CA2553008C - Synthetic aperture radar (sar) system - Google Patents
Synthetic aperture radar (sar) system Download PDFInfo
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- CA2553008C CA2553008C CA2553008A CA2553008A CA2553008C CA 2553008 C CA2553008 C CA 2553008C CA 2553008 A CA2553008 A CA 2553008A CA 2553008 A CA2553008 A CA 2553008A CA 2553008 C CA2553008 C CA 2553008C
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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
- G01S13/904—SAR modes
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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
- G01S13/904—SAR modes
- G01S13/9056—Scan SAR mode
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- Variable-Direction Aerials And Aerial Arrays (AREA)
Abstract
In a multi-channel Synthetic Aperture Radar (SAR), a SAR group antenna with several receivers is reproduced by means of time-multiplexing a receiver, wherein each receive channel is assigned an antenna segment and, parallel to multiplexing a receiver, a different amplitude allocation is given to the antenna. More "virtual" channels than are physically present are provided, which is achieved by an increased pulse repetition frequency (PRF). The amplitude allocation of the antenna can be assigned during transmission, during receiving and during transmission and receiving. Multiplexing can be advantageously expanded by operating receivers in parallel mode and modifying the so-called "burst-mode technique", which is otherwise used in Scan-SAR systems. Used in multi-channel SAR and SAR interferometry, especially satellite applications.
Description
The invention relates to a Synthetic Aperture Radar (SAR) System for mapping ground strips with several, namely a, receive channels, each of which is connected to a different antenna segment of a SAR group antenna featuring a antenna segments and being mounted on a carrier platform moving along above the ground, wherein the antenna segments of the SAR
group antenna, in order to receive the pulse echo signals of SAR impulses previously sent out via the SAR group antenna, feature spatially separated phase centers, thus assigning each receive channel to a different phase center.
Basic SAR systems only feature one receive channel consisting of the antenna, RF amplifier, mixer, receiver, digitization unit and a data storage unit. The antenna is generally designed as a reflector antenna or planar antenna, which is usually divided into antenna segments, with their high-frequency signals being summed up by a summing unit. An example for this technique is the SAR on the ERS satellite of the ESA.
Other advanced systems, such as the ASAR instrument on the ENVISAT satellite, feature an Electronically phase-controlled antenna consisting of a multitude of transmit/receive modules and facilitating a pivoting of the antenna diagram. The high-frequency signals of the receiver units are also summed up by a summing unit, so that eventually there is only a single receive channel present as well.
More advanced SAR systems for SAR interferometry, which are used, for example, to produce terrain models or measure marine currents, require two completely equipped receive channels and spatially separated phase centers of the antennas assigned to these channels.
Furthermore, multi-channel techniques have been developed which only obtain a desired directional response pattern of the antenna after the data have been recorded by the signal processor. So-called group antennae are used, which are usually arranged directly joining each other. Each individual antenna is assigned a complete receive path, i.e. including mixer, digitizer and data storage unit. The phase control of the antenna is then carried out by phase-impacting the multi-channel data in the connected signal processor.
The detection and measuring of moving objects which first have to be made visible before the static background in the radar image requires more than two receive channels. Such an application requires at least= three receive channels. In order to filter the unmoving image background, the data of one receive channel are subtracted from those of the other two channels. Only the signal portions of the moving objects remain in the data of these two channels; their speed can be determined by means of so-called along-track interferometry.
Thus far, multi-channel group antennas could not be used in practical applications of SAR satellite systems, since the expenses, i.e. costs, weight and energy consumption, for an entire series of receiver units, usually at least three or four, are too great.
group antenna, in order to receive the pulse echo signals of SAR impulses previously sent out via the SAR group antenna, feature spatially separated phase centers, thus assigning each receive channel to a different phase center.
Basic SAR systems only feature one receive channel consisting of the antenna, RF amplifier, mixer, receiver, digitization unit and a data storage unit. The antenna is generally designed as a reflector antenna or planar antenna, which is usually divided into antenna segments, with their high-frequency signals being summed up by a summing unit. An example for this technique is the SAR on the ERS satellite of the ESA.
Other advanced systems, such as the ASAR instrument on the ENVISAT satellite, feature an Electronically phase-controlled antenna consisting of a multitude of transmit/receive modules and facilitating a pivoting of the antenna diagram. The high-frequency signals of the receiver units are also summed up by a summing unit, so that eventually there is only a single receive channel present as well.
More advanced SAR systems for SAR interferometry, which are used, for example, to produce terrain models or measure marine currents, require two completely equipped receive channels and spatially separated phase centers of the antennas assigned to these channels.
Furthermore, multi-channel techniques have been developed which only obtain a desired directional response pattern of the antenna after the data have been recorded by the signal processor. So-called group antennae are used, which are usually arranged directly joining each other. Each individual antenna is assigned a complete receive path, i.e. including mixer, digitizer and data storage unit. The phase control of the antenna is then carried out by phase-impacting the multi-channel data in the connected signal processor.
The detection and measuring of moving objects which first have to be made visible before the static background in the radar image requires more than two receive channels. Such an application requires at least= three receive channels. In order to filter the unmoving image background, the data of one receive channel are subtracted from those of the other two channels. Only the signal portions of the moving objects remain in the data of these two channels; their speed can be determined by means of so-called along-track interferometry.
Thus far, multi-channel group antennas could not be used in practical applications of SAR satellite systems, since the expenses, i.e. costs, weight and energy consumption, for an entire series of receiver units, usually at least three or four, are too great.
The conferencE; paper "Conceptual Studies for Exploiting the TerraSAR-X Dual Receive Antenna" by J. Mittermayer and H. Runge, IGARSS 2003, Toulouse/France, 07/21/03-07/25/03, IEEE 2003 International Geoscie;nce and Remote Sensing Sympo-sium, IEEE, 2003, contains the description of a dual-channel SAR receive system for the German remote exploration satellite TerraSAR-X. For redundancy purposes, this satellite features two complete receive channels, just like the Canadian Radarsat-2. The complete antenna is used for transmitting. For receiving, however, the antenna is divided into two separate parts in alc>ng-track. The signals of both receive antenna halves are separately detected and recorded in the two receive channels. This well-known SAR antenna allows along-track interferometry.
The task of t:he present invention is to create a technique which facilitates the implementation of a SAR multi-receive channel system with relatively little effort, wherein the above-mentioned satellites with two receive channels each can also be upgraded to three, four or theoretically even more receive channels, practically without any additional equipment-related expenses. Th.e present invention should be able to be applied not only in SAR satellites but also in SAR
radar equipment on other carrier platforms, such as aircrafts or drones.
According to the present invention, which relates to a Synthetic Aperture Radar (SAR) system of the type mentioned above, this task is solved as follows: the receive channels, which are connected to one antenna segment each, are assigned to a common SAR receiver and operated in the manner of a time-multiplex switching as time-multiplex channels in the SAR receiver, so that specific amplitude allocations can be activated via the various antenna segments and the individual antenna segments can be activated and deactivated, while the pulse repetition frequency (PRF) of the SAR system being operated in time multiplex on t=he receiver side, compared to a standard SP,R system operated without time multiplex, is raised by the factor a; analogously, to produce the corresponding specific amplitude allocations of the SAR group antenna, alternatively or simultaneously corresponding antenna segments of transmit channels can be activated or deactivated in transmission mode.
According to the present invention, the process of time-switching, which may be defined as time multiplexing, allows a receiver to have multiple uses, which increases the number of the "virtual" channels by the factor a. However, multiplexing also requires an increase in the Pulse Repetition Rate (PRF) of the SAR by the factor a. Therefore, an increased Pulse Repetition Rate PRF can save on expenses for hardware, i.e. actually physically present receive channels. It is essential that a different antenna phase center is provided for each channel. For this purpose, the antenna aperture has to be shifted. In the present invention, this is achieved by deactivating antenna segments. This technique is also referred to as "Aperture Switching".
According to an advantageou:~ advanced embodiment of the present invention, an additional factor b is obtained by the parallel use of several receivers which are assigned to certain antenna segment groups, e.g. the front and back half of the antenna in the moving direction of the carrier platform.
The task of t:he present invention is to create a technique which facilitates the implementation of a SAR multi-receive channel system with relatively little effort, wherein the above-mentioned satellites with two receive channels each can also be upgraded to three, four or theoretically even more receive channels, practically without any additional equipment-related expenses. Th.e present invention should be able to be applied not only in SAR satellites but also in SAR
radar equipment on other carrier platforms, such as aircrafts or drones.
According to the present invention, which relates to a Synthetic Aperture Radar (SAR) system of the type mentioned above, this task is solved as follows: the receive channels, which are connected to one antenna segment each, are assigned to a common SAR receiver and operated in the manner of a time-multiplex switching as time-multiplex channels in the SAR receiver, so that specific amplitude allocations can be activated via the various antenna segments and the individual antenna segments can be activated and deactivated, while the pulse repetition frequency (PRF) of the SAR system being operated in time multiplex on t=he receiver side, compared to a standard SP,R system operated without time multiplex, is raised by the factor a; analogously, to produce the corresponding specific amplitude allocations of the SAR group antenna, alternatively or simultaneously corresponding antenna segments of transmit channels can be activated or deactivated in transmission mode.
According to the present invention, the process of time-switching, which may be defined as time multiplexing, allows a receiver to have multiple uses, which increases the number of the "virtual" channels by the factor a. However, multiplexing also requires an increase in the Pulse Repetition Rate (PRF) of the SAR by the factor a. Therefore, an increased Pulse Repetition Rate PRF can save on expenses for hardware, i.e. actually physically present receive channels. It is essential that a different antenna phase center is provided for each channel. For this purpose, the antenna aperture has to be shifted. In the present invention, this is achieved by deactivating antenna segments. This technique is also referred to as "Aperture Switching".
According to an advantageou:~ advanced embodiment of the present invention, an additional factor b is obtained by the parallel use of several receivers which are assigned to certain antenna segment groups, e.g. the front and back half of the antenna in the moving direction of the carrier platform.
5 Therefore, when using this advantageous advanced embodiment, n = a x b "virtual" receivers can be provided, of which only b have to be physically present. Consequently, a SAR group antenna with n receivers can be reproduced in a mufti-channel synthetic aperture radar system (SAR).
The above application of the advanced embodiment has great practical significance, since the factors a and b can only be increased within narrow limits in practice. As is generally known, if the pulse repetition frequency PRF of a Synthetic Aperture Radar System (SAR) is increased (in this case by the factor a), the obtainable image strip width is reduced.
Moreover, as previously described, it is not possible, for reasons of expense, to provide a satellite with any number of receive chains (here factor b) in the form of actually physically present hardware. However, as illustrated by the practical embodiment examples TerraSAR-X and Radarsat-2, the pulse repetit_Lon frequency PRF can be increased by the factor a = 2. On the other hand, two complete receive chains are provided each (factor b = 2), since generally all important components are provided in two (redundant) units in satellites.
Therefore, by using the technique described by the invention, four virtual receive channels can be reproduced for these satellites, for example, thereby facilitating many interesting applications which could not be implemented with only two receive channels.
Multiplexing a receiver in order to facilitate a multi-channel technique is a well-known procedure which is also used in radar technology. The US patent specification 5 966 092 mentions this possibility for a mono-pulse radar dealing with locating purposes, in particular directional reference of radar targets. It must be pointed out in this context, however, that the above invention involves basic switching (multiplexing) of radar channels which is not associated with the switching of antenna apertures (aperture switching), a5 is the case in the present invention. Instead, each Channel is assigned a separate antenna in the above invention.
Another examp7_e for receiver multiplexing is indicated in the German patent specification DE-1001 20 536 C2 in connection with an active obstacle warning radar system. This also involves a receiver being switched between various antenna elements (channels). In this invention, the antenna elements are approached one by one.
In both multiplexing systems known from US-5 966 092 and DE-1001 20 536 C2, the antenna elements are sequentially put through to the receiver.
In this present invention, however, the entire SAR group antenna remains connected to the receiver during multiplexing.
Using specific diagrams of the amplitude allocation of the SAR group antenna, the phase centers of the antenna can be shifted in a SAR system according to the invention, which is also exemplified in detail in several dependent claims.
During this process, Large areas of the antenna should preferably remain active, thus contributing to reception and therefore to a high antenna yield.
Certain switch patterns for t:he antenna segments, passive unit extensions to the active radar antenna, and the use of the so-called SAR burst-mode technique are to be considered as advantageous advanced embodiments of the mufti-channel synthetic aperture radar according to the invention, which is also stated in the dependent claims.
Thus, already existing SAR satellite systems can be operated as mufti-channel systems when using the present invention, and in future designs, a multitude of expensive, heavy and energy-consuming receivers can be dispensed with.
In an advantageous embodiment, the SAR group antenna is an electronically phase-controlled antenna with a multitude of transmit/receive modules. During time multiplexing, the amplification of the respective receive part and/or transmit part of the transmit/receive modules in the areas of the antenna segments which are to be deactivated for reception is switched to zero, so that the specific amplitude allocations can be activated and individual antenna segments can be activated and deactivated via the various antenna segments.
It is therefore important for the above-mentioned advanced embodiment of the present invention that each of the many transmit/receive modules (TR modules) can be individually controlled in its amplification (amplitude weighting of TR
modules). If the amplification is set to zero during transmitting or receiving, the element concerned is practically deactivated and does not contribute to the summation. If the transmit/receive modules at the edge of a planar antenna, e.g, in front and back, are deactivated, the phase center of the antenna can be shifted. So far, this technical trick has not been used in practice, since no application was known for it. In the present invention, it is used to assign different phase centers to a SAR group antenna. Since planar antennae in TR-module technology generally contain several hundred modules, a fine control of the phase center is possible.
The SAR group antenna may also be designed as a passive planar antenna, in which case microwave switches are planned for time multiplexing in the connections of the receiver to the antenna segments, so 'that the specific amplitude allocations can be activated and individual antenna segments can be activated and deactivated via the various antenna segments.
In practice, basic passive planar antennae do not offer as much flexibility as electronically phase-controlled group antennae for this purpose, since they mostly only allow for larger units, so-called "panels" or "leafs" to be activated and deactivated, if the necessary microwave switches are provided.
Advantageous and functional advanced embodiments are the subjects of claims referring to patent claim 1, either directly or indirectly.
The following is a detailed explanation of the invention based on design examples illustrated by drawings. It should be noted that all these designs, except for the design shown in Fig.l, can in principle be implemented with TerraSAR-X.
The illustrations show:
Fig.l shows the schematic design of a typical SAR planar group antenna, in which transmission is carried out as a whole and receiving involves four different aperture allocations and four steps, Fig.2 shows the schematic design of a SAR along-track interferometer implemented by multiplexing a receive channel and aperture-switching the group antenna, Fig.3 shows the schematic design of a SAR across-track interferometer implemented by multiplexing a receive channel and aperture-switching the group antenna, Fig.4 shows a schematic dE~sign involving an active SAR
satellite main antenna with a dual-receive antenna unit extension, Fig.5 shows the schematic design of a virtual 4-channel SAR with double-multiplexing, aperture-switching and a dual-channel receiver, 5 Fig.6 shows the schematic design of a SAR along-track interferometer capable of recording two interferometric data sets with different baseline lengths (multi-baseline interferometer), 10 Fig.7 shows the schematic design of a combined along- and across-track SAR interferometer, and Fig. 8 shows the schematic design of a three-channel SAR
system with double multiplexing, aperture switching and a dual-channel receiver.
The essential fact about a group antenna is that it allows scanning at different spatial positions. For this purpose, the phase centers of the individual channels are spatially separated; in principle, this <:an involve the transmit and/or receive antennae. With a typical group antenna, spatial scanning occurs simultaneously in the individual channels.
Fig.l shows a schematic representation of a typical SAR
planar group antenna suitable both for transmitting and receiving, wit=h its longitudinal axis extending in the moving direction of the carrier platform. In this example, the whole group antenna is used for transmitting. When receiving the radar pulses scattered back from the ground, different antenna segments of the group antenna aperture are deactivated, i.e., switched off, for each of the pulses sub-commutated in the standard PRF (pulse repetition frequency).
The deactivated antenna segments are marked grey in Fig.l and all other figures, while the activated antenna segments are marked white. Using this so-called "aperture switching" and time-multiplexing with the quadruple standard PRF, a 4-antenna-segment group antenna with four equidistant phase centers is reproduced. Only one receiver is used for this procedure. Fig.l illustrates which antenna segments must be deactivated during receiving in order to obtain the desired four phase centers.
The switch between the virtual channels must occur within one pulse repetition interval of the standard SAR system. This means that the pulse repetition frequency PRF of the SAR
system must be increased by the factor which corresponds to the number of receive channe:Ls. An increase of the pulse repetition frequency PRF can only be achieved, among other things, by limiting the image strip width. In fact, the scenario shown in Fig.l cannot be implemented with the TerraSAR-X satellite, since this satellite only allows a double increase of the pulse repetition frequency PRF at most. This entails a bisection of the strip width, which can be accepted, since the additional applications facilitated by a group antenna result in a grEeat gain in quality.
Specifically, the example in F.ig.l involves all four impulses being sent out one by one by the group antenna, with all its four antenna segments activated. In the receiving phase of the backscattered first impulse, only the first antenna segment (far right) of the SAR group antenna is activated, in the receiving phase of the backscattered second impulse, only the second antenna segment is activated (second one from the right), in the receiving phase of the backscattered third impulse, only the third antenna segment is activated (second one from the left), and in the receiving phase of the backscattered fourth impulse, only the fourth antenna segment is activated (far left), with the assumed moving direction of the carrier platform being from left to right.
In a basic planar group antenna application without transmit/receive modules, antenna segments of the group antenna are activated or deactivated in order to provide the different phase centers of thE: channels. With passive group antennae using a summing unit for the different antenna segments, this can be done by closing or opening a microwave switch. With a so-called phased array antenna, that is, an electronically phase-controlled group antenna, the amplification is simply set to zero in the receive path to deactivate the transmit/receive modules.
In the schematic representations of the examples given below and described based on Fig.2 to Fig.8, the grey antenna segments of the group antenna surface are once again deactivated and the white antenna segments activated. It is expressly pointed out that in order to shift the antenna phase centers, alternatively or simultaneously corresponding parts of the transmit antenna can be deactivated.
Fig.2 shows a SAR along-track interferometer arrangement which is provided by switching the active antenna aperture segments with only one receiver. In this example, transmission occurs using the full group antenna area, that is, both antenna segments lined up in the longitudinal direction of the antenna. In the receive path, the left antenna segment of the group antenna is deactivated at the first impulse, and the right antenna segment of the group antenna is deactivated at the second, additional impulse. An antenna segment, in this context, corresponds to one half of the group antenna, with the separation line between the two antenna segments running at right angle to the longitudinal direction of the group antenna and also at right angle to the moving direction of the carrier platform.
Fig.3 shows the implementation of a SAR across-track interferometer arrangement, in which transmission is also carried out using the full group antenna area, that is, both antenna segments, here in parallel to each other, and in which the receiver is switched from impulse to impulse between the upper and lower antenna segment, where the respectively opposite antenna segment is deactivated. An antenna segment, in this context, corresponds to one half of the group antenna, with the sf_paration line between the two antenna segments running in the longitudinal direction of the group antenna and also in the moving direction of the carrier platform.
Aperture switching therefore facilitates very different applications with the same device ("hardware").
To balance out the drop in the performance of the synthetic aperture radar caused by this, the antenna construction can be expanded. However, since the transmit/receive modules of active, electronically phase-controlled group antennae are complex, expensive and heavy, according to an advantageous advanced design of the invention, that part of the antenna segment of the main radar antenna which has been allocated active transmit/receiver modules can be supplemented with purely passive antenna segments. These are inexpensive and can be manufactured with little weight. Depending on the application, the main antenna then has an extension of purely passive receiver antennae in the moving direction of the carrier platform and/or at right angle to the moving direction of the carrier platform.
For example, i.n Fig.4, two passive antenna segments have been added in the moving direction of the carrier platform, namely, one each on both sides of the active main antenna, which consists of two antenna segments lined up in the moving direction of the carrier platform. In this way, the overall length of the group antenna is doubled. Thus, when receiving in a 4-channel SAR system, one can work with antenna segments of half the main antenna length instead of a quarter of the main antenna length (see Fig.1).
Since radar satellites must have a compact design for the start in a carrier rocket, the additional passive antenna segments can be opened in orbit or extended from the satellite body. Anothe r advantage of the extended group antenna is the fact that it provides a greater baseline length for interferometric measurements and a longer SAR
group antenna in general.
A significant characteristic of the invention is the fact that the time-multiplex antenna segment switching technique can be combined with a multi-channel receiver in the form of a group antenna. This advantageous combination is used in the 5 example shown in Fig.5. In this example, which is compatible with TerraSAR-X, the pulse repetition frequency PRF is doubled (multiplex factor a = 2). Furthermore, two complete receivers are used, one of which is assigned to the left antenna half, while the other one is assigned to the right .
10 Each antenna half consists of two antenna segments. In this manner, a 4--channel system as shown in Fig.l can be reproduced.
The total number of the implemented channels is obtained by 15 multiplying the multiplex factor a and the number b of the actually physically present receive channels. The antenna half assigned to an actual receive channel - in the example of Fig.5, these are the first and the second channel - is impacted with a different amplitude allocation at the sub-commutated pulses, as described above. This occurs simultaneously in each receive channel.
In the example of Fig.5, the first and second impulses are sent out by the group antenna, with all four antenna segments activated. The first receive channel is assigned the two SAR
group antenna segments belonging to the left antenna half, while the second receive channel is assigned the two antenna segments belonging to the right antenna half. The two left antenna segments thus form a first antenna segment group and the two right antenna segments. form a second antenna segment group. To receive the first impulse, the antenna segments located on the left of both antenna segment groups are deactivated and the antenna segments located on the right are activated, whereas in order to receive the second impulse, the antenna segments located on the right of both antenna segment groups are deactivated and the antenna segments located on the left are activated.
For practical implementations, the combination of the time-multiplex antenna segment switching technique with a multi-channel receiver in the form of a group antenna is often of vital significance, since both the number of the receivers and the multiplex factor are usually strictly limited. For example, the TerraSAR-X satellite has two receive channels, as described in the contribution "Conceptual Studies for Exploiting t:he Terra:>AR-X Dual Receive Antenna" by J. Mittermayer and H. Runge, IGARSS 03, Toulouse/France, 07/21/03-07/25/03, IEEE 2003 International Geoscience and Remote Sensing Symposium, IEEE, 2003, and the pulse repetition frequency PRF can be increased at most by the factor two, as is the case in the so-called "Dual Polarization Mode."
By applying t:he techniques described above, a 4-channel group antenna may therefore be implemented, for example, with the TerraSAR-X satellite.
This opens up a variety of new application opportunities to similar satellites. Apart from the group antenna technique, two different SAR along-track interferometry baseline lengths may also be provided, as shown in Fig.6, which are useful for solving ambiguities in the evaluation of interferometric measurements.
In the example of Fig.6, bot=h the first and the second impulses are sent out by the group antenna, with all four antenna segments activated. The first receive channel is assigned the t:wo SAR group antenna segments belonging to the left antenna half, and the second receive channel is assigned the two antenna segments belonging to the right antenna half.
The two left antenna segments thus form a first antenna segment group and the two right antenna segments form a second antenna segment group. To receive the first impulse, the left antenna segment in the first (left) antenna segment group is deactivated and the right antenna segment is activated, while in the second (right) antenna segment group, the left antenna segment. is activated and the right antenna segment deactivated, thus resulting in a short SAR along-track interferometry baseline length, due to the closeness of the two activated antenna segrnents. In contrast, to receive the second impulse, the left antenna segment is activated in the first (left) antenna segment group and the right antenna segment is deactivated, while in the second (right) antenna segment group the left antenna segment is deactivated and the right antenna segment activated, thus resulting in a long SAR
along-track interferometry baseline length, due to the distance between the two activated antenna segments.
Furthermore, by applying the combination, created by the present invention, of the time-multiplex antenna segment switching technique with a multi-channel receiver in the form of a SAR group antenna, as shown in Fig.7, for example, by 1~
means of the TerraSAR-X satel7_ite, combined SAR along-and-across-track interferomet.ry is facilitated. This combination makes it possible, among other things, to determine the speed and height of a moving object simultaneously. Due to the short across-track baseline in this TerraSAR-X case, the altitude of flying objects can be determined without ambiguities from the interferometric phase. Ground clutter can be eliminated by forming the difference between the two along-track receive channel pairs.
It is possible to detect high-flying objects if a threshold limit is set in the signal processor for the across-track phase value. Pixels that consist of brightness ("pixel brightness" or "amplitude") and along-track and across-track phase and also have an across-track phase which exceeds a certain value, can thus be identified as high-flying planes.
The speed component at right angle to the moving direction of the carrier platform can be determined from the along-track phase. The fourth receive channel is used to eliminate the static radar ~~ignal by means of subtraction.
In the example of Fig.7, both the first and the second impulses are :>ent out by the SAR group antenna, with all four antenna segments activated. The first receive channel is assigned the two SAR group antenna segments belonging to the left antenna half, while the second receive channel is assigned the two antenna segments belonging to the right antenna half, with the two antenna segments assigned to the first or second channel being arranged beside each other not lengthwise to the antenna, but transverse to the longitudinal direction of the group antenna, that is, parallel to each 1 '9 other. The two left antenna segments form a first antenna segment group and the -two right antenna segments form a second antenna segment group. To receive the first impulse, the upper antenna segment in Fig.7 is deactivated in both antenna segment groups and the lower antenna segment in Fig.7 is activated, while in order to receive the second impulse, the upper antenna segment in Fig.7 is activated in both antenna segment groups and the lower antenna segment in Fig.7 is deactivated.
Fig.8 shows the implementation of a three-channel SAR group antenna by means of time-multiplexing, using the factor 2 and two complete receive channels. Both the first and the second impulse are sent out by the SAR group antenna, which is completely activated. To implement the middle phase center relevant for :receiving the second impulse, the portions from the receive channels of the front and back antenna half in the moving direction of the carrier platform must be added up. In the three-channel SAR arrangement, the active antenna area during receiving amounts to a third of the total group antenna area, as opposed to a fourth in the four-channel SAR
arrangement.
An additional and advantageous possibility to increase the number of channels, besides time-multiplexing and receive channels operated in parallel, is a modification of the so-called ScanSAR technique. The ;ScanSAR technique has been used to expand the strip width of a synthetic aperture radar system by impacting a partial strip only with a certain number of pulses ("bursts"). After each "burst", there is a switch to another partial strip.
The functional advanced embodiment of the invention referred to here suggests that this technique be used to obtain a higher number of channels without increasing the strip width.
5 For a limited time only, the strip is illuminated at ground level with a number of radar impulses ("bursts") and then switched, by means of aperture switching, to a different phase center of the group antenna.
10 Using the number of bursts within the illumination time of the target (synthetic aperture time), an additional multiplier is created for the number of channels that can be implemented.
15 Equivalent to the multiplex procedure described above, by using additional impulses sub-commutated in the standard pulse repetition frequency PRE' (Fig.l), a different part of the radar group antenna is activated with each burst. This procedure can easily be combined with both the multiplex 20 technique described above and with the multi-channel method, thus producing an even greater number of "virtual" channels.
A disadvantage of this burst. method is that a different spectral area of the objects to be imaged is recorded with each burst. If an interferogram is created, this leads to a decorrelation in so-called area diffusers. However, since it has been shown that pixel diffusers remain coherent even across a larger angle area, such objects can be used to perform interferometric measurements.
The above application of the advanced embodiment has great practical significance, since the factors a and b can only be increased within narrow limits in practice. As is generally known, if the pulse repetition frequency PRF of a Synthetic Aperture Radar System (SAR) is increased (in this case by the factor a), the obtainable image strip width is reduced.
Moreover, as previously described, it is not possible, for reasons of expense, to provide a satellite with any number of receive chains (here factor b) in the form of actually physically present hardware. However, as illustrated by the practical embodiment examples TerraSAR-X and Radarsat-2, the pulse repetit_Lon frequency PRF can be increased by the factor a = 2. On the other hand, two complete receive chains are provided each (factor b = 2), since generally all important components are provided in two (redundant) units in satellites.
Therefore, by using the technique described by the invention, four virtual receive channels can be reproduced for these satellites, for example, thereby facilitating many interesting applications which could not be implemented with only two receive channels.
Multiplexing a receiver in order to facilitate a multi-channel technique is a well-known procedure which is also used in radar technology. The US patent specification 5 966 092 mentions this possibility for a mono-pulse radar dealing with locating purposes, in particular directional reference of radar targets. It must be pointed out in this context, however, that the above invention involves basic switching (multiplexing) of radar channels which is not associated with the switching of antenna apertures (aperture switching), a5 is the case in the present invention. Instead, each Channel is assigned a separate antenna in the above invention.
Another examp7_e for receiver multiplexing is indicated in the German patent specification DE-1001 20 536 C2 in connection with an active obstacle warning radar system. This also involves a receiver being switched between various antenna elements (channels). In this invention, the antenna elements are approached one by one.
In both multiplexing systems known from US-5 966 092 and DE-1001 20 536 C2, the antenna elements are sequentially put through to the receiver.
In this present invention, however, the entire SAR group antenna remains connected to the receiver during multiplexing.
Using specific diagrams of the amplitude allocation of the SAR group antenna, the phase centers of the antenna can be shifted in a SAR system according to the invention, which is also exemplified in detail in several dependent claims.
During this process, Large areas of the antenna should preferably remain active, thus contributing to reception and therefore to a high antenna yield.
Certain switch patterns for t:he antenna segments, passive unit extensions to the active radar antenna, and the use of the so-called SAR burst-mode technique are to be considered as advantageous advanced embodiments of the mufti-channel synthetic aperture radar according to the invention, which is also stated in the dependent claims.
Thus, already existing SAR satellite systems can be operated as mufti-channel systems when using the present invention, and in future designs, a multitude of expensive, heavy and energy-consuming receivers can be dispensed with.
In an advantageous embodiment, the SAR group antenna is an electronically phase-controlled antenna with a multitude of transmit/receive modules. During time multiplexing, the amplification of the respective receive part and/or transmit part of the transmit/receive modules in the areas of the antenna segments which are to be deactivated for reception is switched to zero, so that the specific amplitude allocations can be activated and individual antenna segments can be activated and deactivated via the various antenna segments.
It is therefore important for the above-mentioned advanced embodiment of the present invention that each of the many transmit/receive modules (TR modules) can be individually controlled in its amplification (amplitude weighting of TR
modules). If the amplification is set to zero during transmitting or receiving, the element concerned is practically deactivated and does not contribute to the summation. If the transmit/receive modules at the edge of a planar antenna, e.g, in front and back, are deactivated, the phase center of the antenna can be shifted. So far, this technical trick has not been used in practice, since no application was known for it. In the present invention, it is used to assign different phase centers to a SAR group antenna. Since planar antennae in TR-module technology generally contain several hundred modules, a fine control of the phase center is possible.
The SAR group antenna may also be designed as a passive planar antenna, in which case microwave switches are planned for time multiplexing in the connections of the receiver to the antenna segments, so 'that the specific amplitude allocations can be activated and individual antenna segments can be activated and deactivated via the various antenna segments.
In practice, basic passive planar antennae do not offer as much flexibility as electronically phase-controlled group antennae for this purpose, since they mostly only allow for larger units, so-called "panels" or "leafs" to be activated and deactivated, if the necessary microwave switches are provided.
Advantageous and functional advanced embodiments are the subjects of claims referring to patent claim 1, either directly or indirectly.
The following is a detailed explanation of the invention based on design examples illustrated by drawings. It should be noted that all these designs, except for the design shown in Fig.l, can in principle be implemented with TerraSAR-X.
The illustrations show:
Fig.l shows the schematic design of a typical SAR planar group antenna, in which transmission is carried out as a whole and receiving involves four different aperture allocations and four steps, Fig.2 shows the schematic design of a SAR along-track interferometer implemented by multiplexing a receive channel and aperture-switching the group antenna, Fig.3 shows the schematic design of a SAR across-track interferometer implemented by multiplexing a receive channel and aperture-switching the group antenna, Fig.4 shows a schematic dE~sign involving an active SAR
satellite main antenna with a dual-receive antenna unit extension, Fig.5 shows the schematic design of a virtual 4-channel SAR with double-multiplexing, aperture-switching and a dual-channel receiver, 5 Fig.6 shows the schematic design of a SAR along-track interferometer capable of recording two interferometric data sets with different baseline lengths (multi-baseline interferometer), 10 Fig.7 shows the schematic design of a combined along- and across-track SAR interferometer, and Fig. 8 shows the schematic design of a three-channel SAR
system with double multiplexing, aperture switching and a dual-channel receiver.
The essential fact about a group antenna is that it allows scanning at different spatial positions. For this purpose, the phase centers of the individual channels are spatially separated; in principle, this <:an involve the transmit and/or receive antennae. With a typical group antenna, spatial scanning occurs simultaneously in the individual channels.
Fig.l shows a schematic representation of a typical SAR
planar group antenna suitable both for transmitting and receiving, wit=h its longitudinal axis extending in the moving direction of the carrier platform. In this example, the whole group antenna is used for transmitting. When receiving the radar pulses scattered back from the ground, different antenna segments of the group antenna aperture are deactivated, i.e., switched off, for each of the pulses sub-commutated in the standard PRF (pulse repetition frequency).
The deactivated antenna segments are marked grey in Fig.l and all other figures, while the activated antenna segments are marked white. Using this so-called "aperture switching" and time-multiplexing with the quadruple standard PRF, a 4-antenna-segment group antenna with four equidistant phase centers is reproduced. Only one receiver is used for this procedure. Fig.l illustrates which antenna segments must be deactivated during receiving in order to obtain the desired four phase centers.
The switch between the virtual channels must occur within one pulse repetition interval of the standard SAR system. This means that the pulse repetition frequency PRF of the SAR
system must be increased by the factor which corresponds to the number of receive channe:Ls. An increase of the pulse repetition frequency PRF can only be achieved, among other things, by limiting the image strip width. In fact, the scenario shown in Fig.l cannot be implemented with the TerraSAR-X satellite, since this satellite only allows a double increase of the pulse repetition frequency PRF at most. This entails a bisection of the strip width, which can be accepted, since the additional applications facilitated by a group antenna result in a grEeat gain in quality.
Specifically, the example in F.ig.l involves all four impulses being sent out one by one by the group antenna, with all its four antenna segments activated. In the receiving phase of the backscattered first impulse, only the first antenna segment (far right) of the SAR group antenna is activated, in the receiving phase of the backscattered second impulse, only the second antenna segment is activated (second one from the right), in the receiving phase of the backscattered third impulse, only the third antenna segment is activated (second one from the left), and in the receiving phase of the backscattered fourth impulse, only the fourth antenna segment is activated (far left), with the assumed moving direction of the carrier platform being from left to right.
In a basic planar group antenna application without transmit/receive modules, antenna segments of the group antenna are activated or deactivated in order to provide the different phase centers of thE: channels. With passive group antennae using a summing unit for the different antenna segments, this can be done by closing or opening a microwave switch. With a so-called phased array antenna, that is, an electronically phase-controlled group antenna, the amplification is simply set to zero in the receive path to deactivate the transmit/receive modules.
In the schematic representations of the examples given below and described based on Fig.2 to Fig.8, the grey antenna segments of the group antenna surface are once again deactivated and the white antenna segments activated. It is expressly pointed out that in order to shift the antenna phase centers, alternatively or simultaneously corresponding parts of the transmit antenna can be deactivated.
Fig.2 shows a SAR along-track interferometer arrangement which is provided by switching the active antenna aperture segments with only one receiver. In this example, transmission occurs using the full group antenna area, that is, both antenna segments lined up in the longitudinal direction of the antenna. In the receive path, the left antenna segment of the group antenna is deactivated at the first impulse, and the right antenna segment of the group antenna is deactivated at the second, additional impulse. An antenna segment, in this context, corresponds to one half of the group antenna, with the separation line between the two antenna segments running at right angle to the longitudinal direction of the group antenna and also at right angle to the moving direction of the carrier platform.
Fig.3 shows the implementation of a SAR across-track interferometer arrangement, in which transmission is also carried out using the full group antenna area, that is, both antenna segments, here in parallel to each other, and in which the receiver is switched from impulse to impulse between the upper and lower antenna segment, where the respectively opposite antenna segment is deactivated. An antenna segment, in this context, corresponds to one half of the group antenna, with the sf_paration line between the two antenna segments running in the longitudinal direction of the group antenna and also in the moving direction of the carrier platform.
Aperture switching therefore facilitates very different applications with the same device ("hardware").
To balance out the drop in the performance of the synthetic aperture radar caused by this, the antenna construction can be expanded. However, since the transmit/receive modules of active, electronically phase-controlled group antennae are complex, expensive and heavy, according to an advantageous advanced design of the invention, that part of the antenna segment of the main radar antenna which has been allocated active transmit/receiver modules can be supplemented with purely passive antenna segments. These are inexpensive and can be manufactured with little weight. Depending on the application, the main antenna then has an extension of purely passive receiver antennae in the moving direction of the carrier platform and/or at right angle to the moving direction of the carrier platform.
For example, i.n Fig.4, two passive antenna segments have been added in the moving direction of the carrier platform, namely, one each on both sides of the active main antenna, which consists of two antenna segments lined up in the moving direction of the carrier platform. In this way, the overall length of the group antenna is doubled. Thus, when receiving in a 4-channel SAR system, one can work with antenna segments of half the main antenna length instead of a quarter of the main antenna length (see Fig.1).
Since radar satellites must have a compact design for the start in a carrier rocket, the additional passive antenna segments can be opened in orbit or extended from the satellite body. Anothe r advantage of the extended group antenna is the fact that it provides a greater baseline length for interferometric measurements and a longer SAR
group antenna in general.
A significant characteristic of the invention is the fact that the time-multiplex antenna segment switching technique can be combined with a multi-channel receiver in the form of a group antenna. This advantageous combination is used in the 5 example shown in Fig.5. In this example, which is compatible with TerraSAR-X, the pulse repetition frequency PRF is doubled (multiplex factor a = 2). Furthermore, two complete receivers are used, one of which is assigned to the left antenna half, while the other one is assigned to the right .
10 Each antenna half consists of two antenna segments. In this manner, a 4--channel system as shown in Fig.l can be reproduced.
The total number of the implemented channels is obtained by 15 multiplying the multiplex factor a and the number b of the actually physically present receive channels. The antenna half assigned to an actual receive channel - in the example of Fig.5, these are the first and the second channel - is impacted with a different amplitude allocation at the sub-commutated pulses, as described above. This occurs simultaneously in each receive channel.
In the example of Fig.5, the first and second impulses are sent out by the group antenna, with all four antenna segments activated. The first receive channel is assigned the two SAR
group antenna segments belonging to the left antenna half, while the second receive channel is assigned the two antenna segments belonging to the right antenna half. The two left antenna segments thus form a first antenna segment group and the two right antenna segments. form a second antenna segment group. To receive the first impulse, the antenna segments located on the left of both antenna segment groups are deactivated and the antenna segments located on the right are activated, whereas in order to receive the second impulse, the antenna segments located on the right of both antenna segment groups are deactivated and the antenna segments located on the left are activated.
For practical implementations, the combination of the time-multiplex antenna segment switching technique with a multi-channel receiver in the form of a group antenna is often of vital significance, since both the number of the receivers and the multiplex factor are usually strictly limited. For example, the TerraSAR-X satellite has two receive channels, as described in the contribution "Conceptual Studies for Exploiting t:he Terra:>AR-X Dual Receive Antenna" by J. Mittermayer and H. Runge, IGARSS 03, Toulouse/France, 07/21/03-07/25/03, IEEE 2003 International Geoscience and Remote Sensing Symposium, IEEE, 2003, and the pulse repetition frequency PRF can be increased at most by the factor two, as is the case in the so-called "Dual Polarization Mode."
By applying t:he techniques described above, a 4-channel group antenna may therefore be implemented, for example, with the TerraSAR-X satellite.
This opens up a variety of new application opportunities to similar satellites. Apart from the group antenna technique, two different SAR along-track interferometry baseline lengths may also be provided, as shown in Fig.6, which are useful for solving ambiguities in the evaluation of interferometric measurements.
In the example of Fig.6, bot=h the first and the second impulses are sent out by the group antenna, with all four antenna segments activated. The first receive channel is assigned the t:wo SAR group antenna segments belonging to the left antenna half, and the second receive channel is assigned the two antenna segments belonging to the right antenna half.
The two left antenna segments thus form a first antenna segment group and the two right antenna segments form a second antenna segment group. To receive the first impulse, the left antenna segment in the first (left) antenna segment group is deactivated and the right antenna segment is activated, while in the second (right) antenna segment group, the left antenna segment. is activated and the right antenna segment deactivated, thus resulting in a short SAR along-track interferometry baseline length, due to the closeness of the two activated antenna segrnents. In contrast, to receive the second impulse, the left antenna segment is activated in the first (left) antenna segment group and the right antenna segment is deactivated, while in the second (right) antenna segment group the left antenna segment is deactivated and the right antenna segment activated, thus resulting in a long SAR
along-track interferometry baseline length, due to the distance between the two activated antenna segments.
Furthermore, by applying the combination, created by the present invention, of the time-multiplex antenna segment switching technique with a multi-channel receiver in the form of a SAR group antenna, as shown in Fig.7, for example, by 1~
means of the TerraSAR-X satel7_ite, combined SAR along-and-across-track interferomet.ry is facilitated. This combination makes it possible, among other things, to determine the speed and height of a moving object simultaneously. Due to the short across-track baseline in this TerraSAR-X case, the altitude of flying objects can be determined without ambiguities from the interferometric phase. Ground clutter can be eliminated by forming the difference between the two along-track receive channel pairs.
It is possible to detect high-flying objects if a threshold limit is set in the signal processor for the across-track phase value. Pixels that consist of brightness ("pixel brightness" or "amplitude") and along-track and across-track phase and also have an across-track phase which exceeds a certain value, can thus be identified as high-flying planes.
The speed component at right angle to the moving direction of the carrier platform can be determined from the along-track phase. The fourth receive channel is used to eliminate the static radar ~~ignal by means of subtraction.
In the example of Fig.7, both the first and the second impulses are :>ent out by the SAR group antenna, with all four antenna segments activated. The first receive channel is assigned the two SAR group antenna segments belonging to the left antenna half, while the second receive channel is assigned the two antenna segments belonging to the right antenna half, with the two antenna segments assigned to the first or second channel being arranged beside each other not lengthwise to the antenna, but transverse to the longitudinal direction of the group antenna, that is, parallel to each 1 '9 other. The two left antenna segments form a first antenna segment group and the -two right antenna segments form a second antenna segment group. To receive the first impulse, the upper antenna segment in Fig.7 is deactivated in both antenna segment groups and the lower antenna segment in Fig.7 is activated, while in order to receive the second impulse, the upper antenna segment in Fig.7 is activated in both antenna segment groups and the lower antenna segment in Fig.7 is deactivated.
Fig.8 shows the implementation of a three-channel SAR group antenna by means of time-multiplexing, using the factor 2 and two complete receive channels. Both the first and the second impulse are sent out by the SAR group antenna, which is completely activated. To implement the middle phase center relevant for :receiving the second impulse, the portions from the receive channels of the front and back antenna half in the moving direction of the carrier platform must be added up. In the three-channel SAR arrangement, the active antenna area during receiving amounts to a third of the total group antenna area, as opposed to a fourth in the four-channel SAR
arrangement.
An additional and advantageous possibility to increase the number of channels, besides time-multiplexing and receive channels operated in parallel, is a modification of the so-called ScanSAR technique. The ;ScanSAR technique has been used to expand the strip width of a synthetic aperture radar system by impacting a partial strip only with a certain number of pulses ("bursts"). After each "burst", there is a switch to another partial strip.
The functional advanced embodiment of the invention referred to here suggests that this technique be used to obtain a higher number of channels without increasing the strip width.
5 For a limited time only, the strip is illuminated at ground level with a number of radar impulses ("bursts") and then switched, by means of aperture switching, to a different phase center of the group antenna.
10 Using the number of bursts within the illumination time of the target (synthetic aperture time), an additional multiplier is created for the number of channels that can be implemented.
15 Equivalent to the multiplex procedure described above, by using additional impulses sub-commutated in the standard pulse repetition frequency PRE' (Fig.l), a different part of the radar group antenna is activated with each burst. This procedure can easily be combined with both the multiplex 20 technique described above and with the multi-channel method, thus producing an even greater number of "virtual" channels.
A disadvantage of this burst. method is that a different spectral area of the objects to be imaged is recorded with each burst. If an interferogram is created, this leads to a decorrelation in so-called area diffusers. However, since it has been shown that pixel diffusers remain coherent even across a larger angle area, such objects can be used to perform interferometric measurements.
Claims (16)
1. A synthetic aperture radar (SAR) system for the imaging of ground strips, comprising a plurality, namely a number a, of receiving channels, each of which is connected to a different antenna segment of an SAR array antenna comprising a number a of antenna segments and being mounted on a car-rier platform moving above the ground, wherein, for the reception of pulse echo signals of SAR pulses previously transmitted via the SAR array an-tenna, the antenna segments of the SAR array antenna comprise spatially separated phase centers and, thus, each receiving channel is assigned to a different phase center, and wherein the receiving channels connected to re-spectively one of the antenna segments are assigned to a common SAR re-ceiver and are operated in the manner of time-multiplex switching as time-multiplex channels in the SAR receiver, characterized in that, via the various antenna segments adapted to be indi-vidually switched on and off, special schemes of the amplitude allocation of the SAR array antenna can be switched on, thus allowing a shifting of phase centers of the SAR array antenna, wherein the pulse repetition frequency (PRF) of the SAR system which on the receiving side is operated in the time-multiplex manner, is increased by the factor a in comparison to an SAR system operated without time multiplexing, and that, in parallel thereto, a plurality, namely a number b, of SAR receivers are provided which are assigned to a respective different one of antenna segment groups of the SAR array antenna comprising a number of b antenna segment groups for receipt, said antenna segment groups respectively including a plurality of antenna segments.
2. The synthetic aperture radar (SAR) system according to claim 1, char-acterized in that, during the transmission phases for emitting the SAR
pulses via the various antenna segments of the SAR array antenna, differ-ent amplitude distributions can be switched on and, for this purpose, the individual antenna segments can be switched on and off.
pulses via the various antenna segments of the SAR array antenna, differ-ent amplitude distributions can be switched on and, for this purpose, the individual antenna segments can be switched on and off.
3. The synthetic aperture radar (SAR) system according to claim 2, char-acterized in that, during the transmission phases for emitting the SAR
pulses, all antenna segments of the SAR array antenna are switched on.
pulses, all antenna segments of the SAR array antenna are switched on.
4. The synthetic aperture radar (SAR) system according to any one of claims 1 to 3, characterized in that the SAR array antenna is a passive planar an-tenna and that, for time-multiplex switching, microwave switches are pro-vided in the connections from the receiver to the antenna segments, so that the provided amplitude distributions can be switched on via the various an-tenna segments, and individual antenna segments can be switched on and off in the process.
5. The synthetic aperture radar (SAR) system according to any one of claims 1 to 3, characterized in that the SAR array antenna is an electronically phase-controlled antenna comprising a plurality of transmission/reception modules and that, in time-multiplex switching, the amplification of the respective re-ceiving portion and/or transmitting portion of the transmission/reception modules is switched to zero in the ranges of the antenna segments for which reception and respectively transmission has to be deactivated so that the provided amplitude distributions can be switched on via the various an-tenna segments, and individual antenna segments can be switched on and off in the process.
6. The synthetic aperture radar (SAR) system according to any one of claims 1 to 5, characterized in that the SAR array antenna is controlled in such a manner by the time-multiplex switching that the ground strip is illuminated only for a defined time period by a number of SAR pulses (bursts) from an antenna segment and then, for the same defined time period and thus for an identical number of SAR pulses, the process is switched to another an-tenna segment.
7. The synthetic aperture radar (SAR) system according to any one of claims 1 to 6, characterized in that, in application for SAR-along-track interferome-try, antenna segments arranged serially in the moving direction of the car-rier platform are switched, i.e. are switched on and off, in the time-multiplex manner.
8. The synthetic aperture radar (SAR) system according to any one of claims 1 to 6, characterized in that, in application for SAR-across-track interferome-try, antenna segments arranged transversely to the moving direction of the carrier platform are switched, i.e. are switched on and off, in the time-multiplex manner.
9. The synthetic aperture radar (SAR) system according to any one of claims 1 to 6, characterized in that an SAR array antenna provided for a total of four receiving channels is configured in such a manner that it comprises two an-tenna segment arrays for reception, arranged serially in the moving direc-tion of the carrier platform and formed by respectively one antenna half and assigned to respectively one receiver, wherein said two antenna segment groups respectively include two antenna segments which are also arranged in the moving direction of the carrier platform and which by the time-multiplex switching are switched on and respectively off in such a manner that, of said total of four antenna segments of the whole SAR array antenna that are arranged serially in the moving direction of the carrier platform, the first and third antenna segments are, from received pulse to received pulse, alternately switched on and off, respectively, in opposition to the second and fourth antenna segments.
10. The synthetic aperture radar (SAR) system according to any one of claims 1 to 6, characterized in that an SAR array antenna provided for a total of three receiving channels is configured in such a manner that it comprises two antenna segment groups for reception, arranged serially in the moving direction of the carrier platform and formed by respectively one antenna half and assigned to respectively one receiver, wherein said two antenna segment groups together include three antenna segments which are also arranged in the moving direction of the carrier platform and which by the time-multiplex switching are switched on and respectively off in such a manner that, of said total of three antenna segments of the whole SAR ar-ray antenna that are arranged serially in the moving direction of the carrier platform, the two outer antenna segments are, from received pulse to re-ceived pulse, alternately switched on and off, respectively, in opposition to the central antenna segment and the received signals of the two antenna segment groups are added to each other while the central antenna segment is switched on for reception.
11. The synthetic aperture radar (SAR) system according to any one of claims 1 to 6, characterized in that the SAR array antenna forms a main antenna both for transmission and for reception of the SAR pulses and that, apart from said main antenna, additional passive SAR receiving antennae or aper-tures are provided which are adapted to be switched on and off for use as further antenna segments for generating special amplitude allocation.
12. The synthetic aperture radar (SAR) system according to any one of claims 1 to 6, characterized in that, in the moving direction of the carrier platform, two different SAR-along-track interferometry base line lengths are formed by an SAR array antenna provided for a total of four receiving channels which comprises two antenna segment groups for reception, arranged seri-ally in the moving direction of the carrier platform and formed by respec-tively one antenna half and assigned to respectively one receiver, wherein said two antenna segment groups respectively include two antenna seg-ments which are also arranged in the moving direction of the carrier plat-form and which by the time- multiplex switching are switched on and re-spectively off in such a manner that, for generating a short base line length, during a first received SAR pulse, of said total of four antenna segments of the whole SAR array antenna that are arranged serially in the moving direc-tion of the carrier platform, the second and third antenna segments are switched on and are switched in opposition to the first and fourth antenna segments and, for generating a long base line length, during a second re-ceived SAR pulse, the first and fourth antenna segments are switched on and the second and third antenna segments are switched off in opposition thereto.
13. The synthetic aperture radar (SAR) system according to any one of claims 1 to 6, characterized in that an SAR-along-track interferometry base line length and an SAR-across-track interferometry base line length are formed by an SAR array antenna provided for a total of four receiving channels which comprises two antenna segment groups for reception, arranged seri-ally in the moving direction of the carrier platform and formed by respec-tively one antenna half and assigned to respectively one receiver, wherein said two antenna segment groups respectively include two antenna seg-ments which are arranged transversely to the moving direction of the car-rier platform and which by the time-multiplex switching are switched on and respectively off in such a manner that, during a first received SAR pulse, of said total of four antenna segments of the whole SAR array antenna, the two antenna segments arranged on one side of each of said two antenna segment arrays are switched on and the two other antenna segments are switched off and, conversely, during a second received SAR pulse, the two antenna segments arranged on the other side of each of said two antenna segment groups are switched on and the two other antenna segments are switched off.
14. Use of the synthetic aperture radar (SAR) system according to any one of claims 1 to 13 in a multi-channel SAR array antenna on a satellite carrier platform.
15. Use of the synthetic aperture radar (SAR) system according to any one of claims 1 to 13 in a multi-channel SAR array antenna on a carrier platform formed by an aircraft or a drone.
16. Use of the synthetic aperture radar (SAR) system according to claim 14 or 15, for SAR interferometry.
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| US10871561B2 (en) | 2015-03-25 | 2020-12-22 | Urthecast Corp. | Apparatus and methods for synthetic aperture radar with digital beamforming |
| US10955546B2 (en) | 2015-11-25 | 2021-03-23 | Urthecast Corp. | Synthetic aperture radar imaging apparatus and methods |
| US11378682B2 (en) | 2017-05-23 | 2022-07-05 | Spacealpha Insights Corp. | Synthetic aperture radar imaging apparatus and methods for moving targets |
| US11506778B2 (en) | 2017-05-23 | 2022-11-22 | Spacealpha Insights Corp. | Synthetic aperture radar imaging apparatus and methods |
| US11525910B2 (en) | 2017-11-22 | 2022-12-13 | Spacealpha Insights Corp. | Synthetic aperture radar apparatus and methods |
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| CN105425237B (en) * | 2015-12-10 | 2018-01-09 | 上海机电工程研究所 | Multichannel SAR system channel equalization methods based on sample selection |
| CN107132537B (en) * | 2017-04-28 | 2019-10-22 | 北京空间飞行器总体设计部 | A kind of SAR satellite on-orbit performance method for improving based on electromechanical combination scanning |
| CN109521424B (en) * | 2018-12-21 | 2022-03-04 | 西安电子科技大学 | Satellite-borne sliding spotlight SAR satellite attitude and PRF sequence design method |
| CN109633648B (en) * | 2019-01-22 | 2022-04-29 | 北京航空航天大学 | Multi-baseline phase estimation device and method based on likelihood estimation |
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| US5175554A (en) * | 1980-12-29 | 1992-12-29 | Raytheon Company | All weather tactical strike system (AWTSS) and method of operation |
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| DE19543813A1 (en) * | 1995-11-24 | 1997-05-28 | Bosch Gmbh Robert | Radar system, in particular motor vehicle radar system |
| DE10120536C2 (en) * | 2001-04-26 | 2003-12-24 | Eads Deutschland Gmbh | Radar system for active obstacle warning and imaging of the earth's surface |
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- 2006-07-21 CA CA2553008A patent/CA2553008C/en active Active
- 2006-07-21 EP EP06015227A patent/EP1746437B1/en active Active
Cited By (7)
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| US10871561B2 (en) | 2015-03-25 | 2020-12-22 | Urthecast Corp. | Apparatus and methods for synthetic aperture radar with digital beamforming |
| US10615513B2 (en) | 2015-06-16 | 2020-04-07 | Urthecast Corp | Efficient planar phased array antenna assembly |
| US10955546B2 (en) | 2015-11-25 | 2021-03-23 | Urthecast Corp. | Synthetic aperture radar imaging apparatus and methods |
| US11754703B2 (en) | 2015-11-25 | 2023-09-12 | Spacealpha Insights Corp. | Synthetic aperture radar imaging apparatus and methods |
| US11378682B2 (en) | 2017-05-23 | 2022-07-05 | Spacealpha Insights Corp. | Synthetic aperture radar imaging apparatus and methods for moving targets |
| US11506778B2 (en) | 2017-05-23 | 2022-11-22 | Spacealpha Insights Corp. | Synthetic aperture radar imaging apparatus and methods |
| US11525910B2 (en) | 2017-11-22 | 2022-12-13 | Spacealpha Insights Corp. | Synthetic aperture radar apparatus and methods |
Also Published As
| Publication number | Publication date |
|---|---|
| CA2553008A1 (en) | 2007-01-23 |
| ATE407368T1 (en) | 2008-09-15 |
| DE502006001476D1 (en) | 2008-10-16 |
| EP1746437A1 (en) | 2007-01-24 |
| EP1746437B1 (en) | 2008-09-03 |
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