Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Reexamination Certificate
2001-11-20
2003-06-24
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
C342S114000, C342S191000, C342S192000
Reexamination Certificate
active
06583751
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for measuring the movement of city areas and landsliding zones.
As is already known, a synthetic aperture radar or SAR produces a bi-dimensional image. One dimension of the image is called range and it is a measurement of the line-of-sight distance from the radar to the object illuminated. The other dimension is called azimuth and is perpendicular to the range.
The measuring operation and the range accuracy are obtained by means of a synthetic aperture radar determining as precise as possible the time that has passed from the transmission of one pulse by the radar to receiving the echo of the illuminated object. The range accuracy is determined by the length of the pulse transmitted. Shorter time pulses ensure a finer resolution.
To obtain a fine resolution of the azimuth it is necessary to use a large physical antenna so that the electromagnetic wave transmitted and received is as similar as possible to a pulse (in the ideal case the pulse has the shape of a Dirac delta).
Similar to optical systems (such as telescopes), that need large apertures to obtain fine resolutions of the image, also a SAR-type radar, of normal precision, that works at a much lower frequency than those of the optical systems, needs an enormous antenna with enormous apertures (hundreds of meters), that cannot be installed on any platform. Nevertheless a SAR-type radar installed on an aeroplane can collect information during the flight and then elaborate it as if it were an antenna. The distance that the aeroplane covers, simulating the length of the antenna, is called synthetic aperture.
The SAR-type radar consists of a coherent radar, that is a radar which measures both the module and the phase of the electromagnetic wave reflected, operating at a frequency which is usually between 400 Mhz and 10 Ghz, and is, as previously said, installed on aeroplanes, and also on orbiting satellites at a height between 250 and 800 Km.
The antenna of the radar is directed towards the earth orthogonally to the direction of the movement of the platform (aeroplane or satellite) with an angle, called “Offnadir”, between 20 and 80 degrees in relation to the direction of Nadir, that is, perpendicularly to the earth.
With said system, resolution cells or grids of the earth surface can be generated with a spatial resolution of a few meters. Said cells present a minimum grid of resolution, that is, they have a spacing within which it is possible to distinguish two objects to illuminate.
The most important characteristic of SAR is that it is a coherent image system. It is therefore possible to measure the range difference in two or more SAR images (SAR interferometer) of the same surface with an accuracy of a fraction of the SAR wavelength.
Using focusing techniques that preserve the phase, images are obtained in which every element of the image (pixel) is associated with a complex number resulting from the combination of the backscattering of all the objects belonging to the same ground resolution cell and the phase rotation due to the path.
The phase of every pixel is given by the sum of two terms: the first is the phase of the scatterer &phgr;s and the second is given by &phgr;r=4&pgr;r/&lgr;, where r is the radar—resolution cell distance and &lgr; is the radar wavelength (with &lgr;=c/(2&pgr;f), where f is the operating frequency of the radar and c is the speed of the light). The second phase term contains millions of cycles because the wavelengths are a few centimeters and the radar sensor—resolution cell distance is a few hundred kms, while the displacement connected to the scatterers is fundamentally random and therefore the phase of a single SAR image is practically unusable. However, if we consider the phase difference between two SAR images taken from slightly different viewing angles, the phase term due to the scatterers is cancelled (at least in first approximation if the angle difference is small) and the residual phase term results &phgr;=4&pgr;&Dgr;r/&lgr; where &Dgr;r is the difference of the paths between the sensors and the same ground resolution cell. The phase term still contains a very high number of cycles, that is known apart from the high integer multiple of 2&pgr;, but passing from one resolution cell to an adjacent one, the variation of the phase is usually small enough not to present ambiguity of 2&pgr;. The phase thus deduced is called the interferometric phase and the variation information &Dgr;r (which is measured in fractions of wavelength &lgr;) between pixel of the SAR image is connected thereto. Knowing the position of the two sensors, the measure of &Dgr;r can be used to find the relative elevation between the pixel of the image and therefore generate a digital elevation model (“Digital Elevation Model” or DEM), that is, an electronic reading is taken of the topography of the Earth's surface. On the other hand, if the topography is known, that is a DEM of the area of interest is available (there are special data banks from which one can take these digital models), its contribution to the interferometric phase can be eliminated and possible small surface displacements can be detected. In the case of the satellites ERS-1 and ERS-2 (twin satellites sent into orbit by the European Space Agency, the first, ERS-1, in 1991, the second, ERS-2, in 1995, operating at a frequency of 5.3 GHz, characterised by a 35-day revolution period and by a 20-meter grid resolution), for example, from one passage to the next of the platform (ERS-1 and ERS-2 follow each other at a distance of one day), or of one of the two satellites, several scatterers do not change their behaviour, that is, they keep a high coherence and therefore the cancellation of their phases is practically perfect. This means that the phase measures obtained by means of this technique can measure movements that are even a few millimeters of the Earth's topography.
Nevertheless, the present techniques of differential interferometry have some limits. In fact after a few days, in extended zones, the scatterers lose coherence, that is the scatterers do not remain similar to themselves after a period of time and therefore coherent zones with dimensions exceeding a few resolution cells cannot be identified. In addition, the wavelength of the incident signal and the displacement of it are function of atmospheric conditions. These cause phase rotations that cannot be distinguished from the movements of the ground that are required to be measured.
Another problem is the physical structure of the single scatterer that influences the phase variation in function of the observation direction and therefore of the baseline, that is of the distance between the two satellites projected orthoganally to the view line. If the stable scatterer is a surface that backscatterers and that occupies the entire resolution cell in the range, the phase of the radar echo loses correlation in correspondence with the so called critical baseline (for example in the case of satellites of the ERS type the critical baseline is about 1200 meters). When instead the scatterer is pointwise or is a comer reflector, the phase remains unvaried for much greater baselines.
SUMMARY OF THE INVENTION
In view of the state of the art described, the object of the present invention is to identify a measuring process, which resolves the problems of the present techniques so that the movement of city areas and landsliding zones can be measured in a more reliable manner.
According to the present invention, such object is reached through a process for radar measuring of the movement of city areas and landsliding zones which, having available N>20 images taken with a Synthetic Aperture Radar or SAR over a multi-year period, identifies, for every resolution cell, the scatterers, called permanent scatterers PS, that keep their electromagnetic characteristics unchanged over time, characterized in that said PSs are identified through the following steps:
(a) N−1 differential interferograms are fo
Ferretti Alessandro
Prati Claudio
Rocca Fabio
Nixon & Vanderhye P.C.
Politecnico di Milano
Sotomayor John B.
LandOfFree
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