Matteo Ceriotti

Not all wireless sensor networks are created equal: A comparative study on tunnels

Wireless sensor networks (WSNs) are envisioned for a number of application scenarios. Nevertheless, the few in-the-field experiences typically focus on the features of a specific system, and rarely report about the characteristics of the target environment, especially with respect to the behavior and performance of low-power wireless communication. The TRITon project, funded by our local administration, aims to improve safety and reduce maintenance costs of road tunnels, using a WSN-based control infrastructure. The access to real tunnels within TRITon gives us the opportunity to experimentally assess the peculiarities of this environment, hitherto not investigated in the WSN field. We report about three deployments: (i) an operational road tunnel, enabling us to assess the impact of vehicular traffic; (ii) a nonoperational tunnel, providing insights into analogous scenarios (e.g., underground mines) without vehicles; (iii) a vineyard, serving as a baseline representative of the existing literature. Our setup, replicated in each deployment, uses mainstream WSN hardware, and popular MAC and routing protocols. We analyze and compare the deployments with respect to reliability, stability, and asymmetry of links, the accuracy of link quality estimators, and the impact of these aspects on MAC and routing layers. Our analysis shows that a number of criteria commonly used in the design of WSN protocols do not hold in tunnels. Therefore, our results are useful for designing networking solutions operating efficiently in similar environments.

Generation of Optimal Trajectories for Earth Hybrid Pole Sitters

A pole-sitter orbit is a closed path that is constantly above one of the Earths poles, by means of continuous low thrust. This work proposes to hybridize solar sail propulsion and solar electric propulsion (SEP) on the same spacecraft, to enable such a pole-sitter orbit. Locally-optimal control laws are found with a semi-analytical inverse method, starting from a trajectory that satisfies the pole-sitter condition in the Sun-Earth circular restricted three-body problem. These solutions are subsequently used as first guess to find optimal orbits, using a direct method based on pseudospectral transcription. The orbital dynamics of both the pure SEP case and the hybrid case are investigated and compared. It is found that the hybrid spacecraft allows savings on propellant mass fraction. Finally, it is shown that for sufficiently long missions, a hybrid pole-sitter, based on mid-term technology, enables a consistent reduction in the launch mass for a given payload, with respect to a pure SEP spacecraft.

Displaced geostationary orbit design using hybrid sail propulsion

Due to an increase in number of geostationary spacecraft and limits imposed by east-west spacing requirements, the geostationary orbit is becoming congested. To increase its capacity, this paper proposes to create new geostationary slots by displacing the geostationary orbit either out of or in the equatorial plane by means of hybrid solar sail and solar electric propulsion. To minimize propellant consumption, optimal steering laws for the solar sail and solar electric propulsion thrust vectors are derived and the performance in terms of mission lifetime is assessed. For comparison, similar analyses are performed for conventional propulsion, including impulsive and pure solar electric propulsion. It is shown that hybrid sails outperform these propulsion techniques and that out-of-plane displacements outperform in-plane displacements. The out-of-plane case is therefore further investigated in a spacecraft mass budget to determine the payload mass capacity. Finally, two transfers that enable a further improvement of the performance of hybrid sails for the out-of-plane case are optimized using a direct pseudo-spectral method: a seasonally transit between orbits displaced above and below the equatorial plane and a transit to a parking orbit when geostationary coverage is not needed. Both transfers are shown to require only a modest propellant budget, outweighing the improvements they can establish.

Trajectory and spacecraft design for a pole-sitter mission

This paper provides a detailed mission analysis and systems design of a pole-sitter mission. It considers a spacecraft that is continuously above either the North or South Pole and, as such, can provide real-time, continuous, and hemispherical coverage of the polar regions. Two different propulsion strategies are proposed, which result in a near-term pole-sitter mission using solar-electric propulsion and a far-term pole-sitter mission, in which the electric thruster is hybridized with a solar sail. For both propulsion strategies, minimum propellant pole-sitter orbits are designed. Optimal transfers from Earth to the pole sitter are designed, assuming Soyuz and Ariane 5 launch options, and a controller is shown to be able to maintain the trajectory under unexpected conditions, such as injection errors. A detailed mass budget analysis allows for a tradeoff between mission lifetime and payload mass capacity, and candidate payloads for a range of applications are investigated. This results in a payload of ab...

Automated Multigravity Assist Trajectory Planning with a Modified Ant Colony Algorithm

The paper presents an approach to transcribe a multigravity assist trajectory design problem into an integrated planning and scheduling problem. A modified Ant Colony Optimization (ACO) algorithm is then used to generate optimal plans corresponding to optimal sequences of gravity assists and deep space manoeuvers to reach a given destination. The modified Ant Colony Algorithm is based on a hybridization between standard ACO paradigms and a tabu-based heuristic. The scheduling algorithm is integrated into the trajectory model to provide a fast time-allocation of the events along the trajectory. The approach demonstrated to be very effective on a number of real trajectory design problems.

Solar-Sail Trajectory Design for a Multiple Near-Earth-Asteroid Rendezvous Mission

The scientific interest for near-Earth asteroids as well as the interest in potentially hazardous asteroids from the perspective of planetary defense led the space community to focus on near-Earth asteroid mission studies. A multiple near-Earth asteroid rendezvous mission with close-up observations of several objects can help to improve the characterization of these asteroids. This work explores the design of a solar-sail spacecraft for such a mission, focusing on the search of possible sequences of encounters and the trajectory optimization. This is done in two sequential steps: a sequence search by means of a simplified trajectory model and a set of heuristic rules based on astrodynamics, and a subsequent optimization phase. A shape-based approach for solar sailing has been developed and is used for the first phase. The effectiveness of the proposed approach is demonstrated through a fully optimized multiple near-Earth asteroid rendezvous mission. The results show that it is possible to visit five near-...

An Earth Pole-Sitter Using Hybrid Propulsion

In this paper we investigate optimal pole-sitter orbits using hybrid solar sail and solar electric propulsion (SEP). A pole-sitter is a spacecraft that is constantly above one of the Earths poles, by means of a continuous thrust. Optimal orbits, that minimize propellant mass consumption, are found both through a shape-based approach, and solving an optimal control problem, using a direct method based on pseudo-spectral techniques. Both the pure SEP case and the hybrid case are investigated and compared. It is found that the hybrid spacecraft allows consistent savings on propellant mass fraction. Finally, is it shown that for sufficiently long missions (more than 8 years), a hybrid spacecraft, based on mid-term technology, enables a consistent reduction in the launch mass for a given payload, with respect to a pure SEP spacecraft.

Variable-geometry solar sailing: the possibilities of the quasi-rhombic pyramid

Variable geometry solar sailing potentially offers enhanced delta-V capabilities and new orbital solutions. We propose a device with such capabilities, based upon an adjustable quasi-rhombic pyramid sail geometry, and examine the benefits that can be derived from this additional flexibility. The enabling technology for this concept is the bevel crux drive, which can maintain tension in the solar sail across a wide range of apex angles. This paper explores the concept of such a device, discussing both the capabilities of the architecture and the possibilities opened up in terms of orbital and attitude dynamics.

Distributed reflectivity solar sails for extended mission applications

The dynamics of solar sails with a variable surface reflectivity distribution are investigated. When changing the reflectivity across the sail film, solar radiation pressure forces and torques can be controlled without changing the attitude of the spacecraft relative to the Sun or using attitude control actuators. The reflectivity can in principle be modified using electro-chromic coatings, which are applied here as examples to counteract gravity-gradient torques in Earth orbit and to enable specific shape profiles of a flexible sail film. This ‘optical reconfiguration’ method introduces an adaptive solar sail as a multi-functional platform for novel mission applications.

Optimal law for inclination change in an atmosphere through solar sailing

The aim of this paper is to devise a local optimal strategy for the orbital inclination change of solar sail spacecraft in low Earth orbit, combining the effects of the solar radiation pressure and atmospheric forces. The spacecraft is modeled as a reflective flat plate. The acceleration due to effects of atmospheric forces and solar radiation pressure is computed, depending on the orbital parameters and attitude of the sail. Then, the attitude that maximizes the instantaneous orbital inclination change is found through Gauss’ equations. When either one of these effects dominates over the other (and so, one can be neglected), the analytic expressions are found. When both effects are considered, a numerical optimization is used. An additional constraint is introduced to avoid a decrease in the orbital semimajor axis, and therefore prevent losses of orbital energy, while increasing the inclination. Different regions are identified, depending on whether the atmospheric effects dominate, the solar radiation p...