Guidance, navigation and control of a very small solar sail

With the miniaturization of the electrical components and the advance in aerospace technology, new methods of spatial propulsion have been developed. This thesis project is focused on solar radiation pressure modelling, solar sail kinematics and dynamics and attitude determination and control. Solar sailing is based on the same principle as any conventional propulsion system, Newton's third law. However, solar sails gain momentum from Sunlight photons and thus can provide continuous acceleration. The momentum carried by photons is small and for that reason solar sails must have a large area and be extremely light. Since conventional actuators are not suitable for attitude control in solar sailing, specific methods based on Reflectivity Control Devices (RCDs) have been discussed

System modelling and control

Not Availabl

Event-Triggered Consensus and Formation Control in Multi-Agent Coordination

The focus of this thesis is to study distributed event-triggered control for multi-agent systems (MASs) facing constraints in practical applications. We consider several problems in the field, ranging from event-triggered consensus with information quantization, event-triggered edge agreement under synchronized/unsynchronized clocks, event-triggered leader-follower consensus with Euler-Lagrange agent dynamics and cooperative event-triggered rigid formation control. The first topic is named as event-triggered consensus with quantized relative state measurements. In this topic, we develop two event-triggered controllers with quantized relative state measurements to achieve consensus for an undirected network where each agent is modelled by single integrator dynamics. Both uniform and logarithmic quantizers are considered, which, together with two different controllers, yield four cases of study in this topic. The quantized information is used to update the control input as well as to determine the next trigger event. We show that approximate consensus can be achieved by the proposed algorithms and Zeno behaviour can be completely excluded if constant offsets with some computable lower bounds are added to the trigger conditions. The second topic considers event-triggered edge agreement problems. Two cases, namely the synchronized clock case and the unsynchronized clock case, are studied. In the synchronized clock case, all agents are activated simultaneously to measure the relative state information over edge links under a global clock. Edge events are defined and their occurrences trigger the update of control inputs for the two agents sharing the link. We show that average consensus can be achieved with our proposed algorithm. In the unsynchronized clock case, each agent executes control algorithms under its own clock which is not synchronized with other agents' clocks. An edge event only triggers control input update for an individual agent. It is shown that all agents will reach consensus in a totally asynchronous manner. In the third topic, we propose three different distributed event-triggered control algorithms to achieve leader-follower consensus for a network of Euler-Lagrange agents. We firstly propose two model-independent algorithms for a subclass of Euler-Lagrange agents without the vector of gravitational potential forces. A variable-gain algorithm is employed when the sensing graph is undirected; algorithm parameters are selected in a fully distributed manner with much greater flexibility compared to all previous work concerning event-triggered consensus problems. When the sensing graph is directed, a constant-gain algorithm is employed. The control gains must be centrally designed to exceed several lower bounding inequalities which require limited knowledge of bounds on the matrices describing the agent dynamics, bounds on network topology information and bounds on the initial conditions. When the Euler-Lagrange agents have dynamics which include the vector of gravitational potential forces, an adaptive algorithm is proposed. This requires more information about the agent dynamics but allows for the estimation of uncertain agent parameters. The last topic discusses cooperative stabilization control of rigid formations via an event-triggered approach. We first design a centralized event-triggered formation control system, in which a central event controller determines the next triggering time and broadcasts the event signal to all the agents for control input update. We then build on this approach to propose a distributed event control strategy, in which each agent can use its local event trigger and local information to update the control input at its own event time. For both cases, the trigger condition, event function and trigger behaviour are discussed in detail, and the exponential convergence of the formation system is guaranteed

A study of small scale features in the atmosphere of Saturn

Imperial Users onl

Earth imaging with microsatellites: An investigation, design, implementation and in-orbit demonstration of electronic imaging systems for earth observation on-board low-cost microsatellites.

This research programme has studied the possibilities and difficulties of using 50 kg microsatellites to perform remote imaging of the Earth. The design constraints of these missions are quite different to those encountered in larger, conventional spacecraft. While the main attractions of microsatellites are low cost and fast response times, they present the following key limitations: Payload mass under 5 kg, Continuous payload power under 5 Watts, peak power up to 15 Watts, Narrow communications bandwidths (9.6 / 38.4 kbps), Attitude control to within 5°, No moving mechanics. The most significant factor is the limited attitude stability. Without sub-degree attitude control, conventional scanning imaging systems cannot preserve scene geometry, and are therefore poorly suited to current microsatellite capabilities. The foremost conclusion of this thesis is that electronic cameras, which capture entire scenes in a single operation, must be used to overcome the effects of the satellite's motion. The potential applications of electronic cameras, including microsatellite remote sensing, have erupted with the recent availability of high sensitivity field-array CCD (charge-coupled device) image sensors. The research programme has established suitable techniques and architectures necessary for CCD sensors, cameras and entire imaging systems to fulfil scientific/commercial remote sensing despite the difficult conditions on microsatellites. The author has refined these theories by designing, building and exploiting in-orbit five generations of electronic cameras. The major objective of meteorological scale imaging was conclusively demonstrated by the Earth imaging camera flown on the UoSAT-5 spacecraft in 1991. Improved cameras have since been carried by the KITSAT-1 (1992) and PoSAT-1 (1993) microsatellites. PoSAT-1 also flies a medium resolution camera (200 metres) which (despite complete success) has highlighted certain limitations of microsatellites for high resolution remote sensing. A reworked, and extensively modularised, design has been developed for the four camera systems deployed on the FASat-Alfa mission (1995). Based on the success of these missions, this thesis presents many recommendations for the design of microsatellite imaging systems. The novelty of this research programme has been the principle of designing practical camera systems to fit on an existing, highly restrictive, satellite platform, rather than conceiving a fictitious small satellite to support a high performance scanning imager. This pragmatic approach has resulted in the first incontestable demonstrations of the feasibility of remote sensing of the Earth from inexpensive microsatellites

Systems design study of the Pioneer Venus spacecraft. Volume 1. Technical analyses and tradeoffs, section 7 (part 3 of 4)

The aerodynamic design problems for the Pioneer Venus mission are discussed for a small probe shape that enters the atmosphere, and exhibits good stability for the subsonic portion of the flight. The problems discussed include: heat shield, structures and mechanisms, thermal control, decelerator, probe communication, data handling and command, and electric power

Exploring the surface of Titan

The exploration of Saturn's giant satellite Titan is considered, with particular reference to its surface which is hidden beneath a thick atmosphere. Groundbased observations, in which great progress has been made recently, and the measurements made by the Pioneer and Voyager spacecraft, are reviewed. Concepts for spacecraft to perform in-situ measurements on Titan are reviewed, as is the development of the NASA/ESA Cassini mission, how the mission constrains scientific investigations, and in turn how the mission has been constrained by funding pressures. The capabilities of the Cassini payload for investigating Titan's surface are critically assessed, and the ability of the Surface Science Package (SSP) on the Huygens probe to determine the composition of surface liquids is examined. Some thoughts on payload selection and the value of individual measurements are presented. The development of an impact penetrometer, and the interpretation of penetrometer and accelerometer data to measure surface mechanical properties, is described. It should be noted that Huygens is not a vehicle expressly designed as a lander, so the impact dynamics are complex. Additionally, the examination of the prospects offered by acoustic instrumentation are investigated. Modelling of a number of Titan surface processes is presented, including rainfall, photochemical and meteoric deposition, tidal dissipation in the interior, regolith processes such as volatile heat transport, annealing and aeolian transportation and the effects of tidal and crustal processes on lakes. A key subtopic of the thesis addresses the theme of planetary exploration as a whole, with the interaction between and the limitations of the exploration 'triad' of observations, insitu measurements and theory. Note is made of the remarkably significant role played by individuals and their perceptions