The Copernicus Polar Ice and Snow Topography Altimeter (CRISTAL) is one of the six Copernicus Expansion Missions currently being implemented by the European Space Agency and the European Commission. The CRISTAL Mission will provide enhanced retrievals of sea ice thickness and freeboard, land ice sheet/glacier elevation, inland water level and ocean surface topography, wave-height and wind speed by measurements from sensors with improved spatial resolution and performance. Throughout this paper, the current status for the design, development and validation of the ESA CRISTAL Level-2 products is described.

The Copernicus Imaging Microwave Radiometer (CIMR) is one of the six Copernicus Expansion Missions currently being implemented by the European Space Agency and the European Commission. The mission is specifically designed to provide measurement evidence in support of developing, implementing, and monitoring the impact of the European Integrated Policy for the Arctic. CIMR provides microwave imaging radiometry measurements at low frequency (L-, C-, X-, K- and Ka-band) with relatively high spatial resolution and high radiometric fidelity. In this paper, the current plan for the design, development and validation of the ESA CIMR Level-2 products is described.

External calibration of satellite altimeters is commonly used to assess the errors introduced by the instrument when performing its measurements. The on-ground processing of the altimeter acquisitions over an external calibrator is a key step to assess these errors starting from the knowledge of the exact location of the external calibrator and its characteristics. In this letter, a novel on-ground processing is proposed for high pulse repetition frequency altimeters based on the fully focused synthetic-aperture radar (FF-SAR) concept, which relies on the capability of the instrument to preserve the coherency of the transmitted pulses for sufficiently long time. Starting from the FF-SAR processing, methods are here proposed to perform both absolute range calibration and datation calibration. The effectiveness and performance of the proposed approach are verified by processing the transponder acquisitions of CryoSat.

CryoSat-2 (CS2) is the first mission carrying on board an altimeter instrument able to operate in Synthetic Aperture Radar Interferometric (SARIn) mode. CS2 SARIn acquisitions have been exploited for different scientific applications that take advantage of the capability to determine the across-track angle of the first return and, in particular, they have been proved to reduce the uncertainty of sea ice freeboard retrievals. Nonetheless, the analysis of pan-Arctic freeboard obtained by processing CS2 Baseline C L1b products has shown large negative freeboard estimates in correspondence of the beginning of SARIn acquisitions. Throughout this letter, the SARIn waveforms are analyzed to identify the cause of this behavior. An improvement of the CS2 L1b processor is then prototyped and used to obtain a pan-Arctic freeboard data set where the percentage of negative freeboard is successfully minimized.

The BIOMASS mission gives completely new challenges in external calibration arising from the orbital pattern needed for the tomographic and Pol-InSAR phases of the mission, the strong effects of the ionosphere at P-band, and the lack of pre-existing P-band data except over very limited parts of the globe. Together these create problems that can only be solved by combining infrequent visits to instrumented calibration sites with systematic exploitation of the properties of distributed targets and targets of opportunity. Proposed approaches to performing radiometric and polarimetric calibration are described, together with meeting geolocation accuracy requirements.

Delay/Doppler processing for radar altimeters allows to improve the along-track resolution, but it can be affected by Doppler ambiguities depending on the design of the instrument. If not accounted for, Doppler ambiguities can cause a distortion of the multilooked waveforms and a loss of accuracy in retrieved geophysical parameters. This paper presents a waveform model based approach to define a Doppler ambiguity mask to filter out the samples affected by the ambiguous returns before multilooking, assuming a uniform rough scattering surface. The effectiveness of the proposed method for Doppler ambiguities masking has been verified on Sentinel-6 simulated data and on in-orbit data from CryoSat SARin acquisitions over ocean.

In this paper, the full focused processing in the frequency domain for high Pulse Repetition Frequency (PRF) radar altimeter science data is proposed. Considering a radar altimeter as a Synthetic Aperture Radar (SAR) instrument operating in near nadir-looking geometry, a SAR frequency-domain focusing algorithm, namely Range-Doppler algorithm, is analyzed. Then a properly modified processor for radar altimeters is described and implemented. Simulation set-up using parameters from two missions (CryoSat and Sentinel-6) and a preliminary result from in-orbit CryoSat data confirm the effectiveness of the full focused processing in the frequency domain. The proposed approach, compared to the one formerly proposed in literature [1], is less complex and more computationally efficient.

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 1σ.

Synthetic aperture radar (SAR) altimeters reduce the along-track footprint size exploiting the coherence of the transmitted pulses and achieve at the same time a noise reduction. Consequently, a large effort has been aimed at the formulation of theoretical models that apply to SAR altimeters, in order to fully exploit the improvement in spatial resolution obtained from the along-track aperture synthesis. This letter presents a novel semianalytical waveform model for SAR interferometric altimeters that preserves high accuracy even in the presence of mispointing. Starting from the waveform model proposed by Wingham et al. that provides a unified formulation for pulse-limited and SAR interferometric altimeters which can only be computed numerically, here, we describe a semianalytical approximation for small variations of the mispointing angles around an arbitrary combination of pitch and roll angles (μ̈, θ̈). The proposed semianalytical waveform model allows to reduce the high dimensionality of the model proposed by Wingham et al. and it has been proven to be accurate for variations of mispointing angles up to 0.4 deg around the (μ̈, θ̈). The performance of the proposed formulation has been evaluated on simulated data from Sentinel-6 configuration and on real data from CryoSat-2 SARin acquisitions over ocean.

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.

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.

BIOMASS mission [1] was selected as the 7th Earth Explorer Mission within the frame of the ESA Earth Observation Envelope Programme. Being the BIOMASS instrument a P-band Synthetic Aperture Radar (SAR) and being that frequency bandwidth already in use by different services, the BIOMASS acquisitions are expected to be occasionally affected by Radio Frequency Interference (RFI). In the framework of a comprehensive strategy for the BIOMASS mission to cope with the interference signals, in addition to design choices at both mission and instrument level, in this paper it is presented a study aimed at verifying the feasibility of an on-ground processing approach to mitigate the quality degradation on BIOMASS Level1 data due to RFI.

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.

CryoSat's 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.

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.

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.

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.

A novel framework providing invariance to a class of nonlinear valumetric distortions, such as gamma correction, for QIM-based watermarking techniques 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, AQIM and DM in the logarithmic domain 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.

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.