TOPS Mode Research Articles

Comparison of Doppler-Derived Sea Ice Radial Surface Velocity Measurement Methods From Sentinel-1A IW Data

The near-instantaneous radial velocity of a target can be obtained using the Doppler effect of SAR, which is widely used in ocean current retrieval. However, in sea ice drift velocity measurements, only a Doppler centroid estimation algorithm in frequency domain has been studied, so whether there is a better algorithm is worth exploring. In this study, based on Sentinel-1A IW data, three Doppler centroid estimation algorithms applied to ocean current retrieval are selected. Combined with the characteristics of the TOPS mode, made two applicability adjustments to each algorithm, and finally applied the three algorithms to sea ice radial surface velocity measurements. The first adjustment is to explore and determine the optimal parameters. The second adjustment is to use parallel computing to improve the efficiency, which is improved by an average of 43.55%. In addition, the deviation of Doppler centroid estimation bias correction is verified using rainforest data, and the deviation is controlled at approximately 3 Hz. Based on the three algorithms, five sets of experiments are carried out in this study. By analyzing and comparing the results of each algorithm, it is found that the results of the three algorithms are relatively consistent, among which the correlation Doppler estimation algorithm has the advantages of high efficiency and high precision and is the most suitable method for sea ice drift measurement among the three methods. However, for SAR images with abnormal speckles caused by human activities, the sign Doppler estimation algorithm can effectively remove abnormal speckles and ensure the smoothness of the image with better adaptability.

Ground Displacement Variation Around Power Line Corridors on the Loess Plateau Estimated by Persistent Scatterer Interferometry

As the world's largest producer and consumer of electricity, China has deployed a project to transport energy resources from West China to East China through extra-high-voltage transmission lines to reduce carbon emissions. In the Loess Plateau, the safety of transmission lines has been frequently threatened by natural hazards such as landslides and ground subsidence. However, it is prohibitively expensive to build a monitoring network covering all transmission lines to provide timely early warnings, particularly for sparsely inhabited areas. Based on the large-scale coverage of Sentinel-1 SLC images in IW TOPS mode, we utilized persistent scatterer interferometry to measure the ground target movements along transmission lines with millimeter accuracy. Three regions of interest were selected to reflect the different environmental conditions from the south to north of the Yan'an Region. The displacement variations were compared and evaluated at three spatial scales (region, route, local) from October 2018 to November 2019. On the regional scale, the deformation variation decreased from the south (-36 - 29 mm/yr) to the north (-19 - 20 mm/yr), which was significantly different as determined by the analysis of variance (ANOVA) and can be attributed to the impacts of water-induced subsidence and human activities. On the local scale, the results showed that the deformation rate was less than 10 mm/yr, which suggested a stable status for the experimental line sections. The displacement time series of transmission towers preliminarily revealed seasonal displacement variations, wherein winter and spring were relatively quiet periods. The application of advanced InSAR technologies has excellent prospects in smart grid implementation.

Sentinel-1 TOPS co-registration over low-coherence areas and its application to velocity estimation using the all pairs shortest path algorithm

The C-band Sentinel-1 A/B satellites in TOPS mode provide unprecedented opportunities for continuous radar mapping of the earth with enhanced revisit frequency. The reliability for routine operational services relies on a very stringent azimuth co-registration accuracy. However, while the enhanced spectral diversity (ESD) technique achieves a co-registration accuracy of better than 0.001 pixels, the accuracy might still be degraded over low-coherence areas due to fast decorrelation. When TOPS time series data are available, the abrupt loss of coherence further affects the estimation accuracy of the network ESD. I integrate a double sample over the burst overlap region with an ESD estimator to reduce the ESD phase error and simultaneously mitigate the abrupt loss of coherence in the time series using the all pairs shortest path algorithm. I further use this algorithm to create a spatio-temporal network of co-registered Sentinel-1 SAR stacks for line-of-sight (LOS) velocity estimation over low-coherence areas. I perform InSAR time series analysis and evaluate the effectiveness of the proposed method using data acquired from descending tracks over representative low-coherence regions in south-western China. For those images suffering from significant low coherence, this method can achieve a better co-registration accuracy than those obtained from the previous network ESD method. By cross-validating descending COSMO-SkyMed data, a 39% uncertainty reduction in the estimated LOS velocity can be achieved when applying a co-registered Sentinel-1 stack with high accuracy and spatio-temporal network evolution.

Complete three-dimensional near-field surface displacements from imaging geodesy techniques applied to the 2016 Kumamoto earthquake

The recent development of imaging geodesy, an advanced technique with a high spatial resolution and large-scale coverage, has enabled researchers to obtain multiple high-quality surface displacement estimates at low labor-cost, thereby improving the capability to monitor and manage geological disasters. The different sources (e.g., radar, optical and LiDAR sensors) and analysis approaches (e.g., differential interferometric synthetic aperture radar, DInSAR; multiple-aperture InSAR; pixel offset tracking; and iterative closest point, ICP) in imaging geodesy used to derive displacement estimates have unique benefits and drawbacks. However, the inherent differences among these data sources and methods in the construction of three-dimensional (3D) deformation maps, particularly in the near field, remain poorly understood and require further discussion. In this study, we acquired three pairs of ALOS-2 stripmap mode images, two pairs of Sentinel-1 TOPS mode images and pre- and post-event LiDAR data for the 2016 Kumamoto earthquake to explore the 3D near-field displacements using various imaging geodesy techniques with different types of image information, i.e., SAR phase data, SAR amplitude data and LiDAR point cloud data. Our results show that each image type is independently capable of producing a high-quality 3D deformation map for the 2016 Kumamoto earthquake with an on-fault accuracy of <43 cm determined from field work measurement, and an off-fault accuracy of <14 cm determined from GNSS observations. The major contributors to the uncertainty in the 3D deformation estimates from the SAR phase, SAR amplitude, and LiDAR data methods are the effective Doppler bandwidth, pixel resolution and topographic roughness, respectively. In the near-field deformation region, more secondary fault ruptures were revealed by SAR amplitude and LiDAR information than by the SAR phase information, thereby overcoming the sensitivity of the SAR phase signal to incoherence. Finally, an integrated complete 3D map was generated to constrain the coseismic rupture behavior of the Kumamoto earthquake sequence associated with the Futagawa and Hinagu faults. Our slip model suggests the main slip rupture a length of 32 km and terminated near the rim of the Aso caldera; additionally, the slip pattern delineated by three asperities that was dominated by right-lateral strike-slip with a minor normal slip component at a depth of 7 km. Furthermore, the main rupture triggered two secondary faults with right-lateral strike-slip and normal slip. Our source model yields a geodetic moment of 5.77 × 1019 N·m, which corresponds to Mw 7.11.

Ionospheric correction of ALOS‐2 full‐aperture ScanSAR interferometric data for surface deformation measurement in Beijing

Ionospheric disturbance is one of the most serious factors that limit the accuracy of deformation measurement of interferometric synthetic aperture radar (InSAR) especially for spaceborne SAR systems in low frequency such as L-band. Especially in imaging modes with a large coverage, ScanSAR mode for example, ionospheric disturbance phase screen almost submerges other useful geographical information. In this study, two ALOS-2 (L-band) full-aperture ScanSAR images (October 2015 and October 2016) are used to study ground deformation in Beijing from 2015 to 2016. The split range-spectrum technique is adopted to extract ionospheric distortion. The authors splice the differential interferograms of adjacent subswaths by resampling them to perform the overall ionospheric extraction. Also non-local filtering is used to improve the coherence of interferograms. Spatial deformation distribution that was obtained by differential interferometric synthetic aperture radar (D-InSAR) technique using the two ALOS-2 ScanSAR images is highly consistent with the result obtained by persistent scatterer interferometric synthetic aperture radar (PS-InSAR) technique using Sentinel Tops mode images (October 2015–February 2017). The comparison results show the effectiveness of the ionospheric distortion extraction method used in this study. Moreover, the spatial distribution is also highly consistent with the results before 2014, which means that the deformation trend in Beijing continues.

Investigations on the Coregistration of Sentinel-1 TOPS with the Conventional Cross-Correlation Technique

In Sentinel-1 TOPS mode, the antenna sweeps in the azimuth direction for the purpose of illuminating the targets with the entire azimuth antenna pattern (AAP). This azimuth sweeping introduces an extra high-frequency Doppler term into the impulse response function (IRF), which poses a more strict coregistration accuracy for the interferometric purpose. A 1/1000 pixel coregistration accuracy is required for the interferometric phase error to be negligible, and the enhanced spectral diversity (ESD) method is applied for achieving such accuracy. However, since ESD derives miscoregistration from cross-interferometric phase, and phase is always wrapped to [ − π , π ) , an initial coregistration method with enough accuracy is required to resolve the phase ambiguity in ESD. The mainstream for initial coregistration that meets this requirement is the geometrical approach, which accuracy mainly depends on the accuracy of orbits. In this article, the authors propose to investigate the feasibility of using the conventional coregistration approach, namely the cross-correlation-and-rigid-transformation, as the initial coregistration method. The aim is to quantify the coregistration accuracy for cross-correlation-and-rigid-transformation using the Cramér-Rao lower bound (CRLB) and determine whether this method could eventually help to resolve the phase ambiguities of ESD. In addition, we studied the feasibility and robustness of the cross-correlation plus ESD under different conditions. For validation, we checked whether the cross-correlation plus ESD approach could reach the same coregistration accuracy as geometrical plus ESD approach. In general, for large areas with enough coherence and little topography variance, the cross-correlation method could be used as an alternative to the geometrical approach. The interferogram from the two different approaches (with ESD applied afterward) shows a negligible difference under such circumstances.

Sentinel-1 Support in the GAMMA Software

First results using the new Sentinel-1 SAR look very promising but the special interferometric wide-swath data acquired in the TOPS mode makes InSAR processing more challenging than for normal stripmap mode data. The steep azimuth spectra ramp in each burst results in very stringent co-registration requirements. Combining the data of the individual bursts and sub-swaths into consistent mosaics requires careful “book-keeping” in the handling of the data and meta data and the large file sizes and high data throughputs require also a good performance. Considering these challenges good support from software is getting increasingly important. In this contribution we describe the Sentinel-1 support in the GAMMA Software, a high-level software package used by researchers, service providers and operational users in their SAR, InSAR, PSI and offset tracking work.

Correction to “Multichannel Azimuth Processing in ScanSAR and TOPS Mode Operation” [Jul 10 2994-3008

This correction refers to Eqs. (6), (7), and (19) in the above paper (ibid., vol. 48, no. 7, pp. 2994-3008, Jul. 2010).

First TerraSAR-X TOPS Mode Antenna Pattern Measurements Using Ground Receivers

The antenna model used for correcting the influence of the antenna pattern on synthetic aperture radar (SAR) images requires on-ground validation and in-flight verification. A methodology for the in-flight verification that is based upon the measurement of azimuth antenna patterns using ground receivers has been successfully demonstrated for the operational SAR modes of the TerraSAR-X (TSX) and TanDEM-X (TDX) missions. Recently, the novel (terrain observation by progressive scans) TOPS mode was for the first time implemented as an experimental mode on TerraSAR-X to demonstrate its feasibility in support of its implementation on ESA's Sentinel-1 mission. In this mode, besides scanning in elevation, the antenna beam is steered in flight direction from aft to the front at a constant rate to achieve an enhanced radiometric image performance. This paper discusses the methodology and presents results of the first in-flight antenna characterization of a SAR instrument operating in TOPS mode, in this case TerraSAR-X, using ground receivers. The results demonstrate that the TOPS one-way azimuth antenna pattern can be accurately modeled by the TSX antenna model indicating the general suitability of this approach for the in-flight antenna model verification during TOPS mode operations.

MIMO-TOPS MODE FOR HIGH-RESOLUTION ULTRA-WIDE-SWATH FULL POLARIMETRIC IMAGING

The novel TOPS mode can achieve wide swath imaging coverage at the cost of impaired azimuth resolution. MIMO-SAR systems combined with multi-channel SAR signal reconstruction in azimuth and digital beamforming (DBF) on receive in elevation can overcome the inherent contradiction between swath width and azimuth resolution of conventional SAR systems. This paper derives a novel spaceborne MIMO-TOPS mode for high-resolution ultra-wide-swath full polarimetric imaging. In such an imaging scheme, difierent polarimetric waveforms with difierent elevation beam pointing directions and short time delays are transmitted in a single pulse repetition interval (PRI) by difierent sub-aperture antennas in azimuth. Besides improving the desired signal-to-noise ratio (SNR) and suppressing ambiguous energy in elevation, a novel DBF on receive approach including two steps is adopted to separate difierent echoes corresponding to sub-pulses with difierent polarizations. The design example of a full polarimetric MIMO-TOPS SAR system, which allows for the imaging capacity to cover an ultra wide swath of 400km with a high azimuth resolution of 3m, is given to validate the proposed imaging scheme.

In silico prediction of central nervous system activity of compounds. Identification of potential pharmacophores by the TOPS–MODE approach

The central nervous system (CNS) activity has been investigated by using a topological substructural molecular approach (TOPS–MODE). A discriminant analysis to classify CNS and non-CNS drugs was developed on a data set (302 compounds) of great structural variability where more than 81% (247/302) were well classified. Randic’s orthogonalization procedures was carried out to allow the interpretation of the model and to avoid the collinearity among descriptors. The discriminant model was assessed by a leave-n-out (when n varies from 2 to 20) cross-validation procedure (79.94% of correct classification), an external prediction set composed by 78 CNS/non-CNS drugs (80.77% of correct classification) and a 5-fold full cross-validation (removing 78 compounds in each cycle, 80.00% of good classification). With this methodology was demonstrated that the hydrophobicity increase the CNS activity, while the dipole moment and the polar surface area decrease it; evidencing the capacity of the TOPS–MODE descriptors to estimate CNS activity for new drug candidates. The structural contributions to the CNS activity for two compounds are presented on the basis of fragment contributions. The model has also been able to identify potential structural pharmacophore, showing its possibilities in the lead generation and optimization processes.