Along-track Displacements Research Articles

Along-Track Displacement of Mw 7.8 and 7.6 Kahramanmaraş Earthquakes from Sentinel-1 Offset Tracking and Burst Overlap Interferometry

On 6th February, a pair of significant earthquakes with magnitudes of 7.8 and 7.6 struck Kahramanmaraş, Turkey. The Earthquake InSAR Data Provider (EIDP) promptly produced interferograms and offset tracking results within 3 hours of acquiring Sentinel-1 satellite data. However, it was challenging to unwrap the interferograms correctly due to high displacement gradient near the rupture. Hence, early displacement fields were derived from Pixel Offset Tracking (POT) method both in range and azimuth direction with low resolution depending on pixel sizes of Sentinel-1. We used the Burst Overlap Interferometry (BOI) method to extract the accurate along-track displacement and unwrapped the BOI interferogram using Azimuth Offset Tracking (AOT) data as a guide. Combining the unwrapped BOI interferogram and the AOT data, we derive a high-quality along-track displacement field that illuminates the entire earthquake rupture over 350 km and exhibits ±4 m displacement in the along-track direction.

Space debris remediation using space-based lasers

Centimetre and sub-centimetre debris fragments present a significant threat to many operational satellites in Earth orbit. When debris can be tracked, the preferred approach is collision avoidance to slightly alter the orbit of the at-risk satellite, resulting in an along-track displacement which compounds over several orbits. Although numerous strategies for active debris removal and remediation have been proposed, most such methods involve rendezvous maneuvers with targeted pieces of debris prior to some mechanical interaction. While such approaches are appropriate for removing larger objects which are potential sources of further debris, they are unsuitable for application to fragments which are too small to be accurately tracked from Earth and with which a rendezvous would be challenging.This paper investigates the use of photon pressure and ablation from space-based lasers to directly affect the orbits of small debris fragments, both to lower their orbits and lifetimes, and to reduce collision risk with operational satellites. A mission concept is studied which enables the cataloguing of debris shells of small fragments produced after breakup events, while also allowing the encountered fragments to be illuminated by a laser on board the observation satellite(s). The concept takes advantage of a laser’s ability to impart momentum at a distance by employing opportunistic interaction with passing fragments, assuming no prior knowledge of their orbits and no ability to rendezvous. The impact of the mission over a 10-year operational lifetime is simulated statistically by analysing the dynamics of typical encounters and the achievable impact on fragment orbits. The effects of off-beam-axis components of the applied ΔV and its implications for beam tracking are also investigated, along with the effects of extreme laser fluence on fragment attitude.

Modification of the line-of-sight motion error for squinted SAR after real-time PRF adjustment

The motion error of a multi-rotor unmanned aerial vehicle (UAV) leads to severe image blurring for mini synthetic aperture radar (SAR) systems. The along-track displacement error usually is compensated in advance through real-time pulse repetition frequency (PRF) adjustment. And then, only the cross-track displacement error is considered in traditional motion compensation (MOCO) algorithms. However, the instantaneous range expression based on the geometry of a squinted SAR shows that the residual along-track displacement error after PRF adjustment cannot be ignored, especially for mini-SAR systems. Therefore, the line-of-sight (LOS) motion error is modified by introducing the residual along-track displacement error. And then, the first and second-order factors for the LOS MOCO are redefined. The experimental results show that the proposed method can reduce the LOS motion error and significantly improve the imaging performance of the squinted SAR mounted on a multi-rotor UAV.

Towards big SAR data era: An efficient Sentinel-1 Near-Real-Time InSAR processing workflow with an emphasis on co-registration and phase unwrapping

Sentinel-1 Near-Real-Time (NRT) processing embraces great potential in its applications to a wide range of research topics. Towards higher quality NRT products, it is crucial to avoid data reprocessing and to dynamically update InSAR time series. Although state-of-the-art techniques, enabled by high-performance computers and high-capacity storage platforms, make the highly multi-looked data available online quickly, the technique and procedure for efficiently obtaining relatively high resolution NRT data for a specified region are still lacking. Here we propose a workflow in a review way towards efficient and high-resolution Sentinel-1 NRT InSAR processing. In addition, we are committed to addressing issues that are often overlooked but indispensable in NRT data processing: co-registration and phase unwrapping. Through this workflow, not only NRT results in the radar Line-of-Sight (LOS) direction can be obtained, but also along-track NRT observations in overlap regions of images can be achieved. Unlike deriving LOS time series displacements which requires additional operations after co-registration, along-track displacements, a by-product of co-registration, can be directly obtained without further processing. We test and validate the proposed workflow in three regions with representative geomorphological processes including volcanic activity, city subsidence, and landslides. Experimental results demonstrate that our workflow can perform NRT monitoring at a relatively high resolution and a relatively low computational and storage “burden”. The time efficiency is increased by ∼ 65% and the storage space is saved by ∼ 21.8%.

Unambiguous Recovery of Multistatic SAR Data for Nonzero Cross Track Baseline Case

A wide-swath synthetic aperture radar (SAR) image is known to be achievable by merging multistatic aliased data that is collected with an appropriate along-track displacement. However, in terms of both flexibility requirements like potentiality of both along-track and cross-track (XT) interferometry and design requirements like orbital safety, etc., a XT baseline is not only demanded but also unavoidable. In this work, a method is proposed in order to merge SAR data to recover the Doppler spectrum unambiguously for the nonzero XT baseline case. The algorithm is compared and validated by simulating an azimuth invariant multistatic SAR satellite constellation. The results are promising in terms of dealing with XT baselines.

High-quality three-dimensional displacement fields from new-generation SAR imagery: application to the 2017 Ezgeleh, Iran, earthquake

Mapping the three-dimensional (3D) displacement fields associated with a variety of geological phenomena has been widely performed by exploiting synthetic aperture radar (SAR) imagery, as the result is important for providing insight into the formation mechanisms and potential risks of geological hazards. New-generation SAR sensors, namely ALOS-2 and Sentinel-1, can capture surface deformation with a high coherence in wide-swath mode, thereby providing outstanding across-track displacement accuracies; however, this improvement partially sacrifices the azimuth resolution, which affects the retrieval of 3D surface deformation fields. To explore the feasibility of generating 3D deformation maps with new SAR imagery, we collect two pairs of ALOS-2 ScanSAR and four pairs of Sentinel-1 Terrain Observation by Progressive Scans (TOPS) images for the 12 November 2017 Ezgeleh earthquake. Furthermore, the differential interferometric SAR (DInSAR), pixel offset tracking (POT), multiple-aperture InSAR (MAI), and burst-overlap interferometry (BOI) methods are used to measure the across- and along-track displacements. Compared with the POT and MAI methods, the integration of DInSAR and BOI measurements provides high-quality 3D deformation maps with an accuracy of 4 cm, which is four times and two times better than the accuracies of the POT and MAI methods integrated with DInSAR, respectively. In addition, a significant north–south displacement of 0.76 m is found in our 3D deformation results that was underestimated in the slip distribution model constrained with seismic waveforms or InSAR measurements. Our 3D deformation map of the 2017 Ezgeleh earthquake indicates a southwestward horizontal motion and an upward motion without any corresponding surface rupture that effectively match the behavior of a blind rupture along a northeast-dipping reverse fault. We conclude that combining BOI with DInSAR would provide a better 3D deformation field and should be applied to study future earthquakes.

COSEISMIC DISPLACEMENT ANALYSIS OF THE 12 NOVEMBER 2017 MW 7.3 SARPOL-E ZAHAB (IRAN) EARTHQUAKE FROM SAR INTERFEROMETRY, BURST OVERLAP INTERFEROMETRY AND OFFSET TRACKING

Abstract. Interferometric wide-swath mode of Sentinel-1, which is implemented by Terrain Observation by Progressive Scan (TOPS) technique, is the main mode of SAR data acquisition in this mission. It aims at global monitoring of large areas with enhanced revisit frequency of 6 days at the expense of reduced azimuth resolution, compared to classical ScanSAR mode. TOPS technique is equipped by steering the beam from backward to forward along the heading direction for each burst, in addition to the steering along the range direction, which is the only sweeping direction in standard ScanSAR mode. This leads to difficulty in measuring along-track displacement by applying the conventional method of multi-aperture interferometry (MAI), which exploits a double difference interferometry to estimate azimuth offset. There is a possibility to solve this issue by a technique called “Burst Overlap Interferometry” which focuses on the region of burst overlap. Taking advantage of large squint angle diversity of ~1° in burst overlapped area leads to improve the accuracy of ground motion measurement especially in along-track direction. We investigate the advantage of SAR Interferometry (InSAR), burst overlap interferometry and offset tracking to investigate coseismic deformation and coseismic-induced landslide related to 12 November 2017 Mw 7.3 Sarpol-e Zahab earthquake in Iran.

Sentinel-1 TOPS interferometry for along-track displacement measurement

The European Space Agency’s Sentinel-1 mission, a constellation of two C-band synthetic aperture radar (SAR) satellites, utilizes terrain observation by progressive scan (TOPS) antenna beam steering as its default operation mode to achieve wide-swath coverage and short revisit time. The beam steering during the TOPS acquisition provides a means to measure azimuth motion by using the phase difference between forward and backward looking interferograms within regions of burst overlap. Hence, there are two spectral diversity techniques for along-track displacement measurement, including multi-aperture interferometry (MAI) and “burst overlap interferometry”. This paper analyses the measurement accuracies of MAI and burst overlap interferometry. Due to large spectral separation in the overlap region, burst overlap interferometry is a more sensitive measurement. We present a TOPS interferometry approach for along-track displacement measurement. The phase bias caused by azimuth miscoregistration is first estimated by burst overlap interferometry over stationary regions. After correcting the coregistration error, the MAI phase and the interferometric phase difference between burst overlaps are recalculated to obtain along-track displacements. We test the approach with Sentinel-1 TOPS interferometric data over the 2015 Mw 7.8 Nepal earthquake fault. The results prove the feasibility of our approach and show the potential of joint estimation of along-track displacement with burst overlap interferometry and MAI.

Simulation of the SuperSAR Multi-Azimuth Synthetic Aperture Radar Imaging System for Precise Measurement of Three-Dimensional Earth Surface Displacement

The SuperSAR imaging system, a novel multi-azimuth synthetic aperture radar (SAR) system capable of detecting Earth surface deformation in three dimensions from a single satellite platform, has recently been proposed. In this paper, we investigate the feasibility of detecting precise 3-D surface displacement measurements with the SuperSAR imaging system using a point target simulation. From this simulation, we establish both a relationship between the interferometric SAR phase and the across-track displacement and a relationship between the multiple-aperture interferometry phase and the along-track displacement based on the SuperSAR imaging geometry. The theoretical uncertainties of the SuperSAR measurement are analyzed in the across- and along-track directions, and the theoretical accuracy of the 3-D displacement measurement from the SuperSAR system is also investigated according to both the decorrelation and the squint and look angles. In the case that the interferometric coherence is about 0.8 and that five effective looks are employed, the theoretical 2-D measurement precision values are about 3.67 and 6.35 mm in the across- and along-track directions, respectively, and the theoretical 3-D measurement precision values for 3-D displacement are about 4.05, 4.56, and 3.45 mm in the east, north, and up directions, respectively. The result of this study demonstrates that the SuperSAR imaging system is capable of measuring the 3-D surface displacement in all directions with subcentimeter precision.

An integrated method based on DInSAR, MAI and displacement gradient tensor for mapping the 3D coseismic deformation field related to the 2011 Tarlay earthquake (Myanmar)

Abstract The satellite differential interferometric synthetic aperture radar (DInSAR) technology has been widely applied for mapping the ground deformation associated with the geophysical events such as earthquakes. However, the conventional DInSAR can measure only the one-dimensional (1D) ground displacement along the radar line of sight (LOS), and the crucial displacement measurements, e.g., near the epicenter and along the surface ruptures, are usually not available or degraded in quality due to the significant deformation gradients. With availability of satellite SAR images acquired along ascending and descending orbits, this paper proposes an integrated method to map the three-dimensional (3D) coseismic deformation field by combining DInSAR for detecting LOS displacements, the multiple aperture interferometry (MAI) for detecting along-track displacements, and the displacement gradient tensor (DGT) model for characterizing spatial correlation of ground displacements. The proposed method (termed as InSAR–DGT) was first tested through an experiment of recovering 3D displacements from a simulated coseismic deformation field. We then applied the proposed method to map the 3D coseismic deformation field related to the 2011 Tarlay earthquake (Myanmar) by using the ascending and descending ALOS PALSAR images. Both the results derived for the simulated experiment and the 2011 Tarlay earthquake show that the quality of the 3D coseismic displacements can be raised efficiently by the InSAR–DGT method, and the precisions in the east–west (E–W), north–south (N–S) and up–down (U–D) displacements for the 2011 Tarlay earthquake are increased 22%, 36% and 24%, respectively. The validation indicates that the improved coseismic deformation field for the 2011 Tarlay earthquake is in good agreement with the deformation field simulated from the existing optimized fault model. The number of the missing data points in the 3D coseismic deformation field can be reduced significantly by the InSAR–DGT method. It is revealed that the causative fault (i.e., the west part of the Nam Ma Fault) for the Tarlay earthquake generated a left-lateral slip that was accompanied by a minor normal dip-slip component. The careful interpretation demonstrates that the causative fault structures include the determined main fault segment and the two suspected branched segments at east of the Tarlay town.

Measurement of slow-moving along-track displacement from an efficient multiple-aperture SAR interferometry (MAI) stacking

Multiple-aperture SAR interferometry (MAI) has demonstrated outstanding measurement accuracy of along-track displacement when compared to pixel-offset-tracking methods; however, measuring slow-moving (cm/year) surface displacement remains a challenge. Stacking of multi-temporal observations is a potential approach to reducing noise and increasing measurement accuracy, but it is difficult to achieve a significant improvement by applying traditional stacking methods to multi-temporal MAI interferograms. This paper proposes an efficient MAI stacking method, where multi-temporal forward- and backward-looking residual interferograms are individually stacked before the MAI interferogram is generated. We tested the performance of this method using ENVISAT data from Kīlauea Volcano, Hawai‘i, where displacement on the order of several centimeters per year is common. By comparing results from the proposed stacking methods with displacements from GPS data, we documented measurement accuracies of about 1.03 and 1.07 cm/year for the descending and ascending tracks, respectively—an improvement of about a factor of two when compared with that from the conventional stacking approach. Three-dimensional surface-displacement maps can be constructed by combining stacked InSAR and MAI observations, which will contribute to a better understanding of a variety of geological phenomena.

MIMO Distributed Imaging of Rotating Targets for Improved 2-D Resolution

This letter deals with the multiple-input-multiple-output distributed radar imaging of rotating targets. The distributed system is based on formation-flying platforms, which can be configured with proper cross-track and along-track displacements. The platforms carry active radar systems transmitting almost orthogonal waveforms; exploiting both monostatic and bistatic acquisitions, range and cross-range resolution improvement can be achieved, and a maximum theoretical 2-D resolution cell improvement factor can be obtained significantly greater than the number of flying platforms. The required distributed focusing technique is developed, and its effectiveness and robustness is tested against simulated data. The proposed distributed system could be suitable to the application of ship target imaging, using a reconfigurable formation of platforms, for maritime surveillance in wide sea areas.

Theoretical accuracy of along-track displacement measurements from multiple-aperture interferometry (MAI).

The measurement of precise along-track displacements has been made with the multiple-aperture interferometry (MAI). The empirical accuracies of the MAI measurements are about 6.3 and 3.57 cm for ERS and ALOS data, respectively. However, the estimated empirical accuracies cannot be generalized to any interferometric pair because they largely depend on the processing parameters and coherence of the used SAR data. A theoretical formula is given to calculate an expected MAI measurement accuracy according to the system and processing parameters and interferometric coherence. In this paper, we have investigated the expected MAI measurement accuracy on the basis of the theoretical formula for the existing X-, C- and L-band satellite SAR systems. The similarity between the expected and empirical MAI measurement accuracies has been tested as well. The expected accuracies of about 2–3 cm and 3–4 cm (γ = 0.8) are calculated for the X- and L-band SAR systems, respectively. For the C-band systems, the expected accuracy of Radarsat-2 ultra-fine is about 3–4 cm and that of Sentinel-1 IW is about 27 cm (γ = 0.8). The results indicate that the expected MAI measurement accuracy of a given interferometric pair can be easily calculated by using the theoretical formula.

Feasibility of Along-Track Displacement Measurement From Sentinel-1 Interferometric Wide-Swath Mode

The European Space Agency's Sentinel-1, a C-band imaging radar mission to be launched in mid-2013, will provide a continuity of radar data for monitoring the changing Earth. The azimuth resolution of Sentinel-1's background mode, interferometric wide-swath (IW) mode, is four times lower than that of European remote-sensing satellite (ERS) and Envisat systems. Therefore, the measurement accuracy of along-track displacement from Sentinel-1 IW images presumably will be significantly reduced. In this paper, we test the feasibility of along-track displacement measurement from Sentinel-1 IW mode. We simulate Sentinel-1 IW synthetic aperture radar (SAR) images from the ERS raw data that captured the coseismic deformation of the 1999 Hector Mine earthquake in California. Along-track displacement maps are generated using multiple-aperture interferometric SAR (MAI) and intensity tracking techniques, respectively, and are compared with GPS measurements. The root-mean-square (rms) error between the synthetic Sentinel-1 MAI and GPS measurements is about 9.6 cm, which corresponds to only 0.5 % of the azimuth resolution. The rms error between the along-track displacements from synthetic Sentinel-1 offset tracking and GPS is about 27.5 cm, which is about 1.4 % of the azimuth resolution. These results suggest that the MAI method will still be useful to measure along-track displacements from Sentinel-1 IW InSAR imagery and that it would be difficult to effectively measure the along-track displacements by the Sentinel-1 offset tracking method.