Paola Rizzoli

First 2 years of TanDEM‐X mission: Interferometric performance overview

The TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) mission comprises two nearly identical satellites: TerraSAR-X (TSX, launched in June 2007), and TanDEM-X (TDX, launched in June 2010), which form an innovative and flexible single-pass radar interferometer. The primary objective of the mission is to generate a worldwide and consistent digital elevation model (DEM) with an unprecedented accuracy. After a calibration phase of the TDX satellite, which was performed during the first 3 months after its launch, the two satellites were brought into close formation to begin the bistatic commissioning phase. Then, in December 2010, TanDEM-X started the operational global DEM acquisition in bistatic configuration. During the last 2 years, dedicated analyses on test acquisitions as well as persistent monitoring of the interferometric performance have been carried out, which are the subject of this paper. Key quantities in estimating interferometric performance such as coherence, relative height error, and phase-unwrapping indicators are investigated, showing the outstanding capabilities of TanDEM-X. Then, the main focus is shifted to those critical areas which, for various reasons, have shown unsatisfactory data quality and therefore must be reacquired with optimized imaging geometries in order to fulfill the DEM accuracy requirements. Promising results have been obtained so far, and future strategies to handle the critical data are discussed. This paper will present an overview of the interferometric performance of TanDEM-X, based on investigations performed in the first 2 years of mission operation, and will include results from the bistatic commissioning phase until the end of the first global DEM acquisition.

InSAR and DEM quality monitoring of TanDEM-X

TanDEM-X is an interferometric SAR (InSAR) mission acquiring bistatic images with two satellites. Systematic mapping of the Earths land masses will provide individual interferometric data sets which will be mosaicked and calibrated into a global Digital Elevation Model (DEM). The concept of InSAR and DEM quality monitoring throughout the acquisition and processing phase is presented in this paper.

Global Interferometric Coherence Maps From TanDEM-X Quicklook Data

TanDEM-X is a spaceborne synthetic aperture radar (SAR) mission, whose goal is the generation of a global digital elevation model with unprecedented accuracy, by using SAR interferometry. One of the main parameters for asserting the quality of interferometric products is the coherence between the monostatic and bistatic images. The objective of this letter is to present the first global mosaics of the interferometric coherence generated from the TanDEM-X quicklook data set, achieving a resolution down to 25 × 25 ma. This is an improvement in terms of details by several orders of magnitude, with respect to the previously implemented techniques for monitoring the global TanDEM-X interferometric coherence. Critical performance areas are separately analyzed, focusing on the developed approach for optimizing the acquisition strategy, in order to achieve the final mission requirement. Moreover, TanDEM-X mosaics of the interferometric coherence show to be a promising starting point for land classification on a large scale. Finally, they represent a valuable input for the whole SAR community, allowing for the recognition of suitable test areas for further scientific purposes.

Volume Decorrelation Effects in TanDEM-X Interferometric SAR Data

Among the several factors that may affect the quality of interferometric synthetic aperture radar (SAR) products, volume decorrelation represents the coherence loss contribution due to the presence of multiple scatterers within a single resolution cell, which results in an increase in the interferometric phase uncertainty. In this letter, we investigate the effects of volume decorrelation on X-band TanDEM-X interferometric data. TanDEM-X is the first bistatic spaceborne SAR mission and provides a unique, global, and manifold interferometric data set to be exploited for a variety of scientific and commercial applications. The main goal of this letter is to provide the scientific community with a characterization of volume decorrelation effects occurring at X-band for different land cover types and acquisition geometries. The potentials of volume decorrelation contribution at X-band for land classification are discussed as well and some application examples are presented.

Radar Backscatter Mapping Using TerraSAR-X

Global backscatter data can be used for accurate synthetic aperture radar (SAR) performance estimation and optimizing instrument settings for SAR systems, e.g. the TerraSAR-X mission (TSM) and the TanDEM-X mission (TDM). The goal of this work is the generation of X-band backscatter maps by mosaicking images acquired by the TSM. An algorithm that allows the estimation of the on-ground backscatter, for any required polarization and incidence angle from the available data, is implemented. In this paper, the backscatter map generation algorithm is presented, together with the first results, obtained from the TSX-1 data. The validity of the interpolation models is also discussed, as they will form the basis for a future global statistical analysis and modeling of backscatter behavior in X-band SAR data.

Enhancing Interferometric SAR Performance Over Sandy Areas: Experience From the TanDEM-X Mission

The TanDEM-X mission is served by two X-band synthetic aperture radar (SAR) satellites, which fly in close orbit formation acting as a large and flexible single-pass radar interferometer. The primary goal of the mission is the creation of a consistent and global digital elevation model (DEM). A very good and reproducible performance has been verified for most of the land masses. In this paper, a detailed performance analysis of TanDEM-X data is presented for sandy desert areas, which show a strong impact on the quality of spaceborne SAR surveys. The influence of several acquisition parameters on SAR and interferometric (InSAR) performance is evaluated by means of statistical analyses as well as long-term repeated acquisitions on defined test sites. Alternative processing approaches aiming at improving the quality of the interferometric products are presented, too. From the obtained analyses, a description of the scattering mechanisms occurring at X-band over sandy surfaces is derived, which allows to plan a dedicated reacquisition of such areas with optimized imaging geometry to improve the quality of the final TanDEM-X DEM.

TanDEM-X acquisition and quality overview with two global coverages

TanDEM-X is a spaceborne SAR mission with the goal to derive a global Digital Elevation Model (DEM) of high resolution and unprecedented level of accuracy. This paper gives an overview on the acquisition planning and data analysis after completion of two global coverages. The first part summarizes the DEM acquisition strategy including the satellite formation evolution, coverage status, and the planning concept for further interferometric measurements over difficult terrain. In the second part of the paper, the single acquisitions are analyzed for their interferometric quality, such as coherence and relative height errors. After calibration of systematic baseline offsets and instrument internal effects, the monitoring status of absolute DEM height errors is presented, as well.

Characterization of Snow Facies on the Greenland Ice Sheet Observed by TanDEM-X Interferometric SAR Data

This paper presents for the first time a detailed study on information content of X-band single-pass interferometric spaceborne SAR data with respect to snow facies characterization. An approach for classifying different snow facies of the Greenland Ice Sheet by exploiting X-band TanDEM-X interferometric synthetic aperture radar acquisitions is firstly detailed. Large-scale mosaics of radar backscatter and volume correlation factor, derived from quicklook images of the interferometric coherence, represent the starting point for applying an unsupervised classification method based on the c-means fuzzy clustering algorithm. The data was acquired during winter 2010/2011. A partition of four different snow facies was chosen and interpreted using reference melt data, snow density, and in situ measurements. The variations in the stratification and micro-structure of firn, such as the variations of density with depth and the presence of percolation features, are identified as relevant parameters for explaining the significant differences in the observed interferometric signatures among different snow facies. Moreover, a statistical analysis of backscatter and volume correlation factor provided useful parameters for characterizing the snow facies behavior and analyzing their dependency on the acquisition geometry. Finally, knowing the location and characterization of the different snow facies, the two-way X-band penetration depth over the whole Ice Sheet was estimated. The obtained mean values vary from 2.3 m for the outer snow facies up to 4.18 m for the inner one. The presented approach represents a starting point for a long-term monitoring of ice sheet dynamics, by acquiring time-series, and is of high relevance for the design of future SAR missions as well.

TanDEM-X global DEM quality status and acquisition completion

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements)is an interferometric SAR mission flying two radar satellites in close orbit formation. Its primary goal is the production of a homogeneous global digital elevation model (DEM) of unprecedented accuracy. Since 2010 all land surfaces have been mapped at least twice and difficult terrain even up to four times. While data acquisition for the DEM generation will be concluded in August 2014 it is expected to complete the processing of the global DEM by the end of 2015. This paper gives a status update on the current acquisition planning and presents quality results from a huge data base of more than 400,000 single DEM scenes and 1700 final DEM products.

A method for generating forest/non-forest maps from TanDEM-X interferometric data

In this paper a method for the generation of a global forest/non-forest map from TanDEM-X interferometric data is presented. The quality of interferometric products is strongly influenced by the specific characteristics of the illuminated land cover type. Over forested areas the presence of multiple scatterers at different heights and within a single resolution cell results in an increase of the interferometric phase uncertainty (the so-called volume decorrelation), whose intensity depends on several factors, such as the acquisition geometry, the sensor parameters, and the canopy density. From each TanDEM-X coherence map the volume decorrelation contribution can be estimated, leading to the derivation of a forest/non-forest map. This paper shows the developed approach for discriminating between forested and non-forested areas from TanDEM-X interferometric data, and presents some examples aimed at verifying the validity of the proposed method. From this, mosaics can be generated from the TanDEM-X quicklook data set at a final resolution up to 25 m × 25 m.