Ralf Horn

Very-High-Resolution Airborne Synthetic Aperture Radar Imaging: Signal Processing and Applications

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. Todays 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 todays 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.

Overview of interferometric data acquisition and processing modes of the experimental airborne SAR system of DLR

This paper presents first the relevant parameters of the experimental E-SAR system for single- and repeat-pass SAR interferometry. The implementation of the different interferometric operation modes is described and special attention is paid to the problems posed by the unstable airborne platform. Each interferometric mode is described including the solution of its critical problems. The given examples prove the enormous information content one can obtain from the different imaging modes of airborne SAR interferometry. Finally, future developments concerning polarimetric SAR interferometry and tomography will be addressed.

First 3-D Reconstructions of Targets Hidden Beneath Foliage by Means of Polarimetric SAR Tomography

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).

Estimation of the Surface Velocity Field of the Aletsch Glacier Using Multibaseline Airborne SAR Interferometry

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.

Airborne polarimetric SAR interferometry

The authors show for first time the applicability of polarimetric interferometry techniques on high resolution interferometric scattering matrix airborne data. They point out the main requirements considering system hardware as well as data processing and calibration algorithms which are important for the polarimetric interferometric data evaluation and describe their implementation in the case of the DLRs experimental SAR system (E-SAR).

Investigation of SMAP Fusion Algorithms With Airborne Active and Passive L-Band Microwave Remote Sensing

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.

F-SAR — DLR's new multifrequency polarimetric airborne SAR

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’.

INDREX II - indonesian airborne radar experiment campaign over tropical forest in L- and P-band: first results

The initiative to INDREX-II originated from the recommendation of the POLinSAR 2003 workshop held by the European Space Agency (ESA) in Frascati, Italy. From the participating scientists a gap in the knowledge and performance of polarimetric and interferometric SAR over tropical forest has been identified and a recommendation has been given for an airborne SAR experiment. In this paper the general framework of the executed INDREX-II campaign in November 2004 is outlined and preliminary results are presented and discussed. KeywordsL-band; P-band; Pol-InSAR; repeat pass InSAR; forest height; forest biomass; tropical forest;

High-Resolution SAR Interferometry: Estimation of Local Frequencies in the Context of Alpine Glaciers

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.

System status and calibration of the F-SAR airborne SAR instrument

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.