Othmar Frey | Swiss Federal Institute of Technology (ETH) & University of Zurich (original) (raw)

Papers by Othmar Frey

IEEE Transactions on Geoscience and Remote Sensing, 2022

This article presents the bistatic operation mode and the performance analysis of KAPRI, a terres... more This article presents the bistatic operation mode and the performance analysis of KAPRI, a terrestrial frequency-modulated continuous-wave (FMCW) Ku-band polarimetric radar interferometer capable of acquiring bistatic fullpolarimetric datasets with high spatial and temporal resolution. In the bistatic configuration, the system is composed of two independently operating KAPRI devices, one serving as a primary transmitter and receiver and the other as a secondary receiver. The secondary bistatic dataset is affected by possible offsets between the two devices' reference clocks, as well as distortions arising from the bistatic geometry. To correct for this, we present a two-chirp bistatic FMCW signal model, which accounts for the distortions, and a reference chirp transmission procedure, which allows correcting the clock offsets in the deramped signal time domain. The second challenge of operation of a bistatic polarimetric system is polarimetric calibration since it is not possible to employ purely monostatic targets such as corner reflectors. For this purpose, we developed a novel active calibration device Variable-Signature Polarimetric Active Radar Calibrator (VSPARC), designed for monostatic and bistatic calibration of all polarimetric channels. VSPARC and its associated novel polarimetric calibration method were then used to achieve full calibration of both KAPRI devices with polarimetric phase calibration accuracy of 20 • and 30-dB polarization purity in field conditions. This article thus presents a complete measurement configuration and data processing pipeline necessary for synchronization, coregistration, and polarimetric calibration of bistatic and monostatic datasets acquired by a real-aperture FMCW radar.

2016 IEEE International Geoscience and Remote Sensing Symposium (IGARSS), 2016

SAR tomography can be used as an add-on to persistent scatterer interferometry (PSI) to increase ... more SAR tomography can be used as an add-on to persistent scatterer interferometry (PSI) to increase deformation sampling in urban areas by resolving the frequently occurring layovers that are by definition rejected in the PSI processing. This paper, while focusing on the case of a typical highrise building in layover, quantitatively assesses the potential gain in deformation sampling achieved by the added use of an advanced SAR tomographic technique relative to a PSI approach. At the same time, the quantity of the detected scatterers is weighed against their quality, as assessed on the basis of root-mean-square (RMS) phase deviation between the measurements and the model fit. The quality of the scatterers is also compared with the quality of the persistent scatterers as identified with a PSI approach. The experiments are performed on an interferometric stack of 50 TerraSAR-X stripmap mode images.

The coherent backscatter opposition effect (CBOE) enhances the backscatter intensity of electroma... more The coherent backscatter opposition effect (CBOE) enhances the backscatter intensity of electromagnetic waves by up to a factor of two in a very narrow cone around the direct return direction when multiple scattering occurs in a weakly absorbing, disordered medium. So far, this effect has not been investigated in terrestrial snow in the microwave spectrum. It has also received little attention in scattering models. We present the first characterization of the CBOE in dry snow using ground-based and space-borne bistatic radar systems. For a seasonal snow pack in Ku-band (17.2 GHz), we found backscatter enhancement of 50-60% (+1.8-2.0 dB) at zero bistatic angle and a peak half-width-at-half-maximum (HWHM) of 0.25 •. In X-band (9.65 GHz), we found backscatter enhancement of at least 35% (+1.3 dB) and an estimated HWHM of 0.12 • in the accumulation areas of glaciers in the Jungfrau-Aletsch region, Switzerland. Sampling of the peak shape at different bistatic angles allows estimating the scattering and absorption mean free paths, Λ T and Λ A. In the VV polarization, we obtained Λ T = 0.4 ± 0.1 m and Λ A = 19 ± 12 m at Ku-band, and Λ T = 2.1 ± 0.4 m, Λ A = 21.8 ± 2.7 m at X-band. The HH polarization yielded similar results. The observed backscatter enhancement is thus significant enough to require consideration in backscatter models describing monostatic and bistatic radar experiments. Enhanced backscattering beyond the Earth, on the surface of solar system bodies, has been interpreted as being caused by the presence of water ice. In agreement with this interpretation, our results confirm the presence of the CBOE at X-and Ku-band frequencies in terrestrial snow.

IGARSS 2019 - 2019 IEEE International Geoscience and Remote Sensing Symposium, 2019

In the context of differential synthetic aperture radar interferometry (DInSAR), precise trajecto... more In the context of differential synthetic aperture radar interferometry (DInSAR), precise trajectory estimation of the SAR platform is necessary to minimize residual phase errors induced by inaccurate knowledge of the 3D acquisition geometry. Inertial navigation systems (INS) and global navigation satellite system (GNSS) are usually employed to track the position of the platform. However, their unavoidable inaccuracies lead to motion estimation errors that negatively affect the quality of the processed radar data. To assess the positioning performance in a repeat-pass scenario, we used a navigation-grade INS/GNSS system to precisely track the position and the attitude of a platform moving along a rail and carrying a SAR sensor. We analyse the performance of the positioning solution for different scenarios relevant to repeat-pass DInSAR. Since the position of the platform is nearly perfectly repeated at every pass (zero interferometric baseline), the precision of the estimated position can be assessed and the interferometric performance evaluated.

Riverbank erosion occurs along many of the Earth's river systems, affecting riverine populations ... more Riverbank erosion occurs along many of the Earth's river systems, affecting riverine populations by destroying agricultural land and housing. In this study, we detected past events of riverbank erosion along the Jamuna River in Bangladesh using time series of Sentinel-1 satellite radar imagery, ground range detected (GRD) data with a 12-day revisit 10 cycle, available in the Google Earth Engine (GEE). Eroded land is detected by performing a land cover classification and by detecting land cover changes from vegetated areas before the monsoon to sand or water after the monsoon. Further, settlements are detected as persistent scatterers, and classified as eroded if they are located on eroded land. We found that with Sentinel-1 data, erosion locations can be determined already one month after the end of the monsoon, and hence potentially earlier than using optical satellite images which depend on cloud-free daylight conditions. Further, we 15 developed an interactive GEE-based online tool allowing the user to explore where riverbank erosion has destroyed land and settlements along the Jamuna in five monsoon seasons (2015-2019). The source code of our implementation is publicly available, providing the opportunity to reproduce the results, to adapt the algorithm and to transfer our results to assess riverbank erosion in other geographical settings. 20

IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing

Agile synthetic aperture radar (SAR) platforms such as car-borne and UAV-borne SAR systems requir... more Agile synthetic aperture radar (SAR) platforms such as car-borne and UAV-borne SAR systems require combined inertial navigation systems (INS) and global navigation satellite systems (GNSS) to measure the radar sensor trajectories used for focusing and interferometric processing. Measurement inaccuracies from INS/GNSS systems lead to residual phase errors in the SAR products whose minimization is crucial to derive accurate topographic and deformation information. In this work, we analyze the impact of residual positioning errors on car-borne repeat-pass SAR interferometry at L-band for different INS/GNSS measurement configurations and for the typical car-borne acquisition geometry. The positioning errors are evaluated both during single SAR acquisitions with long integration times and between different acquisitions as a function of the distance of the radar platform from the GNSS reference stations. We show the reduction of interferometric phase errors achievable by additionally using a GNSS receiver mounted in the vicinity of the SAR platform as compared to remote reference stations of the national network of permanent GNSS receivers. Test results obtained in a controlled setup with a rail-based SAR system equipped with a navigation-grade INS/GNSS system show maximum repeat-pass trajectory errors on the order of 1-2 cm using a local GNSS reference station and up to 10-15 cm using the remote reference stations, leading to azimuth and range phase trends in the interferometric products.

Abstract—In this contribution, a car-borne SAR and InSAR experiment is described. The slope of a ... more Abstract—In this contribution, a car-borne SAR and InSAR experiment is described. The slope of a valley was imaged by means of a single-pass InSAR system mounted on a car driving on roads along the bottom of the valley. The GAMMA portable radar interferometer GPRI-II hardware with a modified antenna configuration was used for data acquisition. The experimental setup (1), SAR imagery focused along a slightly curved sensor trajectory (2), and first interferometric results (3) obtained using this configuration are presented. Index Terms—Synthetic aperture radar (SAR), ground-based SAR system, SAR imaging, SAR interferometry, car-borne SAR, CARSAR I.

Airborne Laser Scanning (ALS) has been established as a valuable tool for the estimation of bioph... more Airborne Laser Scanning (ALS) has been established as a valuable tool for the estimation of biophysical vegetation properties such as tree height, crown width, fractional cover and leaf area index (LAI). It is expected that the conditions of data acquisition, such as viewing geometry and sensor configuration influence the value of these parameters. In order to gain knowledge about these different conditions, we test for the sensitivity of vegetation products for viewing geometry, namely flying altitude and scanning (incidence) angle. Based on two methodologies for single tree extraction and derivation of fractional cover and LAI previously developed and published by our group, we evaluate how these variables change with either flying altitude and scanning angle. These are the two parameters which often need to be optimised towards the best compromise between point density and area covered with a single flight line. Our testsite in the Swiss National Park was overflown with two nomin...

The estimation of the atmosphere-induced phase delay variations is often more involved in mountai... more The estimation of the atmosphere-induced phase delay variations is often more involved in mountainous areas due to strong spatial variations of the local atmospheric conditions and propagation paths through the troposphere. Heightdependent phase delay variation owing to vertical stratification of the atmosphere within the same range-azimuth resolution cell cannot be ignored. We propose a regression kriging-based data-driven method whereby phase corrections are applied for differential tomographic focusing at each 3D point of interest along the elevation axis. Experiments are performed on an interferometric stack comprising 32 Cosmo-SkyMed stripmap images acquired between 2008-2013 over the Matter Valley in the Swiss Alps.

IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium, 2018

Repeat-pass differential SAR interferometry (DInSAR) using spaceborne SAR data or stationary terr... more Repeat-pass differential SAR interferometry (DInSAR) using spaceborne SAR data or stationary terrestrial radar data is an established technique to measure surface displacements. However, repeat-pass DInSAR from agile platforms (airborne/car-borne) is challenging due to residual motion errors. This is particularly true for high-frequency radar where motion errors of few millimeters represent a non-negligible fraction of the wavelength. In this paper, an experimental car-borne SAR system is presented. Such a system is complementary to the existing solutions (namely spaceborne, airborne, and terrestrial systems) in terms of geometry of acquisition, and flexibility in the selection of temporal baselines and location of the acquisitions. To meet the need of consistent and precise trajectory information, proper postprocessing procedure must be applied to the raw positioning data collected from the inertial navigation system (INS) and the global positioning system (GNSS). A viable procedur...

2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, 2021

SAR offset tracking, applied on areas with strong temporal decorrelation, requires relatively lar... more SAR offset tracking, applied on areas with strong temporal decorrelation, requires relatively large image templates for cross-correlation to compensate for incoherent radar speckle. Template edge lengths of 64–512 pixels are common. Furthermore, velocity maps are often incomplete because weakly visible features are obscured by uncorrelated speckle. To improve SAR offset tracking, we propose a new robust method which can significantly enhance both the spatial completeness and the resolution of velocity products by assuming a stationary velocity field. The method minimizes the motion blur of moving features which occurs when SAR backscatter time series are multilooked in time. Our velocity-adaptive temporal multilooking strongly reduces speckle without losing spatial resolution which makes the cross-correlation much more robust even for template sizes as small as 30 x 30 pixels. We demonstrate the method by generating a high resolution velocity map of Great Aletsch Glacier in Switzerland.

International Geoscience and Remote Sensing Symposium (IGARSS), 2007

Recently, various attempts have been undertaken to obtain information about the structure of fore... more Recently, various attempts have been undertaken to obtain information about the structure of forested areas from multi-baseline synthetic aperture radar data. Tomographic processing of such data has been demonstrated but the quality of the focused tomographic image is limited by several factors. In particular Fourier-based focusing methods are susceptible to irregular and sparse sampling, two problems, that are unavoidable in case of multi-pass, multi-baseline SAR data acquired by an airborne system. We propose a tomographic focusing method based on the time-domain back-projection algorithm, which maintains the geometric relationship between the original sensor positions and the imaged target and is therefore able to cope with irregular sampling without introducing any approximations with respect to the geometry. We assess the tomographic focusing quality with the help of the impulse response of simulated point targets and an in-scene corner reflector. And, in particular, preliminary results obtained with the newly acquired P-band tomographic data set consisting of eleven flight tracks are presented.

International Geoscience and Remote Sensing Symposium (IGARSS), 2008

Standard focusing of SAR data assumes a straight recording track of the sensor platform. Small no... more Standard focusing of SAR data assumes a straight recording track of the sensor platform. Small non-linearities of airborne platform tracks are corrected for during a motion compensation step while keeping the assumption of a linear flight path. In the following, the processing of SAR data from nonlinear tracks is discussed as may originate from small aircraft or drones flying at low altitude. They fly not a straight track but one dependent on topography, influences of weather and wind, or dependent on the shape of dedicated areas of interest such as rivers or traffic routes. A time-domain backprojection based technique, is proposed and evaluated with the help of experimental data featuring a drop in height, a double bend, a 90-degree curve and a linear flight track. In order to assess the quality of the focused data, close-ups of amplitude images are compared and the coherence is evaluated. The experimental data was acquired by the German Aerospace Center's E-SAR L-band system.

International Journal of Remote Sensing, 2008

... Airborne Laser Scanning (ALS) has been established as a valuable tool for the ... photon flux... more ... Airborne Laser Scanning (ALS) has been established as a valuable tool for the ... photon flux density using hemispherical photography and computerized image analysis techniques ... Gaussian decomposition and calibration of a novel small-footprint full-waveform digitising airborne ...

IEEE Transactions on Geoscience and Remote Sensing, 2010

Two coupled investigations of TerraSAR-X (TSX) high-resolution data are described in this paper: ... more Two coupled investigations of TerraSAR-X (TSX) high-resolution data are described in this paper: geometric validation, and estimation of the tropospheric path delay using measurements of corner reflectors (CRs) placed at different altitudes but nearly identical ranges. The CRs were placed within Alpine and valley sites in Switzerland, where terrain diversity provides ideal territory for geometric validation studies. Geometric validation was conducted using slant-range complex products from the spotlight and stripmap (SM) modes in ascending and descending configurations. Based on the delivered product annotations, the CR image positions were predicted, and these predictions were compared to their measured image positions. To isolate path delays caused by the atmosphere, six TSX SM scenes (∼35 × 50 km) were examined containing four identical CRs with the same ranges and an altitude difference of ∼3000 m. The CR arrangement made it possible to verify the annotated TSX atmospheric path delay by comparing the predicted slant range with the slant range obtained by measuring the reflector image coordinates. Range differences between the high-and low-altitude reflectors helped to quantify small variations in the path delay. Both SM and spotlight TSX products were verified to meet the specified accuracy requirements, even for scenes with extreme terrain variations, in spite of the simplicity of the atmospheric model currently integrated into the processor. Small potential improvements of the geolocation accuracy through the implementation of more comprehensive atmospheric modeling were demonstrated.

IEEE Transactions on Geoscience and Remote Sensing, 2009

Standard focusing of data from synthetic aperture radar (SAR) assumes a straight recording track ... more Standard focusing of data from synthetic aperture radar (SAR) assumes a straight recording track of the sensor platform. Small nonlinearities of airborne platform tracks are corrected for during a motion-compensation step while maintaining the assumption of a linear flight path. This paper describes the processing of SAR data acquired from nonlinear tracks, typical of sensors mounted on small aircraft or drones flying at low altitude. Such aircraft do not fly along straight tracks, but the trajectory depends on topography, influences of weather and wind, or the shape of areas of interest such as rivers or traffic routes. Two potential approaches for processing SAR data from such highly nonlinear flight tracks are proposed, namely, a patchwise frequency-domain processing and mosaicking technique and a time-domain back-projection-based technique. Both are evaluated with the help of experimental data featuring tracks with altitude changes, a double bend, a 90 circ curve, and a linear flight track. In order to assess the quality of the focused data, close-ups of amplitude images are compared, impulse response functions of a point target are analyzed, and the coherence is evaluated. The experimental data were acquired by the German Aerospace Center's E-SAR L-band system.

NAVIGATION

High spatio-temporal variability of atmospheric water vapor affects microwave signals of Global N... more High spatio-temporal variability of atmospheric water vapor affects microwave signals of Global Navigation Satellite Systems (GNSS) and Interferometric Synthetic Aperture Radar (InSAR). A better knowledge of the distribution of water vapor improves both GNSS- and InSAR-derived data products. In this work, we present a collocation framework to combine and retrieve zenith and (relative) slant tropospheric delays. GNSS and InSAR meteorological products are combined aiming at a better retrieval of the atmospheric water vapor. We investigate the combination approach with synthetic and real data acquired in the Alpine region of Switzerland. Based on a closed-loop validation with simulated delays, a few mm accuracy is achieved for the GNSS-InSAR combination in terms of retrieved ZTDs. Furthermore, when real delays are collocated, the combination results are more congruent with InSAR computed products. This research is a contribution to improve the spatio-temporal mapping of tropospheric delays by combining GNSS-derived and InSAR-derived delays.

The impact of potential error sources on geocoded products has been investigated with respect to ... more The impact of potential error sources on geocoded products has been investigated with respect to the high resolution capabilities of the TerraSAR-X sensor. Datum shift parameters, maps, digital terrain and surface models have been identified as external error sources. The accuracy of the geocoded products depends heavily on the quality and availability of this information, which underlies regional variations. Error sources closely related to the sensor are its position, sampling window start time and Doppler centroid frequency. Another error source is given by atmospheric refraction. Ionospheric and atmospheric path delays have a considerable impact. Appropriate modeling can mitigate this effect. Further, high requirements on radiometric accuracy ask for an improved antenna gain pattern correction, which depends on the actual elevation angle and the terrain height.