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Interferometry And Synthesis In Radio Astronomy

The problem of algorithm synthesis for processing of the signals at the output of a spatially distributed antenna system for the formation of radio images from aerospace carriers has been solved. In order to synthesize the optimal algorithm, the maximum likelihood method was used. As a result of analyzing the physical nature of the signal processing operations, a transition to the quasi-optimal processing, which simplifies the technical implementation of the algorithm, is proposed. The main feature of this algorithm lies in the fact is that it allows to receive a radio image in the direction of the nadir within the range of angles that is currently not visible by the radars from aerospace carriers. In this range of angles, an image is usually formed by optical means. A scheme of the radar complex has been developed and modeling of radio image formation has been carried out in accordance with the algorithm.

Quasioptimal Spatiotemporal Signal Processing Algorithm for Radar Imaging

The real-time radar imaging of land cover over large areas is performed from aerospace carriers via imaging equipment operating in the optical, infrared and radio wavebands. The latter waveband allows to carry out the surveys regardless of weather and time of day. However, since radio waves are significantly longer (in comparison with optical and infrared ones), to achieve high-resolution images in spatial coordinates it is necessary to use antennas dozens, hundreds or even thousands of meters long. Naturally, it is rather difficult – and in most cases simply impossible – to implement this large antenna aperture. Therefore, since the middle of the XX century, specially synthesized systems consisting one or several antennas, which can be repositioned according to a given law, has been widely used. Signals recorded in different areas of space are accumulated and then processed altogether. This creates the effect of using large antennas. In this case, there are two main methods of signal processing implemented, respectively, in active radar systems with aperture synthesis and passive aperture synthesis systems. Traditionally, the viewing areas of these systems are correlated – the active systems form images within the range of angles ±(15°-50°) from the nadir, and passive systems within ±15° from the nadir. The physical essence of the images obtained at the outputs of these systems is also different complicating the image formation within the entire region of ± 50° from the nadir. This report combines the features of signal processing in active and passive systems with the aperture synthesis proposing a system of active aperture synthesis. This allows to obtain radar images of high resolution in spatial coordinates within the monitored area that cannot be viewed by active radars, and the physical essence of the images themselves will be close to the essence of the images obtained by active radar systems with synthesized aperture.

Signal Processing Algorithm for Active Aperture Synthesis Systems

The aperture synthesis systems allow imaging with high spatial resolution. These images are important for solving problems of Earth remote sensing. One of the promising directions for development of such systems is the transition to UWB spatiotemporal signals processing. This raises a number of difficulties in the physical interpretation of the main UWB radio systems characteristics due to the fact that the classical definitions were introduced with regard to the narrowband systems description. Thus, there is a need for additional research and further definition of UWB systems characteristics and parameters. Among these characteristics is the spatial-spectral sensitivity domain, which, along with the uncertainty function, determines the quality of the radio system. Unlike in radio systems for narrowband signals processing, for systems with UWB signal processing the spectral sensitivity domain depends on both temporal and spatial frequencies. Analysis of this area is complicated by the fact that the temporal and spatial frequencies in the UWB system are not divided. In this work, the physical essence of the spatial-spectral sensitivity domain was studied and it was shown how its analysis will allow to develop requirements for the choice of antenna system geometry and to justify the expediency of using the working frequency band.

Spectral Aperture Synthesis Method

Context. There are a large number of RCS estimation methods in synthetic-aperture radars (SAR), which differ by precision, RCS recovery time of an observation area and complexity of implementation. At the same time, the optimal method, which is a generalization of all existing ones and characterizes both spatial and temporal optimal signal processing, has not been synthesized. Also, usually problem statements do not take into account the stochastic structure of signals reflected from most underlying surfaces. As a result, further ways of improving resolution, optimal SAR structure and maximum achievable precision of estimation of RCS are not determined. Objective. The goal of the work is to solve the problem of synthesis of the optimal method of RCS surfaces restoration as a statistical characteristic of spatially-inhomogeneous random scattering coefficient in aerospace-based radio engineering systems with moving linear antenna arrays and adaptive spatio-temporal signal processing. Method. Applying the method of maximum likelihood estimation and taking into account a priori information about the statistical characteristics of the received spatio-temporal fields a super-resolution method of RCS estimation on spatial coordinates is derived. The generalized problem statement has shown the optimal method of surface observation that allows to overcome the contradiction between the size of the observation area and the accuracy of the parameter estimates. The obtained method allows to achieve highest resolution (as for SpotLight mode) of radar images for wide area of observation (as for Stripmap mode). It is shown that the general algorithm can be adapted to particular solutions with limited statements of the problem. In contrast to the well-known method of aperture synthesis the processing of the received field in the antenna array and receiver is adaptive and depends on the signal-to-noise ratio. Results. The optimal method of area scanning in onboard SAR with antenna arrays and the corresponding method of adaptive spatio-temporal signal processing can be used to describe the receiving path of cognitive on-board radar for remote sensing. Conclusions. The obtained optimal method can be considered as a modified method of aperture synthesis with a multi-beam spotlight mode with the possibility of adaptive radiation pattern formation and signal time processing. In contrast to the classical method performing matched-filtering of the received signal with the reference signal, the modified method additionally decorrelates signals reflected from the earth’s surface. As a result of this decorrelation the characteristic intervals of speckles (the size of the spotted pattern of the image) will be significantly smaller than with match-filtering. Therefore, their subsequent smoothing with the same efficiency can be performed by windows of smaller width. Such processing together with a multi-beam spotlight mode will significantly increase the resolution of the SAR with an expanded area of view.

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Radar cross-section imaging in synthetic aperture radar with linear antenna array and adaptive receiver

To solve problems of high-precision mapping of spatially extended objects regardless of weather conditions and time of day, radar imaging systems are used. The globality and efficiency of the survey is provided by the placement of radars on aerospace carriers. At the same time, there has been a proliferation of side-scan radars (providing a wide viewing range with a low spatial resolution) and antenna aperture synthesis radars (these provide high resolution in spatial coordinates, which depends on the type of survey). Radars used for imaging are classified according to various characteristics determining their advantages and disadvantages. Among such features, we can also distinguish the radar viewing area, which is currently limited to angles of 15° to 60° to the right and left of the observation in the nadir. The viewing range from -15° to + 15° from the nadir is characterized by a low resolution in range and traditionally is not visible by these radars (the socalled “blind zone”). In this regard, an alternative system for building radar images from aerospace carriers is proposed. It will provide imaging with high angular resolution of the viewing range from -15° to + 15° from the nadir. The main feature of the developed radar lies in the signal processing algorithm that combines methods of active (the presence of probing UWB signal) and passive (aperture synthesis) radiolocation. In addition, the radar implements a new method of “spectral aperture synthesis,” which allows the transition from processing of UWB signals with the continuum spectrum to processing multi-band signal processing.

Nguyen Van Huu

Papers

Active Aperture Synthesis Radar for High Spatial Resolution Imaging

Spectral Aperture Synthesis Method