Synthetic aperture radar method and synthetic aperture radar system

Abstract

Synthetic aperture radar method for remote sensing of the Earth's surface (300), in which radar signals (RS) are emitted by a transmitter (100) moving above the Earth's surface (300) via a transmitting antenna (110) designed exclusively for transmitting, and radar echoes (RE) of the radar signals (RS) reflected on the Earth's surface (300) are received by at least one receiver (200) moving above the Earth's surface (300) and comprising a multi-channel receiving antenna (210) designed exclusively for receiving, wherein the transmitter (100) and the at least one receiver (200) are located on different platforms above the Earth's surface (300), wherein the transmitter (100) and the at least one receiver (200) as well as a signal processing unit (140, 150, 160) above the Earth's surface perform the following steps: a) The transmitter (100) generates signals in a base frequency band and performs a frequency conversion of these signals into a predetermined frequency band using oscillator signals of an oscillator (121) of the transmitter (100), wherein the converted signals are transmitted as the radar signals (RS); b) the at least one receiver (200) receives the radar echoes (RE) of the radar signals (RS) of the transmitter (100) and transforms the received radar echoes (RE) in real time by phase-preserving modulation from an electrical domain (201) into an optical domain (202), in which a number of receive beams (BR1-BR3) are generated from the received radar echoes (RE) and combined to form an optical signal (OS), wherein the optical signal (OS) contains the number of receive beams (BR1-BR3), each of which contains the signal of a beam with a different wavelength; c) the at least one receiver (200) sends the optical signal (OS) via a free-space optic (230) to the signal processing unit (140, 150, 160); d) The signal processing unit (140, 150, 160) processes the received optical signal (OS) by reconstructing the receive beams (BR1-BR3) from the optical signal (OS) and then transforming them from the optical domain (102) to the electrical domain (101); and e) The signal processing unit (140, 150, 160) demodulates the received modulated receive beams (BR1-BR3) and performs a frequency conversion of the demodulated signals into the base frequency band using oscillator signals from the oscillator (121) of the transmitter (100), thereby obtaining SAR raw data from which the signal processing unit (140, 150, 160) generates and stores digital data, wherein an optical frequency comb with a number of lines (T1-T3) with a respective optical frequency is generated in the optical domain (202), wherein the number of generated lines (T1-T3) corresponds at least to the number of receive beams (BR1-BR3), wherein an amplitude modulation of the respective optical frequency of the number of generated lines (T1-T3) with the received radar echoes (RE) of each receive channel (211-219) of the Multi-channel receiving antenna (210) using associated electro-optical modulators (241-249),which results in a modulated optical frequency comb channel (SBLiT1, SBRiT1 - SBLiT3, SBRiT3 (i = 1...N)) for each receiving channel (CH1-CH9).

Description

[0001] The invention relates to a synthetic aperture radar method and a synthetic aperture radar system.

[0002] Synthetic aperture radar (SAR) methods enable the remote sensing of the Earth's surface by detecting radar pulses reflected from the surface. These pulses are emitted by a radar transmitter moving at a constant speed on a platform above the Earth's surface. The term "Earth's surface" is to be interpreted broadly and may also include the surface of another celestial body (preferably another planet) besides Earth.

[0003] SAR methods utilize the fact that, due to the moving platform, the same areas of the Earth or a celestial body are captured in different positions, thereby obtaining amplitude and phase information and ultimately a radar image of the Earth's surface.

[0004] In so-called monostatic SAR systems, radar echoes are acquired on the same platform as the radar signals are transmitted. In other words, a combined transmitter and receiver is used for both transmitting the radar signals and receiving the resulting radar echoes. Monostatic SAR systems have the disadvantage that no radar echo can be received at the time a radar signal is transmitted, leading to so-called blind ranges in the SAR radar images of the surface. Conventional monostatic SAR systems also exhibit a trade-off between azimuth resolution and swath width (see reference [1]).

[0005] To avoid the disadvantages of monostatic SAR radar systems, so-called bistatic or multistatic SAR radar systems are known from the prior art. Based on the principle of separating transmit and receive functions in different platforms, whereby the received radar echo is relayed back to the transmitter by the receiver(s), and where the receiver(s) are constructed in a transponder-like manner (see references [2], [3]), operation with high-resolution wide-swath (HRWS) is possible. Due to the separation of transmit and receive functions in bistatic or multistatic SAR systems, two different local oscillators are provided in the transmitter and receiver, which complicates phase synchronization, as the SAR systems require coherent operation for proper functionality.Furthermore, the receiver requires the implementation of a method that enables the generation of multiple antenna beams in order to extract information from the radar echo across a wide swath (see EP 3 425 422 B1). The generation of the antenna beams should, if possible, be carried out without significantly increasing complexity.

[0006] On the other hand, bistatic or multistatic SAR systems have the advantage that the multiple antenna beams do not introduce gaps into the captured scene, as is the case with monostatic SAR systems. This also means that the duty cycle during transmission can be increased, which reduces the requirements for maximum transmit power. The greatest advantage of separating the transmitter and receiver into different platforms is that the receiver can be designed relatively simply.

[0007] In principle, this principle can be extended to multiple receivers to perform a wide variety of SAR applications. In particular, a scene can then be captured simultaneously from different angles, enabling specialized processing of the acquired raw SAR data. For example, SAR interferometry can be performed to calculate elevation models of the Earth's surface or to monitor object movements on the Earth's surface. Furthermore, SAR tomography can be performed to capture the vertical structure of semi-transparent volume scatterers, such as vegetation.

[0008] A future HRWS system will be based on the concept described in EP 3 425 422 B1. It is planned to use a so-called double-mirror link for synchronization, whereby a reference signal is transmitted directly to the receivers and reflected back on a higher carrier frequency. Furthermore, it is intended that each receiver will use only a single simultaneous beam, which limits the achievable stripwidth.

[0009] The use of optical frequency combs for optical inter-satellite links is known from the publication SCHMIDT, TD [et al.]: Optical Technologies for Future Global Navigation Satellite Systems. In: IEEE / ION Position, Location and Navigation Symposium (PLANS), 2023. pp. 311-317.

[0010] The object of the present invention is to provide a synthetic aperture radar method and a synthetic aperture radar system which enable the simple generation of multiple receive beams on the receiver side and simultaneously eliminate the need for explicit synchronization of the clocks of two or more platforms.

[0011] These problems are solved by a method according to the features of claim 1 and a system according to the features of claim 10. Furthermore, a receiver is proposed which is designed to be used in a method according to the invention.

[0012] Advantageous configurations arise from the dependent claims.

[0013] In the SAR method according to the invention for remote sensing of the Earth's surface, radar signals are transmitted from a transmitter moving above the Earth's surface via a transmitting antenna designed exclusively for transmission. The radar echoes of the radar signals reflected on the Earth's surface are received by at least one receiver, which also moves above the Earth's surface and comprises a multi-channel receiving antenna designed exclusively for reception. The transmitter and the at least one receiver move on different platforms above the Earth's surface; that is, the movements of the transmitter's and the at least one receiver's platforms are independent of each other. Preferably, the transmitter and the at least one receiver are located on different satellites or aircraft (e.g., airplanes). The direction of movement of the respective platforms is usually referred to as the azimuth direction. In contrast, the altitude direction is referred to as the altitude.The corresponding altitude angle is referred to as elevation, and the direction perpendicular to azimuth and elevation is referred to as the range direction. The SAR method according to the invention is a bistatic method in the case of one receiver and a multistatic method in the case of multiple receivers.

[0014] In the process according to the invention, the transmitter and at least one receiver, as well as a signal processing unit located above the Earth's surface, i.e., a signal processing device on a platform (in particular on a satellite or aircraft) above the Earth's surface, perform the steps a) to e) described below. Preferably, the signal processing unit is arranged on the transmitter's platform.

[0015] In step a), the transmitter generates signals in a base frequency band and performs a frequency conversion of these signals into a predetermined frequency band using oscillator signals from an oscillator of the transmitter, whereby the converted signals are transmitted as the radar signals. This corresponds to the conventional process of generating radar signals on the transmitter side.

[0016] In step b), the at least one receiver receives the radar echoes of the transmitter's radar signals and transforms the received radar echoes in real time from an electrical domain into an optical domain by means of phase-preserving modulation. In the optical domain, a number of received beams are generated from the received radar echoes and combined to form an optical signal. The optical signal contains the number of received beams, each of which contains the signal of a beam with a different wavelength. The phase-preserving modulation is characterized by the fact that the phase of the processed or received radar echoes is not shifted by the modulation. Such phase-preserving modulations are known per se from the prior art and are carried out in the present invention after the transformation into the optical domain.In a special embodiment, electro-optical modulators are used for this purpose, as described in more detail below.

[0017] In step c), the at least one receiver transmits the optical signal to the signal processing unit via a free-space optical system. As described, the optical signal contains the number of received beams, each of which contains the signal of a beam with a different wavelength.

[0018] In step d), the signal processing unit processes the received optical signal by reconstructing the received beams from the optical signal and then transforming them from the optical domain into the electrical domain.

[0019] In step e), the signal processing unit demodulates the received modulated receive beams and performs a frequency conversion of the demodulated signals into the fundamental frequency band using oscillator signals from the transmitter's oscillator, thereby obtaining raw SAR data from which the signal processing unit generates and stores digital data. In a manner known per se, the raw SAR data contains data sampling in the form of amplitude and phase values ​​for a multitude of azimuth and range positions. Preferably, the signal processing unit transmits the stored digital data to a ground station on the Earth's surface. There, the data can then be further processed using methods known per se.

[0020] According to the invention, an optical frequency comb with a number of lines, each with a specific optical frequency, is generated in the optical domain of the receiver. The number of generated lines corresponds at least to the number of received beams. In the simplest form, the number of lines can be 1. The number of lines can also be greater than 1. The lines can be generated in a manner known per se by one or more electro-optical modulators (connected in series or in parallel).

[0021] Furthermore, according to the invention, amplitude modulation of the respective optical frequency of the number of generated lines is performed with the received radar echoes of each receiving channel of the multi-channel receiving antenna by means of associated electro-optic modulators, thereby obtaining a modulated optical frequency comb channel for each receiving channel. For example, Mach-Zehnder modulators or dual-parallel Mach-Zehnder modulators can be used as electro-optic modulators.

[0022] The method according to the invention has the advantage that little computing power is required on the receiver side, resulting in low energy consumption for the steps to be performed. The data of each received beam are modulated on a separate carrier of a frequency comb and simultaneously transmitted between the receiver and transmitter via the free-space optics. The proposed method makes it possible to operate with multiple receivers in different parts of the optical spectrum, thus enabling interferometric data acquisition while simultaneously adhering to stricter minimum phase error requirements.

[0023] The method proposes a solution for transmitting large amounts of information via a continuous multi-receiver-beam HRWS-SAR system over a free-space optic link between two platforms. On the side of at least one receiver, this approach involves only a minimal increase in computing power and memory requirements.

[0024] In a suitable configuration, the optical signal transmitted to the signal processing unit via the free-space optics is amplitude-modulated. Compared to frequency modulation, this has the advantage that no information about the phases needs to be exchanged between the receiver and the transmitter.

[0025] Advantageously, the frequency spacing between two adjacent lines (if the number of lines is greater than 1) is greater than or equal to twice the maximum frequency of the radiated transmit signal in the high-frequency band into which the signals generated by the transmitter in the basic frequency band were converted by means of oscillator signals from the transmitter's oscillator.

[0026] Another advantageous embodiment provides that the modulated optical frequency comb channels are fed to an optical beamforming network, through which the modulated optical frequency comb channels are subjected to a division and / or forwarding as well as a phase shift and / or delay of the channel signals on the basis of previously defined beamforming weights in order to output the number of received beams at the respective assigned outputs of the optical beamforming network.

[0027] The beamforming network can, for example, be based on a Blass matrix that uses optical phase shifters. In this case, the receiver preferably has a reflector antenna that directs or deflects the radar echoes onto the multi-channel receiver unit, since the power distribution of the individual beams across the receiver channels only minimally overlaps. This allows for high efficiency.

[0028] Alternatively, the beamforming network can be based on real-time delays. Choosing a beamforming network based on real-time delays simplifies subsequent signal processing, as it eliminates the need to reintroduce a carrier signal. However, if the bandwidth requirement of the system components needs to be kept as low as possible, this variant also allows for the reintroduction of the carrier signal.

[0029] In particular, at each output of the optical beamforming network, a different receiving beam, corresponding to a line with a specific optical frequency, is selected by means of optical filtering. A bandpass filter is used as the filter to ensure that the frequencies relevant for the different receiving beams are selected.

[0030] It is also advantageous to combine all the received beams selected by filtering into the optical signal using wavelength division multiplexing. As described above, the optical signal comprises the number of received beams, each containing the signal of a received beam with a different wavelength. This allows the information from the received beams to be transmitted amplitude-modulated and simultaneously to the signal processing unit via the free-space optics. For example, one or more multimode interference (MMI) couplers can be used as wavelength division multiplexers.

[0031] It may also be possible to insert additional data into the optical signal. This additional data includes, in particular, calibration data and / or position data.

[0032] On the signal processing unit side, the optical signal is processed by wavelength demultiplexing to reconstruct the number of received beams. Then, further processing of the raw SAR data can take place as described above, by generating and storing digital data. A series of optical add-drop blocks can be used for wavelength demultiplexing.

[0033] In general, known WDM methods can be used for wavelength division multiplexing and wavelength demultiplexing, such as lens- and grating-based, cascaded Mach-Zehnder interferometers, arrayed waveguide gratings, add-drop filters, star couplers, etc.

[0034] According to another aspect, a computer program product is proposed that includes instructions which, when the program is executed by a distributed computing system, cause it to execute the procedure according to one or more configuration variants.

[0035] The invention further relates to a SAR system for remote sensing of the Earth's surface, comprising a transmitter moving above the Earth's surface, which, during operation, emits radar signals via a transmitting antenna designed exclusively for transmitting, and at least one receiver moving above the Earth's surface, comprising a multi-channel receiving antenna designed exclusively for receiving, which, during operation, receives radar echoes of the radar signals reflected from the Earth's surface. The transmitter and the at least one receiver are located on different platforms above the Earth's surface. The transmitter and the at least one receiver, as well as a signal processing unit above the Earth's surface, are configured to carry out the method according to the invention or one or more preferred variants thereof.

[0036] A transmitter for sending radar signals, not included in the invention, is configured to operate as a transmitter in the method according to the invention. In other words, the transmitter is configured to perform step a) of the method according to the invention. Furthermore, the transmitter can be configured to operate as a transmitter in one or more preferred embodiments of the method according to the invention, provided that the embodiments relate to steps that are performed by a transmitter.

[0037] The invention further comprises a receiver for receiving radar echoes, wherein the receiver is configured to operate as a receiver in the method according to the invention. In other words, the receiver is configured to perform steps b) and c) of the method according to the invention. The receiver can further be configured to operate as a receiver in one or more preferred embodiments of the method according to the invention, provided that the embodiments relate to steps that are performed by a receiver.

[0038] Furthermore, a signal processing unit is proposed, wherein the signal processing unit is configured to function as a signal processing unit in the method according to the invention. In other words, the signal processing unit is configured to perform steps d) and e) of the method according to the invention. Optionally, the signal processing unit can also be configured to perform one or more preferred variants of the method according to the invention, provided that the variants relate to steps that are performed by the signal processing unit. In particular, the signal processing unit is a component of the transmitter. In this case, the steps performed by the signal processing unit are also performed by the transmitter.

[0039] The invention is described in more detail below with reference to the accompanying figures. These show: Fig. 1 a schematic representation of the MirrorSAR concept, which is known in itself; Fig. 2 a schematic representation of a SAR system according to the invention, comprising a transmitter and a receiver according to the invention; Fig. 3. A general schematic representation of the frequency spectrum at different locations in the Fig. 2 SAR systems shown; Fig. 4 a schematic representation of the frequency spectrum at different points of the Fig. 2 SAR system shown according to a first special embodiment of electro-optical modulators of the receiver according to the invention; Fig. 5 a schematic representation of the frequency spectrum at different points of the Fig. 2 SAR system shown according to a second special embodiment of electro-optical modulators of the receiver according to the invention; Fig. 6. A schematic representation of the frequency spectrum at different points in the Fig. 2 SAR system shown according to a third special embodiment of electro-optical modulators of the receiver according to the invention; Fig. 7 a schematic representation of the frequency spectrum at different points of the in Fig. 2 illustrated SAR system according to a special embodiment of an optical beam shaping network of the receiver according to the invention; Fig. 8 a schematic representation illustrating the design of a beam shaping unit as a Blass matrix; and Fig. 9 a schematic representation of a node in the Blass matrix from Fig. 4.

[0040] Fig. Figure 1 illustrates the basic concept of MirrorSAR, as described, for example, in references [2], [3], or EP 3 425 422 A1. The SAR system 1 comprises a transmitter 100 and a receiver 200. The transmitter 100 and the receiver 200 move above an Earth surface 300. The direction of movement corresponds to the so-called azimuth direction, while the so-called range direction is perpendicular to the azimuth direction and perpendicular to the vertical direction. The corresponding elevation angle is referred to as the elevation.

[0041] The transmitter 100 comprises a transmitting antenna 110, a transmitting unit 120, an analog receiving antenna 130, a demodulation unit 140, a converter 141, and a data storage device 150. The transmitting unit 120 includes a local oscillator 121, a frequency generator 122 for generating signals in a base frequency band, a modulator 123, and an amplifier 124 connected to the transmitting antenna 110. The amplifier is, for example, a so-called HPA, high-power amplifier.

[0042] The receiver 200 comprises a receiving antenna 210, a receiving unit 220, and an analog transmitting antenna 230. The receiving unit 220 comprises an amplifier 221, a frequency generator 252, and a conversion unit 253 connected to the transmitting antenna 230.

[0043] In a manner known to those skilled in the art, the transmitting unit 120 generates signals in the base frequency band using the frequency generator 122 and performs a frequency conversion of these signals into a predetermined frequency band by means of oscillator signals from the oscillator 121. The frequency conversion is carried out by the modulator 123. The converted signals are amplified by the amplifier 124 and transmitted as radar signals RS in the direction of the Earth's surface 300.

[0044] The receiver 200 receives the radar echoes RE of the radar signals RS from the transmitter 100, amplifies them using the amplifier 221, and converts them using the frequency generator 252 and the conversion unit 253. The resulting signal is transmitted back via the analog transmission link from the antenna 230 of the receiver 200 to the receiving antenna 130 of the transmitter 100. The received signal is converted by the converter 141 and demodulated using the demodulator 140. For this purpose, the oscillator frequency of the oscillator 121 is supplied to the demodulator 140. After the frequency conversion of the demodulated signals to the base frequency band using the oscillator signals of the oscillator 121, raw SAR data is obtained, from which digital data is generated and stored in the data memory 150. Transmitter 100 preferably sends the stored digital data to a ground station on the Earth's surface 300.The data can then be further processed using methods that are already known.

[0045] The in Fig. The basic concept of MirrorSAR shown in Figure 1 is that the transmitter 100 emits a linear frequency-modulated radar signal RS, while the receiver 200 acts as a simple transponder that amplifies the received radar echo RE and sends it back to the transmitter 100 via an amplitude-modulated (AM) free-space link.

[0046] An embodiment of the method according to the invention, based on a bisstatic SAR system with a transmitter 100 and a receiver 200 on independent platforms, is described below. Such a system is in Fig. Figure 2 shows that the transmitter 100 and the receiver 200 are located on different satellites or aircraft. The invention can also be extended to more than one receiver as described below, so that the SAR system can also be configured as a multistatic system.

[0047] In Fig. 2 are identical elements with the same reference symbols as in Fig. 1 provided.

[0048] The transmitter 100 comprises a transmitting antenna 110 designed exclusively for transmitting, a transmitting unit 120, an optical receiving terminal in the form of a free-space optic 130, a demodulation unit 140, and an electro-optical demodulation unit 160. The transmitting unit 120 and the demodulation unit 140 are part of an electrical domain 101. The free-space optic 130 and the electro-optical demodulation unit 160 are part of an optical domain 102.

[0049] In a known manner, the transmitter unit 120 comprises an oscillator 121, a frequency generator 122 for generating signals in a base frequency band, a modulator 123, and an amplifier 124, e.g., a high-power amplifier (HPA). The transmitter unit 120 is configured to generate signals in the base frequency band using the frequency generator 122. The signals generated in the base frequency band are frequency-converted to a predetermined frequency band using the modulator 123 and the oscillator signals generated by the oscillator 121. The signals in the predetermined frequency band are amplified by the amplifier 124 and transmitted as radar signals.

[0050] The demodulation unit 140 comprises a number of demodulators 143, 144, each of which receives the oscillator signal generated by the oscillator 121. The demodulators 143, 144 serve to convert demodulated, high-frequency signals received at their input into the base frequency band using the oscillator signals of the oscillator 121, thereby obtaining raw SAR data at the output, which is stored in a memory 150. The number of modulators 143, 144 depends on the number of receive beams provided by the receiver 200.

[0051] The received beams to be processed by the demodulation unit 140 are provided by the electro-optic demodulation unit 160 in the optical domain 102. The electro-optic demodulation unit 160 is connected at its input to the aforementioned optical receiving terminal or free-space optics 130. The amplitude-modulated optical signal OS received by the free-space optics 130 is fed to an amplifier 161. The amplifier 161 is preferably an erbium-doped fiber amplifier (EDFA) or a semiconductor optical amplifier (SOA) if integrated photonics are used. The signal amplified by the amplifier 161 is fed to a demultiplexer 162. The amplifier 161 is an optional component, the choice of which depends on the power and noise budget of the system.

[0052] The demultiplexer 162 processes the received optical signal OS and provides corresponding receive beams, which are fed to optoelectronic demodulators 163 and 164 for reconstruction of the received beams. The optoelectronic demodulators 163 and 164 are connected on their output sides to corresponding demodulators 143 and 144 of the demodulation unit 140 in the electrical domain 102. The number of optoelectronic demodulators 163 and 164 corresponds to the number of modulators 143 and 144 and thus, like their number, depends on the number of receive beams provided in the optical signal OS by the receiver 200. The number of receive beams is subsequently denoted by M.

[0053] The receiver 200 comprises a receiving antenna 210 with a number of antenna elements 211-219, each forming receiving channels CH1-CH9. The number of receiving channels CH1-CH9 is not limited to the embodiment shown here with nine channels. The number of receiving channels CH1-CH9 can be greater or less. Generally, the number of receiving channels is subsequently denoted by N.

[0054] The antenna elements 211-219 form a planar phased array antenna. Alternatively, the antenna elements 211-219 can comprise a phased array that feeds a reflector 210R, as exemplified in Fig. Figure 8 shows this arrangement. This arrangement is called a phased reflector.

[0055] The receiver 200 comprises various components in an electrical domain 201 and an optical domain 202. In the electrical domain 201, each of the antenna elements 211-219 of a receive channel CH1-CH9 is connected to an associated amplifier 221-229, also referred to as a receive channel amplifier. Furthermore, a calibration and position data provision unit 291, described below, is provided in the electrical domain 201.

[0056] The optical domain 202 includes an electro-optic modulation unit 240 with a number of electro-optic modulators 241-249 corresponding to the number N of antenna elements 211-219, an optical frequency comb generation unit 250 (optical comb generator), an optional optical splitter 251, an optical beam forming network 260 (OBFN), a filter unit comprising a number of filters 271-273 (BPF), a wavelength multiplexer 270 (MUX), an amplifier 280 (EDFA), a laser 292 connected to an electro-optic modulator 293, and a free-space optics 230 as an optical transmit terminal.

[0057] The electro-optic modulators 241-249 receive the signal amplified by the receive channel amplifiers 221-229 for each receive channel CH1-CH9. Each electro-optic modulator 241-249 is assigned exactly one receive channel amplifier 221-229 for each channel CH1-CH9. An additional input signal supplied to the electro-optic modulators 241-249 is an optical frequency comb with a number of lines T1-T3, each with its respective optical frequency ( Fig. 3 to 7). In the example described here, the number of lines T1-T3 is three (3). The number of lines can also be greater or less than three. The number of lines T1-T3 corresponds to the number M of receiving beams. The number of lines can be 1.

[0058] The frequency comb with the specified number of lines T1-T3, each with its respective optical frequency, is provided by the optical frequency comb generation unit 250. The frequency comb can be generated by one or more electro-optical modulators connected in series or parallel. The frequency comb is also referred to as an electro-optical frequency comb (EFC). The number of lines T1-T3 is selected proportionally to the number of simultaneous received beams required to image the desired fringe width.

[0059] The generation of frequency combs is known from the prior art and is therefore not explained in detail here. For example, the following variants can be used to generate the optical frequency comb: The optical frequency comb can be generated by an optical comb generator (optical frequency comb generation unit) comprising a single Mach-Zehnder modulator with two drivers, operated by a local oscillator signal. The optical comb generator can also consist of two Mach-Zehnder modulators connected in series by two coherent local oscillator signals and two bias voltages. In another embodiment, the optical comb generator can be a double-parallel Mach-Zehnder modulator operated by a local oscillator signal and up to three bias voltages.In a further embodiment, a Mach-Zehnder modulator can be fed with two harmonic frequencies to generate a flat frequency comb with more than three lines. In all four alternatives, the local oscillator signal can be generated by a multiple of an ultrastable local oscillator. The local oscillator signal can also be generated by an optoelectronic oscillator. Likewise, in a further embodiment, the optical comb generator can be replaced by an array of wavelength-stable or tunable lasers.

[0060] The use of the frequency combs is based on the Fig. 3 to 7 described. The Fig. Figures 3 to 7 show the optical frequency spectrum at selected points A to E of the SAR system in Fig. 2. At point A ("Comb Generation") an example frequency comb is shown, which is located in the Fig. 3 to 7 are produced identically.

[0061] The frequency comb comprises, for example, three lines T1-T3, where the frequency of line T2 is denoted by v0. The frequency spacing between any two adjacent lines T1 and T2, T2 and T3, etc., is identical and preferably greater than or equal to twice the maximum transmit signal frequency. The lines T1-T3 of the frequency comb are generated such that they all have the same amplitude P.

[0062] In the Fig. In the embodiment shown in Figure 2, the frequency comb is routed from the optical frequency comb generation unit 250 via the optical splitter 251 to the electro-optic modulators 241-249 (point (A)). Furthermore, in this example, the optical frequency comb is also fed to the wavelength multiplexer 270 via the optical splitter 251, although this is not mandatory depending on the configuration of the optical beam shaping network 260. For example, the optical splitter 251 can be omitted if the beam shaping network 260 is based on real-time delays.

[0063] The following description refers first to the general representation of the frequency spectrum in Fig. 3.

[0064] Using electro-optic modulators 241-249, the received high-frequency signal of each of the N channels CH1-CH9 is used to modulate the amplitude of the frequency comb. In this modulation step, for each channel i, i = 1...N, the lines T1-T3 of the optical frequency comb are treated as separate line sources, a portion of which is converted into two sidebands SBLiT1, SBRiT1, SBLiT2, SBRiT2, SBLiT3, SBRiT3. This can be seen in point (B) (“Opto-electronic modulation”) in Fig. 3 can be seen. Each left and right sideband SBL, SBR is around the frequency ± f RF The (carrier) frequency of lines T1-T3 is shifted (explicitly shown for line T2). The (carrier) frequencies of lines T1-T3 are shown in the frequency spectrum at point (B) with their original amplitude, although it is clear to a person skilled in the art that some of the power has been converted into the sideband.

[0065] The amplitude-modulated (AM) frequency lines of all N channels are then fed into the optical beamforming network 260 together with the lines T1-T3 of the optical frequency comb. Through this network, the modulated optical frequency comb channels SBLiT1, SBRiT1-SBLiT3, SBRiT3 (i = 1...N) are subjected to splitting and / or forwarding, as well as phase shifting and / or delaying of the signals from the N channels CH1-CH9, based on previously defined beamforming weights. The number M of amplitude-modulated received beams BR1-BR3 is then output at the respective corresponding outputs of the optical beamforming network 270.

[0066] The number of outputs of the optical beamforming network 260 corresponds to the number M of generated lines of the frequency comb. The exact configuration of the optical beamforming network 260 is described below.

[0067] Each of the outputs of the optical beamforming network 260 is equipped with a filter 271-273, e.g., a bandpass filter. The bandpass filter selects the AM-RF signal spectra that now each represent a selected received beam BR1 to BR3 and the spectrum at point (C) (“filtering OBFN outputs”). This is shown in Fig. Figure 3 is shown as an example for the second output or the second filter and the receive beam BR2. The carrier frequency T2 is ignored by the filter. Now, the receive beams BRj, j = 1...M, which are encoded on separate parts of the optical spectrum, together with additional position, calibration, or other data that must be transmitted to the primary satellite (transmitter 100), can be multiplexed onto the same channel (one channel) by the wavelength multiplexer 270, whereby the (carrier) frequencies of lines T1-T3 are reinserted into the frequency spectrum (point (D) "Multiplexing and Reinsertion of Carriers").

[0068] The additional position, calibration, or other data are provided by the calibration and position data provisioning unit 291. Conversion from the electrical to the optical domain is performed by means of the laser 292 and the electro-optic modulator 293.

[0069] The signal amplified by amplifier 280, with the spectrum shown at point (D), is then transmitted as the optical signal OS via the free-space path (FSO link) to transmitter 100. The choice of amplifier depends on the operating wavelength. For example, an erbium-doped fiber amplifier, EDFA, is used in the optical C-band. The preferred embodiment of the method is in the optical C-band, but it can also be implemented in other bands. Semiconductor optical amplifiers (SOAs) can also be used, with EDFAs being more linear and having better noise performance.

[0070] In transmitter 100, the optical and amplitude-modulated signal OS received by receiver terminal 130 is amplified again by amplifier 161 to compensate for any losses over the FSO link. The amplified signal is then fed into demultiplexer 162. Demultiplexer 162 can be configured as an array of add / drop blocks and selects the spectrum containing each received beam BRj, j = 1...M, and feeds it into separate optoelectronic demodulators 163 and 164. Fig. Figure 3 shows the spectrum of the signal fed into one of the optoelectronic demodulators at point (E) (“De-Multiplexing”).

[0071] In the demodulation step, any unknown phase originating from the laser 292 or the local oscillator of the receiver 200 is eliminated. Each received beam is now available in the RF range for demodulation and digitization using the oscillator 121 of the transmitter 100. Since the oscillator 121 was used to generate the transmitted radar signal RS, any phase or frequency deviations are eliminated in this demodulation step, just as in a conventional monostatic SAR system.

[0072] The procedure described above is carried out in a modified manner as described below, depending on the embodiment of the electro-optical modulators 241-249 and the filters 271-273, with reference only to the changes compared to Fig. 3 is addressed.

[0073] Fig. Figure 4 shows the schematic representation of the frequency spectrum at the points shown in Fig. Points (A) to (E) of the SAR system are shown in Figure 2, in which the electro-optic modulators 241-249 of the receiver according to the invention are 200 push-pull Mach-Zehnder modulators operated with a low bias voltage and generating a two-sideband modulation with suppressed carrier T1'-T3' (see point (B) “DSB-CS Modulation”). The sidebands SBLiT1, SBRiT1, SBLiT2, SBRiT2, SBLiT3, SBRiT3 are on each side around the frequency ± f RF about the (carrier) frequency of the suppressed lines from T1' to T3'.

[0074] Furthermore, each filter 271-273 is tuned to select one sideband (e.g. BR2) associated with a modulated comb tone and suppresses all other sidebands (e.g. BR1, BR3) as well as all optical carriers T1'-T3'), i.e. the optical lines generated by the optical frequency comb generating unit 250 are suppressed.

[0075] As can be seen at point (B), the suppressed carriers T1'-T3' have a smaller amplitude compared to point (A). Suppression is advantageous for the following reason: In phase-shifter-based processing, phase shifts in the RF range become apparent when only the modulated RF signal is affected by the phase shift and not the optical carrier. A photodiode used for optoelectric conversion is a component that detects the intensity of the incoming optical radiation. Therefore, the output current of the photodiode is proportional to the optical input field in the following way: Ipd(t)∝|Ein(t)|2=Ein*(t)Ein(t) where I pd (t) the output current and E in(t) are the optical input field. If both the carrier and the modulated signal are uniformly phase-shifted, the phase shift cancels out due to complex conjugate multiplication. Therefore, the non-phase-shifted carrier must be reinserted after the phase shift. The portion of the carrier fed into the phase shifter must be suppressed at some point before reinsertion. This suppression can be performed during the modulation phase or with filters after modulation. This principle also applies to the present case with many channels CH1-CH9 fed by a frequency comb. Here, the phase shifts are performed in the optical beamforming network 260. Therefore, the carrier can be suppressed in the electro-optic modulators 241-249 and / or at the optical bandpass filters 271-273 at the output of the beamforming network 260.The carrier is then reinserted into the wavelength multiplexer 270.

[0076] The further procedure corresponds to that in connection with Fig. 3 described procedure.

[0077] Fig. Figure 5 shows the schematic representation of the frequency spectrum at the points shown in Fig. Points (A) to (E) of the SAR system are shown in Figure 2, in which the electro-optical modulators 241-249 of the receiver according to the invention are 200 push-pull Mach-Zehnder modulators with two drivers, operating in a single-sideband modulation format. The left sidebands SBLiT1, SBLiT2, SBLiT3 are thus suppressed (point (B) “SSB modulation”).

[0078] Furthermore, each filter 271-273 is tuned to select one sideband (e.g. BR2) associated with a modulated comb tone and suppresses all other sidebands (e.g. BR1, BR3) as well as all optical carriers T1-T3, i.e. the optical lines generated by the optical frequency comb generating unit 250 are suppressed (point (C) “Filtering receive beam”).

[0079] The further procedure corresponds to that in connection with Fig. 3 described procedure.

[0080] Fig. Figure 6 shows the schematic representation of the frequency spectrum at the points shown in Fig. Points (A) to (E) of the SAR system are shown in Figure 2, in which the electro-optic modulators 241-249 of the receiver according to the invention are 200 dual-parallel push-pull Mach-Zehnder modulators that generate a single-sideband modulation with suppressed carrier T1'-T3' (see point (B) "SSB-CS Modulation"). The right-hand sidebands SBRiT1, SBRiT2, SBRiT3 are located on each side around the frequency + f RF around the (carrier) frequency of the suppressed lines from T1' to T3'. The left sidebands SBLiT1, SBLiT2, SBLiT3 are suppressed.

[0081] Furthermore, each filter 271-273 is tuned to select one sideband (e.g. BR2) associated with a modulated comb tone and to suppress all other sidebands (e.g. BR1, BR3) as well as all optical suppressed carriers T1'-T3', i.e. the optical lines generated by the optical frequency comb generating unit 250 are suppressed (point (C) "filtering receive beam").

[0082] The further procedure corresponds to that in connection with Fig. 3 described procedure.

[0083] The optical beamforming network 260 can be implemented in various ways. For example, the optical beamforming network 260 can be based either on optical phase shifters or on real-time delay elements. A possible reflector antenna 210R as part of the receiving antenna 210 and the optical beamforming network 260 for this setup are shown as examples in Fig. Figure 8 shows a reflector 210R, in particular a parabolic one, which deflects the received signal RE or RE1 to RE3 onto the antenna elements 211-219.

[0084] The optical beamforming network 260 is configured in this example as an optical Blass matrix network, as described, for example, in references [4] and [5]. The reflector 210R focuses the received energy onto a small subset of the antenna elements, the setup preferably being designed such that the power of two different receiving beams RE1 to RE3 overlaps only slightly, allowing each receiving beam RE1 to RE3 to follow each echo as it crosses the ground surface 300 with minimal interference.

[0085] In the Fig. In the example shown, each receiving beam RE1 to RE3 has a specific optical wavelength onto which the radio frequency signal of the respective receiving beam RE1 to RE3 is modulated. By providing a suitable filter at the output of the optical beam shaping network 260 (see filters 271-273 in [reference missing]), Fig. 2) The receiving beams RE1 to RE3 can be recombined after filtering, with the different beams occupying separate parts of the spectrum.

[0086] Each node 261 of the Blass matrix consists of a tunable coupler, e.g., a Mach-Zehnder interferometer, MZI, 262, and an optical phase shifter, PS, 263. When a phase shifter 263 is used, as in Fig. As shown in Figure 9, the original carrier must be suppressed and reinserted after the phase shifter so that the phase shift becomes visible in the RF range.

[0087] Alternatively, the beamforming network 260 can be based on real-time delays. Choosing a beamforming network 260 based on real-time delays simplifies subsequent signal processing, as the need to reintroduce a carrier signal is eliminated. However, if the bandwidth requirement of the system components needs to be kept as low as possible, this variant also allows for the reintroduction of the carrier signal.

[0088] If the beamforming network 260 is based on real-time delays, the optical splitter 251 can be omitted, and the entire energy of the optical frequency comb 250 is directed to the electro-optic modulators 241-249. Preferably, the electro-optic modulators 241-249 are tuned such that the (carrier) frequencies of the lines T1-T3 and at least one sideband are preserved. The filters 271-273 downstream of the beamforming network 260 are then tuned, in particular, such that they select an optical frequency comb T1-T3 and a corresponding sideband SBLiT1, SBRiT1 - SBLiT3, SBRiT3 (i = 1...N). The propagation delays are implemented, for example, using cascaded optical ring resonators.

[0089] Choosing an OBFN based on true time delays eliminates the need for carrier reinsertion. However, reinsertion can still be performed if one wishes to reduce the bandwidth requirements of system components, particularly optical bandpass filters.

[0090] Fig. Figure 7 shows the effect on the spectrum when the reinsertion stage is removed.

[0091] As with point (D) in the Fig.As can be seen in Figures 2 and 3 to 7, the proposed design of the receiver 200 of the SAR system requires that all paths through the optical beamforming network 260 and the reinsertion path be of equal length to avoid introducing any unknown phase contribution that would manifest as a phase difference between the reinserted carrier and the output of the optical beamforming network 260. These equal path lengths can be precisely controlled for multichannel applications using temperature-controlled integrated photonic microwave circuits. Alternatively, a tuning mechanism based on electro-optical or piezoelectric (mechanical) effects could be employed instead of temperature control.

[0092] The principle described above can also be extended to more than one receiver to enable interferometric capabilities. The optical frequency combs generated on each receiver should then occupy different parts of the spectrum so that they do not overlap in the FSO link between the platforms. This means that the laser 292 on each receiver platform 200 must have a wavelength separation of up to a few nanometers if the optical C-band is preferably used. This also applies to the frequency comb generation unit 250. The transmitter 100 is then able to separate the signals of the individual optical signals generated across the constellation using wavelength demultiplexing, as usual. An expansion of the demultiplexing network, as well as greater storage and signal processing capacity in the transmitter, is then advantageous to process the increased number of channels and data.

[0093] The proposed method enables seamless HRWS-SAR data acquisition with the potential for multistatic extension to facilitate interferometric data acquisition. The previously described problem of gaps in strip acquisition is solved by separating the transmit and receive platforms, thus avoiding interruptions due to transmit events during receive operation.

[0094] The inherently complex synchronization of the local oscillators of the separate platforms is circumvented by routing the signal back to the transmitter via an amplitude-modulated optical carrier. The multiple beams required for imaging an ultrawide swath are generated using an optical frequency comb in conjunction with an optical beamforming network, whereby the data of each beam is modulated onto a separate carrier of the comb and simultaneously transmitted via the free-space link between transmitter and receiver. This method allows for more than one receiver operating in different parts of the optical spectrum, enabling interferometric acquisition at the cost of a high requirement for minimal phase error.

[0095] This method offers a way to transmit the large amount of information generated by a gapless multi-beam HRWS-SAR system over the free-space link between platforms, with only a minimal increase in the computational and memory load on the receiving platform. This is made possible by the simultaneous use of the optical frequency comb, the optical beamforming network, optical bandpass filters, the aforementioned free-space optics, and wavelength division multiplexing and wavelength demultiplexing. The transmission of information from multiple SAR receiving beams is unique compared to other types of information.

[0096] The described method can be used in any airborne or spaceborne SAR constellation with multiple receiving beams. It is particularly suitable for spaceborne missions where high performance is required, but receiver simplicity is important to reduce costs.

[0097] Furthermore, the method can be used as a starting point for the construction of a multistatic system that can generate interferometric data and thus, among other things, create a digital elevation model.

[0098] The use of photonics also allows for a reduction in the size, weight and power of the resulting system, which lowers costs and enables smaller and cheaper missions for space-based SAR procedures. References [1] A. Freeman et al., "The "myth" of the minimum SAR antenna area constraint," IEEE Trans. Geosci. Remote Sens., vol. 38, pp. 320-324, Jan. 2000. [2] G. Krieger et al., „MirrorSAR: A fractionated space radar for bistatic, multistatic and high-resolution wide-swath SAR imaging,“ in 2017 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), 2017, pp. 149-152. [3] G. Krieger, M. Zonno, J. Mittermayer, A. Moreira, S. Huber, and M. Rodriguez-Cassola, „MirrorSAR: A fractionated space transponder concept for the implementation of low-cost multistatic SAR missions,“ in EUSAR 2018; 12th European Conference on Synthetic Aperture Radar, 2018, pp. 1-6. [4] C. Tsokos et al., „Analysis of a Multibeam Optical Beamforming Network Based on Blass Matrix Architecture,“ J. Lightw. Technol., vol. 36, pp. 3354-3372, Aug. 2018. [5] M. Reza et al., „Design and Performance Estimation of a Photonic Integrated Beamforming Receiver for Scan-on-Receive Synthetic Aperture Radar,“ J. Lightw. Technol., vol. 39, pp. 7588-7599, Dec. 2021. Bezugszeichenliste 1 Synthetic Aperture Radar System (SAR System) 100 channels 101 electrical domain 102 optical domain 110 transmitting antenna 120 transmitter units 121 Oscillator 122 Frequency generator for generating signals in the basic frequency band 123 Modulator 124 amplifiers (e.g. high-power amplifier, HPA) 130 Free-space optics (optical reception terminal) 140 demodulation units 141 converters 143 Demodulator 144 Demodulator 150 data storage devices 160 opto-electronic demodulation unit 161 Amplifiers (e.g. erbium-doped fiber amplifier, EDFA) 162 Demultiplexers 163 optoelectronic demodulator 164 optoelectronic demodulator 200 recipients 201 electrical domain 202 optical domain 210 Receiving antenna 210 Reflector 211-219 Antenna elements (receiving channel) 221-229 Receive Channel Amplifier 230 Free-space optics (optical transmitting terminal) 240 electro-optical modulation unit 241-249 electro-optical modulator 250 optical frequency comb generation unit 251 optical splitters 260 optical beam shaping network 261 knots 262 Phase shifters (PS) 263 Phase shifter (PS) 270 wavelength multiplexers 271-273 Filters (e.g. bandpass filters) 280 amplifiers (e.g. erbium-doped fiber amplifier, EDFA) 291 Calibration and Position Data Provision Unit 292 lasers 293 electro-optical modulator 300 Earth's surface RS radar signal RE Radar echo RE1, RE2, RE3 Radar echo MZI Mach-Zehnder interferometer T1-T3 (frequency comb) line of the respective optical frequency T1'-T3' suppressed (frequency comb) line of respective optical frequency SBLiT1-SBLiT3 modulated optical frequency comb channel (left sideband) SBRiT1-SBRiT3 modulated optical frequency comb channel (right sideband) BR1-BR3 Replica of the selected receiving beam OS optical signal

Claims

[1] Synthetic aperture radar method for remote sensing of the Earth's surface (300), in which radar signals (RS) are transmitted by a transmitter (100) moving above the Earth's surface (300) via a transmitting antenna (110) designed exclusively for transmitting, and radar echoes (RE) of the radar signals (RS) reflected on the Earth's surface (300) are received by at least one receiver (200) moving above the Earth's surface (300) and comprising a multi-channel receiving antenna (210) designed exclusively for receiving, wherein the transmitter (100) and the at least one receiver (200) are located on different platforms above the Earth's surface (300), wherein the transmitter (100) and the at least one receiver (200) as well as a signal processing unit (140, 150, 160) above the Earth's surface perform the following steps: a) The transmitter (100) generates signals in a base frequency band and performs a frequency conversion of these signals into a predetermined frequency band using oscillator signals of an oscillator (121) of the transmitter (100), wherein the converted signals are transmitted as the radar signals (RS); b) the at least one receiver (200) receives the radar echoes (RE) of the radar signals (RS) of the transmitter (100) and transforms the received radar echoes (RE) in real time by phase-preserving modulation from an electrical domain (201) into an optical domain (202), in which a number of receive beams (BR1-BR3) are generated from the received radar echoes (RE) and combined to form an optical signal (OS), wherein the optical signal (OS) contains the number of receive beams (BR1-BR3), each of which contains the signal of a beam with a different wavelength; c) the at least one receiver (200) sends the optical signal (OS) via a free-space optic (230) to the signal processing unit (140, 150, 160); d) The signal processing unit (140, 150, 160) processes the received optical signal (OS) by reconstructing the receive beams (BR1-BR3) from the optical signal (OS) and then transforming them from the optical domain (102) to the electrical domain (101); and e) The signal processing unit (140, 150, 160) demodulates the received modulated receive beams (BR1-BR3) and performs a frequency conversion of the demodulated signals into the base frequency band using oscillator signals from the oscillator (121) of the transmitter (100), thereby obtaining SAR raw data from which the signal processing unit (140, 150, 160) generates and stores digital data, wherein an optical frequency comb with a number of lines (T1-T3) with a respective optical frequency is generated in the optical domain (202), wherein the number of generated lines (T1-T3) corresponds at least to the number of receive beams (BR1-BR3), wherein an amplitude modulation of the respective optical frequency of the number of generated lines (T1-T3) with the received radar echoes (RE) of each receive channel (211-219) of the Multi-channel receiving antenna (210) using associated electro-optical modulators (241-249),which results in a modulated optical frequency comb channel (SBLiT1, SBRiT1 - SBLiT3, SBRiT3 (i = 1...N)) for each receiving channel (CH1-CH9). [2] Method according to claim 1, characterized by that the optical signal (OS) is amplitude modulated. [3] Method according to claim 1 or 2, characterized by , that the frequency difference between two adjacent lines (T1-T3) is greater than or equal to twice the maximum frequency of the radiated transmit signal in the high-frequency band. [4] Method according to any one of the preceding claims, characterized by, that the modulated optical frequency comb channels (SBLiT1, SBRiT1 - SBLiT3, SBRiT3 (i = 1...N)) are fed to an optical beamforming network (260) by which the modulated optical frequency comb channels (SBLiT1, SBRiT1 - SBLiT3, SBRiT3 (i = 1...N)) are subjected to a split and / or forwarding as well as a phase shift and / or delay of the channel signals on the basis of previously determined beamforming weights in order to output the number of received beams (BR1-BR3) at each assigned outputs of the optical beamforming network (260). [5] Method according to claim 4, characterized by , that at each output of the optical beam shaping network (260) a different receiving beam (BR1-BR3), which is assigned to a line (T1-T3) with respective optical frequency, is selected by means of an optical filtering, in particular with a bandpass filter. [6] Method according to claim 5, characterized by, that all received beams selected by filtering (BR1-BR3) are combined by wavelength division multiplexing to form the optical signal (OS). [7] Method according to any one of the preceding claims, characterized by , that additional data, including calibration data and / or position data, are inserted into the optical signal (OS). [8] Method according to any one of the preceding claims, characterized by , that the optical signal (OS) in the signal processing unit (140-160) is processed by wavelength demultiplexing to reconstruct the number of received beams (BR1-BR3). [9] Computer program product comprising instructions which, when the program is executed by a distributed computing system, cause it to execute the method according to any of the preceding claims. [10] Synthetic aperture radar system for remote sensing of the Earth's surface (300), comprising a transmitter (100) moving above the Earth's surface (300) and transmitting radar signals (RS) via a transmitting antenna (110) designed exclusively for transmitting, and at least one receiver (200) moving above the Earth's surface (300) and comprising a multi-channel receiving antenna (210) designed exclusively for receiving and receiving radar echoes (RE) of the radar signals (RS) reflected on the Earth's surface (300) during operation, wherein the transmitter (100) and the at least one receiver (200) are located on different platforms above the Earth's surface (300), wherein the transmitter (100) and the at least one receiver (200) as well as a signal processing unit (140, 150, 160) above the Earth's surface are configured to perform the following steps: a) The transmitter (100) generates signals in a base frequency band and performs a frequency conversion of these signals into a predetermined frequency band using oscillator signals of an oscillator (121) of the transmitter (100), wherein the converted signals are transmitted as the radar signals (RS); b) The at least one receiver (200) receives the radar echoes (RE) of the radar signals (RS) of the transmitter (100) and transforms the received radar echoes (RE) in real time by phase-preserving modulation from an electrical domain (201) into an optical domain (202), in which a number of receive beams (BR1-BR3) are generated from the received radar echoes (RE) and combined to form an optical signal (OS), wherein the optical signal (OS) contains the number of receive beams (BR1-BR3), each of which contains the signal of a beam with a different wavelength, wherein an optical frequency comb with a number of lines (T1-T3) with respective optical frequencies is generated in the optical domain (202), wherein the number of generated lines (T1-T3) corresponds at least to the number of receive beams (BR1-BR3).wherein an amplitude modulation of the respective optical frequency of the number of generated lines (T1-T3) is carried out with the received radar echoes (RE) of each receiving channel (211-219) of the multi-channel receiving antenna (210) by means of associated electro-optic modulators (241-249), whereby a modulated optical frequency comb channel (SBLiT1, SBRiT1 - SBLiT3, SBRiT3 (i = 1...N)) is obtained for each receiving channel (CH1-CH9); c) the at least one receiver (200) sends the optical signal (OS) via a free-space optic (230) to the signal processing unit (140, 150, 160); d) The signal processing unit (140, 150, 160) processes the received optical signal (OS) by reconstructing the receive beams (BR1-BR3) from the optical signal (OS) and then transforming them from the optical domain (102) to the electrical domain (101); and e) The signal processing unit (140, 150, 160) demodulates the received modulated receive beams (BR1-BR3) and performs a frequency conversion of the demodulated signals into the base frequency band using oscillator signals of the oscillator (121) of the transmitter (100), thereby obtaining SAR raw data from which the signal processing unit (140, 150, 160) generates and stores digital data. [11] Synthetic aperture radar system according to claim 10, characterized by that the system is set up to carry out a method according to one of claims 2 to 8. [12] Receiver for receiving radar echoes, wherein the receiver (200) is configured to operate as a receiver in a method according to any one of claims 1 to 8.

Images