Atacama Large Millimeter/submillimeter ArrayExpanded Very Large Array

Robert C. Byrd Green Bank TelescopeVery Long Baseline Array

Interplanetary Spacecraft Navigationand Planetary Orbit Determination

Jon RomneyNRAO

VLBASUMMARY: High-Precision Angular Positions of Interplanetary Spacecraft• The VLBA excels in accuracy achievable, and in operational simplicity of

measurements.– Invaluable augmentation of primarily range-based DSN tracking.

• Positional accuracy of 1 nrad demonstrated. [1 nrad = 200 μas]– Complementary to “Delta-DOR”, DSN in-house VLBI technique.

• Can be done any time spacecraft transmitter is visible from the VLBA, in “eavesdrop” mode.

• No special on-board equipment or special scheduling required .• Resolution of phase ambiguities intrinsic to imaging technique.

• Basic technique, VLBA differential astrometry, is well-established.– Enhanced sensitivity from bandwidth expansion project will make closer

reference calibrators available.

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VLBASpacecraft Navigation Pilot Project• Funded by NASA, 2003–04.• Collaboration with various groups at JPL:

– Navigation / Delta-DOR / Missions.– Navigators essential to evaluation of value added by VLBA data.

• 13 test observations, on 6 spacecraft, primarily:– Mars landers: MER-A & -B, in final weeks and days before landing.– Mars orbiters: MGS & Odyssey.– Cassini.

• Correlation wavefront model based on JPL spacecraft ephemerides.– Also included (as standard elements applied in all observations):

• Gravitational bending in solar and planetary potential wells.• Near-field effects:

– Wavefront curvature .– Parallax.

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VLBA• Fundamental project goals demonstrated:

– Feasibility of VLBA positional measurements.– Accuracy 1 nrad; inclusion of VLBA results halved formal error in

navigators’ overall orbit fits (but not used operationally).– Residual errors attributable to inaccuracies in reference calibrator

positions and atmospheric modelling.• Other Accomplishments:

– Designed Ka-band receiver to support DSN downlinks at 33 GHz.• Adaptation of EVLA design.• Also designed X/Ka-band dual-frequency dichroic system, with

both elements fully deployable.– Designed recording system upgrade to Mark 5.

• Actual implementation limited to one station as of end of project.

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VLBA“Quasi-Operational” Participation in Cassini Mission• Mission requested assistance in measuring mass of Iapetus

– Cassini flyby on 2004/10/17.– Required to upgrade prediction of Huygens Probe trajectory during

subsequent descent to Titan.• ToO project, 5 observations over 7 days, using common set of reference

calibrators.• Mission team reported these results substantially improved their overall fit

to trajectory of Cassini’s Iapetus flyby.

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VLBAHuygens Probe Titan Descent

• The VLBA participated in worldwide VLBI observations tracking the descent of the Huygens Probe into the atmosphere of Titan.

• Scientific goal: Measure atmospheric winds and surface characteristics.– (Discussed further in subsequent paper by Gurvits.)

• Mark 5 units funded by NASA and EVN (3 each) in support of this observation were installed at 5 VLBA stations and the GBT.– A major step toward full VLBA Mark 5 implementation.– And an early VLBA partnership.

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VLBAPhoenix Encounter, May 2008• VLBA precision angular position technique developed previously was again applied

as NASA Phoenix spacecraft approached Mars.– Ten observations in 3 weeks preceding encounter on 2008/5/25.– One measurement on each of last 4 days.– Additional final observation on last day, up to termination of X-band

transmission at separation of lander.– Positional reference included Mars orbiters as well as cosmic sources.

• Lead Navigator for MSL characterized VLBA results as a valuable new resource, and said he would like to have more such data available.

• Positional uncertainties for various angular-separation scales:

•

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Typical Angular Linear Angular LinearScale Separation [nrad] [m] [nrad] [m]

MRO ↔ ODY 20 ” 0.14 37 0.51 138

Orbiters ↔ PHX 30 ’ 0.35 94 1.28 344

Spacecraft ↔ Quasars 3 ⁰ 0.87 234 3.20 861

Full VLBA Compact VLBA(8611 km) (2345 km)

VLBA

Slide credit: J. Ulvestad / V. Dhawan 8

VLBAThe Orbit of Saturn• Research motivated by success of earlier Cassini measurements.• Rationale:

– Orbits of outer planetary system not well determined.– Few high-precision measurements based on spacecraft radio tracking.– Relative, Saturn-barycentric orbit of Cassini very well known.– Extended presence of Cassini spacecraft within Saturnian system is an

unprecedented opportunity to refine its ephemeris.• VLBA astrometric study, led by D. Jones (JPL), launched in October 2006.

– Nine epochs observed to date, including first in renewed proposal.– Dual-track analyses, based on phase-referenced VLBA imaging, and on

total delays. Results closely consistent.– New positions for Saturn barycenter incorporated in updated DE422

planetary ephemeris.

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VLBA• Initial 8 runs, plus data from two earlier (2004) and one interspersed

(2009) similar observations, analyzed in recent AJ paper: http://arxiv.org/abs/1012.0264

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VLBA

Slide from D. Jones paper at AAS June 2010 meeting 11

VLBAFuture Outlook for SaturnEphemeris Enhancements• Second series of observations proposed, approved, started.• Cassini mission may be extended to 2017:

– A 20-year space mission!• Error is latitude decreases rapidly as data span approaches 1/4 of the

orbital period, about February 2013.• Next opportunity for the outer Solar System: JUNO (Jupiter orbiter),

scheduled for launch in August 2011, arrival at Jupiter in 2016

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VLBAImpact of Reduced Array – onAll Spacecraft Precision Astrometry• Is unknown.

– … but expected to be small.– Spacecraft targets, as well as desirable calibrators, are point-like.– Many additional calibrators will become available with completion of

Sensitivity Upgrade.– Still need sufficient imaging capability to resolve phase ambiguities.– NASA was interested in array of 6 southwest continental stations.

• Was to be studied as part of Pilot Project.• But funding was shifted to design of X/Ka dichroic system.

• Data available, could be studied at any time.– But to do so now would distract essential effort from completion of Sensitivity

Upgrade project.

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VLBATHANK YOU !

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