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A laser radar experiment.

A Laser Radar Experiment

On April 10, 1990 a group of distinguished engineers and scientists from academia, the three Services, and the Strategic Defense Initiative Organization (SDIO) met at the Pentagon in Washington, DC.

Colonel Ray Ross, Director of Sensor Interceptor Technology, SDIO announced that on March 29, 1990, a successful test had been carried out demonstrating the feasibility of using a laser radar for long range target acquisition and tracking. The test was performed by scientists at MIT's Lincoln Laboratory's Firepond laser radar site in Westford, MA.

The SDIO has eyed the concept of lasers for radar applications from the time of its inception.

The laser's ability to produce high power coherent signals with narrow directive beams, and provide high angular and precise range resolution puts lasers high on the list of technologies to be explored for space-tracking application. Infrared, optical and ultraviolet regions of the frequency spectrum have been considered as possible choices for laser radar systems.

The Lincoln Laboratory group, under the direction of Dr. William E. Keicher, used a [CO.sub.2] laser to collect range Doppler images, since [CO.sub.2] lasers operating at 11.2 [macro]m wavelength are the most powerful and most efficient lasers built to date. A medium power argon ion laser, operating in the blue-green portion of the EM spectrum, was selected for angular tracking of the targets.

The project, code-named Firepond Laser Radar, successfully was tested at Lincoln Laboratory's facility near Westford, MA, shown in Figure 1. The test involved the tracking of a sounding rocket, including a specially-designed payload that was launched from NASA's Wallops Island site on Virginia's Atlantic coast.

The payload was ejected six minutes after the launch of the two-stage, sub-orbital Terrier Malemute sounding rocket (Firefly rocket) during the missile's ascent. The Firefly rocket and payload are shown in Figure 2. The payload consisted of a canister containing an inflatable balloon similar in shape and size to a ballistic missile's re-entry vehicle. The balloon had gas thrusters to provide it with precession motion, simulating a missile or decoy in flight.

The purpose of the experiment was to track the missile and decoy payload with the laser radar. The launch was tracked initially by the Millstone Hill L-band tracking radar and X-band imaging radars also located in Westford, MA. Sensor data from these radars were transferred to the Firepond laser radar that obtained images of the targets from the sounding rocket at a range of 800 km. With this experiment, MIT's Keicher confirmed that a laser radar has the tracking and imaging precision required for mid-course discrimination between decoys and warheads. The experiment also demonstrated sensor data fusion capabilities between microwave and laser radars that are important to battle management and [C.sup.3]. The laser radar continuously tracked the missile and decoy from the time it took over from the microwave radars. The targets reached a maximum altitude of 482 km during their 12-minute flight.

Laser Radar Operation

Laser radars operate very much like microwave radars, but their operating frequencies are 3000 times higher, providing laser radars with remarkably high angular resolution. On the detrimental side, atmospheric absorption losses of optical signals are several orders of magnitude greater than those of microwave signals and laser radars operating in the earth's atmosphere are restricted to 1.06 [macro]m and 10.6 [macro]m wavelengths where lower absorption windows exist. Ground-based radar systems usually operate at ranges below 10 miles, while space-based laser systems may operate at ranges of thousands of miles without degradation.

The isotopic [CO.sub.2] laser radar uses an ultra-stable laser oscillator as a signal source. A medium power argon ion laser, which operates in the blue-green portion of the EM spectrum, is combined with the high power IR beam and both beams are directed out the 1.2 m telescope. The argon ion laser permits angle tracking accuracies on the order of microradians. The [CO.sub.2] radar collects range-Doppler images, also known as inverse synthetic aperture radar images, that are processed in real time. As is true for both microwave and mm-wave synthetic aperture radars, the image resolution is independent of the range to the target and is a function of the size of resolution cell chosen.

Laser Radar Imaging

All pulsed and CW radars, including laser radars, primarily are used as range detectors and target trackers. The high angular resolution of laser radars broadens these system applications to include radar imaging. With their angular resolution of 100 [macro]s, ground vehicles, such as armored vehicles and trucks, can be resolved with an almost photographic quality resolution.

Keicher provided the following laser radar range equation for the target signal power received, Mathematical Expression Omitted where S = target signal power P = transmitted power sigma = laser radar target cross

section rho = target backscattering

coefficient A = aperture area Tau = optical efficiency R = target range lambda = wavelength

Laser Radar Receivers

Like radio receivers that may have a tuned RF section or may be of the heterodyne type, laser receivers also may operate as direct receivers or heterodyne type receivers with LO injection and IF mixing. Direct laser receivers detect the backscattered energy from the target. In heterodyne receivers, the transmitted waveform is frequency multiplied by the target return waveform.

The Firepond Laser Radar

Transmitter, Receiver

and Telescope

The Firepond laser radar telescope, shown in Figure 3, has an aperture of 1.2 m and is used as the laser radar transmitter and receiver antenna. At a range of 750 km, the beam spreads to only 7.5 m in diameter attesting to the extremely high beam control that is possible with coherent laser light. It may be worth noting that the design of the 2.4 m Firepond telescope is similar to that of NASA's ill-fated $1.5 B space-based Hubble telescope, except that the Hubble telescope's aperture has twice the diameter of the Firepond instrument. More important, however, is the fact that the Firepond instrument was fabricated to exact specifications and was tested thoroughly before installation. This telescope has no spherical aberrations in either of of its two beam-reflecting mirrors.

Co-aligned with and mounted above the 1.2 m telescope is the 60 cm aperture Nd YAG laser telescope. This smaller telescope was used to acquire and track the Firefly experimental target.

Results

The 35 ft. long naval Terrier surface-to-air missile and payload were tracked during the launch phase by C-band and X-band microwave radars. The Firepond laser radar acquired the targets as they reached 480 km in altitude, 750 km from the Firepond laser radar site. The laser radar tracked the payload section, as well as the launch missile, showing the ejection of the small (8.5" diameter) canister that inflated into a man-sized cone. The cone, which simulated a re-entry vehicle or decoy, traveled on a trajectory different from that of the launch vehicle. Both were tracked with the laser radar whose imaging capabilities enhanced the radar's target identification capability.

Dr. Keicher pointed out that the 4 ft. diameter aperture laser radar telescope provided the resolution and range accuracy equivalent to that of the massive 120 ft. microwave radar antenna reflector on Millstone Hill, MA.

In a ground installation, the laser radar suffers from the fact that it is not an all-weather radar and cannot be used during cloud or rain conditions. Space-based and airborne laser radars, on the other hand, do not have to contend with adverse weather conditions.

There will be another test of MIT Lincoln Lab's Firepond radar in October 1990 to verify the precision of the laser radar measurements made during the March test. A future (1991) Firebird experiment will focus on target identification and discrimination techniques.

PHOTO : Fig. 1 The Millstone Hill Field site, operated by MIT Lincoln Laboratory and located in Westford, MA, includes the Millstone Hill L-band tracking radar, the Haystack X-band long range imaging radar and the Firepond laser radar.

PHOTO : Fig. 2 Firefly rocket payload.

PHOTO : Fig. 3 Photograph of the Firepond telescope mount and telescopes.
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Title Annotation:MIT Lincoln Laboratory tests Firepond laser radar
Author:Stiglitz, Martin R.; Blanchard, Christine
Publication:Microwave Journal
Date:Sep 1, 1990
Words:1334
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