1,721,074 research outputs found
Experimental investigation of a supersonic backward facing step flow
No full textThe flow over a backward facing step is investigated by means of infrared thermography. An inverse heat transfer procedure is used to calculate the surface heat flux from the measured surface temperature in time by taking into account multi-dimensional and unsteady conduction effects. Additionally schlieren visualization is used to get an overview of the flow field. The backward facing step flow is investigated for laminar, transitional and turbulent separating boundary layers. It is found that for the laminar and transitional cases the Stanton number and normalized separation length are influenced by the step height. For turbulent separation this is only marginally the case however the Stanton number downstream of reattachment increases with step height. For the transitional separation, large heat transfer peaks are measured at reattachment
Exploring Stochastic Variational Gaussian Processes for Trustworthy Monocular Pose Estimation in Space
No full textTwo-line-element propagation improvement and uncertainty estimation using recurrent neural networks
No full textAs space traffic increases, Space Situational Awareness (SSA) is becoming fundamental for safe spaceflight operations. Cost-driven missions based on small satellite platforms would benefit from the availability of alternative tools providing preliminary SSA from publicly available information, such as two-line elements. In this work, we propose an orbit prediction and uncertainty evaluation method based on the well-established TLE differencing technique aided by a machine learning corrector. By designing a Recurrent Neural Network with carefully chosen input parameters, the TLE prediction accuracy is significantly improved, when tested against precise orbital data of real satellites. The prediction error is reduced, on average, by 45% across a prediction window of 16 days which may include manoeuvres. We further show that in combination with a statistical test for equality between error distributions, the differencing technique applied to the corrected TLE allows a reliable variance estimate in most situations. Limitation of the work is the training of a dedicated neural network corrector for each specific space object, which will be deposed as part of our ongoing efforts
Verification Approaches for Nano- and Micro-Satellites
Full text linkThere is growing interest for the development of light, small, high-performance spacecraft (S/C)
platforms for a wide range of missions. In the early stages of the small-satellite era, both nano-satellites
(<10 kg) and micro-satellites (>10 kg) were mainly intended for educational and technology
demonstration goals [1]. Nowadays, they are a consolidate means for Earth observation, where they
dramatically reduce mission costs. We are now at a turning point, where nano- and micro-satellite
systems can accomplish interplanetary missions beyond the boundaries of low Earth orbit (LEO) [2,3].
However, in spite of the substantial increase in low-mass satellites launched since 2013, several statistics
show the low success rate of these commercial off-the-shelf (COTS)-based cost-driven systems. Only half
of nano-satellites succeeded in mission operations after a successful launch in the last 15 years [4].
The low success rate of nano-satellites is acceptable, up to a certain extent for educational or
technology demonstration missions. This may stem from the way in which university-led projects
design and carry out the S/C ground verification process, which lacks repeatability and rigor routinely
found in industry. However, even though a nano-satellite is relatively inexpensive, if the S/C is
launched for commercial or scientific exploration purposes, failure is not really an option.
We should thus ask ourselves what technical challenges and programmatic difficulties must be
faced in order to substantially increase the reliability of nano- and micro-satellite missions. The historical
causes of low-mass satellite failure can be traced back to (a) lack of system-level testing due to schedule
and budget constraints, (b) inadequate thermal design and verification, and (c) use of COTS electronics.
Clearly, rigorous ground verification approaches—tailoring the existing testing standards for traditional
large-/medium-class satellites—are needed to effectively face such challenges.
It is therefore our pleasure to introduce this Aerospace MDPI Special Issue on Verification
Approaches for Nano- and Micro-Satellites, which aims at addressing, at least in part, the above concerns
On-ground experimental verification of magnetic attitude control for nanosatellites
No full textFor reliable verification of attitude determination and control systems, ground-based hardware-in-the-loop simulations are strongly desirable. To this end, a Three-Degrees of Freedom Dynamic Testbed for CubeSats has been developed at the University of Bologna. In this paper, the development of a platform for 1U CubeSats testing is described. Within the facility, disturbance-free environment is guaranteed and a magnetic field is generated by a Helmholtz cage to enable experimental verification of magnetic attitude control systems. A COTS CubeSat mockup is integrated and employed to test detumbling, spin-axis pointing and three-axis attitude control laws relying on magnetorquers only. The experimental results show the facility capabilities in highly demanding magnetic attitude control scenarios
A High Accuracy Horizon Sensor for Small Satellites
No full textThe conceptual design for a novel, high accuracy horizon-sensor is presented to be used on-board Low Earth Orbit small satellites. The concept consists of a multihead infrared sensor capturing images of the Earth limb. By fitting an ellipse to the imaged limb arcs, and exploiting some analytical results available from projective geometry, a closed form solution for computing the attitude matrix is developed. To quantify the expected attitude estimation performance of the proposed method, a simulator is developed which generates Earth images as gathered by a low Earth orbit satellite including also the presence of the atmosphere. The algorithm is developed in a dimensionless framework, requiring the knowledge of the shape of the planetary target, but not of its size. As a result, the solution is less sensitive to the limb shift caused by the atmospheric own infrared radiance.Results show that our sensor concept returns rms errors of few hundredths of a degree or less in determining the local nadir direction. Furthermore, coarse information on the orientation about nadir is also obtained, with accuracy in the order of some degrees for most operating conditions
A Comparative Analysis of GPS-based Algorithms for Real-Time Navigation of Small Satellites
No full textAn analytical approach to autonomous optical navigation for a CubeSat mission to a binary asteroid system
No full textWe consider the autonomous optical navigation problem for a CubeSat mission to a binary asteroid system. We aimed at providing an approach which limits the computational burden, as to be suitable for real time on-board implementation. To this end, we formulate a theoretical framework which makes use of analytical results for the position determination from imaged ellipsoid targets. To assess the performance of the proposed approach, a simulation environment is developed, taking as a scenario the navigation to a triangular lagrangian point of Didymos system, and having as a target 10m rms position accuracy. Alternative formulations of the position fixing are evaluated and compared. Results show the effectiveness of the proposed approach to meet the target position determination accuracy
Retrodictor-corrector filter
Full text linkAbstract not present; this is a Technical Note, only requiring an Introduction sectio
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