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Interference events, both intentional and unintentional, are significant threats to global navigation satellite system (GNSS) service continuity. In the presence of interference, it can be difficult for GNSS receivers to maintain continuous tracking of carrier phase and frequency. To address this issue, in place of a traditional phase-locked loop (PLL), we develop and validate a new estimation-based approach to allow GNSS receivers to correctly estimate carrier phase and frequency under conditions of low carrier-to-noise power. An adaptive Kalman filter forms the core of the estimator, with a multiple-model algorithm to account for discrete navigation data bit transitions. The performance of the estimator in the presence of wideband interference is validated through simulation and experiment, showing performance markedly superior to that of a PLL. [ABSTRACT FROM AUTHOR]

In this article, we describe a new method for detecting global navigation satellite system (GNSS) spoofing using an inertial navigation system. We specifically address the most difficult-to-detect scenario, in which a spoofer replicates the authentic GNSS signal with only additive errors due to the spoofer’s uncertainty in knowledge of the target’s position. We derive an optimal monitor to detect the anomalous temporal structure of the spoofed measurements caused by the spoofer’s target tracking errors. This new monitor uses accumulated Kalman filter innovations projected into the position state domain. We demonstrate how the monitor window length can be set to achieve any required missed detection probability, and we evaluate the performance of the monitor for both white and colored tracking error. Finally, we present a complementary solution separation monitoring concept to detect rapid-onset spoofing and to achieve protection levels in real time. [ABSTRACT FROM AUTHOR]

In this article, we develop a ground monitor capable of detecting anomalous signal-in-space spatial gradients for rising, newly acquired, and reacquired satellites in the ground-based augmentation system using either single or dual frequency. These gradients can be caused by satellite orbit ephemeris faults and ionospheric fronts. The monitor utilizes differential code and carrier phase measurements across multiple reference receiver antennas as the basis for detection. We show that the new monitor significantly improves performance over existing detection algorithms and is capable of meeting Category III precision approach and landing requirements. [ABSTRACT FROM AUTHOR]

In this paper, we describe and evaluate a new monitor that uses inertial navigation system (INS) measurements to detect spoofing attacks on global navigation satellite system (GNSS) receivers. Spoofing detection is accomplished by monitoring the Kalman filter innovations in a tightly coupled INS/GNSS mechanization. Monitor performance is evaluated against worst case spoofing attacks, including spoofers capable of tracking vehicle position. There are two main contributions of this paper. The first is a mathematical framework to quantify postmonitor spoofing integrity risk. The second is an analytical expression of the worst case sequence of spoofed GNSS signals. We then apply these to an example spoofing attack on a Boeing 747 on final approach. The results show that GNSS spoofing is easily detected, with high integrity, unless the spoofer's position-tracking devices have unrealistic, near-perfect accuracy, and no delays. [ABSTRACT FROM AUTHOR]

Differential carrier phase applications that utilize cycle resolution need the probability density function of the baseline estimate to quantify its region of concentration. For the integer bootstrap estimator, the density function has an analytical definition that enables probability calculations given perfect statistical knowledge of measurement and process noise. This paper derives a method to upper bound the tail probability of the integer bootstrapped GNSS baseline when the measurement and process noise correlation functions are unknown, but can be upper and lower bounded. The tail probability is shown to be a non-convex function of a vector of conditional variances, whose feasible region is a convex polytope. We show how to solve the non-convex optimization problem globally by discretizing the polytope into small hyper-rectangular elements, and demonstrate the method for a static baseline estimation problem. [ABSTRACT FROM AUTHOR]

In this paper we introduce a ground monitoring architecture to validate the Integrity Support Message (ISM) parameters to be used by aircraft for Advanced Receiver Autonomous Integrity Monitoring (ARAIM). This work focuses on two critical ISM parameters: Psat, which designates the prior probabilities of satellite faults, and bmax, which is a range domain bound on small faults that may occur at probabilities higher than Psat. We show that the choices of bmax and Psat are not independent. The paper first establishes the relationship between bmax, Psat, Time to Integrity Alert (TIA) and constellation service provider performance commitments. We then provide an example ground monitor design that detects inter-frequency bias faults and code-carrier divergence faults. We show that the performance of the monitor can be used to validate specific bmax and Psat values for ARAIM. [ABSTRACT FROM PUBLISHER]

ABSTRACT This paper describes a Ground Based Augmentation System (GBAS) ground-based monitor capable of instantly detecting anomalous ionospheric gradients. The monitor utilizes differential carrier phase measurements across multiple reference station baselines as the basis for detection. Performance analysis shows that the monitor is highly sensitive to the quality of the carrier phase measurements. Therefore, data collected from a GBAS prototype ground facility is used to quantify the measurement quality and validate the concept monitor. Copyright © 2012 Institute of Navigation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 user's 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The advent of multiple Global Navigation Satellite System (GNSS) constellations will result in a considerable increase in the number of satellites for positioning worldwide. This substantial improvement in measurement redundancy has the potential to radically advance receiver autonomous integrity monitoring (RAIM) performance. However, regardless of the number of satellites, the performance of existing RAIM methods is sensitive to the assumed prior probabilities of individual fault hypotheses. In this paper, a new method is developed using Bayes’ theorem to generate upper bounds on posterior probabilities of individual fault hypotheses given current user measurements. These bounds are used in a Bayesian fault-tolerant position estimator (FTE) that minimizes integrity risk. The detection test statistic is a measurement-based integrity risk bound, which is directly compared with a pre-specified risk requirement. The associated challenge of quantifying continuity risk is resolved using a bounding approach, which is also detailed in this work. The new Bayesian FTE method is shown to be more robust to uncertainty in prior probability of fault occurrence than existing RAIM methods. [ABSTRACT FROM PUBLISHER]