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International Civil Aviation Organization

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1 International Civil Aviation Organization
Presentation 5.1 A Integrity of GNSS Presented by E. Chatre, Rapporteur GNSSP Working Group B CAR/SAM ATN/GNSS Seminar Varadero, Cuba, 6 to 9 May 2002 International Civil Aviation Organization

2 What is integrity for GNSS ?
GPS or GLONASS Satellites may broadcast erroneous signals for several hours causing pseudorange errors GPS or GLONASS may broadcast erroneous clock or ephemeris data causing positioning errors Users want to be guaranteed safe operations Need for monitoring capability Need for timely warning when anything goes wrong Different Solutions exist to provide integrity for civil aviation : ABAS, SBAS and GBAS

3 Autonomous Integrity in ABAS
Users need 4 satellite measurements to determine the 3D position + time Usually many more satellites are available (6 to 12) providing redundancy Receiver Autonomous Integrity Monitoring (RAIM) uses this redundancy to cross check the consistency of measurements and determine any anomaly RAIM provides integrity and instantaneous warning when it is available RAIM uses at least 5 satellites for fault detection and 6 satellites for fault exclusion

4 Availability of Autonomous Integrity
Service availability depends on the phase of flight (NPA is less available than En-route) and needs good satellite geometry RAIM Availability at a particular location and given time can be predicted based on almanac data This is the short term solution adopted by many States and availability can be improved when combining with other sensors like Baro Altimeter or INS Some operational limitation may be required RAIM/ABAS not sufficiently available for APV or precision approach operations Overall integrity achieved relies on some assumptions

5 SBAS and GBAS SBAS and GBAS are designed to cope with GPS and/or GLONASS satellite failures SBAS and GBAS systems monitor and send corrections as necessary and, if not able to determine a correction, send warnings to users Two things still need to be guaranteed : 1. That the corrections have an accuracy sufficient for the phase of flight considered 2. That the data transmitted by the SBAS or GBAS are correct and well received Integrity for GBAS and SBAS consists in ensuring these two things

6 Guarantee that accuracy is sufficient
Estimates of pseudorange residual errors are computed by the ground segment This is broadcast on the datalink to users Users know the satellite geometry and can compute bounds on the horizontal and vertical position errors These bounds are called Protection Levels (HPL and VPL). They provide good confidence (10-7/hour probability) that the true position is within a bubble around the computed position

7 Protection Level Horizontal Protection Level (HPL)
Probability (Position Error HPL)  10-7/hour Horizontal Protection Level (HPL) True Position Computed Position

8 Guarantee that broadcast data are correct
Data generation shall be monitored : Parallel processing chains are usually used based on independent hardware/software Data sources may also be duplicated (several receivers) Integrity monitor is designed to check data before they are sent based on comparison Integrity monitor can be implemented in the position domain to check that the data already broadcast are correct Data transmission is protected using parity check

9 Impact of Integrity loss
When the protection level exceeds the alert limit, integrity is not ensured at a proper level Some problems may affect a small area around a specific airfield Other problems will impact multiple users over wide areas Impact on Air Traffic Services will depend on existence of back-ups (airborne and/or ground) overall ATS infrastructure (radar surveillance…) traffic density procedural mitigation… Need to issue NOTAM whenever possible

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