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