuver is implemented to point the antenna from maximum power.

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