DE102023125978A1 - Attitude estimation of an optical satellite terminal - Google Patents

Abstract

The invention relates to a device for a satellite for estimating a current position of a module (3) arranged on a base body (1) of the satellite, which module has a communication sensor (5) for receiving optical communication signals, comprising a computing unit (7) and an actuator (6) for tilting the module (3) relative to the base body (1) and the communication sensor (5), wherein the computing unit (7) is designed to execute an estimation algorithm based on sensor fusion for estimating a position of the module (3) and to use for the estimation algorithm, as a first data source, a signal dependent on an angle of incidence, occurring at the communication sensor (5), 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 (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.

Description

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.

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.

The US 2016/0043800 A1 relates in this context to a CubeSat module for a CubeSat, the CubeSat module comprising an optical transmitter for transmitting data to a remote terminal; and 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 a range of accuracy that overlaps with a range of accuracy of the coarse aiming module of the CubeSat to establish a communication link between the CubeSat and the remote terminal over a low Earth orbit (LEO) distance.

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.

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.

To determine the current attitude, i.e., the orientation of the satellite, a star sensor mounted on the satellite can be used. This sensor is directed 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 thus the luminous objects it detects. However, 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, in particular mechanical inertial gyroscopes or fiber optic rings, to determine the current attitude.

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 can each 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 accurate results at high satellite rotation rates.

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 results from the features of the independent claims. Advantageous further development lations and embodiments are the subject of the dependent claims.

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 occurring at the communication sensor of a light beam emitted by a transmitter located remotely 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 a position sensor arranged on the base body of the satellite, as well as to use 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 to control a limited search pattern for the communication sensor.

The module serves as a carrier for the communication sensor and is connected to the satellite's main body. Typically, the attitude sensor or several attitude sensors are arranged in or on the satellite's main body. The term attitude sensor is functional, i.e., the purpose of the measured data is to determine the attitude of the satellite's main body; this does not mean that the attitude sensor signal must necessarily contain attitude angles or quaternions. For example, a position is only determined through the temporal integration of a yaw rate measurement. As explained at the beginning, the attitude indicates an orientation, which can be described, for example, by 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.

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 pivot 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.

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.

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.

The estimation algorithm's task is to estimate the current position of the module as realistically as possible. To achieve this, the estimation algorithm uses multiple 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 features, 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.

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 processing 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 communication communication sensor of the module, 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.

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 is received at the communication sensor. The additional data source from attitude sensors arranged 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 of which are received at a lower frequency, for example, 2-5 Hz, by the computing unit for use as an input in the estimation algorithm, while the data from the first data source can have significantly higher data rates, for example, 200 Hz.

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.

However, the estimation algorithm must be initialized beforehand because, prior to correct optical communication from the satellite (which itself requires correct position alignment 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 to establish optical communication. Therefore, even before this fine calibration phase can take place, if it is even planned, the communication sensor must execute a search pattern on the module (= 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 with appropriate initialization of the estimation algorithm with an estimated position obtained (preferably exclusively) from the at least one additional data source. This advantageously allows the search pattern to be performed much more efficiently and the desired position and thus alignment of the module for correct communication from the communication sensor to be achieved more quickly.

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 target attitude of the satellite. For this purpose, a controller can be implemented that continuously performs a target-actual comparison to determine corresponding control variables. The control variables are commands for the satellite's actuators, 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.

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, also precise alignment of the satellite base body.

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.

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.

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: angular rate sensor and/or angle of rotation sensor of an inertial measuring unit of the main body of the satellite; star sensor; sun sensor; horizon sensor; magnetometer;

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.

However, an increased rate for new current data can also be used for the estimation algorithm 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.

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.

Using this embodiment, a calibration of a coordinate system transformation can be performed 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 attitude sensors on the base body, which feed 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 a new calibration of this coordinate system transformation arises in particular when, during the launch of a launch vehicle with the satellite, displacements 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.

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.

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.

Preferably, a crossover is implemented which converts desired orientation changes of the satellite's communication sensor at high frequencies by means of 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 main body of the satellite.

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 meter 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.

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.

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.

Because the estimation algorithm exhibits a certain inertia and tends to converge to reality, continuous execution of the estimation algorithm may 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.

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.

This embodiment is used in particular during data transmission to the communication sensor in the communication phase: Under certain circumstances, it cannot be guaranteed that a light beam with imprinted information is continuously received at the communication sensor. The light beam can, for example, 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 position of the module. If this light beam fails, a high-quality estimate of the module's position is available from the estimation algorithm, since this algorithm previously determines a high-quality estimate from sensor fusion using information originating from the angle of incidence of the light beam and the at least one additional data source from a sensor arranged on the base body. Due to the inherent inertia of an estimation algorithm such as a Kalman filter, it can maintain this high-quality estimate for some time. The estimation algorithm is preferably executed continuously, regardless of whether a light beam is currently falling on the communication sensor or not; the use of the estimation algorithm's result is only carried out temporarily. This advantageously prevents jumps in the determination of the position of the module, such as 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.

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 arranged rotatably 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 a module arranged on the main body of the satellite position sensor, as well as to carry out a control allocation to determine a target orientation of the respective module relative to the base body.

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.

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 occurring at the communication sensor of a light beam emitted by a transmitter located remote from the satellite 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, 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.

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 of a light beam emitted by a transmitter located remote from the satellite occurring at the communication sensor 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 a position sensor arranged on the base body of the satellite.

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.

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.

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.

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 present, and otherwise the result of the estimation algorithm is used as the determined position.

Advantages and preferred developments of the proposed method for estimating a current position of a module arranged on a base body of the satellite result from an analogous and analogous transfer of the statements made above in connection with the proposed device.

Further advantages, features and details emerge from the following description, in which - where appropriate with reference to the drawings tion - at least one embodiment is described in detail. Identical, similar, and/or functionally equivalent parts are provided with the same reference numerals.

• 1: Eine Vorrichtung an einem CubeSat zum Schätzen einer Lage eines Moduls des CubeSats gemäß einem Ausführungsbeispiel der Erfindung.
• 2: Eine Vorrichtung an einem CubeSat zum Schätzen einer Lage eines Moduls des CubeSats gemäß einem weiteren Ausführungsbeispiel der Erfindung.

They show:

• 1 : A device on a CubeSat for estimating a position of a module of the CubeSat according to an embodiment of the invention.
• 2 : A device on a CubeSat for estimating a position of a module of the CubeSat according to another embodiment of the invention.

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

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 has a Kalman filter implemented 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 the angle of incidence of the light beam arriving at the communication sensor 5 can be detected. 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 (whose signals must be integrated to obtain an attitude), or a magnetometer. First, optical communication is established by initializing the estimation algorithm and subsequent search pattern-supported alignment of the module 3. The Kalman filter is initialized with the IMU data of the base body 1. Accordingly, a search pattern executed by the actuator 6 for moving the communication sensor 5 to search for the transmitter is narrowed down accordingly, and optical communication is established. A method for calibrating a stored coordinate system transformation between the module 3 of the satellite and the main body 1 of the satellite is also 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 light beam emitted by a transmitter located remotely 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. The at least one further data source for the estimation algorithm based on the signal of an attitude sensor arranged on the main 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 attitude of the module 3 determined exclusively using the at least one further data source. This calibration takes into account the fact that the 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, displacements and settlements can occur between the satellite stored on the ground and the satellite in orbit due to the high accelerations and/or high temperature changes that occur during the launch of the launch vehicle. This means that a previously correctly known orientation between module 3 and the main body 1 of the satellite must be recalibrated when the satellite is put into orbit and ideally from time to time over the product lifetime of the satellite in orbit. This is because, for example, the sporadic operation of magnetic torquers or other subsystems of the satellite can change the magnetic flux on the satellite and require a new calibration because this influences a magnetometer in the function of an attitude sensor of the main body 1. The calculation is preferably carried out by calculating a coordinate system transformation between a coordinate system fixed to the body of module 3 and a coordinate system fixed to the body of base body 1, or alternatively to body-fixed coordinate systems that are supposedly fixed to the respective position sensors 9. Only with precise knowledge of the orientation between module 3 and the base body 1 of the satellite (or the communication sensor 5 and the respective position sensor 9) can the estimation of the current position of module 3 described above and further below be carried out by the estimation algorithm executed on the computing unit 7. be carried out correctly, since the signals or finished estimates of the position determined by the one or more attitude angle sensors 9 of the base body 1 must also be transformed into the coordinate system of the module 3 or the communication sensor 5 in order to ultimately be able to determine the current position of the module 3 or the communication sensor 5. The 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 in order to transmit optical communication signals itself to a ground station or to another satellite, the target position must also be assumed precisely in order to be able to transmit the optical communication signals directed at the desired receiver. However, the Kalman filter and its position estimation of module 3 can also be used after calibration, for example to correct 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 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 estimation.

2 shows a variant of the satellite, whose base body 1 is similar to that of 1 which has a small cube-shaped base body 1 with an edge length of ten centimeters and is therefore called CubeSat. On the base body 1 of the 2 However, in contrast to the 1 Several modules 3, each with its own communication sensors 5, are arranged. However, since the base body 1 can only assume a single position at a given 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 optimally align each of the modules 3 for communication with a communication partner, such as a ground station or another satellite. The first data source used is a signal dependent on an angle of incidence of a light beam emitted by a transmitter located remotely from the satellite, which occurs at the communication sensor 5, in conjunction with the relative position between the satellite and the transmitter, which is known to the computing unit 7. One or more further data sources for the Kalman filter can be taken from the position sensors 9 arranged on the base body 1 of the satellite. A control allocation to determine a target orientation of the respective module 3 with respect to the base body 1 as well as the base body 1 itself is then carried out.

Although the invention has been illustrated and explained in detail by means of preferred embodiments, the invention is not limited by the disclosed examples, and other variations can be derived therefrom by those skilled in the art without departing from the scope of protection of the invention. It is therefore clear that a multitude of possible variations exist. It is also clear that embodiments mentioned by way of example really only represent examples that are not to be understood in any way as limiting the scope of protection, possible applications, or configuration of the invention. Rather, the foregoing description and the description of the figures enable those skilled in the art to specifically implement the exemplary embodiments. The skilled person, with knowledge of the disclosed inventive concept, can make various changes, for example with regard to the function or the 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

1

Main body of the satellite

3

Satellite module

5

Communication sensor

6

Actuator

7

Computing unit

9

Position sensor on the base body

QUOTES CONTAINED IN THE DESCRIPTION

This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 2016/0043800 A1 [0003]

Claims (10)

Device for a satellite for estimating a 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 an estimation algorithm based on sensor fusion for estimating a position of the module (3) 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 (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, as well as to use 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 control a limited search pattern for the communication sensor (5). 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: rotation 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; Apparatus 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. 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 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 (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). 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). Device according to Claim 5 , wherein the computing unit (7) is designed, after initialization of the estimation algorithm, to transmit information about a trim angle for adjustment on the base body (1) to the attitude control of the base body (1) from the estimated position of the module (3) and depending on the relative position of the transmitter to the satellite. 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. 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. 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 the multivariate estimation of the respective position of a plurality of modules (3) arranged tiltably on and relative to the satellite, each having a respective communication sensor (5), 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 (5), 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). Method for calibrating a stored coordinate system transformation between a module (3) of a satellite and a base body (1) of the satellite, wherein module (3) and Base bodies (1) are arranged fixedly to one another and together form the satellite, wherein an estimation algorithm based on sensor fusion is carried out 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), in conjunction with the relative position between the satellite and the transmitter known to the computing unit (7) is used for the estimation algorithm as the first data source, and at least one further data source is used for the estimation algorithm based on the signal of a position 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.

Images