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RAIM
Receiver Autonomous Integrity Monitoring (RAIM) can be defined as a user algorithm that determines the integrity of the GNSS solution. The RAIM algorithm compares the smoothed pseudo range measurements among themselves to ensure that they are all consistent.
Basically, RAIM algorithms make use of measurements redundancy to check the relative consistency among them (by means of the residuals) and in case of detection, the most likely "failed" satellite is determined. A key assumption usually made in RAIM algorithms for civil aviation isthat only one satellite may be faulty, i.e. the probability of multiple satellite failures is negligible. Another key issue related to RAIM algorithms is that one of their goals is to find measurement errors derived from non-nominal situations.
Many RAIM algorithms follow these steps:
- 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).
Taking into account that the user needs to solve four unknowns (3D position and clock) from the satellites, it follows that:
- Step 4: Visible satellites are not enough to provide integrity,
- Step 5: 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 redundancy to compute the solution with different measurements and confirm that the solution is indeed correct. Therefore the receiver is able to issue a warning but not to provide integrity,
- 6 or more satellites: the receiver is able to detect and perform the exclusion.
The more satellites in view, the more combinations of subsets of 4 satellites are available to detect potentially faulty satellites and the better the geometric observability. Please note that when the number of satellites in view increases, the assumption that the probability of multiple satellites is negligible can be questionable.
Integrity Architectures
The main driver pushing GNSS for integrity comes from the aeronautical domain, for which integrity is a critical requirement, since any failure could lead to losses of lives.
Three different architectures have been proposed to provide integrity to the aviation community[4]:
- SBAS
The Satellite-Based Augmentation System (SBAS) is a differential technique that relies on geostationary satellites to broadcast the augmentation information (e.g. corrections and integrity-related). In addition, SBAS also provides ranging capabilities, thus potentially increasing satellite availability. Being GEO satellites, SBAS coverage is limited to a regional area, e.g. EGNOS in EU or WAAS in US, and currently only supports LPV-200, APV I or II approaches.
- GBAS
Ground-Based Augmentation Systems (GBAS) provides GNSS augmentation based on local ground elements. GBAS is a differential technique in which augmentation information (e.g. corrections and integrity-related information) is transmitted to the receiver via Very High Frequency Data Broadcast (VDB) and therefore it can be used in airports (coverageof around 30 km) for CAT III operations.
- ABAS
Unlike the remaining augmentation systems, Aircraft-Based Augmentation System (ABAS) focuses on integrity only, and not on improving solution accuracy (i.e. no corrections are provided). ABAS supports Non Precision Approaches using GPS L1 and it is mainly limited by the vertical error component.
Within ABAS, two types of techniques are envisaged[4]: 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 back-up for alternative sources, e.g. SBAS.
Airborne Autonomous Integrity Monitoring (AAIM), where GNSS information is complemented with on-board sensors and other components.
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