Receiver Autonomous Integrity Monitoring Availability Research Articles

Development and evaluation of a generalized model of RAIM availability for single-, dual- and multi-satellite faults

Receiver autonomous integrity monitoring (RAIM) fault monitoring and identification requires first to judge the geometric distribution of currently visible satellites based on performance indicators and to determine whether it is suitable for integrity monitoring, that is, to judge the availability of the RAIM algorithm. With the rapid development of the global navigation satellite system (GNSS), the possibility of dual-satellite faults and multiple-satellite faults in GNSS navigation and positioning has greatly increased. The existing literature mainly focuses on RAIM availability evaluation for single-satellite fault or dual-satellite faults, ignoring the occurrence possibility of multiple-satellite faults, and thus there is no unified RAIM availability evaluation model. To this end, we use the characters of the quadratic matrix to derive a generalized model of RAIM availability evaluation, and the model is suitable for situations of single-, dual- and multi-satellite faults. Based on the observation data of Beidou Navigation Satellite System (BDS) from 332 International GNSS Service (IGS) stations distributed globally on 24 March 2021 and the global ephemeris file, the global RAIM availability of BDS is evaluated. The results show that the generalized model of RAIM availability evaluation has a rigorous derivation process, strong applicability and easy implementation. When the cutoff elevation angle is greater than 5°, the average number of BDS visible satellites at 332 IGS stations worldwide is 12.289 and the average geometry dilution of precision value is 8.090. This can fully meet RAIM availability evaluation and integrity monitoring requirements at most IGS stations and regions. The RAIM availability of triple-satellite faults is lower than that of dual-satellite faults, and the RAIM availability of dual-satellite faults is lower than that of single-satellite fault. For the three different faults, the RAIM availability cannot fully satisfy the four stages in aviation all over the world. Among them, the RAIM availability is highest in the en-route (oceanic/continental low density) stage for all three types of faults, which is 93.146%, 74.848%, and 67.576%, respectively, while the non-precision approach stage has the lowest RAIM availability, which is 78.193%, 13.636%, 3.333% for the three types of faults, respectively.

Evaluation of BDS and GPS RAIM availability based on data collected in June 2020

The satellite pseudo-range fault detection with the Receiver Autonomous Integrity Monitoring (RAIM) method is affected by several satellite observations and the geometric distribution of satellites. The poor geometry distribution of satellites will conceal the positioning error caused by the satellite pseudo-range fault, resulting in unreliable detection results. Therefore, the availability evaluation must be made before RAIM to ensure that the fault detection performance will not be affected. On June 23, 2020, China successfully launched the 30th (last) navigation satellite of BeiDou's third-generation navigation satellite system (BDS-3), which is also the 55th BeiDou navigation satellite. Combining all the available satellites of BDS-1, BDS-2 and BDS-3, the positioning performance of BDS can be greatly improved. In order to evaluate the RAIM availability of BeiDou Navigation Satellite System (BDS) and Global Positioning System (GPS) in China, this paper first deduces the mathematical models and their characteristics of the three RAIM availability evaluation methods. Then, the study area (N10°–70°, E60°–150°) is divided into 4536 grid points at intervals of 1 ° × 1 ° in latitude and longitude, and the elevations of these grid points are taken from the global terrain data file. The Horizontal Protection Level (HPL) values of these grid points are calculated during 8–15 June 2020 using BDS and GPS ephemeris data. The RAIM availability differences between the two systems are compared and analysed. The analysis shows the Horizontal Protection Level method (HPLM) based on single-satellite pseudo-range fault is the most practical and convenient. During the 8-day observation period, the HPL values of BDS are significantly smaller than those of GPS in terms of geographic location and observation time, and the variation of HPL time series of BDS is also smaller than that of GPS, which indicates that the RAIM availability of GPS in China is not as good as that of BDS. Most importantly, in the four flight stages of the aircraft's Oceanic/Continental low-density En-route, Continental En-route, Terminal En-route and Non-precision approach (NPA), BDS can completely satisfy its RAIM availability requirement, while GPS can only meet the availability requirement of the En-route (Oceanic/Continental low density) phase, and the availability of the other three phases can at least reach 99.714%.

A CIPSO-FCM-Based RAIM Algorithm for the GPS/BDS Integrated Navigation System

The receiver integrity monitoring of a satellite navigation system directly affects positioning accuracy and train safety. The GPS/BDS (Global Positioning System/BeiDou Navigation System) integrated navigation system can meet the requirements of RAIM (Receiver Autonomous Integrity Monitoring) for the minimum number of visible satellites. However, the probability of multiple satellite simultaneous failures increases in the GPS/BDS integrated navigation system so that when the failures occur, the RAIM algorithm cannot meet the requirements of safety for the receiver when positioning trains. In response to the problem, this paper applies the CIPSO-FCM (Chaotic Immune-vaccine Particle Swarm Optimization_ Fuzzy C-means) algorithm to RAIM to establish a new algorithm. It can predict the HPL (Horizontal Protection Level) value of the GPS/BDS integrated navigation system to verify RAIM availability. It also uses the parity space vector to obtain cluster samples through a pseudo-range observation matrix. The CIPSO algorithm obtains the initial cluster centers by iteration to prevent the FCM algorithm from falling into the local optimum, leading to inconsistent classification. A test based on actual field data shows that the CIPSO-FCM-based RAIM algorithm achieves a probability of 99.98% for detecting and identifying multiple satellite failures with a gross error of 28m, which can meet the safety requirements for the receiver when positioning trains.

Analyses of the sensitivity of multi-constellation advanced receiver autonomous integrity monitoring vertical protection level availability to error parameters and a failure model over China

The availability of advanced receiver autonomous integrity monitoring for vertical guidance down to altitudes of 200 ft (LPV-200) is discussed using real satellite orbit/ephemeris data collected at eight international global navigation satellite system service stations across China. Analyses were conducted for the availability of multi-constellation advanced receiver autonomous integrity monitoring and multi-fault advanced receiver autonomous integrity monitoring, and the sensitivity of availability in response to changes in error model parameters (i.e. user range accuracy, user range error, Bias-Nom and Bias-Max) was used to compute the vertical protection level. The results demonstrated that advanced receiver autonomous integrity monitoring availability based on multiple constellations met the requirements of LPV-200 despite multiple-fault detections that reduced the availability of the advanced receiver autonomous integrity monitoring algorithm; the advanced receiver autonomous integrity monitoring availability thresholds of the user range error and Bias-Nom used for accuracy were more relevant to geographic information than the user range accuracy and Bias-Max used for integrity at the eight international global navigation satellite system service stations. Finally, the possibility of using the advanced receiver autonomous integrity monitoring algorithm for a Category III navigation standard is discussed using two sets of predicted errors, revealing that the algorithm could be used in 79% of China.

Research on GNSS Receiver Autonomous Integrity Monitoring Method Based on M-Estimation

Receiver Autonomous Integrity Monitoring (RAIM) method is an effective means to provide integrity monitoring for users in time. In order to solve the misjudgment caused by the interference of gross error to the least squares algorithm, this paper proposes a RAIM method based on M-estimation for multiconstellation GNSS. Based on five programs, BDS, GPS/BDS, and GPS/BDS/GLONASS at the current stage, the future Beidou Global Navigation Satellite System, and the future GPS/BDS/GLONASS/Galileo system, the new RAIM method is compared with the traditional least squares method by simulation. The simulation results show that, with the increase of constellations, RAIM availability, fault detection probability, and fault identification probability will be improved. Under the same simulation conditions, the fault detection and identification probabilities based on M-estimation are higher than those based on least squares estimation, and M-estimation is more sensitive to minor deviation than least squares estimation.

Operational Receiver Autonomous Integrity Monitoring Prediction System for Air Traffic Management System

The Global Positioning System (GPS) is the key enabling technology for new air traffic management (ATM) systems. However, for civil aviation safety-of-life applications, the sensors of an ATM system using GPS must meet the required navigation performance, especially integrity. According to a new ATM system requirement, the receiver autonomous integrity monitoring (RAIM) prediction system (RPS) is an essential element for predicting GPS service quality for a 72 h period. Many web-based RPSs have been developed and are operated by civil aviation organizations around the world. However, none can autonomously provide the GPS RAIM availability information to the new ATM system. Thus, this study develops an RPS that can be integrated with the new ATM system. Airport operators can obtain the forecast results directly from the ATM system for a specific flight-information region. The system integration is validated using three years of actual operation results of the new ATM system. An evaluation of its support for additional navigation satellite constellations is also made.

Performance analysis of BDS for regional services with consideration on weighted factors

At the current stage, the service performance analysis of BeiDou satellite navigation system (BDS) can provide important references for BDS applications. Accuracy, integrity, availability, and continuity are important indicators of the service capability of the satellite navigation system. In this paper, the relationships among them are analyzed firstly. Secondly, for describing accuracy, position dilution of precision (PDOP) is selected and the quantitative calculation formulas of its availability and continuity are established. Then PDOP of BDS in Asia-Pacific region is simulated and analyzed, the results show that the average availability and continuity of PDOP in the service area during 24 h are up to 99.60% and 0.9947/h, respectively. Thirdly, this paper has a brief introduction to weighted least square algorithm of receiver autonomous integrity monitoring (RAIM) and analyzes the relationships among the probability of false alert, the probability of missed detection, threshold, the minimum detectable error and the number of visible satellites. The threshold and the minimum detectable error will increase with the increasing number of visible satellites when given the false alert probability and missed detection probability, respectively. Finally, the quantitative calculation formula of RAIM availability and continuity is presented and RAIM availability and continuity are simulated and analyzed as well, the results show that the average availability and continuity in the service area are up to 99.95% and 0.9984/h, respectively.

Receiver autonomous integrity monitoring availability and fault detection capability comparison between BeiDou and GPS

This paper used the statistical methods of quality control to assess receiver autonomous integrity monitoring (RAIM) availability and fault detection (FD) capability of BeiDou14 (Phase II with 14 satellites), BeiDou (Phase III with 35 satellites) and GPS (with 31 satellites) for the first time. The three constellations are simulated and their RAIM performances are quantified by the global, Asia-Pacific region and temporal variations respectively. RAIM availability must be determined before RAIM detection. It is proposed that RAIM availability performances from satellites and constellation geometry configuration are evaluated by the number of visible satellites (NVS, NVS > 5) and geometric dilution of precision (GDOP, GDOP < 6) together. The minimal detectable bias (MDB) and minimal detectable effect (MDE) are considered as a measure of the minimum FD capability of RAIM in the measurement level and navigation position level respectively. The analyses of simulation results testify that the average global RAIM performances for BeiDou are better than that for GPS except global RAIM holes proportion. Moreover, the Asia-Pacific RAIM performances for BeiDou are much better than that for GPS in all indexes. RAIM availability from constellation geometry configuration and RAIM minimum FD capability for BeiDou14 are better than that for GPS in Asia-Pacific region in all cases, but the BeiDou14 RAIM availability from satellites are worse than GPS’s. The methods and conclusions can be used for RAIM prediction and real-time assessment of all kinds of Global Navigation Satellite Systems (GNSS) constellation.

Receiver autonomous integrity monitoring (RAIM) performances of combined GPS/BeiDou/QZSS in urban canyon

Multi-GNSS (global navigation satellite system) has become a trend to solve the urban canyon effect in order to meet the requirements of GNSS users, but the probability of mistaken satellites is also increasing with the increase in the number of visible satellites. The receiver autonomous integrity monitoring (RAIM) performance has therefore become very important. In this paper, the minimum RAIM availability (MRA) is proposed to evaluate the RAIM availability of combined GNSS constellations, and the minimal detectable effect (MDE) is applied to assess the RAIM fault detection capability. The significant improvements of RAIM performances in urban canyons when GPS/QZSS (quasi-zenith satellite system) are combined without and with BeiDou are quantified and analyzed by the simulations of the Asia-Pacific region snapshot and temporal variations in Beijing, Tokyo, Shanghai, and Hong Kong with different masking angles. The combining of BeiDou with GPS/QZSS has great realistic significance, especially in the Asia-Pacific region. The evaluation measure of RAIM performances can be used to predict the availability of GNSS constellations under different conditions, and it can be also used to assess the availability and integrity of combined constellations. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Integrity analysis of COMPASS and other GNSS combined navigation

The integrity requirement should be satisfied when GNSS is used in aviation. There are now two ways for integrity monitoring, that is, receiver autonomous integrity monitoring (RAIM) and GNSS integrity channel (GIC) based on augmentation system. The rapid development of the Beidou satellite navigation system (COMPASS) will significantly improve the satellite geometry. Besides, the effects of first order ionosphere delay will be mitigated by the combination of GNSS multi-frequency signals. The availability of RAIM will be enhanced, which makes it possible to provide a worldwide seamless integrity service for aviation by using RAIM. The contribution of COMPASS to the availability of RAIM is analyzed by simulation; and the integrity requirement of which aviation approaching phrase can be satisfied when using COMPASS single system and its combination with other satellite navigation system is analyzed. Moreover, the influence of user range accuracy (URA) of COMPASS on integrity performance is discussed.

Extreme value theory-based integrity monitoring of global navigation satellite systems

Measurements consistency-based Receiver Autonomous Integrity Monitoring (RAIM) is the main technique for monitoring the integrity of global satellite navigation systems at the user-level. Existing RAIM algorithms utilize two tests, in the position domain a test for RAIM availability and in the measurement domain a test for failure detection. These tests involve the computation of three parameters: test statistic, decision threshold, and protection level. The test statistic is based on the actual measurements in the form of the sum of the squared errors (SSE). The decision threshold is chosen on the basis of the statistical characteristics of the SSE including the assumption that the errors are normally distributed. However, in practice residual, error distributions exhibit heavier tails than predicted by the Gaussian model. Therefore, this paper challenges the normality assumption of the residual navigation errors in three ways. First, real data are used to assess its impact on the traditional RAIM algorithm. Second, extreme value theory is applied to the tails, and the generalized extreme value (GEV) distribution is derived to capture residual navigation errors. Third, the performance of the traditional RAIM approach is compared with that employing the GEV distribution. The results demonstrate that the GEV model is a more accurate representation of the distribution of residual navigation errors than the conventional Gaussian model and should be used in the development of integrity monitoring algorithms.

The Effect of Terrain Mask on RAIM Availability

Receiver Autonomous Integrity Monitoring (RAIM) is a method, used by an aircraft's receiver, for detecting and isolating faulty satellites of the Global Navigation Satellite System (GNSS). In order for a receiver to be able to detect and isolate a faulty satellite using a RAIM algorithm, a couple of conditions must be met: a minimum number of satellites, and an adequate satellite geometry. Due to the highly predictable orbits of the GPS satellites, a RAIM availability prediction can be done easily. A number of RAIM methods exist; however, none of them takes into account the precise terrain masking of the satellites for the specific location. They consider a uniform fixed mask angle over the whole horizon. This paper will introduce the variable mask RAIM algorithm in order to show to what extent the terrain can affect the RAIM availability and how much it differs from the conventional algorithms.

A Highly Accurate and Computationally Efficient Method for Predicting RAIM Holes

Receiver Autonomous Integrity Monitoring (RAIM) is a method implemented within the receiver to protect users against satellite navigation system failures. For a receiver to execute a RAIM calculation, two conditions must be met: a minimum number of satellites and adequate satellite geometry. The non-existence of the minimum number of satellites (five) is referred to as a RAIM hole. Current regional and global RAIM availability studies use spatial (grid-based) and temporal sampling intervals driven by a trade-off between accuracy and computation workload. The implication of minimising computational load is that accuracy is compromised and potential RAIM holes remain un-sampled, with potential risk to safety. This paper proposes a direct and computationally efficient method (as opposed to the grid-based search approach) to predict RAIM holes. The method is based on the precise computation of satellite coverage (footprint) boundaries, the intersection points and analysis of the topology of the regions of intersection. Test results show that the proposed method is highly accurate and requires minimal computational load compared to the current approach.

A measurement domain receiver autonomous integrity monitoring algorithm

One of the key approaches to monitor the integrity of Global Satellite Navigation Systems (GNSS) is receiver autonomous integrity monitoring (RAIM). Existing RAIM algorithms utilise two tests in the position domain (for RAIM availability) and measurement domain (for failure detection). This paper proposes an alternative RAIM algorithm, which is based entirely in the measurement domain. This algorithm can be used for sensitivity analyses to support performance specification and system design. It can also be used during actual flight operations where the trigger is the phase of flight and its required navigation performance (RNP) parameters. This is made possible by computationally efficient calculation of the chi-squared parameters. The algorithm reverts to the current approach if the phase of flight is unknown. Simulation results for non-precision approach (NPA) have been used to demonstrate the effectiveness of the proposed algorithm.

RAIM Availability of GPS Augmented with Loran-C and Barometric Altimeter for Use in Nonprecision Approach1

Prior to the adoption of the Wide-Area Augmentation System (WAAS) as the primary radionavigation system for the future, a number of different augmentation techniques for providing enhanced integrity and availability of GPS for use in the National Airspace System were analyzed. This paper examines the availability of Loran-C augmented GPS with receiver autonomous integrity monitoring (RAIM) for use in nonprecision approach. Availability of the augmented system is calculated at approximately 2300 airports over the continental United States. It is demonstrated that GPS availability is significantly enhanced with Loran-C and barometric altimeter augmentation. The percentage of time augmented GPS is unavailable for nonprecision approach is reduced by at least two orders of magnitude relative to nonaugmented GPS. Loran-C is by itself an approved supplemental nonprecision approach landing system that can function as a stand-alone backup if GPS is degraded or unavailable. It is demonstrated that the augmented system represents an improved, redundant, and generally more robust supplemental navigation system; however, the Loran-C augmented system cannot satisfy the requirements for a primary navigation system.

Analyses Supporting FAA Decisions Made During the Development of Supplemental GPS Avonics Requirements*

During 1992, members of the FAA (Federal Aviation Administration) Navigation Satellite Operational Implementation Team (SOIT) developed the requirements for Technical Standard Order (TSO) C-129, which describes capabilities of GPS avionics to be used as a supplemental means of navigation for en route through nonprecision approach phases of flight. During the writing of TSO C-129, the SOIT requested that analyses of certain issues be performed, including the effect of certain types of altimeter aiding on receiver autonomous integrity monitoring (RAIM) availability, and the risk of continuing flight in the event of an unpredicted loss of RAIM after an aircraft crosses the final approach fix (FAF) of a nonprecision approach. This paper summarizes technical analyses used in the development of these requirements. The results showed that altimeter aiding significantly improved RAIM availability, and that the risk of continuing an approach in case of an unpredicted loss of RAIM under certain conditions was acceptable. Operational and technical considerations, including the results of these analyses, led the SOIT to require certain capabilities of GPS avionics to be covered under TSO C-129

RAIM Availability for GPS Augmented with Barometric Altimeter Aiding and Clock Coasting1

In late 1991, the FAA formed a group called the Satellite Operational Implementation Team to accelerate the introduction of satellite navigation and communications into the National Airspace System (NAS). Since then, the team has been addressing major technical and operational issues that need to be resolved before GPS is used in instrument flight rules (IFR) operations in the NAS. One of the most critical criteria for the operational approval of near-term use of GPS in the NAS is availability of receiver autonomous integrity monitoring (RAIM) detection and identification functions. To improve RAIM availability, SOFT members suggested that relatively simple GPS augmentations in the form of barometric altimeter aiding and clock coasting be considered. This paper analyzes GPS RAIM availability with these augmentations and presents the technical analyses used by the team to support its recommendations.

RAIM Availability for Supplemental GPS Navigation

This paper examines GPS receiver autonomous integrity monitoring (RAIM) availability for supplemental navigation based on the approximate radial-error protection (ARP) method. This method applies ceiling levels for the ARP figure of merit to screen out bad detection geometries under worst-case bias conditions. The ARP criterion value for each phase of flight is based on the integrity specifications stated in the RTCA Special Committee (SC)-159 supplemental Minimum Operational Performance Standards (MOPS) for GPS. Applying the ARP criterion, extensive analysis was performed to determine the availability of RAIM over the conterminous United States, North Atlantic, Europe, Central East Pacific, and North Pacific during en route, terminal, and nonprecision approach phases of flight. The 21 primary, optimal 21, and optimized 21 + 3 GPS constellations were examined. The results demonstrate that RAIM is not available 100 percent of the time, even with 24 operational satellites. This is because there are times when only 4 satellites are visible, preventing RAIM detection altogether, and other times when the geometry of the visible satellites has an ARP that exceeds the ARP ceiling value.