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In this paper, the synthetic aperture radar (SAR) calibration for low-frequency missions by means of stable point targets is presented. Calibration at low frequency involves the absolute radiometric calibration, the antenna pattern and pointing characterization and validation, and the distortion system parameters' estimation. The use of traditional instrumentation, such as a polarimetric active radar calibrator, a corner reflector, or an active transponder, may be costly and can reduce the time the instrument is used for operational acquisitions. The purpose of this paper is to evaluate the potentiality in calibration of point targets for which the radar cross section and the time stability have been characterized. Given a calibration site, once that a set of the stable point targets have been detected by the analysis of an interferometric stack of SAR acquisitions, they may be used as passive calibrators for the validation of radiometry, elevation antenna pattern, and pointing estimation. We show that, although less targets are expected to be found in P- or L- band than in C- or X-band, a sufficient amount (about 250 targets per acquisition) can provide an accuracy in antenna pattern estimation of about 0.04 dB, if the target accuracy is 0.1 dB at 1◝. [ABSTRACT FROM AUTHOR]

The availability of accurate trajectory information is paramount for the processing and exploitation of synthetic aperture radar (SAR) data. Considering the particular case of spaceborne SARs designed for repeat-pass interferometric applications, errors in the trajectory translate into phase artifacts that affect the interferometric performance. In this paper, we propose a model-based procedure to calibrate the trajectories of spaceborne SAR systems by the multisquint (MS) phase. The technique allows to estimate the along and the derivative of across track geometric errors. The geometric model of the InSAR phase is derived as a function of positioning errors and the MS phase model as derivative of the InSAR phase geometric model, with respect to the squint angle. We perform a sensitivity analysis of the model in order to define which geometric errors can be estimated by the MS phase, justifying the assumption that the MS phase is very poorly affected by the atmospheric phase screen. We particularly concentrate on the TOPSAR acquisition mode, where the phase is very sensitive to geometric errors. We start from the classical two-image case and then consider the extension to the multibaseline case. Experimental results obtained by processing of interferometric pairs acquired by the Sentinel-1A sensor are reported. [ABSTRACT FROM PUBLISHER]

This paper concerns the performance of the flexible dynamic block adaptive quantizer (FDBAQ), which is the onboard data compression scheme used by Sentinel-1 (S-1), i.e., the C-band synthetic aperture radar (SAR) constellation whose first satellite (Sentinel-1A) has been launched on April 3, 2014. The data rate that results from the fine resolution and the wide swath of S-1 would exceed, without data compression, the S-1 hardware limitations. The FDBAQ has been proposed as an efficient method to reduce the instrument data rate and limit the onboard storage requirement. The capabilities of such a compression scheme have not been tested on real data before. Preflight simulation and analysis were based on a simplified processing scheme and radar backscattering information taken from past missions. The purpose of this paper is to present the first results of the analysis of real acquisitions, comprising thousands of products, in every S-1 acquisition mode, acquired during and after the commissioning phase. Advantages and limits are presented in detail, comparing FDBAQ and the traditional block adaptive quantizer (BAQ) compression scheme, in terms of quantization noise level, signal-to-noise ratio, and average bit and data rates. [ABSTRACT FROM PUBLISHER]

In this paper, a numerical method optimizer based on covariance matching is proposed for synthetic aperture radar (SAR) polarimetric calibration. The method makes use of the information provided by a distributed target and a corner reflector in order to jointly estimate the system polarimetric distortion parameters and the Faraday rotation. A preliminary analysis is conducted to show the expected accuracy values and to identify the intrinsic ambiguities of the problem. Results from simulations are shown to assess the accuracy and convergence of the method. Finally, tests have been conducted on stack of repeated full polarimetric ALOS PALSAR images to check the stability of the retrieved distortion parameters in a realistic case. [ABSTRACT FROM PUBLISHER]

This paper focuses on the generation of very high resolution digital elevation models (DEMs) mainly from single-baseline spaceborne synthetic aperture radar (SAR) systems. The stringent requirements on vertical accuracy demand for frequent and accurate measures of the baselines, subject to mechanical vibrations of the boom that may be severe when long booms are exploited. This paper proposes a solution that exploits an existent coarse-resolution DEM, for example, the Shuttle Radar Topography Mission, as a set of ground control points (GCPs). A computational model and an inversion scheme are introduced to jointly take into account the information coming from the grid of GCP and the local shifts measured during image coregistration. The model properly takes account for the quality of the GCP, its location with respect to the target of interest, and the interferogram noise in correspondence to the GCP. A novel “split-swath” approach is proposed to enhance the quality of the baseline estimates. The mode exploits two narrow strips with significant diversity in incidence angle, achieved, for example, by ScanSAR or TOPSAR mode. An example of design is provided by assuming a Ka-band topographic spaceborne SAR mission. [ABSTRACT FROM PUBLISHER]

In this paper, we revise the relative radiometric calibration of synthetic-aperture-radar stacks which exploits natural persistent scatterers (PSs). We introduce a new model to estimate a slight error in the sensor pointing in elevation and a new coherent method that makes use of phases evaluated by averaging the complex data on the local window. We show that the proposed approach outperforms the conventional one, as it is mostly insensitive to the “differential” biasing that affects the noncoherent permanent scatterer calibration (PScal). Results from processing COnstellation of small Satellites for the Mediterranean basin Observation (COSMO)-SkyMed and European Remote Sensing (ERS) satellites stacks are presented. [ABSTRACT FROM AUTHOR]