Position estimation of an optical satellite terminal

Abstract

The invention relates to a device for a satellite for estimating a current position of a module (3) which is disposed on a main body (1) of the satellite and which has a communication sensor (5) for receiving optical communication signals, the device comprising: a computing unit (7); an actuator (6) for tilting the module (3) relative to the main body (1); and the communication sensor (5), the computing unit (7) being designed to execute an estimation algorithm based on sensor fusion in order to estimate a position of the module (3), and to use, as a first data source for the estimation algorithm, a signal which is dependent on an entrance angle of a light beam emitted by a transmitter remote from the satellite, said entrance angle occurring at the communication sensor (5), in conjunction with a relative position between the satellite and the transmitter, said relative position being known to the computing unit (7), and to use at least one additional data source for the estimation algorithm on the basis of a signal of a position sensor (9) disposed on the main body (1) of the satellite.

Description

[0001] Attitude estimation of an optical satellite terminal

[0002] The invention relates to a device for a satellite for estimating a current position of a module arranged on a base body of the satellite, as well as a method for calibrating a stored coordinate system transformation between a module of a satellite and a base body of the satellite.

[0003] Artificial satellites orbiting the Earth can typically communicate with other satellites or ground stations via radio waves; however, optical communication is increasingly being introduced. In optical communication, light signals are sent from the satellite to the ground station or another satellite, or received from one of them. The light signals serve as information carriers for transmitting a data stream between two satellites, or between a ground station and a satellite.

[0004] In this context, US 2016 / 0043800 A1 relates to a CubeSat module for a CubeSat, wherein the CubeSat module comprises an optical transmitter for transmitting data to a remote terminal; a receiver for detecting an optical beacon from the remote terminal; and a fine aiming module operatively coupled to a coarse aiming module of the CubeSat for aiming the optical transmitter at the remote terminal with an accuracy range that overlaps with an accuracy range of the coarse aiming module of the CubeSat, in order to establish a communication link between the CubeSat and the remote terminal over a low Earth orbit (LEO) distance.

[0005] To establish communication to and / or from a satellite using light signals in the visible or invisible range, a satellite sensor or emitter must be aligned with its communication partner. This, in turn, requires determining the position of a sensor module or emitter module on the satellite, particularly relative to an Earth-based or inertial coordinate system.

[0006] The position of the satellite, sensor module, or receiver module is each an orientation, which is preferably described by attitude angles relative to a reference coordinate system, such as an Earth-fixed coordinate system or an inertial coordinate system. Quaternions, instead of three attitude angles relative to Cartesian coordinate systems, can also be used to avoid singularities in coordinate system transformation operations. As an alternative to the Earth-fixed coordinate system as a reference system, a coordinate system can be used, for example, whose origin coincides with an imaginary Earth center and moves with the Earth in its orbit around the Sun. However, this coordinate system does not participate in the Earth's rotation with a period of approximately 24 hours, but rather has a constant orientation relative to an imaginary inertial coordinate system.

[0007] To determine the current attitude, i.e. orientation of the satellite, a star sensor mounted on the satellite can be used. This sensor is pointed into space and determines the attitude of the satellite based on the known self-luminous objects such as stars visible to the star sensor. If the attitude of the satellite changes, the detection range of the star sensor also changes and with it the luminous objects it detects. However, the measurements made by a star sensor are sometimes affected by a low signal-to-noise ratio, or usable sensor data is not available at all, for example if the star sensor is blinded by another source, such as the sun or by the reflection of sunlight off another satellite. In addition, satellites can be equipped with inertial measuring units to determine the current attitude, in particular mechanical inertial gyroscopes or fiber optic rings.

[0008] All of these attitude determination systems are typically mounted on a satellite's base body, but not on an optical sensor module or emitter module of the satellite, which may exhibit unknown angular deviations relative to the satellite's support structure. Star sensors combined with inertial measurement units take up a certain amount of space and result in high costs, while relatively small satellites are increasingly being used, particularly for low Earth orbits (often called "CubeSats" due to their cube shape). These typically use cost-effective and inevitably space-saving sensor systems such as star sensors or inertial measurement units, whose measurement quality is correspondingly suboptimal. Furthermore, star sensors may not provide any or even accurate results at high satellite rotation rates.

[0009] The object of the invention is to improve the attitude estimation of an optical module on a satellite for the purpose of optical communication. The invention is based on the features of the independent claims. Advantageous developments and refinements are the subject of the dependent claims.

[0010] A first aspect of the invention relates to a device for a satellite for estimating a current position of a module arranged on a base body of the satellite and tiltable relative to the base body, which module has a communication sensor for receiving optical communication signals, comprising a computing unit and the communication sensor and an actuator for tilting the module relative to the base body, wherein the computing unit is designed to execute an estimation algorithm based on sensor fusion for estimating a position of the module and to use for the estimation algorithm as a first data source a signal dependent on an angle of incidence of a light beam emitted by a transmitter located remote from the satellite in conjunction with a relative position between the satellite and the transmitter known to the computing unit,and to use at least one further data source for the estimation algorithm based on a signal from an attitude sensor arranged on the base body of the satellite, as well as to control the actuator depending on a position of the base body used to initialize the estimation algorithm and estimated on the basis of the at least one further data source, to execute a limited search pattern for the communication sensor.

[0011] The module serves as a carrier for the communication sensor and is connected to the main body of the satellite. Typically, the attitude sensor or several attitude sensors are arranged in or on the main body of the satellite. The term attitude sensor is functional, i.e. the purpose of the measured data is to determine the attitude of the main body of the satellite; this does not mean that the attitude sensor signal must necessarily contain attitude angles or quaternions. For example, an attitude is only determined through temporal integration of a yaw rate measurement. As explained at the beginning, the attitude indicates an orientation which can be described, for example, using attitude angles or quaternions relative to a reference coordinate system. The reference coordinate system is preferably an inertial coordinate system, or an inertial coordinate system whose origin moves with the Earth as it orbits the sun, or which is assumed to be fixed to the sun.

[0012] While the base body and module could be constructed as a single piece, the two-piece case is considered here, where the relative orientation between the base body and the module can be adjusted using an actuator. In particular, at such a rotary joint, unknown rotations can occur in applied coordinate system transformations between a coordinate system fixed to the body of the module and a coordinate system fixed to the base body. In this case, as explained below, the most accurate calibration possible with respect to the relative orientation of the module to the base body must be determined.

[0013] Against this background, position estimates of the module, which are performed using the signals from the communication sensor itself, and position estimates of the base body, which are determined using the signals from a position sensor arranged on the base body, must be transformed into a common coordinate system, preferably the coordinate system imaginarily arranged on the module body, in order to be able to determine the position of the module relative to the reference coordinate system explained above. This coordinate system transformation is determined based on the relative orientation between the module and the base body.

[0014] Analogous to the transformation of the attitude estimates, original signals used for the attitude estimation can also be used, such as a rotation rate in the example above, which can be transformed into a desired coordinate system to be used for the attitude estimation after the transformation.

[0015] The estimation algorithm's task is to estimate the module's current position as realistically as possible. To achieve this, the estimation algorithm uses several input variables, referred to as data sources due to their potentially highly heterogeneous and diverse nature. The estimation algorithm is designed to accept input variables of different frequencies, from different sensors or other sources, with different signal-to-noise ratios and other distinguishing characteristics, in order to generate a single, current estimate of the module's position. For example, the estimation algorithm is a Kalman filter or an extended Kalman filter.

[0016] The first data source comes from the signal of the communication sensor itself. If a light beam, preferably a laser beam, is transmitted from a transmitter located far away from the satellite, for example, another satellite or a ground station, to the satellite with the computing unit, the light beam can be received by the module's communication sensor if the satellite is properly aligned and thus the module is properly aligned. By observing the direction of incidence of the light beam relative to the module, in particular relative to the module's communication sensor, the position of the module can be determined if the relative position between the transmitter and the module (and thus, to a good approximation, the entire satellite with the module) is known to the computing unit.

[0017] The first data source can be used as a highly accurate and reliable data source for determining the module's current attitude, at least when a corresponding light signal from a transmitter reaches the communication sensor. The additional data source from attitude sensors located on the satellite's main body, such as an attitude angle sensor of an inertial measurement unit (IMU), is typically fused in its own Kalman filter on the main body. The results are received at a lower frequency, for example, 2-5 Hz, by the computing unit for use as input in the estimation algorithm. The data from the first data source can have significantly higher data rates, for example, 200 Hz.

[0018] Light emitted by a transmitter, which strikes the communication sensor as a light beam, can be used without information in a fine calibration phase, for example, to simply initiate the optimal alignment of the module before the communication phase in which information such as a television signal, internet data, or similar is transmitted. For such a calibration light beam, for example, a greater divergence can be used than for a light beam with imprinted information, i.e. the calibration light beam has a larger expansion angle in its radiation characteristic than a light beam with imprinted information for transmitting a data stream, since locating the light beam at the communication sensor is easier with increased divergence.

[0019] However, the estimation algorithm must be initialized beforehand because, prior to correct optical communication from the satellite (which itself requires correct orientation of the communication sensor), only the data from the additional data source is available. However, this data can already be used to facilitate the search pattern of the communication sensor for establishing optical communication. Therefore, even before this fine calibration phase can take place, if it is even planned, the communication sensor on the module must execute a search pattern (= a scanning-searching change in the orientation of the communication sensor to locate a transmitter signal).This is conventionally performed across the entire possible travel range of the actuator, but can be limited in advance by appropriately initializing the estimation algorithm with an estimated position obtained (preferably exclusively) from at least one additional data source. This advantageously allows the search pattern to be executed much more efficiently and to more quickly achieve a desired position and thus alignment of the module for correct communication of the communication sensor.

[0020] The estimate of the current attitude of the satellite, i.e., the orientation preferably expressed as attitude angles or quaternions, determined by executing the estimation algorithm can be used to determine a correction of the attitude to a desired attitude of the satellite. For this purpose, a controller can be implemented that continuously performs a target-actual comparison to determine corresponding manipulated variables. The manipulated variables are commands for actuators of the satellite, which are designed to change the attitude of the satellite when appropriately controlled. For this purpose, at least one magnetic torquer and / or at least one reaction wheel can be used.

[0021] The adjustment of the module's position relative to the base body, and on longer timescales of the base body with a trim angle, is used to align the communication sensor so that it can optimally receive optical communication signals from a communication partner at a great distance from the satellite. The communication sensor is therefore, for example, an optical sensor based on the well-known principle / design of QPD, PSD, or CCD. If the module includes not only a communication sensor for receiving optical communication signals, but also a transmitter unit for emitting optical communication signals, the radiation direction must also be directed towards the receiver, which requires precise alignment of the satellite module and, for longer periods, precise alignment of the satellite base body as well.

[0022] For this purpose, the control of the module's orientation relative to the base body, which is adjusted by the actuator, can determine a temporally essentially constant angular deviation to an optimal alignment, and this angular deviation can be transferred as the desired orientation change to a position control of the base body. The higher-frequency control, on the other hand, is preferably carried out in the orientation between the module and the base body by means of the actuator, which can efficiently compensate for any wobble of the satellite.

[0023] It is an advantageous effect of the invention that through the sensor fusion of the communication sensor used as the first data source and one or more data sources from sensors of the main body of the satellite that are typically significantly inferior in quality and data rate (for example yaw rate sensor, star sensor, ...), all data sources that are in principle available for determining the current position of the module of the satellite relative to a predetermined reference coordinate system are combined in order to be able to obtain an improved estimate of the current position of the module.

[0024] According to an advantageous embodiment, the at least one position sensor arranged on the main body of the satellite comprises at least one of the following: yaw rate sensor and / or rotation angle sensor of an inertial measuring unit of the main body of the satellite; star sensor; sun sensor; horizon sensor;

[0025] magnetometer;

[0026] According to a further advantageous embodiment, the rate at which new and current data are fed to the estimation algorithm is higher for the first data source than for the at least one further data source.

[0027] However, an increased rate for new current data for the estimation algorithm can also be used by at least one additional data source in a calibration phase in order to be able to locate the light signal of the transmitter as quickly and accurately as possible and to maneuver the module - possibly together with the rest of the satellite - into an optimal position.

[0028] According to a further advantageous embodiment, the computing unit is designed to compare the position determined by the estimation algorithm with a further estimate of the position determined for the same time and exclusively by means of the at least one further data source for deviation and to determine from the deviation a calibration of a coordinate system transformation between a coordinate system fixed to the base body or to a position sensor arranged on the base body and a coordinate system fixed to the module or to the communication sensor.

[0029] Using this embodiment, a calibration of a coordinate system transformation can take place between a coordinate system fixed to the base body and a coordinate system fixed to the module. Since the communication sensor is fixedly mounted on the module, there is generally a fixed orientation between the communication sensor and the module. A calibration of this coordinate system transformation already performed on the ground should remain unchanged. The measurements of the position sensors on the base body, which feed the at least one additional data source, and the measurements of the angle of incidence of the light beam from a remote transmitter on the communication sensor can therefore be referenced to one and the same coordinate system through this coordinate system transformation.However, a need for recalibration of this coordinate system transformation arises, particularly when, during the launch of a launch vehicle carrying the satellite, shifts or offsets occur between the communication sensor and the module, and / or between the module and the base body, and / or between the base body and an attitude sensor arranged on the base body. Such a change in orientation can also occur due to temperature changes, the development of internal material stresses, or other influences. Even if a magnetometer is used as the sensor on the base body, its measurements can be distorted if, for example, a magnetic torquer is used as an actuator to change the attitude of the satellite.Whether one body-fixed coordinate system is positioned on the module or on the module's communication sensor, and whether the other body-fixed coordinate system is positioned on the base body or on a position sensor located on the base body, depends on expected error sources and possible relative movements. This is the designer's responsibility to determine.

[0030] According to a further advantageous embodiment, the computing unit is designed to leave a position control of the base body unaffected when controlling the actuator to limit the search pattern of the communication sensor.

[0031] According to a further advantageous embodiment, the computing unit is designed to transmit information about a trim angle for adjustment on the base body to the attitude control of the base body after initialization of the estimation algorithm from the estimated position of the module and depending on the relative position of the transmitter to the satellite.

[0032] Preferably, a crossover network is implemented that converts desired orientation changes of the satellite's communication sensor at high frequencies using the actuator, i.e., by means of an orientation change between the module and the base body, while lower frequencies are transmitted as trim angle changes to the attitude control of the base body. According to a further advantageous embodiment, the at least one further data source for the estimation algorithm is the result of a further sensor fusion of signals from at least two attitude sensors arranged on the base body of the satellite.

[0033] While the first data source is coupled to the occurring angle of incidence of an optical signal from a distant transmitter received at the communication sensor, for the at least one further data source not only the result of a direct sensor measurement such as by a yaw rate sensor or a direct attitude angle sensor can be used, but also a processed sensor measurement such as the integration of measured yaw rates, a filtered version of a direct sensor signal (for example filtered by a low-pass filter or high-pass filter or band-pass filter), and in addition also an already determined estimated value as a result of a filter such as a Kalman filter, which for example fuses the signals of a star sensor and the time-integrated signals of a yaw rate sensor.

[0034] According to a further advantageous embodiment, the computing unit is designed to initialize the estimation algorithm with an estimate of the position of the base body determined exclusively using the at least one additional data source, and to preset a search pattern of the communication sensor using the initialized estimation algorithm. This allows the search pattern to be designed with fewer safety factors and thus allows for faster search patterns. Furthermore, the risk of missing an optical communication partner is reduced.

[0035] According to a further advantageous embodiment, the computing unit is designed to execute the estimation algorithm continuously during a communication phase of the satellite, regardless of whether the first data source is present because optical communication signals are currently being received at the communication sensor, or whether the first data source is not present because the reception of the optical communication signals is interrupted.

[0036] Since the estimation algorithm exhibits a certain inertia and tends to converge to reality, continuous execution of the estimation algorithm can prove advantageous if the first data source or at least one of the other fails or is interrupted. However, the estimation algorithm's result is not necessarily used continuously.

[0037] According to a further advantageous embodiment, the computing unit is designed to determine the position of the module only from the first data source when the first data source is present, and otherwise to use the result of the estimation algorithm as the determined position.

[0038] This embodiment is used particularly during data transmission to the communication sensor during the communication phase: In this case, it may not be guaranteed that a light beam with imprinted information is continuously received at the communication sensor. For example, the light beam can be switched off if there is currently no information to be transmitted. However, if a light beam falls on the communication sensor, it can be used to determine the module's position.If this light beam fails, a high-quality estimate of the module's position is available from the estimation algorithm, as this algorithm previously determined a high-quality estimate from sensor fusion using information from the angle of incidence of the light beam and at least one other data source from a sensor arranged on the base body. Due to the inherent inertia of an estimation algorithm such as a Kalman filter, this high-quality estimate can be maintained for some time. Preferably, the estimation algorithm is executed continuously, regardless of whether a light beam is currently incident on the communication sensor or not; the estimation algorithm's result is only used temporarily.This advantageously prevents jumps in the determination of the position of the module, which can occur when switching between the position determination purely from the information based on the angle of incidence of the light beam at the communication sensor to the position determination based solely on the information of a position sensor arranged on the base body and vice versa.

[0039] According to a further advantageous embodiment, the computing unit is designed to execute the estimation algorithm based on sensor fusion, which is configured for the multivariate estimation of the respective position of several modules rotatably arranged on and relative to the satellite, each having a respective communication sensor, and to use for the estimation algorithm as a first data source a signal dependent on an angle of incidence of a light beam emitted by a transmitter located remote from the satellite, occurring at the communication sensor, in conjunction with the relative position between the satellite and the transmitter known to the computing unit, and to use at least one further data source for the estimation algorithm based on a signal from an attitude sensor arranged on the main body of the satellite, as well as to carry out a control allocation for determining a target orientation of the respective module relative to the main body.

[0040] The purpose of control allocation is to optimally adjust various degrees of freedom, namely the independently adjustable orientations between the modules and the base body. The term "optimal" in this context specifically means finding a compromise or, if possible, a global optimum in the orientation of the base body, so that the remaining degrees of freedom to the modules can ensure a desired transmit or receive function for each of the modules. This creates a problem that can be solved, for example, through nonlinear optimization.

[0041] A further aspect of the invention relates to a method for calibrating a stored coordinate system transformation between a module of a satellite and a base body of the satellite, wherein the module and base body are arranged fixedly to one another and together form the satellite, wherein an estimation algorithm based on sensor fusion is executed to estimate the position of the module, wherein a signal dependent on an angle of incidence of a light beam emitted by a transmitter located remote from the satellite, occurring at the communication sensor, is used for the estimation algorithm in conjunction with the relative position between the satellite and the transmitter known to the computing unit, and at least one further data source is used for the estimation algorithm based on the signal of an attitude sensor arranged on the base body of the satellite,and wherein, for calibrating the coordinate system transformation, the position determined by the estimation algorithm is compared with a position of the module determined exclusively by means of the at least one further data source.,

[0042] Advantages and preferred developments of the proposed method result from an analogous and analogous transfer of the statements made above in connection with the proposed device.A further aspect of the invention relates to a method for estimating a respective current position of a module arranged on a base body of the satellite, which module has a communication sensor for receiving optical communication signals, comprising executing an estimation algorithm based on sensor fusion for estimating a position of the module, wherein a signal dependent on an angle of incidence occurring at the communication sensor of a light beam emitted by a transmitter located remote from the satellite is used as a first data source for the estimation algorithm in conjunction with a relative position between the satellite and the transmitter known to the computing unit, and at least one further data source is used for the estimation algorithm based on a signal from an attitude sensor arranged on the base body of the satellite.

[0043] According to a further advantageous embodiment, the position determined by the estimation algorithm is compared for deviation with a further estimate of the position determined for the same time and exclusively by means of the at least one further data source, and from the deviation a calibration of a coordinate system transformation between a coordinate system fixed to the base body or to a position sensor arranged on the base body and a coordinate system fixed to the module or to the communication sensor is determined.

[0044] According to a further advantageous embodiment, the estimation algorithm is initialized with an estimate of the position of the base body determined exclusively by means of the at least one further data source, wherein a search pattern of the communication sensor is preset with the initialized estimation algorithm.

[0045] According to a further advantageous embodiment, the estimation algorithm is executed continuously during a communication phase of the satellite, regardless of whether the first data source is present because optical communication signals are currently being received at the communication sensor, or whether the first data source is not present because the reception of the optical communication signals is interrupted.

[0046] According to a further advantageous embodiment, the position of the module is determined only from the first data source when the first data source is available, and otherwise the result of the estimation algorithm is used as the determined position. Advantages and preferred developments of the proposed method for estimating the current position of a module arranged on a base body of the satellite result from an analogous and analogous application of the statements made above in connection with the proposed device.

[0047] Further advantages, features, and details will become apparent from the following description, which – where appropriate with reference to the drawings – describes at least one embodiment in detail. Identical, similar, and / or functionally equivalent parts are provided with the same reference numerals.

[0048] They show:

[0049] Fig. 1 : A device on a CubeSat for estimating a position of a module of the CubeSat according to an embodiment of the invention.

[0050] Fig. 2: A device on a CubeSat for estimating a position of a module of the CubeSat according to another embodiment of the invention.

[0051] The representations in the figures are schematic and not to scale.

[0052] Fig. 1 shows a device for a satellite for estimating the current position of a module 3 arranged on a base body 1 of the satellite. The base body is cube-shaped, hence the name CubeSat for the satellite. The device is arranged on the module 3 itself and has a communication sensor 5 for receiving and transmitting optical communication signals. In addition, a computing unit 7 is arranged on the module 3. The computing unit 7 and the communication sensor 5 have already been integrated into a common unit during the manufacturing phase. The computing unit 7 implements a Kalman filter for estimating the current position of the module 3. The Kalman filter is an estimation algorithm for generating a current position estimate of the module 3 or of the communication sensor 5 arranged on the module 3 from various data sources. A first data source for the Kalman filter is available in that theThe angle of incidence of the light beam arriving at the communication sensor 5 can be detected and, based on this, with the aid of a known relative position between the satellite and the transmitter, the position of the communication sensor 5, or if the approximation is sufficiently accurate, the position of the module 3 on which the communication sensor 5 is arranged, can be determined. A further data source is obtained by using an attitude sensor 9 arranged on the cube-shaped base body 1 of the CubeSat, which can be, for example, a star sensor, a yaw rate sensor (the signals of which must be integrated to obtain an attitude), or a magnetometer. First, the optical communication is established by initializing the estimation algorithm and subsequent search pattern-based alignment of the module 3. The Kalman filter is initialized with the IMU data of the base body 1, and accordingly, a search pattern executed by the actuator 6 for moving theCommunication sensor 5 for the search for the transmitter is narrowed down accordingly and optical communication is established. Furthermore, a method for calibrating a stored coordinate system transformation between the module 3 of the satellite and the base body 1 of the satellite is applied on the computing unit 7. The first data source for the Kalman filter is the signal dependent on an angle of incidence of a transmitter emitted at a distance from the satellite, in conjunction with the relative position between the satellite and the transmitter, which is known to the computing unit 7. The at least one further data source for the estimation algorithm based on the signal of an attitude sensor arranged on the base body 1 of the satellite is also used. To calibrate the coordinate system transformation, the attitude determined by the estimation algorithm is compared with a position determined exclusively by means of the at least one furtherThe position of Module 3 is compared with the position determined from the data source. This calibration takes into account the fact that Module 3 may have an unknown orientation offset relative to the main body 1 of the satellite. While the orientation between Module 3 and the main body 1 of the satellite can be precisely measured before the launch of the launch vehicle with the satellite to transport it into orbit around the Earth and while still on the ground, due to the high accelerations and / or high temperature changes occurring during the launch of the launch vehicle, displacements and settlements can occur between the satellite stored on the ground and the satellite in orbit. This means that a previously correctly known orientation between Module 3 and the main body 1 of the satellite must be recalibrated upon insertion of the satellite into orbit and ideally from time to time over the product lifetime of the satellite in orbit, sinceFor example, due to sporadic operation of magnetic torquers or other subsystems of the satellite, the magnetic flux on the satellite may change and a new calibration may be necessary, as this affects a magnetometer in the function of an attitude sensor of the base body 1. The calculation is preferably carried out by calculating a coordinate system transformation between a coordinate system fixed to the body of the module 3 and a coordinate system fixed to the body of the base body 1, or alternatively to body-fixed coordinate systems that are supposedly fixed to the respective attitude sensors 9. Only with precise knowledge of the orientation between the module 3 and the base body 1 of the satellite (or the communication sensor 5 and the respective attitude sensor 9) can the estimation of the current attitude of the module 3 described above and further below be correctly carried out by the estimation algorithm executed on the computing unit 7, since theThe signals or final position estimates determined by the attitude angle sensors 9 of the base body 1 are to be transformed into the coordinate system of the module 3 or the communication sensor 5 in order to ultimately determine the current position of the module 3 or the communication sensor 5. Calibration is preferably carried out before the operation of the communication sensor 5 in the communication phase for receiving optical communication signals to obtain information, in order to bring the satellite and thus the module into a desired target position before the regular reception of optical communication signals in order to enable optimal alignment for optimal signal reception. If a transmitter is also provided on the module 3 to transmit optical communication signals to a ground station or to another satellite, the target position must also be precisely assumed in order to direct the optical communication signals to the desired receiver.to be able to transmit. However, the Kalman filter and its position estimation of module 3 can also be used after calibration, for example to compensate for a position error to a desired position by continuously operating the Kalman filter, regardless of whether a data source temporarily fails, for example because a star sensor on the base body 1 is blinded and does not provide any usable data, or because no light beam is incident on the communication sensor 5. The current position estimation can then only be made from the first data source without using the result of the Kalman filter, as long as a light beam is incident on the communication sensor 5, and otherwise the continuously running Kalman filter can be used to use its position estimate.

[0053] Fig. 2 shows a variant of the satellite whose base body 1 corresponds to that in Fig. 1, which has a small cube-shaped base body 1 with an edge length of ten centimeters and is therefore called a CubeSat. However, in contrast to the base body 1 in Fig. 1, several modules 3 with respective communication sensors 5 are arranged on the base body 1 in Fig. 2. However, since the base body 1 can only assume a single position at a specific time, the modules 3 are rotatably mounted within certain limits in their orientation relative to the base body 1. The task of the at least one Kalman filter implemented on a computing unit 7 as an estimation algorithm is to estimate both the current position of the first module 3 and the second module 3 as well as the current position of the base body 1 in order to be able to align each of the modules 3 optimally for communication with a communication partner such as a ground station or another satellite.The first data source is a signal dependent on the angle of incidence of a light beam emitted by a transmitter located remote from the satellite, which occurs at the communication sensor 5, in conjunction with the relative position between the satellite and the transmitter known to the computing unit 7. One or more additional data sources for the Kalman filter can be taken from the position sensors 9 arranged on the satellite's base body 1. A control allocation is then performed to determine a target orientation of the respective module 3 relative to the base body 1, as well as of the base body 1 itself.

[0054] Although the invention has been illustrated and explained in detail by preferred embodiments, the invention is not limited by the disclosed examples, and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention. It is therefore clear that a multitude of variations exist. It is also clear that exemplary embodiments are truly only examples and should not be construed as limiting the scope, possible applications, or configuration of the invention in any way.Rather, the preceding description and the description of the figures enable the person skilled in the art to implement the exemplary embodiments in concrete terms. With knowledge of the disclosed inventive concept, the person skilled in the art can make various changes, for example, with regard to the function or arrangement of individual elements mentioned in an exemplary embodiment, without departing from the scope of protection defined by the claims and their legal equivalents, such as further explanations in the description. List of reference symbols.

[0055] 1 Main body of the satellite

[0056] 3 Satellite module

[0057] 5 Communication sensor

[0058] 6 Actuator 7 Computing unit

[0059] 9 Position sensor on the base body

Claims

Patent claims 1. A device for a satellite for estimating the current position of a module (3) arranged on a base body (1) of the satellite and tiltable relative to the base body, said module having a communication sensor (5) for receiving optical communication signals, comprising a computing unit (7) and the communication sensor (5) and an actuator (6) for tilting the module (3) relative to the base body (1), wherein the computing unit (7) is designed to execute a sensor fusion-based estimation algorithm for estimating a position of the module (3) and to use, as a first data source for the estimation algorithm, a signal dependent on an angle of incidence of a light beam emitted by a transmitter located remote from the satellite, occurring at the communication sensor (5), in conjunction with a relative position between the satellite and the transmitter known to the computing unit (7).and to use at least one further data source for the estimation algorithm based on a signal from a position sensor (9) arranged on the base body (1) of the satellite, and to control the actuator (6) depending on a position of the base body (3) used to initialise the estimation algorithm and estimated on the basis of the at least one further data source, to execute a limited search pattern for the communication sensor (5).

2. Device according to claim 1, wherein the at least one position sensor (9) arranged on the base body (1) of the satellite comprises at least one of the following: yaw rate sensor and / or rotation angle sensor of an inertial measuring unit of the base body of the satellite; star sensor; sun sensor; horizon sensor; magnetometer; 3. Device according to one of the preceding claims, wherein the rate at which new and current data are supplied to the estimation algorithm is higher for the first data source than that of the at least one further data source.

4. Device according to one of the preceding claims, wherein the computing unit (7) is designed to compare the position determined by the estimation algorithm with a position determined for the same time and exclusively by means of the to compare a further estimate of the position determined from at least one further data source for deviation and to determine from the deviation a calibration of a coordinate system transformation between a coordinate system fixed to the base body (1) or to a position sensor arranged on the base body (1) and a coordinate system fixed to the module (3) or to the communication sensor (5).

5. Device according to one of the preceding claims, wherein the computing unit (7) is designed to leave a position control of the base body (1) unaffected when controlling the actuator (6) for limiting the search pattern of the communication sensor (5).

6. Device according to claim 5, wherein the computing unit (7) is designed to transmit, after initialization of the estimation algorithm, from the estimated position of the module (3) and depending on the relative position of the transmitter to the satellite, information about a trim angle for adjustment on the base body (1) to the attitude control of the base body (1).

7. Device according to one of the preceding claims, wherein the computing unit (7) is designed to execute the estimation algorithm continuously during a communication phase of the satellite, regardless of whether the first data source is present because optical communication signals are currently being received at the communication sensor (5), or whether the first data source is not present because the reception of the optical communication signals is interrupted.

8. Device according to claim 7, wherein the computing unit (7) is designed to determine the position of the module (3) only from the first data source when the first data source is present, and otherwise to use the result of the estimation algorithm as the determined position.

9. Device according to one of the preceding claims, wherein the computing unit (7) is designed to execute the estimation algorithm based on sensor fusion, which is configured for multivariate estimation of the respective position of several modules (3) arranged on and relative to the satellite such that they can be tilted, with a respective communication sensor (5), and for the Estimation algorithm to use as a first data source a signal dependent on an angle of incidence of a light beam emitted at the communication sensor (5) of a transmitter located remotely from the satellite in conjunction with the relative position between the satellite and the transmitter known to the computing unit (7), and to use at least one further data source for the estimation algorithm based on a signal from an attitude sensor (9) arranged on the base body (1) of the satellite, as well as to carry out a control allocation for determining a desired orientation of the respective module (3) relative to the base body (1).

10. A method for calibrating a stored coordinate system transformation between a module (3) of a satellite and a base body (1) of the satellite, wherein the module (3) and base body (1) are arranged fixedly to one another and together form the satellite, wherein an estimation algorithm based on sensor fusion is executed to estimate the position of the module (3), wherein a signal dependent on an angle of incidence of a light beam emitted by a transmitter located remote from the satellite, occurring at the communication sensor (5), is used as the first data source for the estimation algorithm in conjunction with the relative position between the satellite and the transmitter known to the computing unit (7), and at least one further data source is used for the estimation algorithm based on the signal of an attitude sensor arranged on the base body (1) of the satellite,and wherein, for calibrating the coordinate system transformation, the position determined by the estimation algorithm is compared with a position of the module (3) determined exclusively by means of the at least one further data source.,

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