Aviation Considerations for Multi-Constellation

GNSS

Leo Eldredge, GNSS GroupFederal Aviation Administration (FAA)

December 2008

Federal AviationAdministration

8 December 20082

Introduction

• GPS is an important component of today’s aviation guidance infrastructure– Its role will continue to increase over the coming

years

• Future GNSS constellations will also become important to contributors

• However, their incorporation must be done with great care as the integrity requirements for aircraft guidance are very stringent– Less than 10-7 probability of misleading

information

– International standards define different types of augmentation to achieve this level of integrity

8 December 20083

Integrity Monitoring

• Space-based and ground-based augmentation systems provide independent monitoring of the GPS signals through calibrated ground monitors– Requires ground monitoring network and

communication channel to aircraft

• Receiver Autonomous Integrity Monitoring (RAIM) compares redundant satellite measurements against each other to determine identify and eliminate large faults– Requires a larger number of ranging

measurements

8 December 20084

2

April 23, 2004

© 2004 The MITRE Corporation. All Rights Reserved.

Range Comparison Method (RCM) (Example with 5 SVs in View)

GPS Supplemental Use - 1995

• Key Feature:

– Integrity Determination by the User with RAIM

• Key Enabler

– Requires Redundant Ranging Sources

• Key Benefit

– Provides horizontal guidance for aircraft

• Key Challenge

– Accuracy & Availability

8 December 20085

Key GPS Performance Parametersthat Support Horizontal Guidance

• Good accuracy – Vertical and horizontal accuracies better than 10

m 95%

– Nominal ranging accuracy better than 2 m 95%

• Reliable signals– Low rates of failure - 10-5/hour/satellite and better

• Good coverage– Good distribution of satellites in the sky

• Good signal availability – Rarely more than one primary satellite out at a

time

8 December 20086

Two Civil Frequencies

• The ionosphere creates the largest source of uncertainty affecting today’s use of GPS for aviation

• When GPS L5 becomes widely available it will become possible to to directly remove the ionospheric influence– May allow RAIM to support vertical navigation

• Unfortunately, the two frequency combination increases the effects of other noise sources

• It is desirable to reduce these noise terms and/or add more satellites to offset this increase

8 December 20087

Horizontal and Vertical Navigation

• GNSS vertical accuracy is worse than horizontal – Satellites below the aircraft are blocked by Earth

• Aviation requirements are more strict in the vertical – Vertical maneuvers bring the aircraft closer to the

ground

• Therefore, it is much harder for GNSS to meet aviation vertical guidance requirements

• But, absolute vertical guidance from GNSS offers a strong safety benefit – Avoids manual calibration

– Enables smooth, continuous approach paths

• Want to provide vertical and horizontal guidance

8 December 20088

New Constellations Provide New Opportunity

• RAIM requires a sufficient number of satellites to assure redundancy and accuracy– Availability of horizontal guidance through RAIM not

always 100% with today’s GPS constellation

• If new GNSS constellations provided similar signals and performance to GPS, there would be an opportunity to combine information to expand seamless global navigation– Better horizontal performance and availability

• If these signals are available on multiple civil frequencies there is the strong potential for vertical guidance using RAIM– Much greater utility than available today

8 December 20089

Interoperability of Integrity

• Interoperability should be a goal not just for GNSS signals, but for integrity provision as well– Augmentation systems already internationally coordinated

• Open service signals should target performance comparable to or better than GPS L1 signals today

• Different providers may make different design choices and different assurances– However, it is important to establish a common

understanding of how RAIM depends on GNSS performance and how signals from different services could be combined to improve RAIM

– Augmentation systems also benefit from new constellations

– Cooperation and transparency are essential

8 December 200810

Benefits of Multi-Constellation RAIM

• Combining signals from multiple constellations can provide significantly greater availability and higher performance levels than be achieved individually

• Provides a safety of life service without requiring GNSS provider to certify each system to 10-7 integrity levels

• Creates a truly international solution– All service providers contribute

– Not necessarily dependent on any single entity

– Coverage is global and seamless

8 December 200811

Requirements on New Signals and Constellations

• Assure good nominal signal accuracy– Of order 1 m ranging accuracy

• Perform a fault modes and effects analysis– Understand and make transparent potential

faults and their effects

• Assure low fault rates– Of order 10-5/SV/Hour

• Assure good continuity of signals– Less than 10-5/hour probability of unexpected

outages

• Assure good availability of signals

8 December 200812

Recommendation

• Agree that GNSS constellations should seek to provide open service signals of sufficient quality to support the use of multi-constellation RAIM to allow vertical guidance of aircraft

– Such signals could be incorporated into the augmentation systems as well

8 December 200813

Summary

• RAIM allows for worldwide aviation navigation without requiring additional ground infrastructure

• Additional GNSS constellations can significantly improve performance and availability

• New GNSS constellations should assure that their open service signals support RAIM

• International cooperation and coordination will be essential to achieving this goal

8 December 200814

Back up Slides

FY 3309 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 3231

Long Term Schedule

34 35 - - - 40

TBD/Unfunded L2 Semi-Codeless Transition

Solar Maximum

GPS L5FOCInitial Test

Production

GPS-III (A, B, C)

GPS-III FOCIntegrity on 14 SVs Initial Test

Production

Life-Cycle Extension

WAAS Avionics User Transition PeriodDevelopmentStandards

WAAS Phase IIIL5 Implementation

OperationalPhase IVCutoverL5 Design

User Transition TimeGPS-III (+?)

GPS-III+??FOC Integrity

Production +16 SVs

Solar Maximum

Solar Maximum

8 December 200816

Future Considerations

GLONASS

GPS

Galileo (EU)

Other?

Pathway for Aviation Use of GNSS

Single Frequency User (L1)• ABAS: RAIM (Supplemental) • SBAS: North America, Japan, Europe, India• GBAS: Operational Approval in 2009

Dual Frequency SBAS & GBAS • L1 & L5 for Iono & RFI• 24 SVs Minimum• 10-4 Failure Rate• Improved Failure Descriptions• GBAS for Category-III

2018Dual Frequency ABAS• L1 & L5 for Iono & RFI• 30+ Interoperable SVs (Multiple-GNSS?)• 10-4 Failure Rate• Improved Failure Descriptions• Open Service Safety of Life (SoL)

2030GNSS-Integrated Integrity• GPS III with Integrity (24+ SVs) or Other GNSS SoL• L1 & L5 User for Iono & RFI• 10-7 Failure Rate (Clock, Ephemeris, SDM )• Improved Failure Descriptions• GBAS for Category-III

2003

8 December 200818

Summary

• RAIM Currently Limited to Supplemental Use

• GPS With Augmentation Providing Precision Approach

• GPS Modernization Unlikely to Replace SBAS Before 2040

• Multi-Constellation GNSS Interoperability Key Enabler for ARAIM

• Interoperability of GNSS SoL Services Needs to be Coordinated