In the past, pilots depended on their knowledge of the skies, points of reference on the horizon, and some analogic measuring devices that provided some useful information such as altitude and speed to fly an airplane safely without getting lost.
Of course, all the above are still in use. When flying older aircraft, and when the pilot loses the functionality of the instruments those skills are essential to finding a safe way home.
However, most modern aircraft use new technologies such as the Global Positioning System or GPS. Yet, the system, as well as GPS satellites, are not totally failure-proof.
According to the GPS Standard Positioning Service (SPS) specifications, the probability of failure is approximately 10-4 per hour. Obviously, for civil aviation, there are very strict requirements in terms of precision, integrity, continuity of service, and availability provided by a Global Navigation Satellite System (GNSS) since they have a major impact on perhaps the most critical aspect in aviation, safety.
Legacy Global Navigation Satellite Systems had ground control segments that lacked full-time satellite visibility, so fault detection and exclusion could take critical hours.
Fortunately, a solution to speed up fault detection and exclusion was found in the form of the RAIM system. Within this guide, we will cover what this algorithm is, how it works, and the important role it has for the aviation industry. Keep reading to find out!
Receiver autonomous integrity monitoring (RAIM)
RAIM stands for Receiver Autonomous Integrity Monitoring, and it is used to monitor GPS information for fault detection.
But, what exactly is RAIM, and how does it work? Let us get into the details now.
What is RAIM in aviation?
A simple definition could be that RAIM is an independent integrity monitoring technique applied to in-flight electronics to ensure satellite signals comply with the safety requirements at each flight stage. The technique is based on an algorithm that is integrated into safety-critical applications receivers.
The RAIM algorithm and GPS receiver
GPS satellites sometimes broadcast information that is not completely accurate, thus causing the navigation data to be incorrect. Unfortunately, GPS receivers cannot determine whether the information is accurate or not by standard means. Therefore, the pilot cannot be sure of the accuracy of the GPS position without using RAIM. So, the question is how does RAIM work?
RAIM, fault detection, and proper fault exclusion
Using distance measurement of several satellites, the RAIM system can detect satellite failure to alert the pilot of a possible faulty satellite. While RAIM capabilities do not guarantee GPS positioning accuracy, the GPS failure alert helps the pilots take the corresponding measures.
RAIM takes advantage of the redundancy of distance measurements to produce several GPS position fixes. Then the RAIM algorithm compares those measurements to determine how consistent they are and verify possible positioning failure.
The latter is done when a statistical function determines the measurements are not consistent. The distance measurements are actually pseudo distances taken from a satellite to a receiver and are sometimes called pseudoranges. At this point, it is important to highlight the assumptions and steps taken, as well as the conditions required, to perform RAIM to properly determine when a satellite broadcast slightly incorrect information. These assumptions, steps, and conditions are provided as stated by the ESA Global Navigation Satellite System (GNSS) Science Advisory Committee (GSAC).
For civil aviation, there is a very low probability to find more than one satellite failing. So, it is assumed that the probability is negligible, meaning that only one satellite could fail at a time. Several satellites failing at the same time is considered impossible.
Conditions required when RAIM is employed
Four visible satellites are not enough to provide integrity.
Five visible satellites: if an anomaly is detected, the measurement from that specific satellite is discarded, and therefore only 4 satellites are left. With only four satellites, the receiver does not have enough redundant signals to compute the solution with different measurements and confirm that the solution is indeed correct. Therefore the receiver can issue a warning but not to provide integrity.
Six or more satellites: the receiver can detect and perform the exclusion.
Preliminary step: Compute the navigation solution.
Step 1: Fault detection Mechanism.
Step 2: Isolation of “faulty” satellites.
Step 3: Protection levels computation (although this step may be optional).
Traditional RAIM uses fault detection (FD) only, while newer GNSS receivers also incorporate exclusion (FDE) so they can still operate even when there is a GPS failure.
Also, there is an enhanced version of RAIM that uses at least six satellites to both detect a possible faulty satellite and exclude it. This allows the navigation function to keep working without interruption.
Clearly, the objective of FD is to identify the presence of a positioning failure. When this is identified, proper fault exclusion determines and excludes the source of the failure (and it does not need to identify the specific source of the problem), thus allowing navigation to continue without any problem.
While RAIM and FDE may not be equally available for operations in different regions, there are solutions such as the use of satellites from multiple GNSS constellations, and some integrity architectures. Let’s take a closer look at the latter.
The user algorithm known as RAIM can be improved by augmenting techniques that enhance its availability. These are known as integrity architectures that are proposed to meet the high requirements the civil aviation community has to guarantee the safety of travelers. These are the proposed architectures as per the ESA Global Navigation Satellite System Science Advisory Committee.
SBAS stands for Satellite-Based Augmentation System, a differential technique that relies on geostationary satellites to broadcast the augmentation information like corrections and integrity-related data. Moreover, SBAS provides ranging capabilities, thus potentially increasing satellite availability. Being GEO satellites, SBAS satellites provide coverage that is limited to a specific region. Good examples include EGNOS in the EU or WAAS in the US.
GBAS stands for Ground-Based Augmentation Systems. GBAS provides GNSS augmentation based on local ground elements. GBAS is also a differential technique used to transmit augmentation information to the receiver utilizing a Very High-Frequency Data Broadcast (VDB), and as a result, it can be used in airports with a coverage of around 30 km for CAT III operations.
ABAS, which stands for Aircraft-Based Augmentation System, is an augmentation system that focuses on integrity only and does not improve solution accuracy, which means that no corrections are provided. ABAS supports Non Precision Approaches using GPS L1 and it is mainly limited by the vertical error component.
According to ICAO Standard and Recommended Procedures (SARPS) Annex 10, there are two types of techniques within ABAS which are contemplated:
Receiver Autonomous Integrity Monitoring (RAIM), where only GNSS information is used. RAIM scheme can be included in the satellite navigation airborne equipment, either as the main source of integrity or as a backup for alternative sources, such as SBAS.
Airborne Autonomous Integrity Monitoring (AAIM), where GNSS information is complemented with onboard sensors and other components.
RAIM aviation: some final thoughts
Clearly, without RAIM capability no pilot could be sure of the GPS positioning accuracy. Of course, we must remember that a minimum of five satellites is required to identify a failing satellite, and six are required to also exclude it from the navigation solution when using FDE with the RAIM algorithm.
Also, when any receiver should let you know when the RAIM function is not available for the present time and/or position, as well as for any given time and/or position in the future. In any case, it is possible to get data about satellite outages via the Notice to Air Missions or NOTAM system which usually states the abnormal status of a component of the National Airspace System (NAS). However, the effect of an outage on the intended operation can only be determined by a RAIM prediction program that allows excluding a satellite that is predicted to be out of service.
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Jet pilot @NASA
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