Federica Bordoni

X-band sea-clutter nonstationarity: influence of long waves

In this paper, we deal with the problem of modeling the backscattering from sea surface for low-grazing-angle and high-resolution radar systems. Based on the electromagnetic two-scale model, we analyzed both the amplitude and frequency modulations induced on the small-scale Bragg resonant waves by the large-scale surface tilt and advection due to the swell presence. Evidence of sea-clutter nonstationarity has been verified and the relationship between the variations of clutter spectral features, such as texture, Doppler centroid, and bandwidth, have been studied by processing real sea-clutter data recorded by the IPIX radar of McMaster University, Hamilton, ON, Canada. An autoregressive nonstationary process is proposed and validated to model the physical phenomenon.

Performance Comparison of Reflector- and Planar-Antenna based Digital Beam-Forming SAR

The trend in the conception of future spaceborne radar remote sensing is clearly toward the use of digital beamforming techniques. These systems will comprise multiple digital channels, where the analog-to-digital converter is moved closer to the antenna. This dispenses the need for analog beam steering and by this the used of transmit/receive modules for phase and amplitude control. Digital beam-forming will enable Synthetic Aperture Radar (SAR) which overcomes the coverage and resolution limitations applicable to state-of-the-art systems. On the other hand, new antenna architectures, such as reflectors, already implemented in communication satellites, are being considered for SAR applications. An open question is the benefit of combining digital beam-forming techniques with reflector antennas. The paper answers this question by comparing the system architecture and digital beam-forming requirements of a planar and a reflector antenna SAR. Further elaboration yields the resulting SAR performance of both systems. This paper considers multiple novel aspects of digital beam-forming SAR system design, which jointly flow into the presented system performance.

Multibaseline ATI-SAR for robust ocean surface velocity estimation

An open problem of along-track interferometry (ATI) for synthetic aperture radar (SAR) sensing of ocean surface currents is the need of ancillary wind information for inversion of Doppler centroid measurements, that have to be compensated for the propagation velocity of advancing and/or receding Bragg scatterers. We propose three classes of estimators which exploit multibaseline (MB) ATI acquisition and Doppler resolution for robust data inversion under different degrees of a priori information about the wind direction and the value of the characteristic Bragg frequency. Performance analysis and comparison with conventional ATI show that the proposed MB estimators can produce accurate velocity estimates in the absence of detailed ancillary data.

Ambiguity Suppression by Azimuth Phase Coding in Multichannel SAR Systems

The Azimuth Phase Coding (APC) technique, proposed to suppress range ambiguities in conventional SAR systems, stands out for its low implementation complexity and its effectiveness for point and distributed ambiguities. This paper investigates the possibility of applying the APC to the new, forthcoming generation of multichannel SAR systems, for high resolution and wide swath imaging, based on Digital Beamforming on receive. The extension of APC to multichannel SAR systems is mathematically described. A new metric is defined to quantify the APC performance. A numerical analysis is developed to characterize the influence on the APC behaviors of the main SAR system parameters. Finally, an example of APC performance is provided, by considering two multichannel SAR systems based on a planar and a reflector antenna.

Smart Multi-Aperture Radar Techniques for Spaceborne Remote Sensing

The paper deals with the performance of next generation SAR sensors, here referred to as SMART (Smart Multi Aperture Radar Techniques) equipped with digital beamforming capabilities. Apart from representing a technological jump, these sensors offer the flexibility of actually deciding on the (possibly hybrid) mode(s) of operation on-ground after the data have been acquired. The paper presents the performance of SMART system configurations and modes of operation for a digital beamforming SAR which covers a large swath with a high resolution.

MIMO-SAR and the orthogonality confusion

This paper reviews radar architectures that employ multiple transmit and multiple receive channels to improve the performance of synthetic aperture radar (SAR) systems. These advanced architectures have been dubbed multiple-input multiple-output SAR (MIMO-SAR) in analogy to MIMO communication systems. Considerable confusion arose, however, with regard to the selection of suitable waveforms for the simultaneous transmission via multiple antennas. In this paper, it is shown that the mere use of orthogonal waveforms is insufficient for the desired performance improvement in view of most SAR applications. As a solution to this fundamental MIMO-SAR problem we had previously suggested to exploit the special data acquisition geometry of a side-looking imaging radar equipped with multiple receiver channels in addition to appropriately designed waveforms transmitted by multiple antennas. Here, we extend this approach to a more general set of radar waveforms with special correlation properties that satisfy a short-term shift-orthogonality condition. We show that the echoes from simultaneously transmitted pulses can be separated if the short-term shift orthogonality is combined with digital beamforming on receive in elevation. This enables the implementation of a fully functional MIMO-SAR without correlation noise leakage for extended scattering scenarios.

Advanced digital beamforming concepts for future SAR systems

This paper reviews advanced multi-channel SAR system concepts for the imaging of wide swaths with high resolution. Several novel system architectures employing both direct radiating arrays and reflector antennas fed by a digital array are introduced and compared to each other with regard to their imaging performance. In addition, innovative SAR imaging modes are proposed which enable the mapping of ultra-wide swaths with high azimuth resolution. The new techniques and technologies have the potential to enhance the imaging performance of future SAR systems by one order of magnitude if compared to state of the art SAR sensors like TerraSAR-X, ALOS, Radarsat-2 or Sentinel-1.

Ultra Wide Swath Imaging with Multi-Channel ScanSAR

Multi-channel synthetic aperture radar (SAR) systems enable high-resolution wide-swath imagery thus overcoming the inherent limitation of conventional SAR. A possible realization based on the combination of multi-aperture SAR signal reconstruction in azimuth with digital beamforming on receive in elevation is given in [1]. The present paper turns focus to advanced concepts for the imaging of even wider swaths while still providing high azimuth resolution [2]. In this regard, the operation of multi-channel SAR systems in burst modes like ScanSAR or TOPS is introduced and aspects of applying the multi-aperture reconstruction algorithm to burst mode data are analyzed. The influence of the digital processing network on performance parameters as signal-to-noise-ratio and azimuth ambiguity-to-signal-ratio in multi-channel burst mode systems is considered and embedded in the design example of a ScanSAR system that allows for the imaging of a 400 km wide swath with a geometric resolution of 5 m. Finally, first results for a multi-channel TOPS system are presented and an optimized TOPS processing approach is introduced.

Adaptive scan-on-receive based on spatial spectral estimation for high-resolution, wide-swath Synthetic Aperture Radar

Intensive research is currently ongoing in the field of Smart Multi-Aperture Radar Techniques (SMART) for high-resolution wide-swath Synthetic Aperture Radar (SAR) imaging. This work investigates the possibility of applying direction of arrival estimation methods to spaceborne SMART SAR systems, that employ receive beam steering. In particular, a new algorithm based on the actual spatial distribution of the received signal power is proposed. The performance of the algorithm is evaluated by Monte Carlo simulations and compared with that of the conventional scan-on-receive approach, in different operational scenarios. The Cramér Rao Lower Bound is also reported as a benchmark on the performance.

Multibaseline cross-track SAR interferometry using interpolated arrays

This work deals with the problem of interferometric radar phase estimation in the presence of layover. The focus here is on multichannel interferometric synthetic aperture radar (InSAR) systems with a low number of phase centers and nonuniform array geometry. An interpolated array (IA) approach is proposed in order to apply parametric spectral estimation techniques designed for uniform linear arrays (ULAs). In particular, the interpolated MUSIC and weighted subspace fitting (WSF) algorithms are considered and compared with conventional methods. Performance analysis under different InSAR scenarios is carried out based on Monte Carlo simulations. The Cramer-Rao lower bound (CRLB) for the nonuniform interferometric array is derived and reported as a benchmark on the estimation accuracy.