Optical: systems and elements – Compound lens system – Telescope
Reexamination Certificate
1999-01-28
2001-02-06
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Compound lens system
Telescope
C359S419000
Reexamination Certificate
active
06185037
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical reconnaissance system for observing terrain from a great height, for example, from a geostationary or a low-flying satellite, or from high altitude aircraft.
BACKGROUND AND PRIOR ART
In conventional systems for optical earth observation in a single spectral channel, only a single telescope with one detector array in its focal plane is used. In order to improve performance, i.e., ground resolution, it was necessary to use telescopes with focal lengths as large as possible and simultaneously with an aperture that was as large as possible, assuming constant altitude and size of the detector element.
As is known, focal length is linearly related to height above the ground as well as to resolution. Hence, a doubling of resolution requires a doubling of the focal length and therefore a doubling of the structural length for the same telescope construction. Aperture size is also linearly related to resolution by diffraction. Therefore, for a higher resolution, the size of the aperture of the telescope objective lens must also be increased.
Further, the size of the objective lens, which serves as the light-collecting surface of the optical system, is dependent on the size of the reflecting terrain surface segment or area for a given signal-to-noise ratio. Thus, if the area of the terrain surface segment being visualized is reduced by half, then, for a constant signal-to-noise ratio as well as for a constant exposure time, the area of the aperture must be doubled. Also, the available exposure time, for example, in the case of a satellite in a low orbit, is proportional to the linear dimension of the terrain surface segment in the flight direction, due to the velocity of the satellite of approximately 7 km per second relative to the ground. If the length of the terrain surface segment in the flight direction is reduced by one-half, the exposure time is also reduced by one-half.
The aperture size thus increases with increasing resolution of the telescope, as does focal length. This increases the volume and mass of the telescope and correspondingly increases costs.
SUMMARY OF THE INVENTION
An object of the invention is to provide an optical reconnaissance system for observation of terrain surface from a great height, which offers a more compact and thus more cost-favorable execution with comparable performance data, when compared with conventional systems.
According to the invention, instead of a single telescope, a number of individual telescopes are employed, which are adapted to the smaller requirements with respect to their focal length, aperture, and thus individual resolution. These individual telescopes are aligned approximately parallel with respect to their optical axes, and they define a detector array in their focal plane, each in the conventional manner. The individual telescopes, however, must be capable of being aligned such that during the terrain observation, the terrain surface segment imaged on an arbitrary detector element of any one of the individual telescopes of the detector array is imaged-shifted in a direction on the detector array relative to at least one other individual telescope, and in fact, by a fraction of the width of one detector element in the focal plane.
By the increasing displacement of the terrain surface segment by one fraction of the detector element width on each detector element, slightly different total information can be obtained from the total assembly of detector arrays of the individual telescopes. The images obtained by means of the individual telescopes each contain different information content, although basically the same terrain segment is considered. The displacement of the terrain surface segment in the focal planes of the individual telescopes can be altered by slightly inclining the optical axes of the individual telescopes relative to one another, or also, by keeping the optical axes of the telescopes parallel and displacing the detector arrays increasingly relative to one another in the respective focal planes with respect to the optical axes. In the first case, there is no displacement of the detector arrays in the focal planes, and as mentioned, the alignment is produced by inclining the optical axes, while in the second case, the optical axes can be aligned parallel to one another.
In principle, it is equally possible to arrange the individual telescopes in a square matrix array in columns and rows, or in a hexagonal array, or without any regular order. It is only important that at least one group can be defined in the total assembly of individual telescopes, in which the individual telescopes are thus aligned or formed relative to the arrangement of their detector arrays such that they can be brought functionally into a sequence with increasing displacement of the image of the terrain surface segment in a direction lying in the focal plane. With respect to this displacement, a higher resolution can then be obtained by means of computer methods, than would be produced from the image of a single telescope. For example, if N individual telescopes are brought into a sequence in the described manner, it is appropriate to carry out their alignment or the arrangement of their detector arrays such that the displacement of the image of the terrain surface segment is produced by 1/N of the width of a detector element from one telescope to the next in sequence.
An increase in the resolution in a second direction lying in the focal plane, for example, perpendicular to the already mentioned first direction, can be achieved in that again at least one other group of individual telescopes can be arranged, such that the above-mentioned displacements of the terrain surface segments can be increasingly produced in the second direction on the sequence of the detector arrays.
Thus, in a particularly simple arrangement, increased resolution can be obtained in two directions orthogonal to each other, by arranging the individual telescopes at equal distances from one another in rows and columns in the array. The increasing displacement of the imaged terrain surface segment is produced both in the column direction and orthogonally in the row direction. Thus, the number of rows can deviate from the number of columns. The fraction of the detector element width, by which the displacement is produced, is then dependent on direction and amount to the reciprocal value of the number of telescopes present in the respective rows or columns. The higher this number, the smaller the size of the shift between adjacent elements, and thus the obtainable resolution is also higher.
In the case of an instrument carrier (satellite or airplane) flying over terrain at a considerable velocity, more telescopes are arranged, for example, in the column direction extending in the flight direction than in the row direction oriented perpendicularly thereto. In a geostationary satellite, on the other hand, a telescope array having the same number of individual telescopes in the row and column directions, is recommended.
The simplest embodiment of the invention operating in two directions orthogonal to one another consists of an array of 4×4 individual telescopes. Such an array with a 30-cm aperture diameter for each individual telescope can replace a single larger telescope with approximately a 1-m aperture diameter. These individual telescopes need to be equipped with only a common focal length to produce a length of side of a square terrain surface segment of 50 cm, the focal length being dimensioned for much reduced resolution, i.e., for a terrain surface segment of 2-m edge length.
Thus, a substantially smaller structural size is obtained, with smaller telescopes of smaller aperture size. Large-mirror telescopes do not need to be used and conventional telescopes with considerably smaller resolution are sufficient. The integration times with respect to the detector arrays are prolonged by a factor of 4 in the above-mentioned simple embodiment. The use of conventional detector arrays achieves a good signal-to-nois
Holota Wolfgang
Lutz Reinhold
Daimler-Chrysler AG
Ladas and Parry
Nguyen Thong
Robinson Mark A.
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