This paper addresses several important aspects that need to be considered for the processing of spaceborne 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.

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Sentinel-1B is the second of two C-Band Synthetic Aperture Radar (SAR) satellites of the Sentinel-1 mission, launched in April 2016-two years after the launch of the first satellite, Sentinel-1A. In addition to the commissioning of Sentinel-1B executed by the European Space Agency (ESA), an independent system calibration was performed by the German Aerospace Center (DLR) on behalf of ESA. Based on an efficient calibration strategy and the different calibration procedures already developed and applied for Sentinel-1A, extensive measurement campaigns were executed by initializing and aligning DLR's reference targets deployed on the ground. This paper describes the different activities performed by DLR during the Sentinel-1B commissioning phase and presents the results derived from the analysis and the evaluation of a multitude of data takes and measurements.

A test case showing a comparison of TerraSAR-X and ground-based weather radar data acquired nearly simultaneously (within the same minute) over New York City. A good agreement between rain-cell signatures in (left) the SAR image and (right) the weather radar image can be observed.

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Nowadays, stringent requirements for the quality of SAR data products exist. The radiometric and image quality requirements can only be fulfilled by knowing the disturbances caused by the radar hardware itself. The internal calibration is a mechanism to estimate these disturbances from the real hardware. Unfortunately, most of the current internal calibration schemes interrupt the SAR data acquisition during internal calibration and hence cause a loss of azimuth samples and a degradation of the image quality. The calibration is thus often performed only before and after a data take. However, these two calibration events are hardly sufficient to estimate the instrument parameters during the whole acquisition especially for long data takes, which will become more common in future SAR system. In this paper, an internal calibration scheme is introduced which allows performing the internal calibration during nominal data acquisition by injecting a coded calibration signal into the receiver with very low power. This signal causes only minor degradation of the SAR image but can be retrieved from the data using adequate signal processing. This technique is described, the theoretical performance estimated, and finally demonstrated on real data from the TerraSAR-X mission. A selection of possible coding scheme is also analyzed with respect to their suitability for the proposed internal calibration scheme.

The high flexibility and tight accuracy requirements of modern spaceborne SAR systems require innovative technologies to calibrate and process the SAR images. To perform accurate pattern correction during SAR processing, an antenna model can be used to derive the multitude of different antenna beams generated by active antenna steering. The application of such an antenna model could be successfully demonstrated for the TerraSAR-X mission, launched in 2007. The methodology and the results of the in-orbit verification with an achieved accuracy of better than ±0.2 dB is reviewed in this paper in detail showing its outstanding accuracy. Additionally, the results of the antenna pattern long term monitoring are described pointing out the high stability of the system Index Terms-Active antenna calibration, antenna modelling, TerraSAR-X calibration Flight direction / azimuth Range / Elevation Flight direction / azimuth Range / Elevation Uncorrected image After antenna pattern correction