This paper reports on the European Space Agency (ESA) SARSimHT-NG experiment conducted in 2022 over a well-researched location in southern Italy. The experiment involved extensive ground data gathering coordinated with the German Aerospace Center’s (DLR) F-SAR airborne acquisitions in C- and L-bands. The campaign aimed to simulate and analyse geostationary Synthetic Aperture Radar (GeoSAR) measurements over an agricultural area. Besides, it explored the synergy between GeoSAR systems and other Low Earth Orbit (LEO) satellite missions, namely Sentinel-1 Next Generation (NG) and the Radar Observation System for Europe in L-band (ROSE-L). The hyper-temporal repeat cycle of GEoSAR systems is instrumental in investigating the rapid processes of the water cycle. The concept was developed for the Hydroterra mission proposal submitted to the ESA Earth Explorer 10 call and studied in phase 0. The paper illustrates the soil and vegetation in situ data collected together with the L- and C-band fully polarimetric SAR time series acquired at a very high revisit. Moreover, insights into the retrieval of the surface dynamics of the water cycle are also provided.

In March 2021 a field campaign was conducted in the Austrian Alps exploring the application potential and performance of snow mass (snow water equivalent, SWE) retrievals by means of repeat-pass radar interferometry (RP-InSAR). Multiple repeat-pass acquisitions were obtained with an airborne polarimetric C-band and L-band SAR, covering days without snowfall, as well as two snowfall events of different intensity. SWE maps, deduced from the observed change of the RP-InSAR phase delay in the snowpack, show good agreement with field measurements. Whereas L-band data are well applicable for SWE mapping of both snowfall events, the 2 π phase ambiguity and low coherence prevent the use of C-band data for the snowfall event of high intensity. The analysis of the campaign data is going on, addressing among other topics the application of RP-InSAR for SWE monitoring within the planned L-band SAR mission ROSE-L of the European Copernicus program and by means of geostationary C-band SAR as proposed for the Hydroterra mission.

The idea of the former Earth Explorer 10 mission Hydroterra is to place a SAR system into a geosynchronous orbit to understand rapid processes of the diurnal water cycle over regions of interest in Europe and Africa. The DLR Microwaves and Radar Institute conducted an airborne SAR campaign to support the Hydroterra mission. Airborne data were collected to simulate the long integration time of a geosynchronous SAR image and to evaluate the potential to detect diurnal moisture changes. In this paper the airborne campaign is described and the achieved results are presented.

The GeoWAM project aims to achieve high-resolution and highly-accurate digital elevation models (DEM) to monitor the evolution of tidal flats over the German North Sea. To achieve this, the dual-frequency/dual-baseline (DFDB) DEM generation approach, recently developed by DLR, was further expanded to include L-band data. Based in the experience obtained in the 2019 GeoWAM campaign with X- and S-band only, the addition of L-band for the 2020 campaign helps to deal with the decorrelation found in repeat-pass data and potentially allows the creation of new merged products. For the computation of phase and coherence, a new local fringe frequency approach was included to improve the final generated DEMs.

This paper presents first results for an automatic interpretation of SAR images of sea ice acquired in the Davis Strait off the coast of Baffin Island in 2019. While the study provides multi-frequency and interferometric data collected by the DLR F-SAR airborne SAR sensor, we focus on the analysis of C-band as one of the most commonly used frequencies for sea ice monitoring. We apply an iterative version of polarimetric k-Means which allows to work on the local variance-covariance matrices directly. The obtained clusters show very distinct polarimetric as well as topological properties, which indicates that they are closely related to different sea ice types.

Multi-platform campaigns open up new opportunities and allow a comparison and analysis of SAR signatures of the same area at different frequency bands and incident angles. In this paper, we make use of two different sensor systems: DLR's pulsed F-SAR operating at X-, C- and L-band, and FHR's frequency-modulated continuous-wave (FMCW) MIRANDA35 operating at Ka-band. Main goals of the campaign include the comparison of high-resolution polarimetric signatures at different frequency bands, change detection, synchronized acquisition of ground moving targets and air-moving target indication (AMTI) experiments. We present the planning and implementation of the campaign and first results of the data processing.

AfriSAR is an ESA-funded airborne P-band SAR campaign over the African tropical forests of Gabon that is being carried out by ONERA (July 2015) and DLR (February 2016) in support of the development of the geophysical algorithms of the future BIOMASS mission. Multibaseline fully-polarimetric acquisitions have been designed over four test sites in order to further develop and validate algorithms for the estimation of forest upper canopy height, 3-D structure and biomass by implementing PolSAR (SAR Polarimetry), Pol-InSAR (Polarimetric SAR Interferometry) and TomoSAR (SAR Tomography) P-band measurements. In this paper first data processing results will be shown.

The objective of the NASA Soil Moisture Active Passive (SMAP) mission is to provide global measurements of soil moisture and freeze/thaw states. SMAP integrates L-band radar and radiometer instruments as a single observation system combining the respective strengths of active and passive remote sensing for enhanced soil moisture mapping. Airborne instruments are a key part of the SMAP validation program. Here, we present an airborne campaign in the Rur catchment, Germany, in which the passive L-band system Polarimetric L-band Multi-beam Radiometer and the active L-band system F-SAR of DLR were flown simultaneously on six dates in 2013. The flights covered the full heterogeneity of the area under investigation, i.e., the main land cover types and all experimental monitoring sites. Here, we used the obtained data sets as a test bed for the analysis of three active-passive fusion techniques: 1) estimation of soil moisture by passive sensor data and subsequent disaggregation by active sensor backscatter data; 2) disaggregation of passive microwave brightness temperature by active microwave backscatter and subsequent inversion to soil moisture; and 3) fusion of two single-source soil moisture products from radar and radiometer. Results indicate that the regression parameters β are dependent on the radar vegetation index. The best performance was obtained by the fusion of radiometer brightness temperatures and radar backscatter, which was able to reach the same accuracy as single-source coarse-scale radiometer soil moisture retrieval but on a higher spatial resolution.

The objective of the NASA Soil Moisture Active & Passive (SMAP) mission is to provide global measurements of soil moisture and its freeze/thaw state. The SMAP measurement approach is to integrate L-band radar and radiometer as a single observation system combining the respective strengths of active and passive remote sensing for enhanced soil moisture mapping. Airborne instruments will be a key part of the SMAP validation program. Here, we present an airborne campaign in the Rur catchment, Germany, in which the passive L-band system Polarimetric L-band Multi-beam Radiometer (PLMR2) and the active L-band system DLR F-SAR were flown on six dates in 2013. The flights covered the full heterogeneity of the area under investigation, i.e. all types of land cover and experimental monitoring sites. The obtained data sets are used as a test-bed for the analysis of existing and development of new active-passive fusion techniques.

During the last decade, synthetic aperture radar (SAR) became an indispensable source of information in Earth observation. This has been possible mainly due to the current trend toward higher spatial resolution and novel imaging modes. A major driver for this development has been and still is the airborne SAR technology, which is usually ahead of the capabilities of spaceborne sensors by several years. Today's airborne sensors are capable of delivering high-quality SAR data with decimeter resolution and allow the development of novel approaches in data analysis and information extraction from SAR. In this paper, a review about the abilities and needs of today's very high-resolution airborne SAR sensors is given, based on and summarizing the longtime experience of the German Aerospace Center (DLR) with airborne SAR technology and its applications. A description of the specific requirements of high-resolution airborne data processing is presented, followed by an extensive overview of emerging applications of high-resolution SAR. In many cases, information extraction from high-resolution airborne SAR imagery has achieved a mature level, turning SAR technology more and more into an operational tool. Such abilities, which are today mostly limited to airborne SAR, might become typical in the next generation of spaceborne SAR missions.

The objective of the NASA Soil Moisture Active & Passive (SMAP) mission is to provide global measurements of soil moisture by fused active and passive L-band microwave measurements. With an airborne campaign conducted in the Rur catchment, Germany, in 2012, an active and passive L-band microwave data set is generated. The passive Polarimetric L-band Multi-beam Radiometer (PLMR2) and the active L-band system DLR F-SAR were installed on the DLR Dornier DO 228 aircraft. Despite of the differences between the airborne and the future spaceborne sensors, the data set will serve as a test bed for the analysis of existing and development of enhanced future active/passive data fusion techniques. Moreover, the derivation of (vegetation) parameters for the fusion approaches is planned.

The F-SAR airborne SAR instrument represents the successor of the E-SAR system of the German Aerospace Center (DLR), which had previously been extensively used over the last three decades. Its development was triggered by the current demand for simultaneous acquisitions at different wavelengths and polarisations as well as by the demand for very high resolution in the order of decimetres. F-SAR is a modular development utilising state of the art hardware. As for E-SAR, DLRs Dornier DO228-212 aircraft is the first choice as platform for the new system. With the recently completed X-band single-pass interferometer and the P-band subsystem, F-SAR is now ready for fully replacing the E-SAR. This paper presents the two new subsystems and analyses their performance based on fully polarimetric imagery acquired during recent system test and calibration campaigns.

Launched in June 2010, TanDEM-X is an interferometric mission with the main goal of providing a high-resolution, global and unprecedentedly accurate digital elevation model (DEM) of the Earth by means of single-pass X-band SAR interferometry. Despite its usual quasi-monostatic configuration, TanDEM-X is the first genuinely bistatic SAR system in space. During its monostatic commissioning phase, the system was operated in pursuit monostatic mode. During that time, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes were conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. In particular, this article includes results of the first bistatic acquisition and the first single-pass interferometric (mono/bistatic) acquisition with TanDEM-X, addressing their innovative aspects and focussing on the analysis of the experimental results. Even in the absence of essential synchronisation and calibration information, bistatic images and interferogramswith similar quality to pursuit monostatic have been obtained. Some months later, with TanDEM-X already in its operational DEM-acquisition phase a further challenging bistatic acquisition carried out in cooperation with DLR's airborne radar F-SAR has been carried out. The objective was to acquire fully polarimetric interferometric data with a high range of available bistatic angles. A dedicated commanding of both TanDEM-X and F-SAR was required, including irregular sampling schemes, partially missing bistatic echo reception and bistatic synchronisation, which pose a number of technological challenges in SAR data processing before the calibrated bistatic SAR images are obtained. This article reports about these two set of experiments.

SAR tomography (SARTom) is an imaging technique that allows multiple phase center separation in the vertical direction, leading to a 3-D reconstruction of the imaged scene. The retrieval of volume structure information (e.g., for forest classification) and the solution of the layover problem are two of the most promising applications. In this letter, SARTom, in combination with polarimetry (PolSARTom), is exploited to image and to extract characteristic features (e.g., shape and height) of targets hidden beneath foliage. This analysis is applied to L-band airborne data acquired by the E-SAR system of the German Aerospace Center (DLR) during a tomographic campaign that took place in September 2006 on the test site of Dornstetten (Germany).

The F-SAR airborne SAR instrument represents the successor of the E-SAR system of the German Aerospace Center (DLR), which has been extensively used in the last three decades. Its development was triggered by the current demand for data being simultaneously acquired at different wavelengths and polarisations as well as by the demand for very high resolution in the order of decimetres. F-SAR is a modular development utilising the most modern hardware and commercial off the shelf components. As for E-SAR DLRs Dornier DO228-212 aircraft is the first choice as platform for the new system. Although the F-SAR system is still under development, it is already taking over some of the operational duties of the old E-SAR system. This paper will analyse the performance of the current system, based on the multi-frequency and fully polarimetric imagery acquired during several campaigns in the last two years. Since F-SAR is using a fixed antenna mount without gimbal, precise radiometric calibration is particularly challenging, especially in the shorter wavelengths. Therefore, special emphasis is placed on the system calibration and the associated quality control including the achieved spatial resolution and radiometric accuracy in the different bands.

The Microwaves and Radar Institute of the German Aerospace Center (DLR) is known for consistent work on the field of airborne synthetic aperture radar and its application. In April 2008 the 20th anniversary of the maiden flight of the well-known E-SAR system was celebrated. E-SAR has been maintained well over the time. It provided valuable knowledge to the science community, especially in the past 10 years. However, it became more and more obvious that a technological renewal was inevitable. Consequently the development of a new SAR system was put on line under the name `F-SAR'.

Following the success of the first bistatic spaceborne-airborne experiment between TerraSAR-X and F-SAR carried out in November 2007, DLR has performed a second bistatic experiment in July 2008 with new challenging acquisitions. Furthermore, the existing bistatic processing chain has been updated with two significant improvements: a) clock offset synchronisation is now performed without the use of reference targets, and b) SAR imaging is done using a fast focussing technique. The new SAR imaging algorithm, based on the fast factorised backprojection algorithm, has proved very good focussing qualities while dramatically reducing (up to a factor 100 with respect to direct backprojection) the overall computational load. The new processing chain is tested using the image of the first TerraSAR-X experiment. Results of a dualpol acquisition performed during the second TerraSAR-X/F-SAR experiment and showing the first dual-pol bistatic spaceborne-airborne images are also presented in this paper.

This paper presents a methodology to process airborne interferometric synthetic aperture radar (SAR) data to measure surface velocity fields (SVFs) of temperate glaciers, and applies it to data acquired over the Aletsch glacier. The first part of this paper deals with the main limitation in airborne interferometric SAR to retrieve reliable interferometric products, namely, the existence of the so-called residual motion errors - inaccuracies on the order of a few centimeters in the navigation system. An extended multisquint approach is proposed for their estimation in the case of nonstationary scenes. The second part of this paper expounds an efficient methodology to derive SVFs with airborne systems, where the line-of-sight displacement is estimated using differential interferometry and the along-track component by estimating the azimuth coregistration offsets. The necessary steps to finally obtain the 3-D SVF are also presented, as well as the possibility of combining different acquisition geometries. Airborne interferometric SAR data acquired by the Experimental SAR system of the German aerospace center over the Aletsch glacier, located in the Swiss Alps, are used to evaluate the performance of the proposed approach. The motion of the corner reflectors deployed in the scene is retrieved with an accuracy between 1 and 5 cm/day using L-band data.

Following an original proposal by the authors to the TerraSAR-X (TSX) scientific coordination board, a spaceborne-airborne bistatic experiment was successfully performed early November 2007. TSX was used as transmitter and DLR's new airborne radar system, F-SAR, as receiver; due to the capability of the latter to acquire data quasi-continuously, no echo window synchronisation is needed. Monostatic data were also recorded during the acquisition. This paper includes description and results of the spaceborne-airborne bistatic experiment, with special focus on data processing and image comparison. Given the acquisition scenario, with two-channel sampling and transmitter and receiver clocks operating independently, data processing must necessarily follow a three-step strategy: 1) channel balancing, 2) data synchronisation and 3) bistatic SAR processing. Since neither absolute range nor Doppler references are available in the bistatic data set, synchronisation is done with the help of calibration targets on ground and based on the analysis of the acquired data compared to expected data. Due to the variant nature of the bistatic acquisition and the required precision for the processing, data are processed using a bistatic backprojection approach.

Synthetic aperture radar (SAR) interferometric data offer the opportunity to measure temperate glacier surface topography and displacement. The increase of the resolution provided by the most recent SAR systems has some critical implications. For instance, a reliable estimate of the phase gradient can only be achieved by using interferogram local frequencies. In this paper, an original two-step method for estimating local frequencies is proposed. The 2-D phase signal is considered to have two deterministic components corresponding to low-resolution (LR) fringes and high-resolution (HR) patterns due to the local microrelief, respectively. The first step of the proposed algorithm consists in the LR phase flattening. In the second step, the local HR frequencies are estimated from the phase 2-D autocorrelation function computed on adaptive neighborhoods. This neighborhood is the set of connected pixels belonging to the same HR spatial feature and respecting the ldquolocal stationarityrdquo hypothesis. Results with both simulated TerraSAR-X interferograms and real airborne E-SAR images are presented to illustrate the potential of the proposed method.

In continuation of the BACCHUS project, aimed at establishing a reference high quality geographic information system for vineyards, an airborne SAR survey has been carried out in fall 2005 in the Frascati area, near Rome (Italy) to demonstrate the potential of airborne radar remote sensing in vineyard characterization. This contribution reports on the polarimetric L-band and dual polarization C-band SAR data acquisition campaign supported by ESA and carried out on two dates in October 2005 (the first during the grape harvest and the other after the vintage completion).

The paper reviews our current state for river surface current measurements using SAR techniques. New results from an airborne SAR campaign conducted in 2005 are presented. \nThe paper discusses the problem that due to the relative radial motion between the moving water surface and the sensor, the reflected energy from the river is misplaced on the river bank in the SAR image. Here it overlays with the stationary clutter like forests or agriculture fields. If the clutter amplitude is much stronger than the amplitude from the moving river surface an ATI measurement of the surface velocity becomes impossible, because the phase is dominated by the stationary clutter. We propose to use clutter suppression techniques like the Displaced Phase Centre Antenna (DPCA) or the Space Time Adaptive Filtering (STAP) method in order to cancel out the stationary signal. First results obtained with the DPCA method are presented. \n

The launch of the advanced high resolution radar satellite TerraSAR-X in summer 2006 opens new possibilities for the demonstration of traffic monitoring from space. DLR is currently developing an operational traffic processor for the TerraSAR-X ground segment. The paper presents results from an airborne SAR GMTI campaign that was part of a study for algorithm development and processor design. DLR’s E-SAR sensor was used in an Along-Track Interferometry (ATI) mode to image vehicles in controlled and uncontrolled situations. The paper gives an overview on the experiment and presents the results of across- and along-track velocity estimation. Adapted SAR processing techniques were applied to enhance the peak energy of the moving objects in the focused SAR images. The paper presents first results and discusses the techniques.

Among the family of exponential distributions the normal or Gaussian distribution plays a fundamental role in natural environments-above all-due to its symmetrical properties. In distributed terrain the size, position and orientation of scatterers within the ground resolution cell of a SAR system is Gaussian distributed as well as their backscatter over a series of radar pulses; particularly when the size of scatterers is of similar magnitude as the radar wavelength and their number is greater then 10. Now, the motion of the SAR system influences the statistics of backscatter in such a way that its symmetrical properties are lost to asymmetry. The more the structure of terrain changes from homogeneous to heterogeneous (from pasture to forest) and from heterogeneous to extremely heterogeneous (from forest to urban) the more the statistics of SAR backscatter deviates from Gaussian. Hence, another distribution, the inverse Gaussian, arises most naturally for backscatter of distributed targets under the assumption that the single scatter cell keeps its Gaussian behavior. (The term inverse refers to the cumulant generating function of this distribution being the inverse of the Gaussian one.).