Low Thrust Missions Guidance & Navigation: the Smart 1 ...trajectory.estec.esa.int/Astro/3rd-astro-workshop... · The SMART1 OBAN Experiment Page 18 Workshop on Astrodynamics Tools

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Page 1 Workshop on Astrodynamics Tools and Techniques

ASTRIUM

Low Thrust Missions Guidance & Navigation: the Smart 1 OBAN

experiment results & perspectivesWorkshop on Astrodynamics Tools and Techniques


ASTRIUMObNAV Overview

• Camera Sizing (for planet and asteroidencounters)

• Image Processing Modes• Navigation• Guidance• Validation using real images

– Asteroids (Pid du Midi)– Extended Objects (SMART1)

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ASTRIUMObNav Camera

• Preliminary analysis of the Autonavcamera design

– Detector technology CCD still prefered but CIS

likely to be a better choice.

– Field of View: Small field of view selected

(typically 1°)

– Cruise phase camera sizing: Between 6 and 8

kg, About 7.2 W power consuption

• Very large dynamic requirements fromcruise phase to encounter

– Cruise camera can be used for encounter

(Asteroid and Planets)

– Smaller camera sufficient for planet encounters

(like AMIE)

Window

Front Baffle

Aft Baffle

Detector8-Lenses Optical Combination

Window

Front Baffle

Aft BaffleAft Baffle

Detector8-Lenses Optical Combination


ASTRIUMInterplanetary Optical Navigation

• Potential Beacons– Sun, Planet, Asteroids

• Cruise phase:– Use distant asteroids beacons

– Two are required for 3D position estimation.

• Encounter phase:– The target body is used (and

its Moons if any)

– Good estimation of cross trackposition error, poor along trackestimation.

Vehicle position uncertainty

LOS Measurement error of beacon 1

LOS Measurement error of beacon 2

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ASTRIUMObNav Performance RequirementCruise Phase Statistical Analysis

Mag 12

Mag 12 allows to navigate up to 3 AU from the SUN

Mag 11 allows to navigate up to 2.5 AU from the SUN


ASTRIUMImage ProcessingAttitude Measurement Modes

Stars and beacon in NavCam

Beacon in NavCamStars in NavCam Stars in NavCam

Stars in STRBeacon in NavCam

Beacon brightness lower or similar to star brightness

Beacon brightness larger than star brightness, but still allowing measurement of stars with exposure time adaptation.

Beacon is too bright to enable star measurement with NavCam, the star tracker is used.

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ASTRIUMImage ProcessingBeacon Measurement Modes

COM COB

LIMB Measurement COB

Beacon is punctual and faint:Beacon position is measured using post-integration of several successive frames (MTI)

Beacon is punctual with brightness similar to stars: direction measured using same method as for stars.

Beacon is extended but of small size: Planets COM is computed based on COB measurement and a model for phase correction.

Beacon is extended and fit in the FOV.Planets COM is computed based on limb contour measurement.

Beacon is extended and do not fit in the FOV:Limb measurement mode

(Only for planets)


ASTRIUMNavigation

• The navigation solution is computed in an inertial frame centred on the target body. – Allows to avoid including the body ephemeris

uncertainties in the filter state vector, keeping it minimal.

• The selected state vector is composed of:– Position: r=(x,y,z)

– Velocity: v=(vx, vy, vz)

– Pressure radiation factor b

– Thrust scale factor k

vr&r=r

KKK ντ+

−=&

V

n

i i

i

i

iiCB m

TKrr

mb

rr

rrrr

rr p

νµµ rrr

r

r

rr

rrr&r +++

+

−−

−−= ∑=

31

333v

bb ν=&

modelnoiseECRV:,, KbV ννν

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ASTRIUMGuidance

• Two types of guidance laws have been used in the study:– Fixed Time Guidance Law in cruise phase

– Variable Time Guidance Law in encounter phase• Manœuvres are planned:

– At regular intervals in cruise phase: Typically every 7 daysduring electrical manœuvres and every 50 days on coastarcs.

– In logarithmic progressionduring encounter

1−= nn kdd

1−nd

Target

718.2== ekOptimal ratio is


ASTRIUMOn Board Software Architecture

• Navigation Experts execution is managed by the executive:– Image processing– Navigation– Guidance– ACS

• Late encounter tasks are trigerred by cyclicprocessings:– Center Of Brightness

measurement– Time Of Flight estimation

On Board SW

NavigationReal Time

ObNav Executive

Navigation Planning &

Planning Expert

ImageProcessingReal Time

Image ProcessingExperts

AOCSReal Time

Camera PlanningExpert

CameraManagement

NAVIGATION CAMERAAOCS

TM/TC

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ASTRIUMPerformance validation results with real images

Pic du Midi Smart1


ASTRIUMPIC DU MIDI ExperimentAsteroid based navigation with real images• Images of asteroids have

been collected from the pic du midi observatory withstars in the background.

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ASTRIUMPIC DU MIDI EXPERIMENTVirtual planet concept

Virtual planet

Earth

"Pic du Midi" Latitude

Earth Position = virtual orbit of "Pic du Midi"

•Build a dynamic systemwith the same trajectory ofthe observatory

•Virtual planet at « pic du midi » latitude.•Virtual spacecraftorbiting like theobservatory around thevirtual planet.


ASTRIUMArequipa Sample Image

Arequipa

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ASTRIUMPic du Midi ExperimentNavigation results


ASTRIUMPic du Midi ExperimentNavigation results• Results are good (coherent with expectations) with no

initial error• Worst results with large initial conditions mailny due to

the limited size of the arc.

Mean position error (km)

Final position error (km)

Mean velocity error (m/s)

Final velocity error (m/s)

No initial error,zero covariance matrix 0.1609 0.31771 0.02121 0.029435

No initial error,large covariance matrix

(σpos = 2000 km, σvel = 100 m/s)369.0427 453.7128 15.7738 34.8932

[-500 +500 +500] km [+8 +8 0] m/s initial error,large covariance matrix

(σpos = 2000 km, σvel = 20 m/s)485.424 163.3984 23.1338 11.6822

[-6000 +6000+ 6000] km [+60 +60 0] m/s initial error,large covariance matrix

(σpos = 10000 km, σvel = 100 m/s)1702.3863 345.1974 64.2736 18.2879

[-10000 +10000 +10000] km [+100 +100 0] m/s initial error,large covariance matrix

(σpos = 15000 km, σvel = 150 m/s)2838.9317 424.7043 110.2864 39.6694

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ASTRIUM

The SMART1 OBAN Experiment


ASTRIUMExperiment Overview

• SMART1 AMIE Camera used as a navigation camera.

• Many images have beencollected (Moon, Earth andstars)

• The navigation experimentwas limited to the only arc without EP thrust wheresufficient number ofimages for orbit estimation (six) have been collected.

OBAN7-8OBAN11-12

OBAN25-32

OBAN41-48

OBAN49-55

OBAN17-24

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ASTRIUMSmart1 Experiment


ASTRIUMSMART1 Experiment

• Navigation is based on extended objectsobservation.

• Moon and Earth are too brights to enableobservation of stars with the AMIE camera.– Star trackers shall be used to estimate pointing

direction.• One of the key performance driver is the accuracy

of the AMIE pointing direction estimation, drivenby:– Star tracker accuracy (mainly spatial noise)– Thermal stability between star tracker and AMIE.

• Calibration of AMIE/STR alignment bias shall beachieved

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ASTRIUMSMART1 ExperimentAMIE/STRs Alignment calibration

Faintest star (positive)

Faintest star (negative)

Brightest star (positive)

Brightest star (negative)

The two stars are HIP 85927 and HIP 86170. The faintest is Mag 4.27 and the brightest Mag 1.6.


ASTRIUMSMART1 ExperimentAMIE/STRs Alignment stability assessment

• Based on observation of inter STR stability• Temporal, spatial noise and thermal distortion was

assessed.• Up to 1600 µrad have been observed between

the two STRs• Thermal distorsion

wrt AMIE cameracan not be assessed(except throughnavigation errors)

Canopus

EarthMoon

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ASTRIUMSMART1 ExperimentPerformances• Thermal distortion and SMART1 operational constraints

did not allow to check performance limits.• Simulation results are in accordance with prediction

considering thermal distortion.• It is mandatory to ensure very high thermal stability

between STR and AMIE (typ <50µrad).

25 m/s440 km15km,20m/s

5 m/s230 km5km,2m/s

Velocity Error (Norm)

Position Error (Norm)

Case

Achieved Performances


ASTRIUM

From AutoNav to Don Quijote

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ASTRIUMDON QUIJOTE SPECIFICITIES

• DON QUIJOTE Mission includes two vehicles:– An orbiter learning the asteroid environment and orbit

before and after impact.– An impactor to modify the asteroid velocity through a

high velocity impact

• Impactor shall impact the asteroid at a specifiedpoint with an accuracy of 50m (3σ)


ASTRIUMDON QUIJOTE SPECIFICITIES

• Asteroid orbit uncertainties apply for the orbiterrendez-vous but removed for the impactor.– The impactor can be guided to the asteroid through

ground based radiometric navigation with an accuracy of a few tens of km.

• For the targeted relative navigation accuracy, the impactor will perform manoeuvres up to very close to impact.– No attitude manoeuvres will be possible. – 5DOF thruster configuration required (Attitude+cross

track)

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ASTRIUMDON QUIJOTE

• Thanks to the orbiter, high accuracy guidance by ground means is possible.– AutoNav can be triggered only a few hours before

impact.– Medium aperture/size navigation camera requested


ASTRIUMDON QUIJOTETargeting error allocation

NavCamFOV

Star trackeraccuracy

LOSaccuracy

Thermalstability

Thrust level

Manoeuvreduration

Lastmanoeuvre

distance

Targetingerror

1000km

50 m 3σ

20 N10µrad

<40s

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ASTRIUMDON QUIJOTE

• Although challenging, requested performances (50m 3σ) is compatible with state of the art S/C and avionics.– Small thrust level– State of the art star tracker– Medium size navigation camera– Low processing load.

• As for SMART experiment, thermal stability is likely to be the most critical but:– Apply on a few thousand seconds instead of days.– Apply for stable pointing.


ASTRIUMConclusion

• AutoNav navigation and guidance concept is fully applicable to DON QUIJOTE.

• Lesson learns on SMART1 allows to identify critical contributors.

• 5DOF thrusting capability allows to perform manoeuvres closer (100s before impact instead of 3 hours) and is the main reason for performance improvement.

• The requested 50m can be achieved with state of art sensors but are at the limit of what can be achieved with a conventional S/C.

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ASTRIUMAutonomous Navigation

• AutoNav developed the key technology allowing to perform autonomous navigation and guidance.

– Allowing to achieve level of performances not possible with ground based navigation techniques.

– Providing autonomy in low thrust cruise phases reducing the ground segment load by routine operation.

Documents

Low Thrust Missions Guidance & Navigation: the Smart 1 ...trajectory.estec.esa.int/Astro/3rd-astro-workshop... · The SMART1 OBAN Experiment Page 18 Workshop on Astrodynamics Tools