Michele Martone

TanDEM-X: DEM acquisition in the third year era

TerraSAR-X add-on for digital elevation measurement (TanDEM-X) is a space-borne X-band SAR mission designed to derive a digital elevation model (DEM) of the Earth’s land surface with an unprecedented relative vertical accuracy of 2 m at a 12 m posting. To achieve this goal, the two satellites fly in close controlled formation with the opportunity to adjust flexible along- and across-track baselines. A combination of multiple acquisitions with different baseline lengths is required to allow stable phase unwrapping and to achieve the high height accuracy. This paper provides an overview of the TanDEM-X acquisition plan for the third year over Antarctica, mountainous areas and deserts. Critical aspects of the areas to be acquired are presented and the acquisition strategy is illustrated. Moreover, the formation flight and the baselines are compared to the ones of the previous years in terms of their impact on the height of ambiguity and on the performance with respect to the interferometric quality.

The TanDEM-X DEM Mosaicking: Fusion of Multiple Acquisitions Using InSAR Quality Parameters

Since 2010, TanDEM-X and its twin satellite TerraSAR-X fly in a close orbit formation and form a single-pass synthetic aperture radar (SAR) interferometer. The formation was established to acquire a global high-precision digital elevation model (DEM) using SAR interferometry (InSAR). In order to achieve the required height accuracy of the TanDEM-X DEM, at least two global coverages have to be acquired. However, in difficult and mountainous terrain, up to five coverages are present. Here, acquisitions from ascending and descending orbits are needed to fill gaps and to overcome geometric limitations. Therefore, a strategy to properly combine the available height estimates is mandatory. The objective of this paper is the presentation of the operational TanDEM-X DEM mosaicking approach. In general, multiple InSAR DEM heights are combined by means of a weighted average with the height error as weight. Apart from this widely used mosaicking approach, one big challenge remains with the handling of larger height discrepancies between the input data, which are mainly caused by phase unwrapping errors, but also by temporal changes between acquisitions. In the case of inconsistencies, the TanDEM-X mosaicking approach performs a grouping into height levels. A priority concept is set up to evaluate the different groups of heights considering the number of DEMs and several InSAR quality parameters: the height error, the phase unwrapping method, and the height of ambiguity. This allows the identification of the most reliable height level for mosaicking. This fusion concept is verified on different test areas affected by phase unwrapping errors in flat and mountainous terrain as well as by height discrepancies in forests. The results show that the quality of the final TanDEM-X DEM mosaic benefits a lot from this mosaicking approach.

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.

Quantization Effects in TanDEM-X Data

TerraSAR-X add-on for Digital Elevation Measurement (TanDEM-X) is an innovative spaceborne bistatic SAR system comprising the twin satellites TerraSAR-X and TanDEM-X (TSX and TDX, respectively). The primary objective of the mission is the generation of a worldwide, timely, and consistent digital elevation model (DEM) in a bistatic synthetic aperture radar (SAR) configuration with unprecedented accuracy. For TanDEM-X and for future spaceborne SAR missions, an increasing volume of onboard data is going to be demanded. This is due to the employment of large bandwidths, high pulse repetition frequencies, and multiple polarizations, which implies inevitably hard requirements in terms of onboard memory and downlink capacity. In this scenario, SAR raw data quantization represents an essential aspect. The data rate employed for the digitization of the recorded radar signal affects both the amount of data to be stored and transmitted to the ground and the quality of the resulting SAR products. In this paper, the impact of quantization on TanDEM-X monostatic and interferometric data is evaluated. Key quantities in estimating interferometric and SAR performance, such as coherence and phase errors, are investigated in detail. Based on the obtained results, an optimization of the resource-allocation strategy for the global DEM acquisition of TanDEM-X is discussed.

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.

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.

First Characterization and Performance Evaluation of Bistatic TanDEM-X Experimental Products

TanDEM-X is an innovative bistatic and interferometric mission comprising the two twin synthetic aperture radar (SAR) satellites TerraSAR-X and TanDEM-X, which fly in a configurable close formation with the main goal to generate a global, high-accurate, and homogeneous digital elevation model (DEM) of the Earths surface. The DEM is acquired in bistatic stripmap single polarization mode. On top of that, due to the flexibility offered by operating both satellites in different interferometric and imaging constellations, combinations of several experimental modes are possible. A dedicated science phase of the mission started in October 2014 and opened the full exploitation of such experimental modes. These unique data will support a variety of scientific remote sensing applications and will demonstrate new and innovative SAR techniques. In this paper, the expected SAR and interferometric performance of the operationally processed experimental modes available during the bistatic global DEM acquisition phase is modeled and characterized with in-orbit data. Key parameters for the determination of the quality of an SAR product, such as image resolution or noise equivalent sigma zero, or for the interferometric performance estimation, such as coherence or relative height error, are analyzed. Finally, possibilities to increase the quality of the generated DEMs are addressed.

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.