LIDAR for Missile Defense

LADAR/LIDAR exploits the scattering characteristics and shorter wavelength and high directivity of laser energy to image objects at much finer resolution than is readily attainable with longer wave millimeter and microwave systems. 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.

Target identification and tracking systems that employ a number of tracking vehicles to track and/or destroy targets generally require high resolution imaging to identify a specific aimpoint on a target that differs from the target's centroid. It may be desirable to track an aimpoint on a target, rather than a centroid, because the lethality of the tracking vehicle can be improved, resulting in reduced cost, size, and/or weight. Some conventional target identification systems use long-wave (LW) diffraction techniques to identify and/or track a target. The resolution of these long-wave diffraction techniques is limited by aperture size and wavelength, among other things, making these techniques impractical for small tracking vehicles, such as miniature kill vehicles, to track a target's aimpoint other than a centroid.

Some higher resolution systems that use shorter wavelengths for imaging may have better diffraction limits for tracking a separate aimpoint, but have a limited passive acquisition range and may require external illumination to acquire targets. Some lower resolution systems that track a target's centroid do not need high resolution because they do not identify a separate aimpoint. These lower resolution systems may require the tracking vehicles to have a higher kill radius. This may result in heavier and/or more expensive tracking vehicles.

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. The military usages of both lidar systems and lasers have included range finding. As range finders, the U.S. Army has used lidar systems on battlefields to determine the distance to a target, such as an enemy tank. In a range finder application, a laser transmits a pulse while a receiver (often little more than a lens) registers a pulse when back-scattered light is picked up by the receiver. A computer portion of the system measures the time interval between the time when the laser pulse is emitted and the reflected pulse is sensed. Because the speed of light is known, a measurement of the round-trip distance between the laser pulse and the receiver indicates distance to target.