14 research outputs found
A differential algebra based importance sampling method for impact probability computation on Earth resonant returns of Near Earth Objects
A differential algebra based importance sampling method for uncertainty
propagation and impact probability computation on the first resonant returns of
Near Earth Objects is presented in this paper. Starting from the results of an
orbit determination process, we use a differential algebra based automatic
domain pruning to estimate resonances and automatically propagate in time the
regions of the initial uncertainty set that include the resonant return of
interest. The result is a list of polynomial state vectors, each mapping
specific regions of the uncertainty set from the observation epoch to the
resonant return. Then, we employ a Monte Carlo importance sampling technique on
the generated subsets for impact probability computation. We assess the
performance of the proposed approach on the case of asteroid (99942) Apophis. A
sensitivity analysis on the main parameters of the technique is carried out,
providing guidelines for their selection. We finally compare the results of the
proposed method to standard and advanced orbital sampling techniques
A low-order automatic domain splitting approach for nonlinear uncertainty mapping
This paper introduces a novel method for the automatic detection and handling
of nonlinearities in a generic transformation. A nonlinearity index that
exploits second order Taylor expansions and polynomial bounding techniques is
first introduced to rigorously estimate the Jacobian variation of a nonlinear
transformation. This index is then embedded into a low-order automatic domain
splitting algorithm that accurately describes the mapping of an initial
uncertainty set through a generic nonlinear transformation by splitting the
domain whenever some imposed linearity constraints are non met. The algorithm
is illustrated in the critical case of orbital uncertainty propagation, and it
is coupled with a tailored merging algorithm that limits the growth of the
domains in time by recombining them when nonlinearities decrease. The low-order
automatic domain splitting algorithm is then combined with Gaussian mixtures
models to accurately describe the propagation of a probability density
function. A detailed analysis of the proposed method is presented, and the
impact of the different available degrees of freedom on the accuracy and
performance of the method is studied
Impact probability computation of Near-Earth Objects using Monte Carlo Line Sampling and Subset Simulation
This work introduces two Monte Carlo (MC)-based sampling methods, known as
line sampling and subset simulation, to improve the performance of standard MC
analyses in the context of asteroid impact risk assessment. Both techniques
sample the initial uncertainty region in different ways, with the result of
either providing a more accurate estimate of the impact probability or reducing
the number of required samples during the simulation with respect to standard
MC techniques. The two methods are first described and then applied to some
test cases, providing evidence of the increased accuracy or the reduced
computational burden with respect to a standard MC simulation. Finally, a
sensitivity analysis is carried out to show how parameter setting affects the
accuracy of the results and the numerical efficiency of the two methods
Performance assessment of the multibeam radar sensor birales for space surveillance and tracking
Near-Earth space has become progressively more
crowded in active satellites, inactive spacecraft and
debris. Consequently, an international effort is currently
being devoted to improving the performance of the
network of optical and radar sensors for space objects
monitoring. Within this framework, the use of the novel
bistatic radar sensor BIRALES is investigated in this
work, which makes use of a multibeam receiver. The
tailored orbit determination algorithm is described,
which receives as input the data processed by the
acquisition system, that digitally assembles measured
radar echoes. The performances of the orbit
determination process are assessed on a set of numerical
simulations carried out on the NORAD catalogue, using
a dedicated simulator of the sensor.peer-reviewe
Impact probability computation for NEO resonant returns through a polynomial representation of the Line of Variations
A differential algebra based representation and propagation of the Line of Variations for Near Earth Objects impact monitoring is presented in this paper. The Line of Variations is described at the initial epoch by a high-order polynomial that is propagated forward in time. An Automatic Domain Splitting algorithm is embedded in the numerical integrator, in such a way that when the polynomials truncation error becomes too large, the line is split as many times as necessary to meet accuracy requirements. The Line of Variations is propagated forward in time until an intersection with a properly defined target plane occurs for all the generated subdomains. The subdomains are then projected onto the target plane to compute the impact probability by numerically integrating an associated one-dimensional probability density function. The proposed approach is applied to several test-cases to assess the performance of the method for the different possible shapes of the initial confidence region. Starting from a case of direct encounter, the technique is tested up to the case of a resonant return, which features critical nonlinearities
Perturbed Initial Orbit Determination
An algorithm for robust initial orbit determination (IOD) under perturbed
orbital dynamics is presented. By leveraging map inversion techniques defined
in the algebra of Taylor polynomials, this tool is capable of not only
returning an highly accurate solution to the IOD problem, but also estimating a
range of validity for the aforementioned solution in which the true orbit state
should lie. Automatic domain splitting is then used on top of the IOD routines
to ensure the local truncation error introduced by a polynomial representation
of the state estimate remains below a predefined threshold to meet the
specified requirements in accuracy. The algorithm is adapted to three types of
ground based sensors, namely range radars, Doppler-only radars and optical
telescopes by taking into account their different constraints in terms of
available measurements and sensor noise. Its improved performance with respect
to a Keplerian based IOD solution is finally demonstrated with large scale
numerical simulations over a subset of tracked objects in low Earth orbit.Comment: submitted to Astrodynamic
Multifidelity Orbit Uncertainty Propagation using Taylor Polynomials
A new multifidelity method is developed for nonlinear orbit uncertainty propagation. This
approach guarantees improved computational efficiency and limited accuracy losses compared
with fully high-fidelity counterparts. The initial uncertainty is modeled as a weighted sum of
Gaussian distributions whose number is adapted online to satisfy the required accuracy. As
needed, univariate splitting libraries are used to split the mixture components along the direction of maximum nonlinearity. Differential Algebraic techniques are used to propagate these
Gaussian kernels and compute a measure of nonlinearity required for the split decision and
direction identification. Taylor expansions of the flow of the dynamics are computed using a
low-fidelity dynamical model to maximize computational efficiency and corrected with selected
high-fidelity samples to minimize accuracy losses. The effectiveness of the proposed method is
demonstrated for different dynamical regimes combining SGP4 theory and numerical propagation as low- and high-fidelity models respectively
A Multifidelity Approach to Robust Orbit Determination
This paper presents an algorithm for the preprocessing of observation data
aimed at improving the robustness of orbit determination tools. Two objectives
are fulfilled: obtain a refined solution to the initial orbit determination
problem and detect possible outliers in the processed measurements. The
uncertainty on the initial estimate is propagated forward in time and
progressively reduced by exploiting sensor data available in said propagation
window. Differential algebra techniques and a novel automatic domain splitting
algorithm for second-order Taylor expansions are used to efficiently propagate
uncertainties over time. A multifidelity approach is employed to minimize the
computational effort while retaining the accuracy of the propagated estimate.
At each observation epoch, a polynomial map is obtained by projecting the
propagated states onto the observable space. Domains that do no overlap with
the actual measurement are pruned thus reducing the uncertainty to be further
propagated. Measurement outliers are also detected in this step. The refined
estimate and retained observations are then used to improve the robustness of
batch orbit determination tools. The effectiveness of the algorithm is
demonstrated for a geostationary transfer orbit object using synthetic and real
observation data from the TAROT network.Comment: submitted to Acta Astronautic
C60@Lysozyme: Direct observation by nuclear magnetic resonance of a 1:1 fullerene protein adduct
Integrating carbon nanoparticles (CNPs) with proteins to form hybrid functional assemblies is an innovative research area with great promise for medical, nanotechnology, and materials science. The comprehension of CNP-protein interactions requires the still-missing identification and characterization of the 'binding pocket' for the CNPs. Here, using Lysozyme and C-60 as model systems and NMR chemical shift perturbation analysis, a protein-CNP binding pocket is identified unambiguously in solution and the effect of the binding, at the level of the single amino acid, is characterized by a variety of experimental and computational approaches. Lysozyme forms a stoichiometric 1:1 adduct with C-60 that is dispersed monomolecularly in water. Lysozyme maintains its tridimensional structure upon interaction with C-60 and only a few identified residues are perturbed. The C-60 recognition is highly specific and localized in a well-defined pocket