Television – Back scatter reduction
Reexamination Certificate
1999-09-03
2004-12-28
Philippe, Gims (Department: 2613)
Television
Back scatter reduction
C348S081000, C356S004010
Reexamination Certificate
active
06836285
ABSTRACT:
BACKGROUND
1. Field of the invention
This invention relates generally to optoelectronic systems for imaging objects from an elevated or slightly elevated observing instrument. Such imaging systems include but are not limited to mast-mounted systems for obtaining warning of shallow hazards ahead of a water craft, aircraft-carrier landing aids, and refinements in airborne imaging platforms. A related aspect of the invention provides intensity equalization across a fan-shaped probe beam, and has general industrial applications.
2. Related Art
Shallow-angle marine observation systems—A particular difficulty of all marine observational systems, even visual systems, is the problem of interference by the water surface. Reflections at the surface, whether of ambient radiation or of probe beams, tend to be confused with signals or signatures of the hazards or other objects of interest.
Another noteworthy problem with such systems is the limited range of known apparatus and methods. In the past, short range has been seen as essentially an inherent limitation of mast-mounted or other only-slightly-elevated equipment.
It is known to use light detection and ranging (“LIDAR”) for such purposes.
FIG. 1
illustrates an experimental deployment shown by Anderson, Howarth and Mooradian (“
Grazing Angle LIDAR for Detection of Shallow Submerged Objects
”, Proc. International Conference on Lasers, 1978).
Anderson et al. did a pier-based experiment with a single-pixel PMT detector and no scanner. Basically they verified the laws of physics, namely (1) Snell's Law predicting deflection of the light into the water, and (2) the laws of radiative transfer—the light detection and ranging or “LIDAR” equation—predicting enough returning photons to support a detection. There was no suggestion of an entirely practical implementation for such an idea.
More specifically, the Anderson paper describes use of grazing-incidence LIDAR for detection of shallow objects. The group detected a target of diameter about 80 centimeters (2½ feet), to depths of nearly 5 meters (15 feet) at a range of 130 meters (400 feet) from a pier.
The experimental demonstration used a narrow-beam LIDAR and a photomultiplier-tube detector. The laser L (
FIG. 1
) and receiver R were mounted in a hut-like enclosure E on a pier structure S in the ocean, at distance F of about 330 m (1100 feet) forward from the beach.
The LIDAR transceiver L-R was at a height H of about 13 m (40 feet) above the ocean surface O. At the pier the benthic depth D
1
was some 5 m (15 feet) and at the target T the depth D
2
was about 8 m (25 feet).
A winch W on the pier operated a chain CH around a first pulley P
1
, fixed by a clamp CL to the pier S. The chain extended out to the floating target T via a second pulley P
2
, which was tethered to an anchor A (in the form of concrete-filled 55-gallon drums)—thus enabling some variation in range R as desired, the nominal value of the range R being 120 m (400 feet). The severely constrained range associated with these experiments is exemplary of the limitations of shallow-angle object surveillance heretofore.
We are aware of these patents for mast-mounted television cameras used for imaging objects from slightly elevated positions: U.S. Pat. Nos. 3,380,358 and 3,895,388. Pertinent LIDAR-related patents include:
U.S. Pat. No. 4,862,257 of Ulich,
U.S. Pat. No. 4,920,412 of Gerdt,
U.S. Pat. No. 5,013,917 of Ulich
U.S. Pat. No. 5,034,810 of Keeler,
U.S. Pat. No. 5,091,778 of Keeler,
U.S. Pat. No. 5,257,085 of Ulich,
U.S. Pat. No. 5,450,125 of Ulich,
U.S. Pat. No. 5,384,589 of Ulich and
U.S. Pat. No. 5,506,616 of Scheps.
The most relevant of these are the last three Ulich patents mentioned.
Ulich et al. use a streak tube for time-resolved fluorescence (wavelength vs. time), not imaging (angle vs. time). In fact, their text particularly cites use of a streak tube in a nonimaging mode. Furthermore they use a laser blocking filter to specifically reject the in-band response.
Thus the prior art fails to deal incisively, or effectively, with the previously mentioned problems of interference arising from surface reflection. Utilization of a slit by the Ulich group is for spectral dispersion, not imaging.
The '589 Ulich patent, “Imaging LIDAR System”, makes one reference to a ship-based application, but does not develop the idea further. The system is described only with reference to gated, intensified cameras.
Airborne-hazard alert for water craft—LIDAR is also usable for obtaining information about airborne objects, whether threatening hostile objects or otherwise. A separate system for such purposes, however, is costly and occupies significant space in the command center of a water craft.
Aircraft-carrier operations—In addition to detection of floating and airborne obstacles (e.g. mines and other hazards), another marine-related problem that would benefit from visibility aids is that of aircraft-carrier landing. This problem is particularly acute at night, and in fog or other turbid-atmosphere conditions.
The difficulty of such operations is compounded by the high speeds involved, the fact that not only the aircraft but also the carrier is in motion. A further complication sometimes is the need for a degree of discreet or covert character in the traffic. Radio guidance may be of limited practicality in such circumstances.
Airborne surveillance—Still another use of LIDAR systems that has been developed heretofore is airborne surveillance of objects submerged in the ocean or in other bodies of water. U.S. Pat. No. 5,467,122—commonly owned with the present document—sets forth many details of a surveillance system that is particularly aimed at monitoring relatively large areas of the ocean.
In that system, typically imaging is limited to detection from altitudes of at least 160 m (500 feet) and looking straight down into the water with the center of the probe beam. Still, there is some off-axis detection for positions well away from the track of the airborne platform.
Wave noise, and distortion: Wave noise and the resultant image distortion represent one of the severest limitations for airborne surveillance, even in the clearest ocean waters. These concerns have not been adequately addressed with existing airborne LIDAR systems. According to a comparative-evaluation field test in 1997, object-classification capability and the ability to reject false alarms in hazard detection have yet to be achieved to the satisfaction of the United States government.
Both the shapes and the positions of submerged objects are distorted by uncorrelated refractions of different parts of the probe/return beam, due to irregularity of the water surface. Heretofore no effort has been directed to overcoming either the positional error or the relative vagueness of object shapes obtained with this technology.
Uncertainties in coverage: Current systems also provide inadequate information about the fraction of the undersea environment that is actually being screened. The root problem is that wave focusing and defocusing of rays from a LIDAR system cause gaps in the coverage at different depths.
That is to say, inherently certain volumes of water receive and reflect very little light, which means that objects within those volumes cannot be detected. The difficulty here is that existing systems cannot accurately estimate the extent of these effects at different depths, and therefore cannot generate good area-coverage estimates at those depths.
There is no reliable measurement of how well—in particular, how uniformly—the system is illuminating and imaging each layer of water. Such systems resort to a statistical model, based on a single estimate of sea state, to estimate how many passes over the same patch of water are necessary to assure proper coverage.
This model is hard to validate—and the estimate of sea state may or may not be accurate or timely. Errors in the sea-state estimate force present systems to make either too many passes over the same area, which results in poor effective area-coverage rates, or too few passes, which may leave
Bowker J. Kent
Gleckler Anthony D.
Lubard Stephen C.
McLean John W.
Sitter, Jr. David N.
Areté Associates
Lippman Peter
Philippe Gims
LandOfFree
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