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.

Active Aperture Synthesis Radar for High Spatial Resolution Imaging

The algorithm of spatio-temporal ultrawideband (UWB) signal processing for radiometric imaging is synthesized and investigated. Analytical expressions for the limiting error of the radiometric image estimate and ambiguity function of system are derived. The possibility of ambiguity function formation with one main lobe in the UWB cross-correlation and compensation system with ultra-sparse antenna array is substantiated. The simulation examples of radiometric imaging are shown.

Algorithm for radiometric imaging by ultrawideband systems of aperture synthesis

Using modern information about the statistical synthesis of radio-engineering systems, the optimal algorithm for the formation of the radiometric imaging (RMI) is synthesized and investigated with three-antenna ultra-wideband radiometric complex (RMC). Expressions for the limiting error and the fluctuation sensitivity of RMC are found.

Optimal algorithm for estimation of radio brightness of an extended source of radio thermal radiation in the ultra-wideband radiometric complex with a three-element-antenna system

The methods of filling the domain of spatio-spectral sensitivity (SSS) are considered The spectral method for filling the SSS domain in aperture synthesis systems is proposed It is shown that instead of filling the SSS domain of the serial synthesis using one radio-interferometer with variable base expedient to use fixed antennas and ultra-wideband (UWB) signals processing The criterion for the uniqueness of the main lobe of the ambiguity function of the two-antenna array using UWB signals processing is obtained

Spectral method for the spatio-spectral sensitivity domain filling in aperture synthesis system

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 optimal algorithm of multiband spatio-temporal radiometric signal processing is synthesized. Analytical expressions both for the potential fluctuation sensitivity of the system designed according to the algorithm and for the ambiguity function of multiband radiometric systems are deduced. An example of the theory application is demonstrated.

Nguen Van Kiem

Papers

Optimal signal processing for radiometric imaging with multi-antenna & multi-band passive radars