557 research outputs found
Analytical ray-tracing in planetary atmospheres
Ground-based astro-geodetic observations and atmospheric occultations, are
two examples of observational techniques requiring a scrutiny analysis of
atmospheric refraction. In both cases, the measured changes in observables are
geometrically related to changes in the photon path and the light time of the
received electromagnetic signal. In the context of geometrical optics, the
change in the physical properties of the signal are related to the refractive
profile of the crossed medium. Therefore, having a clear knowledge of how the
refractivity governs the photon path and the light time evolution is of prime
importance to clearly understand observational features. Analytical studies
usually focused on spherically symmetric atmospheres and only few aimed at
exploring the effect of the non-spherical symmetry on the observables. In this
paper, we analytically perform the integration of the photon path and the light
time of rays traveling across a planetary atmosphere. We do not restrict our
attention to spherically symmetric atmospheres and introduce a comprehensive
mathematical framework which allows to handle any kind of analytical studies in
the context of geometrical optics. To highlight the capabilities of this new
formalism, we carry out five realistic applications for which we derive
analytical solutions. The accuracy of the method of integration is assessed by
comparing our results to a numerical integration of the equations of
geometrical optics in the presence of a quadrupolar moment . This shows
that the analytical solution leads to the determination of the light time and
the refractive bending with relative errors at the level of one part in
and one part in , for typical values of the refractivity and the
parameter at levels of and , respectively
Integrated model for beach nourishment design, post-nourishment monitoring and beach-maintenance refills
The Grain-size Nourishment Model (GNM) is a new numerical model that forecasts in 3D the littoral features following the nourishment intervention. Its predictions concern: (a) beach and shoreface morphology; (b) shoreline position; (c) geometry of the artificial deposit; (d) sediment amount for the intervention; (e) geographic distribution of sedimentological parameters (mean size, sorting, percentage of sand and mud). Theory and principles of calculation are presented in this paper together to two applications of the model
Experimental verification of a simple method for accurate center of gravity determination of small satellite platforms
We propose a simple and relatively inexpensive method for determining the center of gravity (CoG) of a small spacecraft. This method, which can be ascribed to the class of suspension techniques, is based on dual-axis inclinometer readings. By performing two consecutive suspensions from two different points, the CoG is determined, ideally, as the intersection between two lines which are uniquely defined by the respective rotations. We performed an experimental campaign to verify the method and assess its accuracy. Thanks to a quantitative error budget, we obtained an error distribution with simulations, which we verified through experimental tests. The retrieved experimental error distribution agrees well with the results predicted through simulations, which in turn lead to a CoG error norm smaller than 2mm with 95% confidence level
MATNet: Multi-Level Fusion and Self-Attention Transformer-Based Model for Multivariate Multi-Step Day-Ahead PV Generation Forecasting
The integration of renewable energy sources (RES) into modern power systems
has become increasingly important due to climate change and macroeconomic and
geopolitical instability. Among the RES, photovoltaic (PV) energy is rapidly
emerging as one of the world's most promising. However, its widespread adoption
poses challenges related to its inherently uncertain nature that can lead to
imbalances in the electrical system. Therefore, accurate forecasting of PV
production can help resolve these uncertainties and facilitate the integration
of PV into modern power systems. Currently, PV forecasting methods can be
divided into two main categories: physics-based and data-based strategies, with
AI-based models providing state-of-the-art performance in PV power forecasting.
However, while these AI-based models can capture complex patterns and
relationships in the data, they ignore the underlying physical prior knowledge
of the phenomenon. Therefore, we propose MATNet, a novel self-attention
transformer-based architecture for multivariate multi-step day-ahead PV power
generation forecasting. It consists of a hybrid approach that combines the AI
paradigm with the prior physical knowledge of PV power generation of
physics-based methods. The model is fed with historical PV data and historical
and forecast weather data through a multi-level joint fusion approach. The
effectiveness of the proposed model is evaluated using the Ausgrid benchmark
dataset with different regression performance metrics. The results show that
our proposed architecture significantly outperforms the current
state-of-the-art methods with an RMSE equal to 0.0460. These findings
demonstrate the potential of MATNet in improving forecasting accuracy and
suggest that it could be a promising solution to facilitate the integration of
PV energy into the power grid
Analytical study of the radio signals propagation in planetary atmospheres
The ESA JUICE (JUpiter ICy moons Explorer) mission is planned for launch in 2022 and arrival at Jupiter around 2030. The mission is dedicated to the study of the giant gaseous and its largest moons. While the spacecraft will probe the Jovian system it will be occulted by the atmosphere of Jupiter or its satellites as seen from antennas on Earth. Such a configuration offers a great opportunity to study remotely the physical properties of the occulting atmosphere using radio links as the probe is being occulted. Indeed, non-unity index of refraction causes the electromagnetic waves to depart from the straight line and also impacts the propagation speed of the waves. Both changes modify the wave frequency and conversely, from the time variation of the Doppler measurements the index of refraction profile can be retrieved. In the literature, there are different approaches devoted to the retrieval of the refractive profile from these observables. Let mention, i) the analytic formulation of the Abel inversion which is employed for spherically symmetric atmospheres, and ii) the ray tracing method which is a numerical integration of the fundamental equations of optics and which is well suited for atmospheres with more complicated shapes. Both possess their own advantages and inconveniences. For instance, to invert a complete set of data, the ray tracing method requires more computational time than the Abel transformation. In return, the Abel inversion is based on the spherical symmetry assumption while the ray tracing technique can handle non-radial gradient in the refractive profile. In the context of the future occultations of JUICE by Jupiter, we discuss the benefit of a new formalism based on a full reformulation of the fundamental equations of optics. This new approach let to provide a very comprehensive description of the light trajectory inside a planetary atmosphere with no assumption on the refractive profile. In the special case where the departure from the spherical symmetry is small, we present an analytic solution which is well suited for the data processing of radio occultation experiments. Indeed, this solution can handle the effect of a non-spherically symmetric atmosphere with a low computational cost. We use this solution to process the Cassini Doppler data acquired during an occultation by the oblate atmosphere of Saturn. The validity of the proposed approach is assessed comparing the results with other studies available in the literature
The Juventas CubeSat in Support of ESA\u27s Hera Mission to the Asteroid Didymos
The European Space Agency’s planetary defense Hera mission will launch to the Didymos binary asteroid system in 2023 (with bodies nicknamed Didymain and Didymoon). Once in vicinity of the asteroid, two 6U CubeSats will be deployed to contribute to the asteroid research and mitigation assessment objectives of the Hera mission. This paperwill describe the Juventas CubeSat, equipped with a low frequency radar payload to characterize the internal structure of Didymoon. Juventas is designed to be operated using the Hera mothercraft as a proxy. This mission architecture creates a new paradigm for CubeSats, requiring high levels of mission autonomy while operating in the challenging environment of a small-body binary asteroid. Juventas will utilize the inter-satellite link to Hera for performing radio science experiments, augmenting the characterization of the asteroid gravity field. Once the radar science and radio science observation objectives have been met, Juventas will perform an attempted landing on the surface of Didymoon to research its dynamical properties
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