Walton R. Williamson

Sensor Fusion Applied to Autonomous Aerial Refueling

DOI: 10.2514/1.34589 A methodology for performing sensor fusion using differential Global Positioning System receivers, inertial measurements, wireless communication, and electro-optical measurements is presented. An estimation structure basedonanextendedKalman filterisdevelopedtoestimate thepreciserelativeposition,velocity,andattitudeoftwo aircraft flying incloseproximity. Theprojectisfocusedonthedevelopmentofthismethodologyforautomaticaerial refueling in which a boom control system uses the output of the sensor fusion to guide the boom into the receiver receptacle. Results from a real-time, hardware-in-the-loop demonstration are presented which show estimator performance and feedback control capability on a scaled model.

An Instrumentation System Applied to Formation Flight

As part of a NASA dryden autonomous formation flight program for improved drag reduction of multiple F/A-18 aircraft, a new instrument, the formation flight instrumentation system (FFIS), for the precise estimation of the relative position, velocity, and attitude between two moving aircraft without the aid of ground-based instruments, was developed. The FFIS uses a global position system (GPS) receiver and an inertial navigation sensor (INS) instrumentation package on each aircraft combined with a wireless communication system for sharing measurements between vehicles. An extended Kalman filter structure blends the outputs of each GPS/INS in a distributed manner so as to maximize the accuracy of the relative state estimates. Differential carrier phase GPS measurements are used to provide high accuracy relative range measurements to the filtering algorithm. A multiple hypothesis Wald test for estimating the integer ambiguity between the two moving vehicles was developed as part of this project. The FFIS was tested in a hardware-in-the-loop simulation (HIL Sim) before being tested in actual F-18 flight tests. Test results validated the FFIS performance. Flight test results showed that the Wald test accurately estimates the integer ambiguity and that relative range estimates using least squares provide accurate position estimates with a mean of approximately 7 cm and a standard deviation of 13 cm

Fault Detection and Isolation for Deep Space Satellites

A method for detecting faults in the navigation and control system of deep space satellites is presented. A new method for computing the probability of a fault given multiple different types of residuals processors is presented. The method uses the Shiryayev sequential probability ratio test to estimate the probability of the presence of a fault signal given the residuals generated from either parity relationships or fault detection filters, a fault map of the impact of each fault signal on the residuals, and an adaptive fault estimation scheme that enables processing with fewer residuals. This new methodology is applied to the detection of the fault signals in the attitude control system and navigation system of deep space satellites. First a sensor fusion process is presented for blending star tracker data, gyro data, accelerometer data, and information from the vehicle control system to form the best estimate of the navigation state. Then a set of fault detection filters are developed that detect and uniquely identify faults in each of the sensors or actuators. Decision-making is handled through the sequential processing. Simulation results for a single-satellite system are presented.

Controllable Drogue for Automated Aerial Refueling

This paper presents a new method for controlling and stabilizing an automated refueling drogue. The drogue automatically stabilizes in the presence of winds or receiver forebody effects using drogue canopy manipulation. This paper reports on a characterization and simulation study in which the stability derivatives of a low-speed drogue were calculated based on wind-tunnel data. A simulation was developed using a pendulum-based hose model combined with the aerodynamic drogue model. An active feedback control system is shown to stabilize and control the drogue to hold a commanded steady-state offset.

Optimization and Implementation of Periodic Cruise for a Hypersonic Vehicle

The optimal cruise trajectory for a detail designed hypersonic waverider vehicle is determined. Two possible local extrema are found representing the steady state and periodic cruise trajectories. The optimal steady state cruise is determined by minimizing the instantaneous fuel rate per distance subject to the dynamics being in equilibrium. The approximately optimal periodic cruise is determined by minimizing the fuel used over an optimal range period subject to a periodicity condition on the initial and terminal values of the altitude, velocity, and ∞ight path angle, assuming that the vehicle weight is given and held flxed over the range period. It is shown that if a door is placed over the inlet during the power-ofi phase increasing the drag by 50%, but increasing the lift by 35%, the periodic ∞ight over the cruise region is 13.27% better than ∞ying in steady state. Results for mechanizing the periodic ∞ight by a linear guidance rule allow the constant vehicle weight assumption to be removed, but retain the periodic cruise performance.

Fault Detection for Deep Space Satellites

A method for detecting failures in the navigation and control system of deep space satellites is presented. First a sensor fusion process for blending star tracker data, gyro data, accelerometer data, and information from the vehicle control system to form the best estimate of the navigation state is presented. Then a set of fault detection filters are developed that detect and uniquely identify failures in each of the estimation processes. Decision making is handled through the sequential processing. Simulation results for a single satellite system are presented.