Samer Khanafseh

Autonomous airborne refueling of unmanned air vehicles using the global positioning system

Autonomous airborne refueling requires that the position of the receiving aircraft relative to the tanker be known very accurately and in real time. To meet this need, highly precise carrier-phase differential global positioning system solutions are being considered as the basis for navigation. However, the tanker introduces severe sky blockage into the autonomous airborne refueling mission, which reduces the number of visible global positioning system satellites and hence degrades the positioning accuracy. In this paper, we analyze the autonomous airborne refueling navigation problem in detail, define an optimal navigation architecture, and quantify navigation system availability. As part of this work, a high-fidelity dynamic sky-blockage model is developed and used to plan autonomous airborne refueling flight tests. The flight tests were conducted to obtain time-tagged global positioning system and attitude data that were processed offline to validate the blockage model.

Navigation, Interference Suppression, and Fault Monitoring in the Sea-Based Joint Precision Approach and Landing System

The United States Navy seeks the capability to land manned and unmanned aerial vehicles autonomously on an aircraft carrier using GPS. To deliver this capability, the Navy is developing a navigation system called the Sea-Based Joint Precision Approach and Landing System (JPALS). Because standard GPS is not sufficiently precise to land aircraft on a shortened, constantly moving runway, Sea-Based JPALS leverages dual-frequency, carrier-phase differential GPS navigation. Carrier phase measurements, derived from the sinusoidal waveforms underlying the GPS signal, are very precise but not necessarily accurate unless the user resolves the ambiguity associated with the sinusoids periodicity. Ensuring the validity of ambiguity resolution is the central challenge for the high-integrity, safety-critical JPALS application. Based on a multi-year, multi-institution collaborative study, this paper proposes a navigation and monitoring architecture designed to meet the guidance quality challenge posed by Sea-Based JPALS. In particular, we propose a two-stage navigation algorithm that meets the aggressive integrity-risk requirement for Sea-Based JPALS by first filtering a combination of GPS observables and subsequently exploiting those observables to resolve the carrier ambiguity. Because JPALS-equipped aircraft may encounter jamming, we also discuss interference mitigation technologies, such as inertial fusion and array antennas, which, with appropriate algorithmic modifications, can ensure integrity under Radio Frequency Interference (RFI) conditions. Lastly, we recommend a fault monitoring strategy tailored to the two-stage navigation algorithm. Monitoring will detect and isolate rare anomalies such as ionosphere storms or satellite ephemeris errors which would otherwise corrupt ambiguity resolution and positioning in Sea-Based JPALS.

GPS spoofing detection using RAIM with INS coupling

In this work, we develop, implement, and test a monitor to detect GPS spoofing attacks using residual-based Receiver Autonomous Integrity Monitoring (RAIM) with inertial navigation sensors. Signal spoofing is a critical threat to all navigation applications that utilize GNSS, and is especially hazardous in aviation applications. This work develops a new method to directly detect spoofing using a GPS/INS integrated navigation system that incorporates fault detection concepts based on RAIM. The method is also capable of providing an upper bound on the proposed monitors integrity risk.

A new approach for calculating position domain integrity risk for cycle resolution in carrier phase navigation systems

A new theoretical approach is described to quantify position-domain integrity risk for cycle ambiguity resolution problems in satellite-based navigation systems. It is typically conservatively assumed that all incorrectly fixed cycle ambiguities cause hazardously large position errors. While simple and practical, this conservative assumption can unnecessarily limit navigation availability for applications with stringent requirements for accuracy and integrity. In response, a new method for calculating the integrity risk for carrier phase navigation algorithms is developed. In this method we evaluate the impact of incorrect fixes in the position domain and define tight upper bounds on the resulting navigation integrity risk. Furthermore, a mechanism to implement this method with partially-fixed cycle ambiguity vectors is also derived. The improvement in navigation availability using the new method is quantified through covariance analysis performed over a range of error model parameters.

Detection and Mitigation of Reference Receiver Faults in Differential Carrier Phase Navigation Systems

In this paper, a methodology is developed to evaluate differential carrier phase navigation architectures subject to reference receiver faults. Carrier phase measurements can be used to provide high accuracy estimates of a users position. But in applications that involve safety-of-life, such as in precision approach for autonomous shipboard landing, integrity also plays a critical role. One source of integrity risk is the potential for GPS reference receiver failure. Integrity risk in these situations is typically mitigated by equipping the reference station with redundant receivers. However, various approaches to utilize redundant carrier phase measurements from multiple reference receivers are possible. In this paper, we describe two new methods: an averaging approach where different position solutions are averaged in the position domain, and a coupled estimation approach where the measurements from all reference receivers are coupled in the range domain and used to estimate a unified position solution. Furthermore, we investigate the impact of using these methods on accuracy and integrity from several perspectives, including availability performance, cycle resolution capabilities, implementation complexity, and computational efficiency.

Simulation of Powered Resonance Tubes: Effects of Pressure Ratio and Freestream Flow ©

Flow simulations have been performed as part of our effort to better understand powered resonance tube behavior. Scaled simulations of the powered resonance tube have produced reasonable correspondence to laboratory experiments, in terms of the frequency and amplitude of the resonant response. The simulations suggest new insights into the complexity and details of the flowfield. The simulations show that the flow in the integration slot is primarily on the resonance tube side, with almost no flow on the supply tube side of the integration slot. The numerical results suggest that the acoustic waves from the resonance in the resonance tube drive an unsteady separation at the supply tube. The unsteady separation at the supply tube in turn drives the observed large oscillations in the shock structure. The unsteady separation seems to be a key aspect of the resonance phenomena. Very recently it has been discovered that for shallow resonance tubes, the pressure ratio affects the response frequency. Also, the resonance tube is found to impose strong pressure disturbances in a Mach 0.5 boundary layer flow. The presence of the Mach 0.5 external stream and boundary layer reduces the resonance frequency by about 12%, relative to the case without an external stream.

Synthesis of empirical and theoretical approaches toward the establishment of GBAS σpr_gnd

The objective of this paper is to describe recent progress in the area of the Ground Based Augmentation System (GBAS) σ pr_gnd establishment to ensure aircraft precision approach navigation integrity. In particular, this paper details: (1) the development and testing of a new adaptive binning algorithm for processing GBAS reference station empirical data to account for ranging error non-stationarity; (2) an introduction of a practical empirical method to quantify and compensate for the effects of seasonal variations of ranging error; and (3) the synthesis of these empirical results with existing theoretical ground-multipath models toward a quantitative establishment σ pr_gnd for GBAS.

High Bandwidth Powered Resonance Tube Actuators with Feedback Control

High bandwidth actuators are required if Active Flow Control (AFC) is to be used at various locations on an aircraft and over a range of flight speeds. The actuator selected for bandwidth enhancement was the Powered Resonance Tube (PRT) that is an adaptation of the Hartmann Whistle. The actuator consists of a high speed jet aimed at the open end of a cylindrical resonance tube closed at the other end by an air tight piston. The depth of the resonance tube determines the hquency of the signal produced by the actuator. The high bandwidth capability of this device was previously demonstrated by Raman et al. (2002) by using a computer controlled open loop Look Up Table @UT) approach to change the depth of the resonance tube. The LUT approach was prone to errors under certain conditions. The present work overcomes difficulties associated with the LUT approach by using closed loop control using both Single Input Single Output (SISO) and Multi Input Multi Output (MIMO) control. The parameter changed in the SISO approach was the depth of the resonance tube, whereas the MlMO approach changed both the depth and the spacing between the supply jet and the resonance tube. The “spacing’ served as a tuning parameter that ensured that the resonance tube actuator produced high levels of pressure perturbation at all fkequencies. For both methods, using the LUT result as an initial estimator produced better results that using the Q y t e r Wavelength Theory (QW).

Overbounding position errors in the presence of carrier phase multipath error model uncertainty

In this paper, a methodology is developed to overbound the error in position estimation for carrier phase navigation systems subject to multipath error mis-modeling. In high accuracy and high integrity applications, carrier phase positioning systems with cycle resolution usually rely on time-filtering, in many cases using Kalman filters. Naturally, for a computed integrity risk to be meaningful, it must be guaranteed that the predicted state estimation error covariance always bounds the true covariance. The Kalman filter ensures state error covariance bounding provided that the measurement errors and system dynamics are accurately modeled. However, uncertainties in the dynamic model of the multipath error states always exist. In response, the focus of this work is to quantify and bound position estimate errors subject to uncertainties associated with the time correlation of multipath error.

Bounding Integrity Risk for Sequential State Estimators with Stochastic Modeling Uncertainty

A new method is introduced to upper bound integrity risk for sequential state estimators when the autocorrelation functions of measurement noise and disturbance inputs are subject to bounded uncertainties. Integrity risk is defined as the probability of the state estimate error exceeding predefined bounds of acceptability. In the first part of the paper, a new expression is derived that relates the measurement noise and disturbance input autocorrelation functions to the state estimate error vector. Using this relation, an efficient algorithm is developed in the second part of the paper to upper bound the estimation integrity risk when each input autocorrelation function is known to lie between upper and lower bounding functions. Numerical simulations for a one-dimensional position and velocity estimation problem are conducted to demonstrate the practical feasibility and effectiveness of this new bounding method.