Development of GBAS Ionosphere
Anomaly Monitor Standards toSupport Category III Operations
Matt Harris, Tim Murphy, Susumu Saito
Presentation for ENRI International Workshop on ATM / CNSTokyo, Japan

November 11, 2010

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Presentation Schedule Background Previous Results
 Simulation Details
High Fidelity Plasma Bubble vs. GBAS Simulations

 Latest Results
 Conclusions

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Background

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Motivation
 Ionospheric Anomalies Affect GBAS
– Travelling ionosphere disturbances & equatorial plasma bubbles

 New Methodology for GBAS to Support Cat III Airworthiness– Primarily due to ionospheric anomalies– Additional airborne and reference iono monitors standardized– Airworthiness based on set of Air and Ground functional requirements

 Are the latest GBAS standards sufficient to protect a GBAS
user from position errors due to ionospheric anomalies inCategory III weather conditions?
– This extension of previous work aims to assist in validation of thebaseline GBAS standards (ICAO SARPS, Annex 10, and RTCA DOs)
 Is a maximum vertical error ~10m feasible?
 What are worst case range errors for GBAS signal model use?
 Is the worst multiple satellite impact bounded by worst two satellites?
4Background Industry Ionospheric Anomaly Threat Models– Wedge gradient models North American anomalies– Plasma bubble models equatorial anomalies

 Simulation Methodology– Range domain wedge simulation– Three dimensional plasma bubble simulation– Search exhaustively for worst case phasing / timing / error Air and Ground Monitors Simulated– RTCA DO-253C and Draft Changes to ICAO SARPs Annex 10– Airborne: CCD, RAIM, dual position solution monitor, geometry screens( VPL, LPL, Svert, Svert2, Slat, Slat2 )– Reference: CCD, absolute gradient Assumptions– 10-9 missed detection probability simulated– Plasma bubbles are a relatively regular occurrence5Previous Results

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Example Results: Worst Position Error

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Example Results: Worst Range Errors

8Initial Bubble Results: Worst Vertical Error

Maximum Vertical Position Error (m)Maximum Vertical Error vs. Dist from Reference, Approach Direction, Max Svert2, Pmd
AirCCD + RefCCD + RAIM, Max Svert1=4, FASVAL=10m, AirSpeed=80m/s
NO RAIM for ADDITION, NO GROUND GRADIENT MONITOR
D=6km
RW-18
RW-09
RW-27
RW-36

x : Geometry Screening Onlyo : CCD & RAIM Pmd = 10-9
o : CCD & RAIM Pmd = 0.5

Max Svert2<6Max Svert2<5Max Svert2<4Plasma Bubble Ground Speed (m/s)

9Simulation Details

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Background Ionosphere Simulation

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Background Ionosphere Simulation

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Typical Plasma Bubble•Fan shaped N-S depletion region with contours that follow magnetic field•Finger shaped W-E cross section•Maximum height at magnetic equator•Irregular transition regions
13Plasma Bubble Simulation Model•Modeled as rectangulardepletion in W-E
direction•Dipole magnetic fieldmodel used for N-S
shape•Constant, linear
boundary lengthNS•Constant depletion
region shape moves
zonally14

Bubble Scenario Visualization
 Three identical bubbles in each simulation, at fixed intervals

100m/s

100m/s

100m/s

W=100km
T=20km80m/s+129.62E+133.62E

200km
1500kmdepletion+134.6E

Reference

Magnetic Equator+135E+137.62E

200km
1500km

200km
1500kmdepletion

depletion

Simulated GPS Satellite Geometry

background ionosphere(highly active)

15Bubble Scenario Visualization, RWY 09(additional plasma bubbles +/- 4 degrees
longitude relative to the center bubble)

100m/s

W=100km
T=20km80m/s
HE=1500km+133.62E+134.6E

Reference

Magnetic Equator+135E

depletion

background ionosphere(highly active)
Simulated GPS Satellite Geometry

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Example Results for a Single Scenario
GEOMETRY SCREENING

ALL GEOMETRIES
VAL

Vertical Position Error (m)Vertical Position Error (m)VAL

Vertical Position Error (m)GPS Time (s)VAL

GPS Time (s)ANOMALY MONITORING
Worst Errors Remaining at Threshold(X km from Reference)

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GPS Time (s)Results

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Latest Results, RWY 09

Worst Case Vertical Position Error (meters)

Maximum Vertical Position Error at 5.6km from Reference (meters)
at Three Reference Station Latitudes (0, 22, and 30 degrees - magnetic)
Vertical
Projection:

Svert2<6

Svert2<5

Svert2<4Plasma Bubble Ground Speed (m/s)

19Latest Results, RWY 09

Worst Case Vertical Position Error (meters)

Maximum Vertical Position Error at 5.6km from Reference (meters)
at Three Reference Station Latitudes (0, 22, and 30 degrees - magnetic)•Max Error at 30deg
North is larger thanacceptable•Due to 50 meter
ionosphere delaydifference over 20 km
transition region =2500mm/km

Plasma Bubble Ground Speed (m/s)

20Latest Results, RWY 09, Grad < 500mm/km

Worst Case Vertical Position Error (meters)

Maximum Vertical Position Error at 5.6km from Reference (meters)
at Three Reference Station Latitudes (0, 22, and 30 degrees - magnetic)
Vertical
Projection:

Svert2<6

Svert2<5

Svert2<4Plasma Bubble Ground Speed (m/s)

21Latest Results, RWY 09, Grad < 500mm/km

Worst Case Vertical Position Error (meters)

Maximum Vertical Position Error at 5.6km from Reference (meters)
at Three Reference Station Latitudes (0, 22, and 30 degrees - magnetic)•Max Error at all
latitudes is acceptable•If a 500mm/km limiton the ionosphere
gradient is notreasonable, ICAO
SARPs draft change
recommends further
analysisPlasma Bubble Ground Speed (m/s)

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Conclusions

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Conclusions


The maximum vertical or horizontal position errors induced
by an ionospheric anomaly that will persist (with aprobability of greater than 10-9) after all the ionosphericanomaly mitigations have been applied can be limited toless than 10 meters.
– Errors on the order of 10 meters or less have been shown to resultin an airplane still landing in the safe landing box [1]. The maximum
error can be reduced somewhat by using more aggressive geometry
screening.

A 5 km baseline siting restriction appears to provide
adequate performance.
– Although some possibility to relax this restraint still exists, it isrecommended that 5 km be adopted as the baseline and that future
work be undertaken during the operational validation phase todetermine if this siting restriction can be relaxed or if additional siting
flexibility can be achieved in some other way.
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Conclusions, ContinuedThe general rule that the maximum error in thepseudorange domain of 2.75 meters (postulated in [9])appears to hold for plasma bubbles as well as the wedge
model. This maximum error characterization can be usedin the formulation of a fault model for use in airworthinessassessments.
The conservative approach of accounting for multiple
satellites by geometry limiting based on Svert2 or Slat2
appears valid since no more severe effects have beenfound using the high fidelity 3-D plasma bubble inconjunction with a satellite geometry simulation than were
found with the pseudorange domain wedge model as
scaled by Svert2 limits.

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