Stefan V. Baumgartner

Bistatic TerraSAR-X/F-SAR Spaceborne–Airborne SAR Experiment: Description, Data Processing, and Results

We report about the first X-band spaceborne-airborne bistatic synthetic aperture radar (SAR) experiment, conducted early November 2007, using the German satellite TerraSAR-X as transmitter and the German Aerospace Centers (DLR) new airborne radar system F-SAR as receiver. The importance of the experiment resides in both its pioneering character and its potential to serve as a test bed for the validation of nonstationary bistatic acquisitions, novel calibration and synchronization algorithms, and advanced imaging techniques. Due to the independent operation of the transmitter and receiver, an accurate synchronization procedure was needed during processing to make high-resolution imaging feasible. Precise phase-preserving bistatic focusing can only be achieved if time and phase synchronization exist. The synchronization approach, based on the evaluation of the range histories of several reference targets, was verified through a separate analysis of the range and Doppler contributions. After successful synchronization, nonstationary focusing was performed using a bistatic backprojection algorithm. During the campaign, stand-alone TerraSAR-X monostatic as well as interoperated TerraSAR-X/F-SAR bistatic data sets were recorded. As expected, the bistatic image shows a space-variant behavior in spatial resolution and in signal-to-noise ratio. Due to the selected configuration, the bistatic image outperforms its monostatic counterpart in almost the complete imaged scene. A detailed comparison between monostatic and bistatic images is given, illustrating the complementarity of both measurements in terms of backscatter and Doppler information. The results are of fundamental importance for the development of future nonsynchronized bistatic SAR systems.

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.

Fast GMTI Algorithm For Traffic Monitoring Based On A Priori Knowledge

In this paper, a fast a priori knowledge-based ground moving target indication and parameter estimation algorithm applicable to single- as well as to multichannel synthetic aperture airborne radar data is presented. The algorithm operates directly on range-compressed data. Only the intersection points of the moving vehicle signals with the a priori known road axes, which are mapped into the range-compressed data array, are evaluated. For moving vehicle detection and parameter estimation, basically only a single 1-D fast Fourier transformation has to be performed for each considered road point. Hence, the required computational power is low, and the algorithm is well suited for real-time traffic monitoring applications. The proposed algorithm enables the estimation of the position and velocity vectors of detected moving vehicles independent of the number of channels. A single-channel synthetic aperture radar system may be sufficient in case of fast moving vehicles. The paper includes a detailed performance assessment together with experimental results that demonstrate the applicability in a real-world scenario.

An autonomous, non-cooperative, wide-area traffic monitoring system using space-based radar (TRAMRAD)

To meet the challenges of ever increasing road traffic and the associated economic and sociological impacts, new techniques and technologies for better traffic management are needed. The TRAMRAD project (Traffic Monitoring with space-based Radar) aims to profit from research and development in earth observation and advances in radar remote sensing techniques to define a future space-based sensor system for the wide-area monitoring of road traffic. The paper describes the requirements for the system and the concepts being investigated. In particular, it discusses the complex detection conditions, the requirements on the radar instrument and the methods for processing the data. Possible system concepts are described and their capabilities are discussed.

Large along-track baseline SAR-GMTI: First results with the TerraSAR-X/TanDEM-X satellite constellation

In the paper first ground moving target indication (GMTI) and parameter estimation results obtained with the spaceborne TerraSAR-X/TanDEM-X satellite constellation are presented and discussed. For processing a dual-platform GMTI algorithm developed by the authors was used. This algorithm enables the estimation of the true geographical positions, the velocities and the headings of the detected targets with high accuracy. The algorithm is verified and evaluated using ground truth reference data.

Real-time road traffic monitoring using a fast a priori knowledge based SAR-GMTI algorithm

Radar systems operating on high altitude platforms can provide traffic information over wide areas, independent of sunlight illumination and weather conditions. In the paper, a novel a priori knowledge based ground moving target indication (GMTI) and parameter estimation algorithm applicable on single- as well as on multi-channel synthetic aperture radar (SAR) data is presented. Only the intersection points of the moving vehicle signals with the a priori known road axes, which are mapped into the range-compressed data domain, are evaluated. The algorithm needs low computational load and is hence well suited for real-time traffic monitoring applications.

Multichannel SAR-GMTI in Maritime Scenarios With F-SAR and TerraSAR-X Sensors

This paper explores the ground moving target indication (GMTI) capabilities of the German Aerospace Centers state-of-the-art airborne (F-SAR) and spaceborne (TerraSAR-X) synthetic aperture radars (SARs) when operating over maritime scenarios. The performance of classical dual channel GMTI techniques, such as displaced phase center antenna (DPCA) and along-track interferometry (ATI), as well as the promising adaptive techniques, like extended DPCA (EDPCA) and imaging space-time adaptive processing (ISTAP) have been analyzed on the basis of experimental acquisitions with both sensors. The objective of the paper is to highlight the limitations and challenges to be considered when processing real, multichannel GMTI data from pioneering SAR sensors for maritime surveillance. Different calibration or channel balancing strategies, on the basis of the digital balancing (DB) method, are studied, considering their impact on SAR-GMTI performance. An adaptive SAR processor, accounting for target kinematics and based on a matched filter bank (MFB) approach, is integrated in the SAR-GMTI processing chain in order to retrieve refocused images of the moving vessels.

SAR Traffic Monitoring Using Time-Frequency Analysis for Detection and Parameter Estimation

In the paper a ground moving target indication (GMTI) algorithm operating on preprocessed range-compressed SAR data is presented. For preprocessing range cell migration correction for stationary targets in most practical cases is sufficient. Moving target signal detection and extraction, adaptive range cell migration correction, position and across as well as along-track velocity estimation is then performed without a priori knowledge by using matched filter banks and the fractional Fourier transform. The proposed algorithm is able to cope with multi-component linear frequency modulated signals as they arise in real traffic scenarios containing a number of moving road vehicles.

Bistatic spaceborne-airborne experiment TerraSAR-X/F-SAR: data processing and results

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 DLRs 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.

A priori knowledge-based Post-Doppler STAP for traffic monitoring applications

In this paper an extension of our “Fast GMTI Algorithm for Traffic Monitoring Based on A Priori Knowledge” [1,2] to an arbitrary number of M receiving (RX) channels is presented. This is done by incorporating Post-Doppler space-time adaptive processing into the processing chain. In contrast to our original dual-channel algorithm this additionally allows for robust estimation of the direction-of-arrival (DOA) angles of the detected signals. As a consequence false detections can be recognized and discarded. In the paper the processing chain is explained and performance estimation results for DLRs multi-channel airborne F-SAR system are presented and discussed.