SUBSURFACE IMAGING RADAR
TECHNICAL FIELD
The invention relates to a subsurface imaging radar device comprising a transmitting
unit and a receiving unit, the transmitting unit being arranged to transmit a first radio
wave signal in a lobe towards a selected ground area at a selected elevation angle Q to
the ground area. The invention also refers to a method for such a subsurface imaging
radar device.
BACKGROUND ART
In the arid and open areas of many current conflicts burying objects is a commonplace
element of military tactics. The rationale is that this is often the single way of
concealing them, and very simple to do in e.g. sandy terrain. These objects can be
mines, concealed weapons or tunnels and bunkers. Correspondingly there is a strong
requirement for efficient means of detecting these types of buried objects.
The circumstances and purposes for buried object detection vary. Still surveillance
capacity linked to a high probability of detection is a general concern. For instance a
military transport en route along a road must possess a possibility to detect the mines
which may harm it when traveling at some reasonable speed. In contrast after a peace
treaty there is very strong requirement for efficient demining requiring all mines to be
found and deactivated. They may be spread over large areas, and not always in a
fashion which is well controlled. In this case there is no real time demand though the
surveillance task is often so large that surveillance capacity must be large. Searching
for concealed weapons, is often delimited to certain areas and may not have any
immediate real time requirement. However there may be a strong pressure to obtain
results within definite deadlines so surveillance capacity is a concern in this case too.
An emerging application area is the restoration of former military storage and training
areas to civilian land use. The areas can be severely polluted by unexploded
ordnance, and harmful waste. The location of waste deposits may have been forgotten
through the dramatic organizational changes in e.g. Eastern Europe.
When surveillance requirements are large the use of handheld mine detection devices
would be inefficient. Also self-moving detection devices depending on magnetostatic or
electrostatic effects (thus measuring the ground permeability or dielectricity constant)
have low surveillance capacity. The reason is that static fields decline at short ranges,
calling for careful and slow movements in the detection process. In contrast, radar is
based on electromagnetic radiation. Since range attenuation of electromagnetic
radiation is smaller than that of electrostatic fields, radar seems to be the principle to
be preferred for large coverage subsurface object detection.
Subsurface objects may be small, and their signatures very weak. Therefore a
detection device must sense only a small portion of the ground where the disturbance
of the ground due to the presence of an object will be relatively noticeable. A problem
with radar operating at larger surveillance ranges is therefore how to obtain sufficient
resolution, isolating small volumes of the ground. The principle of synthetic aperture
radar, SAR, is a well-known method to obtain high 2-dimensional resolution of the
ground surface.
A Synthetic Aperture Radar, SAR, is preferably used from air though ground based
systems are also feasible. An airborne SAR produces two-dimensional images
perpendicular to the aircraft path of flight. One dimension in the image is called range
(or cross track) and is a measure of the "line-of-sight" distance from the radar to the
target. Range measurement and resolution are achieved in synthetic aperture radar in
the same manner as most other radars: Range is determined by precisely measuring
the time from transmission of a pulse to receiving the echo from a target and, in the
simplest SAR, range resolution is determined by the transmitted signal bandwidth, i.e.
large bandwidth signals yield fine range resolution.
The other dimension is called azimuth (or along track) and is perpendicular to range
over the ground surface. It is the ability of SAR to produce fine azimuth resolution that
differentiates it from other radars. To obtain fine azimutti resolution, a physically large
antenna is needed to focus the transmitted and received energy into a sharp beam.
The sharpness of the beam defines the azimuth resolution. Similarly, optical systems,
such as telescopes, require large apertures (mirrors or lenses which are analogous to
the radar antenna) to obtain fine imaging resolution. Since SARs are much lower in
frequency than optical systems, even moderate SAR resolutions require an antenna
physically larger than can be practically carried by an airborne platform: antenna
lengths several hundred meters long are often required. However, airborne radar could
collect data while flying this distance and then process the data as if it came from a
physically long antenna. The distance the aircraft flies in synthesizing the antenna is
known as the synthetic aperture. A narrow synthetic beamwidth results from the
relatively long synthetic aperture, which yields finer resolution than is possible from a
smaller physical antenna.
While this section attempts to provide an intuitive understanding, SARs are not as
simple as described above. For even moderate azimuth resolutions, a target's range to
each location on the synthetic aperture changes along the synthetic aperture. In SAR
the energy reflected from the target must be "mathematically focused" to compensate
for the range dependence across the aperture prior to image formation. When the
aperture is large the SAR can give resolution near the radar wavelength. The focusing
is highly sensitive to geometry assumptions and objects will vanish in the SAR image
unless these assumptions are made correctly.
However, the previously known radar or SAR systems cannot be, or have limited
detection capability, when used for underground detection since the electromagnetic
energy cannot penetrate the ground sufficiently, but is reflected over the surface.
EP1965223A1 describes the use of diffraction limited SAR giving large integration
angle and a short depth of field which gives that energy from underground targets is
focused independently at different depths to enable 3D imaging.
US2007/0024489A1 discloses signal processing methods and systems for ground
penetrating radar from elevated platforms to obtain subsurface images. The
depression angle, frequency, and polarization can all be adjusted for the soil conditions
at hand. In particular, the depression angle is set at the "pseudo-Brewster angle" for
improved ground penetration.
The object of the present invention is to provide an improved radar that can be used
for improved underground imaging.
SUMMARY OF THE INVENTION
The present invention relates to a Synthetic Aperture Radar for detection of targets
below ground.
The core of the invention is a linear combination of two SAR images obtained
simultaneously. One SAR image being obtained with horizontally polarised radio
waves, the other with vertically polarised radio waves. The inventor has realised that
both polarizations respond with very different intensity to surface detail, whereas their
underground responses occur with more similar intensities. The linear combination is
adjusted in a particular manner such that a difference between the two differently
obtained images practically cancels out surface clutter, but subsurface signals will not
cancel each other out. The precise method and algorithm(s) for linear combination
uses an adaptive "minimum energy" polarimetric difference SAR image, removing the
surface influence.
Detection method(s)
The present invention provides a method of detection underground objects based on
an inventive polarization change detection algorithm (PCD algorithm) that applies to
low frequency synthetic aperture radar (SAR) at frequencies below 500 MHz. The
wavelength at these frequencies is greater or equal to 0.6 m.
The radar backscattering occurring for bare ground at these wavelengths can be
related to the Fourier components of the ground elevation profile, by the theory of
Bragg scattering. For most types of bare ground and the wavelengths considered, the
elevation amplitudes of the individual Fourier components are only a fraction of the
wavelength. For this reason also the polarization effects become well-modeled by the
so-called "small perturbation model" (SPM), soundly established in the theory of
electromagnetic rough surface scattering. In the case of bare ground PCD may be
designed to deterministically rely on this SPM model or alternatively be designed as an
adaptive algorithm, statistically matching the data to the model. The latter approach
has the advantage of also incorporating deviations from the SPM. For instance, the
method may allow for the ground surface to be lightly vegetated, but only when using
the approach wherein data is statistically matched by an adaptive algorithm. The SPM
model is then no longer valid, and the situation difficult to model theoretically, but it
would still remain at least approximately true that backscattering amplitudes would
stand in a fixed ratio between vertical and horizontal polarization when going from pixel
to pixel, with the vertical amplitudes significantly stronger.
The PCD algorithm serves the purpose of eliminating the ground surface
backscattering in applications of subsurface target detection. As described, surface
backscattering is caused by the roughness of the ground surface. It competes and in
many cases overwhelms any response from subsurface objects. In fact, since surface
and subsurface responses add with a random phase difference, any underground
response may diminish the net ground response just as it may increase it. It follows
that thresholding the net response as a means for finding subsurface response is
deemed to be inefficient.
Coherent change detection - CCD - is a known process of cancelling surface
backscattering taking phase information into account. The cancellation is achieved by
subtracting the phase/amplitude information of one SAR image from that of another
over the same ground scene. CCD requires that the scene has been overflown twice
with target deployments changed in between but that other parameters (e.g. humidity
of the ground) has not changed. It cannot be an overly efficient cancelation method
since much of the multiplicative noise structure (speckle and side lobes - these effects
can be significant) will be independent between the overflights and will not cancel.
The PCD algorithm of the present invention relies on simultaneous or intertwined
horizontally and vertically polarized measurements during the same flight. It thus does
not have the efficiency limitations of CCD. On the other hand it implies a net reduction
of target response with 6 - 10 dB, which although a drawback can be compensated
(according to the radar equation) by shortening of surveillance ranges by 40% to 50%.
Thus, according to a first aspect there is provided a method of removing surface clutter
in SAR radar imaging of subsurface targets, the method comprising the following
steps:
- selecting a first frequency and an incidence angle ) for a radar signal such that the
ratio of surface backscattering to subsurface target backscattering is significantly larger
for vertical polarization than for horizontal;
- obtaining vertically and horizontally polarized SAR images based on the same SAR
path exploiting the selected first frequency and incidence angle for a vertically
polarized and a horizontally polarized radar signal;
- weighting and differencing the vertically and horizontally polarized SAR images so
that the surface backscattering completely cancels between the two images and only
the combination of the target backscattering components remains.
The method, wherein the first frequency of the radar signal and the incidence angle are
chosen such that the wavelength of the vertically polarized radar signal is greater or
equal than the surface roughness.
The method, wherein the incidence angle is chosen to be as low as possible but
without shadows arising.
The method wherein the incidence angle is chosen to be larger than zero (horizontal
incidence) and less than the Brewster angle.
The method, wherein the horizontal and vertically polarized radar signals are
generated by a horizontal and a vertical antenna that conduct registrations in a so
called ping-pong mode.
The method, wherein the first frequency is in the interval of 25 - 500 MHz.
The method, wherein the first frequency is in the interval of 130 - 360 MHz.
The method, wherein the transmitting and receiving components have been adapted to
work within a range of 25m to 5000 m.
The method, wherein the transmitting and receiving components have been adapted to
work within a range of 100m to 500m
The method, wherein an adaptive minimum energy method is used to weighting and
differencing the vertically and horizontally polarized SAR images so that the surface
backscattering nearly or completely cancels between the two images and only the
combination of the target backscattering components remains.
The method, wherein the method comprises the following steps:
- with the aid of an aircraft in flight (205);
- obtain (210) a HH complex SAR image F [x,y);
- obtain (215) a W complex SAR image Fv(x,y);
- select (305, 350), within the images, an area T that appear to be homogenous;
- find (310) a in such that
) - F r x,y d y = min
- form (320) a ground clutter suppressed SAR image AF(x,y)by forming the
expression
AF{x, y ) = FH x, y)- Fv
The method, wherein subsurface targets subsequently is detected by applying e.g.
CFAR thresholding, ICD or CCD methods on AF.
According to a second aspect there is provided a SAR system for providing SAR
images having removed surface clutter to improve detection of subsurface targets, the
system comprising
- a first transmitter;
- a first antenna;
- a first duplexer ;
- a first receiver;
for obtaining a horizontally polarized SAR image, and
- a second transmitter;
- a second antenna;
- a second duplexer ;
- a second receiver;
for obtaining a vertically polarized radar image, the system further comprises
- an incidence angle unit for providing incidence angle data to an
- analytical coefficient calculator, which is configured to calculate coefficients
H,g C V,g H,t V,t
J
to be used in the calculations of
- a linear combiner, the system further comprises
- a linear combiner control unit for controlling the linear combiner to linearly combine
the images from the first receiver and the second receiver to form a ground clutter
suppressed SAR image (AF(x, y)) by forming the difference between the horizontal
polarization SAR image and the product of ymin or c / c and the vertical polarization
SAR image, as selected by an operator and conveyed by the linear combiner control
unit , the system further comprises
- a gamma-min (ymin) finder unit, for finding and providing to the linear combiner a
ymin of an energy function, the system also has
- a selection unit for selecting a homogenous test area T as input to the gamma-min
finding unit.
The system, further comprising a target detector and a display unit for detecting and
visualizing detected targets to an operator.
The system, further comprising a ping-pong control unit connected to the transmitters
to make the transmitters send in ping-pong mode.
]
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and its specific embodiment will now be described in detail with the aid
of the following drawings of which
Figure 1 is a view of a helicopter with a low frequency synthetic aperture radar
equipped with antennas of different polarity
Figure 2a is a flowchart of a general method of detecting subsurface targets using SAR
Figure 2b is a flowchart of an analytical method of detecting subsurface targets using
SAR
Figure 3a is a flowchart of an adaptive method of detecting subsurface targets using
SAR
Figure 3b is a more detailed flowchart of the adaptive method of figure 3a
Figure 4a shows an example of complex valued response from target and ground
surface.
Figure 4b shows the responses of figure 4a re-weighted by selecting weighting
coefficients so as to cancel the ground surface response.
Figure 4c shows a diagram of intensity [dB] of radar signal vs., incidence angle using
SPM theory and Fresnel reflection coefficients to calculate target and surface response
as a function of incidence angle and to see the net attenuation of the target response
when the surface response is cancelled. It is observed that the attenuation is relatively
independent of incidence angle.
Figure 5 shows relations to radar cross section or reflectivities at different polarizations
and for surface and target scattering elements, measured at any point in intensity SAR
images.
Figure 6 shows, in a block diagram representation, a SAR system for obtaining SAR
images having removed surface clutter t o improve detection of subsurface
targets.
DETAILED DESCRIPTION
Definitions
The following terms will be used with the associated meanings throughout this
document if not otherwise explicitly stated.
Surface roughness; surface roughness is a measure of roughness of a ground
surface; there are two well established criteria a surface roughness:
Rayleigh Criterion : if h < 8 cos , the surface is smooth
Fraunhofer Criterion : if h < 32 cos , then the surface is smooth
where h : standard deviation of surface roughness
A: wavelength
Q: incident angle
Incident angle; incident angle is the angle between longitudinal direction of
incident radar signal and the average normal direction to the ground surface
Ping-pong mode; a radar system having a first and a second combined
transmitting and receiving antenna and accompanying transmitters and
receivers, can be made to operate in ping-pong mode, i.e., the transmitter of the
second antenna does not send until the receiver of the first antenna has received
an echo from a signal transmitted by the transmitter of the first antenna, and
vice versa.
General
A low frequency SAR radar is arranged to provide a horizontally polarized channel as
well as a vertically polarized channel. The channels are arranged to be in line with
respect to the direction of flight in order to each provide a SAR image pixel by pixel
completely coincident except for the difference in polarization, i.e., if the same
polarization had been used, there had been two entirely identical images.
Open land often has a roughness in which average height differences over a distance
of one to a few meters is only a fraction of that distance. Radar wavelength of low
frequency radar is of the order of one to a few meters. It is known, and a consequence
of Maxwell's equations, that when roughness in this manner is small compared to the
wavelength of the radar signal, backscattering of the radar signal with vertical
polarization, is much stronger than backscattering of a horizontal signal. There is a
relationship between backscatter at vertical and horizontal polarization at these
particular conditions, which substantially depends on the incident angle, and only
weakly depends on the dielectric constant, and do not depend on either the roughness
or wavelength. This fact implies that a radar image of the ground surface will be very
nearly identical for the horizontal and vertical polarization, with the only but significant
difference that the vertically polarized image has much higher intensity.
If there are radar targets below the surface, these will also be found in the two SAR
images. Radar strength of underground targets will vary between the channels but not
according to the same mathematical laws as the surface reflexes. For underground
targets the conditions are guided by Fresnel reflection coefficients, which entails that
the vertical polarization provides a greater intensity. The intensity difference is however
less for underground targets than for backscattering from the surface.
Because environmental conditions at ground surface and below facilitates it in the
above taught manner, backscattering from the ground surface can be eliminated in the
SAR image by seeking a linear combination of the differently polarized images. This
involves to arrange to assign backscatter from the surface the same amplitude but
opposite sign as the backscatter being differently polarized. Backscattering from the
underground object will thereby be reduced, but only to a level that can be accepted.
Compensation for this reduction is achieved by employing the radar system at a
shorter distance, with the crucial advantage that competing surface clutter thereby to a
great deal have been eliminated.
Because surface clutter in many cases is the main reason why subsurface targets
cannot be distinguished, the method disclosed in the present application should be of
great importance in applications intended to identify subsurface targets.
System overview
The PCD algorithm requires a horizontal polarization transmitted and received (HH)
and a vertical polarization transmitted and received (W) SAR image of the ground
which from every aspect of data collection are as similar as possible. Thus:
> Antennas may have a common phase center or a phase centre displaced
along the flight axis. In the latter case phase centers should be adjusted
to a common phase center with the separation between the two taken
into account in SAR processing motion compensation;
> A realization may either be based on a common radar transceiver
toggling between the H- and V-polarization antennas or one channel for
either and operating in parallel. The latter case has however the
drawback of picking up any unwanted cross polarization response.
Figure 1 shows a helicopter 100 fitted with a low frequency SAR system equipped with
H-polarization and V- polarization antennas 110, 115, 120, 125 for intertwined HH and
W SAR images.
Figure 2a shows a flowchart of a general method of detecting subsurface targets using
SAR. During flight 205 horizontal polarization SAR image 2 0 and vertical polarization
SAR image 215 are obtained. The two images are linearly combined 220 by the use of
certain method (s) to remove ground response and thereby accentuate
underground/subsurface response. Subsequently subsurface targets may be detected
manually from display picture or detected 230 with the aid of applying e.g. CFAR
thresholding, ICD or CCD methods.
Mathematic formulas
This section discloses polarimetry formulas for surface and target backscattering
modification to semi-transparent surface
According to the small perturbation model (SPM) in the theory of electromagnetic
rough surface scattering, the complex valued (including phase) HH and W SAR
images has the following structure (below index of refraction n may be assumed real -
imaginary part affects very little for relevant soils, incidence angle c )
wherein
F (x,y) is the horizontal polarization SAR image
F (x,y) is the vertical polarization SAR image
· is a horizontal polarization specific SPM rough surface backscattering coefficient
v' is a vertical polarization specific SPM rough surface backscattering coefficient
f g x ) js SAR image contribution from rough surface
f '1is a horizontal polarization specific 2-way amplitude transmission loss equals
Polarization specific 1-way power transmission loss
·' is a vertical polarization specific 2-way amplitude transmission loss equals
Polarization specific 1-way power transmission loss
t x y is SAR image contribution from subsurface targets
Further, polarization specific SPM rough surface backscattering coefficients have the
following structure:
More, further polarization specific 2-way amplitude transmission loss equals
polarization specific 1-way power transmission loss
wherein
n is index of refraction (may be assumed real)
is incidence angle
and expressions squared in the two equations above is Fresnel reflection coefficient.
In this context it could be noted that "dense" equals n=5.5 and "light" equals n=3, which
summarizes variability of most dry soils, at frequencies about 100 MHz.
Polarimetric Change Detection Principle
The present invention provides a method for creating a so called polarimetric change
image. Such a polarimetric change image is obtained in two main steps. The steps
efficiently remove the ground response but keeps the subsurface response. Figure 2b
shows a flowchart of such a method of detecting subsurface targets using SAR. It may
be called the "analytical" method.
The main steps, in addition to obtaining horizontally and vertically polarized images,
and forming 255, 260 coefficients as described above, are:
1. Multiplying 265 the second equation with quotient.... .
F H ( y)= H ,gfg ) + ,tft ( y)
Fv (x y)= v,gfg( y)+ cv,tft ( y)
2 . Subtracting 265 the two equations from each other forming a polarimetric
change image AF(x,y) also called a ground clutter suppressed SAR image.
These steps result in a desired cancellation of ground response and also in a change
in subsurface target response.
Interpretation of PCD with respect to radar cross section
Relations to radar cross section or reflectivities at different polarizations and for
surface and target scattering elements, measured at any point in intensity SAR images
will be explained in the following.
PCD target attenuation understood
The independent ratio of H- and V- polarization responses from ground surface and
target can be used to suppress the latter at the price of a certain attenuation affecting
the target response.
Figure 4a shows an example of complex valued response from target and ground
surface. Incidence is from above left, and dotted arrow in first quadrant and unbroken
line arrow in third quadrant represents target polarization difference due to diffuse
surface reflection. Unbroken line arrow in second quadrant and dotted line arrow in
fourth quadrant represents ground polarization difference due to refraction loss caused
by Fresnel specular reflection coefficients.
Figure 4b shows the responses of figure 4a re-weighted by selecting weighting
coefficients so as to cancel the ground surface response.
Figure 4c shows a diagram of intensity [dB] of radar signal vs. incidence angle using
SPM theory and Fresnel reflection coefficients to calculate target and surface response
as a function of incidence angle and to see the net attenuation of the target response
when the surface response is cancelled. It is observed that the attenuation is relatively
independent of incidence angle, as also can be seen from the equation below.
Figure 5 shows relations to radar cross section or reflectivities at different polarizations
and for surface and target scattering elements, measured at any point in intensity SAR
images.
Adaptive PCD algorithm
The present application discloses two basic methods for detecting subsurface targets
- one deterministic/analytic method, as described above, relying on the fact that
ground index of refraction is known and
- one adaptive method, not requiring such knowledge but assuming index of refraction
being the same over an area.
Whilst attenuation of the subsurface target response is almost independent of index of
refraction, the surface V to H ratio depends significantly on the index of refraction, see
figure 4a; this strongly favors the adaptive method.
Figure 3a and 3b shows a flowchart of the adaptive method. The method relies on that
subsurface targets are sporadic and that they will not energy-wise affect the SAR
image. The method comprises the following steps:
- selecting 330, 335 suitable SAR frequency and incidence angle such that the ratio of
surface backscattering to subsurface target backscattering is significantly larger for
vertical polarization than for horizontal ;
- with the aid of an aircraft in flight (205);
- obtaining 210, 340 a HH complex SAR image FH
- obtaining 215, 345 a W complex SAR image F (x,y);
- Selecting 305, 350 a homogenous test area T around a potential target location;
- Finding 310, 355 a ymin such that an energy function E formed as the integral over
the area t of the square of the difference between the horizontal polarization SAR
image and the product of yrnin and the vertical polarization SAR image, is minimized;
E = (x,y)- F (x,yfdxdy = min
- forming 320, 360 a ground clutter suppressed SAR image AF(x, y) by forming the
difference between the horizontal polarization SAR image and the product of ymin and
the vertical polarization SAR image;
AF{x, y ) = FH x, y)- Fv {x, y )
Subsequently subsurface targets may be detected 325 by applying e.g. CFAR
thresholding, ICD or CCD methods on AF.
System
Figure 6 shows a block diagram of a system for detection of subsurface targets using
one of or both methods as described above. In the following first transmitter, first
antenna, first duplexer and first receiver are for horizontal polarization signals, while
second transmitter, second antenna, second duplexer and second receiver are for
vertical polarization signals.
The system comprises first chain for obtaining a horizontally polarized radar image,
i.e., a first transmitter 615, a first antenna 605, a first duplexer 610 and a first receiver
620. Further it comprises a second chain for obtaining a vertically polarized radar
image, i.e., a second transmitter 635, second antenna 625, second duplexer 630 and
second receiver 640.
The system further comprises an incidence angle selection unit 650 for provid
incidence angle t o a analytical coefficient calculator 655, which calculates
coefficients
H,g V,g C H,t C V,t
as explained above.
Further the system comprises a linear combiner control unit 660 for controlling a linear
combiner 665 to linearly combine the images from the first receiver 620 and the
second receiver 640 to form a ground clutter suppressed SAR image AF(x, y) by
forming the difference between the horizontal polarization SAR image and the product
of ymin or cH,,/ cv,t and the vertical polarization SAR image, as selected by an operator
and conveyed by the linear combiner control unit 660.
The system also comprises a gamma-min ymin finder unit 670, for finding and
providing to the linear combiner, a ymin according to what has been explained for
minimizing the energy function E according to the adaptive method as explained
above. The system also has a selection unit 675 for selecting a homogenous test area
T as input to gamma-min finding unit 670.
The system may further be provided with a target detector 680 and a display unit 685
for detecting and visualizing detected targets to an operator.

CLAIMS
1. A method of removing surface clutter in SAR radar imaging of subsurface targets,
comprising:
- selecting a first frequency and an incidence angle ( ) for a radar signal such
that the ratio of surface backscattering to subsurface target backscattering is
significantly larger for vertical polarization than for horizontal;
- obtaining vertically and horizontally polarized SAR images based on the same
SAR path exploiting the selected first frequency and incidence angle for a
vertically polarized and a horizontally polarized radar signal;
- weighting and differencing the vertically and horizontally polarized SAR images
so that the surface backscattering completely cancels between the two images
and only the combination of the target backscattering components remains.
2. The method according to claim 1 wherein the first frequency of the radar signal
and the incidence angle are chosen such that the wavelength of the vertically
polarized radar signal is greater or equal than the surface roughness.
3. The method according to claim 1 or claim 2 wherein the incidence angle is
chosen to be as low as possible but without shadows arising.
4. The method according to anyone of claims 1 to 3, wherein the incidence angle is
chosen to be larger than zero (horizontal incidence) and less than the Brewster
angle.
5. The method according to anyone of claims 1 to 4 wherein the horizontal and
vertically polarized radar signals are generated by a horizontal and a vertical
antenna that conduct registrations in a so called ping-pong mode.
6. The method according to claim 1 or 2 wherein the first frequency is in the interval
of 25 - 500 MHz.
7. The method according to claim 6 wherein the first frequency is in the interval of
130 - 360 MHz.
8. The method according to claim 1 or 2 wherein the transmitting and receiving
components have been adapted to work within a range of 25m to 5000 m.
9. The method according to claim 8 wherein the transmitting and receiving
components have been adapted to work within a range of 00m to 500m
10. The method according to any of the preceding claims wherein an adaptive
minimum energy method is used to weighting and differencing the vertically and
horizontally polarized SAR images so that the surface backscattering nearly or
completely cancels between the two images and only the combination of the
target backscattering components remains.
. The method according to claim 10 wherein the method comprises the following
steps:
- with the aid of an aircraft in flight (205);
- obtain (210) a HH complex SAR image FH(x, y);
- obtain (215) a W complex SAR image Fv (x, y);
- select (305, 350), within the images, an area t that appear to be homogenous;
- find (310) a min such that
- form (320) a ground clutter suppressed SAR image AF(x, y) by forming the
expression
AF(x, y)= FH {x, y)- Fv x,y)
12. The method according to claim 11 wherein subsurface targets subsequently is
detected (325) by applying e.g. CFAR thresholding, ICD or CCD methods on Af.
13. A SAR system (600) for providing SAR images having removed surface clutter to
improve detection of subsurface targets, comprising
- a first transmitter (615),
- a first antenna (605),
- a first duplexer (610)
- a first receiver (620),
for obtaining a horizontally polarised SAR image, and
- a second transmitter (635),
- a second antenna (625),
- a second duplexer (630),
- a second receiver (640),
for obtaining a vertically polarized radar image, the system further comprises
- an incidence angle unit (650) for providing incidence angle data to an
- analytical coefficient calculator (655), which is configured to calculate
coefficients
to be used in the calculations of
- a linear combiner (665), the system further comprises
- a linear combiner control unit (660) for controlling the linear combiner (665) to
linearly combine the images from the first receiver (620) and the second receiver
(640) to form a ground clutter suppressed SAR image (AF(x, y)) by forming the
difference between the horizontal polarization SAR image and the product of ymin
or CH,g / Cv, g and the vertical polarization SAR image, as selected by an operator
and conveyed by the linear combiner control unit (660), the system further
comprises
- a gamma-min (ymin) finder unit (670), for finding and providing to the linear
combiner a ymin of an energy function, the system also has
- a selection unit (675) for selecting a homogenous test area T as input to the
gamma-min finding unit (670).
14. The system according to claim 13 further comprising a target detector (680) and
a display unit (685) for detecting and visualizing detected targets to an operator.
15. The system according to claim 14 or 15 further comprising a ping-pong control
unit connected to the transmitters (615, 635) to make the transmitters (615, 635)
send in ping-pong mode.