Method, transceiver station, identification means and system for identifying an object

Abstract

The invention relates to a method for identifying an object (10), a transceiver station (30), identification means (12) and a system for identifying the object, which comprises a transceiver station (30) with a source (32) for generating a pulse train of pulses of electromagnetic radiation, and an identification means arranged on the object (10). The source (32) sends pulses from the pulse train in the direction of the object (10). A receive signal (52) is transmitted back from the identification means (12) to the transceiver station (30) in response to each transmit signal (50). Two polarisation states of the received signal (52) offset in time by the transit time of the received signal from the object to the transceiver station (30) are detected with two detectors (40) , and an actual identification pattern of the object is formed by aggregating the intensities of the polarisation states of several successive received signals (52). A time shift of the receive signals (52) relative to a trigger signal (42) synchronised with the pulse train of the source (32) is also detected.

Description

[0001] Description

[0002] title

[0003] Method, transceiver station, identification means and system for identifying an object

[0004] State of the art

[0005] The invention relates to a method for identifying an object, in particular a satellite in space, a transceiver station for identifying an object and 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.

[0006] 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 the operation of satellites, but also space situational awareness, collision avoidance, and the recovery of satellites without precise orbital data. The greatest contribution to space situational awareness currently comes from radar measurements. Radar measurements can directly identify 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. The operators of these small satellites often only receive orbital data with a delay, and this data is not always assigned to a specific satellite.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.

[0007] 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.

[0008] US 2016 / 0173196 A1 describes a system and method for identifying a passive target, comprising a laser transmitting source and receiving unit for detecting a laser beam reflected from the target. At least one retroreflector with a waveplate is used as the identification filter. Using a polarization modulator, pulse sequences with different polarizations can be generated, with the polarization of the reflected pulse train being analyzed. The variation diversity of the identification pattern can be increased by emitting laser beams with different wavelengths and by attaching wavelength-sensitive retroreflectors. Disclosure of the Invention

[0009] The object of the invention is to provide a method for identifying an object, in particular a satellite in space, which is robust against air turbulence, angle-tolerant, inexpensive and resource-saving and which allows a large number of objects to be clearly identified.

[0010] A further task is to specify a transmitting-receiving station for carrying out such a method.

[0011] A further task is to provide an identification means for carrying out such a procedure.

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

[0013] A further object is to provide a computer program product for carrying out such a method.

[0014] 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.

[0015] According to one aspect of the invention, a method for identifying an object is proposed, in particular a satellite in space, which comprises at least one transceiver station with a source for generating a pulse train of pulses of electromagnetic radiation and at least one identification means arranged on the object, wherein the source transmits the pulses of the pulse train as a transmission signal in the direction of the object and an electromagnetic reception signal is radiated back to the transceiver station by the at least one identification means of the object in response to each transmission signal.

[0016] At least two polarization states of the received signal are detected with at least two detectors, offset in time by the travel time of the received signal from the object to the transmitting-receiving station, and an actual identification pattern of the object is formed by aggregating the detected intensities of the polarization states of several consecutive received signals, whereby in addition to the polarization states, a temporal shift of the individual received signals relative to a trigger signal synchronized with the pulse train of the source is also detected, and the identification pattern is extended by the information of the temporal shift of the received signal.

[0017] According to a favorable embodiment of the method, the pulses of the pulse train generated by the source can be converted into a multiple pulse, or at least a double pulse with different polarization states. Various methods for generating multiple pulses are described below. It can then be advantageous if the temporal shift of the individual pulses of the multiple pulse can be determined relative to a trigger signal synchronized with the pulse train of the source. Each individual pulse from the multiple pulse forms a received signal.

[0018] This ensures that the time required to capture a complete identification pattern is as short as possible. The impact of any turbulence between the transmitting / receiving station and the object that could disrupt the identification pattern can be minimized.

[0019] The trigger signal is conveniently generated by the pulse train from the source. For example, via a photodiode that detects part of the pulse train. Such a photodiode can be arranged behind a partially transparent mirror, for example.

[0020] It is advantageous if the actual identification pattern is accumulated over several pulses of the pulse train emitted by the source and, if necessary, averaged.

[0021] The object is then conveniently identified by comparing the actual identification pattern with a target identification pattern. Alternatively, the data forming the actual identification pattern can also be evaluated and compared with target parameters as a single parameter (e.g., the angle of the major axis of the polarization ellipsoid resulting from two polarization states).

[0022] Conveniently, the source for generating pulses in a pulse train can comprise an ultrashort pulse laser. The advantage of using laser radiation over radiation from radio transmitters is that laser radiation can be bundled much more effectively and thus focused into a small solid angle. At great distances from objects, such as satellites in space, significantly greater signal strengths and thus system ranges can be achieved. Furthermore, interference can occur when using radio transmitters, which is why only a limited number of radio licenses (each for a specific frequency) can be issued. Because the laser radiation is directional, no problems arise from interference, and no licensing is currently necessary.

[0023] As an identification means, it may be advantageous to use a retroreflector, triple mirror or triple prism to reflect the light emitted by the source antiparallel back to its source.

[0024] The reflected received signal can be detected in a time-resolved manner using a telescope and a detector, and using a time measuring device synchronized with the source, the distance of the retroreflector, and thus of the object, from the source can be calculated from the propagation time of the signal from the source to the detector.

[0025] Furthermore, using this or another timing unit synchronized with the source, the temporal shift of the individual received signals of a multiple pulse relative to each other or relative to the transit time of a single pulse can be recorded. This temporal shift is used to uniquely identify the received signal within the identification pattern.

[0026] According to the invention, the object is equipped with at least one passive optical assembly as an identification means, which comprises at least one retroreflector and other passive optical components, for example, polarization optics such as polarizers, waveplates, or wavelength-dependent transmission filters. This assembly 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 transmitted signal, which is also modulated in these properties, in such a way that unambiguous identification of the object, in particular a satellite, is possible by measuring these properties in the reflected received signal.

[0027] The received signal is redirected by this optical assembly toward the source, limited only by the diffraction of the transmitted signal. This increases the range and signal strength of the transmitted signal and thus also of the received signal.

[0028] 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.

[0029] The at least one identification means of the object can advantageously comprise a retroreflector assembly with a retroreflector and a first optical element, in particular a λ / 4 wave plate, as well as a second optical element, in particular a wire grid polarizer or a λ / 4 wave plate, wherein an optical axis of the second optical element is rotated by an angle relative to an optical axis of the first optical element. As a result, the polarization state of the received signal can be changed in a suitable manner so that the identification means and thus the object can be identified based on the received signal. According to the method according to the invention, this can optionally be carried out with different wavelengths of the transmitted signal in order to increase the number of combinations or identification patterns accordingly by the different wavelengths.Wavelength-sensitive elements are arranged in the identification means.

[0030] According to a favorable embodiment of the method, at least two identification devices spaced apart from one another can be arranged on the object, and, based on a propagation time difference of the reflected signal from each identification device, temporally separated received signals are reradiated as multiple pulses to the transceiver station in response to each transmitted signal. This approach describes one possibility for converting the pulses generated by the source into multiple pulses.

[0031] The at least two identification means spaced apart from one another can be mounted, for example, as two or more purely passive optical assemblies, such as a combination of retroreflectors, optical filters, and polarization optics, at a distance from the object to be detected, such as a satellite. The distance is expediently large enough that the received signal reflected back by both identification means can be individually detected with a high degree of probability due to the difference in the propagation times of the pulses.

[0032] The probability of selectively detecting the received signals is determined by the typically expected propagation time difference between the identification devices. This propagation time difference depends on the angle of the transmitter / receiver unit relative to the object and can vary depending on the rotation and position of the object. According to a favorable embodiment of the method, the source can comprise a laser. In particular, the source can comprise a pulsed laser that emits laser pulses with a pulse length of no more than 1 nanosecond and a pulse repetition rate of at least 100 pulses per second.

[0033] According to a favorable embodiment of the method, laser pulses can be emitted with a pulse duration that is small compared to the maximum time difference of the light reflected by the at least two identification means.

[0034] For the described application, the laser can emit short laser pulses, for example picosecond pulses, in order to temporally resolve the signal from various retroreflector assemblies mounted on the satellite.

[0035] In addition, a high repetition rate and a low beam divergence of the laser are advantageous in order to accumulate the recorded identification patterns in a short time and thus increase the number of detected photons reflected back by the retroreflector.

[0036] According to a favorable embodiment of the method, the transceiver station can comprise a polarization state generator configured such that each pulse of the pulse train generated by the source is converted into a multiple pulse, and each individual pulse from the multiple pulse can be transmitted as a transmission signal, each with a different transmission signal polarization state. This type of conversion of pulses generated by the source into multiple pulses can be combined with the previously mentioned method by using multiple identification means on the object. The number of pulses within the received signal multiple pulse is then determined by the number of pulses within the transmitted signal multiple pulse multiplied by the number of identification means.

[0037] Conveniently, the transmission signals emitted as multiple pulses with different polarization states can then be emitted as exactly two transmission signals, with one transmission signal having a right circular polarization state (RC) and the other transmission signal having a left circular polarization state (LC).

[0038] 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.

[0039] According to a favorable embodiment of the method, the multiple pulses with different polarization states can be emitted at a distance that is large compared to the distance of the multiple pulses that are reflected back by the at least two identification means in response to each transmission signal.

[0040] Advantageously, the spacing of the multiple pulses emitted as transmission signals can then be adapted to the spacing of the multiple pulses re-emitted by the at least two identification means such that the multiple pulses re-emitted by the at least two identification means can just be resolved. According to a favorable embodiment of the method, an actual identification pattern of the object can be formed by combining the intensities I1, I2, I3, I4 detected during the determination of the polarization of at least two multiple pulses re-emitted by the identification means, which are offset in time from one another due to the multiple pulses of the transmission signals.

[0041] 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.

[0042] 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.

[0043] 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. According to a favorable embodiment of the method, the transmitted 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 source, which transmits transmission signals of different wavelengths during the different transmission phases.

[0044] 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.

[0045] 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.

[0046] 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.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 source of electromagnetic radiation for emitting a pulsed transmission signal as a pulse train to the object and a telescope for receiving received signals reflected by the object. The receiving station comprises a polarization state analyzer for determining the received signal polarization states of the received signals reflected by the object. The polarization state analyzer comprises a time measurement unit for detecting the polarization and the temporal shift of the pulses of a multiple pulse at a reference time synchronized with the pulse train of the source.

[0047] The ground-based transceiver station, for example, has a laser source that emits laser light parallel to the optical axis of a telescope. 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. Additionally, 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.

[0048] It is advantageous if the source comprises a laser capable of emitting laser pulses with a pulse length of no more than 1 nanosecond and a pulse repetition rate of at least 100 pulses per second. At the transceiver station, the reflected light is collected by a telescope, focused, and transmitted via the polarization state analyzer (PSA) to at least two detectors, each of which, for example, is a single-photon detector.

[0049] The detector can thus register the received signal with high sensitivity.

[0050] Conveniently, the polarization state can include a timing unit synchronized with the source's pulse train. Using the timing unit, temporal shifts of the individual multiple pulses relative to a reference time can be measured.

[0051] It is advisable to use fast photodiodes or photomultipliers as detectors with evaluation electronics that can resolve time domains in the sub-nanosecond or picosecond range. This allows the polarization of the multiple pulses within the pulse train generated by the source to be recorded temporally.

[0052] It is particularly advantageous if the polarization state analyzer can fully capture the polarization of the multiple pulse for each pulse emitted by the source.

[0053] For this purpose, it is particularly advantageous if the polarization state analyzer spatially separates the received signal according to the polarization state and provides a detector for each polarization state to receive the received signal. In particular, each detector can be designed as a single-photon detector.

[0054] For example, spatial separation can be achieved using beam splitters with polarization filters or, even better, polarization cubes.

[0055] According to a favorable embodiment of the transmitting-receiving station, the polarization state analyzer can comprise at least one beam splitter with a polarization filter and at least two fast photodetectors.

[0056] In a favorable design, the transceiver station can include a timing unit for recording the propagation time between the transmission of the transmitted signal and the reception of the received signal. The timing unit can be used to assign the propagation time of the photon, measured between a photodiode at the laser output and the detector on the telescope, to each detected photon.

[0057] According to a favorable embodiment of the transceiver station, the source can comprise a laser configured to emit laser pulses with a pulse length of at most 1 nanosecond and a pulse repetition rate of at least 100 pulses per second. According to a favorable embodiment of the transceiver station, the polarization state generator can be configured to split each transmitted signal of the pulse train into multiple pulses with different polarization states.

[0058] Such a polarization state generator can expediently comprise at least one beam splitter, in particular at least two beam splitters, a wave plate and a delay line.

[0059] It is advantageous if the delay line is designed in such a way that the pulse spacing of the multiple pulse generated by the polarization state generator is greater than the maximum propagation time difference of the light reflected by several identification means arranged on the object.

[0060] According to a favorable embodiment of the transceiver station, the polarization state generator can be designed to set the polarization state of one pulse of the multiple pulse as right circularly polarized (RC) and the polarization state of another pulse of the multiple pulse as left circularly polarized (LC).

[0061] The 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.

[0062] This allows the polarization state of the individual pulses of the transmitted signal to be changed using suitable optical components such as a λ / 4 wave plate or wire grid polarizers, in order to identify the identification medium with which the signals interacted. According to a favorable embodiment of the transceiver station, the source can be designed to emit a transmitted signal with at least two different wavelengths. In particular, the source can have at least two lasers with different wavelengths. For example, a laser with two different wavelengths can be used as the source, which emits transmitted signals of different wavelengths at different transmission phases. 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.

[0063] According to a further aspect of the invention, an identification means for identifying an object, in particular a satellite in space, is proposed, at least comprising at least one retroreflector assembly, which retroreflector assembly is designed to set 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.

[0064] According to the invention, the object to be identified is equipped with a passive optical assembly as an identification means, which comprises at least one retroreflector and other passive optical components, for example, polarization optics such as polarizers, waveplates, or wavelength-dependent transmission filters. This assembly 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 source, which is also modulated in these properties, in such a way that unambiguous identification of the satellite is possible by measuring these properties in the reflected received signal. The transmitted signal can advantageously be a pulsed laser beam.

[0065] According to a favorable embodiment of the identification means, the retroreflector assembly can comprise a retroreflector and a first optical element, in particular a λ / 4 wave plate, arranged first in the direction of incidence of the transmitted signal, as well as a second optical element, in particular a wire grid polarizer or a λ / 4 wave plate, 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.

[0066] The retroreflector assembly advantageously 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, as well as the angle, various retroreflector assemblies can be produced, which are polarimetrically identifiable by remote detection. According to a favorable embodiment, the identification means can comprise at least one spectral filter with which a transmitted beam with at least one wavelength can be filtered out. Thus, transmitted beams with multiple wavelengths can be assigned to different identification means.

[0067] 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.

[0068] 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.

[0069] 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 source for generating a pulse train of pulses of electromagnetic radiation, as well as at least one identification means arranged on the object. The source transmits the pulses of the pulse train as a transmission signal in the direction of the object, and an electromagnetic reception signal is reflected back to the transceiver station by the at least one identification means of the object in response to each transmission signal. The computer program product comprises at least one computer-readable storage medium containing program instructions that can be executed on a computer system and cause the computer system to execute a method as described above.At least two polarization states of the received signal are detected with at least two detectors, offset in time by the propagation time of the received signal from the object to the transceiver station, and an actual identification pattern of the object is formed by aggregating the detected intensities of the polarization states of several consecutive received signals. In addition to the polarization states, a temporal shift of the individual received signals relative to a trigger signal synchronized with the source's pulse train is also detected, and the identification pattern is expanded to include information about the temporal shift of the received signal (52).

[0070] The intensity of at least two polarization states of the received signal is determined with temporal resolution with respect to the multiple pulses. An actual identification pattern of the object is formed by combining the detected intensities of the polarization states of the received signal. The object is identified by comparing the actual identification pattern with a desired identification pattern.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] Drawings

[0077] 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 conveniently consider the features individually and combine them into useful further combinations. The figures show, by way of example:

[0078] Fig. 1 is a schematic overview of a system for identifying an object, in particular a satellite in space, according to an embodiment of the invention;

[0079] Fig. 2 is a schematic representation of a transmitting-receiving station of the system according to Figure 1;

[0080] Fig. 3 is a schematic representation of a system with two identification means for identifying an object, with a polarization state analyzer for simultaneously measuring different polarization states of the received signal, and with a polarization state generator for generating double pulses from each pulse of the pulse train emitted by the source;

[0081] Fig. 4 is a schematic representation of a retroreflector assembly of an identification means of the system according to Figure 1;

[0082] Fig. 5 shows measured intensities of a received signal for a system according to Figure 3;

[0083] Fig. 6 calculated symmetry parameters for twelve different retroreflector assemblies with normal incidence of the transmitted signal;

[0084] Fig. 7 shows measured symmetry parameters for twelve different retroreflector assemblies with normal incidence of the transmitted signal; Fig. 8 shows measured symmetry parameters of a retroreflector assembly as a function of the angle of incidence of the transmitted signal; and

[0085] Fig. 9 measured symmetry parameters of another retroreflector assembly as a function of a rotation angle on the retroreflector assembly.

[0086] Embodiments of the invention

[0087] 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.

[0088] 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.

[0089] Figure 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.

[0090] 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.

[0091] Figure 2 shows a schematic representation of a transceiver station 30 of the system 100 according to Figure 1.

[0092] The transceiver station 30 has a source 32 of electromagnetic radiation for transmitting a transmitted signal 50 to the object 10 and a telescope 34 for receiving a received signal 52 reflected by the object 10. The source 32 emits a linearly polarized transmitted signal 50. The source 32 can comprise a laser, which is particularly designed to emit laser pulses with a pulse length of at most 1 nanosecond and a pulse repetition rate of at least 100 pulses per second. The laser pulses emitted by the laser form a laser pulse train.

[0093] A partial intensity of the laser pulses is detected by a detector 42 to generate a trigger signal. The trigger signal is used to synchronize the temporal shift of the pulses from the received signal 52 with the pulses emitted by the source.

[0094] The transmission signal 50 leaving the source 32 passes through a polarization state generator 36, which is provided to provide the transmission signal 50 with different transmission signal polarization states. The polarization state generator 36 generates a double pulse, wherein the first pulse of the double pulse is provided with a first transmission signal polarization state and the second pulse of the double pulse is provided with a second transmission signal polarization state. The polarization state generator 36 is configured, 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.

[0095] The polarization state generator 36 can advantageously be configured to split each transmission signal 50 of the pulse train into multiple pulses with different polarization states. For this purpose, the polarization state generator 36 can, for example, comprise at least two beam splitters 70, 72, a wave plate, and a delay line 71.

[0096] The polarization state generator 36 may comprise a polarization beam splitter 70, as shown in Figure 3, for splitting the laser beam into two arms. A delay line 71 is provided in one arm to temporally delay the two pulses split by the beam splitter 70. The polarization state generator 36 comprises another polarization beam splitter 72 to recombine the two beam paths.

[0097] Ideally, the delay line 71 is so long that the pulses have a propagation time difference that is greater than the maximum by several identification means 12 arranged on the object 10.

[0098] A polarization state analyzer 38 is provided at the output of telescope 34 to select several received signal polarization states RC, LC of the received signal 52 reflected by object 10 and to transmit them spatially separated to different detectors 40. Such a polarization state analyzer 38 is also shown in more detail in Fig. 3. The polarization state and the time information are transmitted to the evaluating computer 46 via a time measurement unit 44. The detectors 40 are provided to receive the received signal 52 and can be designed, in particular, as single-photon detectors.

[0099] The timing unit 44 is electrically connected to each 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.

[0100] The 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 source 32 can have at least two lasers 32 with different wavelengths for this purpose. The identification of the object at at least two different wavelengths can furthermore be carried out via a plurality of transceiver stations 30, whose transmission signal 50 is generated with lasers 32 of different wavelengths. Furthermore, the generation of electromagnetic radiation with different wavelengths can be achieved by using non-linear crystals (e.g., for frequency doubling).

[0101] The computer 46 can be part of a data processing system for executing a data processing program comprising computer-readable program instructions to perform a method for identifying the object 10. For this purpose, a computer program product can 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. Figure 4 shows a schematic representation of a retroreflector assembly 13 of an identification means 12 of the system 100 according to Figure 1.

[0102] 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.

[0103] 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 zeroth-order waveplate is advantageously used for the λ / 4 waveplate, which has a low dependence of the phase shift on the temperature and the angle of incidence.

[0104] 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.

[0105] The transmitted signal 50 is incident at an angle of incidence and a rotation angle θ of the first optical element 16 against the direction of incidence 24 of the retroreflector assembly 13. By selecting the central optical element 18 and the angle α of the second optical element 18, various retroreflector assemblies 13 can be realized, which can be identified by polarimetric analysis of the received signal 52.

[0106] 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.

[0107] 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 source 32 of electromagnetic radiation and at least one identification means 12 arranged on the object 10, comprises the steps described below.

[0108] The source 32 generates a pulse train of electromagnetic radiation using a laser. Furthermore, the source 32 generates a multiple pulse from each pulse within the pulse train, consisting of several consecutive transmission signals with different transmission signal polarization states. For example, one pulse can be transmitted with a first transmission signal polarization state RC and another pulse with a different transmission signal polarization state LC. The source 32 transmits the transmission signal 50, prepared as a multiple pulse, with the electromagnetic radiation and different transmission signal polarization states within the multiple pulse in the direction of the object 10. An electromagnetic reception signal 52 is re-radiated from the at least one identification means 12 of the object 10 in response to each of the transmission signals 50 within the multiple pulse, directed toward the transceiver station 30.

[0109] The intensity of the polarization states I1, I2, I3, I4 of the received signal 52 is detected from at least one first received signal polarization state RC, LC and at least one second, different received signal polarization state RC, LC.

[0110] This detection of the intensities is conveniently carried out by aggregating the number of measured photons over several pulses of a pulse train, whereby both the polarization state and the temporal shift of the received signal 52 are determined in comparison to the propagation time of a single pulse.

[0111] 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 object 10 is then identified 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 Figures 4 to 8.

[0112] 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.

[0113] The source 32 can advantageously comprise a laser which emits laser pulses with a pulse length of at most 1 nanosecond and a pulse repetition rate of at least 100 pulses per second, in particular for measuring the transit time t and for realizing different polarization states.

[0114] Figure 5 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 for each retroreflector CCR1, CCR2 result from the number of detected photons in the interval τ1, τ2, τ3, τ4 for the received signal polarization states RC and LC measured using the two detectors 40, aggregated over several pulses of the pulse train emitted by the source.

[0115] Photons and their travel time t are detected in the polarization state analyzer 38 as a function of the arrival time and the detector. Due to their different distances from the transmitting / receiving station 30, the two retroreflectors CCR1, CCR2 can be differentiated by the deviation Δt from the average travel time t. Therefore, each detected photon (indicated in Figure 5 as x symbols and deviations dT from the average travel time t) can be assigned to one of the two retroreflector assemblies CCR1, CCR2. For each detector, there are four different, temporally offset measurement signals, since the two temporally offset transmitted signal polarization states are combined with the temporally offset received signals of the two retroreflectors. Thus, four intensities h, I2, I3, I4 are measured for each retroreflector CCR1, CCR2: I1 = I(Tx = RC, Rx = RC)

[0116] I2= I(Tx = RC, Rx = LC)

[0117] I3= I(Tx = LC, Rx = RC)

[0118] I4= l(Tx = LC, Rx = LC)

[0119] As can be seen in Figure 5, for the first retroreflector assembly CCR1, only intensity I3, i.e., for the emission of left-circularly polarized light (Tx=LC) and detection of right-circularly polarized light (Rx=RC), was measured at a detector 40 designated D1 in the figure. For the retroreflector assembly CCR2, identical intensities I1 (Tx=RC and Rx=RC) were measured at one detector 40, designated D1 in the figure, and I4 (Tx=LC and Rx=LC) at the other detector 40, designated D2 in the figure.

[0120] With the definitions I a = I1+ l2and I b = I3+ I4, the following symmetry parameters P1, P2, P3 can be derived from these intensities I1, I2, I3, I4: which each have a value range from 1 to -1.

[0121] The symmetry parameters P1, P2, P3 of the retroreflector assembly 13 described in Figure 4 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. The Müller matrix of a waveplate ("Linear Retarder") M LR (θ , δ) with a fast axis along 0 and the phase shift δ (e.g. 90° for a λ / 4 wave plate) is given by: M LR (θ , δ)

[0122] The Müller matrix of a polarizer M P (θ) with the polarization axis along 0 is:

[0123] For metal-coated retroreflectors and small angles of incidence Φ, the Müller matrix is ​​approximately that of an ideal mirror:

[0124] The Müller matrices of the retroreflector assemblies M WP (with second wave plate) and M P (with polarizer) are:

[0125] MCCR,WP(θ, α) = M LR (- θ ,90°) M LR (-(θ + α) , 90°) M CCR M LR ((θ

[0126] + a) , 90°) M LR (0 , 9O°)

[0127] M CCR , P (θ, α) = M LR (- θ , 90°) M P (-(θ + α)) M CCR M P (θ + α) M LR( θ , 90°) 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 0 on the light path to the retroreflector corresponds to an angle of -θ on the return path, since the optical component transmits in the opposite direction.

[0128] For the intensities I1 and I2, right circularly polarized light with the Stokes vector (l,0,0,l) T 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. T 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 n1 of the Stokes vector for the entire system.

[0129] I1= M P (0°)M LR (+45° , 90°)M CCR WP / P (θ, α)(l, 0,0,1) τ n1

[0130] 12= M P (0°)M LR (— 45° , 90°)M CCR WP / P (θ, α)(l,0,0,l) τ n1

[0131] 13= M P (0 o )M LR (+45 o , 90 o )M CCR WP / P (θ, α)(l,0,0, — l) τ n1

[0132] 14= M P (0°)M LR (— 45 o , 90°)M CCR WP / P (θ, α)(l,0,0, — l) T n1

[0133] This results in the symmetry parameters:

[0134] Modules with second waveplate: P1 = 0, p2= - p3= cos(4α) Modules with polarizer: Pi = sin(2a), p2= p3= - sin(2α).

[0135] 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 radiation. 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 distinguish the assemblies as easily as possible (even taking measurement errors into account). Figure 6 shows 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) 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. In Figure 7, symmetry parameters P1, P2, P3 (with the reference numerals 60 for the symmetry parameter P1, 62 for the symmetry parameter P2, 64 for the symmetry parameter P3) measured with a test arrangement are plotted against each other for twelve different retroreflector assemblies with the parameters given 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 clearly agree well 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 ​​of the symmetry parameters P1, P2, and P3.

[0136] Additionally, the angular dependence of the symmetry parameters, both with respect to the angle of incidence Φ and the rotation 9, was measured for selected retroreflector assemblies. This is important because the orientation of the object, especially the satellite, is generally unknown during identification.

[0137] In Figure 8, 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.

[0138] In Figure 9, measured symmetry parameters P1, P2, P3 of the further individual retroreflector assembly n = 1 are plotted on the retroreflector assembly as a function of the rotation angle Q. Here, too, the symmetry parameters P1, P2, P3 are largely independent of the rotation angle Q. The angle-independent curves of the symmetry parameters P1, P2, P3 demonstrate that the symmetry parameters P1, P2, P3 represent robust parameters for identifying objects using the method according to the invention.

[0139] 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.

[0140] 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 calculated using the binomial coefficient. 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 numerals

[0141] 10 objects

[0142] 12 means of identification

[0143] 13 Retroreflector assembly

[0144] 14 Retroreflector

[0145] 16 First optical element

[0146] 18 Second optical element

[0147] 20 Optical axis

[0148] 22 Optical axis

[0149] 24 Direction of incidence

[0150] 30 Transceiver Station

[0151] 32 Source

[0152] 34 telescope

[0153] 36 polarization state generator

[0154] 38 Polarization state analyzer

[0155] 40 detector

[0156] 42 Photodiode for trigger signal

[0157] 44 Time measurement unit

[0158] 46 computers

[0159] 48 Mount

[0160] 50 transmission signal

[0161] 52 Reception signal

[0162] 60 symmetry parameters P1

[0163] 62 Symmetry parameter P2

[0164] 64 symmetry parameters P3

[0165] 70 polarization beam splitters (PBS)

[0166] 71 Deceleration section

[0167] 72 polarization beam splitters (PBS)

[0168] 73 polarization beam splitters

[0169] 100 systems

[0170] 200 Earth's surface

Claims

Claims 1. Method for identifying an object (10), in particular a satellite in space, with a system (100) comprising at least one transmitting / receiving station (30) with a source (32) for generating a pulse train of pulses of electromagnetic radiation and at least one identification means (12) arranged on the object (10), wherein the source (32) transmits the pulses of the pulse train as a transmit signal (50) in the direction of the object (10) and an electromagnetic receiving signal (52) is reflected back to the transmitting / receiving station (30) from the at least one identification means (12) of the object (10) in response to each transmit signal (50), wherein the time delay of the received signal from the object (10) to the transmitting / receiving station (30) is offset by the transit time of the received signal.at least two polarization states of the received signal (52) are detected by at least two detectors (40) and an actual identification pattern of the object is formed by aggregating the detected intensities of the polarization states of several successive received signals (52), wherein, in addition to the polarization states (52), a temporal shift of the individual received signals (52) relative to a trigger signal (42) synchronized with the pulse train of the source (32) is also detected and the identification pattern is extended by the information of the temporal shift of the received signal (52).

2. Method according to claim 1, wherein the pulses of the pulse train generated by the source (32) are converted into a multiple pulse, at least a double pulse with different polarization states, in particular wherein the time shift of the individual pulses from the multiple pulse is determined relative to a trigger signal (42) synchronized with the pulse train of the source (32).

3. Method according to one of the preceding claims, wherein at least two spaced-apart identification means (12) are arranged on the object (10) and, due to a time difference of the reflected transmitted signal, temporally separated received signals from each identification means are reflected back to the transmitting-receiving station (30) as multiple pulses in response to each transmitted signal (50), thus converting the pulses generated by the source (32) into multiple pulses.

4. Method according to any of the preceding claims, wherein laser pulses are emitted with a pulse duration that is small compared to the maximum transit time difference of the reflected transmitted signal due to the arrangement of the at least two identification means (12).

5. Method according to one of the preceding claims, wherein the transmit-receive station (30) comprises a polarization state generator (36) configured such that each pulse of the pulse train generated by the source (32) is converted into a multiple pulse and each individual pulse from the multiple pulse is transmitted as a transmit signal (50) with a different transmit signal polarization state.

6. The method of claim 5, wherein exactly two transmit signals (50) are emitted, one of the transmit signals (50) having the polarization state RC and the other transmit signal (50) having the polarization state LC.

7. Method according to claim 3 and one of claims 5-6, wherein the transmit signal multiple pulses with different polarization states are emitted at a distance that is large compared to the distance of the receive signal multiple pulses that are reflected back in response to each transmit signal (50) by the at least two identification means (12).

8. Method according to claim 7, wherein the spacing of the multiple pulses emitted as transmit signals (50) is adapted to the spacing of the multiple pulses of the received signal reflected by the at least two identification means (12) such that the multiple pulses reflected by the at least two identification means (12) can just be resolved.

9. Transmit-receive station (30) for identifying an object (10), in particular a satellite in space, comprising at least one source (32) of electromagnetic radiation for transmitting a pulsed transmit signal (50) as a pulse train to the object (10) and comprising a telescope (34) for receiving received signals (52) reflected from the object (10), wherein the receiving station comprises a polarization state analyzer (38) for determining the received signal polarization states of the received signals (52) reflected from the object (10), wherein the polarization state analyzer (38) comprises a time measurement unit (44) to detect the polarization and the time shift of the pulses of a multiple pulse to a reference time synchronized with the pulse train of the source (32).

10. Transceiver station according to claim 9, wherein the polarization state analyzer (38) comprises at least one beam splitter (73) with polarization filter, and at least two fast photodetectors (40).

11. Transceiver station according to claim 9 or 10, wherein a time measuring unit (44) is provided for recording a transit time (t) between sending the transmit signal (50) and receiving the receive signal (52).

12. Transceiver station according to one of claims 9-11, wherein the source (32) comprises a laser configured to emit laser pulses with a pulse length of at most 1 nanosecond and a pulse repetition rate of at least 100 pulses per second.

13. Transceiver station according to one of claims 9-12, wherein a polarization state generator (36) is provided which is configured to divide each transmit signal (50) of the pulse train into multiple pulses with different polarization states.

14. Transmitting-receiving station nnaacchh Claim 13, wherein the The polarization state generator (36) comprises at least two beam splitters (70, 72), a wave plate and a delay section (71).

15. Transceiver station according to one of claims 13-14, wherein the polarization state generator (36) is configured to set the polarization state of one pulse of the multiple pulse as right circularly polarized (RC) and the polarization state of another pulse of the multiple pulse as left circularly polarized (LC).

16. Identification means (12) for identifying an object (10), in particular a satellite in space, comprising at least one retroreflector assembly (13) designed as a retroreflector assembly (13) to set a received signal polarization state 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. Identification means according to claim 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 an incident direction (24) of the transmitted signal (50), and a second optical element (18), in particular a wire mesh polarizer or a λ / 4 wave plate, arranged in the incident direction (24) behind the first optical element (16), wherein an optical axis (22) of the second optical element (18) is rotated by an angle (α) relative to an optical axis (20) of the first optical element (16), whereby the received signal polarization state of the received signal (52) is adjustable.

18. System (100) for identifying an object (10), in particular a satellite in space, comprising a method according to any one of claims 1 to 8, comprising at least one transmit-receive station (30) according to any one of claims 9 to 15 and at least one identification means (12) according to any one of claims 16 to 17, which is arranged on the object (10).

19. Computer program product for identifying an object (10), in particular a satellite in space, comprising a system (100) comprising at least one transmit-receive station (30) with at least one source (32) for generating a pulse train of pulses of electromagnetic radiation and at least one identification means (12) arranged on the object (10), wherein the source (32) transmits the pulses of the pulse train as a transmit signal (50) in the direction of the object (10) and an electromagnetic receive signal (52) is directed back to the transmit-receive station (30) from the at least one identification means (12) of the object (10) in response to each transmit signal (50), wherein the computer program product comprises at least one computer-readable storage medium comprising program instructions that are executable on a computer system and cause the computer system to execute a method, in particular according to at least one of claims 1 to 8.wherein, at least two polarization states of the received signal (52) are detected with at least two detectors (40) at intervals of the transit time of the received signal from the object (10) to the transmitting / receiving station (30), and an actual identification pattern of the object is generated by aggregating the detected intensities of the polarization states of several successive, Received signals (52) are formed, and in addition to the polarization states, a temporal one is also considered. The shift of the individual received signals (52) relative to a trigger signal (42) synchronized with the pulse train of the source (32) is detected and the identification pattern is extended by the information of the temporal shift of the received signal (52).

20. Data processing system for executing a data processing program which includes computer-readable program instructions for executing a method for identifying an object (10), in particular according to at least one of claims 1 to 8.

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