1 COROT Science Week, Paris, 13-16 May 2002 COROT mission Orbit parameters ðTwo orbit models are...

1COROT Science Week, Paris, 13-16 May 2002

COROT missionCOROT mission

Orbit parameters Two orbit models are used at system level

inertial polar circular orbit right ascension of the ascending node : = 12.5° ( ± 180 ) altitude 826 km ( a = 7204 km ) altitude 900 km ( a = 7278 km ) preferred for phase properties (orbit cycle of 7 / 14 days)

The altitude will be chosen as a compromise solution instrument/satellite performances (straylight, pointing) duty cycle (radiation fluxes) satellite-to-ground TC/TM link capacity



Orbit parameters The orbit will not be kept phased after commissioning

risk of sun glare in case of semi-major axis correction maneuver semi-major axis drift over 5 years : - 7 km (atmospheric drag) orbit period stability over 6 months : better than 1 s

Eclipse

Xs+

Thruster along Xs

Sun direction



Orientation of the satellite - flight domain

South

North

Orbit plane

Perpendicular to the orbit plane

boresightRoll angle

Sun



The sky observed by COROT

0°

30°

60°

90°

120°

150°

180°

210°

240°

270°

300°

330°

0h2h4h6h8h10h12h14h16h18h20h22h

SummerZone of observationcentered at 18h50

WinterZone of observationcentered at 6h50

Galaxy



Satellite design / axes

Zs+

Xs+

Ys+

Equipment bayUpper compartment with sensitive equipmentFine thermal regulation subsystem



Platform design “PROTEUS Evolution” family

series of 5 platforms upgraded electrical and AOCS chains

Li-Ion battery higher capacity (80 A h)

no more problem of power supply in Safe Hold Mode lower thermal dissipation

the battery sidewall can withstand any solar incidenceno need to rotate on the boresight axis after 5 months

New Magneto Torquer Bars higher capacity (180 A m2)

better convergence of the Safe Hold Mode equipment driven by a proportional control law

no more pointing disturbances due to MTB activations

Other features : new star trackers (SODERN), 2-antenna GPS



New mission schedule Thermal constraints shrunk to payload constraints

the Ys+ satellite wall (focal unit radiator) must be in the shadeas much as possible

No more 180° rotation on Xs between CP and EP No more EP2 critical thermal configuration for payload design Several possibilities for the scheduling

Exploratory Programs can be carried out either at the beginningor at the end of a 6-month period

an alternate schedule CP1, EP1, CP2, EP2 is operationally recommended

Focal unit radiator temperature worst cases in 1b and 2b 1b and 2b zones crossed by the Line of Equinoxes temperature depending on direction of observation and roll angle



Previous schedule“Peace and Love”

Line of nodes

Summer

Winter

Autumn

Solar declinationup to +23°

Ys+

Solar declinationdown to –23°

Central Program 2

Exploratory Programs 1 & 2

180° rotation on Zs

180° rotation on Xs

180° rotation on Xs


Satellite axesin a fixed orbital reference frame ROF

XJ2000

YJ2000

XOF

ZOF

Equatorial plane

12.5°

Earth orbit

Central Program 1

Line of Equinoxes

Spring

S

Xs+

Zs-

Xs+

Zs-

Ys+

Xs+

Zs-

Ys+

Zs-

Xs+ Ys+

Anticenter (6h50)Center (18h50)



Updated schedule“Apple pie”

Line of nodes

Summer

Winter

Autumn

Solar declinationup to +23°

Solar declinationdown to –23°

Central Program 2

Exploratory Programs 1 & 2



Satellite axesin a fixed orbital reference frame ROF

XJ2000

YJ2000

XOF

ZOF

Equatorial plane

12.5°

Earth orbit

Central Program 1

Line of Equinoxes

Spring

S

Xs+

Zs-

Ys+

Zs-

Xs+ Ys+

1b

1a 2b

2a

Center (18h50) Anticenter (6h50)



Performance management Performance management consists in choosing the most favorable edge for

each observing run a slight drop in periodic performances (compatible with the requirements) can be tolerated

for the EP observing runs white noise bphot = f(1/ Tobs) in Fourier space

spectrum analysis less sensitive to periodicperturbations (hidden lines) in EP runs

i 2 Ai / ( bphot (T)) 1 / Qi < 100 Hz

To define a scenario, the users shall have a series of criteria direction of observation roll angle to optimize the projection of the targets onto the CCD criticity of the thermal regulation (level, variability) function of the roll angle criticity of the straylight intensity if any



Focal unit configuration

0E2

CCD A1 CCD E1

CCD E2CCD A2 XV

YV

Left

Left

Left

Left

Right

Right

Right

Right

Buffer dump direction

Frame transfer direction

0E1

0A1

0A2

3.05°

2.70°

Ys+

Zs+



Spacecraft roll domainwinter

Ys+

Zs+

S E

CP and EP n°2

Objective : ± 20°

angle for optimum power budget : = arctan (-tan sin) = 5.25°


Spacecraft roll domain summer

Ys+

Zs+

CP and EP n°1

E S

Objective : ± 20°

angle for optimum power budget : = arctan (-tan sin) = 5.25°




Spacecraft roll domain The ± 20° requirement may prove to be difficult to meet The following points must be checked

power budget (solar flux incidence) CNESLi-Ion battery likely to improve the power budget

masking of the star trackers’ field of view by the Earth ASPIAccommodation of the SED-16 star trackers to be worked on

payload thermal constraints CNES, Soditech+20° or -20° reachable for a given observing run TBC

Set of conclusions available in September


System progress reportSystem progress report

Technical status Major instrument sub-system PDR held in the coming months

mechanical, thermal and optical architecture in progress much work on straylight rejection and thermal regulation performances

System engineering activity currently focused on command an control interfaces on-board software light curve corrections and data processing ground segment architecture

Ground Segment & System Review in November 2002

Contract with the launcherto be signed this year


AOCS performancesAOCS performances

Pointing and AOCS Stringent pointing stability requirements

coupled attitude/photometry noise if the image spot moves random : 0.5 arcsec (1 sigma) periodic : 0.2 arcsec (amplitude) for 2-ppm spectral lines in [0.1 ; 1] mHz

Instrument used for angle error measurements random and periodic sensor errors divided by 10 thermo-elastic variations between star tracker and payload frames removed

Small gaps of perturbations (< 3 % of the time) should remain during : eclipse entries/exits, MTB activations and solar panels rotations

Amplitude ( line of sight)Perturbation f

PROTEUS COROTThermo-elastic f0 1" 0"Sensor errors f0 6" 0,03"Gravity Gradient 2f0 0,08"Sensor random noise 1" < 0.08"Eclipses (transitory) 5"MTB commands 18" (could be reduced) 1999 preliminary budget



Pointing and AOCS

AOCS loop modified

ecartometric data generated by each seismology channel (frequency 1 Hz)

2 stars used by the ecartometric algorithm (least square method)

breathing corrected by real time focal length estimate

COROT payload

Gyroscopes

Star Tracker

Estimator

Kalman

Filter

Controller Actuators

Wheels

MTB

Target quaternion

Sensors

Chain 2

Chain 1

A1

A2

E1

E2

PROTEUS

1 or

2


Requirements at spacecraft levelThe PSF movement on the CCD surface is split up into 3 spacecraft rotations Random requirements (1 ) (inertia Iyy, Izz >> Ixx)

0.3 arcsec on Ys, Zs 24 arcsec on Xs

Periodic requirements (0-peak amplitude) 0.1 arcsec on Ys, Zs 4.4 arcsec on Xs

Requirements at instrument levelBased on temporary worst case estimates Random requirements (1 ) lever effect : 1,000 pixels

0.09 arcsec on Ys, Zs pixel size : 2.32 arcsec

15 arcsec on Xs

Thermo-elastic periodic requirements (0-peak amplitude) 0.06 arcsec on Ys, Zs 9 arcsec on Xs


Zs

Xs

Ys



Spacecraft dynamic simulations (1) work undertaken by CNES and ASPI

CNES as prime ASPI as industrial architect

objectives characterization of each perturbation (environment, hardware) consolidation of the requirement set reference data for further system analyses

6-month activity run in 3 steps preliminary analysis simulation software upgrade simulation campaign

results available since December 2002



Spacecraft dynamic simulations (2) preliminary analysis

kinematic filter replaced by a dynamic Kalman filter(state vector including position, speed, drift, perturbation torque)gyrometer noise divided by 3, robust for inertial pointing

choice of the reaction wheel set configuration choice of a 0.05 Hz bandwidth after noise/stability trade-off

controller noise outside the scientific bandwidth worst case identification for subsequent simulations

solar wings at 90° and Sun in the orbit plane

PASIFAE simulation software upgrade dynamic filter implementation MTB proportional control law

Simulations assessment of each external/internal perturbation torque global simulations for system analysis



Dynamic filterKinematic filter



0.05 Hz0.005 Hz

Scientific bandwidth



Random noise budgetSimulation-based

The requirements are met in any case Typical 2D value of 0.3 arcsec

Random (1 sigma)Items X Y Z

Environment Perturbations 0,38 0,115 0,102+ MTB commandsRWS command quantif ication noise 0,056 0,134 0,081Gyro Unit noise 0,466 0,019 0,018Payload noise 1,907 0,112 0,094

Typical noise = quadratic sum 2 0,21 0,162

Simulation Worst Case 3,056 0,261 0,217

Requirements 24 0,3 0,3



Scientific bandwidth Periodic noise budget The instrument harmonic

errors are not rejected 9 arcsec on Xs at 0

0.06 arcsec on Ys, Zs at 0

Many perturbation lines on Ys and Zs due to external environment gravity gradient at 20

Earth magnetic fieldeven harmonics at 20, 40, 60

Most of 2D pointing noise requirements are met

Frequency band polluted< 100 Hz



Conclusion The simulations give hope for a random noise of 0.3 arcsec (1) The duty cycle is improved (+ 2.7 %) by the removal of the MTB

periodic perturbations Despite several lines due to gravity gradient and magnetic torque

in [0.1 ; 1] mHz, the spectrum pollution is less than 100 Hz The periodic requirements should be met after sensibility study

and consolidation of the payload thermo-optical performances angle error measurement simulations in progress improvement expected from real time focal length estimate

if 6 mv 8 and 500 pixels between stars at least

Payload Requirements EstimatesRandom Ys, Zs 0.09 arcsec 0.01 arcsecRandom Xs 15 arcsec 4 arcsecPeriodic Ys, Zs 0.06 arcsec 0.01 arcsecPeriodic Xs 9 arcsec 1 arcsec



Other works in progress Optical distortion variability under assessment CNES/LAM

for seismology channel : to consolidate the angle error budget for exoplanet channel : to check the amplitude of the border/chromatic noise (in the

field of view) set of optical performances under verification point by point

Mission mode architecture study CNES/ASPI inventory of AOCS loop modifications Command & Control Transition from the PROTEUS standard mode DHU performances and channel switching feasability FDIR

Multi-mode AOCS simulator implementation CNES Safe Hold Mode simulations (Monte Carlo) validation of the Mission mode performances

1 COROT Science Week, Paris, 13-16 May 2002 COROT mission Orbit parameters ðTwo orbit models are...

Documents

Transcript of 1 COROT Science Week, Paris, 13-16 May 2002 COROT mission Orbit parameters ðTwo orbit models are...