Franz Meyer

Traffic monitoring with spaceborne SAR-Theory, simulations, and experiments

This paper reviews the theoretical background for upcoming dual-channel radar satellite missions to monitor traffic from space and exemplifies the potentials and limitations by real data. In general, objects that move during the illumination time of the radar will be imaged differently than stationary objects. If the assumptions incorporated in the focusing process of the synthetic aperture radar (SAR) principle are not met, a moving object will appear both displaced and blurred. To study the impact of these (and related) distortions in focused SAR images, the analytic relations between an arbitrarily moving point scatterer and its conjugate in the SAR image have been reviewed and adapted to dual-channel satellite specifications. Furthermore, a specific detection scheme is proposed that integrates complementary detection and velocity estimation algorithms with knowledge derived from external sources as, e.g., road databases. Results using real SAR data are presented to validate the theory.

Spaceborne Traffic Monitoring with Dual Channel Synthetic Aperture Radar Theory and Experiments

This paper revises the theoretical background for upcoming dual-channel Radar satellite missions to monitor traffic from space. As it is well-known, an object moving with a velocity deviating from the assumptions incorporated in the focusing process will generally appear both displaced and blurred in the azimuth direction. To study the impact of these (and related) distortions in focused SAR images, the analytic relations between an arbitrarily moving point scatterer and its conjugate in the SAR image have been reviewed and adapted to dual-channel satellite specifications. To be able to monitor traffic under these boundary conditions in real-life situations, a specific detection scheme is proposed. This scheme integrates complementary detection and velocity estimation algorithms with knowledge derived from external sources as, e.g., road databases.

Results from an airborne SAR GMTI experiment supporting TerraSAR-X traffic processor development

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.

An Integral Detection Scheme for Moving Object Indication in Dual-Channel High Resolution Spaceborne SAR Data

Upcoming SAR satellite missions like TerraSAR-X or RADARSAT-2 will deliver high resolution dual channel SAR data with large coverage. These new missions - together with rising interest in area-wide traffic monitoring - motivate spaceborne GMTI as an attractive alternative to conventional traffic data acquisition. However, a moving object appears distorted in the SAR image since the well-known stationary world assumption in the SAR focusing process is violated. In this paper, a detection approach is presented, which considers simultaneously the effects of azimuthal and radial motion of an object. The mathematical framework of this detector combines information of the measured signal, the expected signal, and their variances. Furthermore, the performance of the proposed algorithm is analysed using experimental airborne SAR data.

Topography and displacement of polar glaciers from multi-temporal SAR interferograms

This paper describes a new technique to simultaneously estimate topography and motion of polar glaciers from multi-temporal SAR interferograms. The approach is based on a combination of several SAR interferograms in a least-squares adjustment using the Gauss- Markov model. For connecting the multi-temporal data sets, a spatio-temporal model is proposed that describes the properties of the surface and its temporal evolution. Rigorous mathematical modeling of functional and stochastic relations allows for a systematic description of the processing chain. It also is an optimal tool to parameterize the statistics of every individual processing step, and the propagation of errors into the _nal results. Within the paper theoretical standard deviations of the unknowns are calculated depending on the con_guration of the data sets. The in_uence of gross errors in the observations and the effect of non-modeled error sources on the unknowns are estimated. A validation of the approach based on real data concludes the paper.

Methods for Small Scale Ionospheric TEC Mapping from Broadband L-Band SAR Data

Ionospheric propagation effects have a significant impact on the signal properties of low-frequency SAR systems. All effects are a function of the density of free electrons in the ionosphere, integrated along the ray-path of the SAR signal, the so-called total electron content (TEC). Methods based on two-frequency GPS observations allow measuring TEC in the ionosphere with a coarse spatial resolution only. In this paper we speculate about the potential of broadband L- band SAR systems for ionospheric TEC mapping. As a basis, the dispersive nature of the ionosphere and its effect on broadband microwave radiation are theoretically derived and analyzed. It is shown that phase advance and group delay can be measured by interferometric and correlation techniques, respectively. The achievable accuracy suffices to map small scale ionospheric TEC disturbances. A differential TEC estimator is proposed that separates ionospheric and tropospheric contributions.

Long-Term and Seasonal Subidence Rates in Urban Areas from Persistent Scatterer Interferometry

As it is well known, space-borne radar interferometry (InSAR) is an appealing and powerful technique to measure the earths topography and surface deformation with high accuracy and high spatial coverage and sampling. Even though the potential of InSAR was extensively proven within the last years and has been widely recognized throughout the scientific community, its value and applicability to geodynamic monitoring problems is severely limited by the influence of temporal decorrelation and electromagnetic path delay variations. The invention of the multi-image Persistent Scatterer (PS) technique in the late 1990s was a big step forward towards a high accuracy observation of slow surface motion and object deformation over long temporal time spans as it enables the identification, isolation, and estimation of millimeter surface deformation processes from space. This approach proves to be particularly suitable for the observation of the deformation regimes of cities from wide (deformation of extended areas) to small scales (deformation of isolated buildings). In this paper the functionality and the basic workflow of the original PS technique is described. Starting from this overview, every processing step is explained in detail and recent advances are mentioned. Based on this the performance of Persistent Scatterer Interferometry to measure linear as well as nonlinear surface deformation with high accuracy and high spatial sampling is exemplified by a case study from the city of Munich. A summary and an outlook concludes the paper.

Detecting moving targets in dual-channel high resolution spaceborne SAR images with a compound detection scheme

Traffic data acquisition from space has evolved to an important task over the last years. Future SAR satellite missions will provide high resolution dual-channel SAR data and therefore a possibility to collect traffic parameters of a large area from space. In this paper a detection approach for vehicles will be presented, which considers simultaneously the effects moving objects suffer from in the SAR image. The performance of the proposed detection scheme is analyzed using experimental airborne SAR data.

A stability analysis of the lambda estimator for solving the ambiguity problem in persistent scatterer interferometry

Persistent Scatterer Interferometry is a well-known technique to obtain displacement rates in urban areas from a stack of SAR interferograms. Besides the original method introduced by A. Ferretti, C. Prati and F. Rocca in the late 1990s, which estimates the displacement rates and DEM corrections by an ensemble coherence maximization approach (periodogram) based on a common master image, several other algorithms have been introduced in the past few years. One of these approaches has been developed at DLR It incorporates the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) method that was originally developed for fast GPS double difference ambiguity estimation. In this paper different parameters are tested to investigate robustness and performance of this estimation method. At first the effects of a reduced number of observations and varying reference points on the estimation with LAMBDA are analyzed while in the second part a direct comparison between LAMBDA and ensemble coherence maximization (periodogram) is performed.