An experimental car-borne SAR system: Measurement setup and positioning error analysis

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 procedure is here presented and first results discussed

Sentinel-1 Support in the GAMMA Software

AbstractFirst 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

Coherent Backscatter Enhancement in Bistatic Ku- and X-Band Radar Observations of Dry Snow

The coherent backscatter opposition effect (CBOE) enhances the backscatter intensity of electromagnetic waves by up to a factor of 2 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 spaceborne bistatic radar systems. For a seasonal snowpack in the Ku-band (17.2 GHz), we found backscatter enhancement of 50 %-60 % (+1.8-2.0 dB) at a zero bistatic angle and a peak half-width at half-maximum (HWHM) of 0.25°. In the 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, 3T and 3A. In the VV polarization, we obtained 3T = 0.4 ± 0.1 m and 3A = 19 ± 12 m at the Ku-band and 3T = 2.1 ± 0.4 m and 3A = 21.8 ± 2.7 m at the X-band, assuming an optically thick medium. 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

Coherent Backscatter Enhancement in Bistatic Ku- and X-Band Radar Observations of Dry Snow

The coherent backscatter opposition effect (CBOE) enhances the backscatter intensity of electromagnetic waves by up to a factor of 2 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 spaceborne bistatic radar systems. For a seasonal snowpack in the Ku-band (17.2 GHz), we found backscatter enhancement of 50 %-60 % (+1.8-2.0 dB) at a zero bistatic angle and a peak half-width at half-maximum (HWHM) of 0.25°. In the 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, 3T and 3A. In the VV polarization, we obtained 3T = 0.4 ± 0.1 m and 3A = 19 ± 12 m at the Ku-band and 3T = 2.1 ± 0.4 m and 3A = 21.8 ± 2.7 m at the X-band, assuming an optically thick medium. 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

Monte Carlo Simulations of Metasomatic Enrichment in the Lithosphere and Implications for the Source of Alkaline Basalts

One hypothesis for the origin of alkaline lavas erupted on oceanic islands and in intracontinental settings is that they represent the melts of amphibole-rich veins in the lithosphere (or melts of their dehydrated equivalents if metasomatized lithosphere is recycled into the convecting mantle). Amphibole-rich veins are interpreted as cumulates produced by crystallization of low-degree melts of the underlying asthenosphere as they ascend through the lithosphere. We present the results of trace-element modelling of the formation and melting of veins formed in this way with the goal of testing this hypothesis and for predicting how variability in the formation and subsequent melting of such cumulates (and adjacent cryptically and modally metasomatized lithospheric peridotite) would be manifested in magmas generated by such a process. Because the high-pressure phase equilibria of hydrous near-solidus melts of garnet lherzolite are poorly constrained and given the likely high variability of the hypothesized accumulation and remelting processes, we used Monte Carlo techniques to estimate how uncertainties in the model parameters (e.g. the compositions of the asthenospheric sources, their trace-element contents, and their degree of melting; the modal proportions of crystallizing phases, including accessory phases, as the asthenospheric partial melts ascend and crystallize in the lithosphere; the amount of metasomatism of the peridotitic country rock; the degree of melting of the cumulates and the amount of melt derived from the metasomatized country rock) propagate through the process and manifest themselves as variability in the trace-element contents and radiogenic isotopic ratios of model vein compositions and erupted alkaline magma compositions. We then compare the results of the models with amphibole observed in lithospheric veins and with oceanic and continental alkaline magmas. While the trace-element patterns of the near-solidus peridotite melts, the initial anhydrous cumulate assemblage (clinopyroxene ± garnet ± olivine ± orthopyroxene), and the modelled coexisting liquids do not match the patterns observed in alkaline lavas, our calculations show that with further crystallization and the appearance of amphibole (and accessory minerals such as rutile, ilmenite, apatite, etc.) the calculated cumulate assemblages have trace-element patterns that closely match those observed in the veins and lavas. These calculated hydrous cumulate assemblages are highly enriched in incompatible trace elements and share many similarities with the trace-element patterns of alkaline basalts observed in oceanic or continental setting such as positive Nb/La, negative Ce/Pb, and similiar slopes of the rare earth elements. By varying the proportions of trapped liquid and thus simulating the cryptic and modal metasomatism observed in peridotite that surrounds these veins, we can model the variations in Ba/Nb, Ce/Pb, and Nb/U ratios that are observed in alkaline basalts. If the isotopic compositions of the initial low-degree peridotite melts are similar to the range observed in mid-ocean ridge basalt, our model calculations produce cumulates that would have isotopic compositions similar to those observed in most alkaline ocean island basalt (OIB) and continental magmas after ~0·15 Gyr. However, to produce alkaline basalts with HIMU isotopic compositions requires much longer residence times (i.e. 1–2 Gyr), consistent with subduction and recycling of metasomatized lithosphere through the mantle. EM magmas cannot readily be explained without appealing to other factors such as a heterogeneous asthenosphere. These modelling results support the interpretation proposed by various researchers that amphibole-bearing veins represent cumulates formed during the differentiation of a volatile-bearing low-degree peridotite melt and that these cumulates are significant components of the sources of alkaline OIB and continental magmas. The results of the forward models provide the potential for detailed tests of this class of hypotheses for the origin of alkaline magmas worldwide and for interpreting major and minor aspects of the geochemical variability of these magmas

Synthetic aperture radar imaging in the time domain for nonlinear sensor trajectories and SAR tomography

Synthetic aperture radar (SAR) systems are used to obtain geospatial information for a broad range of applications, such as measuring geo- and biophysical parameters, topographic mapping, monitoring of land subsidence, landslides, and crustal deformation, as well as disaster mapping. In recent years, advanced SAR acquisition modes of growing complexity have been proposed in order to gain more flexibility in terms of usable sensor constellations and acquisition scenarios, as well as in an attempt to increase the number of observables to allow for a more reliable image and parameter inversion. These new imaging modes require more flexible SAR image reconstruction algorithms. Within the scope of this dissertation, a novel time-domain back-projection (TDBP) based SAR image processing software was developed and investigated in terms of two nonstandard data acquisitions scenarios: 1) SAR imaging along highly nonlinear sensor trajectories, and 2) high-resolution tomographic imaging of a forest at L-band and P-band. To this end, two airborne SAR experiments were designed, which were flown by the German Aerospace Center’s E-SAR system in September 2006. By means of the experimental data involving highly nonlinear sensor trajectories it was shown that the TDBP focusing algorithm yields a superior image quality as compared to a combined chirp scaling and mosaicking approach. The results of the study indicate that, in general, the TDBP algorithm imposes virtually no restrictions on the shape of the sensor trajectory. It is therefore an attractive method for efficient mapping along curvilinear objects of interest, such as traffic routes, rivers, or pipelines. A second emphasis of this dissertation is on SAR tomography of forest environments. In order to explore in detail the back-scattering behavior of radar signals within a forest a non-model-based TDBP tomographic imaging approach was pursued. In particular, three different direction-of-arrival estimation techniques, multi look beamforming, robust Capon beamforming, and MUSIC beamforming, were implemented in order to focus the two multi-baseline airborne SAR data sets at L-band and P-band. In terms of focusing quality, an unprecedented level of detail was obtained using the proposed TDBP-based tomographic imaging approach. Gaps in the canopy due to features like small forest roads are well visible in the tomographic image, for instance. Thus, the three-dimensional tomographic SAR imagery provides a good basis to investigate the back-scattering properties of the forested area at L-band and P-band. With three prospective spaceborne SAR remote sensing missions, BIOMASS at P-band, Tandem-L, and DESDynI, both at L-band, which are all aimed at global mapping and monitoring of carbon stock by assessing the above ground biomass of forests, establishing a good understanding of the interaction of microwaves at L-band and P-band with forests is critical in order to develop reliable biomass products. By means of a detailed analysis of the high-quality three- dimensional SAR data products obtained by tomographic processing, including cross-validation with airborne laser scanning data, a substantial contribution towards an improved understanding of the interaction of microwaves at L-band and P-band with forest environments was achieved within this work. Radarsysteme mit synthetischer Apertur (SAR) werden zur Gewinnung von raumbezogener Information für eine breite Palette von Anwendungen benutzt, wie beispielsweise zur Messung von geo- und biophysikalischen Parametern, zur Ableitung von Höheninformation, zur Uberwachung von Absenkungen, Hangrutschungen und Bewegungen der Erdkruste, sowie zur Schadenserfassung nach Naturkatastrophen. Im Hinblick auf eine Verbesserung der Flexibilität bezüglich verwendbarer Sensorkonstellationen und möglicher Datenakquisitionsszenarien, sowie mit dem Ziel, die Anzahl der Beobachtungen zu erhöhen, um eine zuverlässigere Bildgebung und Parameterinversion zu erreichen, haben in den letzten Jahren neue, komplexe SAR Datenakquisitionsmodi grosse Beachtung gefunden. Diese neuen SAR Modi verlangen flexiblere SAR Bildrekonstruktionsalgorithmen. Im Rahmen dieser Dissertation wurde eine neue SAR Prozessierungssoftware, welche auf dem “time-domain back-projection” (TDBP) Prinzip basiert, entwickelt und anhand von zwei nicht standardmässigen Aufnahmemodi getestet und evaluiert: 1) SAR Aufnahmen entlang von hochgradig nichtlinearen Sensortrajektorien, und 2) hochauflösende tomographische Aufnahmen eines Waldes im L- und P-Band. Die entsprechenden Daten wurden mittels zweier Experimente erhoben, welche im September 2006 mit dem flugzeuggestützten E-SAR Sensor des Deutschen Zentrums für Luft- und Raumfahrt durchgeführt wurden. Mithilfe der experimentellen Daten, welche von hochgradig nichtlinearen Sensortrajektorien aufgenommen wurden, konnte gezeigt werden, dass der TDBP Algorithmus, im Gegensatz zu einem kombinierten “Chirp Scaling”- und Mosaikierungsansatz, trotz nichtlinearer Fluggeometrie keine Verschlechterung der Bildqualität aufweist. Die Ergebnisse der Studie zeigen, dass der vorgestellte TDBP Algorithmus grundsätzlich kaum Einschränkungen bezüglich der Aufnahmegeometrie unterliegt. Die Prozessierungsmethode ermöglicht somit eine effiziente Datenerfassung entlang kurvenförmiger Gebiete und Objekte, wie zum Beispiel Verkehrswege, Flüsse oder Pipelines. Der zweite Schwerpunkt dieser Dissertation liegt auf der tomographischen Abbildung von Waldgebieten mittels “Multibaseline” SAR Daten. Um das Rückstreuverhalten von Radarsignalen innerhalb eines Waldes zu untersuchen wurde ein nicht-modellbasierter, auf dem TDBP Verfahren beruhender Ansatz verfolgt. Insbesondere wurden drei verschiedene “Direction-of-Arrival” Schätzverfahren, Multilook Beamforming, Robust Capon Beamforming und MUSIC implementiert, um die beiden “Multibaseline” SAR Datensätze im L- und P-Band zu fokussieren. Hinsichtlich der Fokussierungsqualität konnte mit dem vorgestellten TDBP-basierten Ansatz ein bisher unerreichter Detaillierungsgrad in der tomographischen Fokussierung erzielt werden. So sind beispielsweise Lücken im Kronenschluss des Waldes über Forstwegen im fokussierten tomographischen Bild immer noch gut erkennbar. Damit bieten die dreidimensionalen tomographischen SAR Bilder eine geeignete Grundlage, um die Rückstreueigenschaften des aufgenommen Waldes in den Frequenzen L-Band und P-Band zu untersuchen. Im Hinblick auf drei in Aussicht stehende satellitengestützte SAR Fernerkundungsmissionen, BIOMASS im P-Band, Tandem-L und DESDynI, im L-Band, welche unter anderem auf eine globale Kartierung und Uberwachung des in Form von Waldbiomasse gespeicherten Kohlenstoffbestandes ausgerichtet sind, ist ein vertieftes Verständnis der Wechselwirkung von Mikrowellen im L-Band und P-Band mit Wald unerlässlich, um zuverlässige Produkte zu entwickeln. Im Rahmen dieser Arbeit konnte aufgrund einer detaillierten Analyse der erzeugten, dreidimensionalen SAR Datenprodukte, einschliesslich einer Validierung mit hochauflösenden Laserscanning-Daten, ein wesentlicher Beitrag zu einem besseren Verständnis der Rückstreueigenschaften von Wald mit Mikrowellen im L- und P-Band geleistet werden

Polarimetric Calibration of the Ku Band Advanced Polarimetric Radar Interferometer (KAPRI)

Differential interferometry using ground-based radar systems permits to monitor displacements in natural terrain with high flexibility in location, time of acquisition, and revisit time. In combination with polarimetric imaging, discrimination of different scattering mechanisms present in a resolution cell can be obtained simultaneously with the estimation of surface displacement. In this paper, we present the preprocessing steps and the calibration procedure required to produce high-quality calibrated polarimetric single-look complex imagery with KAPRI, a new portable Ku-band polarimetric radar interferometer. The processing of KAPRI data into single look complex images is addressed, including the correction of beam squint and of azimuthal phase variations. A polarimetric calibration model adapted to the acquisition mode is presented and used to produce calibrated polarimetric covariance matrix data. The methods are validated by means of a scene containing five trihedral corner reflectors. Data preprocessing is assessed by analyzing the oversampled response of a corner reflector, and the polarimetric calibration quality is verified by computing polarimetric signatures and residual calibration parameters