Ladar Illuminates Optical Sensors
Super-sensitive imaging systems cover new ground.
In the near future, laser-based detection systems will allow military aircraft to identify enemy ground vehicles accurately in battle zones and permit spacecraft and robotic vehicles to navigate safely through unfamiliar terrain. The technology is built around highly sensitive optical detectors that measure minute amounts of reflected laser light. These systems do three-dimensional modeling of scanned objects in real time, offering missile defense systems the capability to differentiate between re-entry vehicles and decoys.
Laser detection and ranging, or ladar, technology dates back to the invention of the laser in the 1960s, but three-dimensional image generation was not achieved until the early 1980s, experts say. Although ladar is now used for weather mapping and astronomy, these systems are large and stationary. Recent developments in miniaturization and increased computer processing capabilities have led to prototype devices small enough to operate on aircraft and in ground vehicles.
One of the centers focusing on ladar research in the United States is the Massachusetts Institute of Technology’s Lincoln Laboratory in Lexington. According to Richard Heinrichs, leader, laser and sensor applications group, Lincoln Laboratory, the original impetus for ladar research was a U.S. government request for three-dimensional imaging technology for ballistic missile defense. Ladar is very useful in this application because it can create an accurate outline of an object, permitting interceptor systems to discriminate between cone-shaped re-entry vehicles and spherical decoys—something traditional radar cannot do, he says.
The heart of Lincoln Laboratory’s ladar systems is a charge coupled device (CCD) technology similar to the one found in digital cameras. In a camera, when a flash goes off, light reflected from the illuminated object enters the device’s aperture and places an image onto the CCD array. Each element in the array consists of a pixel that measures the amount and intensity of light on that part of the image. In a ladar system, a laser serves as the flash to illuminate the object.
Three-dimension-capable ladars also can determine an object’s distance. The device functions much like a military laser target designator by using pulses of light to repeatedly measure range and report changes in distance. A laser pulse is typically a nanosecond to half a nanosecond long, and it is this reflected light that is imaged by the detector array. Unlike a digital camera in which each pixel measures light intensity, Lincoln’s ladar CCDs also determine range on a pixel-by-pixel basis by timing the light’s time of arrival—how long it takes for light to travel from each pixel to the object and back again.
Heinrichs notes that ladar devices have been built for nearly 20 years. These older devices typically use a scan mirror and a single detector instead of a CCD array. This kind of system scans by moving the mirror slightly, taking the range, moving the mirror again and repeating the process to build up an image. The difficulty with this approach is that it takes time to create an image because the mirror must move rapidly to scan with any speed. In contrast, Lincoln Laboratory’s technology permits each individual pixel in the detector array to measure range, allowing the CCDs to generate three-dimensional images at a very high rate.
Although other groups are developing ladar systems with CCD arrays, Lincoln’s devices differ in their use of avalanche photo diodes. A standard solid-state photographic device, the diodes are applied somewhat differently from other types of optical detectors—they operate in the Geiger mode. In this mode, the system actually counts photons instead of merely measuring light intensity. An avalanche photo diode normally operates by running a bias voltage across the arrays. When a photon enters the array, it will ionize an electron, or free it so that it can move around in the device and create current that is measured as a signal.
In an avalanche photo diode, increasing the voltage level accelerates electrons so they will run into the device’s lattice structure, generating more electrons that will then move around in the lattice, creating an amplification. “That’s why they call it the avalanche process. One electron will generate two more, and they will generate others and you will get this kind of cascade effect,” Heinrichs explains.
The laboratory takes this a step further by increasing the voltage level to push the avalanche process into a runaway effect. Because of this sensitivity, a single photon will generate more electrons until a spark is measured on the detector, creating a highly sensitive imaging device. Heinrichs notes that because only one photon per pixel is necessary, very small, low-energy lasers can be used for illumination. “That means we can live with much less laser light, enabling these systems to be much smaller than other technologies would require,” he says.
Researchers also have developed new arrays that are 32 x 32 pixels, or roughly 1,000 pixels per detector. These highly sensitive devices already have been built into prototype three-dimensional ladars. Lincoln scientists are now experimenting with operating the arrays at different wavelengths and with building larger devices containing more pixels to create a megapixel array, Heinrichs explains.
Ladar also is capable of greater resolution than traditional radar systems. Because optical methods rely on light reflected from an object, image resolution is proportional to the frequency of radiation used. Higher frequencies and shorter wavelengths produce better image resolution, which is one advantage optics-based systems have over radio frequency systems. Optical systems also do not need extensive modifications to achieve increased performance. Heinrichs observes that military systems do not require extremely high resolution. “We’re talking about a few centimeters or inches of resolution. This is good enough in most cases to identify vehicles, map regions or tell whether an object is a cone or a sphere,” he says.
Ladar also is very effective for tracking moving objects, Heinrichs observes. Lincoln Laboratory’s systems operate at frame rates in the kilohertz range, permitting rapid measurements. In addition, it is easier for computers to analyze three-dimensional imagery than it is to write programs to image or analyze standard image data automatically. By using a common orientation system to rotate an image, an operator can compare a variety of shapes. This decreases the processing burden on the system, making it easier to conduct real-time image discrimination. “You can always transform three-dimensional image data. You can numerically change it or rotate it to a common coordinate system. In a sense, it doesn’t matter what angle I am actually looking at—I can take that image and rotate it,” he says.
Besides missile defense, another major military application is to provide military aircraft the capability to identify objects on the ground quickly. “Is that a tank down there, or is it a school bus? If it is a tank, is it one of ours or one of theirs?” he asks.
One problem with standard forward-looking infrared and infrared imagers is that they can have difficulty identifying objects, Heinrichs explains. But ladar can create detailed images and silhouettes down to the turret shapes of individual tanks. When this capability is combined with a computer algorithm, it could provide automatic vehicle detection and identification, he says.
The military also is interested in the technology’s mapping capability. For example, in an urban operation, if terrorists were suspected of operating in a building, accurate ladar maps could be used to do three-dimensional modeling of the area, permitting ground forces to find the best ways to approach the structure without being seen.
NASA could use ladar’s terrain mapping capabilities for space exploration. Heinrichs notes that one of the recent NASA Mars probes that was lost while attempting to land near the planet’s south pole region probably fell into a crevasse. A three-dimensional imager could provide highly accurate maps of potential landing areas, offering mission planners more accurate site selection data. NASA also is investigating ladar applications for spacecraft docking systems, he says.
The National Institute of Standards and Technology is interested in ladar for vehicle navigation. This could be applied to both robotic and manned vehicles. While much progress has been made using standard camera images for guidance, a great degree of ambiguity remains in making computers turn data into a picture for navigation, Heinrichs says. A three-dimensional image would help determine an object’s exact distance and the shape of the terrain.
Lincoln Laboratory researchers are continuing to develop, manufacture and test advanced prototype systems. Heinrichs believes it is still several years before these ladar-based imaging systems will enter military and commercial use. While stressing that there is no guarantee that full production will occur in this time, he believes the capability will exist within the next two to three years because much of the basic research and developmental groundwork has already been demonstrated.
Additional information on the Massachusetts Institute of Technology’s Lincoln Laboratory is available on the World Wide Web at www.ll.mit.edu.
Satellite Tracking System Hunts Asteroids
Lincoln Laboratory researchers are applying charge coupled device (CCD) technology to study the solar system. Telescopes fitted with sensitive detector arrays are proving to be highly effective in spotting asteroids. Located at the laboratory’s White Sands, New Mexico, research facility, the telescopes have discovered nearly 70 percent of all recently detected near-earth asteroids, explains Grant Stokes, associate head of the aerospace division of the laboratory.
The technology evolved from U.S. Air Force requirements for a ground-based satellite tracking system that could detect objects beyond the range of phased-array missile warning radar. Lincoln Laboratory scientists improved an existing Air Force system that used 1970s era television cameras as detectors. These were replaced with sensitive CCD arrays that improved sensitivity by a factor of 10, he says.
In the early 1990s, scientists noted that the technology had other applications besides satellite detection. Working with the U.S. Air Force Science Advisory Board and NASA, the laboratory’s White Sands facility began actively tracking heavenly bodies in March 1998. Called the Lincoln near earth asteroid research program (LINEAR), the CCD-equipped telescopes also can detect comets far from the sun, before their tails activate from the solar heat.
This capability has led to significant changes in comet science, Stokes says. Amateur astronomers used to discover most comets, but LINEAR scientists now find them far from Earth before their tails have turned on. The majority of these objects can be imaged while they are still dark in the outer solar system. This early detection is helpful because there are waiting lists and forms to submit to use the Hubble orbital space telescope or the Keck Observatory in Hawaii. In the past, when comets were detected, it was often too late to submit the paperwork in time to gain access to orbital facilities. Detection with LINEAR telescopes provides scientists more time to study these objects, he says.