Cristian Rossi

Interferometric processing of TanDEM-X data

Since July 2010, TerraSAR-X and TanDEM-X jointly acquire interferometric data. Starting their common commissioning phase with a so called pursuit monostatic configuration with 3 seconds time lag between the two passes, they were later put in a close formation in October 2010, acting since then as the first freely configurable bistatic SAR interferometer in space. All operational acquisitions were processed from instrument raw data to DEMs from day one of the data taking on by one single processing system: the Integrated TanDEM-X Processor (ITP) (see [1],[2]). Data take analysis, common parameter calculation, synchronization, bistatic focusing, filtering, co-registration, phase unwrapping and geocoding are all performed in one sequence inside this processor. This approach allows a high precision processing by passing all applied corrections and determined parameters from one step to the next. Specifically the geometric & phase accuracy and stability of the instruments, the processor and the auxiliary data (i.e. the millimetric precision of the baseline products) provide an unprecedented level of relative and absolute geometric accuracy in the bistatic operation. While many challenges of bistatic processing of the TanDEM-X data are encountered, the benefits of this single pass acquisition mode can be used to derive additional information from the data itself for further processing and calibration. In this paper, we will outline the bistatic interferometric processing steps of the ITP and focus on the aspects of geometric accuracy in DEM generation.

Paddy-Rice Monitoring Using TanDEM-X

This paper evaluates the potential of spaceborne bistatic interferometric synthetic aperture radar images for the monitoring of biophysical variables in wetlands, with a special interest on paddy rice. The assessment is made during the rice cultivation period, from transplanting to harvesting time (May to October) for fields around Gala lake (Turkey), one of the largest and most productive paddy rice planting area in the country. Detailed ground truth measurements describing biophysical parameters are collected in a dedicated campaign. A stack of 16 dual-pol TanDEM-X images is used for the generation of 32 digital elevation models (DEMs) over the studied area. The quality of the data allows the use of the interferometric phase as a state variable capable to estimate crop heights for almost all the growing stages. The early vegetative rice stage, which is characterized by flooded fields, cannot be represented by the interferometric phase due to a low signal-to-noise ratio but can be easily detected by amplitude and interferometric coherence thresholding. A study on the impact of the polarization in the signal backscatter is also performed. An analysis of the differences between HH and VV DEMs shows the varying signal penetration for the two polarizations at different growing stages. The validation with reference data demonstrates the capability to establish a direct relationship between interferometric phase and rice growth. The very high coherence of TanDEM-X data yields elevation estimates with root-mean-square error in a decimetric level, supporting temporal change analysis on a field-by-field basis.

Quality Assessment of Surface Current Fields From TerraSAR-X and TanDEM-X Along-Track Interferometry and Doppler Centroid Analysis

All existing examples of current measurements by spaceborne synthetic aperture radar (SAR) along-track (AT) interferometry (ATI) have suffered from short baselines and corresponding low sensitivities. Theoretically, the best data quality at X-band is expected at effective baselines on the order of 30 m, i.e., 30 times as long as the baselines of the divided-antenna modes of TerraSAR-X. In early 2012, we had a first opportunity to obtain data at near-optimum baselines from the TanDEM-X satellite formation. In this paper, we analyze two TanDEM-X interferograms acquired over the Pentland Firth (Scotland) with effective AT baselines of 25 and 40 m. For comparison, we consider a TerraSAR-X dual-receive-antenna (DRA)-mode interferogram with an effective baseline of 1.15 m, as well as velocity fields obtained by Doppler centroid analysis (DCA) of single-antenna data from the same three scenes. We show that currents derived from the TanDEM-X interferograms have a residual noise level of 0.1 m/s at an effective resolution of about 33 m × 33 m, while DRA-mode data must be averaged over 1000 m × 1000 m to reach the same level of accuracy. A comparison with reference currents from a 1-km resolution numerical tide computation system shows good agreement in all three cases. The DCA-based currents are found to be less accurate than the ATI-based ones but close to short-baseline ATI results in quality. We conclude that DCA is a considerable alternative to divided-antenna mode ATI, while our TanDEM-X results demonstrate the true potential of the ATI technique at near-optimum baselines.

Interferometric processing and products of the TanDEM-X mission

Started in June 2010, the TanDEM-X satellite joined the TerraSAR-X satellite in space to perform the conjoint interferometric TanDEM-X mission to acquire a truly global Digital Elevation Model (DEM) of unprecedented accuracy [1]. Since the very first interferometric acquisitions, the Integrated TanDEM-X Processor (ITP) delivered operationally “Raw”-DEMs and complex products of mono- and bistatic data. The RawDEMs are scenes of about 50 km × 30 km, generated for a dedicated DEM Mosaicking and Calibration Processor (MCP) which produces the final DEM. The so-called Coregistered Single-look Slant-range Complex (CoSSC) products are provided for each of these scenes in different flavors for production internal purposes and system performance monitoring as well as for scientific use. The capabilities of the ITP go far beyond the primary mission objective of DEM generation alone: it also provides the operationally available end-user products from different experimental modes as e.g. pursuit monostatic, dual polarization bistatic data, alternating bistatic in single and dual polarization and different bistatic and alternating bistatic spotlight modes. This paper focuses on the accuracy of the generated products, the ITPs contribution to the achieved accuracy of the data and the direct effect of it on the use and interpretation of RawDEM heights for temporal change detection. Also the basic characteristics of the operational experimental products are introduced..

Polarization Impact in TanDEM-X Data Over Vertical-Oriented Vegetation: The Paddy-Rice Case Study

It has been recently shown that the TanDEM-X mission is capable of tracking the plant growth of rice paddies. The precision of the elevation measure depends on the physical interaction between the synthetic aperture radar (SAR) signal and the canopy. In this letter, this interaction is studied by considering the signal polarization. In particular, the vertical and horizontal wave polarizations are compared, and their performance in the temporal mapping of the crop height is analyzed. The temporal elevation difference analysis shows a monotonically increasing trend within the reproductive stage of the canopy, with maximum height discrepancies between polarizations of about 9 cm. From an operational point of view of InSAR-based vegetation height measurements, this letter demonstrates that the oriented structure of the canopy shall be considered not only in polarimetric InSAR studies but also in the interpretation of bistatic spaceborne interferometric elevation models.

Surface current retrieval from TerraSAR-X data using Doppler measurements

The purpose of this paper is to investigate the estimation of surface currents directly on stripmap TerraSAR-X data, as an alternative to Along Track Interferometry. The algorithm relies on efficient baseband magnitude-based Doppler estimation, preventing the estimate from possible biases like azimuth ambiguities and strong target reflections. The validation of the algorithm is made with acquisitions over the Elbe estuary river in Germany and the Eyjafjallajökull volcano in Iceland.

Processing of bistatic TanDEM-X data

On June 21st, 2010, the German radar satellite TanDEM-X was launched and successfully placed in an orbit approaching the TerraSAR-X satellite until both systems will fly in close formation and will establish the only available bi-static interferometer in space. The primary TanDEM-X mission goal is to generate a global Digital Elevation Model (DEM) with a relative point-to-point height accuracy of 2 meters for moderate terrain at 12 m posting. For that purpose interferometric SAR data will be acquired over a period of 3 years in parallel to the operational running TerraSAR-X mission. Systematic processing of SAR raw data to so-called Raw-DEMs is performed by one single processing system, the Integrated TanDEM Processor (ITP). The final global DEM is then calibrated and mosaicked by a second system, the Calibration and Mosaicking Processor (MCP). The scope of this paper is to present an overview of ITP functionalities and to summarize the first processing results.

High-Resolution InSAR Building Layovers Detection and Exploitation

Layover affects the quality of urban interferometric synthetic aperture radar (InSAR) digital elevation models. Moreover, it is generally difficult to interpret because of the superposition of several contributions in a single SAR pixel. In this paper, a novel technique for the extraction of building layovers is first presented. It makes use of the geocoding stage embedded in the InSAR processor. It is shown that building layovers create a regular pattern in the mapping counter, a map describing the number of occurrences of a SAR pixel in the elevation model. Its exploitation yields a generation of a layover map without the use of external supports. The integration in the processor with a limited additional computational load and the capability to isolate layover signatures are additional benefits. Layover patches are then individually analyzed toward a better understanding of the complex urban signal return. A spectral estimation framework is employed to assess the slopes superimposed in the patches. Fringe-frequency estimation is involved. A set of simulations made for a nonparametric (fast Fourier transform) and a parametric (multiple signal classification) technique is performed prior to testing on real data. It is demonstrated that in X-band, for a single interferogram, just one layover contributor, when it dominates over the others, can be extracted with a sufficient accuracy. The algorithms are tested on a TanDEM-X spotlight acquisition over Berlin (Germany).

Framework for Fusion of Ascending and Descending Pass TanDEM-X Raw DEMs

A novel method for calculating optimum incidence angle for the TanDEM-X system using any available digital elevation model (DEM) for the given area is proposed in this study. This method includes the plotting of slopes and aspect of the test area in a statistical way and applying mathematical approach using acquisition geometry in ascending and descending pass TanDEM-X data to optimize the incidence angle for obtaining precise DEM. Furthermore, the TanDEM-X raw DEMs in ascending and descending pass over Mumbai, India are combined using a simple weighted fusion algorithm and the quality of fused DEM thus generated is enhanced. The method adopted for fusion is just an experimental study. The problem of optimum weight selection for fusion has been addressed using height error map and a robust layover shadow mask calculated in “Integrated TanDEM-X Processor” (ITP) during TanDEM-X DEM generation. The height error map is calculated from the interferometric coherence with geometrical considerations and the robust layover and shadow map is calculated using TanDEM-X DEM and the corresponding slant range. Results show a significant reduction in the number of invalid pixels after fusion. In the fused DEM, invalids are only 2.14%, while ascending and descending pass DEMs have 5.02% and 6.34%, respectively. Statistical analysis shows a slight improvement in standard deviation of the error in fused DEM by 8% in urban area and about 5% for the whole scene. Only slight improvement in accuracy of fused DEM can be attributed to the coarse resolution of the SRTM-X DEM used as reference.

Interferometric processing algorithms of TanDEM-X data

The purpose of this paper is to provide an algorithmic overview of the interferometric processing embedded in the Integrated TanDEM-X Processor (ITP), settled to the generation of the raw digital elevation model (DEM). The main processing blocks are described, with a focus on the spectral matching of the azimuth spectra, the high-precision coregistration, the dual-baseline phase unwrapping and the geocoding of the products. The robustness of the algorithms is demonstrated through a dual-pass TerraSAR-X scenario.