Optics: measuring and testing – Angle measuring or angular axial alignment – Apex of angle at observing or detecting station
Reexamination Certificate
2001-03-19
2002-05-28
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Angle measuring or angular axial alignment
Apex of angle at observing or detecting station
C250S203200, C250S206200
Reexamination Certificate
active
06396577
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to air defense systems, and more particularly pertains to a single unified system that may detect, track and even destroy airborne objects, even objects designed to move relatively undetected through conventional radar systems.
2. Description of the Prior Art
Radar systems using radio frequency waves are well known for detecting and tracking objects in the atmosphere of the Earth for the purpose of air defense. Such systems are used for guiding manned aircraft and unmanned weapon systems to the objects, and for destruction of the objects, if necessary. Thus, the conventional air defense systems have typically employed one system for detecting and tracking objects in the atmosphere, and a relatively separate system for destroying, or otherwise rendering ineffective, objects representing a threat to the protected ground area below the area of atmosphere being patrolled.
Technology has been developed that employs relatively higher frequency, shorter wavelength waves in the ultraviolet, visible, and infrared region of the electromagnetic spectrum are sometimes referred to as “lidar” systems, which stands for “LIght Detection And Ranging” or “Laser Infrared raDAR”, depending upon the particular source consulted. The lidar systems transmit and receive relatively short frequency electromagnetic radiation.
The basic instruments of a lidar system are a transmitter, a receiver, and a detector. A lidar system's transmitter is typically a laser-generating apparatus, while the receiver typically includes optical equipment, in contrast to the radio wave transmitters and receivers of radar systems. Different types of lasers can be employed for the transmitter, depending upon the power and wavelength of the electromagnetic wave employed in the lidar system. Laser emissions are produced when high-voltage electricity causes a quartz flash tube to emit an intense burst of light, exciting some of the atoms in a ruby crystal to higher energy levels. At a specific energy level, some atoms emit particles of light called photons. At first, the photons are emitted in all directions. Photons from one atom stimulate emission of photons from other atoms and the light intensity is rapidly amplified. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification. The photons leave through the partially silvered mirror at one end, and these photons comprise the laser light emission. An important fact to note is that the photons are energy. Therefore, when two laser beams are crossed, most of the photons will pass through the intersecting beam and continue on the same course as before they crossed.
The receiver of a lidar system detects the light waves scattered back to the receiver by objects in the path of the photons of the laser emission from the laser of the transmitter. The receiver records the scattered light received by the receiver at fixed time intervals. Lidar systems typically use sensitive detectors called photomultiplier tubes to detect the back-scattered light waves. The photomultiplier tubes initially convert the individual quanta of light, or photons, received by the receiver into electric currents, and then convert the electrical currents into digital photocounts that can be stored and processed on a computer. The electric currents generated by the receivers are normally in the range of picoamps.
The photocounts received by the receiver can be recorded for fixed time intervals during the return pulse of photons. The times can be converted to vertical heights above the ground, referred to as range bins, because the speed of light is a known constant. A range bin can be determined from a return pulse time. Range-gated photocounts (e.g., those photocounts that lie within a small range interval) can be stored and analyzed by a computer.
So far, the primary uses of lidar systems have been for detection of weather phenomena and pollutants in the atmosphere. The National Aeronautics and Space Administration (NASA) has also used a lidar system to map the topography of Mars. The military applications of lidar systems have included using them as range-finders to determine the distance to a target, and for missile defense. In a test in June 2000, the U.S. Air Force trained a high energy lidar laser on a missile for several seconds while tracking it with radar, and destroyed it in mid-air.
The four basic types of lidar systems are used primarily to measure pollutants in the air and to measure wind conditions. The types of lidar systems are similar in that all of the systems use lasers for transmitters and telescopes for receivers. However, each type of lidar system employs a different kind of light scattering.
One type of lidar system, the DIAL system, which stands for DIfferential Absorption Lidar, aims a laser at high and low regions of the atmosphere to measure the amount of ozone. Because light is absorbed at different wavelengths at different altitudes, a measurement of the difference in absorption of light can determine the amount of ozone present.
Another type of lidar system, the LITE system, which stands for LIdar Technology Experiment, is used to detect clouds and aerosols from space. It was used for the first time on NASA shuttle mission STS-64 in September 1994. The LITE system uses elastic scattering of light to measure aerosol particles in clouds. Elastic scattering means that the scattered light waves are at the same frequency as the incident light waves from the laser of the transmitter.
Yet another lidar system, the GALE system, which stands for Giant Aperture Lidar Experiment, measures wind, temperature and ocean waves using resonance fluorescence scattering. When sodium atoms in the atmosphere are illuminated by lidar laser emitted light waves at a precise wavelength, the sodium atoms radiate light waves that are measurable by receivers. By slightly changing the wavelength of the transmitted light, the shift of the spectral line from. its central wavelength can be measured. The shift of the central wavelength is known as the Doppler shift. The Doppler shift can be used to measure wind speeds and currents that could be important for airplanes trying to avoid turbulent winds.
Still another lidar system, the PCL, or Purple Crow Lidar, system measures temperature, waves and water vapor. The PCL system measures temperature with the same kind of sodium resonance-fluorescence scattering as in the GALE system. It also uses Rayleigh scattering from air molecules to measure temperature. Rayleigh scattering refers to the fact that different kinds of light scatter more strongly than others do. Blue light, for example, scatters five times more strongly than red light. The amount and color of the scattering depends on the kinds of molecules the light strikes. Oxygen, for example, produces significant scattering of blue light, which explains the blue sky of Earth's atmosphere. The PCL system employs a receiver called a liquid mirror telescope. The liquid mirror telescope contains mercury or gallium that is spun to achieve a parabolic surface that can be used for lidar light wave measurement.
One prior use of lidar systems was NASA's Multi-center Airborne Coherent Atmospheric Wind Sensor (MACAWS). MACAWS is an experimental design that uses an airborne, pulsed, scanning, coherent Doppler lidar that remotely senses the distribution of wind velocity and aerosol back-scatter within three-dimensional volumes in the troposphere and lower stratosphere. The MACAWS components included a frequency stable, pulsed, transverse-excited, atmospheric pressure (TEA) CO2 laser transmitter producing 0.6-1.0 Joules per pulse between 9 to 11 microns (nominally 10.6 microns and 0.7 J) at a pulse repetition frequency (PRF) of 1 to 30 Hz (nominally 20 Hz); a coherent receiver employing a cryogenically-cooled HgCdTe infrared detector; a 0.3 m off-axis paraboloidal telescope shared by the transmitter and receiver in a monostatic configuration, a ruggedized optical table
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