Steffen Wollstadt

TOPS Interferometry With TerraSAR-X

This paper presents results on SAR interferometry for data acquired in the Terrain Observation by Progressive Scans (TOPS) imaging mode. The rationale to retrieve accurate interferometric products in this mode is expounded, emphasizing the critical step of coregistration. Due to the particularities of the TOPS mode, a high Doppler centroid is present at burst edges, demanding a very high azimuth coregistration performance. A coregistration accuracy of around one tenth of a pixel, as it is usually recommended for stripmap interferometric data, could result in large undesired azimuth phase ramps in each TOPS burst. This paper presents two approaches based on the spectral diversity technique to precisely estimate this coregistration offset with the required accuracy and evaluates their performance. The effect of squint at burst edges in terms of an undesired impulse response shift during focusing and the impact on the interferometric coregistration performance is also addressed. Repeat-pass TOPS data acquired experimentally by TerraSAR-X are used to validate the proposed approaches.

On the Processing of Very High Resolution Spaceborne SAR Data

This paper addresses several important aspects that need to be considered for the processing of spaceborne synthetic aperture radar (SAR) data with resolutions in the decimeter range. In particular, it will be shown how the motion of the satellite during the transmission/reception of the chirp signal and the effect of the troposphere deteriorate the impulse response function if not properly considered. Further aspects that have been investigated include the curved orbit, the array pattern for electronically steered antennas, and several considerations within the processing itself. For each aspect, a solution is proposed, and the complete focusing methodology is expounded and validated using simulated point targets and staring spotlight data acquired by TerraSAR-X with 16-cm azimuth resolution and 300-MHz range bandwidth.

The TerraSAR-X Staring Spotlight Mode Concept

The paper investigates the possibility to enhance the TerraSAR-X (TSX) azimuth resolution by means of staring spotlight imaging in combination with an extended azimuth pattern steering. The current TSX spotlight modes are briefly reviewed, and the azimuth steering limitations are discussed. Based on a realistic TSX azimuth pattern simulation and performance estimation using a Kepler orbit and the WGS84 reference ellipsoid, the key performance parameters are estimated for an increasing azimuth pattern steering angle span. The azimuth ambiguity performance is identified to be the driving performance parameter. Experimental TSX staring spotlight high-resolution images are presented. They demonstrate and verify the envisaged performance estimation. The paper concludes with a concept for a high-resolution TSX staring spotlight mode that serves as baseline for the upcoming operational implementation.

TerraSAR-X System Performance Characterization and Verification

This paper presents results from the synthetic aperture radar (SAR) system performance characterization, optimization, and verification as carried out during the TerraSAR-X commissioning phase. Starting from the acquisition geometry and instrument performance, fundamental acquisition parameters such as elevation beam definition, range timing, receiving gain, and block adaptive quantization setting are presented. The verification of the key performance parameters-ambiguities, impulse-response function, noise, and radiometric resolution-is discussed. ScanSAR and Spotlight particularities are described.

Bidirectional SAR Imaging Mode

This paper introduces the bidirectional synthetic aperture radar (BiDi SAR) imaging mode, i.e., the simultaneous imaging of two directions by one antenna into one receiving channel, and presents short-term time series of images and interferograms acquired by the TerraSAR-X and TanDEM-X satellites. A comparison to alternative approaches for the acquisition of short-term time series is provided. The BiDi acquisition geometry is defined, and a TerraSAR-X BiDi antenna pattern is analyzed. BiDi raw data are simulated, sampled with different pulse repetition frequency values, and compared with real BiDi raw data. The spectral separation of simultaneously acquired forward- and backward-looking images is explained. This paper presents the image results of BiDi acquisitions with TerraSAR-X and TanDEM-X satellites flying with 20-km along-track separation. This pursuit configuration allowed for the acquisition of up to six short-term repeated images and up to three interferograms in a single pass. An overview of potential applications for the new BiDi SAR imaging mode concludes this paper.

High precision SAR focusing of TerraSAR-X experimental staring spotlight data

This paper addresses several innovative steps needed in the chirp scaling and extended chirp scaling (ECS) algorithms in order to process staring Spotlight TerraSAR-X (TSX) images with 21 cm azimuth resolution and 300 MHz range bandwidth. The aspects that need of special consideration are the 2D phase truncation in frequency domain of ECS, the element pattern of the antenna array, the curved orbit, the stop-and-go approximation, and the troposphere. All these aspects are expounded in detail and a solution is given for each of them. The suggested corrections are applied at raw data level, hence easing the integration within the existing TSX processor. Real data acquired by TSX in the experimental staring Spotlight mode are used to validate the proposed methodology.

Taxi: A versatile processing chain for experimental TanDEM-X product evaluation

TanDEM-X is a high-resolution interferometric radar mission with the main goal of providing a global digital elevation model (DEM) of the Earth surface by means of single-pass X-band SAR interferometry. It is, moreover, the first genuinely bistatic spaceborne SAR mission, and, independently of its usual quasi-monostatic configuration, includes many of the peculiarities of bistatic SAR. An experimental, versatile, and flexible interferometric chain has been developed at DLR Microwaves and Radar Institute for the scientific exploitation of TanDEM-X data acquired in non-standard configurations. The paper describes the structure of the processing chain and focusses on some essential aspects of its bistatic part. Some experimental results performed with TerraSAR-X demonstrate the flexibility of the implemented processor.

Staring spotlight imaging with TerraSAR-X

This paper presents the outcome of a staring spotlight experiment with TerraSAR-X for considerably enhanced azimuth resolution. The commanding is discussed in terms of calculation of the azimuth steering angle. The performance estimation results are given for TerraSAR-X parameters and a steering angle range from -2.2° to +2.2°. The first TerraSAR-X staring spotlight images are presented.

Scalloping Correction in TOPS Imaging Mode SAR Data

This letter presents an investigation on scalloping correction in the Terrain Observation by Progressive Scan (TOPS) imaging mode for synthetic aperture radar systems with electronically steered phased array antennas. A theoretical simulation of the scalloping is performed, and two correction methods are introduced. The simulation is based on a general cardinal sine (sinc) antenna model as well as on the TerraSAR-X (TSX) antenna model. Real TSX data acquired over rainforest are used for demonstration and verification of the scalloping simulation and correction. Furthermore, a calibration approach, taking into account the special TOPS imaging mode properties, is introduced.

TerraSAR-X Staring Spotlight Mode Optimization and Global Performance Predictions

For the TerraSAR-X mission, a new staring spotlight mode has been implemented delivering very high azimuth resolution. Detailed performance analyses have been conducted to optimize the commanding parameters for this mode. Compared to the previously available TerraSAR-X imaging modes, staring spotlight requires operating the instrument with minimum margins and at the edge of the specifications, the radar has been designed for. Therefore, an additional step-a global acquisition simulation and analysis-is introduced during the operationalization to ensure the suitability of the derived commanding parameters on a global scale. This paper gives an overview of SAR performance analyses conducted for the mode optimization and implementation phases and presents a novel global performance assessment approach, which is generally applicable for the verification of operational SAR modes. Additionally, measurement results and exemplary acquisitions are shown.