Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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
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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
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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
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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.
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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
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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,
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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.
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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
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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
12
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(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
21
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
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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
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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
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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.
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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.
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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
