Method for identifying an object, transceiver station, identification means, system for identifying an object, computer program product and a data processing system

Abstract

Method for identifying an object (10), in particular a satellite in space, with a system (100) which comprises at least one transmitting-receiving station (30) with at least one transmitting source (32) of electromagnetic radiation and at least one identification means (12) arranged on the object (10), wherein the transmitting source (32) transmits a transmitting signal (50) with the electromagnetic radiation in the direction of the object (10), wherein an electromagnetic reception signal (52) is radiated back to the transmitting-receiving station (30) by the at least one identification means (12) of the object (10) in response to the transmission signal (50), wherein the transmission signal (50) is transmitted in a first transmission phase with at least one first transmission signal polarization state (RC, LC) and in a second transmission phase with at least one second, different transmission signal polarization state (RC, LC), and the intensity (I1, I2, I3, I4) of the reception signal (52) is detected in each case with at least one first reception signal polarization state (RC, LC) and at least one second, different reception signal polarization state (RC, LC) when the transmission signal (50) is transmitted in the first transmission phase with the at least first transmission signal polarization state (RC, LC) and in the second transmission phase with the at least second transmission signal polarization state (RC, LC), wherein a time measuring unit (44) detects a transit time (t) between transmission of the transmission signal (50) and reception of the reception signal (52), wherein an actual identification pattern of the object (10) is formed by combining the detected intensities (I1, I2, I3, I4) of the received signal (52), and wherein the object (10) is identified by comparing the actual identification pattern with a target identification pattern.

Description

State of the art

[0001] The invention relates to a method for identifying an object, in particular a satellite in space. The invention further relates to a transceiver station, an identification means, a system for identifying an object, in particular a satellite in space, as well as a computer program product and a data processing system for carrying out such a method.

[0002] Due to the increasing use of space, including commercial ones, there is great interest in methods for the ground-based detection of space objects. This not only simplifies satellite operation, but also space situational awareness, collision avoidance, and the recovery of satellites without precise orbital data. The largest contribution to space situation awareness currently comes from radar measurements. Radar measurements can directly detect large satellites based on their external dimensions. However, this is generally not possible for microsatellites, such as CubeSats, especially since numerous satellites are sometimes deployed into space during a single rocket launch.

[0003] The operators of these small satellites often receive orbital data only with a delay and this data is not always assigned to a specific satellite.

[0004] There is currently no standard technology to facilitate the identification of space objects from Earth. One existing option is to equip satellites with radio transmitters combined with GPS receivers. These radio signals can be received using large radio telescopes. An alternative is the use of pulsed, coded optical laser diodes on the satellite and detection using a telescope and single-photon detector at a ground station.

[0005] DE 102013101730 A1 describes a method for the unique identification of an object, in which a unique identification means is arranged on or in an object designed as a spacecraft. A transmission signal comprising electromagnetic waves is sent from a transmitting station to the identification means of the spacecraft located in space. An identification signal comprising electromagnetic waves, returned by the identification means of the spacecraft in response to the transmission signal, is received at a receiving station. The identification signal then makes it possible to identify the spacecraft.

[0006] US 2016 / 0173196 A1 describes a system and method for contactless identification of a passive target using a laser radiation transmitter and receiver. The target can be on land, in the air, in space, or in water. The receiver registers laser radiation reflected back from a retroreflector with a delay filter, whose polarization properties are evaluated in a processor. Disclosure of the invention

[0007] The object of the invention is to provide a resource-saving method for identifying an object, in particular a satellite in space, which allows a large number of objects to be uniquely identified.

[0008] A further task is to specify a transmitting-receiving station for carrying out such a method.

[0009] A further task is to provide an identification means for carrying out such a procedure.

[0010] A further task is to specify a system for carrying out such a procedure.

[0011] A further object is to provide a computer program product for carrying out such a method.

[0012] A further task is to specify a data processing system for carrying out such a method.

[0013] The objects are achieved by the features of the independent claims. Advantageous embodiments and advantages of the invention emerge from the further claims, the description, and the drawings.

[0014] A method is proposed for identifying an object, in particular a satellite in space, using a system comprising at least one transceiver station with at least one transmission source of electromagnetic radiation and at least one identification means arranged on the object, wherein the transmission source transmits a transmission signal containing the electromagnetic radiation in the direction of the object. In this case, an electromagnetic reception signal is re-radiated back to the transceiver station by the at least one identification means of the object in response to the transmission signal. The transmission signal is transmitted in a first transmission phase with at least one first transmission signal polarization state and in a second transmission phase with at least one second, different transmission signal polarization state.

[0015] The intensity of the received signal is detected with at least one first received signal polarization state and at least one second, different received signal polarization state when the transmitted signal is transmitted in the first transmission phase with at least one first transmitted signal polarization state and in the second transmission phase with at least one second transmitted signal polarization state. A time measurement unit detects a propagation time between the transmission of the transmitted signal and the reception of the received signal. An actual identification pattern of the object is formed by combining the detected intensities of the received signal. The object is identified by comparing the actual identification pattern with a desired identification pattern.

[0016] Advantageously, identification with a type of license plate of space objects / satellites is made possible by extending the technology of satellite laser ranging (SLR) in such a way that a clear identification of the space object becomes possible.

[0017] In satellite laser ranging, the object is equipped with a retroreflector, typically a triple mirror or triple prism, which reflects a typically pulsed signal, such as a laser beam, emitted by a transmitting / receiving station, antiparallel back to its transmitting source. The reflected received signal can be detected with time resolution using a telescope and a detector, and the distance of the retroreflector, and thus the object, from the transmitting source can be calculated from the signal's travel time.

[0018] According to the invention, the object is equipped with a passive optical assembly as an identification means, which has at least one retroreflector and further passive optical components, for example polarization optics such as polarizers, wave plates or wavelength-dependent transmission filters.

[0019] This module modifies the properties of the transmitted signal reflected as a received signal—namely, a combination of wavelength-dependent transmission, polarization, and the temporal progression of the reflected intensity—of an incoming transmitting source, which is also modulated in these properties, in such a way that unambiguous detection of the object, especially a satellite, is possible by measuring these properties in the reflected received signal. The transmitted signal can advantageously be a pulsed laser beam.

[0020] A key difference from existing technologies for identifying space objects is that one or more purely passive optical assemblies, such as a combination of retroreflectors, optical filters, and polarization optics, are mounted on the object to be detected, such as a satellite. The passive optical design facilitates integration, especially into a satellite, and enables a long and resource-efficient operating life.

[0021] The received signal is redirected by this optical assembly toward the transmitting source, limited only by the diffraction of the transmitted signal. This increases the range and signal strength of the transmitted signal and thus also the received signal.

[0022] According to the method according to the invention, a polarimetric SLR is carried out, which can optionally be carried out at several wavelengths of the transmitted signal, in which the intensity and propagation time of the signal are recorded systematically and with a high repetition rate at different transmitted and received polarization states of the signal.

[0023] The at least one object identification means can advantageously comprise a retroreflector assembly with a retroreflector and a first optical element, in particular a λ / 4 waveplate, as well as a second optical element, in particular a wire-grid polarizer or a λ / 4 waveplate, wherein an optical axis of the second optical element is rotated by an angle relative to an optical axis of the first optical element. This allows the polarization state of the received signal to be changed in a suitable manner, so that the identification means and thus the object can be identified based on the received signal.

[0024] Due to the large distance between a satellite and a ground station serving as a transmitting / receiving station, it is possible that not the entire reflected intensity of the transmitted signal is captured as the received signal, but only a portion of the diffraction pattern of the laser radiation reflected by the retroreflector. This also allows for the advantageous determination of the intensities of the received signal in a suitable manner, which can be used to identify the object. Instead of a stationary transmitting / receiving station, a portable transmitting / receiving station can be provided on a ship, an aircraft, or the like.

[0025] The advantage of using light radiation over radio transmitters is that light radiation can be bundled much more effectively and thus focused into a narrow solid angle. At great distances from objects, such as satellites in space, significantly greater signal strengths and thus system ranges can be achieved.

[0026] Advantageously, this proposes a purely passive optical method that requires no electrical power supply from the satellite. It also eliminates the need for complex electronic circuitry, which is prone to failure, since the electronics on the satellite are inevitably exposed to ionizing space radiation.

[0027] As described, the method according to the invention can advantageously be used to identify a satellite in space, with the identification means being arranged on the satellite and the transceiver station being located on the Earth's surface. However, it is equally possible for the method to be used, for example, to identify aircraft, ground vehicles, or ships, with the transceiver station in these applications also being located on the ground or on aircraft or satellites.

[0028] According to a favorable embodiment of the method, the intensity of the received signal can be detected depending on the propagation time between the transmission of the transmitted signal and the reception of the received signal and / or depending on the wavelength of the received signal. This advantageously allows multiple identification devices on an object to be differentiated based on the different propagation times of the signals. This facilitates the assignment of the received signal to the individual identification devices, making it more likely that an object can be identified.

[0029] According to a favorable embodiment of the method, the transmission signal can be transmitted in at least two different transmission phases with different wavelengths. For example, a laser with two different wavelengths can be used as the transmission source, which transmits transmission signals of different wavelengths during the different transmission phases.

[0030] Alternatively, two lasers with different wavelengths can be used, each emitting pulses at a time offset. This allows the received signals to be assigned to the different transmission phases and the respective propagation times to be determined.

[0031] For this purpose, the at least one identification means can have at least one spectral filter with which a transmission beam with at least one wavelength can be filtered out. Thus, transmission beams with multiple wavelengths can be assigned to different identification means.

[0032] According to a favorable embodiment of the method, the transmission source can comprise a laser. In particular, the transmission source can comprise a laser that emits laser pulses with a pulse length of no more than 100 nanoseconds and a pulse repetition rate of at least 100 pulses per second.

[0033] For the described application, the laser can emit short laser pulses, for example, picosecond pulses, to temporally resolve the signal from various retroreflector assemblies mounted on the satellite. In addition, a high repetition rate and low beam divergence of the laser are advantageous for detecting many photons reflected back from the retroreflector in a short time.

[0034] According to a favorable embodiment of the method, one of the transmission signal polarization states of the transmission signal in one of the transmission phases can have right-circularly polarized radiation and the other of the transmission signal polarization states in the other of the transmission phases can have left-circularly polarized radiation.

[0035] This allows the individual pulses of the transmitted signal to be changed in their polarization state using suitable optical components such as λ / 4 wave plates or wire grid polarizers in order to identify the identification medium with which the signals interacted.

[0036] According to a favorable embodiment of the method, symmetry parameters can be determined from the measured intensities of the received signal to determine the actual identification pattern. The symmetry parameters can be used to create dimensionless and standardized quantities for evaluating the intensities of the received signal. This allows identification patterns to be defined in a suitable manner, by means of which the object can be identified.

[0037] According to a further aspect of the invention, a transceiver station for identifying an object, in particular a satellite in space, is proposed, comprising at least one transmission source of electromagnetic radiation for transmitting a transmission signal to the object and a telescope for receiving a reception signal reflected by the object. A polarization state generator is provided to transmit the transmission signal in a first transmission phase with a first transmission signal polarization state and in a second transmission phase with a second, different transmission signal polarization state. Furthermore, a polarization state analyzer is provided to select a reception signal polarization state of the reception signal reflected by the object, wherein a time measurement unit is provided for detecting a propagation time between the transmission of the transmission signal and the reception of the reception signal.

[0038] The transmitting / receiving station, which is set up as a ground station, has, for example, a laser as a transmission source that emits laser light parallel to the optical axis of a telescope.

[0039] For the described application, the laser should emit short laser pulses (picosecond pulses) to temporally resolve the signal from various retroreflector assemblies on the satellite. In addition, a high repetition rate and low beam divergence of the laser are advantageous for detecting many photons reflected back from the retroreflector in a short time.

[0040] A polarization state generator (PSG) is placed at the laser output, which switches the originally linearly polarized laser between two transmit signal polarization states, for example, right-circular (RC) and left-circular (LC), within a short time (e.g., 10-20 ms). This polarization modulation can be triggered, for example, by a delay generator and can be achieved using electronically switchable liquid crystal waveplates. An alternative is to move conventional waveplates, for example, made of birefringent quartz crystal, with different orientations into the laser beam using a high-speed filter wheel.

[0041] The modulated, circularly polarized laser beam then hits the satellite and is reflected back to the transmitting / receiving station as a received signal by one or more retroreflector assemblies.

[0042] At the transceiver station, the reflected light is collected by a telescope, focused, and transmitted via a polarization state analyzer (PSA) to a detector, such as a single-photon detector. Similar to a PSG, the PSA transmits either right- or left-circularly polarized portions of the laser radiation to the detector.

[0043] Because a time measurement unit is provided to record a transit time between the transmission of the transmitted signal and the reception of the received signal, the transit time of the photon, measured between a photodiode at the output of the laser and the detector on the telescope, can be assigned to each detected photon by means of the time measurement unit.

[0044] According to a favorable embodiment of the transceiver station, the transmitting source may comprise a laser. In particular, the transmitting source may comprise a laser configured to emit laser pulses with a pulse length of no more than 100 nanoseconds and a pulse repetition rate of at least 100 pulses per second.

[0045] According to a favorable design of the transceiver station, the polarization state generator can be configured to set one of the transmit signal polarization states to right circular polarization and the other of the transmit signal polarization states to left circular polarization in the other of the transmit phases. This allows the individual pulses of the transmit signal to be changed in their polarization state using suitable optical components such as a λ / 4 wave plate or wire grid polarizers, thus enabling the identification medium with which the signals interacted to be identified.

[0046] According to a favorable design of the transceiver station, a detector can be provided for receiving the received signal. In particular, the detector can be designed as a single-photon detector. Thus, each detected photon can be assigned to a retroreflector assembly as an identification means. The detector can thus register the received signal with high sensitivity.

[0047] According to a favorable design of the transceiver station, a delay unit can be provided to trigger the polarization state generator. The delay unit can advantageously synchronize the transmission source, the polarization state generator, and the polarization state analyzer, thus enabling precise time-of-flight detection of the individual detected photons. This allows the intensities of the received signal to be conveniently assigned to the various identification means.

[0048] In a favorable embodiment of the transceiver station, the polarization state generator can comprise an electronically switchable liquid crystal waveplate. This allows the transmit polarization state of the transmitted signal to be switched at a suitable high frequency.

[0049] According to a favorable embodiment of the transceiver station, the polarization state generator can alternatively comprise a waveplate that can be pivoted into the transmission signal. In particular, the polarization state generator can comprise a waveplate that can be pivoted into the transmission signal by means of a filter wheel. In this way, the transmission polarization state of the transmission signal can be suitably switched at high frequency.

[0050] According to a favorable embodiment of the transceiver station, the transmission source can be configured to emit a transmission signal with at least two different wavelengths. In particular, the transmission source can comprise at least two lasers with different wavelengths. For example, a laser with two different wavelengths can be used as the transmission source, which transmits transmission signals of different wavelengths during different transmission phases. Alternatively, two lasers with different wavelengths can be used, each emitting pulses at a different time interval.

[0051] This allows the received signals to be assigned to the different transmission phases and the respective propagation times to be determined.

[0052] According to a further aspect of the invention, a system for identifying an object, in particular a satellite in space, using a method as described above is proposed, comprising at least one such transceiver station and at least one such identification means arranged on the object.

[0053] Advantageously, this proposes a purely passive optical method that requires no electrical power supply from the satellite. It also eliminates the need for complex electronic circuitry, which is prone to failure, since the electronics on the satellite are inevitably exposed to ionizing space radiation.

[0054] According to a favorable embodiment of the system, the identification means may comprise at least one retroreflector assembly, which retroreflector assembly is designed to adjust a received signal polarization state and / or a wavelength of a received signal in response to a received transmitted signal and to return the received signal antiparallel to the transmitted signal.

[0055] Advantageously, the object to be identified is equipped with a passive optical assembly as an identification means, which has at least one retroreflector, as well as other passive optical components, for example polarization optics such as polarizers, wave plates or wavelength-dependent transmission filters.

[0056] This module modifies the properties of the reflected transmission signal—namely, a combination of wavelength-dependent transmission, polarization, and the temporal progression of the reflected intensity—of an incoming transmission source, which is also modulated in these properties, in such a way that unambiguous detection of the satellite is possible by measuring these properties in the reflected reception signal. The transmission signal can advantageously be a pulsed laser beam.

[0057] According to a favorable embodiment of the system, the retroreflector assembly can comprise a retroreflector and a first optical element, in particular a λ / 4 waveplate, arranged first in a direction of incidence of the transmitted signal, as well as a second optical element, in particular a wire grid polarizer or a λ / 4 waveplate, arranged behind the first optical element in the direction of incidence. An optical axis of the second optical element can be rotated by an angle relative to an optical axis of the first optical element, thereby adjusting the received signal polarization state.

[0058] The retroreflector assembly preferably consists of a metal-coated retroreflector, an outer λ / 4 waveplate, and another optical element, a wire-grid polarizer or another λ / 4 waveplate, which is mounted between the outer waveplate and the retroreflector. The optical axis of the optical element is rotated by an angle relative to the optical axis of the outer λ / 4 waveplate. By selecting the central optical component (wire-grid polarizer or λ / 4 waveplate) and the angle, various retroreflector assemblies can be produced, which can be identified polarimetrically using remote detection.

[0059] According to a favorable embodiment, the identification means can comprise at least one spectral filter with which a transmission beam with at least one wavelength can be filtered out. Thus, transmission beams with multiple wavelengths can be assigned to different identification means.

[0060] According to a further aspect of the invention, a computer program product for identifying an object, in particular a satellite in space, is proposed, comprising a system comprising at least one transceiver station with at least one transmission source of electromagnetic radiation and at least one identification means arranged on the object, wherein the transmission source transmits a transmission signal with the electromagnetic radiation in the direction of the object. The computer program product comprises at least one computer-readable storage medium containing program instructions that are executable on a computer system and cause the computer system to carry out a method as described above. In response to the transmission signal, an electromagnetic reception signal is re-radiated back to the transceiver station by the at least one identification means of the object.The transmission signal is transmitted in a first transmission phase with at least one first transmission signal polarization state and in a second transmission phase with at least one second, different transmission signal polarization state.

[0061] The intensity of the received signal is detected with at least one first received signal polarization state and at least one second, different received signal polarization state when the transmitted signal is transmitted in the first transmission phase with at least one first transmitted signal polarization state and in the second transmission phase with at least one second transmitted signal polarization state. An actual identification pattern of the object is formed by combining the detected intensities of the received signal. The object is identified by comparing the actual identification pattern with a desired identification pattern.

[0062] The computer program product comprises software code sections that can be loaded directly into the memory of a digital computer and that carry out the method when the software code sections are executed by the computer.

[0063] The computer program product can be formed by a computer program or, in addition to the computer program, can comprise at least one additional component. The at least one additional component can be implemented as hardware and / or software. An example of the at least one additional component implemented as hardware is a storage medium that is readable by the digital computer and / or on which the software code sections are stored.

[0064] An example of the at least one additional component embodied as software is a cloud application program that is configured to distribute the software code sections to different processing units, in particular different computers, of a cloud computing system, wherein each of the processing units is configured to execute one or more software code sections.

[0065] In particular, the software code sections can be used to carry out the method as described above when the software code sections are executed by the processing units of the cloud computing system.

[0066] According to a further aspect of the invention, a data processing system for executing a data processing program is proposed, which comprises computer-readable program instructions in order to carry out a method for identifying an object, in particular as described above. drawing

[0067] Further advantages will become apparent from the following description of the drawings. The figures illustrate exemplary embodiments of the invention. The figures, the description, and the claims contain numerous features in combination. Those skilled in the art will also expediently consider the features individually and combine them into useful further combinations.

[0068] Examples include: Fig. 1 a schematic overview of a system for identifying an object, in particular a satellite in space, according to an embodiment of the invention; Fig. 2 a schematic representation of a transmitting-receiving station of the system according to Fig. 1; Fig. 3 a schematic representation of a retroreflector assembly of an identification means of the system according to Fig. 1; Fig.4 measured intensities of a received signal from two different retroreflector assemblies for different received signal polarization states; Fig. 5 calculated symmetry parameters for twelve different retroreflector assemblies with normal incidence of the transmitted signal; Fig. 6 measured symmetry parameters for twelve different retroreflector assemblies with normal incidence of the transmitted signal; Fig. 7 measured symmetry parameters of a retroreflector assembly as a function of the angle of incidence of the transmitted signal; and Fig. 8 measured symmetry parameters of another retroreflector assembly as a function of a rotation angle on the retroreflector assembly. Embodiments of the invention

[0069] In the figures, components of the same type or function similarly are designated by the same reference numerals. The figures are merely examples and are not to be construed as limiting.

[0070] The directional terminology used below, including terms such as "left," "right," "top," "bottom," "before," "behind," "after," and the like, is intended solely to enhance understanding of the figures and is in no way intended to limit the scope of the invention. The components and elements depicted, as well as their design and use, may vary according to the considerations of a person skilled in the art and may be adapted to specific applications.

[0071] Fig. 1 shows a schematic overview of a system 100 for identifying an object 10, in particular a satellite 10 in space, according to an embodiment of the invention.

[0072] In this example, the system 100 comprises a transceiver station 30 located on the Earth's surface 200, from which a transmission signal 50, for example a laser beam, is transmitted to a satellite 10 on which two identification devices 12 are located. A reception signal 52 is returned from these identification devices 12 and received and evaluated by the transceiver station 30. The identification devices 12 are preferably passive optical assemblies with retroreflectors.

[0073] Fig. 2 shows a schematic representation of a transceiver station 30 of the system 100 according to Fig. 1.

[0074] The transceiver station 30 has a transmission source 32 of electromagnetic radiation for transmitting a transmission signal 50 to the object 10 and a telescope 34 for receiving a reception signal 52 reflected from the object 10. The transmission source 32 emits a linearly polarized transmission signal 50, designated Lin.

[0075] The transmission source 32 may comprise a laser which is designed in particular to emit laser pulses with a pulse length of at most 100 nanoseconds and a pulse repetition rate of at least 100 pulses per second.

[0076] The transmission signal 50 leaving the transmission source 32 passes through a polarization state generator 36, which is provided to transmit the transmission signal 50 in a first transmission phase with a first transmission signal polarization state RC and in a second transmission phase with a second, different transmission signal polarization state LC.

[0077] The polarization state generator 36 is designed, for example, to set the first transmission signal polarization state RC of the transmission signal 50 to be right circularly polarized and the second transmission signal polarization state LC to be left circularly polarized and then to continuously switch the transmission polarization state RC, LC alternately.

[0078] For this purpose, the polarization state generator 36 can comprise an electronically switchable liquid crystal waveplate for switching the transmitted signal polarization states RC and LC. Alternatively, it is also possible for the polarization state generator 36 to comprise a conventional waveplate, for example, made of birefringent quartz crystal with various orientations, which is moved into the laser beam of the transmitted signal 50, for example, using a high-speed filter wheel.

[0079] A delay unit 42 is provided to trigger the switching function of the polarization state RC, LC of the polarization state generator 36. The delay unit 42 is connected to the transmission source 32 to control this function, so that the switching function runs synchronously with the transmission of a transmission signal pulse.

[0080] A polarization state analyzer 38 is provided at the output of telescope 34 to select a received signal polarization state RC, LC of the received signal 52 reflected by object 10 and forward it to a detector 40. The delay unit 42 is also connected to the polarization state analyzer 38 to synchronize the selection of the received signal polarization state RC, LC of the received signal 52 with the transmitted signal polarization state RC, LC of the outgoing transmitted signal 50. The time information is transmitted to the evaluating computer 46 via a time measurement unit 44.

[0081] The detector 40 is provided for receiving the received signal 52 and can in particular be designed as a single-photon detector.

[0082] The timing unit 44 is electrically connected to the detector 40 and is provided for detecting a transit time t of the transmitted signal 50 between the transmission of the transmitted signal 50 and the reception of the received signal 52. The transit time t of an incoming received signal 52 is also transmitted to the computer 46.

[0083] The transmission source 32 can be further configured to emit a transmission signal 50 with at least two different wavelengths in order to have further combination possibilities for identifying the object 10, which can interact in a suitable manner with the identification means 12 of the object 10. In particular, the transmission source 32 can have at least two lasers 32 with different wavelengths for this purpose.

[0084] The identification of the object at at least two different wavelengths can further be carried out via several transmitting-receiving stations 30, whose transmission signal 50 is generated with lasers 32 of different wavelengths.

[0085] The computer 46 may be part of a data processing system for executing a data processing program comprising computer-readable program instructions for performing a method for identifying the object 10. For this purpose, a computer program product may comprise at least one computer-readable storage medium comprising program instructions executable on the computer 46 and causing the computer 46 to execute the method.

[0086] Fig. 3 shows a schematic representation of a retroreflector assembly 13 of an identification means 12 of the system 100 according to Fig. 1.

[0087] The identification means 12 comprises a retroreflector assembly 13 which is designed to set a received signal polarization state RC, LC of a received signal 52 in response to a received transmitted signal 50 and to return the received signal 52 antiparallel to the transmitted signal 50.

[0088] The retroreflector assembly 13 comprises a retroreflector 14, which is designed as a metal-coated triple prism. Furthermore, the retroreflector assembly 13 comprises a first optical element 16 in the form of a λ / 4 waveplate, arranged first in a direction of incidence 24 of the transmitted signal 50, and a second optical element 18 in the form of a wire grid polarizer or another λ / 4 waveplate, arranged behind the first optical element 16 in the direction of incidence 24. A true zero-order waveplate can advantageously be used for the λ / 4 waveplate, which has a low dependence of the phase shift on the temperature and the angle of incidence.

[0089] The optical axis 22 of the second optical element 18 is rotated by an angle α relative to the optical axis 20 of the first optical element 16, whereby the received signal polarization state RC, LC of the received signal 52 can be adjusted.

[0090] The transmission signal 50 is incident at an angle of incidence ϕ and a rotation angle θ of the first optical element 16 against the direction of incidence 24 of the retroreflector assembly 13.

[0091] By selecting the central optical element 18 and the angle α of the second optical element 18, different retroreflector assemblies 13 can be realized, which can be identified by polarimetric analysis of the received signal 52.

[0092] In an embodiment not shown, the identification means can comprise at least one spectral filter with which a transmission beam 50 having at least one wavelength can be filtered out. Thus, the retroreflector assembly 13 can adjust a wavelength of a reception signal 52 in response to a received transmission signal 50 and return the reception signal 52 antiparallel to the transmission signal 50.

[0093] The method according to the invention for identifying the object 10, in particular a satellite in space, with the system 100, which comprises the transceiver station 30 with the transmission source 32 of electromagnetic radiation and at least one identification means 12 arranged on the object 10, comprises the steps described below.

[0094] The transmission source 32 transmits a transmission signal 50 with the electromagnetic radiation in the direction of the object 10. From the at least one identification means 12 of the object 10, an electromagnetic reception signal 52 is radiated back to the transceiver station 30 in response to the transmission signal 50.

[0095] The transmitted signal 50 is transmitted in a first transmission phase with at least one first transmitted signal polarization state RC, LC and in a second transmission phase with at least one second, different transmitted signal polarization state RC, LC. The intensity I1, I2, I3, I4 of the received signal 52 is detected in each case with at least one first received signal polarization state RC, LC and at least one second, different received signal polarization state RC, LC when the transmitted signal 50 is transmitted in the first transmission phase with at least one first transmitted signal polarization state RC, LC and in the second transmission phase with at least one second transmitted signal polarization state RC, LC.

[0096] An actual identification pattern of the object 10 is formed by combining the detected intensities I1, I2, I3, I4 of the received signal 52. The identification of the object 10 is then performed by comparing the actual identification pattern with a desired identification pattern. To determine the actual identification pattern, symmetry parameters can be determined from the measured intensities I1, I2, I3, I4 of the received signal 52, as shown in the Fig. 4 to 8.

[0097] The intensity I1, I2, I3, I4 of the received signal 52 is detected depending on a propagation time t between the transmission of the transmitted signal 50 and the reception of the received signal 52 and / or depending on a wavelength of the received signal 52. This allows different identification means 12 on an object 10 to be distinguished. For this purpose, the transmitted signal 50 can be transmitted in at least two different transmission phases with different wavelengths.

[0098] The transmission source 32 can advantageously comprise a laser which, in particular for measuring the transit time t and for realizing different polarization states, emits laser pulses with a pulse length of at most 100 nanoseconds and a pulse repetition rate of at least 100 pulses per second.

[0099] In this case, the first transmission signal polarization state RC of the transmission signal 50 can advantageously have right-circularly polarized radiation and the second transmission signal polarization state LC can have left-circularly polarized radiation.

[0100] In Fig.Figure 4 shows measured intensities I1, I2, I3, I4 of a received signal 52 from two different retroreflector assemblies 13 designated CCR1, CCR2 (CCR = corner cube reflector) for different received signal polarization states RC, LC. The intensities I1, I2, I3, I4 result from the number of detected photons in the interval τ1, τ2, τ3, τ4 for each retroreflector CCR1, CCR2.

[0101] Photons and their travel time t are detected at different, periodically switched settings in the time intervals τ1 to τ4, on the polarization state generator 36 and on the polarization state analyzer 38. The two retroreflectors CCR1, CCR2 can be differentiated due to the different distances to the transmitting-receiving station 30 via the deviation Δt from the average travel time t. Therefore, each detected photon (in Fig.4 as x-symbols in time intervals τ1 to τ4 and deviations Δt from the mean travel time t) are assigned to one of the two retroreflector assemblies CCR1, CCR2. Four different polarization settings are measured, since the polarization state analyzer 38 (labeled Tx) and the polarization state analyzer 38 (labeled Rx) are each switched between the transmission of right-circularly (RC) and left-circularly (LC) polarized light.

[0102] For each retroreflector CCR1, CCR2, 4 intensities I1, I2, I3, I4 are measured: I1=I(Tx=RC, Rx=RC) I2=I(Tx=RC, Rx=LC) I3=I(Tx=LC, Rx=RC) I4=I(Tx=LC, Rx=LC)

[0103] As in Fig.4, the first intensity I1 for the combination Tx=RC, Rx=RC measured only signals for the second retroreflector assembly CCR2, while the second intensity I2 for the combination Tx=RC, Rx=LC measured only signals for the first retroreflector assembly CCR1. The third intensity I3 for the combination Tx=LC, Rx=RC measured only a few signals for the first retroreflector assembly CCR1, while the fourth intensity I4 for the combination Tx=LC, Rx=LC measured only signals for the second retroreflector assembly CCR2.

[0104] With the definitions I a = I1 + I2 and I3 = I3 + I4, the following symmetry parameters P1, P2, P3 can be derived from these intensities I1, I2, I3, I4: P1=Ia−IbIa+Ib,P2=I1−I2I1+I2,P3=I3−I4I3+I4 which each have a value range from 1 to -1.

[0105] The symmetry parameters P1, P2, P3 of the Fig.The retroreflector assembly 13 described in Figure 3 can be calculated for various angles α and, depending on whether a wire-grid polarizer or a λ / 4 waveplate is mounted in the assembly. This can be done, for example, using the Müller matrix arithmetic according to Russell A. Chipman, "Handbook of Optics, Chapter 22: Polarimetry," Optical Society of America, McGraw-Hill, New York, 1995, and is described below as an example.

[0106] The Müller matrix of a wave plate (“Linear Retarder”) M LR (θ ,δ) with a fast axis along θ and the phase shift δ (e.g. 90° for a λ / 4 wave plate) is given by: MLR(θ, δ)=[10000cos2(2θ)+sin2(2θ)cos δsin(2θ)cos(2θ)(1−cos δ)−sin(2θ)sin δ0sin(2θ)cos(2θ)(1−cos δ)sin2(2θ)+cos2(2θ)cos δcos(2θ)sin δ0−sin(2θ)sin δ−cos(2θ)sin δcos δ]

[0107] The Müller matrix of a polarizer M P(θ) with the polarization axis along θ is: MP(θ)=0.5[1cos(2θ)sin(2θ)0cos(2θ)cos2(2θ)sin(2θ)cos(2θ)0sin(2θ)sin(2θ)cos(2θ)sin2(2θ)00000]

[0108] For metal-coated retroreflectors and small angles of incidence ϕ, the Müller matrix is ​​approximately that of an ideal mirror: MCCR=[1000010000−10000−1]

[0109] The Müller matrices of the retroreflector assemblies M WP (with second wave plate) and M P (with polarizer) are: MCCR,WP(θ, α)=MLR(−θ, 90°)MLR(−(θ+α), 90°)MCCRMLR((θ+α),90°)MLR(θ, 90°) MCCR,P(θ, α)=MLR(−θ, 90°)MP(−(θ+α))MCCRMP(θ+α)MLR(θ, 90°)

[0110] This takes into account that the polarizer and the waveplate mounted directly on the retroreflector are rotated by the angle α of the first waveplate. Furthermore, an angle of θ on the light's path to the retroreflector corresponds to an angle of -θ on the return path, since the optical component transmits in the opposite direction.

[0111] For the intensities I1 and I2, right circularly polarized light with the Stokes vector (1,0,0,1) τ emitted by the transmitting-receiving station, where the symbol τ stands for the transpose of the vector. For the intensities I3 and I4, left-circularly polarized light with the Stokes vector (1,0,0, -1) is used. τ emitted. After reflection by the retroreflector assembly, the light passes through a λ / 4 plate and a polarizer in the polarization state analyzer 38. The measured intensities are then the first element n̂1 of the Stokes vector for the entire system. I1=MP(0°)MLR(+45°,90°)MCCR,WP / P(0,α)(1,0,0,1)τn^1 I2=MP(0°)MLR(−45°,90°)MCCR,WP / P(0,α)(1,0,0,1)τn^1 I3=MP(0°)MLR(+45°,90°)MCCR,WP / P(0,α)(1,0,0,−1)τn^1 I4=MP(0°)MLR(−45°,90°)MCCR,WP / P(0,α)(1,0,0,−1)τn^1

[0112] This results in the symmetry parameters: Assemblies with second wave plate: P1=0, P2=−P3=cos(4α) Assemblies with polarizer: P1=sin(2α), P2=P3=−sin(2α).

[0113] The following table lists the calculated symmetry parameters P1, P2, and P3 of 12 different retroreflector assemblies No. 1 to No. 12 at normal incidence ϕ = 0° of the laser beam. The angles α were chosen so that the values ​​of the symmetry parameters P1, P2, and P3 are as far apart as possible, in order to easily distinguish the assemblies (even taking measurement errors into account). Assembly No. α Optical element in front of the retroreflector P1 P2 P3 1 0° λ / 4-wave plate 0 1 -1 2 12° λ / 4-wave plate 0 0.67 -0.67 3 18° λ / 4-wave plate 0 0.31 -0.31 4 26.5° λ / 4-wave plate 0 -0.28 0.28 5 33° λ / 4-wave plate 0 -0.67 0.67 6 45° λ / 4-wave plate 0 -1 1 7 -45° Wire mesh polarizer -1 1 1 8 -20° Wire mesh polarizer -0.64 0.64 0.64 9 -10° Wire mesh polarizer -0.34 0.34 0.34 10 10° Wire mesh polarizer 0.34 -0.34 -0.34 11 20° Wire mesh polarizer 0.64 -0.64 -0.64 12 45° Wire mesh polarizer 1 -1 -1

[0114] In Fig. 5, the calculated symmetry parameters P1, P2, P3 (with reference numerals 60 for the symmetry parameter P1, 62 for the symmetry parameter P2, 64 for the symmetry parameter P3) from the table for the twelve different retroreflector assemblies (n = 1 to 12) are plotted against each other at normal incidence ϕ = 0° of the transmitted signal 50. The 12 different retroreflector assemblies can be clearly identified based on the values ​​for the symmetry parameters P1, P2, P3.

[0115] In Fig.Figure 6 plots the symmetry parameters P1, P2, and P3 measured using a test setup (referenced 60 for the symmetry parameter P1, 62 for the symmetry parameter P2, and 64 for the symmetry parameter P3) for twelve different retroreflector assemblies with the parameters specified in the table for the angle α and the optical element in front of the retroreflector, also at normal incidence ϕ = 0° of the transmitted signal 50. The measured values ​​of the symmetry parameters P1, P2, and P3 are clearly in good agreement with the calculated values. This allows the 12 different retroreflector assemblies n = 1 to 12 to be identified with sufficient accuracy based on the values ​​for the symmetry parameters P1, P2, and P3.

[0116] Additionally, the angular dependence of the symmetry parameters, both with respect to the angle of incidence ϕ and the rotation θ, was measured for selected retroreflector assemblies. This is important because the orientation of the object, especially the satellite, is generally unknown during identification.

[0117] In Fig. 7, measured symmetry parameters P1, P2, P3 (with reference numerals 60 for the symmetry parameter P1, 62 for the symmetry parameter P2, 64 for the symmetry parameter P3) of the individual retroreflector assemblies n = 6 are plotted as a function of the angle of incidence ϕ of the transmitted signal 50. This results in a curve of the symmetry parameters P1, P2, P3 that is largely independent of the angle of incidence ϕ.

[0118] In Fig.Figure 8 shows the measured symmetry parameters P1, P2, and P3 of the other individual retroreflector assembly n = 1 plotted against the retroreflector assembly as a function of the rotation angle θ. Here, too, the symmetry parameters P1, P2, and P3 are largely independent of the rotation angle θ.

[0119] The angle-independent courses of the symmetry parameters P1, P2, P3 show that the symmetry parameters P1, P2, P3 represent robust parameters for identifying objects according to the method according to the invention.

[0120] Using the symmetry parameters P1, P2, P3 determined from the intensity measurements, actual identification patterns can be created which, by comparison with stored target identification patterns that can be determined in advance by calculation, enable the identification of unknown objects, in particular satellites in space.

[0121] The number of distinguishable objects depends on the number n of distinguishable retroreflector assemblies (e.g., n=12 for 12 different assemblies) and the number k of assemblies mounted on the object and distinguishable over the runtime. The number of combinations can be determined using the binomial coefficient (n+k−1k) can be calculated. For two assemblies mounted on the object, this results in 78 combinations. If one or two assemblies are mounted, this results in 78 + 12 = 90 combinations. For up to three assemblies, this results in 364 + 78 + 12 = 454 combinations. To further increase the number of combinations, the inventive method can also be extended to multiple wavelengths. The number of retroreflector assemblies can then be increased using spectral filters. At two wavelengths, up to four retroreflector assemblies would be sufficient to make 20,474 satellites identifiable. Reference symbol 10 objects 12 means of identification 13 Retroreflector assembly 14 Retroreflector 16 First optical element 18 Second optical element 20 Optical axis 22 Optical axis 24 Direction of incidence 30 Transceiver Station 32 Broadcast source 34 telescope 36 polarization state generator 38 Polarization state analyzer 40 detector 42 Delay unit 44 Time measurement unit 46 computers 48 Mount 50 transmission signal 52 Reception signal 60 symmetry parameters P1 62 Symmetry parameter P2 64 symmetry parameters P3 100 systems 200 Earth's surface

Claims

[1] Method for identifying an object (10), in particular a satellite in space, with a system (100) which comprises at least one transmitting-receiving station (30) with at least one transmitting source (32) of electromagnetic radiation and at least one identification means (12) arranged on the object (10), wherein the transmitting source (32) transmits a transmitting signal (50) with the electromagnetic radiation in the direction of the object (10), wherein an electromagnetic reception signal (52) is radiated back to the transmitting-receiving station (30) by the at least one identification means (12) of the object (10) in response to the transmission signal (50), wherein the transmission signal (50) is transmitted in a first transmission phase with at least one first transmission signal polarization state (RC, LC) and in a second transmission phase with at least one second, different transmission signal polarization state (RC, LC), and the intensity (I1, I2, I3, I4) of the reception signal (52) is detected in each case with at least one first reception signal polarization state (RC, LC) and at least one second, different reception signal polarization state (RC, LC) when the transmission signal (50) is transmitted in the first transmission phase with the at least first transmission signal polarization state (RC, LC) and in the second transmission phase with the at least second transmission signal polarization state (RC, LC), wherein a time measuring unit (44) detects a transit time (t) between transmission of the transmission signal (50) and reception of the reception signal (52), wherein an actual identification pattern of the object (10) is formed by combining the detected intensities (I1, I2, I3, I4) of the received signal (52), and wherein the object (10) is identified by comparing the actual identification pattern with a target identification pattern. [2] Method according to claim 1, wherein the intensity (I1, I2, I3, I4) of the received signal (52) is detected as a function of the propagation time (t) between transmission of the transmitted signal (50) and reception of the received signal (52) and / or as a function of a wavelength of the received signal (52). [3] Method according to claim 2, wherein the transmission signal (50) is transmitted in at least two different transmission phases with different wavelengths. [4] Method according to one of the preceding claims, wherein the transmission source (32) comprises a laser, in particular a laser which emits laser pulses with a pulse length of at most 100 nanoseconds and a pulse repetition rate of at least 100 pulses per second. [5] Method according to one of the preceding claims, wherein one of the transmission signal polarization states (RC, LC) of the transmission signal (50) in one of the transmission phases has right-hand circularly polarized radiation and the other of the transmission signal polarization states (RC, LC) in the other of the transmission phases has left-hand circularly polarized radiation. [6] Method according to one of the preceding claims, wherein symmetry parameters are determined from the measured intensities (I1, I2, I3, I4) of the received signal (52) to determine the actual identification pattern. [7] Transceiver station (30) for identifying an object (10), in particular a satellite in space, with at least one transmission source (32) of electromagnetic radiation for transmitting a transmission signal (50) to the object (10) and with a telescope (34) for receiving a reception signal (52) reflected by the object (10), wherein a polarization state generator (36) is provided to transmit the transmission signal (50) in a first transmission phase with a first transmission signal polarization state (RC, LC) and in a second transmission phase with a second, different transmission signal polarization state (RC, LC), wherein a polarization state analyzer (38) is provided to select a received signal polarization state (RC, LC) of the received signal (52) reflected by the object (10), wherein a time measuring unit (44) is provided for detecting a transit time (t) between transmission of the transmission signal (50) and reception of the reception signal (52). [8] Transceiver station according to claim 7, wherein the transmission source (32) comprises a laser, in particular a laser designed to emit laser pulses with a pulse length of at most 100 nanoseconds and a pulse repetition rate of at least 100 pulses per second. [9] Transceiver station according to one of claims 7 to 8, wherein the polarization state generator (36) is designed to set one of the transmission signal polarization states (RC) of the transmission signal (50) in one of the transmission phases as right circularly polarized and the other of the transmission signal polarization states (LC) in the other of the transmission phases as left circularly polarized. [10] Transceiver station according to one of claims 7 to 9, wherein a detector (40) is provided for receiving the received signal (52), in particular wherein the detector (40) is designed as a single-photon detector. [11] Transceiver station according to one of claims 7 to 10, wherein a delay unit (42) is provided for triggering the polarization state generator (36). [12] Transceiver station according to one of claims 7 to 11, wherein the polarization state generator (36) comprises an electronically switchable liquid crystal wave plate. [13] Transceiver station according to one of claims 7 to 11, wherein the polarization state generator (36) comprises a wave plate which is pivotable into the transmission signal (50), in particular which is pivotable into the transmission signal (50) with a filter wheel. [14] Transceiver station according to one of claims 7 to 13, wherein the transmission source (32) is designed to transmit a transmission signal (50) with at least two different wavelengths, in particular wherein the transmission source (32) has at least two lasers (32) with different wavelengths. [15] System (100) for identifying an object (10), in particular a satellite in space, with a method according to one of claims 1 to 6, comprising at least one transceiver station (30) according to one of claims 7 to 14 and at least one identification means (12) which is arranged on the object (10). [16] System according to claim 15, wherein the identification means (12) comprises at least one retroreflector assembly (13), which retroreflector assembly (13) is designed to set a received signal polarization state (RC, LC) and / or a wavelength of a received signal (52) in response to a received transmitted signal (50) and to return the received signal (52) antiparallel to the transmitted signal (50). [17] System according to claim 15 or 16, wherein the retroreflector assembly (13) comprises a retroreflector (14) and a first optical element (16), in particular a λ / 4 wave plate, arranged first in a direction of incidence (24) of the transmission signal (50), and a second optical element (18), in particular a wire grid polarizer or a λ / 4 wave plate, arranged behind the first optical element (16) in the direction of incidence (24), wherein an optical axis (22) of the second optical element (18) is rotated by an angle (α) with respect to an optical axis (20) of the first optical element (16), whereby the received signal polarization state (RC, LC) of the received signal (52) can be adjusted. [18] System according to one of claims 15 to 17, further comprising at least one spectral filter with which a transmission beam (50) with at least one wavelength can be filtered out. [19] Computer program product for identifying an object (10), in particular a satellite in space, with a system (100) which comprises at least one transceiver station (30) with at least one transmission source (32) of electromagnetic radiation and at least one identification means (12) arranged on the object (10), wherein the transmission source (32) transmits a transmission signal (50) with the electromagnetic radiation in the direction of the object (10), wherein the computer program product comprises at least one computer-readable storage medium which comprises program instructions which are executable on a computer system and cause the computer system to carry out a method according to at least one of claims 1 to 6, wherein an electromagnetic reception signal (52) is radiated back to the transmitting-receiving station (30) by the at least one identification means (12) of the object (10) in response to the transmission signal (50), wherein the transmission signal (50) is transmitted in a first transmission phase with at least one first transmission signal polarization state (RC, LC) and in a second transmission phase with at least one second, different transmission signal polarization state (RC, LC), and the intensity (I1, I2, I3, I4) of the reception signal (52) is detected in each case with at least one first reception signal polarization state (RC, LC) and at least one second, different reception signal polarization state (RC, LC) when the transmission signal (50) is transmitted in the first transmission phase with the at least first transmission signal polarization state (RC, LC) and in the second transmission phase with the at least second transmission signal polarization state (RC, LC), wherein an actual identification pattern of the object (10) is formed by combining the detected intensities (I1, I2, I3, I4) of the received signal (52), and wherein the object (10) is identified by comparing the actual identification pattern with a target identification pattern. [20] Data processing system for executing a data processing program comprising computer-readable program instructions for carrying out a method for identifying an object (10) according to at least one of claims 1 to 6.

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