Martina Gabele

First Spaceborne Demonstration of Digital Beamforming for Azimuth Ambiguity Suppression

Over the past years, the use of multiple antenna apertures combined with digital beamforming (DBF) has been spotlighted as a promising solution for the fundamental restriction for high-resolution and wide-swath spaceborne synthetic aperture radar (SAR) imaging. In this paper, we present the first spaceborne experiment of a DBF technique on receive, using the TerraSAR-X dual receive antenna mode. For this experiment, we implemented a DBF module, which includes the reconstruction filter as a digital beam former and associated signal processing for calibration and channel balancing. The experimental results exhibit the successful ambiguity suppression capability of the DBF and validate the high potential of the DBF both for advanced future and current SAR systems.

Fore and Aft Channel Reconstruction in the TerraSAR-X Dual Receive Antenna Mode

The TerraSAR-X satellite is a high-resolution synthetic aperture radar (SAR) system launched in June 2007 which provides the option to split the antenna in along-track direction and sample two physical channels separately. Modern SARs are equipped with active phased array antennas and multiple channels. In order to keep costs low, TerraSAR-X uses the redundant receiver unit for the second channel such that fore and aft channel signals are combined by a hybrid coupler to form sum and difference channel data. The dual receive antenna (DRA) mode can either be used to acquire along-track interferometric data or to acquire signals with different polarizations at the same time (Quad-Pol). Fore and aft channel reconstruction is necessary if ground moving target indication (GMTI) algorithms such as the displaced phase center antenna technique or along-track interferometry shall be applied, and in order to separate the horizontally and vertically polarized received signal components. The proposed approach uses internal calibration pulses from different calibration beams in order to estimate and compensate the hardware impact. The theoretical framework together with the results from the experimental data evaluation for the fore and aft channel reconstruction of the TerraSAR-X DRA mode are presented. The impact of the receive hardware transformation matrix estimation accuracy on errors in the reconstructed fore and aft channel image data is studied, and first examples on the GMTI capability of the TerraSAR-X DRA mode are given.

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.

GMTI Performance of a High Resolution Wide Swath SAR Operation Mode

In this paper an operation mode for space-based SAR/GMTI is suggested which uses a rectangular antenna array and employs subpulse beamsteering on transmit in order to yield wide swath coverage. A signal modeling in range Doppler domain is introduced that is suitable for studying the SAR/GMTI performance. Simulation results are given.

A new method to create a virtual third antenna from a two-channel SAR-GMTI system

Two-channel SAR-GMTI systems are suboptimal for moving target motion parameter estimation. Indeed, the ATI phase estimate of the across-track velocity component for a moving target is biased to lower values depending on the target signal to clutter ratio and the target across-track velocity. Additional antenna diversity can introduce additional degrees of freedom that can eliminate the bias problem. Aperture switching is an accepted method to virtually increase the number of channels without adding new hardware. One such mode is the RADARSAT-2 toggle mode (S. Chiu and C. Gierull, 2006). This paper proposes a new processing method to create a similar effective phase center configuration as the RADARSAT-2 Toggle mode from already recorded two-channel SAR data. This is achieved by delaying and combining the recorded two-channel measurements. The combination operation manifests not only a third phase center halfway between the phase centers of the two-channel system, but also a different antenna length of the virtual third antenna which requires a modification of the DPCA-ATI processing algorithm. The DPCA-ATI performance of the new mode is assessed and compared to ATI from the original two-channel mode.

Impact of Road Vehicle Accelerations on SAR-GMTI Motion Parameter Estimation

In this paper the problem of reliable along-track velocity estimation is addressed. It is known that the neglection of across-track accelerations can bias the estimates of along-track velocities, but up to now it is unknown which vehicle accelerations appear in real traffic scenarios. On this account an experiment to measure accelerations was conducted. The acceleration measurement results, which are presented and discussed in this paper, imply that accelerations must not be neglected in the along- track velocity estimation step if accurate estimates are required. Finally, some basic ideas are given which enable a reliable separation of along-track velocity and across- track acceleration and so improve the estimation accuracy.

TerraSAR-X Dual Receive Antenna mode — Channel reconstruction and impact on the GMTI performance

TerraSAR-X is a high resolution synthetic aperture radar (SAR) satellite which provides the option to split the antenna in along-track direction and sample two physical channels separately. This so-called dual receive antenna (DRA) mode is implemented by use of a Magic-T hybrid junction which provides the sum and difference channel data instead of the fore and aft channel data. One way to exploit this spatial degree of freedom is to reconstruct the fore and aft channel data in the processing. The critical issue with the fore and aft channel reconstruction is the fact, that for space-based radar systems very high precision calibration is required. This is even more difficult since the wide bandwidth pulses employed by the radar makes the receive hardware transformation frequency dependent. In this paper a fore and aft channel reconstruction approach is described based on an estimation of the receive hardware transformation matrix by use of internal calibration pulses and calibration beams. The quality gain of the fore and aft channel reconstruction is demonstrated with experimental data and results on the ground moving target indication (GMTI) capability are shown.

Study of System Design Parameters for Space-Based Traffic Monitoring

This paper addresses the ground moving target indication (GMTI) performance and the impact of system design parameters on space-based multi-channel synthetic aperture radar (MSAR) systems as they suffer from the high speed of the radar platform leading to a wide spread clutter spectrum. Opposite to classical GMTI systems near future space-based SAR systems offer only few simultaneous channels and relatively low pulse repetition frequency (PRF). The impact of PRF, antenna size, antenna separations and number of channels on detection performance is studied. The analysis is based on a post-Doppler approach of the optimal processor.