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We present a framework for GNSS spoofing detection combining an arbitrary number of metrics while guaranteeing a fixed maximum false alert probability. The detection test assumes a simple form that makes it suitable for real time applications. We define criteria for metrics to be used within this framework and demonstrate compatibility with a range of commonly used metrics. We achieve a more than 70% reduction in worst-case missed detection probability compared to conventional metric combination techniques. [ABSTRACT FROM AUTHOR]

Fault detection algorithms (such as advanced RAIM) developed for aviation can be applied in situations where the probabilities of fault are arbitrarily high. However, with large probabilities, the user receiver needs to protect against a large number of fault modes (which results from the combination of simultaneous independent faults). In baseline ARAIM algorithms, the user receiver must compute a fault-tolerant position solution for each fault mode (computed using the subset of the available measurements that would not be affected by the fault mode), or at least its error covariance. After showing how to use a solution separation algorithm without computing the fault-tolerant position solutions, we present a technique to obtain upper bounds on the subset error covariance for a given subset size. This upper bound does not require the computation of every subset error covariance and can be computed without additional matrix inversions. We evaluate the potential of this technique by applying it to ARAIM scenarios. [ABSTRACT FROM AUTHOR]

Due to the ever-growing threat of GPS spoofing, it has become necessary for the aviation sector to develop an effective means of detection. This article focuses on two complementary spoofing-detection techniques that are available on commercial GPS receivers and thus require no additional hardware to operate. The primary methodology for detection is using a combination of radio power monitoring metrics, levering both automatic gain control and C/N0 measurements, along with multiple correlations for signal distortion to provide a best practices spoofing-detection algorithm, which is able to distinguish between interference and spoofing. The article first assesses nominal statistics for both metrics compiled from more than 250 h of nominal data collected from multiple wide area augmentation system stations. These data are compared to previous collections to validate the thresholds and false alarms rates and establish a complete testing methodology. These tests and thresholds are then assessed with the Texas spoofing test battery series of GPS spoofing data sets to confirm detection capabilities. Finally, these tests and thresholds are applied to assess the GPS signal of six extended flights over the United States to assess the performance on an aircraft. [ABSTRACT FROM AUTHOR]

Two critical parameters for advanced receiver autonomous integrity monitoring are the probability of satellite fault and the probability of constellation fault. This paper provides specific definitions for each of these fault types. We describe how these faults are evaluated and how to estimate their probability occurrence. Providing a precise definition of what constitutes a fault is essential so that all observers are able to agree on whether or not one has occurred. [ABSTRACT FROM AUTHOR]

We present a method to determine Gaussian overbounding distributions used in integrity analysis. This method overcomes the limitations of previous techniques by combining them, through the determination of an intermediate symmetric and unimodal overbounding distribution. This method is the basis of a MATLAB toolset that computes strict Gaussian overbounding distributions for any sample distribution. We apply the method to the determination of the overbounding distribution of the GPS clock and ephemeris errors for the advanced receiver autonomous integrity monitoring. [ABSTRACT FROM AUTHOR]

We describe a simple method to determine a Global Navigation Satellite System (GNSS) position estimator for advanced receiver autonomous integrity monitoring (RAIM) that improves availability upon the commonly used least squares estimator. The idea consists in searching the estimator among all affine combinations of the all-in-view least squares estimator and a fault-tolerant estimator?the one corresponding to the most difficult fault to mitigate. Availability simulations show a significant improvement in vertical guidance coverage levels for a dual constellation scenario. [ABSTRACT FROM PUBLISHER]

Multipath and jamming interference affects the accuracy, availability, and continuity of global navigation satellite systems. The U.S. Global Positioning System and the Russian Global'naya Navigatsionnaya Sputnikovaya Sistema (GLONASS) are being joined by the European Galileo and the Chinese BeiDou. An increasing number of satellites in multiple constellations enable users to use high-mask-angle antennas (HMAAs) to mitigate interference signals coming from a low-elevation angle. This paper studies the optimal antenna mask angle that maximizes the suppression of interference but still maintains the performance of a single constellation with a low-mask-angle antenna. This paper first proves a novel lower bound on the expectation of dilution of precision (DOP) and derives closed-form formulas that relate the lower bound to the antenna mask angle and the number of satellites. Then, through extensive simulations, a variety of optimal mask angles are obtained with respect to different constellation settings, different DOP metrics, and different assumptions of range accuracy. The numerical results highly agree with our theory. Both of them show that two constellations can match the performance of one constellation with a 5 ^\circ–14 ^\circ higher mask, and three constellations can match the performance of one constellation with an 11 ^\circ–23 ^\circ higher mask, depending on the DOP metric and the range error model used. The numerical results also show that using HMAAs is more beneficial to users interested in positioning accuracy than to users interested in time transfer accuracy. [ABSTRACT FROM PUBLISHER]

GPS technology is essential for future intelligent air transportation systems such as the Next Generation Air Transportation System (NextGen) of the United States. However, observed deep and frequent amplitude fading of GPS signals due to ionospheric scintillation can be a major concern in expanding GPS-guided aviation to the equatorial area where strong scintillation is expected. Current civil GPS airborne avionics track signals at a single frequency (L1 frequency) alone because it was the only civil signal available in the frequency band for aviation applications. The first GPS Block IIF satellite was launched in May 2010. This next-generation satellite transmits a new civil signal at the L5 frequency, which can be used for air transportation. This paper investigates a possible improvement in the availability of GPS-based aircraft landing guidance down to 200 ft above the runway, which is also known as Localizer Performance with Vertical Guidance (LPV) 200, under strong ionospheric scintillation when dual-frequency signals are available. Based on the availability study, this paper proposes and justifies a GPS aviation receiver performance standard mandating fast reacquisition after a very brief signal outage due to scintillation. In order to support a temporary single-frequency operation under a single-frequency loss due to scintillation, a new vertical protection level (VPL) equation is proposed and justified. With this new performance requirement and new VPL equation in place, 99% availability of LPV-200 would be attainable, rather than 50% at the current standards, even under the severe scintillation scenarios considered in this paper. [ABSTRACT FROM PUBLISHER]

Knowledge of the Global Positioning System (GPS) signal-in-space (SIS) anomalies in history has a great importance for not only assessing the general performance of GPS SIS integrity but also validating the fundamental assumption of receiver autonomous integrity monitoring (RAIM): at most one satellite fault at a time. The main purpose of this paper is to screen out all potential SIS anomalies in the last decade by comparing broadcast ephemerides and clocks with precise ones. Validated broadcast navigation messages are generated from 397,044,414 navigation messages logged by on average 410 International GNSS Service (IGS) stations during the period 6/1/2000?8/31/2010. Both IGS and National Geospatial-Intelligence Agency (NGA) precise ephemerides/clocks are used as truth references. Finally, 1256 potential SIS anomalies are screened out. These anomalies show an improving SIS integrity performance in the last decade, from tens or hundreds of anomalies per year before 2003 to on average two anomalies per year after 2008. Moreover, the fundamental assumption of RAIM is valid because never have two SIS anomalies or more occurred simultaneously since 2004. [ABSTRACT FROM AUTHOR]

Satellite navigation has been used for aircraft navigation for more than 50 years. In the last ten years, the capabilities of satellite navigation have been expanded to more demanding phases of flight, in particular vertical guidance down to 200 ft, thanks to the implementation of augmentation systems. In this paper, we attempt to predict the state of satellite navigation in the next 15 years. We will start by reviewing the challenges that must be addressed by satellite navigation for aircraft guidance. Then, we will describe the current techniques that enable satellite navigation for aviation and the level of performance they achieve today. This will be followed by a description of the upcoming changes to satellite navigation, which include the launch of new constellations and the introduction of new civil signals. Despite these developments, satellite navigation is inherently vulnerable to radio-frequency interference so that backup navigation systems are still necessary. Nonetheless, these improvements will have a great impact on the availability and level of service achieved by satellite navigation, in particular enabling worldwide coverage of vertical guidance. [ABSTRACT FROM PUBLISHER]

A unaugmented Global Positioning System (GPS)-based flight inspection system (FIS) that performs airborne inspection of the instrument landing system (ILS) is introduced. This novel system relies on a TV positioning system (TVPS) that measures the horizontal position over the runway threshold and a radar altimeter (RA) to determine a reference point that is able to remove the GPS biases. Because of the near-real-time nature of flight inspection, it is possible to reconstruct an accurate approach trajectory even though the reference point occurs after the completion of the trajectory. The precise relative positioning (PRP) algorithm is applied to achieve the desired results. It includes a first-order fit to the ionospheric error, based on code and carrier measurements, that enables a significant reduction in this error source. This system has an autonomous integrity feature, called FIS-RAIM (receiver autonomous integrity monitoring), that is equipped in this system to protect against GPS satellite failures that may cause significant positioning errors. The FIS-RAIM is specifically designed for a flight inspection problem, and much of its detail is also described. Experimental flight tests strongly suggest that the system architecture and algorithm could meet FIS accuracy requirements for CAT III ILS with better performance than current flight inspection systems in terms of cost and efficiency. The system is not dependent on any GPS augmentation system; therefore the unaugmented GPS-based FIS provides low cost and high efficiency with worldwide availability. [ABSTRACT FROM AUTHOR]

In the 2020 time frame, the Global Positioning System (6PS) will be fully modernized, and other satellite navigation systems will be operational. With an additional layer of fault detection, these systems will provide vertical guidance worldwide. This capability will be born of three important technologies. First and foremost, avionics will receive signals on two frequencies: LI/EI and L5/ESa. This frequency diversity will do much to obviate the impact of ionospheric storms that troubles aviation use of GPS today. Secondly, a multiplicity of data broadcasts will be available to convey integrity information from the ground to the airborne users. These will include the navigation satellites themselves, geostationary satellites, and possibly terrestrial transmitters. However, the most important change will be the most subtle. The fault monitoring burden will be split between the aircraft and the supporting ground systems in a new way relative to the fault-detection techniques used in 2008. This new integrity allocation and the associated architectures are the subject of this paper. [ABSTRACT FROM AUTHOR]

In safety-of-life applications of satellite navigation, the protection level (PL) equation translates what is known about the pseudo-range errors into a reliable limit on the positioning error. The current PL equations for satellite-based augmentation systems (SBAS) rely on Gaussian statistics. This approach is very practical: the calculations are simple and the receiver computation load is small. However, when the true distributions are far from Gaussian, such a characterization forces an inflation of the PLs that degrades performance. This happens in particular with errors with heavy tail distributions or for which there is not enough data to evaluate the distribution density up to small quantiles. We present a way of computing the optimal protection level when the pseudo-range errors are characterized by a mixture of Gaussian modes. First, we show that this error characterization adds a new flexibility and helps account for heavy tails without losing the benefit of tight core distributions. Then, we state the positioning problem using a Bayesian approach. Finally, we apply this method to PL calculations for the wide area augmentation system (WAAS) using real data from WAAS receivers. The results are very promising: vertical PLs are reduced by 50% without degrading integrity. [ABSTRACT FROM AUTHOR]

This paper develops an empirical confidence bound for barometric altimeter altitude errors and shows how this bound may improve the performance of GPS-based approach and landing systems. This empirical bound is developed using historical meteorological data collected at a set of geographically diverse locations over a thirty year period. The confidence bound developed is shown to provide a Gaussian overbound on altimeter altitude errors in standard atmospheric conditions between a 10-5 and 10-6 confidence level. This confidence bound is integrated into the standard methodology for analyzing the performance of GPS-based landing systems and the results of a performance trade study using the confidence bound are presented. The results show that incorporating the empirical barometric altimeter confidence bound provides an increase in the coterminous United States (CONUS) service volume for lateral precision with vertical guidance (LPV) type approaches. While this increase is approximately 2% for an L1 single-frequency GPS user, it jumps to roughly 40% for an L5 single-frequency user. [ABSTRACT FROM AUTHOR]

Strong ionospheric scintillation due to electron density irregularities inside the ionosphere is commonly observed in the equatorial region during solar maxima. Strong amplitude scintillation causes deep and frequent Global Positioning System (GPS) signal fading. Since GPS receivers lose carrier tracking lock at deep signal fading and the lost channel cannot be used for the position solution until reacquired, ionospheric scintillation is a major concern for GPS aviation in the equatorial area. Frequent signal fading also causes frequent reset of the carrier smoothing filter in aviation receivers. This leads to higher noise levels on the pseudo-range measurements. Aviation availability during a severe scintillation period observed using data from the previous solar maximum is analyzed. The effects from satellite loss due to deep fading and shortened carrier smoothing time are considered. Availability results for both vertical and horizontal navigation during the severe scintillation are illustrated. Finally, a modification to the upper bound of the allowed reacquisition time for the current Wide Area Augmentation System (WAAS) Minimum Operational Performance Standards (MOPS) is recommended based on the availability analysis results and observed performance of a certified WAAS receiver. [ABSTRACT FROM PUBLISHER]

Among the receiver autonomous integrity monitoring (RAIM) algorithms treating multiple failures, multiple hypothesis solution separation algorithms (MHSS)—a type of solution separation algorithm—offer several advantages: First, the link between threat model, upper bound on the position error—the protection level and probability of hazardously misleading information is an easy and straightforward one; second, the calculation of the protection level does not involve complex steps. One of the critical steps in this algorithm is the allocation of the integrity and continuity budgets among the failure modes, as it determines the overall performance of the algorithm. After describing the baseline MHSS approach, we present an algorithm that simultaneously allocates the integrity and continuity budget among the failure modes to obtain the minimum protection level per satellite geometry. Then, we show how slope-based RAIM and solution separation RAIM are related through a little-known formula, which both unifies and highlights the differences between the two approaches. Finally, we apply the algorithm to evaluate the performance of RAIM for vertical guidance for a dual constellation, and find that even with a very large prior probability of satellite failure, vertical guidance can be achieved worldwide with high availability. [ABSTRACT FROM AUTHOR]

Techniques available to sustain dual-frequency ionosphere performance when a dual-frequency airborne Global Positioning System (GPS)/wide area augmentation system (WAAS) user loses all but one GPS frequency while descending into radio frequency interference (RFI) are investigated. We are particularly interested in the case where the user transitions from L1–L5 to having L5-only since the uncertainty of the L5-only ionospheric delay estimation is larger than the case for L1-only. The goal is to provide the techniques necessary for single-frequency users to sustain a performance similar to those of dual-frequency users. The proposed techniques are 1) the code and carrier divergence technique, 2) the WAAS ionosphere threat model technique, and 3) the maximum ionospheric delay gradient model technique. The results show that all three proposed techniques provide good ionospheric delay estimation for the full duration of approach. [ABSTRACT FROM PUBLISHER]