Nutation in Ulysses Third Orbit

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Nutation in Ulysses Third Orbit

• Daniel Presti, Colin Goulding

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Presentation Overview

• The Ulysses mission
• Ulysses nutation anomaly
• Nutation in the third orbit
• Analysis of the past nutation season and
  predictions for the upcoming season
• Operations concepts

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Mission Overview

• 6 October 1990 Launch (Space Shuttle Discovery)
• 1992 Jupiter flyby and injection into polar
  orbit

• 1994-1995 First set of polar passes
• 2000-2001 Second set of polar passes
• 2006-2008 Third set of polar passes

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The Ulysses Spacecraft

• Spin-stabilised at 5 rpm
• Thermally isolated
• Two internal platforms
• Power provided by RTG
• Two 35 m wire booms
• One axial boom
• One radial boom
• Attitude control provided by hydrazine thrusters
• High-gain antenna (1.65 m) for X-Band downlink
  and S-Band uplink

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Instruments and Investigations
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The Nutation Anomaly

• Caused by thermal distortions of the axial boom,
  when partially illuminated by the Sun
• Occurs around perihelion
• Spacecraft wobbles about its spin axis,
  coupling energy into wire booms

• Can grow exponentially, if left uncontrolled

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Nutation Forcing Function (NFF)

• Quantifies the intensity of nutation (build-up
  rates)
• Based only on solar aspect angle (SAA) and range
  (R)

• SAA determines the fraction of the boom
  illuminated by the Sun
• R determines the intensity of the incident
  radiation

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Nutation Control Closed Loop CONSCAN

• Onboard attitude control system
• Requires a stable uplink from a ground station
  for closed-loop control
• Estimates the Earth off-pointing (amplitude and
  phase) by measuring physical properties of the
  receivers AGC signal (conical scanning)
• Fires thrusters to re-align the spin axis with
  the Earth
• Reduces the Earth off-pointing
• Damps the nutation (secondary effect)
• Two pre-defined selectable deadbands (wide and
  narrow)

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The Third Orbits Nutation Season

• From February 2007 to February 2008
• NFF comparable to second orbits season
• Lower RTG power
• Minimal power margin
• Mutually-exclusive operations
• Reduced DSN ground station coverage
• Large gaps with uncontrolled nutation build-up

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Analysis of Previous Nutation Season

• Based on high-rate attitude data from the 2001
  season (instantaneous Earth off-pointing and
  nutation angle)
• Empirical characterisation of the nutation
  build-up rates and CONSCANs nutation damping
  performance

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MAGIC

• Maximum Allowable Gaps In CONSCAN operations have
  been defined, based on predicted nutation
  build-up rates

• As gaps in nutation control increase
• Nutation increases above acceptable levels
  (science-defined)
• Loss of downlink due to off-pointing
• Severe increase in unwanted oscillations in wire
  booms
• Loss of spacecraft?

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Ground Station Coverage Requirements

• Maximum acceptable nutation limit defined as 0.5
  half-cone (science-driven)
• Resulting gaps in station coverage based on
  maximum gaps in CONSCAN, minus an operational
  overhead

• Almost-continuous coverage required at the peak
  of the nutation season

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Typical Nutation Operations Sequence

• Two nutation-control CONSCAN session per pass
• Uplink of commands during CONSCAN sessions
• High-rate playback of recorded data in between
  CONSCAN sessions
• Gap in coverage (lt MAGIC) with nutation build-up
  and data recording

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DSN Visibility

• DSN designed primarily for in-ecliptic spacecraft
• Non-continuous visibility between March and July
  2007

• DSN gap exceeds maximum allowable gap!

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Use of Kourou

• The ESA Kourou station can fill the gap almost
  entirely

• Negotiations for the use of other stations for
  nutation control are currently taking place

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Conclusions

• The Ulysses extended mission operations are
  challenging due to the diminishing resources
• The nutation anomaly increases the challenge
  significantly
• Nutation control has to take place with reduced
  coverage and mutually-exclusive spacecraft
  operations
• A detailed characterisation of the past nutation
  season took place, leading to
• An improved understanding and quantification of
  the nutation phenomenon
• A proven need for additional ground resources
  during critical phases