Michelangelo Villano

Errata: Digital Beamforming on Receive: Techniques and Optimization Strategies for High-Resolution Wide-Swath SAR Imaging

Synthetic Aperture Radar (SAR) is a well-proven imaging technique for remote sensing of the Earth. However, conventional SAR systems are not capable of fulfilling the increasing demands for improved spatial resolution and wider swath coverage. To overcome these inherent limitations, several innovative techniques have been suggested which employ multiple receive-apertures to gather additional information along the synthetic aperture. These digital beamforming (DBF) on receive techniques are reviewed with particular emphasis on the multi-aperture signal processing in azimuth and a multi-aperture reconstruction algorithm is presented that allows for the unambiguous recovery of the Doppler spectrum. The impact of Doppler aliasing is investigated and an analytic expression for the residual azimuth ambiguities is derived. Further, the influence of the processing on the signal-to-noise ratio (SNR) is analyzed, resulting in a pulse repetition frequency (PRF) dependent factor describing the SNR scaling of the multi-aperture beamforming network. The focus is then turned to a complete high-resolution wide-swath SAR system design example which demonstrates the intricate connection between multi-aperture azimuth processing and the system architecture. In this regard, alternative processing approaches are compared with the multi-aperture reconstruction algorithm. In a next step, optimization strategies are discussed as pattern tapering, prebeamshaping-on-receive, and modified processing algorithms. In this context, the analytic expressions for both the residual ambiguities and the SNR scaling factor are generalized to cascaded beamforming networks. The suggested techniques can moreover be extended in many ways. Examples discussed are a combination with ScanSAR burst mode operation and the transfer to multistatic sparse array configurations.

Tandem-L: A Highly Innovative Bistatic SAR Mission for Global Observation of Dynamic Processes on the Earth's Surface

Tandem-L is a proposal for a highly innovative L-band SAR satellite mission for the global observation of dynamic processes on the Earths surface with hitherto unparalleled quality and resolution. It is based on the results of a pre-phase A study which started in 2013 and is currently undergoing a phase-A study. Thanks to the novel imaging techniques and the vast recording capacity with up to 8 terabytes/day, it will provide vital information for solving pressing scientific questions in the biosphere, geosphere, cryosphere, and hydrosphere. By this, the new L-band SAR mission will make an essential contribution for a better understanding of the Earth system and its dynamics. Tandem-L will, moreover, open new opportunities for risk analysis, disaster management and environmental monitoring by employing especially designed acquisition modes and techniques in combination with a reconfigurable tandem satellite configuration and an L-band SAR instrument with advanced digital beamforming techniques.

Staggered SAR: High-Resolution Wide-Swath Imaging by Continuous PRI Variation

Synthetic aperture radar (SAR) is a remote sensing technique, capable of providing high-resolution images independent of weather conditions and sunlight illumination. This makes SAR very attractive for the systematic observation of dynamic processes on the Earths surface. However, conventional SAR systems are limited in that a wide swath can only be achieved at the expense of a degraded azimuth resolution. This limitation can be overcome by using systems with multiple receive apertures, displaced in along track, but a very long antenna is required to map a wide swath. If a relatively short antenna with a single aperture in along track is available, it is still possible to map a wide area: Multiple swaths can be, in fact, simultaneously imaged using digital beamforming in elevation, but “blind ranges” are present between adjacent swaths. This paper considers an innovative concept, staggered SAR, where the pulse repetition interval (PRI) is continuously varied. This concept allows the imaging of a wide continuous swath without the need for a long antenna with multiple apertures. The choice of the sequence of PRIs and the preprocessing of the raw data are discussed in detail, showing how the staggered SAR is even less affected by ambiguities of pointlike or extended targets with respect to a system with constant PRI, which simultaneously maps multiple swaths. Some system design examples are finally presented and compared.

A Novel Processing Strategy for Staggered SAR

Staggered synthetic aperture radar (SAR) is an innovative concept, where the pulse repetition interval (PRI) is continuously varied. This, in combination with digital beamforming (DBF) on receive, allows high-resolution imaging of a wide continuous swath without the need for a long antenna with multiple apertures. However, staggered-SAR systems require a mean pulse repetition frequency (PRF) much larger than the signal Doppler bandwidth and allow the use of transmitted pulses of limited length. This letter proposes a novel processing strategy for staggered SAR data, which allows a reduction of the mean PRF and the use of longer transmitted pulses. The performance obtained with the proposed novel strategy is evaluated and compared with a conventional SAR system operating with constant PRI.

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.

Signal: SAR for ice, glacier and global dynamics

SIGNAL is an innovative earth exploration mission proposal with the main objective to estimate accurately and repeatedly topography and topographic changes associated with mass change or other dynamic effects on glaciers, ice caps and polar ice sheets. Elevation measurements are complemented with glacier velocity measurements, providing valuable additional information for a better understanding of the hydrology of glacierized basins and of the Arctic and Antarctic water cycle. SIGNAL is capable of monitoring all critical regions with a high spatial resolution and an adequate revisit time. This paper gives an overview about the actual mission design status and provides a brief description of the topography (DEM - digital elevation map) self-calibration strategy and the estimated global interferometric performance.

Impact of Azimuth Ambiguities on Interferometric Performance

The impact of azimuth ambiguities on interferometric performance in terms of phase bias and standard deviation of the interferometric phase is analyzed, resorting to the interferogram statistics for jointly circular Gaussian processes. The theoretical results are validated through simulation and compared with measurements on a TanDEM-X interferogram, affected by azimuth ambiguities.

On-board Doppler filtering for data volume reduction in spaceborne SAR systems

High-resolution wide-swath (HRWS) synthetic aperture radar (SAR) systems based on digital beamforming (DBF) in elevation are very attractive for the observation of dynamic processes on the Earths surface. However, HRWS systems are inherently associated with a huge data volume. Moreover, in order to comply with azimuth ambiguity requirements, a pulse repetition frequency (PRF) much higher than the required processed Doppler bandwidth is often desirable. The data volume can be drastically reduced, if on-board Doppler filtering and decimation are performed prior to downlink. A finite impulse response (FIR) filter with a relatively small number of taps suffices to completely suppress the additional ambiguous components and recover the original impulse response, provided that the filters transfer function is compensated for in the processing. This strategy is also applicable to staggered-SAR systems, where on-board Doppler filtering and resampling can be jointly implemented.

Spectral-Based Estimation of the Local Azimuth Ambiguity-to-Signal Ratio in SAR Images

An innovative technique to estimate the local azimuth ambiguity-to-signal ratio (AASR) in synthetic aperture radar (SAR) images is presented. Unlike the backscatter-based (BB) technique, the proposed one, which is based on the spectral properties of the image, does not require that the areas responsible for the ambiguity lie within the focused image. Analysis of real TerraSAR-X data shows that the estimates of the proposed technique are consistent with the BB ones. Moreover, according to simulations, the proposed technique seems to provide more accurate estimates than the BB method, especially for high values of the local AASR.

Antenna array for passive radar: Configuration design and adaptive approaches to disturbance cancellation

We consider the selection of an antenna array configuration, composed of a small number of omnidirectional elements, to be exploited for passive radar sensors. Based on properly identified pattern characteristics and design criteria for practical applications, a suitable planar configuration is selected that allows both angular selectivity and direct signal attenuation. The selected configuration is further optimized in terms of sidelobe level by resorting to appropriate amplitude tapering. Moreover, three different approaches are investigated for antenna-based adaptive disturbance cancellation, and a comparative performance analysis is carried out. Simulation results show that an effective clutter suppression is obtained if the direct signal from the transmitter is attenuated by means of spatial adaptive cancellation, and the multipath echoes from stationary obstacles are removed by means of temporal adaptive cancellation. In particular, the approach based on the Sidelobe Canceller is shown to yield good performance while requiring a limited system complexity.