Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Patent
1997-04-09
1999-05-18
Pihulic, Daniel T.
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
G01S13/90
Patent
active
059054565
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the field of observation satellite imaging, and more particularly space radar imaging.
BACKGROUND OF THE INVENTION
Space radar imaging is different from optical imaging, which is well known, and which relies on physical principles close to those of human vision. The image acquired by a space radar differs firstly in that its contents differs from that obtained by means of a conventional optical system since the scene is observed by the space radar at a decimeter wavelength (3 cm to 25 cm) which is much greater than the micrometer wavelength used in traditional optics. The image also differs in the way in which it is acquired, since a space radar is an active instrument which includes its own source for illuminating the scene, thereby making observation possible by night as well as by day, and by an appropriate choice of wavelength, making observation insensitive to cloud. It therefore provides "all-weather" imaging which gives numerous operational advantages. However, since it is difficult to deploy antennas that are more than 10 meters long in space, the natural angular resolution of space radars is very poor, being of the order of half a degree (whereas the human eye is about ten times better). Radar imaging therefore does not rely on the natural resolution of the instrument. Resolution in the direction perpendicular to the flight direction is obtained by analysis on board the satellite of echo return times (the principle on which all radars are based), and in the direction parallel to the flight direction, by a digital process known as "aperture synthesis", which process is performed on the ground and requires large amounts of computation. Each of the lines of a radar "image" corresponds to a pulse emitted by the radar and transformed into a run of samples ordered by their return times giving column indices for the image. The samples are complex numbers representative of the amplitude and of the phase of the reflected wave. The above-mentioned digital process of aperture synthesis conserves the complex nature of the signals processed, such that, in addition to its main operational quality, i.e., that of being insensitive to cloud and being capable of operating at night, a space radar installed on a satellite makes it possible to measure small displacements by means of the technique of interferometry.
The phase of the reflected wave contains information concerning the position, the distribution, and the radioelectric nature of elements constituting the scene illuminated by the radar, also known as "targets" (such as pebbles, branches, etc.). By comparing radar images taken at different dates and under almost identical conditions, position information can be isolated from other information by constructing an interferogram. Radar interferometry was proposed and tested more than 20 years ago, and reference may usefully be made to the article published by L. C. Graham in IEEE Proceedings, Vol. 62, No. 6, Jun. 1974, entitled "Synthetic interferometer radar for topographic mapping".
Phase information is influenced by three factors, of which the first two are unknown: associated with the electromagnetic properties of the targets; within a common image element or "pixel". The resultant phase of a pixel is the result of a complex combination of the contributions from the various targets present within the pixel, and weighted by their respective amplitudes; and of observation conditions.
When implementing radar interferometry techniques, it is assumed that the first two factors, although unknown, are stable over time. For the first factor, this assumption implies that the targets are physically stable, and for the second factor, it implies geometrical stability restraining possible variation in the angle of incidence of the radar between two passes of the satellite. Thus, the surface state of the ground must not change excessively between acquiring two images (which rules out the surface of the sea, for example), and the satellite must follows its earlier
REFERENCES:
patent: 5332999 (1994-07-01), Prati et al.
patent: 5726656 (1998-03-01), Frankot
Centre National d'Etudes Spatiales
Pihulic Daniel T.
LandOfFree
Method of evaluating non-euclidean effects affecting an image ac does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of evaluating non-euclidean effects affecting an image ac, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of evaluating non-euclidean effects affecting an image ac will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-1762837