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|Year of Publication||2011|
As traditional RAIM techniques continue evolving to cope with different requirements and tailored to different applications, the GNSS Evolutionary Architecture Study (GEAS) panel has proposed an Advanced RAIM technique able to guarantee LPV-200 operation worldwide, up to 2030 timeframe (i.e. excluding GPS III effort).
In its report of 2010, the GNSS Evolutionary Architecture Study (GEAS) panel recommends a path to achieve worldwide LPV-200 capability for air navigation. For that purpose, the following challenges are identified:
- Faults: usually reflected as satellite clocks run-off or upload of faulty navigation data (observed about 3 times/year for GPS),
- Rare normal conditions: general space weather conditions (e.g. ionospheric anomalies) that introduce Hazardous Misleading Information (HMI),
- Constellation weakness: too few satellites are well positioned for navigation purposes,
- Radio Frequency Interference (RFI): intentional or not, may lead to GNSS blockage.
The following Table identifies the challenges tackled by the different single frequency technologies.
|Constellation/challenge||Fault||Rare Normal||Constellation Weakness||RFI|
|RAIM||detect||detect||most vulnerable (>27 sat)||vulnerable|
|GBAS||detect||detect||vulnerable (> 23 sat)||vulnerable|
|SBAS||detect||detect||vulnerable (> 23 sat)||vulnerable|
Improving these systems to dual frequency would:
- Tackle rare normal conditions,
- Tackle most of unintentional RFI, such as those caused by DME (Distance Measuring Equipment).
As implemented currently in aircrafts, Receiver Autonomous Integrity Monitoring (RAIM) only supports lateral navigation. In order to cover vertical guidance, Advanced RAIM (ARAIM) techniques were recommended to include:
- Frequency diversity (e.g. using dual frequency measurements on L1/L5 for GPS),
- Geometry diversity (e.g. using as many GNSS constellation as possible in order to reach the required levels of satellite availability),
- Use of an Integrity Support Message (ISM) that would convey safety assertions associated with each of the core GNSS to the sovereign responsible for a given airspace,
- less likely threats to GNSS such as faults in the Earth Orientation Parameters (EOP) or any fault related to the generation of the ISM.
The GEAS report stated that worldwide vertical guidance based on ARAIM is feasible using a joint constellation of 24 Galileo satellites and 21 GPS satellites. This finding provides relief from a major concern for aviation (since only 21 operational GPS satellites were guaranteed by the DoD and that Galileo might run into some budgetary constraints).
The main reasons for which conventional RAIM techniques are considered not adequate for LPV-200 operations are:
- Design assurance level required (severe-major/hazardous for LPV-200 and only major for LNAV),
- Alert limits for LPV-200 are smaller when compared to LNAV’s,
- Multiple faults should now be considered for LPV-200,
- LPV-200 operations have more stringent accuracy requirements.
For these reasons, GEAS proposed a modified RAIM algorithm called Advanced RAIM – ARAIM which is based on the following assumptions:
- Satellite ranging error characteristics: instead of being assumed as zero mean Gaussian (conventional RAIM), now the biases are also considered, contributing as a sum of biased Gaussian distributed quantities.
- Satellite Integrity Failure Models: the integrity failure rate is assumed to be 10-5/hour/satellite (instead of 10-4/hour/satellite) and that the GPS Control Segment informs the user within one hour of the fault onset. It is further assumed that the rate of faults causing multiple simultaneous satellite integrity failures is 1.3x10-8/approach.
- requirement is now allocated to three cases: fault-free, single fault and multiple faults (in conventional RAIM, it was allocated equally among the first two cases).
The computation of Protection Levels (PL) for ARAIM considers the following contributions:
- Contributions of the pseudo-range residuals scaled to the position domain, weighed by a K factor that contains the Probability of Hazardous Misleading Information - P(HMI),
- Contributions of the bias terms,
- Contribution from the nominal conditions defined by the continuity/ accuracy requirements.
Furthermore, it is recommended that the approving authority provides additional information on the overbounding distributions (both for nominal conditions and expected un-faulted behavior):
- URA (User Range Accuracy) term describing the overbounding portion of the distribution that can be root sum squared with the other error overbounding distributions,
- URA term to contribute to the overbounding bias value,
- Constant value independent of URA.
Finally, ARAIM is considered available if:
- VPL is the Vertical Protection Level,
- VAL is the Vertical Alert Limit,
- n represents the VPL computed by excluding the measurement.
- ^ a b c Phase II of the GNSS Evolutionary Architecture Study, February 2010
- ^ Blanch, J., Ene, A., Walter, T., Enge, P. “An Optimized Multiple Solution Separation RAIM Algorithm for Vertical Guidance”. Proceedings of the ION GNSS 2007, Fort Worth, TX, September 2007.
- ^ Lee2, Y.C. and McLaughlin, M. P., “Feasibility Analysis of RAIM to Provide LPV 200 Approaches with Future GPS,” in Proceedings of ION GNSS-2007, Fort Worth, TX, September 2007