Note: Descriptions are shown in the official language in which they were submitted.
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Title:
Interferometric Microwave Radar Method
The invention relates to an interferometric microwave radar
method,-particularly including a synthetic aperture (SAR),
wherein in phase difference analysis undertaken by coherent
demodulation of the echo signals of pulse radar
transmission signals received in two different positions,
any resulting phase ambiguities are resolved by phase
unwrapping in the form of analyzing two interferograms
based on different wavelengths, from which a so-called
delta k interferogram of substantially greater wavelength
is obtained, serving as a value for estimating the absolute
phase without further unwrapping.
In radar interferometry the phase is analyzed as measured
in a radar system with coherent demodulation of the
received signal. Since the distance resolution of a
microwave radar is much coarser than the wavelength,
although this phase furnishes highly accurate relative
distance sensing between neighboring pixels, it produces no
absolute values for pixel sensing. Apart from this, phase
sensing is available in principle only in the range -180
to +180 , and is, in other words, ambiguous.
Practically all applications of radar interferometry suffer
hitherto from this ambiguity, e.g. in generating digital
terrain models, mapping geological deformations or in
sensing vehicle velocities.
This ambiguity is resolved in the majority of the
applications by unwrapping the phase by analyzing the
gradients to the neighboring pixels.
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Most of the known phase unwrapping techniques attempt to
achieve a consistently correct solution solely from
ambiguously sensed values, basically making it impossible
to come to a physically unambiguous correct solution. The
reason for this is that various physical constellations can
produce the same sensed values, experience having shown
that such problematic constellations materialize not only
in theory but often also in nature.
The sole alternative is accordingly to take into account
external prior knowledge in the process of phase
unwrapping. This necessitates being previously aware of the
sensing parameter with the accuracy of the wavelength X, as
is, however, only very seldom the case.
One possibility of establishing at least a constant phase
offset of the full image reads from the paper by S.N.
Madsen: "On absolute phase determination techniques in SAR
interferometry", SPIE Conference on Radar Sensor
Technology, April 19-21, 1995, pages 393-401. To compute
the absolute distance between the radar sensor and a
backscatter object with wavelength accuracy, use is made of
the minimally different wavelengths within the radar
frequency spectrum. Referring now to FIG. 1 there is
illustrated how for this purpose an interferogram is formed
in each case from the upper and lower sideband, a so-called
delta k interferogram being obtained from these two
interferograms fl and f2. This delta k interferogram
corresponds to an interferogram having the substantially
greater wavelength 1(1/X1-1/X2) which can serve as a value
for estimating the absolute phase in broad ranges without
further phase unwrapping by scaling with the ratio of the
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carrier frequency fo to the distance of the frequency bands
B/2. For radar sensors as used nowadays this ratio is a
very large number between 1000 and 3000, this being the
reason why the error in the estimate is correspondingly
high. It is because of this that the relative unwrapped
phase is subtracted from the interferogram fa and the
difference is averaged over the full image, i.e. over
several million pixels.
One indication of being able to derive even the absolute
phase of single pixels by this known technique reads from
the paper by Engen, G., Guneriussen, T., Overein O.; "New
Approach for Snow Water Equivalent (SWE) estimation using
repeat pass interferometric SAR", IGARSS 2003.
Unfortunately, because of the small differences in
wavelength this known interferometric technique is highly
prone to error, is seldom mentioned in pertinent literature
and also finds hardly any application in actual practice.
Techniques for phase unwrapping as known hitherto in
interferometric radar are thus unreliable and in general no
measure of the error can also be stated. Although the value
as measured in the interferogram is precise to a
millimeter, the real value may be imprecise by multiples of
the wavelength, i.e. centimeters or even several meters.
Accordingly, making use of the delta k interferometric
technique for phase unwrapping has hitherto been a failure
for lack of accuracy, it finding mention hitherto only for
establishing the constant absolute phase offset value of
the image as a whole (Madsen) as well as the absolute phase
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of discrete, very strong single point scatterers (G. Engen
et al.).
The invention is thus based on the objective of reliably
resolving the phase ambiguities with zero error in
interferometric radar techniques as correctly indicated
for each and every pixel of an image without undue
additional complication technically.
In accordance with the invention there is provided an
interferometric microwave radar method with a synthetic
aperture (SAR), wherein, upon a phase difference analysis
performed after a coherent demodulation of echo signals of
a pulsed radar transmission signal transmitted from a
transmitter end, said echo signals being received with
their spectrum at a receiver end at two different
positions resulting in two received signals, any occurring
phase ambiguities are resolved by phase unwrapping
realized by analyzing two interferograms formed in
different wavelength ranges, from which a so-called delta-
k-interferogram of a substantially greater wavelength is
obtained, which, without any further unwrapping serves as
an absolute phase estimate, the spectrum of the received
signal therefore being split into two sub-bands, each
including a respective one of the two different wavelength
ranges, and the interferograms occurring in those two sub-
bands being calculated to obtain said delta-k-
interferogram, comprising following steps of:
generating two separate radar transmission pulse trains on
the transmitter end when forming the pulsed radar
transmission signal, said two radar pulse transmission
trains being driven in separate different wavelength
ranges within a predetermined band width range and each
pulse of which is always emitted simultaneously and in
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parallel or sequenced in time in thus forming together a
train of two-wavelength pulses, and
employing the delta-k-interferogram calculated at the
receiver end as an estimate of absolute phase and thus
also as an absolute measured value of distance sensed for
pixels.
The invention thus involves precisely optimizing an
interferometric radar method for delta k interferometry.
The technical means for achieving this are modest and can
be easily supplemented in interferometric radar sensors.
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The method in accordance with the invention will now be
detailed with reference to the drawings in which:
FIG. 1 is a block diagram of an assembly for
5 implementing the interferometric radar technique
with delta k interferometry as known from the
aforementioned paper by S.N.Madsen:
FIG. 2 is a graph showing the frequency spectrum of a
parallel two-frequency pulse as a function of
time for the improved delta k interferometry by
the method in accordance with the invention;
FIG. 3 is likewise a graph showing the frequency
spectrum of a sequential two=frequency pulse as a
function of time for the improved delta k
interferometry by the method in accordance with
the invention;
FIG. 4 is a rough block diagram of an interferometric
radar system for implementing the invention with
a SAR processor, the optimizations achieved by
the invention for the delta k interferometry
being highlighted shaded;
FIG. 5 is a diagram showing the receiver spectrum of the
radar signal with 1/6 of the total bandwidth in
the sub-bands;
FIG. 6 is a diagram showing the receiver spectrum of the
radar signal decimated by the factor 3, the
repetition spectra being highlighted heavily
shaded, and
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FIG. 7 is a block diagram of a circuit for implementing
processing of the interferometric radar data in
the method in accordance with the invention, the
phase constant n being derived from
interferometric delta k processing.
In the interferometric radar method in accordance with the
invention the conventional radar transmitter is modified
into a two-frequency transmitter by modulating the carrier
signal with a two-frequency chirp. As compared to prior art
as explained at the outset, this achieves, for the same
channel bandwidth, a substantially greater spacing of the
frequency channels in thus significantly enhancing the
accuracy of the delta k interferometry method.
Concentrating the transmitter power to a narrow bandwidth
improves the signal-to-noise ratio.
Referring now to FIG. 2 there is illustrated as a function
of time the frequency spectrum of a parallel two-frequency
pulse, i.e. existing simultaneously with its single pulses,
as emitted by the radar transmitter. For each single pulse
of the two parallel pulse trains a linear modulated chirp
is used over the pulse duration, for example. With the one
single pulse the frequency linearly increases during the
pulse duration about the frequency fl whereas with the
other pulse the linear increase is about the higher
frequency f2.
Even larger differences in frequency are achievable as long
as the electronic modules used in common permit. Thus,
common reflector antennas can be used, for example,
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signalled with horns in different frequency bands, for
instance X-band and Ku-band.
Referring now to FIG. 3 there is illustrated how likewise
the two single pulses of the two pulse trains can be
transmitted one after the other in time, as evident from
the frequency spectrum graph as a function of time.
The conventional radar receiver is modified in application
of the interferometric radar method in accordance with the
invention into a two-frequency receiver having a common
arialog/digital conversion followed by a sub-sampler.
Referring now to FIG. 4 there is illustrated in a
diagrammatic circuit diagram a radar system working with a
synthetic aperture (SAR) for implementing the method in
accordance with the invention with the modules modified for
improved delta k interferometry, indicated shaded. FIG. 5
shows in addition the spectrum of the received radar
signal.
Using two separate sub-band filters in the receiver makes
for an improved signal-to-noise ratio as compared to use of
a wideband filter as employed in prior art should
decimation in the analog/digital conversion of the
interferometric signal be provided. Decimation to the
effectively used bandwidth of the channels is possible when
the width and spacing of the channels as well as the
decimation ratio are suitably selected.
Referring now to FIG. 6 there is illustrated in this case
how the signal of a wideband receiver, but also how the sum
of two narrow band receivers can be decimated by a factor
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of 3 without the repetition spectra (shown heavily shaded)
overlapping. This decimation is a major advantage over
prior art delta k interferometry as aforementined since the
data rate is reduced to a fraction for the same total
bandwidth. This data reduction is very important especially
when radar interferometry is employed in satellite systems.
If use is made of various frequency bands with a difference
of more than a few hundred MHz, it is of advantage to
sample the analog data separately. Mixing the two channels
in this case is done digitally in capturing the data
stream, in thus enhancing the signal quality whilst
facilitating separating the frequency channels in
processing.
Referring now to FIG. 7 there is illustrated how the data
is processed in a block diagram, whereby the same as in
FIG. 4 SLC stands for "single look complex image", in other
words the focussed image product as usual in a complex
visualization. To make for a better understanding, deriving
the phase constant n from interferometric delta k
processing is illustrated here merely incompletely and in a
simplified way, it being somewhat more complicated in the
embodiment as actually achieved.
For the improved interferometric radar method in accordance
with the invention there exists a whole series of
applications of considerable potential as regards marketing
and scientific importance.
Thus, highly accurate digital elevation models even of
difficult mountainous areas in which hitherto the phase
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ambiguity was non-resolvable can now be produced by space
satellites.
It can also be used for aerospace sensing the velocity of
traffic, glaciers and ocean currents, it being particularly
in the case of single vehicles that even higher velocities
outside of the ambiguity interval can now be resolved by
application of the interferometric radar method in
accordance with the invention.
It will be appreciated that the method as described can be
put to use, of course, not only on satellites but also in
aircraft.
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