Ocean Surface Observations Using the TanDEM-X Satellite Formation

The TanDEM-X SAR satellite formation permits improved ocean surface observations by means of bistatic along-track interferometry (ATI) when compared to single-satellite systems. The flexible imaging geometry of its two cooperating SAR sensors forms an interferometer that can achieve very high sensitivity to motions of objects on ground. This way, radar imaging of surface currents with unprecedented accuracy, high spatial resolution and wide coverage at the same time becomes possible. We demonstrate the capabilities of the sensor in the contexts of tidal current mapping, measurement of thermohaline and wind-driven ocean currents as well as detection of areas with surface films. We have developed a dedicated postprocessing system for TanDEM-X image products that allows extracting surface current information from the data. By this paper, we address bistatic data acquisition and processing aspects for sea surface imaging with TanDEM-X like interferometric baseline geometry, temporal decorrelation, and phase calibration. We present a variety of examples of data evaluation that clearly demonstrate the application potential of the methodology

Measurements of Sea Surface Currents in the Baltic Sea Region Using Spaceborne Along-Track InSAR

The main challenging problems in ocean current retrieval from along-track interferometric (ATI)-synthetic aperture radar (SAR) are phase calibration and wave bias removal. In this paper, a method based on differential InSAR (DInSAR) technique for correcting the phase offset and its variation is proposed. The wave bias removal is assessed using two different Doppler models and two different wind sources. In addition to the wind provided by an atmospheric model, the wind speed used for wave correction in this work is extracted from the calibrated SAR backscatter. This demonstrates that current retrieval from ATI-SAR can be completed independently of atmospheric models. The retrieved currents, from four TanDEM-X (TDX) acquisitions over the 6resund channel in the Baltic Sea, are compared to a regional ocean circulation model. It is shown that by applying the proposed phase correction and wave bias removal, a good agreement in spatial variation and current direction is achieved. The residual bias, between the ocean model and the current retrievals, varies between 0.013 and 0.3 m/s depending on the Doppler model and wind source used for wave correction. This paper shows that using SAR as a source of wind speed reduces the bias and root-mean-squared-error (RMSE) of the retrieved currents by 20% and 15%, respectively. Finally, the sensitivity of the sea current retrieval to Doppler model and wind errors are discussed

River Run Off Measurement With SAR Along Track Interferometry

The paper summarizes the need for global space borne river run-off measurements. It reports about an airborne SAR experiment aimed to measure the surface velocity of the river Isar in Bavaria / Germany. The results from two different SAR techniques, including Along Track Interferometry (ATI) show good correspondence. Finally suggestions for further studies are given

Combined InSAR and Terrestrial Structural Monitoring of Bridges

This paper examines advances in InSAR satellite measurement technologies to understand their relevance, utilisation and limitations for bridge monitoring. Waterloo Bridge is presented as a case study to explore how InSAR data sets can be combined with traditional measurement techniques including sensors installed on the bridge and automated total stations. A novel approach to InSAR bridge monitoring was adopted by the installation of physical reflectors at key points of structural interest on the bridge, in order to supplement the bridge’s own reflection characteristics and ensure that the InSAR measurements could be directly compared and combined with insitu measurements. The interpretation and integration of InSAR data sets with civil infrastructure data is more than a trivial task, and a discussion of uncertainty of measurement data is presented. Finally, a strategy for combining and interpreting varied data from multiple sources to provide useful insights into each of these methods is presented, outlining the practical applications of this data analysis to support wider monitoring strategies.Author funding under EPSRC (UK) Award 1636878, co-author funding under Research Council of Norway (RCN Grant no. 237906)

Retrieval of Ocean Surface Currents and Winds Using Satellite SAR backscatter and Doppler frequency shift

Ocean surface winds and currents play an important role for weather, climate, marine life, ship navigation, oil spill drift and search and rescue. In-situ observations of the ocean are sparse and costly. Satellites provide a useful complement to these observations. Synthetic aperture radar (SAR) is particularly attractive due to its high spatial resolution and its capability to extract both sea surface winds and currents day and night and almost independent of weather.The work in this thesis involves processing of along-track interferometric SAR (ATI-SAR) data, analysis of the backscatter and Doppler frequency shift, and development of wind and current retrieval algorithms. Analysis of the Doppler frequency shift showed a systematic bias. A calibration method was proposed and implemented to correct for this bias. Doppler analysis also showed that the wave contribution to the SAR Doppler centroid often dominates over the current contribution. This wave contribution is estimated using existing theoretical and empirical Doppler models. For wind and current retrieval, two methods were developed and implemented.The first method, called the direct method, consists of retrieval of the wind speed from SAR backscatter using an empirical backscatter model. In order to retrieve the radial current, the retrieved wind speed is used to correct for the wave contribution. The current retrieval was assessed using two different (theoretical and empirical) Doppler models and wind inputs (model and SAR-derived). It was found that the results obtained by combining the Doppler empirical model with the SAR-derived wind speed were more consistent with ocean models.The second method, called Bayesian method, consists of blending the SAR observables (backscatter and Doppler shift) with an atmospheric and an oceanic model to retrieve the total wind and current vector fields. It was shown that this method yields more accurate estimates, i.e. reduces the models biases against in-situ measurements. Moreover, the method introduces small scale features, e.g. fronts and meandering, which are weakly resolved by the models.The correlation between the surface wind vectors and the SAR Doppler shift was demonstrated empirically using the Doppler shift estimated from over 300 TanDEM-X interferograms and ECMWF reanalysis wind vectors. Analysis of polarimetric data showed that theoretical models such as Bragg and composite surface models over-estimate the backscatter polarization ratio and Doppler shift polarization difference. A combination of a theoretical Doppler model and an empirical modulation transfer function was proposed. It was found that this model is more consistent with the analyzed data than the pure theoretical models.The results of this thesis will be useful for integrating SAR retrievals in ocean current products and assimilating SAR observables in the atmospheric, oceanic or coupled models. The results are also relevant for preparation studies of future satellite missions

Investigating SAR algorithm for spaceborne interferometric oil spill detection

The environmental damages and recovery of terrestrial ecosystems from oil spills can last decades. Oil spills have been responsible for loss of aquamarine lives, organisms, trees, vegetation, birds and wildlife. Although there are several methods through which oil spills can be detected, it can be argued that remote sensing via the use of spaceborne platforms provides enormous benefits. This paper will provide more efficient means and methods that can assist in improving oil spill responses. The objective of this research is to develop a signal processing algorithm that can be used for detecting oil spills using spaceborne SAR interferometry (InSAR) data. To this end, a pendulum formation of multistatic smallSAR carrying platforms in a near equatorial orbit is described. The characteristic parameters such as the effects of incidence angles on radar backscatter, which support the detection of oil spills, will be the main drivers for determining the relative positions of the small satellites in formation. The orbit design and baseline distances between each spaceborne SAR platform will also be discussed. Furthermore, results from previous analysis on coverage assessment and revisit time shall be highlighted. Finally, an evaluation of automatic algorithm techniques for oil spill detection in SAR images will be conducted and results presented. The framework for the automatic algorithm considered consists of three major steps. The segmentation stage, where techniques that suggest the use of thresholding for dark spot segmentation within the captured InSAR image scene is conducted. The feature extraction stage involves the geometry and shape of the segmented region where elongation of the oil slick is considered an important feature and a function of the width and the length of the oil slick. For the classification stage, where the major objective is to distinguish oil spills from look-alikes, a Mahalanobis classifier will be used to estimate the probability of the extracted features being oil spills. The validation process of the algorithm will be conducted by using NASA’s UAVSAR data obtained over the Gulf of coast oil spill and RADARSAT-1 dat

Ice dynamics and mass balance in the grounding zone of outlet glaciers in the Transantarctic Mountains

The Antarctic grounding zone has a disproportionately large effect on glacier dynamics and ice sheet stability relative to its size but remains poorly characterised across much of the continent. Accurate ice velocity and thickness information is needed in the grounding zone to determine glacier outflow and establish to what extent changing ocean and atmospheric conditions are affecting the mass balance of individual glacier catchments. This thesis describes new satellite remote sensing techniques for measuring ice velocity and ice thickness, validated using ground measurements collected on the Beardmore, Skelton and Darwin Glaciers and applied to other Transantarctic Mountain outlet glaciers to determine ice discharge. Outlet glaciers in the Transantarctic Mountains provide an important link between the East and West Antarctic Ice Sheets but remain inadequately studied. While long-term velocities in this region are shown here to be stable, instantaneous velocities are sensitive to stresses induced by ocean tides, with fluctuations of up to 50% of the mean observed in GPS measurements. The potential error induced in averaged satellite velocity measurements due to these effects is shown to be resolvable above background noise in the grounding zone but to decrease rapidly upstream. Using a new inverse finite-element modelling approach based on regularization of the elastic-plate bending equations, tidal flexure information from differential InSAR is used to calculate ice stiffness and infer thickness in the grounding zone. This technique is shown to be successful at reproducing the thickness distribution for the Beardmore Glacier, eliminating current issues in the calculation of thickness from freeboard close to the grounding line where ice is not in hydrostatic equilibrium. Modelled thickness agrees to within 10% of ground penetrating radar measurements. Calibrated freeboard measurements and tide-free velocities in the grounding zones of glaciers in the western Ross Sea are used to calculate grounding zone basal melt rates, with values between 1.4 and 11.8 m/a⁻¹ in this region. While strongly dependent on grounding line ice thickness and velocity, melt rates show no latitudinal trend between glaciers, although detailed error analysis highlights the need for much improved estimates of firn density distribution in regions of variable accumulation such as the Transantarctic Mountains

Sea Surface Current Measurements Using Along-Track Interferometric SAR

Ocean currents affect the weather, the climate and the marine ecosystem. Observing ocean currents is important for understanding the upper-ocean layer dynamics and its interaction with the other components of the climate system. In-situ measurements are sparse and their deployment and maintenance is costly. Satellite remote sensing with large spatial coverage offers a good complement to the in-situ observations. In this work we have studied the spaceborne Along-Track Interferometric SAR (ATI-SAR) for measuring sea surface currents. The measurement principle is based on the fact that the phase difference between two SAR acquisitions is directly related to radial (line-of-sight) velocity of the illuminated surface. Previous studies based on similar systems were carried out in areas with well defined and strong tidal currents ( ~1 - 3 m/s). In this work we demonstrate thecapability of ATI-SAR, through several study cases, in areas with weak currents ( <0.5 m /s). This is challenging for the satellite measurements of surface currents because it requires very accurate processing and retrieval algorithms. In addition, it has been found that wave motion contribution, systematically dominates the measured ATI-SAR radial velocity in these weak current areas. Estimation of the wave motion contribution relies on high-resolution and accurate wind data. Thus, a wind speed retrieval algorithm from SAR is needed to support the ATI-SAR current retrieval. We have shown that with an appropriate processing of the ATI-SAR phase and with applying the necessary corrections to the measured velocity a good agreement with ocean circulation models is achieved (rmse =0.1 m /s). These corrections include phase calibration and wind compensation to correct for instrument and geophysical systematic errors, respectively. Finally, a novel method for removing the wind direction ambiguity, based on the ATI-SAR phase, is presented. In previous methods, the wind ambiguity removal was based on external information, e.g. an atmospheric model or on visual observation of wind shadows

Locating Ice Sheet Grounding Lines Using Satellite Radar Interferometry and Altimetry

In this thesis, I use synthetic aperture radar (SAR) and radar altimeter data to make new observations of Antarctic and Greenland ice sheet grounding lines. I use ERS SAR data acquired between 1992 and 2011 to map the Petermann Glacier grounding line on 17 occasions using quadruple difference interferometric SAR (QDInSAR). Over the 19-year period, the grounding line position varied by 0.5 km, on average, with no significant trend over time. Although tidal forcing explains a fraction (34 %) of the movement, localised variations in the glacier thickness could explain it all were they to alter the glaciers hydrostatic balance as they advect downstream – a hitherto unconsidered possibility that would reduce the accuracy with which changes in grounding line position can be detected. Next, I developed a new technique for detecting grounding lines using differential range direction offset tracking (DRDOT) in incoherent SAR data. I then applied this technique to a sequence of 11 TerraSAR-X images acquired in 2009 over Petermann Glacier. The DRDOT technique is able to reproduce the shape and location of the grounding line with an estimated lateral precision of 0.8 km and, although this is 30 times poorer than QDInSAR, provides a complementary method given the paucity of coherent SAR data. Finally, I developed another new method for detecting the grounding line as the break in ice sheet surface slope computed from CryoSat-2 elevation measurements. I then applied this technique to map grounding lines in the sectors of Antarctica buttressed by the Filchner-Ronne, Ekström, Larsen-C, and Amundsen Sea ice shelves. The technique is able to map the grounding line to within 4.5 km, on average, and, although this is far poorer than either QDInSAR or DRDOT, it is computationally efficient and can succeed where SAR-based methods fail, offering an additional complementary approach