Andrea Nascetti

High-Resolution SAR Radargrammetry: A First Application With COSMO-SkyMed SpotLight Imagery

The availability of new high-resolution radar spaceborne sensors offers new interesting potentialities for the acquisition of data useful for the generation of Digital Surface Models (DSMs). Two different approaches may be used to generate DSMs from Synthetic Aperture Radar (SAR) data: the interferometric and the radargrammetric one. At present, the importance of the radargrammetric approach is rapidly growing due to the new high-resolution imagery [up to 1 m Ground Sample Distance (GSD)] which can be acquired by COSMO-SkyMed, TerraSAR-X and RADARSAT-2 in SpotLight mode. The defined and implemented model is related to COSMO- SkyMed SpotLight imagery in zero-Doppler geometry; it performs a 3-D orientation based on two range and two zero-Doppler equations, allowing for the least squares estimation of some calibration parameters, related to satellite position and velocity and to the range measure. The model has been implemented in SISAR (Software per Immagini Satellitari ad Alta Risoluzione), a scientific software developed at the Geodesy and Geomatic Institute of the University of Rome “La Sapienza”. Starting from this model, based on a geometric reconstruction, also a tool for the Rational Polynomial Coefficients (RPCs) generations has been implemented. To test the effectiveness of the new model, a stereo pair over the test sites of Merano (Northern Italy) has been orientated using the rigorous model and the RPCs one, and first results of radargrammetric DSM generation are presented; they display the possibility to reach an overall average accuracy of 3.5 m.

Fast terrain modelling for hydrogeological risk mapping and emergency management: the contribution of high-resolution satellite SAR imagery

Geomatic tools fast terrain modelling play a relevant role in hydrogeological risk mapping and emergency management. Given their complete independence from logistic constraints on the ground (as for airborne data collection), illumination (daylight), and weather (clouds) conditions, synthetic aperture radar (SAR) satellite systems may provide important contributions in terms of digital surface models (DSMs) and digital elevation models (DEMs). For this work we focused on the potential of high-resolution SAR satellite imagery for DSM generation using an interferometric (InSAR) technique and using a revitalized radargrammetric stereomapping approach. The goal of this work was just methodological. Our goal was to illustrate both the fundamental advantages and drawbacks of the radargrammetric approach with respect to the InSAR technique for DSM generation, and to outline their possible joint role in hydrogeological risk mapping and emergency management. Here, it is worth mentioning that radargrammetry procedures are independent of image coherence (unlike the interferometric approach) and phase unwrapping, as well as of parsimony (only a few images are necessary). Therefore, a short time is required for image collection (from tens of minutes to a few hours), thanks to the independence from illumination and weather. The most relevant obstacles of the technique are speckle and the lack of texture impact on image matching, as well as the well-known deformations of SAR imagery (layover and foreshortening), which may produce remarkable difficulties with complex morphologies and that must be accounted for during acquisition planning. Here, we discuss results obtained with InSAR and radargrammetry applied to a COSMO-SkyMed SpotLight triplet (two stereopairs suited for radargrammetry and InSAR, sharing one common image) acquired over suburbs of San Francisco (United States), which are characterized by mixed morphology and land cover. We mainly focused on urban areas and zones covered by bare soil and rocks. Image processing was performed using the well-known commercial software SARscape® for InSAR, and the radargrammetric suite implemented in SISAR, software developed at the Geodesy and Geomatic Division of the University of Rome “La Sapienza”. Global accuracies were approximately 5 m using both approaches. However, several differences in terrain morphology reconstruction were determined and are underlined and evaluated here, as well as a possible way to further enhance the results using the integration of InSAR and radargrammetry.

Digital surface models from ZiYuan-3 triplet: performance evaluation and accuracy assessment

At the beginning of 2012, a new optical satellite, called ZiYuan-3 (ZY-3), was launched from the Taiyuan Satellite Launch Centre. This article is aimed at the analysis and the assessment of the digital surface models (DSMs) and orthophotos from a ZY-3 triplet acquired over the city of Bolzano and the surrounding areas. The imagery processing chain was executed with four different software packages. In all cases the Rational Polynomial Functions (RPFs) orientation model was used, with rational polynomial coefficients (RPCs) supplied by the vendors. The accuracies of the extracted DSMs were compared with the public light detection and ranging (lidar) DSM. The DSM extracted with each software package was used to generate the corresponding orthophoto. The orthophoto accuracy was assessed against 1:5000/1:10000 maps. The results obtained show that ZY-3 imagery allow us to generate DSMs with an accuracy, in terms of root mean square error (RMSE), which can reach 1.5–4.2 m in the flat area, 3.9–6.9 in the urban area, and 5.7–8.2 m in the mountain area. These results are better than Shuttle Radar Topography Mission (SRTM). Regarding orthophoto generation, all software packages supply similar results with a significant bias of about 20 m, mainly caused by vendors’ RPC inaccuracy.

Open source tool for DSMs generation from high resolution optical satellite imagery: development and testing of an OSSIM plug-in

ABSTRACT The fully automatic generation of digital surface models (DSMs) is still an open research issue. From recent years, computer vision algorithms have been introduced in photogrammetry in order to exploit their capabilities and efficiency in three-dimensional modelling. In this article, a new tool for fully automatic DSMs generation from high resolution satellite optical imagery is presented. In particular, a new iterative approach in order to obtain the quasi-epipolar images from the original stereopairs has been defined and deployed. This approach is implemented in a new Free and Open Source Software (FOSS) named Digital Automatic Terrain Extractor (DATE) developed at the Geodesy and Geomatics Division, University of Rome ‘La Sapienza’, and conceived as an Open Source Software Image Map (OSSIM) plug-in. DATE key features include: the epipolarity achievement in the object space, thanks to the images ground projection (Ground quasi-Epipolar Imagery (GrEI)) and the coarse-to-fine pyramidal scheme adopted; the use of computer vision algorithms in order to improve the processing efficiency and make the DSMs generation process fully automatic; the free and open source aspect of the developed code. The implemented plug-in was validated through two optical datasets, GeoEye-1 and the newest Pléiades-high resolution (HR) imagery, on Trento (Northern Italy) test site. The DSMs, generated on the basis of the metadata rational polynomial coefficients only, without any ground control point, are compared to a reference lidar in areas with different land use/land cover and morphology. The results obtained thanks to the developed workflow are good in terms of statistical parameters (root mean square error around 5 m for GeoEye-1 and around 4 m for Pléiades-HR imagery) and comparable with the results obtained through different software by other authors on the same test site, whereas in terms of efficiency DATE outperforms most of the available commercial software. These first achievements indicate good potential for the developed plug-in, which in a very near future will be also upgraded for synthetic aperture radar and tri-stereo optical imagery processing.

DSM generation from very high optical and radar sensors: Problems and potentialities along the road from the 3D geometric modeling to the Surface Model

The availability of new high resolution optical and radar spaceborne sensors offers new interesting potentialities for the acquisition of data useful for the generation of Digital Surface Models (DSMs).

Exploiting performance of different low-cost sensors for small amplitude oscillatory motion monitoring: Preliminary comparisons in view of possible integration

We address the problem of low amplitude oscillatory motion detection through different low-cost sensors: a LIS3LV02DQ MEMS accelerometer, a Microsoft Kinect v2 range camera, and a uBlox 6 GPS receiver. Several tests were performed using a one-direction vibrating table with different oscillation frequencies (in the range 1.5–3 Hz) and small challenging amplitudes (0.02 m and 0.03 m). A Mikrotron EoSens high-resolution camera was used to give reference data. A dedicated software tool was developed to retrieve Kinect v2 results. The capabilities of the VADASE algorithm were employed to process uBlox 6 GPS receiver observations. In the investigated time interval (in the order of tens of seconds) the results obtained indicate that displacements were detected with the resolution of fractions of millimeters with MEMS accelerometer and Kinect v2 and few millimeters with uBlox 6. MEMS accelerometer displays the lowest noise but a significant bias, whereas Kinect v2 and uBlox 6 appear more stable. The results suggest the possibility of sensor integration both for indoor (MEMS accelerometer

How geodesy can contribute to the understanding and prediction of earthquakes

Earthquakes cannot be predicted with precision, but algorithms exist for intermediate-term middle-range prediction of main shocks above a pre-assigned threshold, based on seismicity patterns. Few years ago, a first attempt was made in the framework of project SISMA, funded by Italian Space Agency, to jointly use seismological tools, like CN algorithm and scenario earthquakes, and geodetic methods and techniques, like GPS and SAR monitoring, to effectively constrain priority areas where to concentrate prevention and seismic risk mitigation. We present a further development of integration of seismological and geodetic information, clearly showing the contribution of geodesy to the understanding and prediction of earthquakes. As a relevant application, the seismic crisis that started in Central Italy in August 2016 with the Amatrice earthquake and still going on is considered in a retrospective analysis of both GPS and SAR data. Differently from the much more common approach, here, GPS data are not used to estimate the standard 2D velocity and strain field in the area, but to reconstruct the velocity and strain pattern along transects, which are properly oriented according to the a priori information about the known tectonic setting. SAR data related to the Amatrice earthquake coseismic displacements are here used as independent check of the GPS results. Overall, the analysis of the available geodetic data indicates that it is possible to highlight the velocity variation and the related strain accumulation in the area of Amatrice event, within the area alarmed by CN since November 1st, 2012. The considered counter examples, across CN alarmed and not-alarmed areas, do not show any spatial acceleration localized trend, comparable to the one well defined along the Amatrice transect. Therefore, we show that the combined analysis of the results of intermediate-term middle-range earthquake prediction algorithms, like CN, with those from the processing of adequately dense and permanent GNSS network data, possibly complemented by a continuous InSAR tracking, may allow the routine highlight in advance of the strain accumulation. Thus, it is possible to significantly reduce the size of the CN alarmed areas.

Unsupervised flood extent detection from SAR imagery applying shadow filtering from SAR simulated image

The present paper is focused on a potential method for unsupervised flood extent classification approach based, on the extraction of radar shadows from SAR simulated image, generated by means of image metadata and an available DSM covering the area of interest. The SAR simulation process is carried out using the SAR Simulator tool, implemented in the free and open SAR PlugIn developed for the Opticks platform. Different tests have been performed using SGF RADARSAT-2 imagery in order to assess the effectiveness of the proposed approach.

DSM generation from optical and SAR high resolution satellite imagery: Methodology, problems and potentialities

The actual high resolution optical and Synthetic Aperture Radar (SAR) satellite sensors offer interesting potentialities for Digital Surface Models (DSMs) generation. Both optical and SAR imagery are characterized by proper deformations and noise due to the different acquisition geometries and processes, which have to be duly taken into account during the DSM generation procedure in order to fully exploit the aforementioned potentialities. The aim of this work is to evaluate the performances of high resolution optical and SAR imagery for DSMs generation over the same testfield area where a dense network of GCPs and LiDAR DSM are available as ground truth data. The image processing and DSMs generation are carried out with the packages SISAR (Software Immagini Satellitari ad Alta Risoluzione) and SAT-PP (SATellite image Precision Processing) while an additional comparison is performed using PCI Geomatica 2012.

Monitoring ground displacements at centimeter level exploiting TerraSAR-X range measurements

The goal of this work is to exploit the slant-range measurements reaching centimetre accuracies using only the amplitude information of SAR data acquired by TerraSAR-X satellite sensor. The leading idea is to evaluate the positioning accuracy of well identifiable and stable natural and man-made Persistent Scatterers (PSs) along the SAR line of sight. New Earth observation SAR (Synthetic Aperture Radar) satellite sensors, as COSMO-SkyMed, TerraSAR-X and PAZ, acquire imagery on any point of the Earth with high resolutions, in terms of phase and amplitude value. Thanks to this higher amplitude resolution (up to 0.20 m pixel resolution in the Staring SpotLight mode for TerraSAR-X and PAZ) and to the use of on board dual frequency GPS receivers, which allows to determine the satellite orbit with an accuracy at few centimetres level, the SAR images offer the capability to achieve, in a global reference frame, positioning accuracies in the meter range and even better.