CA2827279A1 - Synthetic aperture radar for simultaneous imaging and ground moving target indication - Google Patents
Synthetic aperture radar for simultaneous imaging and ground moving target indication Download PDFInfo
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Classifications
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
- G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
- G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
- G01S13/9021—SAR image post-processing techniques
- G01S13/9023—SAR image post-processing techniques combined with interferometric techniques
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
- G01S13/88—Radar or analogous systems specially adapted for specific applications
- G01S13/89—Radar or analogous systems specially adapted for specific applications for mapping or imaging
- G01S13/90—Radar or analogous systems specially adapted for specific applications for mapping or imaging using synthetic aperture techniques, e.g. synthetic aperture radar [SAR] techniques
- G01S13/9021—SAR image post-processing techniques
- G01S13/9029—SAR image post-processing techniques specially adapted for moving target detection within a single SAR image or within multiple SAR images taken at the same time
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01Q—ANTENNAS, i.e. RADIO AERIALS
- H01Q3/00—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system
- H01Q3/26—Arrangements for changing or varying the orientation or the shape of the directional pattern of the waves radiated from an antenna or antenna system varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture
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- Radar, Positioning & Navigation (AREA)
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- Electromagnetism (AREA)
- Computer Networks & Wireless Communication (AREA)
- General Physics & Mathematics (AREA)
- Radar Systems Or Details Thereof (AREA)
Abstract
The invention relates to a synthetic aperture radar for simultaneous imaging and moving target indication, with an antenna configuration (10) that comprises at least one linear antenna (12, 14), formed from a plurality of subapertures (RX_N+1-RX_2*N, RX1-RX_N) arranged in a row in the flight direction (18), for receiving reflected signals, imaging means (20) embodied for generating SAR images by SAR/HRWS processing (22, 24, 26) of the separately recorded signals received from the individual subapertures (RX_N+1-RX_2*N, RX1-RX_N), and moving target indication means (28) embodied for estimating the velocity of a moving target by transforming (30) the received signals from the individual subapertures (RX_N+1-RX_2*N, RX1-RX_N) to the azimuthal frequency range, filtering (32) the transformed received signals for signal selection, and correlating (34) the selected signals for focusing on a moving target.
Description
SYNTHETIC APERTURE RADAR FOR SIMULTANEOUS IMAGING AND
GROUND MOVING TARGET INDICATION
The invention relates to a synthetic aperture radar (SAR) that can be operated simultaneously as a conventional SAR for imaging and as an interferometric SAR
for ground moving target indication (GMTI SAR).
A synthetic aperture radar (SAR) scans an object, such as the earth's surface, for example, using a moved antenna, via which pulsed signals, i.e., pulses that are emitted at defined time intervals, and echo signals, that is, the pulsed signals reflected from the scanned objects, are received. The direction of motion of the antenna is also referred to as the azimuth or along track. For each region that is illuminated and scanned by the antenna, an image of the scanned object is calculated by an SAR processor through corresponding data processing of the echo signals. An SAR is used, for example, for surveying and mapping the earth's surface by means of satellites, that is, for imaging. An SAR can also be used for ground moving target indication. An SAR used for this purpose is referred to as GMTI (Ground Moving Target Indicator) SAR. The terms SAR, SAR instrument, SAR sensor, and SAR platform are used herein as synonyms.
In what follows, the principles of the geometry of GMTI SAR and frequency/direction coupling in such a system will be described.
Fig. 1 shows the basic geometry of a GMTI SAR instrument, including the trajectory of the instrument, a moving target on the ground and the specific point target on the ground that, at the moment shown, produces the same Doppler frequency on the receiving side of the GMTI SAR instrument as the moving target with the radial velocity thereof.
Fig. 2 shows a diagram of frequency/direction coupling, which has as its ordinate the Doppler frequency f and as its abscissa the sine of the azimuthal viewing direction a from the instrument in flight onto the ground. The solid line shown
GROUND MOVING TARGET INDICATION
The invention relates to a synthetic aperture radar (SAR) that can be operated simultaneously as a conventional SAR for imaging and as an interferometric SAR
for ground moving target indication (GMTI SAR).
A synthetic aperture radar (SAR) scans an object, such as the earth's surface, for example, using a moved antenna, via which pulsed signals, i.e., pulses that are emitted at defined time intervals, and echo signals, that is, the pulsed signals reflected from the scanned objects, are received. The direction of motion of the antenna is also referred to as the azimuth or along track. For each region that is illuminated and scanned by the antenna, an image of the scanned object is calculated by an SAR processor through corresponding data processing of the echo signals. An SAR is used, for example, for surveying and mapping the earth's surface by means of satellites, that is, for imaging. An SAR can also be used for ground moving target indication. An SAR used for this purpose is referred to as GMTI (Ground Moving Target Indicator) SAR. The terms SAR, SAR instrument, SAR sensor, and SAR platform are used herein as synonyms.
In what follows, the principles of the geometry of GMTI SAR and frequency/direction coupling in such a system will be described.
Fig. 1 shows the basic geometry of a GMTI SAR instrument, including the trajectory of the instrument, a moving target on the ground and the specific point target on the ground that, at the moment shown, produces the same Doppler frequency on the receiving side of the GMTI SAR instrument as the moving target with the radial velocity thereof.
Fig. 2 shows a diagram of frequency/direction coupling, which has as its ordinate the Doppler frequency f and as its abscissa the sine of the azimuthal viewing direction a from the instrument in flight onto the ground. The solid line shown
2 indicates the track of all static point targets (so-called background =
clutter), as drawn during a pass of the instrument. Depending on the azimuthal direction of an observed point target, in SAR imaging operation said target will produce a signal with a clearly assigned Doppler frequency in a receiving antenna.
located on the ground, which is moving at a constant velocity in a radial direction.
u denotes the projection of this constant velocity within the slant distance plane.
The smaller u is, the closer the dashed line is to the solid line; for u =0 the lines would coincide, and the moving target would be a static target.
A main goal of the signal processing carried out on the ground is to separate these two curves for lul > 0 (so-called clutter suppression). The lowest velocity at which a given moving target can still be separated from the static clutter is an essential performance feature of a GMTI SAR (so-called minimum detectable velocity = M DV).
In what follows, the prior art for GMTI SAR will be specified in greater detail and the associated set of problems described.
A conventional GMTI SAR antenna architecture is schematically illustrated in Fig.
clutter), as drawn during a pass of the instrument. Depending on the azimuthal direction of an observed point target, in SAR imaging operation said target will produce a signal with a clearly assigned Doppler frequency in a receiving antenna.
located on the ground, which is moving at a constant velocity in a radial direction.
u denotes the projection of this constant velocity within the slant distance plane.
The smaller u is, the closer the dashed line is to the solid line; for u =0 the lines would coincide, and the moving target would be a static target.
A main goal of the signal processing carried out on the ground is to separate these two curves for lul > 0 (so-called clutter suppression). The lowest velocity at which a given moving target can still be separated from the static clutter is an essential performance feature of a GMTI SAR (so-called minimum detectable velocity = M DV).
In what follows, the prior art for GMTI SAR will be specified in greater detail and the associated set of problems described.
A conventional GMTI SAR antenna architecture is schematically illustrated in Fig.
3. This architecture consists of one transmitting aperture TX and three receiving subapertures RX1, RX2, RX3 arranged in flight direction 18, all having the length L in flight direction 18. Fig. 4 shows a schematic illustration of the corresponding frequency/time coupling diagram for this architecture. The signals of all subapertures RX1-RX3 are recorded in flight and are processed on the ground.
In what follows, a principal method of processing will be described in simple terms.
The signals from subapertures RX1 and RX2 are combined with one another after suitable pre-processing using a frequency-dependent digital filter such that the pair of apertures forms the antenna pattern for the receiving antenna, for example.
This pattern is optimized for maximum suppression of clutter and for maximum amplification of the moving target signal. This process is repeated with the signal pair from subapertures RX2 and RX3, so that again, only the moving target signal remains. A comparison of the two signals filtered in this manner results in phase information, from which the change in radial slant distance AR that occurs during AR
the time At and therefore the radial velocity of the moving target u = ¨ can be At determined. At is the time that elapses between the passing of the combined phase center of RX1+RX2 and the combined phase center of RX2+RX3 on the L .
moving target: At = ¨2v. v is the velocity of the GMTI SAR platform, and the factor 2 is based upon the bistatic nature of the antenna system.
The following problems occur with this system.
1. The separation of clutter signal from moving target signal by beam shaping with two phase centers results in losses in the latter. These losses are greater the lower the velocity of the moving target is, i.e., the closer the moving target track moves to the clutter track in the frequency/time coupling diagram. This results in a so-called blind velocity band, as illustrated in Fig.
In what follows, a principal method of processing will be described in simple terms.
The signals from subapertures RX1 and RX2 are combined with one another after suitable pre-processing using a frequency-dependent digital filter such that the pair of apertures forms the antenna pattern for the receiving antenna, for example.
This pattern is optimized for maximum suppression of clutter and for maximum amplification of the moving target signal. This process is repeated with the signal pair from subapertures RX2 and RX3, so that again, only the moving target signal remains. A comparison of the two signals filtered in this manner results in phase information, from which the change in radial slant distance AR that occurs during AR
the time At and therefore the radial velocity of the moving target u = ¨ can be At determined. At is the time that elapses between the passing of the combined phase center of RX1+RX2 and the combined phase center of RX2+RX3 on the L .
moving target: At = ¨2v. v is the velocity of the GMTI SAR platform, and the factor 2 is based upon the bistatic nature of the antenna system.
The following problems occur with this system.
1. The separation of clutter signal from moving target signal by beam shaping with two phase centers results in losses in the latter. These losses are greater the lower the velocity of the moving target is, i.e., the closer the moving target track moves to the clutter track in the frequency/time coupling diagram. This results in a so-called blind velocity band, as illustrated in Fig.
4, within which the desired moving target signal is contrasted insufficiently from the undesirable clutter signal. The width of the blind velocity band determines the lowest still measurable velocity of the moving target MDV.
In an advantageous design, this width must be as small as possible.
2. The time lag At and therefore the length L of the subapertures determines the greatest clearly detectable velocity range of the moving target. With long subapertures, this range can be too small for some applications.
3. The pulse repetition frequency must be chosen as greater than for the underlying SAR instrument so that the moving target signal will not fold and will not penetrate again into the blind velocity band from the other side, as illustrated in the diagram of Fig. 5. However, an increased PRF (pulse repetition frequency) will lead to a generally undesirable narrowing of the maximum detectable ground strip width, and therefore to incompatibilities with a simultaneous SAR mode of the instrument.
To correct the limitation according to 1), the literature proposes systems having the following modifications.
A): Subapertures RX1-RX3 are spread apart in flight direction 18, so that the phase centers of adjacent subapertures are spaced from one another by the distance B1, as shown in Fig. 6. This results in a narrowing of the central blind velocity band. However, this has the disadvantage of additional (also narrow) blind velocity bands at higher velocities.
B): To avoid the disadvantage under A), the resulting gap can be filled by additional subapertures of equal length. This will narrow the central blind velocity band without producing additional, periodic blind velocity bands.
Other improvements of A) are obtained by partially filling in the gaps with individual subapertures and with optimized spacing, however, gaps remain.
C): The use of longer subapertures RX1-RX3 without gaps ¨ as illustrated in Fig. 7 ¨ will also result in a narrowing of the central blind velocity band without producing new blind velocity bands. However, the clearly definable velocity range of the moving target will also be decreased due to the lengthening of spacing line B1 between the centers of adjacent subapertures.
With conventional architectures, the pulse repetition frequency is chosen such that the track of the fastest moving target will remain unfolded. Otherwise, it will become folded, as illustrated, e.g., in Fig. 5. If no reconstruction method is applied, folding will result in the loss of a part of the signal energy.
In what follows, prior art SAR systems that have a reduced pulse repetition frequency, so-called HRWS (High Resolution Wide Swath) SAR systems, which are used for imaging, will be described.
For SAR instruments without moving target indication, the following method for
In an advantageous design, this width must be as small as possible.
2. The time lag At and therefore the length L of the subapertures determines the greatest clearly detectable velocity range of the moving target. With long subapertures, this range can be too small for some applications.
3. The pulse repetition frequency must be chosen as greater than for the underlying SAR instrument so that the moving target signal will not fold and will not penetrate again into the blind velocity band from the other side, as illustrated in the diagram of Fig. 5. However, an increased PRF (pulse repetition frequency) will lead to a generally undesirable narrowing of the maximum detectable ground strip width, and therefore to incompatibilities with a simultaneous SAR mode of the instrument.
To correct the limitation according to 1), the literature proposes systems having the following modifications.
A): Subapertures RX1-RX3 are spread apart in flight direction 18, so that the phase centers of adjacent subapertures are spaced from one another by the distance B1, as shown in Fig. 6. This results in a narrowing of the central blind velocity band. However, this has the disadvantage of additional (also narrow) blind velocity bands at higher velocities.
B): To avoid the disadvantage under A), the resulting gap can be filled by additional subapertures of equal length. This will narrow the central blind velocity band without producing additional, periodic blind velocity bands.
Other improvements of A) are obtained by partially filling in the gaps with individual subapertures and with optimized spacing, however, gaps remain.
C): The use of longer subapertures RX1-RX3 without gaps ¨ as illustrated in Fig. 7 ¨ will also result in a narrowing of the central blind velocity band without producing new blind velocity bands. However, the clearly definable velocity range of the moving target will also be decreased due to the lengthening of spacing line B1 between the centers of adjacent subapertures.
With conventional architectures, the pulse repetition frequency is chosen such that the track of the fastest moving target will remain unfolded. Otherwise, it will become folded, as illustrated, e.g., in Fig. 5. If no reconstruction method is applied, folding will result in the loss of a part of the signal energy.
In what follows, prior art SAR systems that have a reduced pulse repetition frequency, so-called HRWS (High Resolution Wide Swath) SAR systems, which are used for imaging, will be described.
For SAR instruments without moving target indication, the following method for
5 decreasing PRF is available (see also European Patent EP 1 241 487 B1).
As with conventional GMTI SAR, N>1 subapertures are provided in azimuth, and the received signals thereof are recorded separately. The PRF can then be decreased by a factor of approximately N. The resulting folding of the received signal is illustrated in Fig. 8 for N=2.
For every Doppler frequency there are N point targets in azimuth, the signals of which are ambiguously superposed. However, the N subaperture antenna diagrams can be combined during processing by beam shaping in the frequency range (i.e., the N signals can be filtered during pruing using digital filter), such that, depending on the choice of the weights of the digital filter, all but one of these point target responses are suppressed. In this manner, a separation of the signals and therefore subsequently a reconstruction of the entire azimuthal spectrum of a fixed point target are possible.
Fig. 9 shows the frequency/time coupling with the additional presence of a moving target and a decreased pulse repetition frequency (in the example, by a factor of 2). The signal tracks of both the static targets and the moving target are folded.
One significant difference between the two instrument functions "SAR for imaging"
and "SAR for moving target indication" consists in the requirements relating to the Doppler band width of the received signals that must be processed by the radar instrument; these requirements are significantly more stringent for the second function than for the first. This difference has heretofore led to the use of different pulse repetition frequencies (PRF) and has prevented the definition of a common radar mode that does not result in limitations in at least one of the two modes.
As with conventional GMTI SAR, N>1 subapertures are provided in azimuth, and the received signals thereof are recorded separately. The PRF can then be decreased by a factor of approximately N. The resulting folding of the received signal is illustrated in Fig. 8 for N=2.
For every Doppler frequency there are N point targets in azimuth, the signals of which are ambiguously superposed. However, the N subaperture antenna diagrams can be combined during processing by beam shaping in the frequency range (i.e., the N signals can be filtered during pruing using digital filter), such that, depending on the choice of the weights of the digital filter, all but one of these point target responses are suppressed. In this manner, a separation of the signals and therefore subsequently a reconstruction of the entire azimuthal spectrum of a fixed point target are possible.
Fig. 9 shows the frequency/time coupling with the additional presence of a moving target and a decreased pulse repetition frequency (in the example, by a factor of 2). The signal tracks of both the static targets and the moving target are folded.
One significant difference between the two instrument functions "SAR for imaging"
and "SAR for moving target indication" consists in the requirements relating to the Doppler band width of the received signals that must be processed by the radar instrument; these requirements are significantly more stringent for the second function than for the first. This difference has heretofore led to the use of different pulse repetition frequencies (PRF) and has prevented the definition of a common radar mode that does not result in limitations in at least one of the two modes.
6 One problem addressed by the present invention is that of devising an SAR
which can be operated simultaneously as a conventional SAR for imaging and as an interferometric SAR for moving target indication.
This problem is solved by an SAR having the features of claim 1. Additional embodiments of the invention are the subject matter of the dependent claims.
According to the invention, this problem is solved by using an antenna system that comprises, on the receiving side, a series of subapertures, each with separate recording of received signals, and by implementing suitable signals processing, in particular, by implementing suitable signal processing methods that can be applied to the recorded SAR raw data, i.e., to the reflected signals that are received and recorded separately for each subaperture, particularly during processing on the ground. The suitable signals processing implements two operating modes of the SAR according to the invention: a conventional SAR
operating mode for imaging and a mode for moving target indication. This allows the invention especially to achieve simultaneously = a low PRF and therefore a broad detected strip of ground, = a high, clear velocity measurement range for the moving target, = a low minimum velocity for indicating the moving target and = a high azimuthal resolution of the SAR operating mode, as long as a corresponding number of receiving-side subapertures of the antenna system of the SAR according to the invention is provided. In other words, the antenna configuration with multiple subapertures and separate received signal recording of the subapertures allows an SAR according to the invention to be operated simultaneously in an imaging mode and a moving target indication mode, with the mode being distinguished by the processing of signals. In this
which can be operated simultaneously as a conventional SAR for imaging and as an interferometric SAR for moving target indication.
This problem is solved by an SAR having the features of claim 1. Additional embodiments of the invention are the subject matter of the dependent claims.
According to the invention, this problem is solved by using an antenna system that comprises, on the receiving side, a series of subapertures, each with separate recording of received signals, and by implementing suitable signals processing, in particular, by implementing suitable signal processing methods that can be applied to the recorded SAR raw data, i.e., to the reflected signals that are received and recorded separately for each subaperture, particularly during processing on the ground. The suitable signals processing implements two operating modes of the SAR according to the invention: a conventional SAR
operating mode for imaging and a mode for moving target indication. This allows the invention especially to achieve simultaneously = a low PRF and therefore a broad detected strip of ground, = a high, clear velocity measurement range for the moving target, = a low minimum velocity for indicating the moving target and = a high azimuthal resolution of the SAR operating mode, as long as a corresponding number of receiving-side subapertures of the antenna system of the SAR according to the invention is provided. In other words, the antenna configuration with multiple subapertures and separate received signal recording of the subapertures allows an SAR according to the invention to be operated simultaneously in an imaging mode and a moving target indication mode, with the mode being distinguished by the processing of signals. In this
7 manner, the additional subapertures that are required for the moving target indication mode at low PRF can also be used in the imaging mode to increase the sensitivity of the SAR instrument.
One embodiment of the invention relates to a synthetic aperture radar (SAR) for simultaneous imaging and moving target indication having an antenna configuration that comprises at least one linear antenna, formed from multiple subapertures arranged in a row in the flight direction for receiving reflected signals, imaging means designed for generating SAR images by means of SAR/HRWS processing of the separately recorded received signals from the individual subapertures, and moving target indication means designed for estimating the velocity of a moving target by transforming the separately recorded received signals from the individual subapertures in the azimuthal frequency range, filtering the transformed received signals for signal selection, and correlating the selected signals for focusing on a moving target. Due to the special antenna configuration and the processing of the separately recorded received signals for each subaperture by the imaging means and by the moving target indication means, an SAR of this type can be used both for recording SAR
images and for indicating moving targets, without substantial limitation of imaging or of moving target indication.
In one embodiment of the SAR it can be provided that a multiplicity of linear antennas of an antenna configuration is arranged offset from one another in the flight direction. The offset of the linear antennas allows the adjustment of a clear velocity measurement range of a moving target to be indicated, especially randomly and independently of the length of the subapertures in the flight direction.
In a special embodiment, the antenna configuration can comprise precisely two linear antennas, each formed from four subapertures arranged in a row in the flight direction, said linear antennas being arranged offset from one another in the direction of flight, wherein in each case two subapertures of each linear antenna
One embodiment of the invention relates to a synthetic aperture radar (SAR) for simultaneous imaging and moving target indication having an antenna configuration that comprises at least one linear antenna, formed from multiple subapertures arranged in a row in the flight direction for receiving reflected signals, imaging means designed for generating SAR images by means of SAR/HRWS processing of the separately recorded received signals from the individual subapertures, and moving target indication means designed for estimating the velocity of a moving target by transforming the separately recorded received signals from the individual subapertures in the azimuthal frequency range, filtering the transformed received signals for signal selection, and correlating the selected signals for focusing on a moving target. Due to the special antenna configuration and the processing of the separately recorded received signals for each subaperture by the imaging means and by the moving target indication means, an SAR of this type can be used both for recording SAR
images and for indicating moving targets, without substantial limitation of imaging or of moving target indication.
In one embodiment of the SAR it can be provided that a multiplicity of linear antennas of an antenna configuration is arranged offset from one another in the flight direction. The offset of the linear antennas allows the adjustment of a clear velocity measurement range of a moving target to be indicated, especially randomly and independently of the length of the subapertures in the flight direction.
In a special embodiment, the antenna configuration can comprise precisely two linear antennas, each formed from four subapertures arranged in a row in the flight direction, said linear antennas being arranged offset from one another in the direction of flight, wherein in each case two subapertures of each linear antenna
8 are operated in an HRWS mode of the synthetic aperture radar. By operating two subapertures in HRWS mode in each case, the strip width for this antenna configuration can be nearly doubled, so that a large strip width with a simultaneously large measurement range for the moving target velocity can be achieved to general advantage.
In one particularly special embodiment, the antenna configuration can be designed for operation in the Ka band at 35.5 GHz, each of the subapertures can have a length L of 2 m, and the offset between the two linear antennas can be 0.5 m.
The antenna configuration can also comprise precisely one linear antenna formed from seventeen subapertures arranged in a row in the flight direction, wherein in each case, four subapertures are operated in an HRWS mode of the synthetic aperture radar. In this configuration, in principle, multiple linear antennas are combined to form one long linear antenna, which is advantageous particularly with sufficiently small subapertures of different partial antennas in that it allows the antenna configuration to be less costly in design.
The imaging means can comprise a coherent addition unit, which is designed for coherently combining multiple single look complex SAR images generated by SAR/HRWS processing in order to generate a single SAR image with maximum potential sensitivity.
The moving target indication means can further comprise a frequency filter for separating the separately recorded received signals from the individual subapertures into independent data sets for independent estimates of moving target velocities.
For filtering, the moving target indication means can have M digital optimal filters for each linear antenna, for separating M branches of a folded moving target signal in the frequency/time coupling diagram. For each Doppler frequency, each
In one particularly special embodiment, the antenna configuration can be designed for operation in the Ka band at 35.5 GHz, each of the subapertures can have a length L of 2 m, and the offset between the two linear antennas can be 0.5 m.
The antenna configuration can also comprise precisely one linear antenna formed from seventeen subapertures arranged in a row in the flight direction, wherein in each case, four subapertures are operated in an HRWS mode of the synthetic aperture radar. In this configuration, in principle, multiple linear antennas are combined to form one long linear antenna, which is advantageous particularly with sufficiently small subapertures of different partial antennas in that it allows the antenna configuration to be less costly in design.
The imaging means can comprise a coherent addition unit, which is designed for coherently combining multiple single look complex SAR images generated by SAR/HRWS processing in order to generate a single SAR image with maximum potential sensitivity.
The moving target indication means can further comprise a frequency filter for separating the separately recorded received signals from the individual subapertures into independent data sets for independent estimates of moving target velocities.
For filtering, the moving target indication means can have M digital optimal filters for each linear antenna, for separating M branches of a folded moving target signal in the frequency/time coupling diagram. For each Doppler frequency, each
9 ambiguous branch of a moving target track can be suppressed by an optimal filter provided for this purpose and the clear branch of the moving target track can be determined.
The moving target indication means can have one correlation filter for each linear antenna for correlation and focusing on a moving target. Each correlation filter can be configured to match a desired moving target signal.
The moving target indication means can further have means for along-track interferometric processing, embodied for conjugate multiplication of the image signal values generated by correlation and originating from the linear antennas, and for generating phase information as output data that are provided for estimating the velocity of a moving target.
The synthetic aperture radar can further be embodied such that transmitted signals are transmitted using a transmitting aperture at different pulse repetition frequencies, in particular, at at least two different pulse repetition frequencies, which are particularly varied from trajectory segment to trajectory segment over the entire integration period of the synthetic aperture radar. In this manner, a moving target, which at one pulse repetition frequency is covered by a blind band, can be identified at another pulse repetition frequency. The precision of moving target detection can therefore be improved by using different pulse repetition frequencies.
Further advantages and potential applications of the present invention will be discussed in the following description, in conjunction with the embodiment examples illustrated in the set of drawings.
In the description, in the claims, in the abstract and in the drawings, the terms and the associated reference symbols in the list of reference symbols at the end of this document are used.
The drawings show in Fig.1 the basic geometry of a GMTI SAR instrument, including the trajectory of the instrument, a moving target on the ground and a point target on the ground;
Fig. 2 a diagram illustrating frequency/direction coupling;
5 Fig. 3 an example of a conventional GMTI SAR antenna system;
Fig. 4 a frequency/time coupling diagram of a subaperture pair in the GMTI SAR
antenna system according to Fig. 3;
Fig. 5 folding processes at low pulse repetition frequencies (left slow moving target, right rapid moving target);
The moving target indication means can have one correlation filter for each linear antenna for correlation and focusing on a moving target. Each correlation filter can be configured to match a desired moving target signal.
The moving target indication means can further have means for along-track interferometric processing, embodied for conjugate multiplication of the image signal values generated by correlation and originating from the linear antennas, and for generating phase information as output data that are provided for estimating the velocity of a moving target.
The synthetic aperture radar can further be embodied such that transmitted signals are transmitted using a transmitting aperture at different pulse repetition frequencies, in particular, at at least two different pulse repetition frequencies, which are particularly varied from trajectory segment to trajectory segment over the entire integration period of the synthetic aperture radar. In this manner, a moving target, which at one pulse repetition frequency is covered by a blind band, can be identified at another pulse repetition frequency. The precision of moving target detection can therefore be improved by using different pulse repetition frequencies.
Further advantages and potential applications of the present invention will be discussed in the following description, in conjunction with the embodiment examples illustrated in the set of drawings.
In the description, in the claims, in the abstract and in the drawings, the terms and the associated reference symbols in the list of reference symbols at the end of this document are used.
The drawings show in Fig.1 the basic geometry of a GMTI SAR instrument, including the trajectory of the instrument, a moving target on the ground and a point target on the ground;
Fig. 2 a diagram illustrating frequency/direction coupling;
5 Fig. 3 an example of a conventional GMTI SAR antenna system;
Fig. 4 a frequency/time coupling diagram of a subaperture pair in the GMTI SAR
antenna system according to Fig. 3;
Fig. 5 folding processes at low pulse repetition frequencies (left slow moving target, right rapid moving target);
10 Fig. 6 one example A) of a modified, known GMTI SAR antenna system;
Fig. 7 one example C) of a modified, known GMTI SAR antenna system;
Fig. 8 frequency/time coupling in an HRWS SAR;
Fig. 9 frequency/time coupling in scenarios with moving targets and a decreased pulse repetition frequency;
Fig. 10 beam shaping in the frequency range with digital filtering according to one embodiment example of the invention;
Fig. 11 an embodiment example of a two-level antenna configuration according to the invention;
Fig. 12 a block diagram of imaging means with SAR processing according to one embodiment example of the invention;
Fig. 7 one example C) of a modified, known GMTI SAR antenna system;
Fig. 8 frequency/time coupling in an HRWS SAR;
Fig. 9 frequency/time coupling in scenarios with moving targets and a decreased pulse repetition frequency;
Fig. 10 beam shaping in the frequency range with digital filtering according to one embodiment example of the invention;
Fig. 11 an embodiment example of a two-level antenna configuration according to the invention;
Fig. 12 a block diagram of imaging means with SAR processing according to one embodiment example of the invention;
11 Fig. 13 a block diagram of moving target indication means with moving target processing according to one embodiment example of the invention;
Fig. 14 a first embodiment example of an antenna configuration according to the invention; and Fig. 15 a second embodiment example of an antenna configuration according to the invention.
In the following description, equivalent, functionally equivalent and functionally associated elements are identified using the same reference symbols. In what follows, absolute values are indicated only by way of example, and are not understood as having a limiting effect on the invention.
The invention is based on an antenna configuration that is capable of separating every branch of the moving target track from the other branches thereof and from the different branches of the clutter track. For this purpose, a sufficient number of subapertures must be arranged in a row as with conventional GMTI SAR. Each received signal from a subaperture is recorded separately and is provided for processing on the ground.
For each Doppler frequency and for each ambiguous branch of the moving target signal a digital filter is calculated, which, after the associated filtering of the subaperture signals (i.e., the combination of subaperture beams to form the shaped overall beam), leaves only this branch and suppresses all undesirable signals at this frequency.
Fig. 10 shows an example of beam shaping in the frequency range with digital filtering. An optimal filter can be chosen as the digital filter, taking into consideration the noise output in the signals. The more subapertures having the length L that are used, the better the undesirable signals are suppressed without
Fig. 14 a first embodiment example of an antenna configuration according to the invention; and Fig. 15 a second embodiment example of an antenna configuration according to the invention.
In the following description, equivalent, functionally equivalent and functionally associated elements are identified using the same reference symbols. In what follows, absolute values are indicated only by way of example, and are not understood as having a limiting effect on the invention.
The invention is based on an antenna configuration that is capable of separating every branch of the moving target track from the other branches thereof and from the different branches of the clutter track. For this purpose, a sufficient number of subapertures must be arranged in a row as with conventional GMTI SAR. Each received signal from a subaperture is recorded separately and is provided for processing on the ground.
For each Doppler frequency and for each ambiguous branch of the moving target signal a digital filter is calculated, which, after the associated filtering of the subaperture signals (i.e., the combination of subaperture beams to form the shaped overall beam), leaves only this branch and suppresses all undesirable signals at this frequency.
Fig. 10 shows an example of beam shaping in the frequency range with digital filtering. An optimal filter can be chosen as the digital filter, taking into consideration the noise output in the signals. The more subapertures having the length L that are used, the better the undesirable signals are suppressed without
12 losses in the desired signal, and at the same time, the narrower the blind velocity band becomes.
The antenna configuration according to the invention can consist of two such linear arrangements of N subapertures, which are arranged one above the other and are offset randomly in the flight direction, as illustrated in Fig. 11 by the embodiment example of a two-level antenna arrangement or configuration 10 according to the invention. In addition to a transmitting aperture TX, the arrangement 10 has two linear antennas 12 and 14, each of which has multiple subapertures RX1-RX_N and RX_N+1-RX_2*N, respectively, arranged one behind the other or in a row in flight direction 18 of the SAR instrument for receiving reflected signals transmitted by the transmitting aperture. The two linear antennas 12 and 14 formed in this manner are arranged offset in the flight direction by an offset B2. This offset B2 allows a clear velocity measurement range of the moving target to be adjusted randomly and independently of the subaperture length L. If this length is already sufficiently small, the two partial antennas can be combined to form a single antenna arrangement that is longer by one or more subapertures.
If this velocity measurement range is large and the PRF is low, the folding of the moving target signal can run multiple times around the frequency band; at any velocity at which the moving target track coincides with the black static track, a so-called blind velocity will occur.
According to the invention, it is thus possible first to make the blind bands as small as desired by using a sufficient number N of subapertures. Second, it is possible to record at least two looks in azimuth with different PRF.
Since the blind velocities of one and the same moving target are based upon the PRF at which said target is recorded, if a target is covered in one look by a blind band, it will be visible in the next look. For this purpose, it is necessary only for the PRF to be adjusted appropriately.
The antenna configuration according to the invention can consist of two such linear arrangements of N subapertures, which are arranged one above the other and are offset randomly in the flight direction, as illustrated in Fig. 11 by the embodiment example of a two-level antenna arrangement or configuration 10 according to the invention. In addition to a transmitting aperture TX, the arrangement 10 has two linear antennas 12 and 14, each of which has multiple subapertures RX1-RX_N and RX_N+1-RX_2*N, respectively, arranged one behind the other or in a row in flight direction 18 of the SAR instrument for receiving reflected signals transmitted by the transmitting aperture. The two linear antennas 12 and 14 formed in this manner are arranged offset in the flight direction by an offset B2. This offset B2 allows a clear velocity measurement range of the moving target to be adjusted randomly and independently of the subaperture length L. If this length is already sufficiently small, the two partial antennas can be combined to form a single antenna arrangement that is longer by one or more subapertures.
If this velocity measurement range is large and the PRF is low, the folding of the moving target signal can run multiple times around the frequency band; at any velocity at which the moving target track coincides with the black static track, a so-called blind velocity will occur.
According to the invention, it is thus possible first to make the blind bands as small as desired by using a sufficient number N of subapertures. Second, it is possible to record at least two looks in azimuth with different PRF.
Since the blind velocities of one and the same moving target are based upon the PRF at which said target is recorded, if a target is covered in one look by a blind band, it will be visible in the next look. For this purpose, it is necessary only for the PRF to be adjusted appropriately.
13 With a space-borne SAR with high azimuthal resolution, the integration time of the radar is generally considerably longer than the coherence time of the echo of the moving target; therefore, two looks or more are possible. The variation of the PRF
is compensated for SAR image generation, e.g. by the loss-free interpolation of the raw data.
The antenna configuration according to the invention supplies raw SAR data, which can be used both for moving target indication (moving target processing) and for static SAR image evaluation (SAR and/or imaging processing). For this purpose, a suitable dual processing is proposed. M subapertures are required for SAR image generation at the desired PRF, and N =--- 2 = n = Msubapertures are provided on both levels of the antenna system. The factor of 2 is required in principle due to the presence of the two tracks in the frequency/time coupling diagram; the factor of n must be chosen as large enough to achieve a required blind band width.
SAR processing is first implemented independently for every M subaperture.
Fig.
12 shows a block diagram of imaging means 20 with SAR processing according to an embodiment example of the invention in which M=2, n=1. The N received signals RX1-RX_N (first linear antenna 12) and N received signals RX_N+1-RX_2*N (second linear antenna 14) recorded separately for each subaperture are supplied in pairs to SAR/HRWS processing units 22 and 24, respectively (with adjustable time lag). Each of units 22 and 24 generates an SLC (single-look complex) SAR image from the supplied received signals from the subaperture pairs.
The resulting 4-n SLC SAR images each represent a so-called look and can be further processed in the known manner. To produce a single image with maximum sensitivity, the 4n looks are coherently combined using a corresponding addition unit 26. Processing of the subaperture pairs respectively displaced in the flight direction should be displaced on the time axis for the purpose of co-registration by
is compensated for SAR image generation, e.g. by the loss-free interpolation of the raw data.
The antenna configuration according to the invention supplies raw SAR data, which can be used both for moving target indication (moving target processing) and for static SAR image evaluation (SAR and/or imaging processing). For this purpose, a suitable dual processing is proposed. M subapertures are required for SAR image generation at the desired PRF, and N =--- 2 = n = Msubapertures are provided on both levels of the antenna system. The factor of 2 is required in principle due to the presence of the two tracks in the frequency/time coupling diagram; the factor of n must be chosen as large enough to achieve a required blind band width.
SAR processing is first implemented independently for every M subaperture.
Fig.
12 shows a block diagram of imaging means 20 with SAR processing according to an embodiment example of the invention in which M=2, n=1. The N received signals RX1-RX_N (first linear antenna 12) and N received signals RX_N+1-RX_2*N (second linear antenna 14) recorded separately for each subaperture are supplied in pairs to SAR/HRWS processing units 22 and 24, respectively (with adjustable time lag). Each of units 22 and 24 generates an SLC (single-look complex) SAR image from the supplied received signals from the subaperture pairs.
The resulting 4-n SLC SAR images each represent a so-called look and can be further processed in the known manner. To produce a single image with maximum sensitivity, the 4n looks are coherently combined using a corresponding addition unit 26. Processing of the subaperture pairs respectively displaced in the flight direction should be displaced on the time axis for the purpose of co-registration by
14 the respective time lag value for the subaperture group. This can be adjusted by inputting a correspondingly chosen time lag value into the units 24.
Moving target processing is first implemented independently for the two antenna planes. Fig. 13 shows a block diagram of moving target processing means 28 with moving target indication according to one embodiment example of the invention.
The 2*N raw data, i.e., the 2*N received signals RX1-RX_N from the first linear antenna 12, separately recorded for each subaperture, and the separately recorded received signals RX_N+1-RX_2*N from the second linear antenna 14 are each supplied to range pre-processing means 30 for generating "looks in range". This pre-processing is optional. During pre-processing, the raw data are separated remotely by means of a frequency filter into individual data sets ("looks"), which later result in independent estimates of the moving target velocity.
An averaging of these independent estimated values results in improved precision. The decrease in the radar bandwidth that results from the formation of looks is useful in cases in which the SAR images need to have high resolution, in other words, the instrument has high bandwidth. However, in most cases moving target indication requires only moderate resolution.
The range pre-processing means 30 also transform the received signals RX1-RX_N from the first linear antenna 12, separately recorded for each subaperture, and the separately recorded received signals RX_N+1-RX_Th from the second linear antenna 14 to the azimuthal frequency range.
After transformation of the 2*N received signals to the azimuthal frequency range, the N received signals and/or channels of each linear antenna 12 and 14 are subjected to M different digital optimal filters 32 in order to separate the M
The separated M branches are then focused using a correlation filter 34, which is adapted to the desired moving target signal. The correlation filter 34 can also be adjusted for this purpose by moving target parameter 38.
Finally, the image signal values generated by the correlation filter 34 and 5 originating from the two partial antennas 12 and 14 are conjugate multiplied with one another by means of All (along-track interferometry) processing means 36 in the conventional All mode, in order to arrive at the desired phase information and therefore radial velocity measurement. Finally, the All processing means 36 supply, as output data, estimates of the moving target velocity for the different 10 looks 40.
The means illustrated in the block diagrams of Fig. 12 and 13 can be implemented as hardware, for example, in the form of special circuits, more particularly, ASICs (application specific integrated circuits), FPGAs (field programmable gate arrays), or as software, for example, run on a powerful standard microprocessor and/or
Moving target processing is first implemented independently for the two antenna planes. Fig. 13 shows a block diagram of moving target processing means 28 with moving target indication according to one embodiment example of the invention.
The 2*N raw data, i.e., the 2*N received signals RX1-RX_N from the first linear antenna 12, separately recorded for each subaperture, and the separately recorded received signals RX_N+1-RX_2*N from the second linear antenna 14 are each supplied to range pre-processing means 30 for generating "looks in range". This pre-processing is optional. During pre-processing, the raw data are separated remotely by means of a frequency filter into individual data sets ("looks"), which later result in independent estimates of the moving target velocity.
An averaging of these independent estimated values results in improved precision. The decrease in the radar bandwidth that results from the formation of looks is useful in cases in which the SAR images need to have high resolution, in other words, the instrument has high bandwidth. However, in most cases moving target indication requires only moderate resolution.
The range pre-processing means 30 also transform the received signals RX1-RX_N from the first linear antenna 12, separately recorded for each subaperture, and the separately recorded received signals RX_N+1-RX_Th from the second linear antenna 14 to the azimuthal frequency range.
After transformation of the 2*N received signals to the azimuthal frequency range, the N received signals and/or channels of each linear antenna 12 and 14 are subjected to M different digital optimal filters 32 in order to separate the M
The separated M branches are then focused using a correlation filter 34, which is adapted to the desired moving target signal. The correlation filter 34 can also be adjusted for this purpose by moving target parameter 38.
Finally, the image signal values generated by the correlation filter 34 and 5 originating from the two partial antennas 12 and 14 are conjugate multiplied with one another by means of All (along-track interferometry) processing means 36 in the conventional All mode, in order to arrive at the desired phase information and therefore radial velocity measurement. Finally, the All processing means 36 supply, as output data, estimates of the moving target velocity for the different 10 looks 40.
The means illustrated in the block diagrams of Fig. 12 and 13 can be implemented as hardware, for example, in the form of special circuits, more particularly, ASICs (application specific integrated circuits), FPGAs (field programmable gate arrays), or as software, for example, run on a powerful standard microprocessor and/or
15 digital signal processor (DSP) or combined with special hardware such as ASICs or FPGAs and software, run, for example, on a standard DSP. More particularly, in the case of processing on the ground and on a flying platform with an antenna configuration as illustrated, for example, in Fig. 11, the means can be implemented, for example, as a conventional computer, which is configured with software for implementing the processing described herein and which can be expanded with special hardware such as correspondingly powerful digital signal processing circuits. The flying platform with the antenna configuration according to the invention can have means for recording the received signals from the subapertures separately and for supplying them for further processing, for example, by transmission to a ground station, which carries out the processing.
Partial processing can also take place on the flying platform.
In what follows, a first, special embodiment example of the antenna configuration according to the invention will be described (embodiment example 1 in the table below).
Partial processing can also take place on the flying platform.
In what follows, a first, special embodiment example of the antenna configuration according to the invention will be described (embodiment example 1 in the table below).
16 The antenna configuration 10 operated in the Ka band (35.5 GHz) and illustrated in Fig. 14 has two linear antennas 12, 14, each having 4 subapertures RX1-RX4 and/or RX5-RX8 having the length L 2m,=
arranged in a row in the flight direction 18. The azimuthal resolution of the SAR image is 1 m. To improve (double) the narrow strip width that is possible at this high resolution only using conventional means, two subapertures each are operated in the HRWS mode (M=2, n=1).
The second level of 4 subapertures is offset by 62 = 0.5 m in the flight direction.
Approximately the performance values listed in table 1 are achieved. The advantage of configuration 10 consists in a wide strip width in combination with a very large moving target velocity measurement range.
arranged in a row in the flight direction 18. The azimuthal resolution of the SAR image is 1 m. To improve (double) the narrow strip width that is possible at this high resolution only using conventional means, two subapertures each are operated in the HRWS mode (M=2, n=1).
The second level of 4 subapertures is offset by 62 = 0.5 m in the flight direction.
Approximately the performance values listed in table 1 are achieved. The advantage of configuration 10 consists in a wide strip width in combination with a very large moving target velocity measurement range.
17 Table 1 (all moving target velocities indicated in slant distance) Parameter Embodiment Example 1 Embodiment Example 2 SAR azimuthal resolution 1 m 0.5 m PRF 3.8 kHz 1.9 kHz Maximum strip width with 45 km 90 km a 450 angle of incidence Clear velocity range 64 m/s 32 m/s Typical minimum velocity 2 m/s 1 m/s (max. ¨5dB signal loss) PRF blind velocities nx15.8m/s nx8m1 s Width of a PRF blind 25% 25%
band as a % of blind velocity In what follows, a second special embodiment example of the antenna configuration according to the invention will be described (embodiment example In this example, a narrower clear velocity measurement range for the moving target will suffice. The offset between the two linear antenna configurations can therefore measure lm. At the same time, the subaperture length should be L=lm in order to produce a high azimuthal resolution for the SAR instrument. The upper
band as a % of blind velocity In what follows, a second special embodiment example of the antenna configuration according to the invention will be described (embodiment example In this example, a narrower clear velocity measurement range for the moving target will suffice. The offset between the two linear antenna configurations can therefore measure lm. At the same time, the subaperture length should be L=lm in order to produce a high azimuthal resolution for the SAR instrument. The upper
18 Finally, significant advantages of the invention will be summarized in the following.
The high pulse repetition frequency (PRF) that is required in conventional GMTI
SAR instruments for detecting fast moving targets has thus far required that the instrument be designed for imaging narrow strips of ground. This is a disadvantage. Moreover, it prevents the design of a single instrument that has a simultaneous recording mode for both GMTI SAR and for conventional imaging SAR, which is generally designed for wide strips.
In contrast, the present invention offers the following advantages:
1. The invention specifies how the PRF of a conventional GMTI SAR
instrument can be decreased by introducing additional subapertures on the receiving side. Each of these subapertures has separate signal recording.
The strip width to be imaged and/or the maximum velocity of a detectable moving target can thereby be increased.
2. The invention specifies how a single instrument can be designed for both GMTI SAR and conventional SAR functionality and can be operated in a single imaging mode. Differentiation is made first during processing. The additional subapertures that are required for the GMTI SAR mode at low PRF can be used in the SAR mode in order to increase the sensitivity of the instrument.
3. The invention specifies which processing sequences may be used in order to arrive at both SAR image data and moving target indication data using one and the same set of raw data.
4. The maximum velocity of a detectable moving target can be adjusted independently of the length of the subapertures.
The high pulse repetition frequency (PRF) that is required in conventional GMTI
SAR instruments for detecting fast moving targets has thus far required that the instrument be designed for imaging narrow strips of ground. This is a disadvantage. Moreover, it prevents the design of a single instrument that has a simultaneous recording mode for both GMTI SAR and for conventional imaging SAR, which is generally designed for wide strips.
In contrast, the present invention offers the following advantages:
1. The invention specifies how the PRF of a conventional GMTI SAR
instrument can be decreased by introducing additional subapertures on the receiving side. Each of these subapertures has separate signal recording.
The strip width to be imaged and/or the maximum velocity of a detectable moving target can thereby be increased.
2. The invention specifies how a single instrument can be designed for both GMTI SAR and conventional SAR functionality and can be operated in a single imaging mode. Differentiation is made first during processing. The additional subapertures that are required for the GMTI SAR mode at low PRF can be used in the SAR mode in order to increase the sensitivity of the instrument.
3. The invention specifies which processing sequences may be used in order to arrive at both SAR image data and moving target indication data using one and the same set of raw data.
4. The maximum velocity of a detectable moving target can be adjusted independently of the length of the subapertures.
19 5. The blind velocities that occur at low pulse repetition frequencies are displaced in the invention by varying the PRF in alternating azimuthal looks.
As a result, moving targets that in one look are covered by a blind velocity are visible in the other look.
REFERENCE SIGNS AND ACRONYMS
10 Antenna configuration 11 Antenna configuration 5 12 First row of subapertures for receiving reflected signals 14 Second row of subapertures for receiving reflected signals 16 Single row of subapertures for receiving reflected signals 18 Flight direction
As a result, moving targets that in one look are covered by a blind velocity are visible in the other look.
REFERENCE SIGNS AND ACRONYMS
10 Antenna configuration 11 Antenna configuration 5 12 First row of subapertures for receiving reflected signals 14 Second row of subapertures for receiving reflected signals 16 Single row of subapertures for receiving reflected signals 18 Flight direction
20 SAR processing means 10 22 SAR/HRWS processing unit 24 SAR/HRWS processing unit with time lag 26 Coherent addition unit 28 Moving target processing means Range pre-processing means 15 32 Digital optimal filter 34 Correlation filter 36 ATI processing means 38 Moving target parameter Velocity estimate of a moving target for a specific look 20 RX1 - RX_N N subapertures of the first row 12 RX_N+1 ¨ RX_2*N N subapertures of the second row 14 TX Transmitting aperture Aperture length in flight direction 18 B2 Offset between the two rows 12 and 14 in flight 25 direction 18 ATI Along-Track Interferometry GMTI Ground Moving Target Indicator HRWS High Resolution Wide Swath MDV Minimum Detectable Velocity 30 PRF Pulse Repetition Frequency SAR Synthetic Aperture Radar
21 SLC single look complex
Claims (11)
1. A synthetic aperture radar for simultaneous imaging and moving target indication, having an antenna configuration (10) which comprises at least one linear antenna (12, 14) formed from a plurality of subapertures (RX_N+1-RX_2*N, RX1-RX_N) arranged in a row in the flight direction (18) for receiving reflected signals, imaging means (20), embodied for generating SAR images by means of SAR/HRWS processing (22, 24) of the separately recorded signals received from the individual subapertures (RX_N+1-RX_2*N, RX1-RX_N), and moving target indication means (28), embodied for estimating the velocity of a moving target by transforming (30) the separately recorded signals received from the individual subapertures (RX_N+1-RX_2*N, RX1-RX_N) to the azimuthal frequency range, filtering (32) the transformed received signals for signal selection, and correlating (34) the selected signals for focusing on a moving target.
2. The synthetic aperture radar according to claim 1, characterized in that a plurality of linear antennas (12, 14) in an antenna configuration (10) are arranged offset from one another (B2) in the flight direction (18).
3. The synthetic aperture radar according to claim 2, characterized in that the antenna configuration (10) comprises precisely two linear antennas (12, 14), each formed from four subapertures (RX1-RX4, RX5-RX8) arranged in a row in the flight direction (18), which antennas are arranged offset from one another (B2) in the flight direction (18), wherein in each case two subapertures of each linear antenna are operated in an HRWS mode of the synthetic aperture radar.
4. The synthetic aperture radar according to claim 3, characterized in that the antenna configuration (10) is designed to operate in the Ka band at 35.5 GHz, each of the subapertures has a length L of 2 m, and the offset (B2) between the two linear antennas (12, 14) is 0.5 m.
5. The synthetic aperture radar according to claim 1 or 2, characterized in that the antenna configuration (11) has precisely one linear antenna (16) formed from seventeen subapertures (RX1-RX17) arranged in a row in the flight direction (18), wherein in each case, four subapertures are operated in an HRWS mode of the synthetic aperture radar.
6. The synthetic aperture radar according to any one of claims 1 to 5, characterized in that the imaging means (20) have a coherent addition unit (26), embodied for coherently combining a plurality of single look complex SAR images generated by SAR/HRWS processing (22, 24) in order to generate a single SAR image with maximum possible sensitivity.
7. The synthetic aperture radar according to any one of claims 1 to 6, characterized in that the moving target indication means (28) further has a frequency filter (30) for separating the separately recorded signals received from the individual subapertures (RX_N+1-RX_2*N, RX1-RX_N) into independent data sets for independent estimates of moving target velocities.
8. The synthetic aperture radar according to any one of claims 1 to 7, characterized in that for filtering, the moving target indication means (28) has M digital optimal filters (32) for each linear antenna (12, 14) for separating M branches of a folded moving target signal in the frequency/time coupling diagram.
9. The synthetic aperture radar according to claim 8, characterized in that the moving target indication means (28) has a correlation filter (34) for each linear antenna (12, 14) for the purpose of correlation and focusing on a moving target.
10. The synthetic aperture radar according to any one of claims 1 to 9, characterized in that the moving target indication means (28) further has means for along-track interferometric processing (36), embodied for the conjugate multiplication of image signal values generated by correlation (34) and originating from the linear antennas (12, 14), and for generating phase information as output data that are provided for estimating the velocity of a moving target.
11.The synthetic aperture radar according to any one of claims 1 to 10, characterized in that it is embodied for transmitting transmitted signals with a transmitting aperture (TX) at different pulse repetition frequencies, particularly at at least two different pulse repetition frequencies, which are varied particularly from trajectory segment to trajectory segment over the entire integration period of the synthetic aperture radar.
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| DE102012021010.8 | 2012-10-26 | ||
| DE102012021010.8A DE102012021010B4 (en) | 2012-10-26 | 2012-10-26 | Synthetic aperture radar for simultaneous image acquisition and moving target detection |
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| CA2827279C CA2827279C (en) | 2020-09-22 |
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| EP (1) | EP2725382B1 (en) |
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| Publication number | Publication date |
|---|---|
| EP2725382A1 (en) | 2014-04-30 |
| DE102012021010B4 (en) | 2022-02-03 |
| DE102012021010A1 (en) | 2014-04-30 |
| EP2725382B1 (en) | 2017-01-18 |
| CA2827279C (en) | 2020-09-22 |
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