Critical Monitoring of the

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Critical Monitoring of the Cassini Saturn
Orbit Insertion Maneuver S. W. Asmar, D. V.
Johnston, E. Maize, R. T. Mitchell Jet
Propulsion Laboratory, California Institute of
Technology 19 May 2004 Montreal, Canada 
   
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May 5, 2004
May 5, 2004
3.44 billion km so far 29.7 million km to go
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Communication Strategy DuringSaturn Orbit
Insertion

• Prior to ascending ring plane crossing (ARPC)
• Switch to the low gain antenna
• Used as a beacon during burn
• Turn telemetry modulation off
• Increase carrier power
• Turn spacecraft to place high gain antenna into
  dust ram direction
• Becomes dust shield
• Cross the ring plane and turn to the initial burn
  attitude
• Perform the Saturn Orbit Insertion (SOI) engine
  burn
• After the burn, switch to the HGA for a brief
  call home
• Perform post-SOI science observations without
  communications
• Turn to descending ring plane crossing (DRPC)
  attitude
• Then turn to earth and playback SOI data

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Orbit Insertion Characteristics

• Burn start July 1, 112 UTC
• 0735 PM PDT June 30th
• Burn duration 96.4 minutes
• Turn rate 0.008/sec
• Rotation during burn 46
• Nominal ?V 626 m/s
• Periapsis July 1, 239 UTC
• Burn end July 1, 248 UTC
• Ring plane crossings
• July 1, 0047 0434 UTC

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SOI Communication Conditions

• Low signal levels
• Low gain antenna
• Attitude optimized for engine burn not for
  antenna pointing
• Interruptions during occultation by rings
• Frequency Dynamics
• Gravity of Saturn Doppler shift 400 kHz
• Engine burn Doppler shift 11 kHz at 2.5
  Hz/sec
• Timing errors manifested as deviations from
  predicted signal frequency
• Minimizing timing uncertainties allow using
  narrower receiver bandwidth
• Other conditions
• One-way light time 1 hour and 24 minutes
• Near solar conjunction

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Ring occultation not shown
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Signal Acquisition Strategy

• Standard services of Deep Space Network
• Tracking receiver
• May lose lock if level drops below threshold
• Radio Science Receiver (RSR)
• Open-loop receiver (no lock) tuned by prediction
  file
• Enhanced Radio Science Processor
• Custom-made for SOI enhanced visualization
  monitoring at JPL
• Backup special application processor (developed
  for rover landing)
• Fast FFT computations of RSR data under low
  signal conditions
• Carrier arraying
• 70-m 2 34-m stations. Increase 1.5 dB

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Radio Science Receiver

• Open-loop receiver tuned via frequency
  predictions derived from navigation solution
• Configured from JPL
• Four receivers available at Canberra complex
• Flexibility due to user-selected recording
  bandwidths sampling
• Multiple bandwidths simultaneously
• Multiple predicts simultaneously
• Tuning predictions from tracking services
  cross-checked with predictions from science team
  software which incorporates atmospheric model

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Radio Science Receiver Configuration

• RSR1A (data used for enhanced processing)
• Frequency predict includes model of engine burn
• Records bandwidths 1, 2, 4, and 50 kHz (RCP)
• RSR1B
• Frequency predict includes model of engine burn
• Records bandwidths 1, 2, 4, and 50 kHz (possibly
  LCP)
• RSR2A (Array and EDA)
• Frequency predict includes model of engine burn
• Records bandwidths 1, 2, 4, and 50 kHz (RCP)
• RSR2B
• Frequency predict do NOT include engine burn
• Records bandwidths 1, 2, 16, and 100 kHz (RCP)

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Navigation Deliverables

• Navigation team refining model
• Cruise data
• Trajectory correction maneuvers
• Error analysis
• Navigation to deliver uncertainty analysis with 3
  sigma error
• Agree on baseline updates of trajectory files

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Enhanced Radio Science Processor

• Real-time analysis of RSR data via Enhanced Radio
  Science Processor (developed by Radio Science
  Systems Group)
• User-selectable integration time, number of
  averages, etc.
• Spectra via Fast Fourier Transform
• Signal level and frequency plotted over duration
  of activity
• Display pseudo-resids difference between
  actual received frequency and case of no-burn to
  see the effect of the main engine firing

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Blue no burn Yellow burn Red actual
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SOI Demonstration

• Verification and validation
• Spacecraft and Deep Space Network systems
• RSR acquired Cassini signal in a demo Summer 2003
• Spacecraft was turned but no engine burn
• Two sets of frequency predicts used
• Nominal trajectory accurately model expected
  signal frequency received at ground station
• Model expected Doppler shift due to the engine
  burn
• Deviation 9 kHz from over 95 minutes
• Average drift rate of 2 Hz/sec

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Demo Data
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Demo Data
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Conclusion

• Placing the Cassini spacecraft in orbit around
  Saturn is challenging but well-planned
• Demonstrated process in place to receive
  communication indicators during the event
• Navigation data continually updated to minimize
  uncertainties in timing and frequency prediction
• Precision Doppler instrumentation in the Deep
  Space Network ready to support critical
  monitoring
• Special Radio Science displays allow managers and
  public to assess progress and success

Orbital tour of Saturn, its satellites and
environment starts on 1 July