Note: Descriptions are shown in the official language in which they were submitted.
CA 02553008 2006-07-21
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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
CA 02553008 2006-07-21
2
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.
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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
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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
CA 02553008 2006-07-21
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
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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.
CA 02553008 2006-07-21
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.
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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.
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9
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,
CA 02553008 2006-07-21
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
CA 02553008 2006-07-21
11
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
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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,
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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
CA 02553008 2006-07-21
14
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.
CA 02553008 2006-07-21
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
CA 02553008 2006-07-21
16
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
CA 02553008 2006-07-21
17
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
CA 02553008 2006-07-21
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
CA 02553008 2006-07-21
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.
CA 02553008 2006-07-21
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.
