TanDEM-X Satellite Research Articles

Characterization of the giant landslide at Wenjiagou by the insar technique using TSX-TDX CoSSC data

A landslide map is the most basic element for any landslide assessment, especially for giant landslides. However, due to remoteness and hazards, it is difficult to know the characteristics of these landslides after great earthquakes. In this study, the Coregistered Single look Slant range Complex(CoSSC) synthetic aperture radar (SAR) data from TerraSAR-X and its twin satellite TanDEM-X are employed to estimate a high-accuracy digital elevation model (DEM) for the giant landslide in Wenjiagou on Feb. 1, 2013, which was one of the biggest landslides triggered by the 2008 Wenchuan great earthquake. Then, compared to the DEM from SRTM, the elevation change can be estimated, and the characteristics of the Wenjiagou landslide, such as the source and deposit area, are highlighted. Based on the estimation of elevation change, the initial and deposit volumes are estimated as 5.09 × 107 m3 and 4.47 × 107 m3 respectively which are in agreement with previous field investigations by other researchers. And the results also show that using bistatic space-borne SAR data is an effective means for timely assessment of the giant landslide.

Statistical Tests for a Ship Detector Based on the Polarimetric Notch Filter

Ship detection is an important topic in remote sensing and Synthetic Aperture Radar has a valuable contribution, allowing detection at night time and with almost any weather conditions. Additionally, polarimetry can play a significant role considering its capability to discriminate between different targets. Recently, a new ship detector exploiting polarimetric information was developed, namely the Geometrical Perturbation Polarimetric Notch Filter (GP-PNF). This work is focused on devising two statistical tests for the GP-PNF. The latter allow an automatic and adaptive selection of the detector threshold. Initially, the probability density function (pdf) of the detector is analytically derived. Finally, the Neyman-Pearson (NP) lemma is exploited to set the threshold calculating probabilities using the clutter pdf (i.e. a Constant False Alarm Rate, CFAR) and a likelihood ratio (LR). The goodness of fit of the clutter pdf is tested with four real SAR datasets acquired by the RADARSAT-2 and the TanDEM-X satellites. The former images are quad-polarimetric, while the latter are dual-polarimetric HH/VV. The data are accompanied by the Automatic Identification System (AIS) location of vessels, which facilitates the validation of the detection masks. It can be observed that the pdf's fit the data histograms and they pass the two sample Kolmogorov-Smirnov and χ 2 tests.

Monitoring forest carbon in a Tanzanian woodland using interferometric SAR: a novel methodology for REDD.

BackgroundREDD+ implementation requires establishment of a system for measuring, reporting and verification (MRV) of forest carbon changes. A challenge for MRV is the lack of satellite based methods that can track not only deforestation, but also degradation and forest growth, as well as a lack of historical data that can serve as a basis for a reference emission level. Working in a miombo woodland in Tanzania, we here aim at demonstrating a novel 3D satellite approach based on interferometric processing of radar imagery (InSAR).ResultsForest carbon changes are derived from changes in the forest canopy height obtained from InSAR, i.e. decreases represent carbon loss from logging and increases represent carbon sequestration through forest growth. We fitted a model of above-ground biomass (AGB) against InSAR height, and used this to convert height changes to biomass and carbon changes. The relationship between AGB and InSAR height was weak, as the individual plots were widely scattered around the model fit. However, we consider the approach to be unique and feasible for large-scale MRV efforts in REDD+ because the low accuracy was attributable partly to small plots and other limitations in the data set, and partly to a random pixel-to-pixel variation in trunk forms. Further processing of the InSAR data provides data on the categories of forest change.The combination of InSAR data from the Shuttle RADAR Topography Mission (SRTM) and the TanDEM-X satellite mission provided both historic baseline of change for the period 2000–2011, as well as annual change 2011–2012.ConclusionsA 3D data set from InSAR is a promising tool for MRV in REDD+. The temporal changes seen by InSAR data corresponded well with, but largely supplemented, the changes derived from Landsat data.

SATELLITE-BASED RADAR MEASUREMENTS FOR VALIDATION OF HIGHRESOLUTION SEA STATE FORECAST MODELS IN THE GERMAN BIGHT

Abstract. Remote sensing Synthetic Aperture Radar (SAR) data from TerraSAR-X and Tandem-X (TS-X and TD-X) satellites have been used for validation and verification of newly developed coastal forecast models in the German Bight of the North Sea. The empirical XWAVE algorithm for estimation of significant wave height has been adopted for coastal application and implemented for NRT services. All available TS-X images in the German Bight collocated with buoy measurements (6 buoys) since 2013 were processed and analysed (total of 46 scenes/passages with 184 StripMap images). Sea state estimated from series of TS-X images cover strips with length of ~200km and width of 30km over the German Bight from East-Frisian Islands to the Danish coast. The comparisons with results of wave prediction model show a number of local variations due to variety in bathymetry and wind fronts

Multi-sensor mapping of West African land cover using MODIS, ASAR and TanDEM-X/TerraSAR-X data

Land cover information plays an elementary role for regional water and land management, and is an essential variable for the assessment of ecosystem services and regional climate impact. This paper describes the generation of a regionally optimized land cover dataset for West Africa with a spatial resolution of 250m, which is based on earth observation data from three optical and radar instruments. The choice of sensors is based on their individual strengths and weaknesses in assessing specific land surface types. Annual profiles of the optical Moderate Resolution Imaging Spectroradiometer (MODIS) are analyzed for the classification of vegetated classes including agriculture. The classification approach builds on random forest classification with learning data extracted from higher resolution land cover maps. Envisat Advanced Synthetic Aperture Radar (ASAR) Wide Swath (WS) time series are used, in combination with MODIS data, to delineate permanent and seasonal water bodies. Here, an approach integrating threshold classification and morphological operations is applied. Built-up areas of different densities are identified based on a seamless coverage of radar imagery collected by the satellites TanDEM-X and TerraSAR-X. The detection of settlements is based on an unsupervised classification scheme which exploits texture metrics and backscattering amplitudes of the fine resolution radar sensors. The accuracy assessment of the multi-sensor land cover map yields an overall accuracy of 80% at legend level 1 (9 classes) and 73% at the more detailed legend level 2 (14 classes). Comparisons with available wall-to-wall datasets of the region demonstrate the valuable information content of the presented West African land cover map.

Forest biomass change estimated from height change in interferometric SAR height models.

BackgroundThere is a need for new satellite remote sensing methods for monitoring tropical forest carbon stocks. Advanced RADAR instruments on board satellites can contribute with novel methods. RADARs can see through clouds, and furthermore, by applying stereo RADAR imaging we can measure forest height and its changes. Such height changes are related to carbon stock changes in the biomass. We here apply data from the current Tandem-X satellite mission, where two RADAR equipped satellites go in close formation providing stereo imaging. We combine that with similar data acquired with one of the space shuttles in the year 2000, i.e. the so-called SRTM mission. We derive height information from a RADAR image pair using a method called interferometry.ResultsWe demonstrate an approach for REDD based on interferometry data from a boreal forest in Norway. We fitted a model to the data where above-ground biomass in the forest increases with 15 t/ha for every m increase of the height of the RADAR echo. When the RADAR echo is at the ground the estimated biomass is zero, and when it is 20 m above the ground the estimated above-ground biomass is 300 t/ha. Using this model we obtained fairly accurate estimates of biomass changes from 2000 to 2011. For 200 m2 plots we obtained an accuracy of 65 t/ha, which corresponds to 50% of the mean above-ground biomass value. We also demonstrate that this method can be applied without having accurate terrain heights and without having former in-situ biomass data, both of which are generally lacking in tropical countries. The gain in accuracy was marginal when we included such data in the estimation. Finally, we demonstrate that logging and other biomass changes can be accurately mapped. A biomass change map based on interferometry corresponded well to a very accurate map derived from repeated scanning with airborne laser.ConclusionsSatellite based, stereo imaging with advanced RADAR instruments appears to be a promising method for REDD. Interferometric processing of the RADAR data provides maps of forest height changes from which we can estimate temporal changes in biomass and carbon.Electronic supplementary materialThe online version of this article (doi:10.1186/s13021-014-0005-2) contains supplementary material, which is available to authorized users.

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.

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.

Fusion of Monostatic/Bistatic InSAR Stacks for Urban Area Analysis via Distributed Scatterers

Interferometric synthetic aperture radar (SAR) is a powerful technique providing meter-precision elevation maps and millimeter-precision surface displacement maps. Since 2007, the high-resolution SAR satellite TerraSAR-X allows monitoring of even single buildings from space using advanced monostatic repeat-pass stacking techniques. Furthermore, the launch of its twin satellite TanDEM-X in 2010 facilitates bistatic single-pass SAR interferometry. The main objective of this mission is the generation of a global digital elevation model. It also provides a configurable SAR platform for demonstrating new interferometric techniques and applications. However, in dense urban areas, standard TanDEM-X elevation models are inaccurate because ambiguities in radar layover areas cannot be solved. This letter describes the potential of joint monostatic and bistatic (motion-free and atmosphere-free) SAR interferometric stacking for an improved scene elevation and surface deformation estimation in complex urban areas. It involves exploiting distributed scatterers (DSs) using an advanced high-resolution small-baseline subset algorithm. Since most of the scatterers within a radar image can be classified as DSs, there is an increasing focus on an optimal processing of DSs for urban area monitoring. The fusion technique and an application test case are presented using a high-resolution spotlight mixed TerraSAR-X/TanDEM-X data stack of Las Vegas, USA.

Atmosphere sounding by GPS radio occultation: First results from TanDEM-X and comparison with TerraSAR-X

On 21 June 2010 the TerraSAR-X satellite was joined by the TanDEM-X satellite. A Global Positioning System (GPS) radio occultation (RO) experiment using the twin satellites has been carried out to estimate the precision of GPS atmospheric soundings. For the Day Of Year (DOY) 330–336, 2011, we analyze phase and amplitude data recorded by GPS receivers separated by a few hundred meters in a low earth orbit and derive collocated atmospheric refractivity profiles. In the altitude range 10–20km the standard deviation between TerraSAR-X and TanDEM-X refractivity does not exceed 0.15%. The standard deviation is rapidly increasing for lower and higher altitudes; close to the surface and at an altitude of 30km the standard deviation reaches 0.8% and 0.5%, respectively. Systematic deviations between TerraSAR-X and TanDEM-X refractivity in the considered altitude range (0–30km) are negligible. The results confirm the anticipated high precision of the GPS RO technique. However, the difference in the retrieved refractivity in the lower troposphere for different Open Loop (OL) signal tracking parameters, altered onboard TanDEM-X for DOY 49–55, 2012, calls for an in depth analysis. At the moment we can not exclude that a potential bias in the OL Doppler model introduces a bias in our retrieved refractivity at altitudes <8km.

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.

Bidirectional SAR Imaging Mode

This paper introduces the bidirectional synthetic aperture radar (BiDi SAR) imaging mode, i.e., the simultaneous imaging of two directions by one antenna into one receiving channel, and presents short-term time series of images and interferograms acquired by the TerraSAR-X and TanDEM-X satellites. A comparison to alternative approaches for the acquisition of short-term time series is provided. The BiDi acquisition geometry is defined, and a TerraSAR-X BiDi antenna pattern is analyzed. BiDi raw data are simulated, sampled with different pulse repetition frequency values, and compared with real BiDi raw data. The spectral separation of simultaneously acquired forward- and backward-looking images is explained. This paper presents the image results of BiDi acquisitions with TerraSAR-X and TanDEM-X satellites flying with 20-km along-track separation. This pursuit configuration allowed for the acquisition of up to six short-term repeated images and up to three interferograms in a single pass. An overview of potential applications for the new BiDi SAR imaging mode concludes this paper.

The TerraSAR-X/TanDEM-X Formation Acquisition–from Planning to Realization

On June 21, 2010 the TanDEM-X satellite (TDX) was injected into orbit at 15,700 km distance from its twin satellite TerraSAR-X (TSX), which has been in orbit since 2007. Already one month later TDX acquired a formation with TSX in order to build up the first single-pass radar interferometer in space. Within three years of close formation flying with flexible baselines ranging from 150 m to a few kilometers the twin satellites will collect interferometric radar measurements for the generation of a global digital elevation model with unprecedented accuracy. This paper elaborates on the TDX pre-launch analysis performed in the fields of collision assessment during orbit injection and target formation acquisition. To avoid a critical close approach shortly after TDX separation, the risk of collision between the already flying TSX satellite and the newly injected elements (DNEPR upper-stage, gas dynamic shield, and TDX satellite) had to be carefully analyzed. Further, the paper discusses a fuel-saving formation acquisition strategy, for which the maneuver budget is analyzed as a function of launch day and launch injection accuracy. Finally, flight results are presented to illustrate the successful formation acquisition realized in July 2010 and the formation reconfiguration process from the 20 km wide formation into the 300–400 m close formation performed in October 2010. This reconfiguration marked the start of the bi-static TDX/TSX instrument operation.

Operational TanDEM-X DEM calibration and first validation results

In June 2010, the German TanDEM-X satellite was launched. Together with its twin satellite TerraSAR-X it flies in a close formation enabling single-pass SAR interferometry. The primary goal of the TanDEM-X mission is the derivation of a global digital elevation model (DEM) with unprecedented global accuracies of 10m in absolute and 2m in relative height. A significant calibration effort is required to achieve this high quality world-wide. In spite of an intensive instrument calibration and a highly accurate orbit and baseline determination, some systematic height errors like offsets and tilts in the order of some meters remain in the interferometric DEMs and have to be determined and removed during the TanDEM-X DEM calibration. The objective of this article is the presentation of an approach for the estimation of correction parameters for remaining systematic height errors applicable to interferometric height models. The approach is based on a least-squares block adjustment using the elevation of ICESat GLA14 data as ground control points and connecting points of adjacent, overlapping DEMs as tie-points. In the first part its implementation in DLR’s ground segment is outlined. In the second part the approach is applied and validated for two of the first TanDEM-X DEM test sites. Therefore, independent reference data, in particular high resolution reference DEMs and GPS tracks, are used. The results show that the absolute height errors of the TanDEM-X DEM are small in these cases, mostly in the order of 1–2m. An additional benefit of the proposed block adjustment method is that it improves the relative accuracy of adjacent DEMs.

German satellite TanDEM-X launch a success

German satellite TanDEM-X launch a success

Definition of ICESat Selection Criteria for Their Use as Height References for TanDEM-X

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The TanDEM-X satellite synthetic aperture radar (SAR) mission, which is the result of the partnership between the German Aerospace Center (DLR) and Astrium GmbH, has the goal to deliver a high-precision global digital elevation model (DEM). The X-band SAR interferometry-derived DEMs contain absolute and relative height errors that have to be minimized with the help of height references in order to achieve the specified accuracies. ICESat laser altimetry data are suited for this task, due to their accuracy and global distribution. In order to gain experience in the comparison between a radar-derived DEM and ICESat GLA14 elevation data, an X-band DEM was acquired over a test region with the experimental airborne radar system of DLR in Oberpfaffenhofen. Additionally, a laser DEM of the area was used to verify the height accuracy claimed by previously published ICESat studies over different terrain types and after applying different selection threshold criteria. The analyses described in this paper are the basis for the definition of a suitable global ICESat selection strategy and include the computation of the density of selected ICESat samples over the Earth. These aspects are crucial for a successful TanDEM-X DEM generation. </para>

SURFACE PARAMETER ESTIMATION USING BISTATIC POLARIMETRIC X-BAND MEASUREMENTS

The main purpose of this paper is to separately estimate the important surface parameters (soil moisture and roughness) by using full polarimetric bistatic measurements. The results provide a basis for new satellite application of future bistatic measurement systems such as the TanDEM-X satellite mission. Initially, bistatic X-band measurements, which have been recorded in the Bistatic Measurement Facility (BMF) at the DLR Oberpfafienhofen, Microwaves and Radar Institute, will be presented. The bistatic measurement sets are composed of soils with difierent well-known statistical roughness scales and difierent moistures. The BMF has been calibrated using the Isolated Antenna Calibration Technique (IACT). The validation of the calibration was achieved by measuring the re∞ectivity of fresh water. In the second part, the assessment of the surface parameters (soil moisture and surface roughness) using the well calibrated data introduced in the former related part, will be detailed. The validation of the specular algorithm by estimating the soil moisture of two surfaces with difierent roughness scales will be reported. Additionally, a new technique using the coherent term of the Integral Equation Method (IEM) to estimate the soil roughness will be presented, as well as the sensitivity of phase and re∞ectivity with regard to moisture variation and therefore the penetration depth was evaluated. Current results demonstrate a non-linear relationship