International Civil Aviation Organization

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

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

• Probability (Position Error ?HPL) ? 10-7/hour

Horizontal Protection Level (HPL)
Computed Position
True 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