Michele Scagliola

Hyperbolic RDM for Nonlinear Valumetric Distortions

Quantization-based watermarking techniques are sensitive to valumetric scaling, a wide class of distortions applied to images and videos, such as contrast change or gamma correction. Several methods have been proposed to counteract valumetric attacks, but the common approach to the problem is to take only the linear ones into account. This paper presents an extension to the rational dither modulation (RDM) data-hiding scheme which provides robustness against nonlinear distortions modelled by a power-law attack. The algorithm makes use of proper mapping of the pixel values from the Cartesian to hyperbolic coordinates. This mapping is able to render the problem similar to the classical RDM scheme, since fixed multiplicative scaling is cancelled out while the exponentiation of a nonlinear distortion is transformed into a gain scaling. The validity of the approach has been confirmed by applying the watermarking scheme to Gaussian host and real images; experimental results confirm its intrinsic invariance against the power-law attack. Finally, it will be shown that under the white Gaussian noise addition, the proposed scheme achieves a good bit-error rate (BER). The measured BER is affected by the properties of the embedding domain, as supported by the theoretical analysis given in this paper.

A Dirty Paper Scheme for Hierarchical OFDM

A novel approach is here presented to embed a low-priority data stream into a high-priority data stream for OFDM wireless communications. The main improvement with respect to conventional hierarchical modulations is that the low-priority data stream can be decoded from the received subcarrier symbols without any previous equalization in case of multipath fading channel. As a consequence, the expected performance is independent of the channel estimation method used to equalize the channel. The low-priority signal is superimposed on the high-priority symbols using a dirty paper coding technique. As a result of the analysis of the proposed method, several improvements to reduce the interference on the high-priority symbols are introduced. The performance of our scheme has been compared with that of a conventional hierarchical modulation using DVB-T system parameters, showing a considerable improvement for multipath fading channels.

Pitch Estimation for CryoSat by Analysis of Stacks of Single-Look Echoes

This letter describes a novel approach to retrieve the pitch of the CryoSat satellite by the analysis of Synthetic Interferometric Radar ALtimeter (SIRAL) acquisitions in synthetic aperture radar (SAR) or SAR-interferometric mode over ocean. The power distribution of the single-look echoes, which are gathered for each sample location on the Earth surface, has been verified to be modulated by the along-track antenna pattern. Thus, the actual antenna pointing direction can be retrieved by fitting the model of the along-track antenna pattern on the power distribution of a stack, which is a collection of single-look echoes referred to the same ground location but acquired from different look angles. With the pitch measurement being independent of the attitude information returned by the onboard star trackers, by comparison of the pitch measured from data and of the pitch from the star tracker, the existence of a bias on the pitch can be verified. Exploiting the proposed approach, the pitch returned by star trackers on CryoSat has been found to be biased by about 0.0533°, confirming similar results obtained by other methods proposed in the literature. It is worth noticing that the method for pitch estimation here described is not limited to CryoSat but can be applied to any SAR altimeter without loss of generality.

Geometric Distortion Resilient Watermarking Based on a Single Robust Feature for Still Images

Many watermarks for still images are robust against common signal processing techniques, mainly JPEG compression, noise adding and low-pass filtering, while they are sensitive to geometrical manipulations, that yield desynchronization errors.

CryoSat: Siral calibration and performance

CryoSats Synthetic Interferometric Radar Altimeter (SIRAL) [1] is a Ku-band pulsewidth limited radar altimeter that transmits pulses at a high pulse repetition frequency thus making the received echoes phase coherent and suitable for De-lay/Doppler processing [2]. Moreover SIRAL takes advantage of two antennas mounted across-track for interferometric capability, in order to determine the across-track direction from which the echo is received [3]. The calibration strategy for SIRAL includes both internal calibrations and external calibration [1]. Due to the fact that SIRAL is an interferometric phase coherent pulse-width limited radar altimeter, a proper calibration approach has been developed. In this paper we will describe as first the internal calibration strategy and then the different calibration corrections that are applied to science data. The internal calibration results over more than three years of mission will be presented, analysing their temporal evolution in order to highlight the stability of the instrument over its life. Finally, the external calibration measurements for SIRAL will be presented.

The CoReH2O end to end simulator: Architecture and simulation results analysis

An assessment of the impacts of the SAR system performance parameters on the Level-2 products quality is here presented for CoReH2O, that is one of the candidate Earth Explorer Core Missions and whose main objective is to estimate the water stored in the snow pack. A sensitivity analysis has been conducted exploiting the end-to-end mission performance simulator that has been developed in the framework of the feasibility studies to provide a tool for the evaluation of the mission performance up to Level-2 products. Simulations have been run for a subset of the SAR system instrumental errors, such as additive noise, radiometric bias and speckle noise. The obtained results allow to verify how the accuracy of the retrieved snow parameters, e.g. the SWE, depends on the quality of the Level-1 data and, as a consequence, on the SAR system performance parameters.

Gain-Invariant Dirty Paper Coding for Hierarchical OFDM

A novel approach is here presented to superimpose a low-priority data stream on a high-priority data stream for OFDM systems. The main improvement with respect to conventional hierarchical modulation, which is included with the same purpose in various wireless technologies, is that the low-priority data stream can be decoded from the received subcarrier symbols without any previous equalization for multipath fading channels. As a consequence, the expected performance is nearly invariant of the channel estimation method used to equalize the channel. The low-priority stream is inserted adopting a gain-invariant dirty paper coding based on Rational Dither Modulation, which was proposed for data hiding applications. In this paper an analysis of the developed system is presented and several simulations have been carried out using DVB-T system parameters to verify the validity of the proposed approach. The experimental results show the better performance of the proposed method with respect to that of a conventional hierarchical modulation, particularly when the accuracy of the estimated channel response decreases.

Providing invariance to nonlinear valumetric scaling for quantization basedwatermarking

A novel framework providing invariance to a class of nonlinear valumetric distortions, such as gamma correction, for QIM-based watermarking techniques [1] is presented. Valumetric distortions are quite common in image and video processings and the sensitivity to valumetric scalings represents the main weakness of the watermarking techniques belonging to the quantization based class. The proposed method amounts to perform a mapping of the host samples in a proper transformed domain where the watermark is subsequently embedded using a gain invariant QIM-based technique. The effectiveness of this approach has been verified by applying the watermarking system using RDM [2], AQIM [3] and DM in the logarithmic domain [4] as embedding algorithms. Simulation results provide a useful comparison of the performance of these different techniques within the proposed scheme as well as they confirm the invariance to nonlinear distortions.

Principal components dynamic block quantization for multichannel SAR

This paper concerns the on-board data volume reduction for a single-platform multichannel Synthetic Aperture Radar (SAR) system. In such a system, the unambiguous Doppler spectrum reconstructed on-ground is enlarged at the cost of an increased amount of acquired data. Since the signals received by a multichannel SAR instrument are intrinsically correlated, such correlation can be exploited to represent the signals in a smaller set of components, introducing a controlled distortion. This way, a reduced amount of data can be quantized and transmitted on-ground. Data simulations have been performed to evaluate the performance of the proposed approach. By analysis of the simulation results, efficient reduction of data volume is achieved, even keeping low enough the additional distortion introduced in the reconstructed data.

An extended analysis of along-track antenna pattern compensation for SAR altimetry

One of the main benefits of the along-track processing in Synthetic Aperture Radar altimetry [1] is the speckle reduction that is achieved by averaging all the observations accumulated for a same scattering area. Being the different observations obtained by looking at the same scattering area from different positions of the instrument along the orbit, they result to be modulated by the along-track antenna pattern. By compensating the along-track antenna pattern before averaging, a higher speckle reduction can be achieved. This paper is aimed at presenting this processing method and at evaluating the possible improvement in the sense of the effective number of looks and of the precision of the physical parameters retrieved from the power waveforms.