Shiyong Yan

Mountain glacier displacement estimation using a DEM-assisted offset tracking method with ALOS/PALSAR data

Mountain glaciers, as a sensitive indicator of climate change, are often monitored with all-weather synthetic aperture radar (SAR) technology. In contrast to the differential interferometric SAR approach, which is affected by decorrelation, the offset tracking method is a promising alternative that produces glacier displacement fields directly. The method, however, is often influenced by additional offsets associated with topographic effects in rugged mountain areas. For removing these topographic-related offsets, we present a digital elevation model-(DEM-) assisted offset tracking procedure and employ it with the Advanced Land Observing Satellite (ALOS) Phased Array type L-band SAR (PALSAR) data of Mt. Muztagh Ata. The results demonstrate the advantages of this method by reducing the topographical effects and increasing the accuracy (0.98 m). The approach proposed in this study can increase the accuracy of the results for estimated mountain glacier displacement and also extend the applicable range of the offset tracking method.

Accurate Determination of Glacier Surface Velocity Fields with a DEM-Assisted Pixel-Tracking Technique from SAR Imagery

We obtained accurate, detailed motion distribution of glaciers in Central Asia by applying digital elevation model (DEM) assisted pixel-tracking method to L-band synthetic aperture radar imagery. The paper firstly introduces and analyzes each component of the offset field briefly, and then describes the method used to efficiently and precisely compensate the topography-related offset caused by the large spatial baseline and rugged terrain with the help of DEM. The results indicate that the rugged topography not only forms the complex shapes of glaciers, but also affects the glacier velocity estimation, especially with large spatial baseline. The maximum velocity, 0.85 m∙d−1, was observed in the middle part on the Fedchenko Glacier, which is the world’s longest mountain glacier. The motion fluctuation on its main trunk is apparently influenced by mass flowing in from tributaries, as well as angles between tributaries and the main stream. The approach presented in this paper was proved to be highly appropriate for monitoring glacier motion and will provide valuable sensitive indicators of current and future climate change for environmental analysis.

Large deformation monitoring over a coal mining region using pixel-tracking method with high-resolution Radarsat-2 imagery

ABSTRACT Differential synthetic aperture radar interferometry (D-InSAR) is limited when exploited in high-intensity mining areas, because large deformation gradients lie beyond the maximum measurable value of the D-InSAR technique which breaks the prerequisite for successfully employing of the method. The SAR amplitude-based pixel-tracking method provides an alternative way to efficiently and robustly extract the large deformation distribution particularly when the D-InSAR technique is limited by loss of coherence. In addition, the deformation in the line-of-sight direction and the deformation along the azimuth direction are also presented in this paper with 24-day interval repeat-pass high-resolution Rardarsat-2 imagery. Combining both of these techniques can help to better understand the deformation mechanisms associated with underground mining activities. The accuracies of 0.12 m in slant-range direction and 0.19 m in the azimuth direction were achieved, respectively. Besides, the profiles across the maximum deformation region have verified that the deformation occurred during two acquisition periods is far beyond the theoretical maximum deformation gradient corresponding to high-resolution C-band SAR data. The obtained surface motion infers to the mining activities and assessed damage caused by the large deformation.

Modified four-pass differential SAR interferometry for estimating mountain glacier surface velocity fields

ABSTRACT Conventional four-pass differential synthetic aperture radar interferometry (DInSAR) assumes that there are no significant changes in the ground during the period between the acquisition times of SAR images for topographic DInSAR pairs. This assumption can rarely be satisfied for glacial areas due to their continuous movement. This letter proposes a modified four-pass DInSAR method without an external digital elevation model (DEM), taking into account glacier movement between the acquisition times of SAR images used to form topographic DInSAR pairs. An explicit expression of theoretical formulas for a modified four-pass DInSAR technique was derived for the first time, revealing that four-pass DInSAR considering ground movement of topographic pairs was equivalent to that of conventional four-pass DInSAR with a spatially varying nominal wavelength. Then the proposed method was tested with four Advanced Land Observing Satellite (ALOS) SAR images covering Dongkemadi glacier located on the Tibetan Plateau, China. An experiment with real data showed that the proposed method could obtain glacial flow patterns efficiently, and that the difference between two-pass DInSAR and the proposed method is a result of DEM bias and glacial thinning. The approach presented in this letter proved to be appropriate for monitoring glacial motion and provides a valuable tool for glacier studies, without the need of an external DEM.

Deriving Ice Motion Patterns in Mountainous Regions by Integrating the Intensity-Based Pixel-Tracking and Phase-Based D-InSAR and MAI Approaches: A Case Study of the Chongce Glacier

As a sensitive indicator of climate change, mountain glacier dynamics are of great concern, but the ice motion pattern of an entire glacier surface cannot be accurately and efficiently generated by the use of only phase-based or intensity-based methods with synthetic aperture radar (SAR) imagery. To derive the ice movement of the whole glacier surface with a high accuracy, an integrated approach combining differential interferometric SAR (D-InSAR), multi-aperture interferometry (MAI), and a pixel-tracking (PT) method is proposed, which could fully exploit the phase and intensity information recorded by the SAR sensor. The Chongce Glacier surface flow field is estimated with the proposed integrated approach. Compared with the traditional SAR-based methods, the proposed approach can determine the ice motion over a widely varying range of ice velocities with a relatively high accuracy. Its capability is proved by the detailed ice displacement pattern with the average accuracy of 0.2 m covering the entire Chongce Glacier surface, which shows a maximum ice movement of 4.9 m over 46 days. Furthermore, it is shown that the ice is in a quiescent state in the downstream part of the glacier. Therefore, the integrated approach presented in this paper could present us with a novel way to comprehensively and accurately understand glacier dynamics by overcoming the incoherence phenomenon, and has great potential for glaciology study.

Reconstructing the Vertical Component of Ground Deformation from Ascending ALOS and Descending ENVISAT Datasets—A Case Study in the Cangzhou Area of China

Abstract. Due to the regular revisit periods of SAR platforms and the decorrelation of SAR images, the estimation of ground deformation retrieved from a particular SAR dataset may be of low resolution in both spatial and temporal dimensions. To enhance the spatial and temporal sampling rates in ground deformation estimation, a multiplatform SAR dataset combination strategy is proposed, based on the Multi-Temporal InSAR (MTInSAR) technique and the Same Name target point Pairs (SNPs) combination approach. The combination strategy has been tested and applied in the Cangzhou area, which has suffered from ground subsidence due mainly to groundwater extraction. The derived results show that the deformation monitoring sampling rates can be improved in the space and time dimensions when compared to the results obtained from a single-platform SAR dataset, which makes the multiplatform SAR dataset combination strategy useful for uneven (spatially variable) and nonlinear (temporally variable) deformation detection. Moreover, the results derived from the combination strategy tend to be more reliable and have fewer noise points.

Glacier surface velocity estimation in the West Kunlun Mountain range from L-band ALOS/PALSAR images using modified synthetic aperture radar offset-tracking procedure

Abstract Glacier movement is closely related to changes in climatic, hydrological, and geological factors. However, detecting glacier surface flow velocity with conventional ground surveys is challenging. Remote sensing techniques, especially synthetic aperture radar (SAR), provide regular observations covering larger-scale glacier regions. Glacier surface flow velocity in the West Kunlun Mountains using modified offset-tracking techniques based on ALOS/PALSAR images is estimated. Three maps of glacier flow velocity for the period 2007 to 2010 are derived from procedures of offset detection using cross correlation in the Fourier domain and global offset elimination of thin plate smooth splines. Our results indicate that, on average, winter glacier motion on the North Slope is 1     cm / day faster than on the South Slope—a result which corresponds well with the local topography. The performance of our method as regards the reliability of extracted displacements and the robustness of this algorithm are discussed. The SAR-based offset tracking is proven to be reliable and robust, making it possible to investigate comprehensive glacier movement and its response mechanism to environmental change.

Monitoring Recent Urban Expansion and Urban Subsidence of Beijing Using ENVISAT/ASAR Time Series Datasets

With worldwide economic development and population increases, urban areas create significant stresses on the local, regional and global environment. Information about the spatial and temporal dynamics of the characteristics of urban areas is therefore needed to support sustainable urban development. Time series earth observation data obtained using radar satellites have provided effective data sources for monitoring urban areas. This chapter first describes the development of synthetic aperture radar as well as its important role in the detection and monitoring of urban areas. Then, the fundamental principle of time series radar data in monitoring urban areas is introduced and discussed. Next, to demonstrate the capacity of time series SAR (Synthetic Aperture Radar) imagery for monitoring urban areas using ENVISAT/ASAR (Environmental Satellite /Advanced Synthetic Aperture Radar) time series radar data, Beijing city in China was selected as a test site. Beijing has all of the typical problems of a megacity such as resource, environment and population problems arising from rapid urban expansion during recent decades. A C5.0 rulesets classifier and the Multi Temporal Interferometric Synthetic Aperture Radar (MTInSAR) method were used to map the urban expansion and the millimeter level urban subsidence, respectively and the results were validated via high resolution WorldView optical datasets and leveling benchmark measurement, respectively. The results demonstrate the effectiveness and high accuracy of the time series radar data for monitoring urban areas. Furthermore, the spatial-temporal characteristic of urban expansion and urban subsidence of Beijing city were analyzed. Finally, the mechanisms or driving factors for urban expansion and subsidence are addressed based on economic development, population growth and the impacts of recent Beijing government policy.

Fluctuations and movements of the Kuksai Glacier, western China, derived from Landsat image sequences

Abstract Nine Landsat thematic mapper/enhanced thematic mapper (TM/ETM)+ images from 1998 to 2010 were analyzed to detect variations in the Kuksai Glacier of Mt. Muztagh Ata, western China. The velocities of glacial movement were quantified using the normalized cross-correlation (NCC) method. The surface debris cover of the glacier makes automated glacier outline mapping difficult, but provides useful features for monitoring glacier movement with the NCC method. Six displacement maps of the Kuksai Glacier, with an accuracy of 7 m, were derived from the band 3 of Landsat images. The NCC method is proven to be very effective in monitoring the activity of debris-covered glaciers. The results indicate that the velocity of the Kuksai Glacier is high in the upper portion and decreases downstream. For most of the years studied, the variability in the glacier movements in the middle and upper parts of the glacier, especially at 9 to 16 km upstream from the glacier terminal, is much larger than that in the downstream part. This study demonstrates that glacial movements can be routinely monitored using Landsat images, providing an input to and an opportunity for the detailed study of glacier dynamics.

Kekesayi glacier velocity extraction based on the offsets derived from SAR images

The velocity of glacier is the most important parameter in the study of glaciers and remote sensing is a powerful tool to calculate their surface velocities. Due to persistent cloud cover in this region, it is impossible to acquire enough optical images to provide measurements. However, measurement of the offsets between two SAR images is an effective way to determine surface velocity. In order to do this, offsets both in slant range and azimuth directions are derived from two SAR images. The movement of the glacier during the SAR data acquisition time is calculated after the global part of offsets has been removed by the polynomial fit method. The offsets used for removing the global part are selected on the basis of the Single-to-Noise ratio (SNR) and correlation in area without glaciers but with large topographic changes. The surface velocity of the whole glacier using SAR data will make a significant contribution to the study of glacier dynamics. The Kekesayi glacier can be divided into four parts, based on the velocity map. The results show that the surface velocity of the Kekesayi glacier is different on the different part of the glacier, and offset measurements are an effective method for the study of glaciers.