Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Reexamination Certificate
1999-12-17
2002-06-04
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
C342S160000, C342S195000, C342S196000, C342S352000
Reexamination Certificate
active
06400306
ABSTRACT:
TECHNICAL FIELD
This invention relates to a space-based or airborne synthetic aperture radar (SAR) system with moving target indication (MTI). In particular, this invention relates to such a radar system that employs modern space-time adaptive processing (STAP) techniques to provide subclutter visibility of slow moving targets embedded in surface clutter.
BACKGROUND OF THE INVENTION
Synthetic aperture radar (SAR) systems are commonly employed on airborne and space-based platforms to provide high resolution imaging of the earth's surface and stationary targets. SAR systems are used in a variety of remote sensing applications. Most commonly, single-channel (i.e. one antenna connected to a single receiver) systems are employed. However, dual-channel systems (i.e. two antennas connected to two receivers) are of more recent interest and are used in applications requiring cross-track interferometry (which facilitates height determination) and polarimetric information (useful for identification of image features). SAR systems employ a variety of SAR signal processing techniques known and used by those skilled in the art. A general treatment of such techniques can be found, for example, in Curlander and McDonough, Synthetic Aperture Radar Systems and Signal Processing, Wiley, 1991. These techniques generally assume the objects being imaged are stationary. High resolution images are formed in the range and cross-range (also called azimuth) dimensions of the image using high-bandwidth waveforms, and long dwells which have the effect of creating a large synthetic aperture. Range compression techniques are generally employed to compress the coded waveforms, thereby producing the desired range resolution. Over the duration of the dwell, waveform pulses are transmitted and received coherently and subsequently compressed in azimuth to produce the desired azimuth resolution. Over the duration of the dwell, the moving platform carrying the antenna traverses a large distance relative to the real antenna aperture dimension, thereby forming a synthetic aperture. The resulting fine azimuth resolution is intuitively related to the dimensions of this synthetic aperture analogous to the (coarse) azimuth resolution associated with a real aperture. Depending on the application, but generally speaking, individual scatterers being imaged can walk in range and azimuth due to the motion of the platform. As a result, range and azimuth correction techniques are often required in addition to the range and azimuth compression steps. Other platform motion compensation steps may be additionally required, depending on the system and the application. These steps can either be done in the radar hardware (by adjusting oscillators and sampling times) or in the digital processing.
It is well understood by those skilled in the art that imaging moving targets using conventional SAR's has many problems making performance generally unacceptable for a variety of applications. Target motion can result in significant degradations in signal strength and image resolution, making detection of moving targets difficult or impossible. Furthermore, moving targets are displaced from their true locations in images, requiring additional estimation and correction techniques to be employed. Although there have been approaches suggested to provide a single-channel SAR with moving target detection and imaging (see for example Freeman and Currie, “Synthetic Aperture Radar Images of Moving Targets”, GEC Journal of Research, Vol. 5, No.2, 1987), these approaches are applicable to a limited number of systems (usually airborne systems where a PRF several times larger than the clutter bandwidth may be employed) and applications (targets with sufficient radial velocity relative to the clutter bandwidth so as to move clear of the clutter).
To reliably detect slow and fast moving targets in clutter-limited scenes, moving target indication (MTI) techniques are generally employed, which combine signals from multiple (two or more) channels to suppress unwanted clutter and provide improved moving target detection and parameter estimation. When the radar is not moving (e.g. for ground-based systems), moving targets are easily detected by the simple use of pulse-canceler circuits. Only moving targets will have a Doppler shift away from DC (i.e. zero frequency) which allows them to escape cancellation by the pulse canceler. In airborne radar systems, returns from stationary objects (e.g. the ground or stationary targets) have non-DC Doppler shifts due to the motion of the platform. Those skilled in the art recognize that the mainbeam ground returns span a large clutter Doppler bandwidth that is proportional to the platform velocity and the azimuth beamwidth (resolution) of the antenna. The clutter bandwidth commonly spans the entire signal spectrum, thereby covering moving target returns. As a result, it is necessary to provide subclutter visibility in order to detect small moving targets. Many multi-channel systems and associated signal processing techniques have been developed to provide moving target detection and estimation for airborne radars. See for example iSkoinik, Radar Handbook, Second Edition, Chapter 16, McGraw-Hill Inc., 1990. These techniques are often referred to as AMTI (for airborne moving target indication) techniques. Moving targets can be both airborne targets and ground-based targets. Ground-based targets such as vehicles (including tanks and jeeps) travel slower than airborne targets such as aircraft and missiles. However, relative to the clutter, ground targets and air targets can both move slowly .
The large majority of moving target indication (MTI) systems and techniques have been developed for airborne radar systems. This is evident from the numerous open literature and patent literature publications. Furthermore, there are numerous airborne MTI radars in use today. For systems and techniques designed for operation in an air-to-air or air-to-ground mode, the term AMTI (for airborne moving target indication) has been generally used in the literature. Some airborne literature uses the term GMTI (for ground moving target indication), however, specifically for the air-to-ground mode. Description of space-based MTI radar systems and techniques is virtually nonexistent in the patent literature, and quite limited in the open literature. See for example Nohara et al., “A Radar Signal Processor for Space-Based Radar”, 1993 IEEE National Radar Conference, April 1993, and Nohara, “Design of a Space-Based Radar Signal Processor”, IEEE Trans. Vol. AES-34, No.2, April 1998. Furthermore, there are no known space-based MTI radars in use today. For space-based radars which operate in a space-to-air or space-to-ground mode, the term AMTI has been used for the former mode, and AMTI or GMTI for the latter mode, following airborne systems. Recognizing that for the most part, the same body of signal processing techniques and system elements are employed in air-to-air, air-to-ground, space-to-air and space-to-ground modes, the term MTI is deliberately used herein so as not to limit the scope of the invention to a specific space-to-air, space-to-ground, air-to-air or air-to-ground system or mode of operation, as well as to avoid confusion.
MTI techniques combine multiple channels to cancel or attenuate unwanted clutter. Selected multiple channels are combined by multiplying each selected channel by an appropriate weight and adding the resulting weighted channels together to produce the output, clutter-suppressed channel. If the weights used are fixed (i.e. pre-determined), then the signal processing techniques are referred to as “fixed” MTI techniques. On the other hand, if the weights are computed adaptively (i.e. they depend on the received data) as for example in Brennan et al., “Adaptive Arrays in Airborne MTI Radar”, IEEE Trans. Vol. AP-24, September 1976, then the term “adaptive” MTI techniques is used. Adaptive MTI techniques have the potential to provide greater clutter suppression than fixed MTI techniques; but have higher process
Nohara Timothy Joseph
Weber Peter Thomas
Coleman Henry
Gregory Bernarr E.
Sapone William
Sicom Systems, LTD
Sudol R. Neil
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