Communications: directive radio wave systems and devices (e.g. – Synthetic aperture radar
Reexamination Certificate
2001-06-18
2002-08-27
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Synthetic aperture radar
C342S190000, C342S195000
Reexamination Certificate
active
06441772
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a SAR radar system. The system permits detection of moving objects by means of a low-frequency radar with synthetic aperture (SAR). An important application is detection of moving objects concealed in forest vegetation from an airborne platform. In such a situation, both optical, infrared and microwave radiation are blocked to such an extent that it is not possible to carry out such detection.
2. Description of the Related Art
SAR is a known technique for two-dimensional high-resolution ground mapping. A platform, such as an aircraft or satellite, moves along a nominal straight path and illuminates a large ground area by means of an antenna. Short pulses, or alternatively long coded signals filtered by using pulse compression technique, are transmitted from the antenna and the return signal from the ground is received by the antenna and recorded along the straight path. By signal processing, high resolution is accomplished both along and transversely of the straight path. A condition for this is that the position of the antenna is known or can be calculated within a fraction of the wavelength and that the relative amplitude and phase of the transmitted and received radar signal are known. Moreover, the ground has to be invariable as the aircraft passes. The optimum geometric resolution that can be provided with SAR is determined by centre frequency and bandwidth of the transmitted signal and the aperture angle, over which the ground area is illuminated by the antenna, along the straight path.
The SAR technique has been applied in a very wide frequency range, about 20 MHz-100 GHz which corresponds to wavelengths of 3 mm-15 m. The choice of frequency determines largely which ground secures are to be reproduced since the backscattered return signal is affected above all by structures whose extent is of the wavelength size. Moreover, primarily the wavelength determines the capability of penetrating various ground layers, i.e. the penetration of the wave increases with a decreasing frequency. In connection with, for instance, vegetation, the attenuation is small for frequencies below 100 MHz and great for frequencies above 1 GHz. Thus the capability of penetrating vegetation decreases gradually with an increasing frequency, and a practical limit for detecting objects concealed in vegetation therefore is about 1 GHz. SAR systems which operate below and above 1 GHz, respectively, are in the following referred to as low-frequency and high-frequency systems, respectively.
Swedish Patent 8406007-8 (456,117) and the corresponding PCT Application SE85/00490 resulting in, inter alia, U.S. Pat. Nos. 4,866,446 and 4,965,582, and Swedish Patent Application 9503275-1 and the corresponding PCT Application SE96/01164, which are herewith incorporated by reference, disclose embodiments of low-frequency two-dimensional broadband SAR imaging.
Static objects in forest terrain can be detected with low-frequency SAR, i.e. with a wavelength in the range 0.3-15 m. The low frequencies have the property of penetrating the vegetation layer with little attenuation and only causing a weak back-scattering from the coarse structures of the trees. Thus, static objects, such as stationary vehicles, can be detected also in thick forest by combining low frequencies with SAR technique which gives resolution of wavelength size. This has been scientifically demonstrated in a plurality of experiments in recent years.
As described above, low-frequency SAR cannot detect objects that are moving. The high resolution of SAR arises by the imaging process using signals for a long time of integration. To enable sufficiently high resolution for detection, the radar must observe the object along a path which is of the same order as the distance to the object. This distance can be 20 km, i.e. for typical flying speeds the time of integration is about 100 s. During this time, an object must therefore be static within a fraction of the wavelength, i.e. the fraction of one meter. This fact makes is impossible in practice to detect moving camouflaged objects by using this technique. As a matter of fact, the speed of an object must be less than about 0.1 m/s for the object to be considered stationary.
It is a well-known fact that high-frequency SAR technique can be modified to detect and reproduce moving objects by using an array of narrow-lobe antennae. By arranging the antennae so that the antenna lobes are displaced in parallel it is possible by using signal processing to essentially eliminate all influence on the radar signal deriving from stationary objects. This GMTI function (ground moving target indication) can be implemented essentially in two different ways.
The first method is called DPCA (displaced phase centre antenna), which is used to eliminate stationary objects from the signals from two parallel-displaced antenna elements. This method utilises the fact that the signal, from all stationary objects, in the front and rear element, respectively, is repeated after a time interval in conformity with the platform moving the same distance as the element distance. After a delay, the signals from the stationary objects can thus be eliminated by subtraction. The drawback of this method is that it requires a calibrated and time-invariant radar system. A further problem with DPCA is that blind speeds arise, for which also moving objects are perceived to be stationary. The reason for this is that extinction also occurs when the phase change between the signals is a multiple of 2&pgr;. In practice this involves a demand for maximum antenna separation, which thus affects the detectable minimum radial speed.
The second method is called STAP (space-time-adaptive filtering) and is based on the covariance properties of the time signals for the different elements in the array antenna. The covariance matrix for stationary and moving objects, respectively, is different which is used by linearly combining signals in time and space so that a maximum ratio of desired to undesired signal is obtained. In practice, the co-variance matrix is estimated by taking random samples of the undesired signal, which together with a model of the desired signal forms an adaptive signal-adjusted space-time filter. The STAP technique is not restricted to the elimination of stationary objects as is the DPCA technique. Essentially all forms of undesired signal can be processed in the same manner provided that the covariance matrix can be estimated and that it differs from the desired signal. For example, also intentional or unintentional interference signals can be eliminated by the same method.
BRIEF SUMMARY OF THE INVENTION
The basic question—which the present invention intends to solve—is how to combine the technique of low-frequency SAR and the technique of detection of moving objects (GMTI) to produce signals which penetrate forest vegetation and at the same time permit detection of moving objects. The problem is especially the practical difficulty of providing on an airborne platform a sufficiently large radar antenna at low frequencies which has the same high directivity as a high-frequency narrow-lobe radar antenna so that the described methods for high-frequency SAR having the GMTI function can be used. The restricted physical space on board such a platform means essentially that low-frequency radar antennae are omnidirectional and have a low directly. The absence of directivity has two important consequences for a low-frequency SAR having the GMTI function, which mean that prior-art methods cannot be used.
First, the absence of directivity means a considerable problem of providing optimum performance for the GMTI function. The latter in fact requires that the directional sensitivity of the elements be as equal as possible, which is difficult to achieve if the directivity is low. The reason is that the antenna elements connect electromagnetically to the platform and thus the directional sensitivity changes. Consequently, the directional sensitivity is changed accordin
Hellsten Hans
Ulander Lars
Jacobson & Holman PLLC
Sotomayor John B.
Totalförsvarets Forskningsinstitut
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