Juan Blanch

Worldwide Vertical Guidance of Aircraft Based on Modernized GPS and New Integrity Augmentations

In the 2020 time frame, the Global Positioning System (GPS) 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: L1/E1 and L5/E5a. 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.

RAIM with Optimal Integrity and Continuity Allocations Under Multiple Failures

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.

Baseline advanced RAIM user algorithm and possible improvements

We present a baseline multiple fault and multiconstellation advanced receiver autonomous integrity monitoring user algorithm for vertical guidance. After reviewing the navigation requirements for localizer performance with vertical guidance down to 200 feet, we describe in detail how to process the pseudorange measurements, the nominal error models, and the prior fault probabilities to obtain the protection levels and other figures of merit. In particular, we show how to determine which fault modes must be monitored and a method for performing fault exclusion. Finally, we present a list of possible algorithm improvements and simplifications.

Satellite Navigation for Aviation in 2025

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.

Position error bound calculation for GNSS using measurement residuals

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.

Determination of fault probabilities for ARAIM

Two critical parameters for Advanced Receiver Autonomous Integrity Monitoring (ARAIM) are the probability of satellite fault, Psat, and the probability of constellation fault, Pconst. A satellite fault is one whose root cause is only capable of affecting a single satellite; while a constellation fault has a root cause that is capable of affecting more than one satellite at the same time. This paper provides more specific definitions for each of these fault types. We describe how performance commitments supplied by Constellation Service Providers (CSPs) are used to complete the fault definitions and to estimate their probability of 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. This paper is intended to lead to a framework for an open and transparent system for determining these parameters. This framework is intended to be the basis for an internationally agreed upon process for determining the fault rates that may be safely used for ARAIM.

GNSS Integrity in The Arctic: GNSS Integrity in The Arctic

Growing activity in the Arctic calls for high integrity navigation in this region. This can be achieved using Global Navigation Satellite Systems (GNSS) in conjunction with Satellite Based Augmentation Systems (SBAS) or Advanced Receiver Autonomous Integrity Monitoring (ARAIM). Single frequency GPS-only SBAS is in service in some regions today and is reliant on ground and space infrastructure. ARAIM will be more autonomous and will rely on the multitude of signals and core constellations coming in the future. Here, we examine both SBAS and ARAIM in the context of aviation and maritime requirements in the Arctic. Results demonstrate that the introduction of multi-frequency and multi-constellation to these systems enables navigation safety in the Arctic. SBAS brings aircraft precision approach as well as precise maritime operations such as mapping. ARAIM also supports precision approach in addition to autonomous ice navigation at sea but falls short of precision maritime requirements.

A simple position estimator that improves advanced RAIM performance

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.