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DFMC. Satellite Selection Discussions. IWG # 26. Jed Dennis and Mark Hemstad. Jason Burns. Feb 5-7, 2014. Background. Legacy L1 SBAS: DO-229 Section 1.3.3

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Satellite Selection Discussions

IWG # 26

Jed Dennis and Mark Hemstad

Jason Burns

Feb 5-7, 2014

  • Legacy L1 SBAS: DO-229
    • Section 1.3.3

“The operational concept for GNSS and Space-Based Augmentation Systems is predicated on the combination of the different GNSS elements without pilot intervention. As GNSS is a global system, there should be no flight crew interaction based on airspace, so that the flight crew should not be involved in the selection of different SBASs”

  • SARPS Section 6.2.5 and 6.2.7

“while the State has responsibility to approve the use of one or more SBAS signals in its airspace, due to the inability of current equipment to deselect particular SBAS, States might effectively preclude use of SBAS if the State does not approve use of all SBAS. ”

    • Would presence of 3-4 SBAS deter State approval?
sbas in 2013
SBAS in 2013

Minimal coverage overlap with little to no overlap in service areas

SBAS Selection

  • En Route: No selection guidance
  • Terminal: No selection guidance
  • Approach: SBAS identified in FAS data block
future sbas
Future SBAS
  • Changes with DFMC
    • Direct avionics mitigation of ionosphere
    • Addition of GAGAN and SDCM
    • Potential to use additional core constellations
  • Results in significant regions with 3 SBAS, some regions with 4 SBAS
  • Overlap of coverage in SBAS Service Areas
reference problem
Reference Problem

SBAS Service Areas

Supported by reference network

SBAS # 3

SBAS # 2

Flight Path

  • Coverage from three available SBAS
  • Flight path through airspace of three SBAS providers
  • Which SBAS to use when?

SBAS # 1

GEO coverage arcs

dfmc sbas selection
DFMC SBAS Selection
  • Questions
    • Is SBAS selection required? Desired?
      • DFMC capable of meeting LPV requirements in most of use area
        • Horizontal Alert Limit (HAL): 40 m, Vertical Alert Limit (VAL): 35 m
      • Easily meets en route and terminal requirements
        • En route HAL: 2 nm; Terminal HAL: 1 nm
      • Suggests any SBAS service would be sufficient to meet Performance Based Navigation (PBN) requirements
        • Assumes SBASs provide similar performance
    • Will future operations require tighter horizontal or vertical performance?
      • Automated Dependent Surveillance – Broadcast (ADS-B)
      • Trajectory Based Operations (TBO)
    • How well can automated selection means match SBAS Service Areas?
    • Will standardized selection criteria help with State approval of SBAS?
sbas selection options
SBAS Selection Options

Broadcast Integrity Parameter Methods

Pre-defined Methods

  • Use corrections from one SBAS at a time
  • Select SBAS prior to GEO or ranging source selection
  • Avionics may use any ranging source corrected by SBAS
    • Not required to use all corrected ranging sources
  • SBAS selection independent of ranging source selection
    • Avionics able to track sufficient number of SVs corrected by SBAS
    • Expect can be guaranteed if any of the following are true
      • Each SBAS broadcasts corrections for common core constellation(s)
      • All GNSS broadcast at same frequencies (ie L1/E1 and L5/E5a)
      • Avionics can track all core constellations
  • Used Stanford University MAAST tool
    • DF mode of operations
    • Each SBAS run separately
    • Post-processed for selection assessment
    • Uses all in-view satellites
  • Scenario parameters
    • 24 1-hour time epochs
    • 2 deg by 2 deg grid
  • Constellations
    • GPS: 24 SV DO-229 MOPS constellation
    • Galileo: 27+3 Walker 56°:27/3/1 constellation
  • SBAS Use Areas
    • Areas in which there are or expect will have SBAS Service Areas

Note: Simulation results presented at ION ITM 2014

assessment metrics
Assessment Metrics
  • SBAS selection
    • Percent of associated SBAS Use Area in which associated SBAS was selected every epoch
    • Transition Area: Percent of world in which more than one SBAS was selected over the course of a day
    • Predefined: Percent of associated SBAS Use Area in which designated SBAS meets availability requirements
  • Availability
    • Percent of the world in which selected SBAS provided RNP 0.3 service at least 95% of the time
    • For pre-defined methods, percent of world in which at least one SBAS by itself provided RNP 0.3 service at least 95% of the time
hpl selection criteria
HPL Selection Criteria
  • Choose SBAS based on minimum Horizontal Protection Level



nsv selection criteria
NSV Selection Criteria
  • Choose SBAS that corrects the largest number of satellites observed by the user (Number of Common SVs)



geo selection criteria
GEO Selection Criteria
  • Choose SBAS based on highest GEO elevation angle



d dfre selection criteria
dDFRE Selection Criteria
  • Choose SBAS based on MT-28 covariance (dDFRE)
    • Identify SBAS that has best dDFRE for each satellite
    • Select SBAS that has largest number of selected satellites



d dfre gps and galileo
dDFRE, GPS and Galileo

Smaller transition regions


GPS + Galileo

Expanded Coverage

GPS + Galileo


Second constellation improves results

other methods
Other methods
  • Based on individual performance of SBAS in simulation

Availability lower than best automated selection method

comparison of assessed methods
Comparison of assessed methods
  • Protection Level
    • Provides best results (SBAS selection, availability)
    • Requires processing of multiple SBAS / GEOs
      • Processing requirements
      • Results in requirement for at least 4 GEO tracking channels
        • 2 for Selected SBAS, 2 for alternate SBAS
  • δDFRE
    • Requires sufficient number of ranging sources for positive selection
    • Potential to improve method if prefer over protection level method
  • Number of common SVs
    • Many regions lack clear selection
    • Method requires refinement
  • GEO elevation angle
    • Not for SBAS selection, okay to chose GEO once SBAS selected

HPL Method seems best

  • Broadcast Integrity methods sufficient for SBAS selection
    • Automated, no pilot intervention
    • Slight improvement in availability
    • Protection level assessment has best performance, but other methods are close and acceptable
  • Performance improved with more ranging satellites
  • Trade-off between pilot responsibility, maintenance responsibility and computational burden for SBAS avionics
  • Need to reach concurrence on methods prior to working specific requirements
sbas systems
SBAS Systems









vpl selection criteria
VPL Selection Criteria
  • Choose SBAS based on minimum Vertical Protection Level



sbas selection dependency on flight mode
SBAS Selection Dependency on Flight Mode

Flight Mode

SBAS Selection


  • Requires reassessment during flight
  • Provide Guidance


  • Manufacturer Implementation






Transition to SBAS identified by FAS




Per FAS data block

LPV / LPV-200

sbas assessment triggers
SBAS Assessment Triggers
  • Event Based
    • Mismatch in number of tracked versus augmented SVs
      • Indicator of distance from SBAS Reference Stations (SRS)
        • Good distinction in East-West direction
        • Poor distinction for North-South when multiple SRS in same hemisphere
    • Change in longitude or latitude
      • Set based on distance between SRS and flight speeds
    • Distance flown
      • Set based on distance between SRS and flight speeds
  • Periodic
    • Set time interval
      • Set based on distance between SRS and flight speeds
constellation selection
Constellation Selection
  • Augmented by SBAS in use
  • Augmented by another available SBAS
    • Requires switching SBAS
    • Permitted if not restricted by provider identified in FAS data block
  • As able to support RAIM
ranging source selection
Ranging Source Selection
  • All in view for constellation(s) in use
    • Simplicity, does not require complex algorithm for satellite selection
    • Best for RAIM integrity
    • SBAS integrity uses subset with valid corrections
  • Selection based on tracking limit of equipment
    • Support for operation and integrity method
    • If multi-constellation RAIM, will need an additional SV(s) to resolve constellation biases
  • Goal is to maximize availability en-route, ensure selection of the matching service provider sufficiently before commencing final approach