L1/L5 WAAS Support

Todd Walter

Stanford University

http://waas.stanford.edu

Todd Walter

Stanford University

http://waas.stanford.edu

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Key DFMC MOPS Components

Operational goals (1.3) Equipment classes (1.4) Constellation assumptions (B) Equipment & tracking requirements (2.1, 2.2, 2.3) Environmental assumptions (2.4) Interference environment (C ) Test requirements (2.5) Antenna requirements (separate MOPS) Signal specification (A)

L5 physical layer (A.2 & A.3) Message structure and content (A.4)

Protection Levels (J)

Recently Agreed to L5 MOPS Changes

Expanded PRN maskRemoval of fast correctionsSingle satellite correction message

Smaller quantization errors

Alert message with DFRECIsWhen more than 51 SVs correctedFull DFREs can be sent in 2 messages

Updatable DFRE tableSupport for more SBAS orbit types

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ICD Topics Still Under Discussion

L5 reversionary modeOptional ionospheric messages

Extra clock states in the G matrixSeparate state for each constellationCorrelation of tropospheric error

Clock parameters in MT-12 like message as part of correction

Identification of service regionsEphemeris content validationAuthentication of service

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L5 Reversionary Mode

The FAA is not pursuing ionospheric corrections on L5L5/E5-only RAIM should provide lateral

guidance for terminal through en route

Other ANSPs may elect to define messages to permit vertical guidance in L1/E1-denied areasThese messages are not required to

support dual frequency vertical guidance5

Separate Constellation Clock States

SBAS corrections can align the time offsets between constellationsMay require separate clock terms

Differences in signal structures and satellite filters can create receiver dependent biases

Adding a clock state per constellation addresses dynamic range and biasesSmall increase in protection levels

Recommended by RTCA + EUROCAE6

Inter-GNSS Clock Parameters

Clock offset states can be broadcast in MT-12 like message

MT-12 currently only used for timingNot currently broadcast by WAAS

Could also be applied to corrections to align GNSS timescalesCurrent offsets greater than dynamic range

of the correctionsSplits correction information across different

messages

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Identification of Service Regions

May want to assist user in finding SBAS with highest availabilityInterest expressed in specifying which

SBAS to use in certain areas

May want to broadcast information about other SBAS satellitesCurrently supported in L1, but not used

Additional messages could be defined to meet these goalsNeeds to be coordinated among SBASs

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Ephemeris Content Verification

Addresses issue of ephemeris content change without IODE change

Instead of IODE, correction message would contain a hash of full contentsNeeds a large number of bits to fully assure

unique hash (>24)Adjacent ephemerides must have different hashes

Requires more bits than easily availableLooking into alternate methods to achieve goalPerhaps a low bit count hash with the IODE

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Signal Authentication

FAA is interested in having ability to authenticate SBAS signalLevel of authentication to be studied

Signal onlyData contents

Bandwidth requirements TBDMay only require a message per minuteMay use Q-channel

Needs to be loss tolerantKey management is an issue

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Next StepsDocument agreed upon ICD elements

similar to appendices A & J of MOPSExchange documents to ensure common

understandingComment on descriptions

Identify missing detailsDiscuss options to address identified

issuesUpdate documentation with new

agreements11

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