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Using the VLBA forUsing the VLBA forSpacecraft NavigationSpacecraft Navigation

Jonathan Romney

National Radio Astronomy Observatory

VLBA 10th Anniversary

2003 June 9 – 12

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CollaboratorsCollaborators

John Benson

Walter Brisken

Vivek Dhawan

Craig Walker

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OverviewOverview

Basic Concept

Initial Tests

Proposed Pilot Project

Implementation Phase

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Basic ConceptBasic Concept

Precision Differential AstrometryUse well-established VLBA phase-referencing technique.

Image spacecraft downlink transmitter;measure (time-dependent) offset, relative to suitable

reference sources.

Close AnalogObservations of maser sources, referenced to continuum

calibrators.Primary difference: source moves relative to references.

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Basic ConceptBasic Concept

Conventional Spacecraft NavigationPrimarily 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 Technique45-baseline observation eliminates fringe ambiguity, as in

normal imaging.Provides absolute celestial coordinates directly.

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Basic ConceptBasic Concept

SynthesisPrecise 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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OverviewOverview

Basic Concept

Initial Tests

Proposed Pilot Project

Implementation Phase

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Initial TestsInitial Tests August August – – December 2002December 2002

Experimental TargetStardust.

Mission to sample and return dust from Comet Wild 2: 2004 January 2.

(Successful close flyby of Asteroid Annefrank: 2002 November 2.)

Test ObservationsOctober – December 2002.Imaged Stardust with ~5 mas resolution, 100:1 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 TestsInitial Tests

Experimental Modifications to VLBASchedules 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.

(VLBA’s near field extends to ~100 AU at X-band!)

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OverviewOverview

Basic Concept

Initial Tests

Proposed Pilot Project

Implementation Phase

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Proposed Pilot ProjectProposed Pilot Project

Study Goals – FeasibilityEvaluate 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 ProjectProposed 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 ProjectProposed Pilot Project

StatusDiscussions 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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OverviewOverview

Basic Concept

Initial Tests

Proposed Pilot Project

Implementation Phase

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Implementation PhaseImplementation 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!Thank You!

For exploiting the VLBA’s many capabilities …to achieve all the great science reported in these four days;

…

And for coming to our Anniversary Party!