The COROT scientific mission

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The COROT scientific mission

• Overview
• Experiment designed for high accuracy relative
  stellar photometry, with long continuous
  observing runs
• Two scientific programs, working simultaneously
  on adjacent regions of the skyStellar
  seismology Search for extrasolar planets

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The COROT scientific mission

• Stellar seismology
• Focused on internal hydrodynamic processes
• Frequency analysis of photon flux oscillations
• Photometric relative accuracy of a few 10-6 in
  white light
• Scientific bandwidth 0.1 10 mHz
• Mission designed to acquire up to 10 stars
  simultaneously
• Central program (150-day observing runs)
• Accurate spectrum analysis of 50 bright stars
• resolution 0.1 ?Hz
• Main targets A, F, G, solar-like and ? Scuti
  stars brighter than mv6.5
• Secondary targets extended range of stars
  brighter than mv9
• Exploratory program (20-day observing runs)
• Statistics on excitation processes of 50 bright
  stars
• resolution 0.6 ?Hz
• Wide variety of stars up to mv9 (HR diagram
  scanned)

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The COROT scientific mission

• Search for extrasolar planets
• Focused on telluric planets
• Detection of planetary transits by occultation
• Photometric relative accuracy of a few 10-4 up to
  mv15.5 (ground integration time 1 hour)
• three-color dispersion device for chromatic
  analysis
• Mission designed to acquire up to 12 000 stars
  simultaneously
• Central program (150-day observing runs)
• criterion of phase coherence for periodic dimming
  if Tlt50 days
• criterion of achromatic event for discrimination
  against stellar activity
• targets red dwarves, F to M spectral types,
  magnitude range 12 15.5
• density higher than 1 500 targets per square
  degree
• Statistics on planet detection
• hundreds of Jupiter-like or Uranus-like planets
• between 10 and 40 telluric planets (temperature
  between 200 and 600 K)

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The COROT mission

• Mission constraints (1/2)
• Long duration for central program 150 days
• the line of sight is assigned to keep a same
  direction during 150 days
• 90 duty cycle requirement (no occultation by the
  Earth)
• Inertial polar circular orbit
• altitude between 800 and 900 km
• lower limit fixed by the terrestrial straylight
• upper limit fixed by the size of the South
  Atlantic Anomaly and the flux of protons
  acceptable by the instrument
• 826 km is preferred for phase properties (orbit
  cycle of 7 days)

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The COROT mission

• Mission constraints (2/2)
• Sun glare
• the observation are possible when the Sun is at
  more than 90 of the observed field
• Straylight from the Earth
• the line of sight must remain at more than ?
  20 from the Earth limb
• radius of the observation cone ? arccos (R/a)
  - ? 10
• to be adjusted in flight after effective
  straylight measurements
• Roll domain
• ? 20 on the boresight axis, after alignment of
  the solar arrays for the optimum of power budget
  (roughly North-South axis)
• Helpful to optimize the projection of the targets
  stars onto the CCD(to get targets out of
  smearing columns, for instance)

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The COROT mission

• Orientation of the satellite - flight domain

Sun
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The COROT mission

• The sky observed by COROT

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The COROT mission

• Satellite design / axes

Xs
Zs
Ys
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The COROT mission

• Thermal constraints
• Platform thermal constraints
• in PROTEUS normal flight conditions, the Zs-
  sidewall (NiCd battery)cannot be exposed to high
  solar fluxes for a long time
• as a minimum necessary upgrade, the battery heat
  distributor will be adapted to withstand a solar
  incidence higher than 30 (Flt190 W/m2)
• rotation on the boresight axis Xs before or after
  5 months
• Payload thermal constraints
• the Ys satellite wall (focal block radiator)
  must be in the shadeas much as possible
• Ys exposed to the Sun when the Earth is close to
  the Line of Equinoxes (low solar declination,
  high solar azimuth)
• Taken together, these constraints lead to only
  one possible mission schedule
• 4 rotations in a year
• cycle CP1, EP1, EP2, CP2

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The COROT mission
180 rotation on Zs

• Mission schedule

Spring
Line of Equinoxes
Line of nodes
Satellite axes in a fixed orbital reference frame
ROF
Summer
Earth orbit
Solar declination up to 23
Central Program 2
Central Program 1
S
ZOF
YJ2000
XJ2000
XOF
Winter
Exploratory Programs 1 2
Solar declination down to 23
Ys
Equatorial plane
180 rotation on Xs
180 rotation on Xs
12.5
Autumn
180 rotation on Zs
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The COROT mission

• Proposal for an orbit plane drift (1/2)
• the RAAN is nominally 12.5 0.2
• inclination maneuver after the launch if
  necessary
• inclination maneuver after 2.5 years
• if not strictly polar (i90?i), the orbit plane
  drifts at d?/dt
• the position of the target stars is given as a
  setpoint and the satellite slowly rotates to
  compensate the apparent movement of the sky
• at every moment of an observation phase, the line
  of sight must remain inside the observation cone
  (straylight constraint)
• the authorized observation zone is reduced to the
  intersection of two circles (useful zone
  autocorrelation function)

?(t-t0)
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The COROT mission

• Proposal for an orbit plane drift (2/2)
• examples
• If ?i 0.1, d?/dt 4/year ?V 14 m/s ?
  drifts toward the scenario 2 (7h50)
• if ?i -0.1, d?/dt -4/year? drifts toward
  the scenario 1 (6h10)
• if the observations are well ordered, it allows
  to observe some stars rejected by the choice of
  the orbit plane at ? 6h50
• The drift must be anticipated
• launcher requirements
• thermal studies (drift towards the scenario 1
  technically preferred because of lower Ys
  sidewall fluxes)
• mission analysis and ?V budget
• Decision to be taken by the next Scientific
  Committee

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The COROT mission

• Mission phases
• Launch
• Commissioning Closed by the
  In-Flight Assessment Review after 60 days
• Early Operations Phase (EOP) and satellite
  engineering assessment
• Beginning of Life Calibrations (obturator closed
  then open)
• First station acquisition
• Station acquisition 7 days
• transition - step by step - to the COROT pointing
  mode
• setting up the instrument for both observation
  programs
• Observation 150/20 days
• 4 observation phases in a year CP1, EP1, EP2,
  CP2
• periodically interrupted by solar panels
  rotations and calibrations
• Satellite orientation and housekeeping 1 day
• 4 rotation maneuvers in a year, coupled with
  orbit correctionmaneuvers (semi-major axis) if
  any

Repeated once in operational context
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The COROT mission

• Instrument modes overview

Station acquisition phase
Transition by TC
Transition after anomaly
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On-board treatments

• Photometric chains
• 2 photometric chains
• DPU BEX BCC proximity electronics CCD A
  CCD E
• nominally work in parallel
• 2 channels in a chain
• seismology (integration time 1 s)
• exoplanets (integration time 32 s)

Digital Treatment Units
DPU1
BEX1
BCC1
CCD1A
CCD1E
Chain 1
CCD2A
DPU2
BEX2
BCC2
CCD2E
Chain 2
CS16
TC 1553
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On-board treatments

• Instrument modes
• a mode is defined as a combination of
• the equipment power supply
• the configuration of every channel (seismology,
  exoplanets)
• the software services running
• seismology channels can be in 4 configurations
• Standby
• CCD complete readout IMA
• Large windows (binned) memory plane PMG
• Scientific memory plane PMS
• exoplanets channels can be in 3 configurations
• Standby
• CCD complete readout IMA
• Scientific memory plane PMS

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On-board treatments

• Channels configuration

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On-board treatments

• Constraints
• The total scientific telemetry volume is 900
  Mbits/day
• received by a dedicated S band antenna
  (Villafranca)
• PLTM packets data rate 550 kbits/s
• Complete images cannot be downloaded in a
  permanent way
• Solutions
• CCD readout is windowed
• windowing described in uploaded memory planes
  (PMG, PMS)
• at camera control and extraction units level for
  seismology channels
• at extraction units level only for exoplanets
  channels
• Photometry is integrated on-board within
  pre-defined masks
• CCD complete readout restricted to specific
  operations
• station acquisition
• calibrations

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On-board treatments

• Seismology programThe on-board photometric
  chains are designed to process, for each CCD
• 5 star windows
• maximum size of the window 50x50 pixels
• typical surface of the star mask 350 pixels
• two aperture photometry methods available
• fixed threshold for isolated bright stars (mv lt
  7)
• fixed mask preferred for polluted faint stars
• the image of 2 stars among five can be downloaded
  in 25x25 TM masks,if accumulated during 32 s
• 5 sky reference windows
• maximum size of the window 5 000 pixels
• binning of columns to be chosen among 5 values
  (lt100)
• 2 offset reference windows
• 1 024 exit pixels of each register

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On-board treatments

• Exoplanets program (1/2)The on-board photometric
  chains are designed to process, for each CCD
• 5 000 stars within chromatic masks (three colors)
• maximum surface of the mask 150 pixels
• photometry integrated over 32 x 32 s (17 min)
• up to 36 targets oversampled on request (in case
  a transit is expected)
• 1 000 stars within monochromatic masks
• maximum surface of the mask 150 pixels
• photometry integrated over 32 x 32 s (17 min)
• masks used for faint and cold stars, as well as
  for background references
• 9 small sky reference images
• maximum surface of the image 150 pixels
• acquired every 32 s, without accumulation
• 2 offset reference windows
• 1 024 exit pixels of each register

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On-board treatments

• Exoplanets program (2/2)
• The masks have to be selected among a pre-defined
  list of uploaded patterns (TC constraints)
• The shape of the mask is a function of several
  parameters
• the magnitude of the star
• the temperature of the star
• the position on the CCD (space variations of the
  PSF)
• the contamination of the target by closeby stars
• 256 patterns should allow to fit every possible
  target
• System tool to be developed (EWG) to determine
  the optimal programming of an exoplanet observing
  run
• Simulations of representative fields to be
  carried out