Relative Height Accuracy Research Articles

TanDEM-X: 10 Years of Formation Flying Bistatic SAR Interferometry

On June 21, 2010, the TanDEM-X mission was launched and opened a new era in spaceborne radar remote sensing. The first formation flying radar system was built by extending the TerraSAR-X synthetic aperture radar (SAR) mission by a second, TerraSAR-X-like satellite TanDEM-X. The resulting large single-pass SAR interferometer features flexible baseline selection, enabling the acquisition of highly accurate cross-track interferograms not impacted by temporal decorrelation and atmospheric disturbances. The primary objective of the mission was the generation of a global Digital Elevation Model (DEM) with unprecedented accuracy (12-m horizontal resolution and 2-m relative height accuracy). The main mission phase for DEM data acquisition has been finished in 2014; the processing of the global TanDEM-X DEM was concluded in September 2016. The final DEM product is well within specifications and features an extremely low percentage of void areas. It is of fundamental importance for a wide range of commercial and scientific applications. But the scientific exploitation of TanDEM-X is not limited to the DEM. TanDEM-X has unique capabilities, including along-track interferometry, and new bistatic and multistatic SAR techniques, that support numerous secondary mission objectives. Indeed, some of these experiments were directly performed during the DEM acquisition phase, when suitable satellite formation geometries were available. Moreover, regular acquisitions over selected super test sites enabled multitemporal analyses. A dedicated science phase after the DEM acquisitions included up to 4 km cross-track baselines, operation in the so-called Dual-Receive Antenna mode, as well as a period in pursuit monostatic flight formation. Comparisons of the TanDEM-X DEM with that of SRTM, or among multitemporal TanDEM-X data, revealed dramatic, ongoing, changes in Earth's topography, especially over ice and forests. In the last 3.5 years the mission has further acquired data for a global change layer showing the height changes relative to the first global DEM dataset. The so-called “Change DEM” is planned for release in 2021. Despite being well beyond their design lifetime, both satellites are still fully functional and have enough consumables for several additional years. Therefore, bistatic operations continue with a focus on changes in the cryosphere, biosphere, and densely populated urban areas.

SAR Tomography at the Limit: Building Height Reconstruction Using Only 3–5 TanDEM-X Bistatic Interferograms

Multi-baseline interferometric synthetic aperture radar (InSAR) techniques are effective approaches for retrieving the 3-D information of urban areas. In order to obtain a plausible reconstruction, it is necessary to use more than twenty interferograms. Hence, these methods are commonly not appropriate for large-scale 3-D urban mapping using TanDEM-X data where only a few acquisitions are available in average for each city. This work proposes a new SAR tomographic processing framework to work with those extremely small stacks, which integrates the non-local filtering into SAR tomography inversion. The applicability of the algorithm is demonstrated using a TanDEM-X multi-baseline stack with 5 bistatic interferograms over the whole city of Munich, Germany. Systematic comparison of our result with TanDEM-X raw digital elevation models (DEM) and airborne LiDAR data shows that the relative height accuracy of two third buildings is within two meters, which outperforms the TanDEM-X raw DEM. The promising performance of the proposed algorithm paved the first step towards high quality large-scale 3-D urban mapping.

Quality of Height Models Covering Large Areas

Digital height models (DHM) are a basic requirement for several applications. The generation of DHM is time consuming and expensive, but some nearly worldwide covering height models are available free of charge or commercially and the number and quality are growing. For practical use it is important to have some information about the quality, the accuracy, accuracy characteristics, areas with problems, height definition as digital surface model with the height of the visible surface or as digital terrain model with heights of the bare ground, resolution (point spacing and correlation of neighboured height values), homogeneity and availability. Also morphologic details are important, depending upon the point spacing and relative height accuracy. An overview about the freely available and commercially nearly worldwide covering height models with a satisfying point spacing and accuracy is given. Changes in this area are fast, so only the current status can be described. A really worldwide covering DHM came with the commercial WorldDEM, based on TanDEM-X interferometric synthetic aperture radar (InSAR) from which now also a reduced, free version with 3 arcsec point spacing is available as TDM90. In addition to the highest accuracy in this field with 10 m point spacing, WorldDEM has also very good morphologic details with the exception of city areas. Quality files belonging to WorldDEM include information about areas with problems. Some height models have been analysed by comparison with satisfying reference height models. The characteristics are described as well as the problems of accuracy specifications with different accuracy figures and dependency upon terrain inclination and other parameters, to allow a selection corresponding to the individual requirements. This also includes horizontal shifts or even rotations and higher degree systematic differences between the reference and the analysed DHM.

Landcover-Dependent Assessment of the Relative Height Accuracy in TanDEM-X DEM Products

Digital elevation models (DEMs) are extensively used for a variety of scientific and commercial applications. For the global TanDEM-X DEM, one of the main performance parameters is the relative height accuracy, which is specified to be under 2 m for flat terrain. Land cover types where the radar signal penetrates into a volume, as forest and ice, are excluded from this specification. Knowing the accuracy of the DEM for a specific land cover type is essential for applications relying on it, such as navigation applications, gradient and aspect estimations, and others. This letter is meant to be as add-on the global relative analysis presented in [1] . Here, we present a characterization of the relative height accuracy based on the interferometric coherence assessing the performance for every class defined by the CCI Land Cover Maps at a continental and global basis. This characterization raises the awareness of the advantages and limitations of the DEM for each specific application and helps scientists using the TanDEM-X DEM to interpret their results. In addition, an estimation of the relative height accuracy for acquisitions where volume decorrelation is present is locally performed using the high-frequency component of repeat-pass DEMs differences. For two particular land cover types, both methods are compared. The main source of errors in the DEM generation is clearly associated with strong inhomogeneous signal returns, becoming a driving factor on the actual accuracy of the estimated mean phase center height.

Fusion of Urban TanDEM-X Raw DEMs Using Variational Models

Recently, a new global digital elevation model (DEM) with pixel spacing of 0.4 arcsec and relative height accuracy finer than 2 m for flat areas (slopes $ 20%) and better than 4 m for rugged terrain (slopes $ >$ 20%) was created through the TanDEM-X mission. One important step of the chain of global DEM generation is to mosaic and fuse multiple raw DEM tiles to reach the target height accuracy. Currently, weighted averaging (WA) is applied as a fast and simple method for TanDEM-X raw DEM fusion, in which the weights are computed from height error maps delivered from the Integrated TanDEM-X Processor (ITP). However, evaluations show that WA is not the perfect DEM fusion method for urban areas, especially in confrontation with edges such as building outlines. The main focus of this paper is to investigate more advanced variational approaches such as TV- $ L_{1}$ and Huber models. Furthermore, we also assess the performance of variational models for fusing raw DEMs produced from data takes with different baseline configurations and height of ambiguities. The results illustrate the high efficiency of variational models for TanDEM-X raw DEM fusion in comparison to WA. Using variational models could improve the DEM quality by up to 2 m, particularly in inner city subsets.

Spaceborne Staring Spotlight SAR Tomography—A First Demonstration With TerraSAR-X

With the objective of exploiting hardware capabilities and preparing the ground for the next-generation X-band synthetic aperture radar (SAR) missions, TerraSAR-X and TanDEM-X are now able to operate in staring spotlight mode, which is characterized by an increased azimuth resolution of approximately 0.24 m compared with 1.1 m of the conventional sliding spotlight mode. In this paper, we demonstrate for the first time its potential for SAR tomography (TomoSAR). To this end, we tailored our interferometric and tomographic processors for the distinctive features of the staring spotlight mode, which will be analyzed accordingly. By means of its higher spatial resolution, the staring spotlight mode will not only lead to a denser point cloud but also to more accurate height estimates due to the higher signal-to-clutter ratio. As a result of a first comparison between sliding and staring spotlight TomoSAR, first, the density of the staring spotlight point cloud is approximately $\text{5.1}$ – $\text{5.5}$ times as high; and, second, the relative height accuracy of the staring spotlight point cloud is approximately $\text{1.7}$ times as high.

TANDEM-X MISSION STATUS, PRODUCTS AND PERSPECTIVES

Abstract. TanDEM-X is an innovative single-pass interferometric radar mission, which is comprised of two formation flying satellites. The TerraSAR-X (TSX) satellite was launched on June 15th 2007, and its almost identically constructed twin satellite TanDEM-X (TDX) was launched on 21st of June 2010. Together they supply high-quality radar data in order to serve two main mission goals: Scientific observation of the Earth and provision of remote sensing data for the commercial market (TerraSAR-X mission), and the generation of a global digital elevation model (DEM) of the Earth’s surface (TanDEM-X mission). Between December 2010 and early 2015, radar data of all land surfaces has been acquired and as of September 2016 the final TanDEM-X DEM dataset has been available. This paper provides a final quality assessment of the TanDEM-X global DEM products with respect to the relative and absolute height accuracy and data coverage both at the global and geocell level. In addition, an overview on current mission status is given and new DEM products, which are currently in the scope of the TanDEM-X mission, are described.

The Dual-Baseline Phase Unwrapping Correction Framework for the TanDEM-X Mission Part 1: Theoretical Description and Algorithms

The TanDEM-X mission is the first free flying bistatic SAR mission. It has the primary objective to generate within a short time frame a global digital elevation model (DEM) of 10-m absolute vertical accuracy and 2-m relative height accuracy. For that, the whole land mass has been mapped at least twice with different baselines. The success of the mission depends on the accuracy of the final DEM and therefore on the reliability of the phase unwrapping (PU) algorithm. Hence, a robust and versatile PU method, which is in accordance with the acquisition concept, is necessary. This paper presents a new method that combines bistatic high-resolution interferometric data in order to perform an accurate PU on a huge amount of data. The dual-baseline PU correction (DB-PUC) framework addresses this challenge by correcting errors that occurred during the single-baseline PU procedure. It benefits from the additional information available through the differential interferogram and the stereo-radargrammetric phase , which are used to correct region-wise the ambiguity bands of the misestimated unwrapped phases to be less sensitive to noise and possible temporal changes. The multilevel of the DB-PUC approach makes it flexible, computationally efficient, and well adapted to deal with the various PU error scenarios. This framework is used operationally for the processing of the data of the TanDEM-X mission.

Relative Height Accuracy Estimation Method for InSAR-Based DEMs

Digital elevation models (DEMs) are of fundamental importance for a broad range of commercial and scientific applications. The relative height accuracy of the DEM is one of the most important quality parameters as it refers to the local differences among adjacent elevation values. A direct comparison against a reliable reference DEM is only possible for some single regions where an accurate model similar in resolution is already available. DEMs generated from interferometric synthetic aperture radar (InSAR) can be analyzed with respect to the relative height accuracy based on the data itself. This paper introduces a novel approach to estimate the relative height accuracy. The analysis is based on the well-known statistical characteristics of SAR interferograms and is designed as a specification check for global DEM generation. First results of evaluation of the relative height accuracy are presented for TanDEM-X data.

TANDEM-X MISSION STATUS

Abstract. TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an innovative formation flying radar mission that opens a new era in spaceborne radar remote sensing. Its primary objective is the acquisition of a global Digital Elevation Model (DEM) with unprecedented accuracy (12 m horizontal resolution and 2 m relative height accuracy). This goal is achieved by extending the TerraSAR-X synthetic aperture radar (SAR) mission by a second TerraSAR-X like satellite, TanDEM-X (TDX). Both satellites fly in close orbit formation of a few hundred meters distance, and the resulting large single-pass SAR interferometer features flexible baseline selection enabling the acquisition of highly accurate cross-track interferograms not impacted by temporal decorrelation and atmospheric disturbances. Beyond the global DEM, several secondary mission objectives based on along-track interferometry as well as new bistatic and multistatic SAR techniques have been defined. Since 2010 both satellites have been operated in close formation to map all land surfaces at least twice and difficult terrain even up to four times. While data acquisition for the DEM generation will be concluded by the end of 2014 it is expected to complete the processing of the global DEM in the second half of 2016.

Projekt TanDEM-X radarowych obserwacji powierzchni Ziemi prowadzonych z pułapu satelitarnego - założenia i status misji

TanDeM-X is DLr (Deutsches zentrum fur Luftund raumfahrt) the earth surface observation project using high-resolution sAr interferometry. it opens a new era in space borne radar remote sensing. The system is based on two satellites: TerrasAr-X (TsX) and TanDeM-X (TDX) flying on the very close, strictly controlled orbits. This paper gives an overview of the radar technology and overview of the TanDeM-X mission concept which is based on several innovative technologies. The primary objective of the mission is to deliver a global digital elevation model (DeM) with an unprecedented accuracy, which is equal to or surpass the HrTi-3 specifications (12 m posting, relative height accuracy ±2 m for slope 20% [8]). Beyond that, TanDeM-X provides a highly reconfigurable platform for the demonstration of new radar imaging techniques and applications.

TanDEM-X: The New Global DEM Takes Shape

Abstract-TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an innovative formation flying radar mission that opens a new era in spaceborne radar remote sensing. The primary objective is the acquisition of a global Digital Elevation Model (DEM) with unprecedented accuracy (12 m horizontal resolution and 2 m relative height accuracy). This goal is achieved by extending the TerraSAR-X synthetic aperture radar (SAR) mission by a second, TerraSAR-X like satellite TanDEM-X (TDX) flying in close formation with TerraSAR-X (TSX). The resulting large single-pass SAR interferometer features flexible baseline selection enabling the acquisition of highly accurate cross-track interferograms not impacted by temporal decorrelation and atmospheric disturbances. Beyond the global DEM, several secondary mission objectives based on alongtrack interferometry as well as new bistatic and multistatic SAR techniques have been defined.

Results of the TanDEM-X Baseline Calibration

The TanDEM-X mission based on two satellites provides a radar interferometer in space with the goal to derive a global Digital Elevation Model (DEM) with never achieved quality for a global coverage: a global DEM with a relative height accuracy of 2 m and 10 m absolute. In order to achieve this mission goal, the radial and cross-track distance between both satellites, the so called baseline has to be known with high accuracy. Only then, systematic baseline errors can be detected and compensated for, i.e., an accurate calibration of the global DEM can be ensured. The paper describes the procedure and the results of calibrating the baseline, verifying the outstanding accuracy of this calibration procedure.

TANDEM-X MISSION: OVERVIEW, STATUS AND OUTLOOK

Abstract. TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) opens a new era in spaceborne radar remote sensing [1]–[3], [9], [12]. Realized in the framework of a public-private partnership between the German Aerospace Center (DLR) and EADS Astrium GmbH, a single-pass SAR interferometer with adjustable baselines in across- and in along-track directions was formed by adding a second (TDX), almost identical spacecraft to TerraSAR-X (TSX) and flying the two satellites in a closely controlled formation on average orbit height of 514 km. This allows the acquisition of highly accurate cross-track and along-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry due to temporal decorrelation and atmospheric disturbances. With typical across-track baselines of 150 to 500 m a global Digital Elevation Model (DEM) with 2 m relative height accuracy at a 12 m posting is being generated.

TanDEM-X: A radar interferometer with two formation-flying satellites

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an innovative formation-flying radar mission that opens a new era in spaceborne radar remote sensing. The primary objective is the acquisition of a global digital elevation model (DEM) with unprecedented accuracy (12m horizontal resolution and 2m relative height accuracy). This goal is achieved by extending the TerraSAR-X synthetic aperture radar (SAR) mission by a second, TerraSAR-X like satellite (TDX) flying in close formation with TerraSAR-X (TSX). Both satellites form together a large single-pass SAR interferometer with the opportunity for flexible baseline selection. This enables the acquisition of highly accurate cross-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry due to temporal decorrelation and atmospheric disturbances. Besides the primary goal of the mission, several secondary mission objectives based on along-track interferometry as well as new bistatic and multistatic SAR techniques have been defined, representing an important and innovative asset of the TanDEM-X mission. TanDEM-X is implemented in the framework of a public–private partnership between the German Aerospace Center (DLR) and EADS Astrium GmbH. The TanDEM-X satellite was successfully launched in June 2010 and the mission started its operational data acquisition in December 2010. This paper provides an overview of the TanDEM-X mission and summarizes its actual status and performance. Furthermore, results from several scientific radar experiments are presented that show the great potential of future formation-flying interferometric SAR missions to serve novel remote sensing applications.

Relative height accuracy analysis of TanDEM-X system

Relative height accuracy analysis of TanDEM-X system

Analysis of Correlation and Relative Height Accuracy for Spaceborne Parasitic InSAR System

Analysis of Correlation and Relative Height Accuracy for Spaceborne Parasitic InSAR System

Mapping of lunar surface from side-looking orbital radar images

Side-looking spacecraft radar imagery has thus far been produced only from an orbit around the Moon. This was a part of the Apollo Lunar Sounder Experiment (ALSE) of the Apollo 17 mission in December, 1972. This paper reports results of a radargrammetric evaluation of overlapping Apollo 17 synthetic aperture radar images (wavelength 2 m). The potential to map from single images and to reconstruct 3D stereoscopic models is studied. The relative height accuracy achieved is about ± 100 m and is thus competitive with that obtained with the vidicon camera that is presently used for planetary exploration.