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Published on January 16, 2008

Author: Margherita

Source: authorstream.com

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Using the VLBA for Spacecraft Navigation:  Using the VLBA for Spacecraft Navigation Jonathan Romney National Radio Astronomy Observatory VLBA 10th Anniversary 2003 June 9 – 12 Collaborators:  Collaborators John Benson Walter Brisken Vivek Dhawan Craig Walker Overview:  Overview Basic Concept Initial Tests Proposed Pilot Project Implementation Phase Basic Concept:  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. Basic Concept:  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. Basic Concept:  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. Overview:  Overview Basic Concept Initial Tests Proposed Pilot Project Implementation Phase Initial Tests August – December 2002:  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, 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. Initial Tests:  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. (VLBA’s near field extends to ~100 AU at X-band!) Overview:  Overview Basic Concept Initial Tests Proposed Pilot Project Implementation Phase Proposed Pilot Project:  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.) Proposed Pilot Project:  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. Proposed Pilot Project:  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. Overview:  Overview Basic Concept Initial Tests Proposed Pilot Project Implementation Phase Implementation Phase:  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. 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!

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