Luca Massotti

Drag-Free Control of the GOCE Satellite: Noise and Observer Design

This brief concerns model and noise design relevant to drag-free and attitude control of the European satellite gravity field and steady-state ocean circulation explorer (GOCE) under different control modes. As the model must include accurate dynamics of disturbance and measurement drifts to be rejected/estimated, noise design aims to select, and to mark with the Boolean variables of the control modes, the necessary and sufficient feedback channels (noise estimator) connecting model error to noise, which are the paths through which model state variables can be updated in real-time. Noise design is applied to a generic model encompassing position and attitude control, fed by position, rate, attitude, and acceleration sensors. The resulting closed-loop becomes a state predictor, providing controllable and disturbance states to drag-free and attitude control law, switching smoothly from mode to mode. Noise estimator gains are tuned to robust performance and stability by properly assigning closed-loop eigenvalues. Tuning details and simulated results illustrating the different modes are provided.

A SIMULATION TOOL FOR ON-LINE REAL TIME PARAMETER IDENTIFICATION

This paper describes a simulation tool developed at West Virginia University (WVU) for online aircraft parameter identification (PID) within a specific fault tolerant flight control scheme for the NASA IFCS F-15 program. The simulation package developed by WVU researchers is modular and flexible so that different methods and/or approaches can be used for each of the tasks of the general fault tolerant control system, such as aircraft model, controller, parameter identification method, and on-line data storage. Numerous simulation options are directly available to the user through specific graphical user interface. These options allow to select among different control loop configurations, different versions of the parameter identification method, and different failure scenarios.

A Spaceborne Gravity Gradiometer Concept Based on Cold Atom Interferometers for Measuring Earth’s Gravity Field

We propose a concept for future space gravity missions using cold atom interferometers for measuring the diagonal elements of the gravity gradient tensor and the spacecraft angular velocity. The aim is to achieve better performance than previous space gravity missions due to a very low white noise spectral behavior and a very high common mode rejection, with the ultimate goals of determining the fine structures of the gravity field with higher accuracy than GOCE and detecting time-variable signals in the gravity field better than GRACE.

Modeling and Simulation of Failures for Primary Control Surfaces

This paper describes the results of the extension of an existing failure modeling approach for the longitudinal aircraft dynamics to the lateraldirectional dynamics. In particular, a general formulation is developed for the most common failure scenarios, that is actuator blockage with and without a missing portion of the control surface. The approach consists of modeling the contribution of each individual control device within the expressions for the total external forces and moments using a single “efficiency” parameter that is easily accessible during simulation. Although the methodology can be applied to any force and moment generating control surface the paper describes simulation results relative to failures for elevators/stabilators, ailerons, and rudder occurring separately and simultaneously.

Online parameter estimation issues for the NASA IFCS F-15 fault tolerant systems

This paper focuses on specific issues relative to real-time online estimation of aircraft aerodynamic parameters at nominal and post-actuator failure flight conditions. A specific parameter identification (PID) method, based on Fourier transform, has been applied to an approximated mathematical model of the NASA IFCS F-15 aircraft. In this effort different options relative to the application of this PID method are evaluated and compared. Particularly, the direct evaluation of stability and control derivatives versus the estimation of the coefficients of the state space system matrices evaluation is considered. Furthermore, the options of considering individual control surfaces (left and right) versus total surfaces as inputs to the PID process are discussed. Finally, since the PID method relies on the use of derivative terms, the option of using time domain derivatives versus frequency domain derivatives is also evaluated. Results are presented in terms of the accuracy and reliability of the estimates of selected stability and control derivatives.

Flight Simulator for the Control Law Design of an Innovative Remotely-Piloted Airship

This paper is focused on the development of a refined Flight Simulator for the innovative Lighter-Than-Air vehicle patented by Nautilus S.r.l. Emphasis is placed on the airship modeling and simulator features, especially concerning the different control strategies applied in hovering and forward flight, with and without wind. The availability of a complete and flexible simulation tool is necessary for the design, test and implementation of the most suitable flight control laws for this non conventional remotely-piloted airship. Furthermore, since the Nautilus airship features an absolutely unique command strategy, the NPU Simulator is expected to be extensively used as a pilot trainer.

Automation and Control of Fabry–Pérot Interferometers

Fabry-Peacuterot interferometry (FPI), which was originally invented for spectroscopy, is now evolving as a basic technology for ultrafine dimensional stabilization and measurement. To this end, the light path length of an optical cavity and the wavelength of a laser source injected into the cavity have to be tuned to each other through a set of frequency and/or displacement actuators driven by a sharp and narrow signal-encoding total-cavity detuning. Digital control is essential in facilitating and automating FPI use in view of space applications and routine instrumentation. This paper shows how embedded model control (EMC) technology, which was developed by one of the authors, allows to systematically proceed from fine dynamics and requirements to the EM, which is the core of control design and algorithms. In this framework, all critical control issues have a coordinated solution: disturbance estimation and rejection, command constraints and multiplicity, hybrid dynamics, constraints due to unmodeled dynamics, and performance analysis. Several experimental results are illustrated and discussed in the light of the methodology

Augmentation of a non linear dynamic inversion scheme within the NASA IFCS F-15 WVU simulator

This paper describes the results of a study focused on enhancing the performance of a non linear dynamic inversion scheme augmented with a neural network to cancell the dynamic inversion error. The approach is based on adding a pre-trained neural network providing the values of the aerodynamic stability and control derivatives required by the dynamic inversion calculations, as the aircraft moves throughout its flight envelope. Additionally, a comparison is performed using two different classes of neural networks (Sigma-Pi and EMRAN algorithms) for the cancellation of the dynamic inversion errors. The study is performed using the WVU IFCS F-15 simulation environment. The results show that the updating of the aerodynamic derivatives reduces the error compensating activity of the neural network. Performance improvements in terms of tracking error are observed for some maneuvers; however, a significant sensitivity to the update rate has been noticed.

Orbit and formation control for low-earth-orbit gravimetry drag-free satellites

The paper outlines orbit and formation control of a long-distance (>100u2009km) two-satellite formation for the monitoring of the Earth gravity. Orbit control applies to a single satellite and performs altitude control. Here, formation control is formulated as a control capable of altitude and distance control at the same time. The satellites being placed in a low Earth orbit, orbit, and formation control employ the measurements of a global navigation system. Formation control is imposed by long-distance laser interferometry, which is the key instrument for gravity measurement. Orbit and formation control are low-frequency control systems in charge of canceling bias and drift of the residual drag-free accelerations. Drag-free control is the core of the orbit/formation control since it allows the formation to fly drag-free only subject to gravity. Drag-free control being required to have a bandwidth close to 1u2009Hz, is designed as the inner loop of the formation control. In turn, formation control must not destroy drag-free performance, in which objective demands that formation control be effective only below the 0.2 mHz orbital frequency. Control design is based on a new orbit and formation dynamics, which are compared with the classical Hill–Clohessy–Wiltshire equations. The new dynamic equations are the first step in building the embedded model, which is the core of the control unit. Embedded model derivation is explained only for the orbit control, and briefly mentioned for the formation control. Simulated results are provided. Drag-free results are compared with GOCE experimental data.

The Future of the Satellite Gravimetry After the GOCE Mission

Launched on March 17th 2009 from the Plesetsk Cosmodrome (Northern Russia), GOCE maps the Earth’s gravity field with unprecedented accuracy and resolution and will be of benefit for many branches of Earth science. This paper gives an overview of the European Space Agency’s (ESA) recent technical developments and activities going beyond the GOCE mission and its technology. It describes the outcome of the recent Laser SST concept studies, the Laser metrology concept and the objectives of the ongoing parallel Next Generation Gravimetry Mission studies, together with an overview on the latest technology development studies on atomic clocks and atom interferometry for possible future gravity sensing.