Synthetic aperture radar method
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2024-05-06
- Publication Date
- 2026-03-25
AI Technical Summary
Conventional synthetic aperture radar methods face limitations in covering wide areas due to signal quality degradation and gaps in the range direction, as the radar device cannot receive echoes during transmission, restricting the width of the captured strip.
The method employs the F-scan technique with a moving radar beam and frequency-modulated radar pulses, using multiple pulse types with different frequency bands to illuminate disjoint sections of the Earth's surface, allowing parallel detection and reducing data loss by interleaving pulse types and adjusting pulse repetition rates.
This approach enables the coverage of larger areas with improved signal quality and reduced data loss by receiving radar echoes in shorter intervals and overlapping sections, enhancing the efficiency of SAR data acquisition.
Smart Images
Figure EP2024062426_21112024_PF_FP_ABST
Abstract
Description
[0001] Synthetic aperture radar method
[0002] Description
[0003] The invention relates to a synthetic aperture radar method for remote sensing of the earth's surface via a radar device and a corresponding synthetic aperture radar system.
[0004] Synthetic aperture radar (SAR) techniques enable the remote sensing of the Earth's surface using radar pulses emitted by a radar system, whose radar echoes are reflected from the Earth's surface and received by the radar system. The radar system moves at a constant speed over the Earth's surface in a so-called azimuth direction. The radar echoes are recorded along the so-called range direction, which runs perpendicular to the azimuth direction. The term "Earth's surface" is to be understood broadly in the following and can include not only the Earth's surface but also the surface of another celestial body, particularly a planet.SAR techniques take advantage of the fact that, due to the moving radar system, the same areas of the Earth's surface are detected from different radar positions, which allows amplitude and phase information and ultimately a radar image (also known as a SAR image) of the Earth's surface to be obtained. This creates a synthetic aperture in the azimuth direction.
[0005] In conventional SAR methods, the radar system generates successive radar pulses in transmit mode, directing a fixed radar beam obliquely toward the Earth's surface. The radar returns from these radar pulses are then received by the same radar system in receive mode. The problem with this approach is that the width of the swath of the Earth's surface covered is subject to range limitations. In particular, the signal quality deteriorates when swaths are too wide. Furthermore, gaps occur within each swath because radar returns that arrive at the radar system at the same time as a radar pulse is transmitted cannot be received by the radar system.
[0006] In order to cover a wider area on the Earth's surface, it is known from the document US 9 869 762 B2 to carry out several SAR acquisitions of different strips on the Earth's surface in parallel using a radar device by interleaving sequences of different radar pulses.
[0007] A further development of conventional SAR methods is the so-called F-scan technique, which is described, for example, in the publication WO 2019 / 015911 A1. In this method, the radar beam generated by a radar pulse moves along the range direction towards shorter distances between the radar device and the Earth's surface. At the same time, the radar pulse is frequency-modulated within a specified frequency band so that the position of the moving radar beam correlates with corresponding frequencies of the radar pulse. The F-scan method enables the signals of the radar echo of a radar pulse to arrive at the radar device at essentially the same time, thereby significantly shortening the reception time interval of the radar echoes compared to conventional methods. This, in turn, means that for a specified pulse repetition rate of the successively transmitted radar pulses, the width of the detected strip can be increased.
[0008] The object of the invention is to further develop a synthetic aperture radar method based on the F-scan technique described above in order to further increase the area covered by the earth's surface.
[0009] This object is achieved by the method according to claim 1 and the device according to claim 12. Further developments of the invention are defined in the dependent claims.
[0010] The synthetic aperture radar method according to the invention uses a radar device for remote sensing of the Earth's surface. The radar device moves in an azimuth direction above the Earth's surface. The radar device can be operated in a transmit mode to transmit radar pulses to the Earth's surface and in a receive mode to receive the radar echoes of the radar pulses from the Earth's surface along a range direction perpendicular to the azimuth direction. Preferably, the radar device contains a single antenna device configured both to transmit the radar pulses and to receive the radar echoes. In transmit mode, the antenna device cannot receive radar waves.
[0011] In the synthetic aperture radar method according to the invention, the radar device emits successive radar pulses with associated pulse durations during transmission mode. Each radar pulse generates a radar beam directed toward the Earth's surface and has a waveform in a frequency band. The frequencies contained in the waveform within the frequency band are varied over the pulse duration. At the same time, the radar beam moves along the range direction over the pulse duration toward shorter distances between the radar device and the Earth's surface in order to illuminate a section of the Earth's surface in the range direction.
[0012] The method according to the invention thus uses the F-scan technique based on a moving radar beam and a frequency-modulated radar pulse. The frequency of the radar pulse within the frequency band preferably changes linearly, whereby the frequency can increase or decrease over time depending on the embodiment.
[0013] The radar echo of the respective radar pulse is received by the radar device in receive mode after reflection in the illuminated section within a reception time interval assigned to the respective radar pulse. In other words, each radar pulse is assigned a reception time interval during which the radar echo of the respective radar pulse is received. A reception time interval may also be assigned to multiple radar pulses if necessary. If necessary, a respective reception time interval can be temporarily interrupted by the transmission of one or more subsequent radar pulses, because receiving a radar echo by the radar device is generally not possible when a radar pulse is transmitted.
[0014] In contrast to conventional F-scan technology, the method according to the invention is characterized in that each radar pulse belongs to a pulse type from a predetermined number of different pulse types (i.e., at least two different pulse types), wherein the pulse type is always changed between two consecutive radar pulses and the radar pulses of the same pulse type are transmitted at a predetermined pulse repetition rate. Radar pulses of the same pulse type also have waveforms in the same frequency band and generate radar beams that illuminate the same section in the range direction on the Earth's surface. In contrast, radar pulses of different pulse types have waveforms in different frequency bands and generate radar beams that illuminate different sections in the range direction on the Earth's surface.The term “different frequency bands” is to be understood here and below to mean that for each pair of frequency bands of the different frequency bands, at least some of the frequencies of one frequency band of the pair are different from the frequencies of the other frequency band of the pair. Preferably, each frequency band of the different frequency bands contains at least some frequencies that are not contained in any other frequency band of the different frequency bands. In a particularly preferred embodiment, the different frequency bands are disjoint to one another (i.e., they have no intersections). The term “sections that differ in the range direction” is to be understood here and below to mean that for each pair of sections of the different sections, at least some of one section of the pair has no overlap in the range direction with the other section of the pair.Preferably, each section of the different sections contains at least one partial area in the range direction that is not contained in any other section of the different sections.
[0015] The method according to the invention has the advantage that the use of different pulse types enables the parallel detection of multiple sections of the Earth's surface in the range direction using the F-scan technique. In this way, a large area of the Earth's surface can be detected.
[0016] In a preferred embodiment, the predetermined pulse repetition rate at which the radar pulses of the respective pulse types are transmitted is the same for all pulse types. In other words, the pulses of the different pulse types are transmitted interleaved with a constant offset.
[0017] In a further, particularly preferred embodiment of the method according to the invention, a sequence of consecutive radar pulses of all pulse types corresponding to the predetermined number of different pulse types is repeatedly transmitted without an intervening reception time interval for radar echoes from radar pulses of this sequence, wherein radar echoes from radar pulses of all pulse types of the predetermined number of different pulse types are subsequently received in a reception time interval, wherein the received radar echoes belong to radar pulses from different, chronologically successive earlier sequences. The number of received radar echoes in a reception time interval corresponds to the predetermined number of different pulse types. This means that there are no two radar echoes in a reception time interval that belong to different radar pulses of the same pulse type.All previous sequences generally occur before the sequence followed by the reception time interval. The number of radar pulses in the sequence thus corresponds to the specified number of different pulse types, with each sequence containing one radar pulse of each pulse type. This embodiment is characterized by the fact that the radar echoes of the radar pulses of all pulse types are received in parallel in a common reception time interval, so that the radar echoes can be acquired in a shorter time. The use of different frequency bands for the waveforms of the radar pulses of different pulse types ensures that the radar echoes of the radar pulses of different pulse types can be differentiated from one another.
[0018] In a further embodiment of the method according to the invention, a respective reception time interval is provided exclusively for the radar echo of a radar pulse. By using such separate reception time intervals, a separately processable SAR data acquisition of the section of the Earth's surface associated with the pulse type is generated for each pulse type.
[0019] In a further preferred variant of the method according to the invention, at least some of the different sections, and preferably all of the different sections, are disjoint sections on the Earth's surface, i.e., sections that are disjoint in the range direction (i.e., free of intersections). Preferably, the disjoint sections are selected such that a radar echo originating from a gap between two disjoint sections in the range direction would arrive at the radar device while it is currently transmitting, so that this radar echo generally cannot be received at all.
[0020] In a further embodiment of the method according to the invention, it is also possible for the sections of at least some of the different sections to have an overlap in the range direction.
[0021] In a further preferred variant of the method according to the invention, one or more of the different pulse types each belong to a stripmap SAR mode, in which the radar pulses of the pulse type generate radar beams with main beam directions perpendicular to the azimuth direction. In other words, this embodiment implements a special variant of the known stripmap operating mode, wherein in the stripmap operating mode, the Earth's surface is detected along an elongated strip by radar pulses in a direction perpendicular to the azimuth direction.
[0022] In a further variant of the method according to the invention, one or more of the different pulse types each belong to a spotlight SAR mode, in which the radar pulses of the pulse type generate radar beams that illuminate a fixed target area on the Earth's surface. In other words, this embodiment implements a special variant of a conventional spotlight operating mode, wherein in the spotlight operating mode, the radar beams are also tilted relative to the azimuth direction in order to acquire high-resolution SAR data from a corresponding target area.
[0023] The two previously explained embodiments relating to a stripmap SAR mode and a spotlight SAR mode can also be combined with one another. In particular, one or more pulse types can belong to a stripmap SAR mode, whereas one or more other pulse types belong to a spotlight SAR mode. In a further preferred variant of the method according to the invention, the predetermined pulse repetition rate of the radar pulses of at least one pulse type of the predetermined number of different pulse types is changed one or more times during the implementation of the method. This ensures that so-called blind areas, from which incoming radar echoes cannot be received due to the transmission of a radar pulse, are shifted over time. In this way, data loss due to blind areas can be greatly reduced. Depending on the configuration, the pulse repetition rate can be changed in different ways.In particular, it can remain constant over several pulse repetition intervals before being changed. It is also possible for the pulse repetition rate to change after each pulse repetition interval.
[0024] In a further preferred embodiment of the method according to the invention, for a respective pulse type of at least some of the different pulse types and optionally of all different pulse types, the section in the range direction illuminated by the radar beam generated by the radar pulses of the respective pulse type is changed one or more times during the implementation of the method. Preferably, when the section in the range direction illuminated by the radar beam generated by the radar pulses of the respective pulse type is changed for a respective pulse type, the pulse repetition rate of the radar pulses of the respective pulse type is changed at the same time, so that a gap relating to an area on the earth's surface from which radar echoes cannot be received before the pulse repetition rate is changed due to the transmission of a radar pulse is covered by the changed section.In other words, radar echoes are received in the modified section of the Earth's surface that was a gap before the pulse repetition rate was changed. This way, data loss due to gaps or blind spots can be reduced.
[0025] In addition to the method described above, the invention relates to a synthetic aperture radar system for remote sensing of the earth's surface with a radar device which, during operation, moves in an azimuth direction above the earth's surface, wherein the radar device can be operated in a transmitting mode for transmitting radar pulses to the earth's surface and in a receiving mode for receiving the radar echoes of the radar pulses from the earth's surface along a range direction running perpendicular to the azimuth direction, wherein the synthetic aperture radar system is designed such that, during operation, it carries out the synthetic aperture radar method according to the invention or one or more preferred variants of this method.
[0026] Embodiments of the invention are described in detail below with reference to the attached figures.
[0027] They show:
[0028] Fig. 1 is a schematic perspective view explaining the SAR principle used in the invention;
[0029] Fig. 2 is a schematic diagram illustrating the differences between a conventional stripmap SAR mode without F-scan and a conventional stripmap SAR mode with F-scan;
[0030] Fig. 3 is a schematic perspective view of the conventional stripmap SAR mode with F-scan;
[0031] Fig. 4 is a schematic perspective view showing a
[0032] SAR operating mode according to a first embodiment of the invention;
[0033] Fig. 5 and Fig. 6 are diagrams illustrating characteristics of the pulse types used in the embodiment of Fig. 4; Fig. 7 is a schematic perspective view illustrating a SAR operating mode according to a second embodiment of the invention; and
[0034] Fig. 8 is a schematic perspective view showing a
[0035] SAR operating mode according to a third embodiment of the invention.
[0036] The principle of a SAR measurement is first explained in general terms with reference to Fig. 1. For this purpose, a SAR radar device 100 is used, which comprises a radar antenna, indicated in Fig. 1 as a rectangular antenna array. The radar device 100 moves in the so-called azimuth direction x at a flight speed v at a height h above the earth's surface GR. The dashed line NT indicates the so-called nadir track, which is the projection of the azimuth direction perpendicular to the earth's surface GR. For the movement of the radar device, it is located on a flying object (not shown), which is preferably a satellite, but can also be an aircraft if necessary. The radar device 100 is a combined transmitting and receiving device which transmits radar radiation during transmitting mode and receives radar radiation during receiving mode. It is not possible to receive radar radiation during transmitting mode.
[0037] During transmission, the radar device 100 transmits a radar beam RB based on a radar pulse RP at an angle β to the earth's surface GR. The radar beam RB corresponds to the main lobe of the emitted antenna radiation and is incident on the earth's surface at an angle β. The radar pulses RP are directed at the earth's surface GR in successive pulse repetition intervals with a predetermined pulse repetition frequency. The largest part of the energy of a respective radar pulse RP or its radar beam RB is directed onto an approximately elliptical surface FP on the earth's surface. This surface is generally referred to as the "footprint" of the radar device or the associated radar antenna.
[0038] In Fig. 1, SR denotes the slant distance from the radar device 100 to a point on the Earth's surface. This slant distance can be clearly converted into the so-called ground range, the direction of which is denoted by y in Fig. 1. The ground range corresponds to the projection of the slant distance SR onto the Earth's surface GR and runs perpendicular to the azimuth direction x. In the following, the direction of the ground range is also generally referred to as the range direction. The footprint FP has an extent in the range direction that corresponds to the width of the displayed strip SW. During the movement of the radar device 100, radar echoes of the radar pulses from the strip SW reflected at the Earth's surface are detected. For this purpose, the radar device 100 is switched to receive mode.
[0039] The principle of SAR measurement is based on the fact that a point on the Earth's surface GR is detected multiple times from different angles due to the movement of the radar device 100. Due to the Doppler effect, a frequency shift occurs during the detection of the radar echoes, which can be suitably evaluated, ultimately yielding amplitude and phase information for the points on the Earth's surface at which the radar pulses are reflected, and thus an image point of the Earth's surface. The corresponding calculation of image points of the Earth's surface from the detected radar echoes is well known to those skilled in the art and will therefore not be explained in further detail.
[0040] Fig. 1 shows a conventional stripmap SAR mode (without F-scan), in which the SAR radar device 100 detects an elongated strip SW with a fixed radar beam RB emitted perpendicular to the azimuth direction. A known variation of this stripmap SAR mode is the stripmap SAR mode with F-scan, which is illustrated in Fig. 2 in comparison to the stripmap SAR mode without F-scan. Fig. 2 contains four diagrams D11, D11', D12 and D12', each of which represents sections in a plane perpendicular to the azimuth direction x shown in Fig. 1. The diagram D11 shows the emission of the radar beam RB for the stripmap SAR mode without F-scan, whereas the diagram D12 shows the emission of the radar beam RB for the stripmap SAR mode with F-scan.The diagram DU' shows the wavefronts of the correspondingly transmitted radar pulses RP and the resulting radar echoes EC for the stripmap SAR mode without F-scan, whereas the diagram DI2' shows the wavefronts of the transmitted radar pulses RP and the resulting radar echoes EC for the stripmap SAR mode with F-scan.
[0041] As can be seen from the diagram DU, in stripmap SAR mode without F-scan, a radar beam RB is emitted to illuminate the strip SW, which does not change over time. Due to the fixed radar beam, the wavefronts of the radar pulses RP are emitted without a time offset, which in turn means that the resulting radar echoes EC return to the radar device 100 with a time delay due to the different distances of their reflection points (see diagram DU'). Thus, a long reception time interval is required for the reception of the radar echoes. Furthermore, the conventional stripmap SAR method has the disadvantage of using a wide radar beam, which leads to so-called range ambiguities caused by simultaneously received radar echoes from different radar pulses.
[0042] To improve the stripmap SAR mode shown in diagrams DU and DU', the stripmap SAR operating mode with F-scan is known, shown in diagrams DI2 and DI2' in Fig. 2. A suitable radar antenna design combines linear frequency-modulated pulses RP with spatial beamsteering of the radar beam RB. In other words, the frequency of a radar pulse RP changes over time during transmission, and in parallel, the radar beam RB caused by the radar pulse moves along the range direction, i.e., along the width of the detected strip SW. The corresponding position of the moving radar beam thus correlates with a frequency section of the frequency band of the transmitted radar pulse.
[0043] As can be seen from diagram DI2 in Fig. 2, the F-scan causes the radar beam RB to move in the direction of the arrow P shown. According to diagram DI2', this results in a time offset in the wavefront of the transmitted radar pulses RP. This time offset, in turn, compensates for the time offset of the radar echoes EC that occurs without F-scan. Consequently, the radar echoes EC of the respective pulses RP are received by the radar device 100 at essentially the same time. In this way, the radar echoes can be detected in a significantly shorter reception time interval. Furthermore, the use of a narrow radar beam can improve the signal quality, while at the same time, the movement of the radar beam ensures the detection of a wide swath on the Earth's surface.
[0044] Fig. 3 shows again in perspective the stripmap SAR mode with F-scan from the diagrams DI2 and DI2' of Fig. 2. As can be seen, radar pulses RP are emitted successively with the pulse repetition interval PRI, whereby each radar pulse generates a radar beam RB that covers the strip SW on the Earth's surface due to its movement in the direction of the arrow P.
[0045] The embodiments of the method according to the invention described below represent an improvement of the F-Scan SAR mode just explained. In this mode, the Earth's surface is detected using moving radar beams that cover several different areas on the Earth's surface.
[0046] Fig. 4 shows a first embodiment of an inventive SAR operating mode. According to the method shown therein, sequences of three radar pulses RP each of different pulse types PT1, PT2, and PT3 are transmitted consecutively. Within a sequence, pulse type PT1 is transmitted first, followed by pulse type PT2, and then pulse type PT3. The radar pulses of the same pulse type are transmitted with a fixed, predetermined pulse repetition interval PRI, which is the same for all pulse types. In other words, within a sequence, the individual radar pulses of the different pulse types are transmitted with the same time offset from one another.
[0047] Fig. 4 shows three temporally successive sequences of correspondingly transmitted radar pulses RP of the different pulse types PT1, PT2 and PT3. The radar pulses of the different pulse types differ in that, on the one hand, they are linearly frequency-modulated signals in disjoint frequency bands and, on the other hand, they cover different strips SW1, SW2 and SW3 on the Earth's surface. Each pulse type generates a radar beam RB that moves within the corresponding strip along the range direction y towards the nadir track NT. Specifically, the radar pulse of pulse type PT1 generates a radar beam that illuminates strip SW1, the radar pulse of pulse type PT2 generates a radar beam that illuminates strip SW2, and the radar pulse of pulse type PT3 generates a radar beam that illuminates strip SW3. The gaps between the strips SW1 and SW2 respectively.The gaps in the strips SW2 and SW3 result from the fact that these gaps correlate with time intervals in which the radar device 100 is transmitting radar pulses and therefore cannot receive any radar echoes.
[0048] Each sequence of the three radar pulses of different pulse types is followed by a receive time interval in which the radar echoes of the radar pulses of all pulse types are received by the radar device in receive mode. The radar echoes in the receive time interval belong to radar pulses from different, chronologically successive earlier sequences, which usually lie several pulse repetition intervals before the sequence followed by the receive time interval. In other words, in the receive time interval, the radar echo of the radar pulse of pulse type PT1 belongs to an earlier sequence that lies before the sequence from which the radar echo of the radar pulse of pulse type PT2 originates, while in the receive time interval, the radar echo of the radar pulse of pulse type PT2 belongs to an earlier sequence that lies before the sequence from which the radar echo of the radar pulse of pulse type PT3 originates.Due to the moving radar beams, the radar echoes arrive in a significantly shorter time than in a conventional SAR operating mode without F-scan. By using disjoint frequency bands for the different pulse types, the radar echoes can also be assigned to the corresponding radar pulses. In modified embodiments, such as the variant described below in Fig. 7, in which a stripmap SAR mode is combined with a spotlight SAR mode, it is sufficient if the frequency bands are only different according to the definition given above.
[0049] The embodiment of Fig. 4 enables the detection of a large area of the Earth's surface using multiple strips by sequentially transmitting moving radar pulses of different pulse types, whereas in the conventional F-scan operating mode, only one strip of the Earth's surface is detected at a time. Furthermore, the radar pulses of a sequence are received in a common reception time interval, thereby improving the time efficiency of the method. At the same time, the use of a narrow radar beam for the individual strips achieves a high signal-to-noise ratio and reduces the signal strength of range ambiguities.
[0050] Corresponding SAR images obtained from the radar echoes acquired using the method in Fig. 4 exhibit gaps between the displayed stripes. These gaps can be compensated for using known techniques. For example, the radar pulses of the individual pulse types can be transmitted in consecutive bursts, with the bursts differing in that the acquired stripe is shifted from one burst to the next. This can ensure that all gaps occurring in one burst are covered by corresponding stripes of another burst. Furthermore, the pulse repetition rate can also be varied over time if necessary, which in turn changes the position of the gaps, significantly reducing the data loss caused by the gaps so that the lost data can be reconstructed using interpolation.
[0051] Fig. 5 shows a diagram that plots the pulse shapes of the pulse types from Fig. 4 along the ordinate PU as a function of time t (abscissa). The dotted line LI corresponds to pulse type PT1, the dashed line L2 to pulse type PT2, and the solid line to pulse type PT3. The individual pulse types are rectangular pulses that are transmitted one after the other. The pulse durations of the individual pulse types PT1, PT2, and PT3 are designated PD1, PD2, and PD3, respectively. As can be seen, the pulses are of different lengths. They are transmitted at the beginning of a pulse repetition interval PRI over a period of approximately 100 ps. The pulse interval PRI as a whole is approximately 390 ps long.
[0052] Fig. 6 illustrates the frequency bands (ordinate with frequency f) of the individual pulses of pulse types PT1, PT2, and PT3 over time t (abscissa). The dotted line LI' shows the frequency band FBI of pulse type PT1, the dashed line L2' the frequency band FB2 of pulse type PT2, and the solid line L3' the frequency band FB3 of pulse type PT3. As can be seen, the frequency bands are disjoint from each other, which makes it possible to distinguish between the radar echoes of the radar pulses of different pulse types received together in the reception time interval.
[0053] Fig. 7 and Fig. 8 each show a second and a third embodiment of an inventive SAR operating mode. These embodiments differ from the first embodiment in that the radar echoes of the successively transmitted radar pulses of different pulse types are no longer received in a common reception time interval, but rather a separate reception time interval is used for each pulse type. This means that the radar echoes of the respective pulse types represent separate SAR data acquisitions with independent SAR raw data, from which separate SAR images can then be obtained. In the second embodiment shown in Fig. 7, two pulse types PT1 and PT2 are used, which are transmitted alternately one after the other. The radar pulses RP of the first pulse type PT1 execute a stripmap SAR mode with F-scan, i.e.They generate radar beams RB that move across a wide strip SW1 on the Earth's surface along the range direction y in a plane perpendicular to the azimuth direction x. In contrast, the radar pulses RP of the second pulse type PT2 generate radar beams RB that are all directed at a fixed target area TR on the Earth's surface, whereby this target area has a width SW2 in the range direction that is significantly smaller than the width of the strip SW1. The radar beams of the second pulse type PT2 are used to implement a SAR mode similar to a known spotlight SAR mode, whereby, in contrast to the conventional spotlight SAR mode, the individual radar beams execute a movement (i.e., an F-scan).
[0054] Fig. 8 shows a third embodiment of an inventive SAR operating mode. Analogous to the operating mode shown in Fig. 7, this operating mode also contains two different pulse types PT1 and PT2 with separate reception time intervals for the radar echoes. The first pulse type PT1, analogous to Fig. 7, performs a stripmap operating mode with F-scan in a strip SW1. In contrast to Fig. 7, in the embodiment shown in Fig. 8, the second pulse type PT2 also performs a stripmap operating mode with F-scan, but a strip SW2 of the Earth's surface is detected which, in the range direction y, is disjoint from the strip SW1 detected with the first pulse type PT1.
[0055] The embodiments of the invention described above have a number of advantages. In particular, several pulse types that generate a moving radar beam for detecting the Earth's surface are interleaved, with each pulse type covering a different section of the Earth's surface in the range direction. This makes it possible to detect a broad area of the Earth's surface with high resolution and good signal quality. In one variant, the radar echoes of all pulse types are acquired in a common reception time interval. The radar echoes can be differentiated from one another due to the use of different frequency bands of the radar pulses. By using a common reception time interval for all pulse types, more SAR raw data can be acquired in the same period of time, which means that the SAR acquisition can cover a larger area on the ground.
Claims
Patent claims 1. Synthetic aperture radar method for remote sensing of the earth's surface (GR) via a radar device (100) which moves in an azimuth direction (x) above the earth's surface (GR), wherein the radar device (100) is operable in a transmitting mode for transmitting radar pulses (RP) to the earth's surface (GR) and in a receiving mode for receiving the radar echoes (EC) of the radar pulses (RP) from the earth's surface (GR) along a range direction (y) running perpendicular to the azimuth direction (x), wherein the radar device (100) in the transmitting mode successively transmits radar pulses (RP) with associated pulse durations (PD1, PD2, PD3), wherein a respective radar pulse (RP) generates a radar beam (RB) directed towards the earth's surface (GR) and has a waveform in a frequency band (FB1, FB2, FB3), wherein the waveform contains Frequencies within the frequency band (FBI, FB2, FB3) over the pulse duration (PD1, PD2,PD3) are varied and the radar beam (RB) moves over the pulse duration (PD1, PD2, PD3) along the range direction (y) towards shorter distances between the radar device (100) and the earth's surface (GR) in order to illuminate a section (SW1, SW2, SW3) in the range direction (y) on the earth's surface (GR), wherein the radar echo (EC) of the respective radar pulse (RP) is received by the radar device (100) in receive mode after reflection in the illuminated section (SW1, SW2, SW3) in a receive time interval assigned to the respective radar pulse (RP), characterized in that each radar pulse (RP) belongs to a pulse type (PT1, PT2, PT3) from a predetermined number of different pulse types (PT1, PT2, PT3) and the pulse type (PT1, PT2, PT3) is always changed between two successive radar pulses (RP), wherein the transmission of the Radar pulses (RP) of the same pulse type (PT1, PT2, PT3) with a specified pulse repetition rate,where radar pulses (RP) of the same pulse type (PT1, PT2, PT3) have waveforms in the same frequency band (FBI, FB2, FB3) and, Generate radar beams (RB) which illuminate the same section (SW1, SW2, SW3) in the range direction (y) on the Earth's surface (GR), and wherein radar pulses (RP) of different pulse types (PT1, PT2, PT3) have waveforms in different frequency bands (FBI, FB2, FB3) and generate radar beams (RB) which illuminate different sections (SW1, SW2, SW3) in the range direction (y) on the Earth's surface (GR).
2. Method according to claim 1, characterized in that the predetermined pulse repetition rate is the same for all pulse types (PT1, PT2, PT3).
3. Method according to claim 1 or 2, characterized in that a sequence of successive radar pulses (RP) of all pulse types (PT1, PT2, PT3) corresponding to the predetermined number of different pulse types (PT1, PT2, PT3) is repeatedly transmitted without an intermediate reception time interval for radar echoes (EC) of radar pulses (RP) of this sequence and then, in a reception time interval, radar echoes (EC) of radar pulses (RP) of all pulse types (PT1, PT2, PT3) of the predetermined number of different pulse types (PT1, PT2, PT3) are received, wherein the received radar echoes (EC) belong to radar pulses (RP) from different, temporally successive earlier sequences.
4. Method according to claim 1 or 2, characterized in that a respective reception time interval is provided exclusively for the radar echo (EC) of a radar pulse (RP).
5. Method according to one of the preceding claims, characterized in that at least some of the different sections (SW1, SW2, SW3) in the range direction (y) are disjoint sections (SW1, SW2, SW3) on the earth's surface (GR).
6. Method according to one of the preceding claims, characterized in that the sections (SW1, SW2, SW3) of at least some of the different sections (SW1, SW2, SW3) have an overlap in the range direction (y).
7. Method according to one of the preceding claims, characterized in that one or more of the different pulse types (PT1, PT2, PT3) each belong to a stripmap SAR mode in which the radar pulses (RP) of the pulse type generate radar beams (RB) with main beam directions perpendicular to the azimuth direction (x).
8. Method according to one of the preceding claims, characterized in that one or more of the different pulse types (PT1, PT2, PT3) each belong to a spotlight SAR mode in which the radar pulses (RP) of the pulse type generate radar beams (RB) which illuminate a fixed target area on the earth's surface (GR).
9. Method according to one of the preceding claims, characterized in that the predetermined pulse repetition rate of the radar pulses of at least one pulse type (PT1, PT2, PT3) of the predetermined number of different pulse types (PT1, PT2, PT3) is changed one or more times.
10. Method according to one of the preceding claims, characterized in that for a respective pulse type (PT1, PT2, PT3) of at least a part of the different pulse types (PT1, PT2, PT3), the section (SW1, SW2, SW3) in the range direction (y) which is illuminated by the radar beam (RB) generated by the radar pulses (RP) of the respective pulse type (PT1, PT2, PT3) is changed one or more times.
11. Method according to claim 10, characterized in that when for a respective pulse type (PT1, PT2, PT3) the section (SW1, SW2, SW3) in TI Range direction (y) illuminated by the radar beam (RB) generated by the radar pulses (RP) of the respective pulse type (PT1, PT2, PT3) is changed, at the same time the pulse repetition rate of the radar pulses (RP) of the respective pulse type (PT1, PT2, PT3) is changed, so that a gap which concerns an area on the earth's surface (GR) from which radar echoes (EC) cannot be received before the pulse repetition rate is changed due to the transmission of a radar pulse (RP) is covered by the changed section (SW1, SW2, SW3).
12. Synthetic aperture radar system for remote sensing of the earth's surface (GR) with a radar device (100) which, during operation, moves in an azimuth direction (x) above the earth's surface (GR), wherein the radar device (100) is operable in a transmitting mode for transmitting radar pulses (RP) to the earth's surface (GR) and in a receiving mode for receiving the radar echoes (EC) of the radar pulses (RP) from the earth's surface (GR) along a range direction (y) running perpendicular to the azimuth direction (x), wherein the synthetic aperture radar system is designed such that the radar device (100) in the transmitting mode successively transmits radar pulses (RP) with associated pulse durations (PD1, PD2, PD3), wherein a respective radar pulse (RP) generates a radar beam (RB) directed towards the earth's surface (GR) and a waveform in a frequency band (FBI, FB2, FB3), wherein frequencies contained in the waveform within the frequency band (FBI, FB2, FB3) are distributed over the pulse duration (PD1,PD2, PD3) are varied and the radar beam (RB) moves over the pulse duration (PD1, PD2, PD3) along the range direction (y) towards shorter distances between the radar device (100) and the earth's surface (GR) in order to illuminate a section (SW1, SW2, SW3) in the range direction (y) on the earth's surface (GR), wherein the radar echo (EC) of the respective radar pulse (RP) is received after reflection in the illuminated section (SW1, SW2, SW3) in a reception time interval by the radar device (100) in the reception mode, characterized in that each radar pulse (RP) belongs to a pulse type (PT1, PT2, PT3) from a predetermined number of different pulse types (PT1, PT2, PT3) and between two consecutive radar pulses (RP) the pulse type (PT1, PT2, PT3) is always changed, wherein the transmission of the radar pulses (RP) of the same pulse type (PT1, PT2, PT3) takes place at a predetermined pulse repetition rate, wherein radar pulses (RP) of the same pulse type (PT1, PT2, PT3) have waveforms in the same frequency band (FBI, FB2, FB3) and generate radar beams (RB) which illuminate the same section (SW1, SW2, SW3) in the range direction (y) on the earth's surface (GR), and wherein radar pulses (RP) of different pulse types (PT1, PT2, PT3) have waveforms in different frequency bands (FBI, FB2, FB3) and Generate radar beams (RB) that illuminate different sections (SW1, SW2, SW3) in the range direction (y) on the Earth's surface (GR). 13.Synthetic aperture radar system according to claim 12, which is arranged to carry out a method according to one of claims 2 to 11.