operational mitigation practice to enable the use of gbas on areas influenced by harsh ionosphere...

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Engineering, Test & Technology

Boeing Research & Technology

OPERATIONAL MITIGATION PRACTICE TO ENABLE THE USE OF GBAS ON AREAS INFLUENCED BY HARSH IONOSPHERE PHENOMENA

Authors:

Glaucia Balvedi – Boeing Research & Technology-Brazil (Presenter)

Matt Harris – Boeing Commercial Airplanes-Seattle

William Peterson – Boeing Commercial Airplanes-Seattle

José Alexandre Guerreiro Fregnani – Boeing Research & Technology-Brazil

Osamu Saotome – Instituto Tecnológico de Aeronáutica-Brazil

XV SITRAER – Simpósio de Transporte Aéreo – São Luis do Maranhão, 24-26 de Outubro de 2016

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Agenda

Introduction

GPS and Augmentation Systems

Ground Based Augmentation System – GBAS

Ionospheric Effects on GPS and GBAS

Proposed GBAS Operations in Brazil

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Introduction

• GBAS (Ground Based Augmentation System) is a key enabler for current andfuture precision approach operations;

• In some regions, GBAS meets the stringent needs of aviation; is moreeconomical than conventional ILS (Instrument Landing System) and has lowinstallation, maintenance and lifecycle costs;

• GBAS is not implemented in Brazil due to harsh ionospheric effects, whichaffects Global Positioning System (GPS) signals in a manner that impede thesystem to be safely used in Brazilian territory.

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Agenda

Introduction





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• GPS (Global Positioning System) is a satellite-based radio navigation system,part of the GNSS (Global Navigation Satellite System) core constellations;

• GPS provides position and time information to a used with an appropriatereceiver;

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By Paulsava - Own work, CC BY-SA 4.0,

https://commons.wikimedia.org/w/index.php?curid=47210072

• Satellites transmit range signals and navigationmessages at 1575.42 MHz (L1) and 1227.6 MHz(L2);

• Receiver requires signals from at least fourdifferent satellites: three for triangulation and onefor clock synchronization;

• GPS triangulation works by determining the time ittakes to receive a signal from orbiting satellites.




• GPS accuracy 95% of the time (ICAO Annex 10)

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Global Average

(95% of the time)

Worst Site

(95% of the time)

Horizontal Position Error

Vertical Position Error

13m (43 ft)

22m (72 ft)

36m (118 ft)

77m (253 ft)

• GPS positioning errors sources: multipath, receiver noise, hardware noise,broadcasted ephemeris errors, troposphere and ionosphere propagation;

• Navigation and Approach Aids (NAVAIDs) must meet specified requirements ofaccuracy, continuity, availability and integrity to each phase of flight;

• Performance requirements were established by ICAO.




• Signal-in-space requirements (ICAO Annex 10)

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Typical Operation

Phase of Flight

Accuracy

Horizontal

(95%)

Accuracy

Vertical

(95%) Integrity

Time-to-

alert Continuity Availability

En-route3.7 km

(2 NM)N/A 1-1x10-7/h 5 min

1-1x10-4/h

to 1-1x10-

8/h

0.99 to

0.99999

En-route

Terminal

0.74 km

(0.4 NM)N/A 1-1x10-7/h 15 s

1-1x10-4/h

to 1-1x10-

8/h

0.99 to

0.99999

Initial approach

Intermediate approach

Non-precision approach

(NPA)

Departure

220m

(720 ft)N/A 1-1x10-7/h 10 s

1-1x10-4/h

to 1-1x10-

8/h

0.99 to

0.99999

Approach operations with

vertical guidance (APV-I)

16m

(52 ft)

20m

(66 ft)

1-2x10-7/h

in any

approach

10 s1-8x10-6 per

15 s

0.99 to

0.99999

Approach operations with

vertical guidance (APV-II)

16m

(52 ft)

8 m

(26 ft)

1-2x10-7/h

in any

approach

6 s1-8x10-6 per

15 s

0.99 to

0.99999

CAT I precision approach16m

(52 ft)

6m to 4m

(20 ft to 13 ft)

1-2x10-7/h

in any

approach

6 s1-8x10-6 per

15 s

0.99 to

0.99999

Only en-route, terminal and non-precision approach operations can be safely supported by GPS ranging signals alone.




• Precision approach operations require stringent requirements, demandingaugmentation systems that can restrain navigation errors to accepted levels;

ABAS – Aircraft Augmentation Systems

Receiver Autonomous Integrity Monitoring (RAIM)

Aircraft Autonomous Integrity Monitoring (AAIM)

SBAS – Satellite Based Augmentation Systems

Wide Area Augmentation System (WAAS)

European Geostationary Navigation Overlay Service (EGNOS)

GBAS – Ground Based Augmentation Systems

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Agenda

Introduction





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Ground Based Augmentation System - GBAS

• GBAS is an augmentation system that provides ranging errors corrections andintegrity monitoring of GPS signals;

• Developed to attend civil aviation landing procedures and substitute ILS;

• Current GBAs approved by the Federal Aviation Administration (FAA) monitorsand augment the GPS L1 signals.

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• Space segment

• Airborne segment

• Ground segment

• Reference GPS receivers: installed at surveyed locations




• The difference between the calculated position and the surveyed positionprovides indication on the amount of propagations errors induced bytroposphere and ionosphere, for each reference receiver;

• The processing facility computes pseudo range corrections for each satellitesensed by the reference receivers;

• Differential corrections and integrity information is send to the aircraft over VHFdata link from the ground transmitter.

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• The airborne system applies the correction information in order to obtaincorrection levels smaller than the alert limits corresponding to the current phaseof flight;

• The corrections broadcast by GBAS improve the accuracy of GPS by 4m to0.5m.

• GBAS errors bounds are updated every 0.5s.

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Accuracy: bahavior of the system in presence of errors

Integrity: limit risks (integrity risk, maximum tolerable error, time

to alert)

Continuity: limit risk of losing the service unexpectedly

Availability: fraction of time one has accuracy + integrity +

continuity



Agenda

Introduction





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• Signal propagation behavior: troposphere, ionosphere;

• Influences radio signals, causing tropospheric and ionospheric propagationserrors;

• Under nominal or “quiet” atmosphere, the propagation errors can be modeledand the correction parameters are sufficient to mitigate the errors.

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• However, Brazil is located under the geomagnetic equator, which implies thatthe Brazilian territory is under harsh disturbances in the ionosphere:

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Equatorial Anomaly




• However, Brazil is located under the geomagnetic equator, which implies thatthe Brazilian territory is under harsh disturbances in the ionosphere:

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South Atlantic Anomaly

Plasma Bubbles




• The amount of errors introduced by these anomalies (“threats”) can be muchgreater than the correction broadcasted in GPS navigation message;

• Two main effects on GPS signal propagation: ranging errors and scintillations.

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• Scintillations:

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• For GBAS, the worst condition occurs when the reference receivers and theairborne equipment does not have a common error source;

• The ionosphere disturbances affect differently airborne and ground facility.

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Anomalous Ionosphere

Higher Gradients Lower Gradients

GPS

Ground Station

Broadcasted Pseudorange Correction

Message



Ionospheric Effects on GPS and GBAS• GBAS architecture cannot mitigate the spatially decorrelated data only by

monitoring;

• In order to deal with the decorrelated data, ionospheric “threat models” areapplied to GBAS solution;

• Threat models are developed considering a set of monitored ionosphericgradients versus satellite elevation angle over a territory, in order to define thethreat space;

• At each epoch (specific GPS instant time), the worst threat is assumed to occurin 100% of the time, preventing the aircraft from using unsafe combinations ofGNSS satellites;

• The current GBAS ionospheric threat model (used in commercial GBAS stationsin US) was assembled based on observed ionospheric data collected inContinental United States (CONUS);

CONUS data is not universally applicable!

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• The largest observed gradient in Brazilian territory is two times the upper boundfor the CONUS;

• There is a need for an ionospheric threat model for GBAS in Brazil, which willallow to proceed with the certification process to support GBAs installations inBrazil;

• Efforts have been made by federal aviation agencies and research institutes tomitigate the ionospheric threats, in order to certify the use of GBAS as a landingmethod in all regions of the world including Brazil.

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Agenda

Introduction





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Proposed GBAS Operation in Brazil

• The existing ionospheric threat model (CONUS) does not accurately representthe conditions found over the magnetic equator and over most part of theBrazilian territory;

• Preventing current GBAS operations, thereby compromising airspace efficiencyand capacity in the terminal areas;

• The only currently certified GBAS station uses an ionospheric threat modelinitially developed for mid-latitude locations and is insufficient for Brazil, notmeeting ICAO Annex 10 requirements.

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• Over the Brazilian territory, the ionosphere adverse effects on GPS signals arepronounced after sunset, between 1800 and 0500 local time;

• The phenomena is enhanced from September to March;

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HIGH IONOSPHERE ACTIVITY HIGH IONOSPHERE ACTIVITY

Month




• A suitable operational mitigation practice could be developed in order to allowthe use of GBAS stations in Brazil out of this high activities period until thecomplete development of the Brazilian ionosphere threat model, which couldpermit a wider availability of the GBAS stations;

• Setting the times of GBAS operations from 0600 to 1700 local time at Southern,Southeast, Central and Northeast Brazilian Airports. Airlines trials could beperformed, within those time limitations, using a higher Decision Altitude (DA)than CAT I (for example 500 ft) and obtain control baseline operational data.

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HIGH IONOSPHERE ACTIVITY HIGH IONOSPHERE ACTIVITY

WINDOW OF OPPORTUNITY




• A conservative solution would be to use similar operational approvalmethodologies adopted by the Brazilian local authorities in similar contexts. Forexample, using operational approval standards applied to Required NavigationPerformance Authorization Required (RNP-AR) approach procedurescertification at Santos Dumont Airport (SBRJ).

• Da tryout period all approaches were performed under Visual MeteorologicalConditions (VMC) by the applicant airline. Afterwards, higher decision altitudeswere applied until a solid safety case was built. This safety case could be usedto assist the determination of operational constraints (via Notice to Airman -NOTAM), restricting the period of operations of GBAS approaches

The use of the system can be leveraged by using the system during nominal ionospheric periods of the day.

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Closing Video

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