Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2000-12-06
2002-08-27
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S332000
Reexamination Certificate
active
06441376
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to imaging systems in which two or more complex signals of a region of interest may be combined to yield one or more interferometric images. More particularly, the invention is directed to a method and system for two-dimensional radiometric imaging of a planetary surface region of interest utilizing thermal radiation emitted by the region of interest.
BACKGROUND OF THE INVENTION
Computed imaging systems are utilized in a wide variety of applications. Of particular interest here is the use of radio frequency antennas to collect complex signals employable to obtain high quality images of planetary surfaces.
Such complex images are typically obtained by overhead transmission/reflected receipt of pulses of energy at a predetermined frequency. In the latter regard, microwave radiation has been advantageously employed due to its ability to yield high resolution images in virtually all weather conditions and at all times (i.e., day and night).
While such systems have been utilized with success, they require the use of radiation signal transmission payloads on one or more aircraft or satellites (i.e., “space vehicles”). As may be appreciated, such transmission payloads add significant weight, complexity and cost to an imaging system. Additionally, the use of active transmitters entails significant attendant power requirements. Further, the active transmission of microwave signals toward a region of interest is detectable and may be undesired in certain applications.
SUMMARY OF THE INVENTION
In view of the foregoing, a primary objective of the present invention is to provide an improved imaging system and method that reduces imaging componentry payload and complexity on space vehicles utilized to collect imaging data. Related objectives are to reduce on-board power requirements and componentry costs associated with the obtainment of imaging data on space-borne vehicles.
Another important objective of the present invention is to provide a radiometric imaging system and method that is passive in nature and thereby avoids the active transmission of energy signals to an image region of interest to form a pixel image thereof.
An additional main objective of the present invention is to provide an imaging system and method that reduces the number of space vehicles and associated antennas necessary for generating high-resolution images.
Yet another objective of the present invention is to provide an imaging system and method that provides high-resolution images in inclement weather and day
ight conditions.
The above objectives and additional advantages are realized by the present invention. To do so, the present inventors have recognized that even though thermal emissions from a planetary surface region of interest are of random phase and amplitude, such emissions may be assumed to be largely isotropic and mutually coherent at a receiving antenna (e.g., as received or time-shifted), and may be collected and processed in a manner that allows such randomness to be effectively removed. Relatedly, it has been recognized that thermal radiation collection and processing can be carried out in a manner that reduces the number of antennas necessary to yield high-resolution images. At the outset it should be noted that while the present invention is particularly apt for radiometric imaging applications, certain aspects may also be employable in active imaging arrangements.
The inventive system contemplates a plurality of space vehicles located in known relative positions over a planetary surface region of interest (ROI). At least a corresponding plurality of antennas are mounted on the space vehicles to collect radiation emissions from the ROI (e.g., thermal or blackbody radiation) and provide corresponding thermal emission signals. In turn, processor means (e.g., one or more signal processors) may be utilized (e.g., either on-board the space vehicles and/or more preferably at another location) to combine the thermal emission signals and obtain interferometric fringe signals employable to form a pixel image of the ROI. As will be appreciated, the formation and use of interferometric fringes effectively removes phase randomness from the collected signals.
In one aspect of the invention, the space vehicles may be spaced at different relative distances therebetween, wherein the collection antennas collectively define a “sparse aperture”. For such purposes, the space vehicles may be located so that two or more of the antennas are horizontally and/or vertically offset from each other in relation to the imaged ROI during imaging. Such an arrangement allows the thermal emission signals obtained by the antennas to be processed in varying combinations, wherein each combination yields a different interferometric phase measurement based upon a corresponding different interferometric baseline. As such, the multiple different interferometric phase measurements can effectively “fill-in” an array of interferometric images employable in pixel image formation for the ROI. As will be appreciated, the differential spacing of antennas to collectively define a sparse aperture facilitates reduction of the overall number of space vehicles required to yield high-resolution ROI images.
In a further aspect of the invention, the space vehicles may be positioned in a “nearfield” imaging arrangement to collect thermal emissions from an ROI. That is, the space vehicles may be positioned so that the imaging center axes for at least two of the antennas define an angle &thgr; of at least about 2° therebetween, and more preferably about 2° and 15° therebetween, depending upon the collection center frequency of the antennas. In the latter regard, the antennas may be provided to collect thermal emissions over a collection bandwidth of between about 1 MHz and 1 GHz with a center frequency of between about 1 GHz and 100 GHz. The establishment of a near-field imaging arrangement also facilitates the obtainment of high-resolution ROI images.
In one arrangement, a plurality of antennas may be mounted on a corresponding plurality of satellites located in a known constellation passing over an ROI to be imaged. More particularly, two or more satellites may be located in corresponding repeatable orbits having relatively small differences in eccentricity and/or inclination (e.g., Hill's orbits), wherein the corresponding antennas are horizontally and/or vertically offset in a known geometry relative to the ROI for imaging. By way of example, four satellites may be positioned in known orbits to laterally define a repeatable Y-shaped pattern for sparse aperture imaging. Further, at least two of the satellites may be positioned so that the center imaging axes of the corresponding antennas mounted thereupon define an angle of about 2° to 15° therebetween, thereby yielding a near-field imaging arrangement. In earth imaging applications, the satellites may be disposed in low-earth orbits, wherein the satellites are placed at altitudes and spacings consistent with near-field operations.
In conjunction with noted aspects of the present invention, it should be recognized that the antennas should be provided in a spotlight mode (e.g., via gimbaled mounting) so that they remain pointed at an imaged ROI during an imaging, or “dwell”, time period. Further in this regard, the antennas should be provided to collect thermal emissions from overlapping portions of the ROI in a substantially simultaneous manner to maintain mutual coherence. In turn, the thermal emissions collected from the ROI at each of the antennas may be substantially simultaneously sampled at a predetermined frequency (e.g., at least the Nyquist rate) during a given dwell period, thereby yielding an ROI thermal emission data set comprising each thermal emission signal. Such ROI data sets are combinatively employed by the processor means for image formation.
In the latter regard, and in another general aspect of the present invention, the processor means may be provided to combine, or correlate, at least a first thermal emission signal (e
Alexander Scott David
Glass Carter M.
Golubiewski Arthur Casimir
Harrison Lori K.
Herring Christopher Taylor
Gagliardi Albert
Hannaher Constantine
Lockheed Martin Corporation
Marsh & Fischmann & Breyfogle LLP
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