Patent No. 5822047 Modulator LIDAR system
Patent No. 5822047
Modulator LIDAR system (Contarino, et al., Oct 13, 1998)
Abstract
A modulated LIDAR system is disclosed, in which a laser for generating an tical carrier signal and a microwave generator for generating a coded microwave signal are provided. A modulator is further provided for modulating the carrier signal with the microwave signal, whereby a modulated signal is generated. A method of detecting a reflective surface is also disclosed, in which an optical carrier signal is generated, the carrier signal is modulated with a coded microwave signal, the modulated signal is reflected off of a reflective surface and the reflected signal is recovered.
Notes:
STATEMENT
OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government
of the United States of America for Governmental purposes without the payment
of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
The present invention relates to detection and ranging systems and, more particularly,
to techniques in which LIDAR and RADAR technologies are combined to improve
the resolution and sensitivity of conventional LIDAR.
Since the early 1930's, acoustic echo sounders have dominated the field of underwater
surveying; however, the slow speed of the transporting surface vessel results
in a limited area coverage rate. Shallow water surveying remains a challenge
because survey ships operate in relatively deep waters. Therefore, the need
for faster, more efficient, and more accurate techniques for shallow underwater
surveying has led to the development of airborne light detecting and ranging
("LIDAR") systems. The benefits of LIDAR over SONAR include an increase in coverage
rate and area, and an improved spatial distribution of soundings. G. C. Guenther,
"Airborne Laser Hydrography--System Design and Performance Factors," NOAA Professional
Paper Series, LCN 85-600602, March 1985.
LIDAR systems have been developed for a variety of detection purposes, e.g.,
detection of underwater targets from an airborne platform. The typical LIDAR
system is much like the early RADAR systems. A short pulse of optical radiation
is transmitted, and a receiver measures the reflected return optical power as
a function of time. In principle, the high optical frequency of LIDAR systems
should result in improved target resolution over microwave RADAR, but high attenuation
and dispersion of the optical signal and lack of coherent signal processing
techniques inhibits the exploitation of these benefits in practice. Therefore,
there is a need to develop a method to improve the detection sensitivity of
existing LIDAR systems.
In contrast, the use of RADAR on the ground, in the air, and on the sea to locate
remote objects has been extensive. This technology has experienced many advances
in its history, including sophisticated coherent detection schemes. However,
the benefits of microwave RADAR cannot be used directly for underwater detection
because microwaves do not penetrate water. LIDAR, on the other hand, uses blue-green
optical radiation, which can penetrate the water. However, techniques based
on coherent detection cannot be applied to LIDAR because light rapidly loses
coherency as it travels through water. This disadvantage results in a loss of
system sensitivity and underwater target contrast.
SUMMARY OF THE INVENTION
The current invention provides a system in which both LIDAR and RADAR technologies
are combined to improve the resolution and sensitivity of conventional LIDAR
in the detection and ranging of targets.
The current invention combines the desirable attributes of RADAR and LIDAR to
provide a novel hybrid LIDAR-RADAR system, as shown in FIG. 3. The hybrid LIDAR-RADAR
system achieves superior detection by combining the sophisticated signal processing
techniques for microwave RADAR with the underwater transmission capacity of
LIDAR. Referring to FIG. 3, a conventional LIDAR system between points 1.sub.1
and 1.sub.2 is provided. The laser generates pulsed blue-green optical radiation
that penetrates deep into the water, and the return signal is captured by an
optical detector and investigated for underwater target information.
The application of RADAR to the LIDAR systems is also depicted in FIG. 3. At
point m.sub.1, a microwave envelope is superimposed on the optical carrier by
a high-speed modulator. The blue-green optical carrier then transports the microwave
signal through the water. The reflected optical signal, with the microwave envelope,
is detected by an aerial optical receiver. At point m.sub.2, the microwave signal
is recovered by a high-speed photodetector. Therefore, between points m.sub.1
and m.sub.2, a RADAR system that can be subjected to well-established coherent
signal processing is provided. This approach enables one to transmit a RADAR
signal underwater as a subcarrier to render target detection and ranging. At
the receiver, both the optical carrier (LIDAR signal) and the microwave envelope
(hybrid LIDAR-RADAR signal) can be examined simultaneously from a single measurement.
Other objects, advantages and novel features of the invention will become apparent
from the following detailed description of the invention when considered in
conjunction with the accompanying drawings wherein:
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Each technical article cited herein is hereby incorporated by reference thereto
as if set forth in its entirety. Although a present preferred embodiment of
the invention and variations thereon have been illustrated and described, the
invention is not limited thereto but may be embodied otherwise within the scope
of the following claims.
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