Salvatore D'Addio

The PARIS Ocean Altimeter In-Orbit Demonstrator

Mesoscale ocean altimetry remains a challenge in satellite remote sensing. Conventional nadir-looking radar altimeters can make observations only along the satellite ground track, and many of them are needed to sample the sea surface at the required spatial and temporal resolutions. The Passive Reflectometry and Interferometry System (PARIS) using Global Navigation Satellite Systems (GNSS) reflected signals was proposed as a means to perform ocean altimetry along several tracks simultaneously spread over a wide swath. The bandwidth limitation of the GNSS signals and the large ionospheric delay at L-band are however issues which deserve careful attention in the design and performance of a PARIS ocean altimeter. This paper describes such an instrument specially conceived to fully exploit the GNSS signals for best altimetric performance and to provide multifrequency observations to correct for the ionospheric delay. Furthermore, an in-orbit demonstration mission that would prove the expected altimetric accuracy suited for mesoscale ocean science is proposed.

Consolidating the Precision of Interferometric GNSS-R Ocean Altimetry Using Airborne Experimental Data

This paper revises the precision of altimetric measurements made with signals of the Global Navigation Satellite Systems (GNSS) reflected (GNSS-R) off the sea surface. In particular, we investigate the performance of two different GNSS-R techniques, referred to here as the clean-replica and interferometric approaches. The former has been used in GNSS-R campaigns since the late 1990s, while the latter has only been tested once, in 2010, from an 18-m-high bridge in static conditions and estuary waters. In 2011, we conducted an airborne experiment over the Baltic Sea at 3-km altitude to test the interferometric concept in dynamic and rougher conditions. The campaign also flew a clean-replica GNSS-R instrument with the purpose of comparing both approaches. We have analyzed with detail the data sets to extract and validate models of the noise present in both techniques. After predicting the noise models and verifying these with aircraft data, we used them to obtain the precision of altimetric measurements and to extrapolate the performance analysis to spaceborne scenarios. The main conclusions are that the suggested noise model agrees with measured data and that the GNSS-R interferometric technique is at least two times better in precision than a technique based on using a clean replica of the publicly available GPS code. This represents a factor of at least four times finer along-track resolution. A precision of 22 cm in 65-km along-track averaging should be achievable using near-nadir interferometric GNSS-R observations from a low earth orbiter.

Phase Altimetry With Dual Polarization GNSS-R Over Sea Ice

This paper evaluates the potential use of reflected signals from Global Navigation Satellite Systems as a source of opportunity for the retrieval of absolute ellipsoidal heights over sea ice. Accurate estimation of the surface level would be helpful for the determination of the ice thickness, a key parameter for classification and characterization of sea ice masses. Our analysis is based on altimetric estimations from the coherent differential phase between direct and both cross- and co-polar reflected signals. For this purpose, GPS waveforms have been collected from a fixed platform in Greenland, monitoring the complete process of sea ice formation and melting during a 7-month period. The variability of coherent phase samples and polarimetric measurements are compared with in situ observations to make a realistic rough characterization of the ice cover. The retrieved sea ice surface height estimates are then evaluated against an Arctic tide model, ice surface temperature from moderate-resolution imaging spectroradiometer, and the laser altimetry product from ICESat.

Monitoring sea-ice and dry snow with GNSS reflections

GPS reflected signals have become a source of opportunity for remote sensing of the Earths suface. In this work, we present several capabilities of this technique in two different polar environments: Greenland and Antarctica. The first part is dedicated to the retrieval of sea-ice properties, giving emphasis to the study of the coherent phase for altimetric and roughness estimations, and polarimetric measurements for the determination of the ice salinity variation. The results show good agreement with a tide model and daily ice charts. On the second part, some preliminary results and analysis strategies to retrieve dry snow signatures are presented.

Accurate Characterization of TWTA Distortion in Multicarrier Operation by Means of a Correlation-Based Method

Complex payload architectures, together with the use of high-order digital modulations and advanced coding schemes, call for an accurate evaluation of traveling-wave tube amplifier (TWTA) nonlinearity impairments. To assess in a simple yet accurate manner the level of signal distortion generated by the TWTA nonlinearity, we adopt a correlation-based methodology. The proposed method provides an accurate evaluation of the TWTA intermodulation, particularly in the presence of multicarrier operation with nonconstant envelope modulations.

Modeling and Analysis of GNSS-R Waveforms Sample-to-Sample Correlation

The exploitation of global navigation satellite systems Earth-reflected signals to perform ocean mesoscale altimetry from space has been originally proposed at the beginning of the 1990s. This technique is generally defined as “GNSS-R.” Since then, the versatility and availability of the GNSS-R has been the subject of studies targeting many other earth remote sensing applications, for both ocean and land. GNSS-R observables (called GNSS-R “waveforms”) are typically obtained by performing a complex cross-correlation of the received GNSS reflected signals with a locally generated replica of one of the open access GNSS signals, evaluated over a pre-defined set of delays of the local replica. The knowledge of the statistical properties of GNSS-R waveforms is of fundamental importance in order to define the retrieval algorithms to estimate the geophysical parameters and in order to optimize the accuracy of these estimations. The statistical properties of interest are mainly: 1) the correlation between realizations of GNSS-R waveforms generated at different time instants (generally defined as slow-time correlation) and 2) the correlation between the different delay samples of a given GNSS-R waveform (generally defined as fast-time or sample-to-sample correlation). The modeling and analysis of the slow-time statistical properties has been the subject of previous works. On the other hand, this paper presents for the first time a detailed analytical model describing the sample-to-sample statistics of GNSS-R waveforms. The model has been validated with real measurements of global positioning systems (GPSs) reflected signals collected by UK-DMC receiver, showing excellent agreement with the observations from space. The model is generic and can be easily extended to GNSS-R waveforms from other systems. For altimetry applications, the knowledge of waveform statistics allows to assess the dependence of altimetry performance on critical system/instrument and retrieval parameters such as the sampling frequency, the receiver bandwidth or signal-to-noise-ratio (SNR). This paper also presents an analysis of the impact of these parameters on instrument performance, the conclusions of which are general and constitute an important basis for optimization of future GNSS-R instruments.

Mitigation of Direct Signal Cross-Talk and Study of the Coherent Component in GNSS-R

In Global Navigation Satellite System Reflectometry (GNSS-R), power waveforms or Delay-Doppler Maps (DDM) are incoherently averaged to reduce the standard deviation of the fluctuations caused by the speckle and thermal noise. This letter proposes a simple and innovative processing concept based on the computation of the variance of the complex waveforms. By this approach, the coherent part of the signal is subtracted from the incoherent averaged waveform. It can be useful, for example, in scenarios where reflected signal is contaminated by the direct signal, as ground-based scenarios or in future experiments such as GEROS-ISS due to the nearby multipath of direct signals.

GNSS-R Altimeter Based on Doppler Multi-looking

Measuring ocean mesoscale variability is one of the main objectives of next generation satellite altimeters. Current radar altimeters make observations only at the nadir sub-satellite ground track, which is not sufficient to sample the ocean surface with the required spatial and temporal sampling. The GNSS-R concept has been proposed as an alternative observation system in order to overcome this limitation, since it allows performing altimetry along several points simultaneously over a very wide swath. Latest proposed GNSS-R altimeter configurations allow measuring sea height with an accuracy of few decimeters over spatial scales of 50-100 km, by means of a single-pass. This paper proposes an innovative processing and retracking concept for GNSS-R altimeters based on the acquisition of the full delay-Doppler map (DDM), which allows to acquire multiple waveforms at different Doppler frequencies, whose footprints are located outside the typical pulse-limited region. The proposed processing adapts the Synthetic Aperture Radar (SAR) delay-Doppler concept of spaceborne radar altimeters for use in a GNSS-R system. This processing yields additional multi-look with respect to conventional GNSS-R concepts and translates into an improvement of the altimetry performance estimated to be at least 25%-30%, and even higher, depending on the wanted along-track spatial resolution. The proposed processing can also provide measurements with high spatial resolution at best possible performance, and more generally, offers various possibilities for optimal trade-off between spatial-resolution and height estimation accuracy.

Partial Interferometric Processing of Reflected GNSS Signals for Ocean Altimetry

The bistatically reflected global navigation satellite systems (GNSS) signals have become an attractive tool for spaceborne ocean altimetry. The interferometric processing that can exploit the full bandwidth of the available GNSS signals without the knowledge of the actual ranging codes was proposed for the PARIS IoD mission to improve the ranging precision. This letter presents a novel on-board processing method which utilizes the interferometry of partial GNSS signal components to explore a further improvement of the altimetry precision. The scheme of extracting partial signal components and interferometry procedure, for GPS L1 band as an example, are illustrated. The assessment and comparison of achievable altimetric performance of the proposed method including the altimetric sensitivity, the resolution per pulse, the signal-to-noise ratio, and the overall altimetric precision are also introduced. The extension of the proposed method to other existing and planned GNSS systems and signals would guarantee an improvement in the overall performance of the PARIS concept.

Interferometric GNSS-R achievable altimetric performance and compression/denoising using the wavelet transform: An experimental study

Reflectometry using Global Navigation Satellite Systems (GNSS) opportunity signals was originally conceived for altimetry [1]. However, when the GNSS signals scatter over the Earths surface (or propagates through the atmosphere), the amplitude and polarization characteristics change, which can be used for many other remote sensing applications such as sea state, water level, soil moisture, vegetation height, snow depth monitoring, atmospheric profiling... ([2], [3] among many others). In [4] and [5], recent experimental results from an ESA-sponsored airborne campaign carried out in June and November 2011 in the Baltic sea are presented using conventional and interferometric GNSS-R, respectively. In this work the properties of the measured interferometric GNSS-R waveforms are studied, and the wavelet transform is applied to compress and denoise the waveforms. This is of interest for future missions, such as ESAs proposal of PARIS IoD, to reduce the amount of data to be stored on board and downloaded to Earth.