Integrity is one criteria to evaluate GNSS performance, which was first introduced in the aviation field. It is a measure of trust which can be placed in the correctness of the information supplied by the total system. In recent years, many GNSS-based applications emerge in the urban environment including liability critical ones, so the concept of integrity attracts more and more attention from urban GNSS users. However, the algorithms developed for the aerospace domain cannot be introduced directly to the GNSS land applications. This is because a high data redundancy exists in the aviation domain and the hypothesis that only one failure occurs at a time is made, which is not the case for the urban users. The main objective of this paper is to provide an overview of the past and current literature discussing the GNSS integrity for urban transport applications so as to point out possible challenges faced by GNSS receivers in such scenario. Key differences between integrity monitoring scheme in aviation domain and urban transport field are addressed. And this paper also points out several open research issues in this field.

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GNSS Position Integrity in Urban Environments: A Review of Literature

Global navigation satellite systems (GNSSs) have been the prevalent positioning, navigation, and timing technology over the past few decades. However, GNSS signals suffer from four main limitations.

/pdf/i-hear-therefore-i-know-where-i-am-compensating-for-gnss-zv01i4w7za.pdf

I Hear, Therefore I Know Where I Am: Compensating for GNSS Limitations with Cellular Signals

Multipath is one of the most important error sources in Global Navigation Satellite System (GNSS) carrier-phase-based precise relative positioning. Its theoretical maximum is a quarter of the carrier wavelength (about 4.8 cm for the Global Positioning System (GPS) L1 carrier) and, although it rarely reaches this size, it must clearly be mitigated if millimetre-accuracy positioning is to be achieved. In most static applications, this may be accomplished by averaging over a sufficiently long period of observation, but in kinematic applications, a modelling approach must be used. This paper is concerned with one such approach: the use of ray-tracing to reconstruct the error and therefore remove it. In order to apply such an approach, it is necessary to have a detailed understanding of the signal transmitted from the satellite, the reflection process, the antenna characteristics and the way that the reflected and direct signal are processed within the receiver. This paper reviews all of these and introduces a formal ray-tracing method for multipath estimation based on precise knowledge of the satellite–reflector–antenna geometry and of the reflector material and antenna characteristics. It is validated experimentally using GPS signals reflected from metal, water and a brick building, and is shown to be able to model most of the main multipath characteristics. The method will have important practical applications for correcting for multipath in well-constrained environments (such as at base stations for local area GPS networks, at International GNSS Service (IGS) reference stations, and on spacecraft), and it can be used to simulate realistic multipath errors for various performance analyses in high-precision positioning.

Development and testing of a new ray-tracing approach to GNSS carrier-phase multipath modelling

This paper introduces a new Kalman filter-based method for detecting sensor faults in linear dynamic systems. In contrast with existing sequential fault-detection algorithms, the proposed method enables direct evaluation of the integrity risk, which is the probability that an undetected fault causes state estimate errors to exceed predefined bounds of acceptability. The new method is also computationally efficient and straightforward to implement. The algorithm’s detection test statistic is established in three steps. First, the weighted norms of current and past-time Kalman filter residuals are defined as generalized noncentrally chi-square distributed random variables. Second, these residuals are proven to be stochastically independent from the state estimate error. Third, current-time and past-time residuals are shown to be mutually independent, so that the Kalman filter-based test statistic can be recursively updated in real time by simply adding the current-time residual contribution to a previously ...

Kalman Filter-Based Integrity Monitoring Against Sensor Faults

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.

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

Carrier-phase ranging measurements from Global Positioning System (GPS) and low-Earth-orbiting Iridium telecommunication satellites are integrated in a precision navigation system named iGPS. The basic goal of the system is to enhance GPS positioning and timing performance, especially under jamming. In addition, large satellite geometry variations generated by fast-moving Iridium spacecraft enable rapid estimation of floating cycle ambiguities. Augmentation of GPS with Iridium satellites also guarantees signal redundancy, which enables Receiver Autonomous Integrity Monitoring (RAIM). In this work, parametric models are developed for iGPS measurement error sources and for wide-area corrections from an assumed network of ground reference stations. A fixed-interval positioning and cycle ambiguity estimation algorithm is derived and a residual-based carrier-phase RAIM detection method is investigated for integrity against single-satellite step and ramp-type faults of all magnitudes and start-times. Predicted overall performance is quantified for various ground, space, and user segment configurations.

Analysis of Iridium-Augmented GPS for Floating Carrier Phase Positioning

The concept of Relative Receiver Autonomous Integrity Monitoring (RRAIM) using time differential carrier phase measurements is investigated in this paper. The precision of carrier phase measurements allows for mitigation of integrity hazards by implementing RRAIM monitors with tight thresholds without significantly affecting continuity. In order to avoid the need for cycle ambiguity resolution, time differences in carrier phase measurements are used as the basis for detection. In this work, we examine RRAIM within the context of the GNSS Evolutionary Architecture Study (GEAS), which explores potential architectures for aircraft navigation utilizing the satellite signals available in the mid-term future with GPS III. The objectives of the GEAS are focused on system implementations providing worldwide coverage to satisfy LPV-200 operations, and potentially beyond. In this work, we study two different GEAS implementations of RRAIM. General formulas are derived for positioning, fault detection, and protection level generation to meet a given set of integrity and continuity requirements.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0373463309990403

Carrier phase relative RAIM algorithms and protection level derivation

Methodologies sufficient to monitor GPS satellite orbit ephemeris for category I aircraft precision approach navigation are described. In the absence of a satellite maneuver, it is shown that a monitor based on the projection of previously validated ephemeris parameters is adequate to meet navigation integrity and availability requirements. After scheduled stationkeeping maneuvers, no previously validated ephemerides are available, so a measurement-based method is required. The feasibility of such a monitor is also established.

Orbit ephemeris monitors for local area differential GPS

One troublesome failure mode for Ground Based
Augmentation Systems (GBAS) is the possibility of large
discrepancies between satellite locations in space and the
locations derived by the ephemeris data that they
broadcast. For the Global Positioning System (GPS),
nominal ephemeris errors are typically 10 meters or less,
and it would take large errors (typically greater than 1
km) to be hazardous to GBAS users making precision
approaches to Category I minima. Although most large
errors will be detected by the GBAS ground segment
Message Field Range Test, ephemeris errors orthogonal to
the line-of-sight between a failed satellite and a GBAS
ground station are not detectable by this simple test.
To counter this possibility, RTCA has adopted new
protection levels to quantify the potential impact of
undetected ephemeris failures on user position errors for
both precision approach and terminal area applications.
These equations define position error bounds as functions
of the approximate aircraft location with respect to each
satellite and the GBAS ground station as well as the
magnitude of the satellite orbit error detectable by the
ground station. This Minimum Detectable Error (MDE)
determines the "P-value" that is broadcast by the GBAS
ground station for each satellite it has approved for use.
Several GBAS monitor algorithms have been developed
and tested for use in GBAS installations that lack SBAS
coverage. One of these is a comparison between satellite
positions given by the current satellite ephemeris and the
ephemeris broadcast by the same satellite on its previous
pass. Variants of this "YE-TE" test have been shown to
support GBAS MDE's as low as 1100 meters, which will
minimize the resulting impact on Category I user
availability due to the ephemeris protection level
equations. In addition, means of using raw measurements
to initialize this monitor and to separately verify
ephemerides in real-time are proposed.

Ephemeris Protection Level Equations and Monitor Algorithms for GBAS

Ionospheric delay uncertainty creates the largest restriction to the availability of high integrity satellite navigation for today’s single frequency systems. LAAS, WAAS, and the other SBAS providers are limited in their coverage and service levels by the variability of the ionosphere. With the arrival of the new civil signals at L5, comes the ability to directly estimate and remove the ionospheric delay at any point on the Earth. This allows for new architectures exploiting L1 and L5 to bring airplanes within two hundred feet of the ground anywhere on the globe. The FAA has initiated a study panel, called the GPS Evolutionary Architectural Study (GEAS) to look into future architectures to provide this global service. The GEAS has determined that Time-to-Alert (TTA) will be one of the more difficult challenges for any global monitoring approach. To address this problem, the GEAS is looking at two methods, each of which transfers some of the TTA responsibility onto the aircraft. The first method is called Relative RAIM (RRAIM). It uses precise carrier phase measurements to propagate older code based position solutions forward in time. The veracity of the propagation is checked using RAIM on the very low noise carrier phase measurements. In this way, the overall TTA can be less than a second, but the ground is given tens of seconds to minutes to identify a fault. The second method is Absolute RAIM (ARAIM). This is more similar to existing FDE techniques except that the requirements must be made much more precise in order to support smaller alert limits. Again, the aircraft is able to raise a flag within seconds of receiving faulty data. The ground is allowed to take an hour or longer to identify the fault and remove it from future consideration. The protection level equations for both methods will be evaluated in this paper. In addition to the errors considered in today’s equations, the two new methods will include explicit bias terms to improve the handling of nominal biases and non-gaussian error sources. A critical parameter in the performance of these approaches is the strength of the constellation. The performance of each is evaluated for constellations optimized for 24, 27, and 30 satellites. Further, their performance is evaluated under conditions of satellite outages. RRAIM can perform very well with fewer satellites. ARAIM on the other hand is ideal for integrating in Galileo or other satellite constellations. Both of the methods show great promise for global provision of vertical guidance.

Livio Gratton

Papers

Analysis of Iridium-Augmented GPS for Floating Carrier Phase Positioning

Carrier phase relative RAIM algorithms and protection level derivation

Orbit ephemeris monitors for local area differential GPS

Ephemeris Protection Level Equations and Monitor Algorithms for GBAS

Future Architectures to Provide Aviation Integrity