Using the VLBA for Spacecraft Navigation

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Using the VLBA forSpacecraft Navigation

• Jonathan Romney
• National Radio Astronomy Observatory
• VLBA 10th Anniversary
• 2003 June 9 12

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Collaborators

• John Benson
• Walter Brisken
• Vivek Dhawan
• Craig Walker

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Overview

• Basic Concept
• Initial Tests
• Proposed Pilot Project
• Implementation Phase

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Basic Concept

• Precision Differential Astrometry
• Use well-established VLBA phase-referencing
  technique.
• Image spacecraft downlink transmitter
• measure (time-dependent) offset, relative to
  suitable reference sources.
• Close Analog
• Observations of maser sources, referenced to
  continuum calibrators.
• Primary difference source moves relative to
  references.

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Basic Concept

• Conventional Spacecraft Navigation
• Primarily based on range, measured as integrated
  velocity.
• Supplemented by single-baseline VLBI technique.
• Bandwidth-synthesis-like side tones required to
  resolve fringe ambiguity.
• Yields delay as fundamental observable.
• VLBA Multi-Baseline Technique
• 45-baseline observation eliminates fringe
  ambiguity, as in normal imaging.
• Provides absolute celestial coordinates directly.

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Basic Concept

• Synthesis
• Precise two-dimensional celestial coordinates can
  enhance range-based navigation substantially.
• Overall 3-dimensional navigation capability could
  be simultaneously more precise, less expensive,
  and operationally simpler.

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Overview

• Basic Concept
• Initial Tests
• Proposed Pilot Project
• Implementation Phase

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Initial Tests August December 2002

• Experimental Target
• Stardust.
• Mission to sample and return dust from Comet Wild
  2 2004 January 2.
• (Successful close flyby of Asteroid Annefrank
  2002 November 2.)
• Test Observations
• October December 2002.
• Imaged Stardust with 5 mas resolution, 1001
  dynamic range.
• Phase referencing only partially successful.
• Stardust was in far southern Ecliptic, at -22
  degrees declination.
• Reference sources 2 6 degrees distant.
• Successfully measured position offset once, 12
  mas relative to predicted orbit.

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Initial Tests

• Experimental Modifications to VLBA
• Schedules kluged to point to Stardust.
• Downlink schedule transmitted by fax.
• Correlator software modified to add moving source
  coordinates.
• NAIF/SPICE system previously used for moving
  station in Space VLBI correlation.
• But best ephemeris expected is equivalent to
  SVLBI predicted orbit reconstructed orbit
  would require same input we aim to provide.
• Near-field correction, already implemented in
  correlator model server, tested for first time.
• (VLBAs near field extends to 100 AU at X-band!)

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Overview

• Basic Concept
• Initial Tests
• Proposed Pilot Project
• Implementation Phase

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Proposed Pilot Project

• Study Goals Feasibility
• Evaluate feasibility of VLBA spacecraft
  navigation.
• Achievable angular precision, under various
  observing conditions, for various spacecraft
  characteristics. Goal 0.5 nano-radian.
• Enhancement of overall spacecraft navigation
  resulting from addition of VLBA angular position
  measurements.
• Determine optimal observing strategy.
• Required instrumental sensitivity and density of
  reference sources.
• Estimate impact on VLBA observing load.
• (Expected to be possible to interleave with
  ongoing observations, with minor impact.)

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Proposed Pilot Project

• Study Goals Implementation Plans / budgets
  for
• New equipment.
• 33-GHz receiver systems new spacecraft downlink
  band.
• Wideband Mark 5 recording systems essential for
  sufficiently dense catalog of reference sources.
• Replacement for correlator PBI current
  bandwidth bottleneck.
• Software upgrades.
• Scheduling pointing.
• Rapid bandwidth switching.
• Correlation model accountability.
• New operational procedures.
• Scheduling.
• Shipping.

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Proposed Pilot Project

• Status
• Discussions with NASA began in January 2003.
• NRAO proposal positively received.
• Anticipating NASA will fund Pilot Project in near
  future
• but details of how it will work still being
  worked out.

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Overview

• Basic Concept
• Initial Tests
• Proposed Pilot Project
• Implementation Phase

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Implementation Phase

• Assuming Successful Outcome of Pilot Project
• NRAO will propose implementation phase.
• Two-year duration planned.
• Implementation Project Would Include
• New equipment 3 GHz / new PBI / Mark 5.
• Software upgrades.
• Development of new operational procedures.
• Compilation of 33-GHz reference catalog.

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Thank You!

• For exploiting the VLBAs many capabilities
• to achieve all the great science reported in
  these four days
• And for coming to our Anniversary Party!