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L1/L5 WAAS Support

L1/L5 WAAS Support. Todd Walter Stanford University http://waas.stanford.edu. 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)

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L1/L5 WAAS Support

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  1. L1/L5 WAAS Support Todd Walter Stanford University http://waas.stanford.edu

  2. 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)

  3. Recently Agreed to L5 MOPS Changes • Expanded PRN mask • Removal of fast corrections • Single satellite correction message • Smaller quantization errors • Alert message with DFRECIs • When more than 51 SVs corrected • Full DFREs can be sent in 2 messages • Updatable DFRE table • Support for more SBAS orbit types

  4. ICD Topics Still Under Discussion • L5 reversionary mode • Optional ionospheric messages • Extra clock states in the G matrix • Separate state for each constellation • Correlation of tropospheric error • Clock parameters in MT-12 like message as part of correction • Identification of service regions • Ephemeris content validation • Authentication of service

  5. L5 Reversionary Mode • The FAA is not pursuing ionospheric corrections on L5 • L5/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 areas • These messages are not required to support dual frequency vertical guidance

  6. Separate Constellation Clock States • SBAS corrections can align the time offsets between constellations • May 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 biases • Small increase in protection levels • Recommended by RTCA + EUROCAE

  7. Inter-GNSS Clock Parameters • Clock offset states can be broadcast in MT-12 like message • MT-12 currently only used for timing • Not currently broadcast by WAAS • Could also be applied to corrections to align GNSS timescales • Current offsets greater than dynamic range of the corrections • Splits correction information across different messages

  8. Identification of Service Regions • May want to assist user in finding SBAS with highest availability • Interest expressed in specifying which SBAS to use in certain areas • May want to broadcast information about other SBAS satellites • Currently supported in L1, but not used • Additional messages could be defined to meet these goals • Needs to be coordinated among SBASs

  9. Ephemeris Content Verification • Addresses issue of ephemeris content change without IODE change • Instead of IODE, correction message would contain a hash of full contents • Needs a large number of bits to fully assure unique hash (>24) • Adjacent ephemerides must have different hashes • Requires more bits than easily available • Looking into alternate methods to achieve goal • Perhaps a low bit count hash with the IODE

  10. Signal Authentication • FAA is interested in having ability to authenticate SBAS signal • Level of authentication to be studied • Signal only • Data contents • Bandwidth requirements TBD • May only require a message per minute • May use Q-channel • Needs to be loss tolerant • Key management is an issue

  11. Next Steps • Document agreed upon ICD elements similar to appendices A & J of MOPS • Exchange documents to ensure common understanding • Comment on descriptions • Identify missing details • Discuss options to address identified issues • Update documentation with new agreements

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