Stripmap Mode Research Articles

Evaluation of RISAT-1 SAR Radiometric Calibration Using Extended Amazon Rainforest

The single-frequency Radar Imaging Satellite-1 (RISAT-1) Synthetic Aperture Radar (SAR) (launched in April, 2012) is designed to provide a wide variety of imaging options compared to the prevailing SAR missions. The scale of the calibration task for the RISAT-1 mission is increased with the inclusion of multiple beams and polarizations (single, dual and quad) along with circular/hybrid polarization capabilities, for the first time from space. Significant efforts have been made in the evolution of SAR processor for the provision of radiometrically and geometrically calibrated products to the users. After the commissioning phase, RISAT-1 was put into routine operations in Oct. 2012. Since then extensive beam coverage of Amazon rainforest in various modes was carried out to evaluate the calibration parameters and SAR processor performance. Subsequently, the processor version 1.2.0 was improved to version 1.3.0 and with further modifications to version 1.4.0. This article details the analysis carried out at each processor version, by quality evaluation team, to quantify the parameter behaviour and to give feedback on the processor performance to the SAR processor team. The various radiometric quality parameters evaluated and monitored are shape of the antenna pattern, gamma-naught, backscattering coefficient (sigma-naught), speckle index, radiometric resolution and Noise Equivalent Sigma Zero (NESZ). Data studied for the analysis is in Single Look Complex (SLC) for stripmap mode and multilook (Ground range) for ScanSAR and stripmap mode in dual and circular polarizations. The performance of version 1.4.0 for all the beams (near-to-far) is found to be consistent and meeting the mission specifications for the product.

Synthetic aperture radar image of fractal rough surface

The synthetic aperture radar imaging of fractal rough surface is studied. The natural surface can be very accurately described in terms of fractal geometry. Such a two-dimensional fractional Brownian motion (FBM) stochastic process provides a very sound description of natural surface. The samples of band-limited FBM process are realized by using physical Weierstrass-Mandelbrot function. The parameters of fractal rough surface are discussed and how to choose the value is analyzed. The roughness is mostly determined by the Hurst coefficient or the fractal dimension. In the actual simulation, a fractal rough surface can be seen as the superposition of finite sinusoidal tones, and any scattering measurement is limited to a finite set of scales. In this paper, the surface is described with two-scale model, i. e., locally approximated by plane facets with dimension smaller than that of resolution cells, but much larger than wavelength. Because this paper focuses on the texture of the synthetic aperture radar (SAR) image and the overall image texture is related to the macroscopic scale, the microscopic roughness superimposed on the facets is neglected. For the macroscopic scale scattering problem, a facet Kirchhoff approach is proposed. The fractal rough surface consists of many triangle facets, and the scatter field of each facet can be obtained by the facet Kirchhoff approach. The principle of dimension selection is studied. The dimension of facet must follow the principle that the surface profile is not damaged. At the same time, the facet dimension should be as large as possible in order to increase the efficiency of imaging. After establishing the fractal geometry model and obtaining the field from each facet, the SAR image can be realized through Rang-Doppler method in the stripmap mode. The results show that in the SAR image, the effects of fractal parameters on the rough surface can be obviously observed. The peaks and ravines of rough surface are obviously observed at low fractal dimension or high Hurst coefficient. However, when the fractal dimension gets higher or Hurst coefficient gets higher, the peaks and ravines disappear because the surface becomes rougher and diffuse scattering is enhanced. The effect of fractal parameter on the SAR image can be specifically expressed with entropy and angle second moment. With the increase of fractal dimension D, the texture of SAR image behaves more randomly and disorderly. So the entropy of SAR image becomes larger and the angle second moment of SAR image becomes smaller. The texture of SAR image is also related to the squint angle and frequency of incidence wave. The relative roughness will become larger when the squint angle and frequency of incidence wave become larger. The research on a complete fractal surface SAR imaging system consists of establishing the environmental model, developing the electromagnetic scattering model, and using the SAR imaging technique. The achievements show the characteristics of fractal rough surface SAR image, which have a theoretical support for natural environment remote sensing and the environment parameters inversion.

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.

Mapping and characterization of land subsidence in Beijing Plain caused by groundwater pumping using the Small Baseline Subset (SBAS) InSAR technique

Abstract. InSAR time series analysis is widely used for detection and monitoring of slow surface deformation. In this paper, 15 TerraSAR-X radar images acquired in stripmap mode between 2012 and 2013 are processed for land subsidence monitoring with the Small Baseline Subset (SBAS) approach in Beijing Plain in China. Mapping results produced by SBAS show that the subsidence rates in the area of Beijing Plain range from −97.5 (subsidence) and to +23.8 mm yr−1 (uplift), relative to a presumably stable benchmark. The mapping result also reveals that there are the five subsidence centers formed by surface deformation spreading north to south east of the downtown. An uneven subsidence patten was detected near the Beijing Capital International Airpor, which may be related to loading of buildings and the aircraft.

Sentinel-1A Product Geolocation Accuracy: Commissioning Phase Results

Sentinel-1A (S1A) is an Earth observation satellite carrying a state-of-the-art Synthetic Aperture Radar (SAR) imaging instrument. It was launched by the European Space Agency (ESA) on 3 April 2014. With the end of the in-orbit commissioning phase having been completed at the end of September 2014, S1A data products are already consistently providing highly accurate geolocation. StripMap (SM) mode products were acquired regularly and tested for geolocation accuracy and consistency during dedicated corner reflector (CR) campaigns. At the completion of this phase, small geometric inconsistencies had been understood and mitigated, with the high quality of the final product geolocation estimates reflecting the mission’s success thus far. This paper describes the measurement campaign, the methods used during geolocation estimation, and presents best estimates of the product Absolute Location Error (ALE) available at the beginning of S1A’s operational phase.

Phenological tracking og agricultural feilds investigated by using dual polarimetry tanDEM-X images

Abstract. Remote sensing plays a key role in monitoring and assessing environmental changes. Because of its special imaging characteristics such as high-resolution, capabilities to obtain data in all weather conditions and sensitivity to geometrical and dielectric properties of the features, Synthetic Aperture Radar (SAR) technology has become a powerful technique to detect small scale changes related to earth surface.SAR images contain the information of both phase and intensity in different modes like single, dual and full polarimetric states which are important in order to extract information about various targets. In this study we investigate phenological changes in an agricultural region using high-resolution X-band SAR data. The case study is located in Doroud region of Lorestan province, west of Iran. The purpose is to investigate the ability of copolar and interferometric coherence extracted from TanDEM-X dual polarimetry (HH/VV) in bistatic StripMap mode for tracking the phenological changes of crops during growing season. The data include 11 images acquired between 12.06.2012 and 02.11.2012 and 6 images acquired between 30.05.2013 and 04.08.2013 in the CoSSC format. Results show that copolar coherence is almost able to follow phenological changes but interferometric coherence has a near constant behaviour with fluctuations mainly related to baseline variations.

Landslide deformation monitoring using point-like target offset tracking with multi-mode high-resolution TerraSAR-X data

Many landslides in the Three Georges area have shown active deformations associated with water level fluctuations since the full operation of the Three Georges Dam. Such active deformations of landslide bodies need to be closely monitored for disaster prevention and warning. One way to do this is to employ SAR pixel offset tracking, a powerful technique that can be used for measuring two-dimensional large displacements. As an improvement of the original pixel offset tracking technique, the point-like target (PT) offset tracking method focuses on stable point-like targets and thus can obtain more reliable measurements. In this paper, the PT offset tracking method is employed to investigate historical evolution of the Shuping landslide, using time series TerraSAR-X data acquired in both Stripmap (SM) and High-resolution Spotlight (HS) modes. Artificial corner reflectors (CR) installed at/near the landslide are identified as PTs and used to analyze the spatial–temporal pattern of landslide deformations. Results showed that the maximum accumulative deformation of the Shuping landslide reached more than 1.5m over a time span of two years. A correlation analysis between the deformation trends and the fluctuation of reservoir water level indicated that most deformations of the landslide happened during the water level declining period.

On the Detection of Oil Spill with China's HJ-1C SAR Image

This paper focuses on the analysis and evaluation of the China's HJ-1C SAR satellite with respect to oil spill identification. Six HJ-1C SAR images (S band, VV Pol, StripMap mode, 5 m resolution, Level-2) in a track in December 2012, covering the coastal region of Fujian Province of China, are used in this study.A comparison of the detectability of oil spill in between HJ-1C and Envisat ASAR images was investigated by introducing the Damping Coefficient s based on SAR intensity image.The difference of s was -1.3dB between the two oil spill samples considered in this paper revealed an expected result in accordance with visual inspection of themselves.

Extension of Wavenumber Domain Focusing for spotlight COSMO-SkyMed SAR Data

In this work we describe a method to handle curved orbits in wavenumber domain focusing algorithm for high-resolution SAR data acquired by Low Earth Orbit satellites using spotlight mode. The standard wavenumber domain focusing algorithm make assumptions that start to be invalid when applied to high-resolution spotlight SAR data acquired in spaceborne low Earth orbit (LEO) configurations. The assumption of a hyperbolic range history is no longer accurate for sub-metric spatial resolutions due to the satellite curved orbit. The proposed method is used to estimate the satellite velocity and closest-approach range distance of the rectified effective orbit which minimizes the phase errors over the whole scene coverage. This allows to use all frequency domain focusing kernels developed to focus SAR images acquired with a stripmap mode. We use this method with the ω- k algorithm, and its approximation Fast ω-k (Fω-k), demonstrating its effectiveness to focus COSMO-SkyMed (CSK) SAR images obtaining, respectively, sub-metric and metric azimuth spatial resolution.

X-band SAR 위성영상의 3차원 위치결정 기법 평가 - 레이더 간섭기법, Radargrammetry, RPC를 중심으로

Korea`s first X-band SAR satellite KOMPSAT-5 has been launched in 2013, so the research related to the X-band SAR satellite image is required to increase the utilization of KOMPSAT-5. In this study, we generated a DEM(Digital Elevation Model) using X-band SAR satellite images based on three methods which are InSAR, radargrammetry and RPC(Rational Polynomial Coefficients), and evaluated the performance of each methods. The four stripmap mode TerraSAR-X images taken in Daejeon were used to generate DEM, and accuracy was evaluated using DEM by IKONOS RPC. As results, DEM produced by the InSAR showed the highest accuracy. Also, we knew that RPC could be effective method if you want to create a large area DEM which contains the various elevation.

A New Imaging Method for Circular Trace Scanning SAR with Wide Observation

Circular Trace Scanning SAR is a new strip map mode SAR imaging in recent years, its imaging speed is fast with larger scene. This paper first establishes the imaging geometry right, by the four order approximation of the oblique distance model, combined with the series inversion method, and the removal of the coupling terms in the spectrum in the range Doppler domain, finally complete the azimuth compression. The simulation results prove the validity and feasibility of the imaging.

Image Formation Processing for Sliding Spotlight SAR With Stepped Frequency Chirps

This letter extends the “two-step” focusing approach to sliding spotlight (SS) synthetic aperture radar (SAR) with stepped frequency chirps, where two major points should be looked out. One is the slant-range variation among one group of subpulses introduced by the radar platform position changing. In the stripmap mode, an effective approach to compensate this variation is to multiply each subband echo with a phase ramp in the Doppler domain. Herein, we apply this approach to the SS SAR. The other is the residual Doppler bandwidth after azimuth deramping with a fixed reference function. To remove this residual bandwidth, a range-frequency-dependent reference function (FDRF) combined with azimuth zero-padding was developed in a spotlight SAR with a single subband. In this letter, we extend the FDRF to the multiple-subband SS SAR, where the FDRF also depends on the subband carrier frequency. The main contribution of this letter is that the chirp z transform is employed to acquire an identical azimuth output pixel interval for different range frequencies and different subbands. All the aforementioned processing and the bandwidth synthesis are imbedded into the “two-step” focusing approach, which is validated by simulation results.

An approach to evaluate and monitor RISAT-1 SAR from level-0 raw data

Data quality evaluation of synthetic aperture radar (SAR) products is essential with respect to mission performance and reliability of the data products for users. The generation of any value-added SAR product starts with a level-0 raw data product, so it is vital firstly to analyse and monitor the raw data. In other words, any inconsistency or error occur because of data, the SAR processor requires to ‘quality evaluate’ the raw data first to ensure that the higher level of data products meets the user specifications. Many parameters can be evaluated from the raw data per se before SAR processing. Here, an independent approach is formulated to evaluate the long-term stability and performance of the SAR system from raw data. Quality parameters such as echo data statistics in terms of bias in the mean of Inphase (I) and Quadrature in phase (Q), power imbalance, phase imbalance, antenna pattern in elevation for the qualified swath, Doppler centroid estimation, and chirp phase and amplitude stability were evaluated and monitored for various scenes of an orbit and characterized in detail for different beams covering the near to far range. In the Radar Imaging Satellite (RISAT-1), the SAR beam is formed by a number of transmit/receive modules (TRMs) (total number of active TRMs depends on the look angle), and the consolidated effect of TRMs can be observed from the raw data instead of individual TRMs, which enables examination of the overall behaviour of beams, from near to far range in different orbits, for temporal stability. In this article, for the first time, a novel approach is taken to observe and evaluate the antenna pattern for the 3 dB qualified swath from range-uncompressed raw data and to relatively monitor the different beams from level-0 raw data for the qualified swath, for both extended homogeneous and non-homogeneous targets. Furthermore, the approach is justified in that it is not necessary to perform range compression, and also one can use scenes with heterogeneous targets to estimate and monitor the beamwidth parameter of the antenna pattern. This article focuses on the results obtained from RISAT-1 level-0 raw data from various beams covered in strip-map mode. The methodology to evaluate the RISAT-1 SAR system from raw data is discussed. Analysis is carried out with respect to different beams (60 beam data of different orbits either side, covering near to far nadir swath coverage) of fine resolution strip-map-1 (FRS-1) mode in circularly transmitted and linearly received polarization, with a defined swath of 25 km with 3 m × 2 m (azimuth × range) resolution. The results of evaluation and monitoring analysis show consistency in the identified parameters observed over time for different beams, and they are within specifications.

Multilook SAR Tomography for 3-D Reconstruction and Monitoring of Single Structures Applied to COSMO-SKYMED Data

With reference to the application to the imaging and monitoring of infrastructures and buildings in urban areas, SAR tomography has been mainly developed and tested at full resolution. In this work, we investigate the possibility related to the use of a multilook approach for fine resolution analysis of ground structures that combines SAR tomography and a method, CAESAR, recently proposed for classical DInSAR analysis at coarse resolution over large areas. Shown results, achieved by processing two 3 m spatial resolution (stripmap mode) COSMO-SKYMED datasets relative to the urban areas of Naples and Rome (Italy), clearly indicate that the proposed multilook-based method allows achieving an impressive density of detected scatterers over buildings and infrastructures, much higher than those achievable with standard full-resolution methods.

Deformation Monitoring of Offshore Platform Using the Persistent Scatterer Interferometry Technique

One of the problems that occur during the exploitation of oil and gas is offshore platform deformation. It could occur due to the environments load as well as the extraction of oil and gas itself under the seabed that caused reservoir compaction. Offshore platform deformation may affect the platform structural integrity and cause loss of production, thus it is very important to monitor its occurrences. Offshore platform deformation monitoring has been carried out using the satellite-based Global Positioning System (GPS) technique until recently. Even though the technique has proven its worth for the job, it has however some limitations, the most prominent is it could only monitor selected portion of the offshore platform. Thus, this study presents an attempt of detecting and monitoring the deformation phenomenon of an offshore platform using the Persistent Scatterer Interferometry (PSI) technique. This technique would overcome some of the limitations of the previous (GPS) deformation monitoring technique. A total of 11 high resolution TerraSAR-X images (i.e., 3 m in StripMap mode) were acquired from Aug, 2012 to Apr, 2013 for this purpose, while one of the offshore platforms in South China Sea is being used as monitored platform. Preliminary results showed that detail and sensitive deformations could be detected by this technique. In addition, analysis results in the form of mean deformation velocity map and displacement time series would allow us to further understand the behaviour of offshore platform deformation.

Random forest classification of crop type using multi-temporal TerraSAR-X dual-polarimetric data

The classification maps are required for the management and the estimation of agricultural disaster compensation; however, those techniques have yet to be established. Some supervised learning models may allow accurate classification. In this study, the Random Forest (RF) classifier and the classification and regression tree (CART) were applied to evaluate the potential of multi-temporal TerraSAR-X dual-polarimetric data, on the StripMap mode, for the classification of crop type. Furthermore, comparisons of the two algorithms and polarizations were carried out. In the study area, beans, beet, grasslands, maize, potato and winter wheat were cultivated, and these crop types were classified using the data set acquired in 2009. The classification results of RF were superior to those of CART, and the overall accuracies were 0.91–0.93.

An Efficient Signal Reconstruction Algorithm for Stepped Frequency MIMO-SAR in the Spotlight and Sliding Spotlight Modes

Multiple-input multiple-output (MIMO) synthetic aperture radar (SAR) using stepped frequency (SF) waveforms enables a high two-dimensional (2D) resolution with wider imaging swath at relatively low cost. However, only the stripmap mode has been discussed for SF MIMO-SAR. This paper presents an efficient algorithm to reconstruct the signal of SF MIMO-SAR in the spotlight and sliding spotlight modes, which includes Doppler ambiguity resolving algorithm based on subaperture division and an improved frequency-domain bandwidth synthesis (FBS) method. Both simulated and constructed data are used to validate the effectiveness of the proposed algorithm.

Caracterização de uso e cobertura da terra na Amazônia utilizando imagens duais multitemporais do COSMO-SkyMed

A utilização de imagens de radar é fonte alternativa de informações para subsidiar o monitoramento da região amazônica, visto que as imagens ópticas têm limitações de imageamento em zonas tropicais face a ocorrência de nuvens. Por conseguinte este trabalho teve como objetivo analisar a capacidade das imagens-radar de banda X multitemporais e polarizadas obtidas pelo satélite COSMO-SkyMed (COnstellation of small Satellites for Mediterranean basin Observation), no modo intensidade, isoladamente e agregados às informações texturais, na caracterização temática de uso e cobertura da terra no município de Humaitá/AM. A metodologia empregada consistiu da: análise das imagens duais obtidas em duas aquisições subsequentes, de forma a explorar a potencialidade do conjunto de dados na forma quad-pol intensidade; extração dos atributos texturais a partir da matriz de coocorrência (Gray Level Co-occurrence Matrix) e posterior classificação contextual; avaliação estatística de desempenho temático das imagens intensidade e texturais, isoladas e em grupos polarizados. Dentre os vários resultados alcançados, foi verificado que o grupo formado somente pelas imagens intensidade apresentou o melhor desempenho, comparado àqueles contendo os atributos texturais. Nesta separabilidade, estavam envolvidas as classes de floresta, floresta aluvial, reflorestamento, savana, pasto e queimada, obtendo-se 66% de acurácia total e valor Kappa de 0,55. Os resultados mostraram que as imagens de banda X do COSMO-SkyMed, modo StripMap (Ping-Pong), multipolarizadas, têm potencial moderado para a caracterização e monitoramento da dinâmica de uso e cobertura da terra na Amazônia brasileira.

Application of Multi-temporal TerraSAR-X Data to Map Winter Wheat Planted Areas in Hokkaido, Japan

Winter wheat is an important crop for many countries, and monitoring of its planted area is considered important. Optical sensors have been used to monitor agricultural land, and have shown good classification and monitoring capabilities. However, observations using optical sensors sometimes suffer from interference due to cloud cover or rain. In contrast, synthetic aperture radars (SAR) can be used for Earth observation even under rainy, cloudy or dark conditions, hence SAR is expected to be effective in monitoring agricultural fields and identifying winter wheat fields. The objective of this study is to analyze the potential of TerraSAR-X dual images, in the StripMap mode, for mapping winter wheat planted areas. Using the separability statistic (D), it emerged that the sigma naught acquired in mid-July possesses great potential. The method applied in this study has an overall accuracy exceeding 96% for HH and VV polarization data for identifying winter wheat fields.

Three-dimensional imaging interferometric synthetic aperture ladar

A monostatic strip-map mode interferometric synthetic aperture ladar (SAL) is reported. Using a chirped laser of about 5 mW at 1550 nm wavelength as the illumination source and two cross-track receiving apertures with a baseline of 1.6 mm, the ladar can generate both well-focused two-dimensional SAL images without adopting phase error removing techniques and three-dimensional images by interferometric SAL techniques. Detailed results are illustrated for retro-reflective or diffusive targets at a distance of 2.4 m.