CA2831043C

CA2831043C - Interferometric sar system

Info

Publication number: CA2831043C
Application number: CA2831043A
Authority: CA
Inventors: Francisco Lopez-Dekker; Marwan YOUNIS; Gerhard Krieger; Christoph Schaefer
Original assignee: Airbus Defence and Space GmbH
Current assignee: Airbus Defence and Space GmbH
Priority date: 2012-10-22
Filing date: 2013-10-22
Publication date: 2019-07-09
Anticipated expiration: 2033-10-22
Also published as: DE102012219229A1; EP2722686B1; EP2722686A1; CA2831043A1

Abstract

The invention describes a SAR system designed to transmit two or more pulses (P1, P2) temporally offset within a pulse repetition interval (PRI) in such a way that the pulses (P1, P2) are radiated from phase centers arranged at varying positions transverse to the flight direction, and the pulses (P1, P2) illuminate an identical ground strip (24). The SAR system according to the invention is further designed to separate the received echoes of the pulses (P1, P2) from each other via digital beam shaping.

Description

, INTERFEROMETRIC SAR SYSTEM
Specification The invention relates to a side-scanning SAR system, i.e., a radar system with a synthetic aperture. In particular, the invention relates to a satellite-supported side-scanning SAR system. The invention further relates to a method for operating such a SAR system.
SAR systems enable the remote sensing of the earth's surface to detect radar pulses reflected on the earth's surface, which are emitted by the SAR system moving on a platform over the earth's surface at a constant speed. Such a system applies the knowledge that, as a result of the moved platform, the same regions of the earth are acquired in various positions, thereby making it possible to obtain amplitude phase information, and ultimately a radar image of the earth's surface.
In a SAR system, the achievable resolution and strip width, i.e., the width of the strip detectable by the radar system on the earth's surface in a direction perpendicular to the flight direction of the platform, oppose each other as competing parameters.
Known for increasing resolution at a given strip width of SAR systems is the use of multi-aperture systems, in which radar echoes are acquired simultaneously by several receivers.
Also known is to interferometrically operate a satellite-supported, imaging radar system with a synthetic aperture (synthetic Aperture Radar: SAR). During interferometric operation of the SAR system, two or more transmitting and/or receiving phase centers are used. In a so-called fully interferometric SAR
system, an electromagnetic wave is successively radiated through each of the transmitting phase centers, and the reflected echo is simultaneously received with all phase centers. In a so-called monostatic system, an antenna is used for simultaneously transmitting and = = =

2 receiving. In a so-called basic interferometric system, a single, fixed phase center is used for transmitting.
The quality or performance of an imaging radar system is characterized with a so-.. called weighting factor. The quotient of strip width and azimuth resolution is at its maximum for an optimal interferometric system. This quotient represents the weighting factor. The strip width is understood as a strip of land currently being scanned and acquired by a radar pulse through the movement of the actual antenna.
The azimuth angle characterizes the angle between the flight direction and range direction.
In comparison to a basic interferometric system, a fully interferometric radar system requires either that the pulse repetition frequency of emitted electromagnetic waves (pulses) be doubled, or that the antenna length be doubled. This either reduces the strip width or worsens the resolution. In other words, the fully interferometric operation of a SAR system cuts the rating number roughly in half. Since an attempt is made in SAR systems to minimize the antenna surface, interferometry results in a limitation that even digital beam shaping techniques have thus far been unable to eliminate.
Known from EP 1 241 487 Al is a SAR system that uses digital beam shaping techniques. This side-scanning SAR system encompasses a transmitting aperture and a receiving aperture of varying size, which is separate from the transmitting aperture and divided into several receiving sub-apertures. Each sub-aperture covers the area illuminated by the transmitting aperture. The signal from each sub-aperture is here received in a separate channel. Each channel delivers a separate input signal for subsequent digital signal processing. This method is also known by the name SCORE (Scan-on-Receive), and described in [1], for example. This SAR system known from prior art also has the disadvantage of a diminished weighting factor.
The object of the present invention is to indicate a SAR system that has been structurally and/or functionally improved in such a way that enables a higher

3 geometric azimuth resolution at a given strip width, or a larger strip width at a given azimuth resolution. Another object of the invention is to indicate a method for operating a SAR system.
These objects are achieved in accordance with a first aspect of the present invention, with a SAR system designed to transmit two or more pulses (P1, P2) temporally offset within a pulse repetition interval (PRI) in such a way that the two or more pulses (P1, P2) are radiated from phase centers arranged at varying positions transverse to a flight direction, and the two or more pulses (P1, P2) illuminate an identical ground strip (24), to separate received echoes of the two or more pulses (PI, P2) from each other via digital beam shaping, and which encompasses at least two transmitting antennas (11, 12) and at least one receiving antenna (13, 14), characterized in that a second pulse (P2) following a first pulse (P1) intersects with the first pulse (P1), wherein the received echoes are separated from each other by pulse modulation, and a Scan-on-Receive method is used for separating the received echoes of the two or more pulses (P1, P2).
These objects are further achieved, in accordance with a second aspect of the present invention, with a method for operating a SAR system, in which two or more pulses (PI, P2) are transmitted temporally offset within a pulse repetition interval (PRI) in such a way that the two or more pulses (P1, P2) are radiated from phase centers arranged at varying positions transverse to a flight direction, and the two or more pulses (P1, P2) illuminate an identical ground strip (24), and received echoes of the pulses (PI, P2) are separated from each other via digital beam shaping characterized in that 3a a second pulse (P2) following a first pulse (P1) is transmitted to intersect with the first pulse (P1), wherein the received echoes are separated from each other by a Scan-on-Receive- method.
A SAR system according to the invention is designed to transmit two or more pulses temporally offset within a pulse repetition interval in such a way that the pulses are radiated from phase centers arranged at varying positions transverse to the flight direction, and the pulses illuminate an identical ground strip. The received echoes of the pulses are separated from each other via digital beam shaping.
In a method according to the invention for operating a SAR system, two or more pulses are transmitted temporally offset within a pulse repetition interval, wherein the pulses are radiated from phase centers arranged at varying positions transverse to the flight direction, and the pulses illuminate an identical ground strip. The received pulse echoes are separated from each other via digital beam shaping.
If two or more pulses are radiated temporally offset within a pulse repetition interval from varying phase centers, and the received echoes are separated from each other through the use of digital beam shaping techniques, a SAR system can be operated in a fully interferometric mode, without having to tolerate any reduction in geometric azimuth resolution or strip width. This makes it possible to maximize the weighting factor. The weighting factor can be increased by a factor of 2 by comparison to known SAR systems.
It is best that a temporal offset between two consecutively transmitted pulses be significantly smaller than the pulse repetition interval. A second pulse following a first pulse can here not intersect with the first pulse. Alternatively, a second pulse following the first pulse can intersect with the first pulse. In this variant, the radar echoes of the two pulses are separated from each other via suitable pulse modulation.

= .

4 In a preferred embodiment, the SAR system according to the invention encompasses at least two transmitting antennas and at least one receiving antenna. Using at least two transmitting antennas enables the utilization of interferometry, wherein the number of phase centers when transmitting the pulses depends on the number of transmitting antennas and receiving antennas. An increase in the number of antennas leads to an improved weighting factor and/or can be used for interferometry.
If the SAR system encompasses more than one receiving antenna, the scannable strip width can be increased.
In another expedient embodiment, the number of pulses within the pulse repetition interval is equal at most to the number of transmitting antennas.
In order to arrange the phase centers transverse to a flight direction of the SAR
system, the at least two transmitting antennas can be arranged on one or more booms of a platform. By contrast, the receiving antenna(s) can be arranged on the platform itself. On the platform itself or directly thereupon means that the receiving antenna(s) are not or do not have to be arranged on any boom. One advantage to this approach is that the transmitting antennas are significantly smaller and lighter than the receiving antenna(s). This yields a simpler configuration for the SAR system, since the booms can exhibit a simpler layout.
For example, an arrangement transverse to the flight direction exists when the phase centers are arranged orthogonally relative to the flight direction of the SAR
system.
Slight deviations from an orthogonal alignment are of course also permissible.
It is also possible to arrange the at least two transmitting antennas and the receiving antenna(s) on varying platforms of a SAR system, so as to obtain the phase centers transverse to a flight direction of the SAR system. In this variant, the transmitting antennas can be arranged on the boom(s) of the varying platforms. In like manner, the transmitting antennas can be arranged directly on the varying platforms.
One transmitting and one receiving antenna can be arranged on one platform, and the at =
least one other transmitting antenna can be situated on another platform.
Likewise, each of the antennas can be arranged on a separate platform. In this case, the SAR
system comprises at least three platforms.

5 According to another embodiment, the receiving antenna is an antenna that encompasses an array of receiving units. Likewise, the transmitting antenna can be an antenna that encompasses an array of transmitting units. In particular, the antennas can be reflector antennas, planar antennas or reflector arrays. If the transmitting antenna encompasses an array of transmitting units, it is sufficient for the transmitting antennas to each encompass at least two transmitting units.
By contrast, the receiving antennas comprise a plurality of receiving units, so that a high anisotropy (directivity) can be realized by means of digital beam shaping.
According to another preferred embodiment, the SAR system is designed to use the Scan-on-Receive method for separating the echoes of the pulses. In this method, the echo of two or more pulses from a specific receiving unit of the at least one receiving antenna is received at different, consecutive points in time. As a consequence, each receiving unit (each receiving unit represents one channel) provides a separate signal in temporal succession for each of the received echoes for the subsequent digital signal processing. For example, the method described in [1] can be drawn upon for this purpose, which is implemented in temporal succession for each received echo.
The invention will be described in greater detail below based on an exemplary embodiment in the drawing. Shown on:
Fig. 1 is a schematic, top view of a SAR system according to the invention, Fig. 2 is a perspective view of a SAR system according to the invention, Fig. 3 is a schematic view depicting how two pulses emanating from varying phase centers are generated on the same ground strip, '

6 Fig. 4 is a diagram illustrating the transmission of two temporally offset pulses within a pulse repetition interval, Fig. 5 is a diagram illustrating the antenna amplification as a function of viewing angle, and Fig. 6 is a diagram illustrating the resulting ambiguities as a function of a signal ratio.
To provide a better understanding, the principle of a SAR measurement based upon the radar radiation transmitted by a satellite-supported SAR system will be explained. The SAR system (hereinafter also referred to as satellite) moves in one direction, which is designated as the azimuth direction. As it moves in the azimuth direction, the satellite, whose altitude over the earth's surface is known, continuously transmits radar pulses in the direction toward the earth's surface by way of a transmitting antenna. The radar echo of each transmitted radar pulse is determined by using a receiver to temporally scan the radar radiation reflected on the earth's surface in the so-called range direction, which extends perpendicular to the flight direction or azimuth direction of the satellite.
This results in a plurality of scans, wherein each scan corresponds to the radar echo of a specific radar pulse and a range position. The allocation of a scan to a radar pulse is here represented by an azimuth position, which is the geometric midpoint between the azimuth position of the transmitter when transmitting the radar pulse and the azimuth position of the receiver when receiving the radar echo of the radar pulse.
For example, the transmitter and receiver here comprise part of a combined transceiver antenna, which acts as the transmitter during transmission, and the receiver during reception. In this case, the SAR is a so-called single aperture system, in which the radar echoes are only detected by one single receiver, i.e., not simultaneously by several receivers.

= = '

7 In the evaluation process, only information from the radar radiation within a ground strip (English: swath) illuminated by the transmitted radar pulse is acquired, which can exhibit a width of several kilometers, e.g., several tens or hundreds of kilometers.
The principle of SAR measurement is now based on acquiring respective points on the earth's surface repeatedly from different viewing angles owing to the movement of the satellite. During acquisition of the radar echo, the known Doppler effect leads to a frequency shift that can be suitably evaluated, ultimately yielding amplitude and phase information, and hence a pixel of the earth's surface, for the points on the earth's surface where the radar pulses are reflected. Because the expert is familiar with correspondingly calculating the pixels of the earth's surface from the SAR data, this will not be explained in any further detail. The SAR method uses a plurality of radar pulses of a radar transmitter with a small aperture to simulate a larger synthetic aperture according to the expansion of the radar pulse on the earth's surface.
In such an SAR system, radar pulses are usually transmitted with a fixed pulse repetition frequency PRF (Pulse Repetition Frequency). The pulse repetition frequency depends on the aperture of the receiver, and the velocity with which the satellite moves in the azimuth direction. In order to acquire a broad strip on the earth's surface using a single aperture radar system, the pulse repetition frequency PRF must be kept as low as possible. However, this in turn has the disadvantage of steadily decreasing resolution in the azimuth direction.
Known from prior art for achieving a high resolution in the azimuth direction while at the same time keeping a large distance between the regions is to use so-called multi-aperture radar systems, in which the radar signal is simultaneously acquired by several receivers. However, this system results in a longer antenna in the azimuth direction.
If necessary, it is also possible to increase the pulse repetition frequency, with the danger here being that ambiguities might arise. For example, an ambiguity is present

8 when the radar echo of a radar pulse transmitted first had not yet been completely received before the SAR system transmitted another radar pulse.
The SAR system according to the invention described in greater detail below utilizes interferometry, a technique sufficiently known to the expert involving the evaluation of information from two or more transmitting and/or receiving phase centers.
Typically applied here is a shift in the flight direction, i.e., the azimuth direction, for measuring the velocity (English: along-track interferometry) or perpendicular to the flight direction for altitude measurement (English: across-track interferometry). In known SAR systems, either the strip width or resolution in an azimuth direction is here reduced during transition to fully interferometric operation. In a conventional, fully interferometric system, the two pulses within two PRI periods are radiated by the two transmitting phase centers. This is the so-called "ping pong mode".
Two PRI's are thus required, which reduces the scanning per channel, and hence decreases the resolution. Alternatively, the PRI's could be shortened so as to arrive at the actual resolution. In this case, the strip width is reduced due to timing.
The proposed SAR system solves this problem by having the SAR system transmit two or more pulses within a pulse repetition interval in such a way that the radiated electromagnetic waves are radiated by different phase centers. For example, this can be achieved by providing two spatially separated transmitting antennas. The transmitting antennas are actuated or arranged on a platform in such a way that all pulses transmitted within the pulse repetition interval illuminate the same ground strip. For example, the received echoes of the pulses transmitted temporally offset are separated from each other with methods in digital beam shaping by means of one or more receiving antennas each with several receiving units. This makes it possible to increase the rating factor defined at the outset by a factor of 2.
Fig. 1 and 2 show a schematic and perspective view of a SAR system according to the invention. The satellite-supported SAR system 1 encompasses a platform 10, upon which two booms 19, 20 are arranged at two opposite ends, lying on a common axis. A first transmitting antenna 11 (Txl) and a second transmitting antenna = I =

9 (Tx2) are arranged on the ends of the booms 19, 20. The platform 10 has arranged on it two receiving antennas 13, 14 (Rxl, Rx2), which are larger by comparison to the transmitting antennas 11, 12. The transmitting antennas 11, 12 and the receiving antennas 13, 14 are exemplarily designed as reflector antennas, which have allocated to them respective transmitter supply units 15, 16 as well as receiver supply units 17, 18. Two transmitter supply units 15, 16 are allocated to the transmitting antennas 11, 12 strictly by way of example, wherein the number of transmitter supply units 15, 16 could also be higher. By contrast, a plurality of receiver supply units 17, 18 are allocated to the respective receiving antennas 13, 14 to carry out a digital beam shaping process.
The booms 19, 20 extend orthogonally to the azimuth direction, i.e., the flight direction of the SAR system marked with reference number 22 on Fig. 2. The booms 19, 20 have a length of 10 m, for example. For example, the transmitting antennas 11, 12 extend 1.5 m in the azimuth direction, while extension in the direction of the booms 19, 20 can measure 0.7 m. By contrast, the receiving antennas 13, 14 extend 1.5 m in the azimuth direction, for example, while extension in the direction of the booms 19, 20 can measure 2.0 m.
Even though the exemplary embodiment shown on Fig. 1 and 2 comprises two receiving antennas 13, 14, a SAR system according to the invention could also comprise just a single receiving antenna 13.
For example, power is supplied to the SAR system by means of the solar cells schematically depicted on Fig. 2.
Such a SAR system operates in the Ka band (35 GHz), for example, and is configured for so-called single-pass cross-track-interftromeay. This requires that the earth's surface to be measured be radiated from two phase centers, which are separated by arranging the transmitting antennas on the booms in a direction orthogonal to the flight direction 22. The advantage to this embodiment is that the transmitting antennas 11, 12 are smaller and lighter by comparison to the receiving elements 13, 14, so that the booms 19, 20 can be built smaller and lighter in contrast to SAR systems, in which the receiving antennas are arranged on the booms. As explained, the two transmitting antennas are arranged at the ends of the booms 19, 20 due to the necessity of transmitting two pulses within the pulse repetition interval 5 while illuminating the same ground strip.
By using two receiving antennas 13, 14 instead of the single receiving antenna that is essentially only required, the ground strip can be enlarged by a factor of 2, making it possible to additionally increase the weighting factor.
As explained, the transmitting and receiving antennas 13, 14 are designed as reflector antennas, which each have allocated to them an array of supply elements 17, 18. The plurality of receiver supply elements is required for each receiving antenna on the receiving side to enable the use of digital beam shaping, for example the Scan-On-Receive (SCORE) method known to the expert. The SCORE method is here applied in an identical manner for each pulse transmitted within a pulse repetition interval and the respectively received echo. Because the transmission of pulses, and hence the reception of echoes, is temporally offset, the echoes received by the respective receiving units 17 or 18 can be allocated to one or the other pulse.
The operating procedure will be explained below based on Fig. 3 and 4. Fig. 3 presents a side view of the SAR system I described on Fig. 1 and 2 with illumination cones 25, 26 generated by the transmitting antennas 11, 12 through the emission of transmission pulses PI, P2. The illumination cones exhibit a shared illumination area .. 23 on the earth's surface, which corresponds to the ground strip 24. The flight direction 22 of the satellite runs perpendicularly into the sheet plane. The arrow marked A corresponds to the already mentioned range direction, which is also referred to as the distance direction.
Fig. 4 presents a diagram, from which it may be seen that two transmitting pulses (pulses) PI and P2 are transmitted within a pulse repetition interval PRI in temporal succession from transmitting units 11 (marked Tx 1 ) or 12 (marked Tx2). Even though the pulses P1, P2 on Fig. 4 exhibit no temporal overlap, it is also possible in principle for the pulses PI and P2 to overlap. The extent to which an overlap is possible depends on the temporal resolution of pulses following a pulse compression in the receiver.
The separation of pulse echoes by the receiving units is enabled by the high directivity of the receiving antennas based on the plurality of receiving units 17 or 18 in combination with digital beam shaping. The relevant approach in relation to a respective echo is in principle familiar to the expert, and can be seen from the EP 1 241 487 Al mentioned at the outset, as well as from publication [1], for example.
According to the invention, the procedures described therein, which are to be included in the contents of the specification by way of reference, make it possible to generate a narrow reception beam from each echo, which follows the current direction or arrival of the echo. The two or more reception beams formed in this way are here generated independently of each other.
Fig. 5 shows an example for a contemporary elevation directional diagram, presented as a standardized amplification AP (antenna pattern) over a look angle LA
(look angle). The solid line marked K1 is that of the transmitting antenna, the dashed line marked K2 shows the one-way reception directivity, while the line marked K3 represents the combined pattern. Also depicted are the pulse directions for a desired echo of one of the pulses within the pulse repetition interval PRI at the maximum of curve K3 (marked PT!) and for the interference pulse echo (marked PT2). In this case, the two-way amplification for the interference pulse PT2 measures more than a 20 dB attenuation in relation to the desired second pulse P2.
This implies that a so-called "range-ambiguity-to-signal" ratio RASR of better than minus 20 dB is achieved, which is the typical requirement for a SAR system.
The angular separation between the echoes of the two pulses Pl, P2 is proportional to their temporal delay. For this reason, the RASR level can be easily modulated by changing the distance between the pulses Pl, P2. Fig. 6 presents the resulting RASR
ratio as a function of the ground range (distance from satellite nadir) from the =

satellite to the illuminated region of the floor plate. Other performance metrics of the SAR system are not shown, since these are not influenced by the proposed approach.
For example, the described SAR system is used in interferometric, space-based aperture radar systems, in particular at higher carrier frequencies, as described in the sample application.

= =

Literature [1] M. Suess, M. Zubler, R. Zahn. "Performance Investigation on the High Resolution, Wide Swath SAR System", Proc. European Conference on Synthetic Aperture Radar EUSAR 2002, June 2002, pp. 187-190.

=

Reference List 1 SAR system Platform 5 11 First transmitting antenna (Tx 1) 12 Second transmitting antenna (Tx2) 13 First receiving antenna (Rxl) 14 Second receiving antenna (Rx2) First transmitter supply unit

10 16 Second transmitter supply unit 17 First receiver supply unit 18 Second receiver supply unit 19 Boom Boom 15 21 Power supply 22 Flight direction 23 Illumination area 24 Ground strip Illumination cone of the first transmitting antenna 20 26 Illumination cone of the second transmitting antenna PRI Pulse repetition interval PI Transmission pulse of the first transmitting antenna 11 P2 Transmission pulse of the second transmitting antenna 12 Time 25 K I Curve 1 K2 Curve 2 K3 Curve 3 PT1 Pulse direction for the echo of transmission pulse PI
PT2 Pulse direction for the echo of transmission pulse P2

Claims

1. A SAR system designed to transmit two or more pulses (P1, P2) temporally offset within a pulse repetition interval (PRI) in such a way that the two or more pulses (Pl.

P2) are radiated from phase centers arranged at varying positions transverse to a flight direction, and the two or more pulses (P1, P2) illuminate an identical ground strip (24), - to separate received echoes of the two or more pulses (P1, P2) from each other via digital beam shaping, and - which encompasses at least two transmitting antennas (11, 12) and at least one receiving antenna (13, 14), characterized in that - a second pulse (P2) following a first pulse (P1) intersects with the first pulse (P1), wherein the received echoes are separated from each other by pulse modulation, and - a Scan-on-Receive method is used for separating the received echoes of the two or more pulses (P1, P2).

2. The SAR system according to claim 1, characterized in that a temporal offset between two consecutively transmitted pulses (PI, P2) is significantly smaller than the pulse repetition interval (PRI).

3. The SAR system according to claim 1 or 2, characterized in that, of the two or more pulses (P1, P2), a second pulse (P2) following a first pulse (P1) does not intersect with the first pulse (P1).

4. The SAR system according to claim 1 or 2, characterized in that, of the two or more pulses (P1, P2), a second pulse (P2) following a first pulse (P1) intersects with the first pulse (P1).

5. The SAR system according to claim 1, characterized in that the number of the two or more pulses (P1, P2) within the pulse repetition interval (PRI) is equal at most to the number of transmitting antennas (11, 12).

6. The SAR system according to claim 1, characterized in that the at least two transmitting antennas (11, 12) are arranged on one or more booms of a platform.

7 The SAR system according to claim 1, characterized in that the at least one receiving antenna (13, 14) is arranged directly on a platform.

8. The SAR system according to claim 1, characterized in that the at least two transmitting antennas (11, 12) and the at least one receiving antenna (13, 14) are arranged on varying platforms.

9. The SAR system according to claim 1, characterized in that the at least one receiving antenna (13, 14) is an antenna that encompasses an array of receiving units.

10. A method for operating a SAR system, in which two or more pulses (P1, P2) are transmitted temporally offset within a pulse repetition interval (PRI) in such a way that the two or more pulses (P1 , P2) are radiated from phase centers arranged at varying positions transverse to a flight direction, and the two or more pulses (P1, P2) illuminate an identical ground strip (24), and received echoes of the pulses (P1, P2) are separated from each other via digital beam shaping characterized in that a second pulse (P2) following a first pulse (P1) is transmitted to intersect with the first pulse (P1), wherein the received echoes are separated from each other by a Scan-on-Receive- method.