Performance evaluation results are summarized based on the selected RAIM algorithms. First of all, it has to be noted that the accuracy is independent from the RAIM algorithm itself. Evaluations have been performed with and without the consideration of a consistent bias on pseudoranges on all satellites. The major outcome is that the accuracy requirement is fully met even assuming a single constellation. In addition, it has been demonstrated that – especially considering a single constellation – performance is a function of the geometry and can therefore vary quite significantly. To anticipate, this is also the main driver for the degraded performance for integrity and continuity.
Integrity performance is given with respect to its availability as well as the resulting HPLs. The availability of integrity is the percentage of available samples (over the evaluation period and with a defined sampling rate) for which the HPL complies with the HAL respectively. Continuity performance is evaluated taking into account an exposure period of 15 minutes respective 3 hours. The availability of continuity basically gives the ratio of events
142 Performance Results
(HPL exceeding corresponding HAL) and the total evaluation period taking into account the sampling rate and the continuity exposure period.
The main results are summarized in Table 8-8 allowing for checking against the respective requirement values.
As stated above, accuracy is not shown due its independency from the RAIM algorithms.
Table 8-8: Summary of Performance Results
Scenario Algorithm Avail. of Integrity
[%]
Avail. of Continuity [%]
15 min 3 hours
Requirement 99.8 99.8 99.8
Single Const. (GPS only)
LSR RAIM 96.86 54.40 22.37
Novel RAIM 97.80 67.39 32.81
MHSS RAIM 99.29 98.89 86.68
Dual Const.
(GPS+Gal)
LSR RAIM 99.96 99.94 99.32
Novel RAIM 99.97 99.95 99.45
MHSS RAIM 100.00 100.00 100.00
Triple Const.
(GPS+Gal+GLO)
LSR RAIM N/A N/A N/A
Novel RAIM N/A N/A N/A
MHSS RAIM 100.00 100.00 100.00
The following conclusions can be drawn:
• Novel RAIM shows improved performance compared to LSR RAIM in any case. This is to be expected as Novel RAIM basically must be seen as an extension of the LSR RAIM. Hence, by design, the worst case performance cannot fall below the performance level of LSR RAIM. Especially for the single constellation scenario, a clear improvement can be observed due to more robustness of the Novel RAIM against weak satellite geometries.
• The performance of MHSS RAIM in general is higher than LSR and the Novel RAIM. This can be explained by the design of the algorithm that allows for a probabilistic weighting of the various threat cases and still fulfilling the integrity risk requirement. This is opposed to the other RAIM algorithms that solely are driven by the worst case performance.
• The LSR RAIM and Novel RAIM performance has been evaluated assuming a single constellation and dual constellation. In the Annex A.3 it is shown that the probability of having multiple simultaneous satellite failures exceeds the requirement for the total integrity risk assuming three constellations. For this rea-son, scenarios assuming a third constellation are neglected. The dis-ability of proper multiple fault de-tection capabilities of these two algorithms constitute clearly a limitation. However, the MHSS RAIM has the ability to properly cope with multiple simultaneous satellite failures and therefore results are pre-sented up to the use of three constellations.
• The requirement in terms of integrity is met considering multiple constellations. Insufficient performance levels using a single constellation can be observed for all three RAIM algorithms. Using a single
constel-Conclusion 143
lation reveals partly weak satellite geometries that cause outliers in the HPL time series. Thus, this per-ception underlines the fact that RAIM performance is strongly driven by the satellite geometry.
• The continuity requirement is met only if multiple constellations and an exposure period of 15 minutes are considered. It is clearly shown that the continuity requirement over 3 hours cannot be met by LSR RAIM and Novel RAIM. However, MHSS RAIM satisfies continuity requirements for both the 15 minutes and the 3 hours exposure period assuming multiple constellations.
Also, an additional analysis has been performed to complement and support the performance evaluation anal-yses. The dependency of the elevation mask of the Novel RAIM has been assessed. It is shown that performance is a function of the elevation mask applied at user level. Following the design of the Novel RAIM algorithm, it is shown that optimal performance is achieved if an elevation mask of 20° is used. This elevation mask has been used throughout the performance evaluations.
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9 Advanced RAIM Related Considerations
The situation nowadays of the GNSS is such that no sufficient integrity parameters are provided to the user on a global basis. It is obvious that with the current GNSS design, integrity and continuity cannot be guaranteed to the user. To overcome this limitation, an Advanced RAIM (ARAIM) architecture is discussed in current literature [WG-C ARAIM 2015]. This architecture is supposed to complement current GNSS in order to provide integrity measures to the user. The architecture consists in principle of an additional independent monitoring network. A dedicated central processing facility collects the data from the network and computes an Integrity Support Message (ISM) in order to provide the user with the required integrity parameters. The parameters contained in the ISM shall then be used by the MHSS RAIM algorithm. ARAIM allows a ground system to provide updates regarding the nominal error characterization and fault rates for the multiplicity of contributing satellites and constellations. The infrastructure of ARAIM constitutes a third party that might take over considerable parts of the integrity burden. An important advantage of the ARAIM concept is the potentially reduced complexity of the system compared to already established systems like SBAS.
First, the ARAIM architecture and its design drivers are addressed and presented. System architecture aspects are identified and potential impacts due to the different requirements specified by ICAO and IMO are highlighted.
Second, an overview of common overbounding concepts is presented together with their weaknesses and strengths taking into account maritime user environmental conditions. Third, from a user perspective, the as-sumption of the same model for the error contributions due to multipath and interference for the maritime user as for the aviation might be questioned. Therefore, the option of considering multipath and interference in the allocation of the integrity/continuity risk is still to be assessed. An assessment of the sensitivity of performance with respect to the contribution of multipath and interference is done and its results are outlined in this chapter.