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During recent years the experimental SAR (E-SAR) system of the German Aerospace Center (DLR) has continuously been operated to acquire data for different scientific applications in forestry, agriculture and glaciology. In this context, the data where mainly used to develop new scattering models in preparation of future space-borne missions posing increasing accuracy requirements especially for the processing of repeat-pass interferometric data. Also direct model-free methods like differential SAR interferometry for measuring displacements in the order of the wavelength or SAR tomography for real 3D microwave imaging of scattering volumes determined the development of advanced and very accurate motion compensation techniques. This paper first gives an overview of the updates of the E-SAR system performed during the last couple of years and then focusses on the recently developed airborne interferometric processing methods and their applications.

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. The 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.

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.

Here are described the first successes in obtaining bistatic cross-platform interferograms from a two aircraft SAR acquisition campaign performed in February 2003 by the DLR and ONERA using their E-SAR and RAMSES facilities. The main challenging issues in the signal processing involved are the estimation and compensation of phase drift between the two radars master oscillators (bistatic receive signal is demodulated with a local oscillator which is not coherent with the transmitter one) and the determination of the relative aircraft trajectories -i.e. interferometric baseline-accurate to the millimetre (X-band wavelength is 3 cm, hence 1 mm error in relative position shifts the phase by about 10/spl deg/). The high phase quality of the first interferograms obtained is encouraging, but also shows that full endo-clutter moving target indication capability in multistatic environment requires at least two passive SAR receivers in the vicinity of the transmitting active SAR system.

"ProSmart" is the name of a remote sensing project established in the first phase of a future commercial SAR and hyperspectral scanner satellite mission. Research work in the past has shown the commercial potential of combined SAR and hyperspectral scanner data. It is the purpose of ProSmart now to identify the market needs and to develop a set of highly profitable products. The airborne experimental SAR E-SAR of DLR contributes the high resolution geocoded SAR imagery of L-band (fully polarimetric) and X-band (HH and VV).

Three DLR institutes, forming the cluster "Sensorik fuer Fernerkundung and Navigation", and two italian CNR institutes, I.R.E.C.E. and I.I.V., agreed on carrying out a joint airborne multisensor campaign in the Mt. Etna area in summer 1996. The site is of high geological interest. The deployment of an experimental SAR (E-SAR), a multispectral optical scanner (DAIS) and a wide angle stereo camera (WAAC) resulted in a number of unique data sets, which allow the analysis of the synergy between optical and microwave sensors with regard to the application.

We propose and evaluate an operational algorithm, capable of obtaining a mosaic DEM from a wide area from several interferometric data sets.

This paper presents a detailed study of geocoding algorithms for interferometric SAR data from airborne sensors. They allow to relate a phase value in slant-range coordinates to a position in a cartesian reference system and, hence, a height value on a topographic map. Two basic approaches, which follow forward or backward strategies, are proposed in this paper. Another aspect is related to the kind of the acquisition mode: single- or repeat-pass. In general, a wide variety of techniques and combinations (single or repeat pass, direct or indirect transformations, one or two steps, zero-Doppler or squinted geometry) exist. This paper reviews the state-of-the-art of these combinations, as well as their evaluation, comparison and discussion by means of real data from the DLR’s E-SAR system. The discussion includes the problem of polarimetric data geocoding. In that case, the map-projected height information derived from a X-band single-pass system is used to geocode non-interferometric channels (for example polarimetric L-band data).

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.

A multisensor airborne campaign is carried out in Switzerland in summer 1997. The campaign did not only involve a suite of different sensors but also extensive ground supporting measurements. Amongst the sensor that acquired data over a predefined set of three standard test sites were the hyperspectral imagers DAIS 7915 and CASI, a wide angle airborne camera (WAAC) and a SAR (E-SAR) system as well as an imaging laserscanner. On the ground, geolocation is performed with differential GPS systems and a number of georeferenced ground control points. An active navigation system for the aircraft is used for accurate flight path determination in order to support single- and multi-pass interferometric flights. The thermal ground references consist of a number of targets in the field to verify the thermal performance of the DAIS. Radiometric validation on the ground involves spectroradiometric measurements of a number of selected reference targets, measurements of global flux and radiant temperature, as well as sunphotometer measurements. Conventional field mapping completes the full documentation of the three test sites. The generation of digital surface models using the stereo approach of the WAAC camera and the laserscanner is a goal to support the georeferencing of the different acquired image data. Finally all data are projected onto a common reference system and can be used for further analysis.

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 DLR's experimental SAR system (E-SAR).

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.).

Bistatic configuration is an attractive concept for spaceborne and airborne SAR missions when distributed radars are necessary as for example in the case of interferometric applications. The first reason is the important cost reduction achieved over the multiple radar elements, by having only one transmitter (expensive part) and multiple receivers. The most promising applications are single-pass interferometry with a large baseline and target or surface characterisation from bistatic scattering signature analysis. In a defence context, the improved stealth associated with the receive-only component can provide a wider operational capability. In order to explore the potentials and technical challenges associated with bistatic radar, DLR and ONERA have conducted a joint bistatic airborne radar experiment involving both their SAR systems E-SAR and RAMSES between October 2002 and February 2003. Two main geometrical configurations were flown to explore different scientific and technical objectives. In the first geometrical configuration, the quasi-monostatic mode, the two planes were flying very close to each other to acquire interferometric data in a single-pass cross-platform configuration with large interferometric baselines. The second geometrical configuration was designed to acquire images with a large bistatic angle. The two planes were flying on parallel tracks around 2 km apart, at about the same altitude, with the antennas pointing at the same area on the ground. The authors describe this research programme, including the preparation phase, the analysis of the technological challenges that had to be solved before the acquisition, the strategy adopted for bistatic image processing, the first results and a preliminary analysis of the acquired images.