Thorsten Brehm

MEMPHIS-a fully polarimetric experimental radar

The MEMPHIS polarimetric SAR is able to operate simultaneously at X-, Kaand W-band. After a detailed technical description, several examples are given to demonstrate its capabilities in the side-looking mode, in forward squinted Doppler beam sharpenung (DBS) mode and with across-track interferometry. Of great importance is the polarimetric calibration which is described in some detail.

High resolution millimeter wave SAR interferometry

High resolution millimeter wave synthetic aperture radar (SAR) interferometry is presented using the MEMPHIS multi-baseline InSAR system. A complete processing chain is used to generate digital elevation models starting from the radar raw data. A deeper focus is laid on the phase unwrapping step, which is achieved using the multi-baseline properties of the system. In November 2006, an experiment was realized including two test sites in Switzerland; the actual results are presented and discussed.

Towards airborne single pass decimeter resolution SAR interferometry over urban areas

Airborne cross-track Synthetic Aperture Radar interferometers have the capability of deriving three-dimensional topographic information with just a single pass over the area of interest. In order to get a highly accurate height estimation, either a large interferometric baseline or a high radar frequency has to be used. The utilization of a millimeter wave SAR allows precise height estimation even for short baselines. Combined with a spatial resolution in the decimeter range, this enables the mapping of urban areas from airborne platforms. The side-looking SAR imaging geometry, however, leads to disturbing effects like layover and shadowing, which is even intensified by the shallow looking angle caused by the relatively low altitudes of airborne SAR systems. To solve this deficiency, enhanced InSAR processing strategies relying on multi-aspect and multi-baseline data, respectively, are shown to be necessary.

Processing of MEMPHIS Ka-Band Multibaseline Interferometric SAR Data: From Raw Data to Digital Surface Models

MEMPHIS is an experimental millimeter-wave synthetic aperture radar (SAR) system that acquires cross-track multibaseline interferometric data at high resolution in a single pass, using four receive horns. In this paper, we present the SAR system and navigation data, and propose a processing chain from the raw data input to a digital surface model (DSM) output. This processing chain includes full bandwidth reconstruction of the steppedfrequency SAR data, azimuth focusing with an Extended Omega-K algorithm, generation ofinterferograms for each available baseline, phase unwrapping using the multibaseline data, and phaseto-height conversion. The hardware and processing chain were validated through the analysis of experimental Ka-band data. The SAR image resolution was measured with point targets and found to be ~2% and ~15% coarser than the theoretical value in range and azimuth, respectively. The geolocation accuracy was typically better than 0.1 m in range and 0.2 m in azimuth. Observed depression angledependent interferometric phase errors were successfully removed using a correction function derived from the InSAR data. Investigation of the interferometric phase noise showed the utility of a multibaseline antenna setup; the number of looks and filter size used for the DSM generation were also derived from this analysis. The results showed that in grassland areas, the height difference between the ~2 m-resolution InSAR DSMs and the reference ALS models was 0 ± 0.25 m.

Building damage assessment in decimeter resolution SAR imagery: A future perspective

Damage assessment after natural disasters (e.g. earthquakes) is crucial for initiating effective post disaster relief actions. Synthetic aperture radar (SAR) sensors are an important source of information since they can map the extended areas quickly, in an uncensored manner, and independent from the weather conditions and the solar illumination. The spaceborne SAR sensors TerraSAR-X and COSMO-SkyMed reach spatial resolutions of about 1 meter and permit to analyze urban areas at the level of individual buildings. With this type of data completely destroyed buildings can be detected, while different types of damages can not be distinguished. In this paper we analize a set of decimeter resolution SAR data from an experimental airborne SAR system acquired from an artificial village of different types of destroyed buildings. We show that the increased resolution supports a more reliable identification of destroyed buildings, and allows the classification of destroyed buildings into several basic damage classes. Furthermore, we discuss some initial ideas for the development of novel automatic building damage assessment methods and give an outlook on how this type of data can be efficiently used in damage assessment scenarios.

Processing of MEMPHIS millimeter wave multi-baseline InSAR data

This paper presents a processing method for multi-baseline interferometric data acquired with the MEMPHIS airborne sensor. The processing method ingests the SAR raw data from each receiver and extends up to the generation of digital elevation models (DEMs). Critical steps include the correction of the azimuth phase undulations, the multi-baseline processing and the phase-to-DEM conversion. Methods for resolving the various hurdles were adapted to the MEMPHIS sensor and are presented here. The results obtained for a data take over a test site near Zurich, Switzerland are shown; these results are in a good agreement with comparable LIDAR products.

Multibaseline interferometric SAR at millimeterwaves test of an algorithm on real data and a synthetic scene

Interferometric Synthetic Aperture Radar has the capability to provide the user with the 3-D-Information of land surfaces. To gather data with high height estimation accuracy it is necessary to use a wide interferometric baseline or a high radar frequency. However the problem of resolving the phase ambiguity at smaller wavelengths is more critical than at longer wavelengths, as the unambiguous height interval is inversely proportional to the radar wavelength. To solve this shortcoming, a multiple baseline approach can be used with a number of neighbouring horns and an increasing baselength going from narrow to wide. The narrowest, corresponding to adjacent horns, is then assumed to be unambiguous in phase. This initial interferogram is used as a starting point for the algorithm, which in the next step, unwraps the interferogram with the next wider baseline using the coarse height information to solve the phase ambiguities. This process is repeated consecutively until the interferogram with highest precision is unwrapped. On the expense of this multi-channel-approach the algorithm is simple and robust, and even the amount of processing time is reduced considerably, compared to traditional methods. The multiple baseline approach is especially adequate for millimeterwave radars as antenna horns with relatively small aperture can be used, while a sufficient 3-dB beamwidth is maintained. The paper describes the multiple baseline algorithm and shows the results of tests on real data and a synthetic area. Possibilities and limitations of this approach are discussed. Examples of digital elevation maps derived from measured data at millimeterwaves are shown.

Analysis of a Maximum Likelihood Phase Estimation Method for Airborne Multibaseline SAR Interferometry

It has been shown using simulated data that phase estimation of cross-track multibaseline synthetic aperture radar (SAR) interferometric data was most efficiently achieved through a maximum likelihood (ML) method. In this paper, we apply and assess the ML approach on real data, acquired with an experimental Ka-band multibaseline system. Compared to simulated data, dealing with real data implies that several calibration steps be carried out to ensure that the data fit the model. A processing chain was, therefore, designed, including steps responsible for compensating for imperfections observed in the data, such as beam elevation angle dependent phase errors or phase errors caused by imperfect motion compensation. The performance of the ML phase estimation was evaluated by comparing it to results based on a coarse-to-fine (C2F) algorithm, where information from the shorter baselines was used only to unwrap the phase from the longest available baseline. For this purpose, flat areas with high coherence and homogeneous texture were selected in the acquired data. The results show that with only four looks, the noise level was marginally better with the C2F approach and contained fewer outliers. However, with more looks, the ML method consistently delivered better results: noise variance with the C2F approach was slightly but steadily larger than the variance obtained with ML method.

Millimeter wave propagation above the sea surface during the Squirrel campaign

This contribution reports about experimental activities during the 2011 Squirrel campaign in the Baltic Sea, where a number of in- and outbound runs of the Mittelgrund research vessel were measured by the Fraunhofer FHR MEMPHIS Radar in sea configuration at both Ka and W band. Aboard the employed vessel, four corner-reflectors (CR) at different heights in forward and backward direction where mounted in order to measure the signature of the CRs along the inbound- and outbound trajectories. The parabolic wave equation modeling software TERPEM was used to model the propagation factor and to compare the measurements against the modeling. The characterization of the oceanographic and meteorological input parameters for the modeling was performed by a large number of sensors operated by the WTD 71. The present contribution provides additional evidence of previously observed propagation phenomena under different operating frequencies. The qualitative behavior of the experimental data was confirmed by established modeling and fits quite well at the location of the performed campaign.

Analysis of radar sea clutter data acquired during the MARLENE measurement campaign

Maritime environment, and in particular the sea clutter, has a significant impact on the detection efficiency. Therefore the physical properties of the radar sea clutter must be well mastered. In this paper, we focus on sea clutter data acquired during an experiment called MARLENE, which was held in the Mediterranean Sea region of Toulon, France, in 2014. The aim of this experiment was to characterize the sea environment influence on radar propagation and backscattering for Refractivity From Clutter (RFC) and radar Doppler detection. Three coastal radars, MARSIG (WTD 71), MEMPHIS (FHR) and MEDYCIS (ONERA) were used to acquire sea clutter measurements simultaneously. During the radar measurements, the in-situ oceanic and meteorological conditions were characterized on board of the RV PLANET meteorological ship deployed by WTD 71 and by WAVEWATCH III and SWAN wave models. The paper gives an overview of the MARLENE experiment, presents the radar data and the first results on sea clutter reflectivity and Doppler.