Method for calibrating an SAR sensor
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- DEUTSCHES ZENTRUM FÜR LUFT UND RAUMFAHRT E V
- Filing Date
- 2024-07-29
- Publication Date
- 2026-06-17
Smart Images
Figure EP2024071435_13022025_PF_FP_ABST
Abstract
Description
[0001] P3008PC00 German Aerospace Center e. V. Königswinterer Str. 522-524 53227 Bonn ____________________________________________________________________ Method for calibrating a SAR sensor ____________________________________________________________________ Description The invention relates to a method for calibrating a SAR sensor with one or more receiving channels, wherein each receiving channel comprises a plurality of receiving branches with an antenna, an analog-to-digital converter (ADC) for digitizing the SAR signal received by the antenna into SAR raw data, and a beamforming unit. The beamforming unit is configured to weight the SAR raw data of the plurality of receiving branches with a weighting factor, to combine the weighted SAR raw data of the plurality of receiving branches, and to store the SAR raw data obtained for a respective receiving channel in a data memory for later processing.During SAR satellite missions, calibration of the SAR sensors used is common practice. Examples of the approaches developed for this purpose are described in [2] for Terra-SAR-X, in [3] for TanDEM-X, in [4] for the Cosmo-SkyMed mission, in [5] for the Sentinel-1 mission, and in [6] for the Radarsat-2 mission. Similar calibration approaches have also been applied for the calibration of airborne SAR sensors, namely F-SAR and DBFSAR sensors, as described in [7] and [8]. A well-known approach for external calibration is described in publication [9] and EP 3364212 A1. This calibration approach differs from the previously mentioned techniques in that it is based on the pulse-by-pulse analysis of reference target responses in the range-compressed raw SAR data acquired by the SAR sensor.In contrast to existing methods, neither active targets nor azimuth focusing of the SAR image / SAR recording are required. The calibration approach described in [9] introduces explicit error models and derives calibration corrections through an optimization process. In particular, it explicitly considers the uncertainty in the three-dimensional positions of the antenna phase centers to ensure consistent phase and geometry between the SAR sensor receive channels, which is crucial in multi-channel SAR applications (including Digital Beam Forming (DBF) applications). Furthermore, the optimization approach is suitable for more comprehensive and realistic modeling of error sources. For example, the proposed estimation of antenna pointing errors can derive three-dimensional roll / pitch / yaw corrections instead of the conventional elevation / azimuth estimates.It can also provide separate corrections for the transmit and receive channels if they comprise different antennas. The present invention relates to improvements of the approach described in [9], which are particularly relevant in the case of SAR sensors with integrated digital beamforming technology. The object of the invention is to provide a method, a computer program product and a device that enable improved calibration of a SAR sensor with DBF processing. P3008PC00 A further object of the invention is to improve the accuracy of the antenna alignment estimates obtained during calibration. These objects are achieved by a method according to the features of claim 1, a computer program product according to the features of claim 18 and a device according to the features of claim 19. Advantageous embodiments emerge from the dependent claims.A method for calibrating a SAR sensor with one or more receiving channels is proposed. Each receiving channel comprises a plurality of receiving branches with an antenna and an analog-to-digital converter (ADC) for digitizing the SAR signal received by the antenna into SAR raw data, as well as a beamforming unit. The beamforming unit is configured to weight the SAR raw data of the plurality of receiving branches with a weighting factor, combine the weighted SAR raw data of the plurality of receiving branches, and store the SAR raw data obtained for a respective receiving channel in a data memory for later processing. The method for calibrating the SAR sensor comprises the following steps: In step a), the weighting factors of the receiving branches are adjusted to obtain unweighted SAR raw data for each receiving branch.In step b), a reference target analysis is carried out for each reception branch, in which the signatures of several reference targets detected by the SAR sensor in the unweighted SAR raw data are compared with modeled signatures and a residual response of the signature is determined for each reference target in each raw data set. P3008PC00 In step c), a respective weighting factor is determined for each of the plurality of reception branches based on the reference target analysis, wherein the determined weighting factors represent calibration parameters of the SAR sensor. The proposed method enables simple and improved calibration of a SAR sensor with any number of reception channels with integrated DBF processing. The method is based on the approach described in publication [9] and improves it with regard to applicability and accuracy for SAR sensors with digital beamforming.In order to check and, if necessary, improve the calibration of the individual reception branches that form the input information for DBF processing in the radar sensor, access to each individual reception branch is required. With the known method, this is no longer possible because the DBF processing has already been carried out in the sensor before the data is sent to a ground station. The method is based on the idea of bypassing the DBF processing for the acquisition of calibration data in order to allow access to each individual reception branch for calibration. The steps a) to c) described above are expediently carried out before the SAR sensor is operated. In particular, it is sufficient to carry out steps a) to c) once before the SAR sensor is operated.In a practical embodiment, the weighting factors of the receive branches for acquiring calibration data are adjusted using a multiplexing operation, by cyclically switching between the receive branches or receive branch groups from receive pulse to receive pulse. A receive branch group comprises a combination of signals from multiple receive branches. The multiplexing operation is preferably implemented using binary weighting factors, whereby at the time of a receive pulse, the weighting factor of one receive branch is set to 1 and the weighting factors of the remaining P3008PC00 receive branches or receive branch groups are set to zero. This allows the multiplexing operation to be implemented without the provision of an explicit switching element.It is furthermore expedient if the adaptation of the weighting factors comprises a demultiplexing step, in which the reception branches or reception branch groups arise from the demultiplexing. This allows the SAR raw data determined by a reception branch to be subjected to the reference target analysis. According to a further expedient embodiment, the SAR raw data of the plurality of reception branches or reception branch groups are subjected to respective signal processing. In one expedient embodiment, the signal processing comprises a summation and shifting of the Doppler center frequency of the SAR raw data of the plurality of reception branches or reception branch groups. The shifting of the center frequency followed by a summation is referred to below as Doppler superposition. The adaptation of the weighting factors according to this embodiment is carried out according to the following rule: in which are: ^^^^ ^^^^ ^^^^(^^^^ ^^^^, ^^^^ ^^^^): the weighting factor for the nth receive branch or the nth receive branch group, ^^^^ ^^^^: integer index of the sampling point in the distance direction, starting at zero for the first sample in each received pulse, ^^^^ ^^^^: integer azimuth pulse index in the calibration acquisition, starting at zero for the first transmit pulse of the data acquisition, P3008PC00 N is the number of receive branches or receive branch groups. A further expedient embodiment provides that the adaptation of the weighting factors includes a step of undoing the center frequency shift, in which the receive branches or receive branch groups emerge from this step. This enables the reference target analysis to be carried out in step b) for each receive branch.According to a further expedient embodiment, the beamforming unit is configured to vary the weighting factors only in the azimuth direction when performing the multiplexing operation in order to multiplex the plurality of receive branches or receive branch groups onto the output of the beamforming unit. A further expedient embodiment provides for the determination of correction parameters for the viewing direction of multiple antennas of at least one group of antennas of the SAR sensor. If multiple receive branches of SAR data are acquired simultaneously, the signal-to-noise ratio (SNR) in each individual receive branch is inadequate due to ambiguities and the combined effects of radar interference, instrument noise, and low antenna gain. A low SNR thus impairs the accuracy of the antenna alignment estimate obtained during calibration.The SAR signals received from multiple antennas are subsequently combined, resulting in noise reduction and, consequently, more robust and accurate antenna pointing estimates. The majority of digitized echoes transmitted and received with a specific antenna combination constitute a raw SAR data channel. For each reference target and each receive branch, a P3008PC00 residual response of the signatures is determined based on a reference target analysis, in which the signature of multiple reference targets in the raw SAR data is compared with modeled signatures. The residual responses are fused for each reference target across all receive branches of the antenna group, yielding a fused residual response for each reference target.The correction parameters for the viewing direction of several antennas of at least one group of antennas of the SAR sensor are then determined from the fused residual response. The fusion expediently comprises an addition of the residual responses for each reference target of the group of antennas. The fusion can additionally or alternatively comprise a weighted averaging of the residual responses for each reference target of the group of antennas, with an effective antenna pattern of the group of antennas being processed as the weight. The individual residual responses are expediently fused coherently. A further embodiment provides that the fused residual response and a fused antenna pattern of the group of antennas are processed as input variables for determining the correction parameters.It is furthermore expedient if the coherent superposition, which leads to the acquisition of the fused residual response, includes a precise phase calibration of all channels involved. Therefore, a calibration of the baseline and the phase offset is expediently performed before the calibration of the antenna alignment. Such a highly accurate calibration of the antenna baseline and the phase offset is described, for example, in [9]. According to a second aspect, a computer program product is proposed that includes instructions which, when executed by a computer, cause the computer to carry out the method according to one or more embodiments.P3008PC00 According to a further aspect of the invention, a device for calibrating a SAR sensor with one or more receiving channels is proposed, wherein each receiving channel comprises a plurality of receiving branches with an antenna and with an analog-to-digital converter (ADC) for digitizing the SAR signal received by the antenna into SAR raw data, as well as a beamforming unit, wherein the beamforming unit is configured to weight the SAR raw data of the plurality of receiving branches with a weighting factor, to combine the weighted SAR raw data of the plurality of receiving branches, and to store the SAR raw data obtained for a respective receiving channel in a receiving memory for later processing. The device has a processor configured to carry out the method according to one or more embodiments.The invention is explained in more detail below using exemplary embodiments in the drawing. There show: Fig. 1 a schematic representation of a known multi-channel SAR sensor with integrated DBF technology; Fig. 2 a schematic representation of a channel of an inventive SAR sensor, in which calibration data from each individual antenna is acquired by multiplexing; Fig. 3 a schematic representation of the further processing of the data acquired with the SAR sensor according to Fig. 2; Fig. 4 a schematic representation of a channel of an inventive SAR sensor, in which calibration data from all antennas are acquired simultaneously by performing signal processing using Doppler superposition; P3008PC00 Fig. 5 a schematic representation of the further processing of the data acquired with the SAR sensor according to Fig. 4;6 is a schematic representation of a channel of an inventive SAR sensor, in which calibration data from all antennas are acquired simultaneously, with a combined Doppler superposition of receive branches to receive branch groups and a multiplexing operation being carried out for each receive branch group; Fig. 7 is a schematic representation of the further processing of the data acquired with the SAR sensor according to Fig. 6; and Fig. 8 is a schematic representation of the procedure for improved estimation of an antenna alignment by increasing the signal-to-noise ratio. Fig. 1 is a schematic representation of a known multi-channel SAR sensor with DBF (Digital Beam Forming) technology. The figure illustrates the relevant components of the SAR sensor hardware and the data flow occurring therein. The data flow shown in Fig.The SAR sensor shown in Figure 1 comprises, by way of example, two reception channels CH1, CH2 (generally: CHX, where X is the number of a respective reception channel). The number of reception channels can in principle also be greater than 2. The SAR sensor can also comprise just a single reception channel without limiting the generality. The structure of the reception channels CH1, CH2 is identical, so that in the following, to simplify the description, reference is made only to the first reception channel CH1. The components belonging to a respective channel have an index notation "ab". The first digit "a" of the index indicates the channel P3008PC00 (hereinafter: 1), the second digit "b" the number of a reception branch of the channel. The reception channel CH1 comprises a plurality N of reception branches. The number of reception branches ^^^^ ^^^^. 11 , … , ^^^^ ^^^^ 1 ^^^^can be identical or different for all reception channels. In the embodiment shown in Fig.1, the second reception channel CH2 has, for example, M reception branches ^^^^ ^^^^ 21 , … , ^^^^ ^^^^ 2 ^^^^ Each receiving branch ^^^^ ^^^^ 11 , … , ^^^^ ^^^^ 1 ^^^^ includes an antenna ^^^^ 11 , … , ^^^^ 1 ^^^^ , an analog-digital converter ADC for independent digitization of the signal received by the respective antenna ^^^^ 11 , … , ^^^^ 1 ^^^^ received SAR signal in SAR raw data ^^^^ 11 , … , ^^^^ 1 ^^^^ . The SAR raw data ^^^^ 11 , … , ^^^^ 1 ^^^^ are fed as input information into a beamforming unit DBF1. A memory SP1 is connected to the beamforming unit DBF1, in which data output by the beamforming unit DBF can be stored. The beamforming unit DBF1 is configured to process the SAR raw data ^^^^ 11 , … , ^^^^ 1 ^^^^the majority N of receiving branches ^^^^ ^^^^ 11 , … , ^^^^ ^^^^ 1 ^^^^ with a weight factor 1N with a weight factor ^^^^ 1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^) and the weighted SAR raw data of the majority N of receiving branches ^^^^ ^^^^ 11 , … , ^^^^ ^^^^ 1 ^^^^ The weighted raw SAR data obtained for the receive channel CH1 are stored in the data memory SP1 for later processing. In other words, the signals from several antennas are combined in a known manner for the receive channel CH1. 11 , … , ^^^^ 1 ^^^^ recorded signals with time-variant, complex weighting factors ^^^^ 1 ^^^^(^^^^ ^^^^, ^^^^ ^^^^) combined, with the raw data being stored for each beam-shaping unit DBF1. The data stored in the data storage SP1 can then be transmitted for further analysis, for example, to a processing unit on Earth via downlink. P3008PC00 The method described below improves the external calibration technique known from [9] with regard to applicability and accuracy for multi-channel SAR sensors with integrated digital beam-shaping technology. The notation used in Fig. 1 and in the following description largely corresponds to that used in [9] and is summarized in the following table: Symbol Meaning a XnAntenna of the receive branch n of the receive channel X ADC Analog-to-digital converter. Az Integer azimuth pulse index in the calibration acquisition, starting at zero for the first pulse of the data acquisition. Rg Integer range sample index, starting at zero for the first sample in each range line. ^ ^^^ √−1 D Xn (az, rg) Complex 2D matrix containing the echoes received by antenna n. A Xn (az, rg) Complex weight of the azimuth and range variant applied to the signal received by antenna n in the associated DBF unit. During the prior art data acquisition for SAR imaging, as shown in Fig. 1, the beamforming unit DBF1 of the SAR sensor combines the data received via multiple antennas ^^^^ 11 , … , ^^^^ 1 ^^^^received signals. Only the output of the beamforming unit DBF1 is stored and forwarded to the processing unit on the ground. This creates the problem that, should the data quality prove to be inadequate after processing by the beamforming unit DBF1 on board the SAR sensor, a calibration correction for the receiving branches ^^^^ ^^^^ 11 , … , ^^^^ ^^^^ 1 ^^^^ is required, which must be taken into account by the beamforming unit DBF1. P3008PC00 However, the estimation of such corrections requires access to the individual receive branches into which the beamforming unit DBF1 processes, ie the raw SAR data ^^^^ 11 , … , ^^^^ 1 ^^^^In practice, calibration requires a special calibration data acquisition mode that bypasses the on-board processing by the beamforming unit DBF1 so that the calibration processing can be performed. One difficulty is that this imposes certain restrictions on the beamforming unit in the acquisition modes. For example, a maximum allowable data rate must not be exceeded. Various techniques for acquiring the data required for calibrating the beamforming unit are described below. The techniques do not increase the data rate beyond what is required for the nominal imaging modes, so little or no additional hardware complexity is needed to support calibration data acquisition. The description of each mode is based on a single beamforming unit DBFX of a single receive channel X.As with conventional imaging modes, the beamforming units acquire the data simultaneously and in parallel. Furthermore, it should be noted that the two initially described modes are special cases of a single, generalized approach, which is explained below. The principle underlying the following embodiments is to use the weighting factors ^^^^. ^^^^ ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^) of the reception branches ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ to adapt that for each receiving branch ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ unweighted SAR raw data ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ Subsequently, a reference target analysis is performed for each receiving branch ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ carried out, in which a signature of several reference targets detected by the SAR sensor in the unweighted SAR raw data ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^compared with modeled signatures and a P3008PC00 residual response of the signature is determined for each reference target. Then, a respective weighting factor is determined. ^^^^ ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^) for each of the plurality N of receiving branches ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ based on the reference target analysis, where the determined weight factors ^^^^ ^^^^ ^^^^(^^^^ ^^^^, ^^^^ ^^^^) represent the desired calibration parameters of the SAR sensor. To understand the present invention, it is sufficient to consider only the signal reception path in a SAR sensor, on which processing by the beamforming unit DBFX takes place. In practice, each received signal is also connected to a transmission channel to a transmission antenna. However, the description and implementation of the method is independent of the signal transmission path, so details of the transmission are neither discussed nor included in the notation used. For example, the calibration data acquisition could use the same transmission path as the usual imaging mode. Although the analysis of reference targets and the derivation of calibration corrections require that the transmission path be known and characterized, these processing steps lie outside the scope of the present invention.In a first alternative, the weight factors ^^^^ are adjusted. ^^^^ ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^) by a so-called receive branch multiplexing, which is explained in more detail below with reference to Figures 2 and 3. Fig.2 shows a schematic representation of such a receive channel CHX, in which data from each individual antenna ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ by multiplexing. Fig. 3 shows a schematic representation of the further processing of the data acquired with the SAR sensor or its reception channel CHX after it has been stored in the data memory SPX. The further processing can, as described, be carried out by a processing unit outside the sensor, e.g., in a data center on Earth. P3008PC00 In this embodiment, the processing of the beamforming unit DBFX is "replaced" by a multiplexer MUX, which cyclically switches between the reception branches D X1 ,…, D XNswitches from pulse to pulse and the corresponding SAR raw data ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ the beamforming unit DBX. The multiplexing process is carried out using binary weights in the weighting factors ^^^^ ^^^^ ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^) of the receiving branches of the beamforming unit is realized: if ^^^^ ^^^^ mod ^^^^ = ( ^^^^ − 1) otherwise , to multiplex the N receive branches to the output of the beamforming unit DBX. For the actual calibration, the approach described in reference [9] is modified in such a way that before the reference target analysis ^^^^ ^^^^( ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ) for each receiving branch ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ a de-multiplexing step DEM is inserted, as shown in Fig. 3. Each receiving branch resulting from the de-multiplexing corresponds to a single receiving branch ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^or input information for the beamforming unit DBFX. The fact that this input information has a significantly lower sampling rate is not a problem, since the target analysis described in [9] does not require SAR imaging and is therefore also capable of processing heavily subsampled data. Multiplexing MUX stores a sequence of raw SAR data in the data memory SPX of the receive channel CHX in sequential order of the receive branches ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ This is visualized in the data memory PSX by "1", "2", ..., "N", "1", "2", ... Fig.3 shows the downstream processing, in which the de-multiplexing DEM for each receive branch ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ obtained SAR raw data sorted and the target analysis ^^^^ ^^^^( ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^) (target analysis). P3008PC00 Fig.4 shows a schematic representation of a receive channel CHX, in which calibration data from all antennas ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ simultaneously, whereby signal processing by Doppler superposition is carried out for all reception branches ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ is performed. Fig. 5 shows a corresponding schematic representation of the further processing of the data acquired with the SAR sensor or its reception channel CHX after it has been stored in the data memory SPX. Further processing can, as described, be performed by a processing unit outside the sensor, e.g., in a data center on Earth. In this embodiment, the normal processing by the beamforming unit DBFX is performed by summing the raw SAR data received at the inputs of the beamforming unit DBFX. ^^^^1 , … , ^^^^ ^^^^ ^^^^the N receiving branches ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ where a different azimuthal phase ramp SV was applied to each input information. This acquisition mode can be implemented by appropriate weighting factors in the coefficient matrices of the beamforming unit DBFX as follows: around the N input receiving branches ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ coherently to form an output information Σ of the beamforming unit DBFX. For calibration, the approach described in [9] is modified such that a de-ramp step ISV (see Fig. 5) is inserted before the analysis of the reference targets. In this approach, the target analysis ^^^^ ^^^^( ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ) is applied multiple times to the same raw SAR data after it has been preprocessed with different azimuth phase ramps. The signal applied to each DBF input receive branch ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^is isolated by a bandpass filter, which is part of the target analysis workflow ^^^^ ^^^^( ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ) (Target Analysis) is as described in [9]. P3008PC00 The alternatives for calibrating the SAR sensor described above can also be combined, as shown in Figures 6 and 7. During data acquisition, as shown in Figure 6, a Doppler overlay combination is applied independently to M groups G1,…,G M of reception branches ^^^^ ^^^^ ^^^^1 , … , ^^^^ ^^^^ ^^^^ ^^^^ applied. Each of the groups G1,…,G M includes, for example, two reception branches ( ^^^^ ^^^^ ^^^^1 , ^^^^ ^^^^ ^^^^2 ) … , ( ^^^^ ^^^^^^^^ ^^^^−1, ^^^^ ^^^^ ^^^^ ^^^^ ) .After the Doppler overlay, in which a different azimuthal phase ramp (SV) was applied to each input, a multiplexer (MUX) switches between the M overlays to obtain the recorded SAR raw data for subsequent calibration. For calibration, the acquired raw data are analyzed before the reference target analysis (^^^^ ^^^^( ^^^^) ^^^^1 , … , ^^^^ ^^^^ ^^^^ ) are successively subjected to demultiplexing (DEM) and then to a de-ramping ISV (see Fig.7). For simplicity, it was assumed in this example that the N receive channels are divided into M equal-sized groups G, which consist of ^^^^ = ⌈ ^^^^ / ^^^^ ⌉ equally sized groups, each with ^^^^ consecutive receive branches. The combined heterodyne multiplexing acquisition mode can be implemented by defining the weighting factors in the beamforming unit DBFX as follows: (^^^^ − 1) mod ^^^^ ^^^^ ^^^^ ^^^^ − 1 ^^^^ ^^^^ �^^^^ if ^^^^ ^^^^ mod ^^^^ =� ^^^^ �Otherwise, Fig. 6 illustrates an example for ^^^^ = 2. The example in Fig. 6 shows, as a sequence, the first application of signal processing of the SAR raw data, wherein the combined signals of two (generally: several) signal branches are then subjected to the step of multiplexing. Alternatively, the method can also first provide for the step of multiplexing and then signal processing can take place as described above. In the following, a further development of the described method is described by determining correction parameters for the viewing direction of several antennas of at least one group of antennas ^^^^, ^^^^, ^^^^, ^^^^, ^^^^, ^^^^, ^^^^ of the SAR sensor described above.The method is based on a reference target analysis ZA (Target Analysis), in which the signature of several reference targets ^^^^, ^^^^, ^^^^ (in general: ^^^^) in the SAR raw data is compared with modeled signatures and for each of the reference targets ^^^^, ^^^^, ^^^^ and each transmit and receive branch ^^^^ ^^^^, ^^^^ ^^^^ (in general: ^^^^ ^^^^) a residual response ^^^^. ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^ )of the signatures is determined. The described correction of antenna pointing errors (also referred to as antenna alignment) is based on a method described in [9] and is fundamentally applicable independently of the calibration data acquisition modes described above. Thus, the improvement is not limited to SAR sensors with integrated beamforming (DBF). It can always be applied when it can be assumed that the antenna alignment error is identical for multiple antennas of a SAR sensor. In the following description of the determination of correction parameters for the viewing direction of the antennas of a SAR sensor, transmission channels must be explicitly considered. The notation used above is expanded as summarized in the following table: Symbol Meaning a nThe nth antenna ADC analog-to-digital converter. P3008PC00 Az Integer azimuth pulse index in the calibration acquisition, starting at zero for the first pulse of the data acquisition. Rg Integer range sampling index, starting at zero for the first sample point in each range line. ^ ^^^ � −1 D st (az, rg) Complex-valued SAR raw data channel for the transmit and receive branch st, which contains the signals received by the antennas a s and a t sent or received echoes. ^^^^ ^^^^ ^^^^ ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^)The noise-filtered residual response (or signature) of the reference target ^^^^ in the raw data Dst (az, rg).C lutter ^^^^ ^^^^ ^^^^ Average residual noise energy in the residual response^^^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^^ ( ^^^^ ^^^^, ^^^^ ^^^^). ^^^^ ^ ^ ^ ^ ^ ^ ^^^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^)The fused residual response of the reference target ^^^^ after combining signatures from multiple SAR raw data channels.C lutter ^^^^ ^^^^ ^^^^ Average residual noise energy in the fused residual response ^^^^ ^^^^ ^ ^^^ ^^^^ (^^^^ ^^^^, ^^^^ ^^^^). ^^^^ ^^^^ Range frequency (frequency within the bandwidth sampled by the ADC) ^ ^^^ ^^^^ ( ^^^^, ^^^^, ^^^^ ^^^^)Diagram of the antenna ^^^^ ^^^^ as a function of the range frequency and the propagation direction, parameterized by angle (^^^^, ^^^^) .^^^^ ^^^^ ^^^^ ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^)Azimuth and range-frequency dependent antenna gain to the reference target ^^^^. Derived from ^^^^ ^^^^ ( ^^^^, ^^^^, ^^^^ ^^^^) in conjunction with the channel-independent geometry of the calibration acquisition. ^ ^^^ ^^^^(^^^^, ^^^^, ^^^^ ^^^^)Effective antenna pattern after combining signals from multiple channels. As shown in Fig. 1, the SAR sensor collects multiple SAR raw data channels via multiple receiving antennas ^^^^ 11 , … , ^^^^ 1 ^^^^ , ^^^^ 21 , … , ^^^^ 2 ^^^^The calibration approach developed in [9] is capable of estimating an independent 3D antenna pointing correction for each individual antenna. However, this is often not required in practice. Given that the individual antennas on Earth have been very carefully characterized and designed, and given the mechanical stiffness of an antenna assembly, a different antenna pointing correction for each individual antenna is usually not necessary. Instead, it is proposed to apply a single, global antenna pointing error, i.e., the same antenna pointing correction for all antennas of the SAR sensor, or to determine different pointing errors for each of a comparatively small number of antenna groups.For example, the same antenna pointing correction can be made for all antennas on the same subpanel of a planar antenna after it has deployed in orbit. In practice, the accuracy of antenna pointing calibration is primarily limited by the signal-to-noise ratio (SNR). As illustrated in Fig. 8, the signal-to-noise ratio can be improved by using residual responses. ^ ^ ^^ ^^ ^ ^ ^ ^ ^ ^^ ), ^^^^ ^^^^ ^^^^( ^^^ ^ ^ ^ ^ ^ ^ ^ ^^ ^ ^^^ ), ^^^^ ^^^^ ^^^^( ^^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ), ^^^^ ∈ ^^^^, ^^^^, ^^^^ of the same reference target ^^^^, ^^^^, ^^^^ (second “column” in Fig.2, where ^^^^ ^^^^( ^^^^ ^ ^ ^ ^^ ^^^^ ), ^^^^ ^^^^( ^^^^ ^ ^ ^ ^^ ^^^^), ^^^^ ^^^^( ^^^^ ^ ^ ^ ^^ ^^^^ ) in the first "column" represents the reference target analysis of the respective SAR raw data channels), which were acquired simultaneously via several antennas ^^^^, ^^^^, ^^^^, ^^^^, ^^^^, ^^^^, are additively combined. This additive combination is visualized in the second and third "columns" of Fig. 8. A single estimate of the viewing direction KPPD is then determined in an orientation estimation step (last "column" in Fig. 8). A set of transmit and receive branches ^^^^ = { ^^^^ ^^^^, ^^^^ ^^^^, ... used for recording SAR raw data channels to determine correction parameters (calibration) includes all antennas of at least one group G. The same antenna ^^^^, ^^^^, ^^^^, ^^^^, ^^^^, ^^^^ can be used in multiple channels ^^^^, but each antenna ^^^^, ^^^^, ^^^^, ^^^^, ^^^^, ^^^^ must appear at least once. Furthermore, it is advisable to record the channels ^^^^ simultaneously or, if multiple P3008PC00 transmit antennas are used, as close in time as possible by interleaving different transmit pulses in a repeating pulse sequence. This simplifies processing because it ensures that the geometry of all channels ^^^^ can be considered identical for estimating the antenna orientation. In addition to these general requirements, specific details may make a particular set of transmit and receive branches preferable to others.For example, a global alignment correction can be derived for an interferometer with two antennas 1 and 2, such that ^^^^ = {1,2}, with channels ^^^^ = {11,12} or with ^^^^ = {11,22}. In practice, the first set of channels ^^^^ = {11,12} has the advantage of using only a single transmitting antenna, antenna 1, so that all channels can be received simultaneously and consequently a higher sampling rate can be used. This example shows that a radar system with two antennas can be calibrated in two different ways. In the first case, one always transmits with antenna 1 and receives simultaneously on antennas 1 and 2 (C = {11,12}). In the second case, one transmits and receives with antenna 1 (-> 11) and then does the same with antenna 2 (-> 22). In another example, a polarimetric SAR instrument uses two antennas ^^^^ = {1,2} to transmit and receive horizontally and vertically polarized signals, respectively.In this case, it may be necessary to use ^^^^ = {11, 22} as a set of channels to facilitate global alignment calibration, since certain commonly used types of reference targets are only detectable in co-polarized channels (ie, 11 or 22), but not in cross-polarized channels (ie, 12 or 21). The method described herein is based on and extends the methods described in Chapter 4.2 of Reference [9], formulating the method as a P3008PC00 optimization problem, which requires the following input information for all transmit and receive branches ^^^^ ^^^^ ∈ ^^^^ and reference targets ^^^^: − ^^^^. ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^): The residual response of a single reference target ^^^^ obtained by correcting systematic fluctuations and noise filtering. − Clutter ^ ^ ^ ^ ^ ^ ^ ^^ ^^^ : the average residual noise energy in the filtered response ^^^^ ^^^^ ^^^^( ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^)) − ^^^^ ^^^^ ( ^^^^, ^^^^, ^^^^ ^^^^) and ^^^^ ^^^^ (^^^^, ^^^^, ^^^^ ^^^^) as antenna patterns of the transmitting and receiving antennas. The content of Chapter 4.2 of Reference [9] is incorporated into the present application by reference. With regard to the actual optimization problem, as formulated in equation (27) of [9], the quantity ^^^^ ^^^^ ^^^^ ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^^ ( ^^^^ ^^^^) from the answer ^^^^ ^^^^ ^^^^( ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^)) and the residual noise energy clutter ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^can be derived using equation (14) from [9]. The antenna patterns ^^^^ ^^^^ ( ^^^^, ^^^^, ^^^^ ^^^^) and ^^^^ ^^^^ ( ^^^^, ^^^^, ^^^^ ^^^^) in conjunction with the channel-independent geometry of the calibration acquisition ^^^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^^ ( ^^^^ ^^^^, ^^^^ ^^^^) represents the azimuth- and range-frequency-dependent antenna gain relative to the reference target, as described in equation (6) of [9]. The strategy of the method proposed in Fig.8 is to use the available residual responses ^^^^ ^^^^ ^^^^ ( ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) ∀ ^^^^ ^^^^ ∈ ^^^^) to a single answer ^^^^ ^^^^ ^^^^( ^^^^ ^^ ^ ^ ^ ^ ^^ ^ ^^^( ^^^^ ^^^^, ^^^^ ^^^^)) with a significantly improved signal-to-noise ratio. As indicated by indices, the combined residual response ^^^^ ^^^^ ^^^^( ^^^^ ^^ ^ ^ ^ ^ ^^ ^ ^^^ (^^^^ ^^^^, ^^^^ ^^^^)) is treated as if it were derived from a SAR raw data channel that uses a (virtual) antenna ^^^^ for both transmission and reception. After providing the other input information required for the P3008PC00 optimization, namely the average residual clutter ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^^ and the antenna diagrams (general: ^^^^ ^^^^(^^^^, ^^^^, ^^^^ ^^^^)), the original method according to [9] for estimating the gaze direction is applied to estimate the gaze direction correction parameters. 5 The fused signature for a respective reference target ^^^^ (representing the majority of reference targets ^^^^, ^^^^, ^^^^) can then be determined as a weighted average as follows: The antenna amplification integrated over the signal bandwidth is used: 15 The residual noise in the fused signature can then be determined as follows: 20 w obei 〈 … 〉 ^^^^ the mean over ^^^^ and 25 denotes the average residual energy. P3008PC00 The combined signature is associated with an effective antenna pattern according to where | ^^^^ |denotes the number of transmit and receive branches in ^^^^. There is a certain degree of flexibility regarding the correction parameters (weights) used to determine the fused quantities. In the above formulation, greater emphasis is placed on signal components with a higher antenna gain in the interest of noise suppression. Other schemes are also conceivable. It should be noted that the individual responses ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) be added coherently to get the result ^^^^ ^^ ^ ^ ^ ^ ^^ ^ ^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ )Coherent superposition ultimately improves the signal-to-noise ratio and thus the accuracy of the direction estimation, with the following properties and advantages: − Additive coherent fusion reduces the effects of thermal noise from the SAR sensor's instruments. − Fusion of signatures with antennas separated in a longitudinal direction reduces azimuth ambiguity. − Fusion of signatures with antennas separated cross-track reduces range ambiguity. − Fusion of signatures with different polarizations reduces the effects of thermal noise and reduces the residual interference (including contributions from range and azimuth ambiguities).P3008PC00 In many cases, the set of antennas combined for improved joint direction estimation will include both longitudinal and transverse baselines and / or different polarizations, resulting in the suppression of range and azimuth ambiguities, as well as thermal noise and residual interference energy. It is important to note that the coherent heterodyne used to obtain ^^^^. ^^ ^ ^ ^ ^ ^^ ^ ^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) requires a certain phase calibration of all participating SAR raw data channels. It is therefore necessary to perform a calibration of the baseline and the relative phase positions before calibrating the antenna alignment. Such a high-precision calibration of the antenna baseline and phase positions is also described, for example, in [9].
[0002] P3008PC00 Referenzen [1] Freeman, A. SAR calibration: An overview. IEEE Trans. Geosci. Remote Sens. 1992, 30, 1107–1121. [2] Werninghaus, R.; Buckreuss, S. The TerraSAR-X Mission and System Design. IEEE Trans. Geosci. Remote Sens.2010, 48, 606–614. [3] Krieger, G.; Moreira, A.; Fiedler, H.; Hajnsek, I.; Werner, M.; Younis, M.; Zink, M. TanDEM-X: A Satellite Formation for High-Resolution SAR Interferome- try. IEEE Trans. Geosci. Remote Sens.2007, 45, 3317–3341. [4] Covello, F.; Battazza, F.; Coletta, A.; Lopinto, E.; Fiorentino, C.; Pietranera, L.; Valentini, G.; Zoffoli, S. COSMO-SkyMed an existing opportunity for observ- ing the Earth. J. Geodyn.2010; 49, 171–180. [5] Torres, R.; Snoeij, P.; Geudtner, D.; Bibby, D.; Davidson, M.; Attema, E.; Potin, P.; Rommen, B.; Floury, N.; Brown, M.; et al. GMES Sentinel-1 mission. Re- mote Sens. Environ.2012, 120, 9–24. [6] Morena, L.C.; James, K.V.; Beck, J. An introduction to the RADARSAT-2 mis- sion. Can. J. Remote Sens.2004, 30, 221–234.[7] Reigber, A.; Scheiber, R.; Jager, M.; Prats-Iraola, P.; Hajnsek, I.; Jagdhuber, T.; Papathanassiou, K.P.; Nannini, M.; Aguilera, E.; Baumgartner, S.; et al. Very- High-Resolution Airborne Synthetic Aperture Radar Imaging: Signal Pro- cessing and Applications. Proc. IEEE 2013, 101, 759–783. [8] Reigber, A.; Jäger, M.; Fischer, J.; Horn, R.; Scheiber, R.; Prats, P.; Notten- steiner, A. System status and calibration of the F-SAR airborne SAR instrument. In Proceedings of the Geoscience and Remote Sensing Symposium (IGARSS), 2011 IEEE International, Vancouver, BC, Canada, 24–29 July 2011; pp.1520– 1523. [9] Jäger, M.; Scheiber, R.; Reigber, A. Robust, Model-Based External Calibration of Multi-Channel Airborne SAR Sensors Using Range Compressed Raw Data. Remote Sens.2019, 11, 2674.
Claims
P3008PC00 Patent claims 1. Method for calibrating a SAR sensor with one or more receiving channels (CH1, CH2), each receiving channel (CH1, CH2) having a plurality (M, N) of receiving branches with an antenna ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) and with an analog-digital converter (ADC) for digitizing the signal received by the antenna ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) received SAR signal into SAR raw data ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) and a beam forming unit (DBF1, DBF2), wherein the beam forming unit (DBF1, DBF2) is configured to process the SAR raw data ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) of the plurality (M, N) of receiving branches with a em Gewichtsfaktor ( ^^^^1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) , ^^^^2 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) ) zu gewichten, die gewichtetenSAR raw data ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) of the plurality (M, N) of reception branches and storing the SAR raw data obtained for a respective reception channel (CH1, CH2) in a data memory (SP1, SP2) for later processing, the method comprising the following steps: a ) Anpassen der Gewichtsfaktoren ( ^^^^1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) , ^^^^2 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) ) der Emp- receiver branches to obtain unweighted SAR raw data ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ); b) performing a reference target analysis per receiving branch, in which the signatures of several reference targets detected by the SAR sensor in the unweighted SAR raw data ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^) is compared with modeled signatures and a residual response of the signature is determined for each reference target in each raw data set; c) determining a respective weight factor ( ^^^^ 1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^), ^^^^ 2 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^)) for each of the plurality (M, N) of receiving branches based on the reference target analysis, wherein the determined weight factors ( ^^^^1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) , ^^^^2 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) ) Kalibrationsparameter des SAR-Sensors re- present. P3008PC00 2. Method according to claim 1, characterized in that steps a) to c), in particular once, are carried out before the operation of the SAR sensor.
3. Method according to one of claims 1 or 2, characterized in that d as Anpassen der Gewichtsfaktoren ( ^^^^1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) , ^^^^2 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) ) der Emp-The multiplexing of the reception branches is carried out by a multiplexing operation by cyclically switching between the reception branches or reception branch groups from reception pulse to reception pulse, each reception branch group comprising a combination of signals from several reception branches.
4. The method according to claim 3, characterized in that the multiplexing operation is carried out with the aid of binary weighting factors ( ^^^^ 1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^), ^^^^ 2 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^)) is implemented.
5. Method according to claim 3 or 4, characterized in that the adaptation of the weighting factors ( ^^^^ 1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^), ^^^^ 2 ^^^^ (^^^^ ^^^^, ^^^^ ^^^^)) comprises a demultiplexing step, in which the reception branches or reception branch groups arise from the demultiplexing.
6. Method according to one of the preceding claims, characterized in that the SAR raw data (^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^2 ^^^^ ) of the plurality (M, N) of receiving branches or receiving branch groups are subjected to a respective signal processing, wherein each receiving branch group comprises a combination of signals from several receiving branches.
7. The method according to claim 6, characterized in that the signal processing comprises summing and shifting the center frequency of the SAR raw data (^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) of the plurality (M, N) of reception branches or reception branch groups. P3008PC00 8. Method according to claim 7, characterized in that the adjustment of the weighting factors ( ^^^^ 1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^), ^^^^ 2 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^)) in accordance with the following regulation: in which are: ^^^^ ^^^^ ^^^^( ^^^^ ^^^^, ^^^^ ^^^^): the weighting factor for the nth receive branch or the nth receive branch group, ^^^^ ^^^^: integer index of the sampling point in the range direction, starting at zero for the first sample in each received pulse, ^^^^ ^^^^: integer azimuth pulse index in the calibration acquisition, starting at zero for the first transmit pulse of the data acquisition; N: the number of receive branches or receive branch groups.
9. Method according to one of claims 6 to 8, characterized in that the adaptation of the weighting factors ( ^^^^ 1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^), ^^^^ 2 ^^^^(^^^^ ^^^^, ^^^^ ^^^^)) comprises a step of undoing the phase shift, in which the receiving branches or receiving branch groups emerge from this step.
10. Method according to one of claims 3 to 9, characterized in that the beamforming unit (DBF1, DBF2) is configured to use the weighting factors (^^^^) when carrying out the multiplexing operation. 1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^), ^^^^ 2 ^^^^ (^^^^ ^^^^, ^^^^ ^^^^)) only in the azimuth direction in order to multiplex the plurality (M, N) of receiving branches or receiving branch groups to the output of the beamforming unit (DBF1, DBF2).
11. Method according to one of the preceding claims, characterized in that a determination of correction parameters for the viewing direction P3008PC00 several antennas of at least one group of antennas (u, v, w, x, y, z) of the SAR sensor.
12. The method according to claim 11, characterized in that based on a reference target analysis, in which the signatures of several reference targets ( ^^^^ ∈ ^^^^, ^^^^, ^^^^) in the SAR raw data channels ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) are compared with modeled signatures, for each reference target ( ^^^^ ∈ ^^^^, ^^^^, ^^^^) and each SAR raw data channel a residual response ( ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) ) of the signatures are determined.
13. Method according to claim 12, characterized in that the residual responses ( ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^( ^^^^ ^^^^, ^^^^ ^^^^)) for each reference target ( ^^^^ ∈ ^^^^, ^^^^, ^^^^) in the SAR raw data channels of the group of antennas (u, v, w, x, y, z) are fused, resulting in a fused residual response ( ^^^^ ^^ ^ ^ ^ ^ ^^ ^ ^^^ (^^^^ ^^^^, ^^^^ ^^^^)) is obtained, from which the correction parameters for the viewing direction of several antennas of at least one group of antennas (u, v, w, x, y, z) of the SAR sensor are determined.
14. Method according to claim 13, characterized in that the fusion comprises an addition of the residual responses (^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^( ^^^^ ^^^^, ^^^^ ^^^^)) for each reference target ( ^^^^ ∈ ^^^^, ^^^^, ^^^^) in the SAR raw data channels of the group of antennas (u, v, w, x, y, z).
15. Method according to claim 13 or 14, characterized in that the fusion comprises a weighted averaging of the residual responses ( ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) ) for each reference target (A, B, C) over the SAR raw data channels of the group of antennas (u, v, w, x, y, z), wherein an effective antenna pattern of the group of antennas (u, v, w, x, y, z) is processed as the weight.
16. Method according to one of claims 13 to 15, characterized in that the individual residual responses ( ^^^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ( ^^^^ ^^^^, ^^^^ ^^^^ ) ) can be coherently merged. P3008PC00 17. Method according to one of claims 13 to 16, characterized in that the fused residual response ( ^^^^ ^^ ^ ^ ^ ^ ^^ ^ ^^^ (^^^^ ^^^^, ^^^^ ^^^^)) and a fused antenna diagram of the group of antennas (u, v, w, x, y, z) are processed as input variables for determining the correction parameters.
18. Computer program product comprising instructions which, when the program is executed by a computer, cause the computer to carry out the method according to one of claims 1 to 17.
19. Device for calibrating a SAR sensor with one or more receiving channels (CH1, CH2), wherein each receiving channel (CH1, CH2) has a plurality (M, N) of receiving branches with an antenna (^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) and with an analog-digital converter (ADC) for digitizing the signal received by the antenna ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21, … , ^^^^ 2 ^^^^ ) received SAR signal into SAR raw data ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) and a beam forming unit (DBF1, DBF2), wherein the beam forming unit (DBF1, DBF2) is configured to process the SAR raw data ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^ ) of the majority (M, N) of receiving branches with a weighting factor ( ^^^^ 1 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^), ^^^^ 2 ^^^^ ( ^^^^ ^^^^, ^^^^ ^^^^)) to weight the weighted SAR raw data ( ^^^^ 11 , … , ^^^^ 1 ^^^^ ; ^^^^ 21 , … , ^^^^ 2 ^^^^) of the plurality (M, N) of reception branches and to store the SAR raw data obtained for a respective reception channel (CH1, CH2) in a data memory (SP1, SP2) for later processing, wherein the device comprises a processor which is configured to carry out the method according to one of claims 1 to 17.