Galileo Avionica- Precision Star Tracker

ESTEC, NoordwijkThe Netherlands6-th March 2003

HYPERIndustrial Feasibility Study

Final Presentation

Precision Star TrackerActivity 3, WP 3100

2 6-th March 2003, ESTEC, Noordwijk, The Netherlands

Agenda

Introduction

1 PST Requirements

2 PST CCD Characteristics

3 PST System Trade-off

4 PST Baseline Configuration

5 PST Optics

6 PST internal baffle

7 PST Accuracy

8 Guide Star Catalogue

9 Conclusions


Introduction 1/2

� PST purpose� To allow the measurement of the Lense-Thirring effect

– This measurement is performed as relative measurement between thePrecision Star Tracker (PST) giving angles between a guide star, fixed ininertial space and an atomic gyroscope direction, which has an extremelyhigh short time sensitivity for rotation rates (angular rates).

– The PST is directed to far-distant guide stars, which are not affected bythe Lense-Thirring effect. They represent a reference for the measurementand for the motions of the satellite and its control.

– The second measurement is performed with an Atomic Sagnac Unit(ASU), which measures the rotations of freely falling atoms relative to aseries of laser beams, whose orientation is rigidly linked to the PSTboresight


Introduction 2/2

� Today GA has the capability to provide Star Sensors for a widevariety of mission requirements and applications, ranging from highaccuracy pointing of scientific instruments and platform, to mediumFOV sensors with AAD capability

� The required accuracy of the HYPER PST is about 1000 times moresevere than most accurate GA star trackers (ISO, XMM).

� This makes this study very challenging


PST Requirements 1/4

� The PST requirements, level 1 from HYP-2-05, are the following:

Req.# Requirement Value (3�)

R1-PST-01 PST internal errors in the frequencyrange between 3.5*10-5 Hz and 5 Hz.

< 1.2 * 10-8 rad(< 2.48 * 10-3 arcsec)

R1-PST-02 External measurement errors (star,aberration, etc) in the frequencyrange between 3.5*10-5 Hz and 5 Hz.

< 1.2 10-9 rad(<0.25 * 10-3 arcsec)

R1-PST-03 PST internal errors in the frequencyrange below 3.5*10-5 Hz

< 1.2 * 10-9 rad(< 0.25 * 10-3 arcsec)

R1-PST-04 External measurement errors (star,aberration, etc) in the frequencyrange below 3.5*10-5 Hz.

< 1.2 * 10-9 rad(< 0.25 * 10-3 arcsec)

R1-PST-05 Timing/Jitter 1 ms



� Level 2 from HYP-2-05� R1-PST-01

R2-PST-01-01 � Optical Distortion Residual Error < 10-10 rad(< 0.02 * 10-3 arcsec)

R2-PST-01-02 � Focal Length Variation with Temperature < 10-10 rad(< 0.02 * 10-3 arcsec)

R2-PST-01-03 � Focal Length Variation with Star Colour < 10-10 rad(< 0.02 * 10-3 arcsec)

R2-PST-01-04 � Photo Response Non-Uniformity Effect onStar Signal and Straylight

< 4.1 * 10-9 rad(< 0.85 * 10-3 arcsec)

R2-PST-01-06 � Dark Current Non-Uniformity < 2.9 * 10-9 rad(< 0.6 * 10-3 arcsec)

R2-PST-01-07 � Centroiding Algorithm Error < 8.2 * 10-9 rad(< 1.7 * 10-3 arcsec)

R2-PST-01-08 � Arithmetic Round-Off < 1.4 * 10-9 rad(< 0.29 * 10-3 arcsec)

R2-PST-01-09 � Noise Equivalent Angle < 6.8 * 10-9 rad(<1.4 * 10-3 arcsec)



� Level 2 from HYP-2-05� R1-PST-02

� R1-PST-03– No PST internal low frequency errors have been identified

� R1-PST-04

� R1-PST-05– No 2nd level errors have been identified

R2-PST-02-01 � Relativistic Aberration < 1.2 * 10-9 rad(< 0.25 * 10-3 arcsec)

R2-PST-04-01 � Star Proper Motion < 10-5 rad(< 2.06 arcsec)

R2-PST-04-02 � Star Parallax Error < 10-5 rad(< 2.06 arcsec)

R2-PST-04-03 � Star Catalogue Error < 10-5 rad(< 2.06 arcsec)



� Other Requirements� PST optics to fit within the following dimension:

– 387x387x700 mm (boresight)

� PST Update rate– 10 Hz

� PST Optical entrance– 190 mm

� PST Guide star catalogue– To have at least 1 guide star always available


PST CCD Characteristics 1/1

� To obtain typical value the following characteristics of the ATMELTH7890 (used by GA ASTR) have been taken into account. Its maincharacteristics are:� Full Well Capacity 2*105 electrons (17 micron pixel size)� dark current 15 pA/cm2

� Quantum Efficiency See figure


PST System Trade-off 1/3

� To identify the baseline PST configuration the following guidelineshave been followed:� IFOV REDUCTION. In order to reduce the contribution of all errors that

can be characterised in terms of fraction of pixels such as Centroidand NEA: this can be obtained by a longer focal length.

� INCREASE OF CCD FWC (Pixel size). In order to avoid CCDsaturation

� CONSIDER LARGE TRACKING MATRIXES. In order to match largePSF produced by high F number (considered odd rows/columnsmatrixes from 3x3 up to 25x25 pixels) and reduce Centroid error interms of fraction of pixel

� Use of GA simulator to evaluate PST performance, especially in termsof Centroid error and NEA



� Used Centroid algorithm:

�

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Col

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1,2,...Ni)1(*)1(*9'

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� N value is tied to the star spot size� Increasing N means to decrease the centroid error

– the ratio pixel size / spot size is reduced: a smaller sampling period is obtained andthen the rounding effect introduced by the physical pixel dimension is reduced

� Increasing N gives rise to greater sensitivity to CCD non-uniformities andnoise

– more pixels and then more error contributions (1 for each pixel)– new outer pixels having a higher weight in the barycentre computation– the centroid computation cannot be considered an “average computation” because

the denominator of is not proportional to the total number of pixels but to thefraction of the star spot energy collected by the tracking matrix

�#**44.2min fd �



� To identify the best N value the following simulation steps havebeen performed:� Increase focal length to reduce the IFOV� Check if the pixel size is able to contain all star signal, otherwise

increase it� Consider N=N0 = and evaluate performance in

terms of Centroid error and NEA� Find optimum N value: increase N value and evaluate if best

performance in terms of Centroid error and of NEA degradation havebeen obtained, then decide if to continue increasing N or not

� Check if Centroid error and NEA are within the requirement� Repeat all from the beginning� stop when increasing focal length does not produce significant

improvement in performance (too low signal to noise ratio)

�#**44.2min fd �


PST Baseline Configuration 1/1

� The identified baseline PST configuration is the following:� Optical configuration: Ritchey-Chretien telescope� Focal length 36 m� F number 190� FOV ±25 arcsec� CCD number of pixels 1024x1024� CCD pixel size 13 micron� IFOV 0.074 arcsec� Integration time 100 ms, jitter < 1 ms� magnitude range 2 � V � 4� Centroid algorithm Based on a 17x17 pixels tracking window


PST Optics 1/5

� The principal aim of the optics study has been to find a configurationwith the smallest number of elements, reducing as small as possiblethe criticality of position errors of the optical elements

� PST OPTICS LAYOUT� Compact 4 mirrors (2 parabolas + 2 flat) configuration� Flat and parallel plate as closure window, secondary mirror holder and

support of flat mirror M4 coating� Minimised secondary mirror magnification (�18 x)� 36 m Effective Focal Length� enough focal plane relief to accommodate the detector� half cone 25 arc seconds FOV� the FOV is limited to avoid mechanical overlap of mirrors M2/M4 and to

avoid interference of the output beam with M1/M3


PST Optics 2/5


PST Optics 3/5


PST Optics 4/5


PST Optics 5/5


PST Internal Baffle 1/2


PST Internal Baffle 2/2

� Irradiance on focal plane, as computed by ASAP 7.1 assuming twodiffusion models (far field point source at the extreme FOV)� Chemglaze Z302 paint and Lambertian 5% reflectance

� Results� No ghosts from mechanics� Flat low level straylight


PST Accuracy 1/4Requirement number Requirement

3�, rad(3� , arcsec)

Estimated value3�, rad

(3� , arcsec)

Frequencycontrib.

R2-PST-01-01: optical distortion 10-10

(0.02*10-3)< 10-10

(<0.02*10-3)(**)

R2-PST-01-02: focal length variation with temperature 10-10

(0.02*10-3)< 10-10

(<0.02*10-3)(**) + 1 orbit

R2-PST-01-03: focal length variation with colour 10-10

(0.02*10-3)< 10-10

(<0.02*10-3)(**)

R2-PST-01-04: PRNU on star and on straylight 4.1*10-9

(0.85*10-3)3.4*10-9

(0.74*10-3)(*)

R2-PST-01-06: CCD DSNU 2.9*10-9

(0.6*10-3)2.9*10-10

(0.06*10-3)(*)

R2-PST-01-07: Centroid error 8.2*10-9

(1.7*10-3)8.2*10-9

(1.7*10-3)(*)

R2-PST-02-01: relativistic aberration 1.2*10-9

(0.25*10-3)< 1.2*10-9

(< 0.25*10-3)1 orbit

R2-PST-01-08: Arithmetic round off 1.4*10-9

(0.3*10-3)< 10-10

(< 0.02*10-3)10 Hz

R2-PST-01-09: NEA 6.8*10-9

(1.4*10-3)6.8*10-9

(1.4*10-3)10 Hz

R2-PST-04-01: Star proper motion 10-5

(2)< 10-5

(< 2)1 year

R2-PST-04-02: Star parallax error 10-5

(2)< 10-5

(< 2)1 year

R2-PST-04-03: Star Catalogue error 10-5

(2)<10-5

(<2)1 year

(*) It depends on how the star moves within the pixel (depends on S/C attitude control system)(**) It depends on how the star moves within the CCD (depends on S/C attitude control system)


PST Accuracy 2/4

� Centroid error curves for baseline configuration


PST Accuracy 3/4

� NEA curves for baseline configuration


PST Accuracy 4/4

� Pixel edge effects induced by CCD non-uniformities� Fig. 1 �PRNU= 0% and �DSNU =0 %� Fig. 2 �PRNU= 1% and �DSNU =10 %

FigFig. 1. 1 FigFig. 2. 2


Guide Star Catalogue 1/8

� The guide stars selection has been performed in accordance with thefollowing criteria:� Declination: -29.5 to 10.5 degrees. In fact the guide star will be in a direction

close to the normal to the orbit plane. The maximum angular distance from thenormal is represented by the directions forming an angle of 10 deg. to theEarth limb: ±(30-10) - 9.5 deg. for 1000 Km orbit altitude, inclination 99.5 deg.

EARTHLIMB

Anti-Sun direction

Orbit Plane(inclination 99.5 deg.)

-29.5 deg.

+10.5 deg.

GUIDESTARS

Anti-Sun direction

File guidestar.ppt



� Selection criteria (continued)� Right ascension: the maximum allowable angular separation between

two consecutive guide stars is 30 deg� Brightest star: V � +2, to avoid CCD saturation� Faintest star: V such that the required sky coverage is guaranteed.� Be non-variable, non-binary and no double� Have an absolute proper motion known (3�) better than 8 arcsec/year

(from R2-PST-04-01)� Have a Right ascension known (Star catalogue error, 3�

�) better than

10-5 rad (i.e. 2 arcsec), (from R2-PST-04-02)� Have a Declination known (Star catalogue error, 3�

�) better than 10-5

rad (i.e. 2 arcsec), (from R2-PST-04-03)� Have a parallax error known (3�

�) better than 10-5 rad (i.e. 2 arcsec),

(from R2-PST-04-02)



� Hipparcos catalogue� To perform the catalogue creation, the following fields of the Hipparcos

catalogue have been taken into account:– H01: Hipparcos Catalogue (HIP) identifier– H02: Proximity flag– H05: V magnitude– H06: Coarse variability flag

– H08: Right ascension (�), degrees (ICRS, Epoch=J1991.25)– H09: Declination (�), degrees (ICRS, Epoch=J1991.25)– H10: Reference flag for astrometric parameters of double and multiple

systems

– H11: Trigonometric parallax �, [milliarcsec]– H12: Right ascension proper motion ��* = ��.* cos(�), ICRS

[milliarcsec/year]

– H13: Declination proper motion ��, ICRS [milliarcsec/year]



� Hipparcos catalogue (continued)– H14: Standard error in Right ascension, �� [milliarcsec]– H15: Standard error in Declination, ��, [milliarcsec]– H16: Standard error of the trigonometric Parallax, �� [milliarcsec]– H17: Standard error in Right ascension proper motion, ��*= �� cos(�),

[milliarcsec/year]

– H18: Standard error in Declination proper motion, ��, [milliarcsec/year]– H37: Colour index, B-V– H76: Spectral type



� Final results� Considering 2 � V � 4 a guide star catalogue has been generated:

Total number of stars: 485360 <= Hip. entry no. <= 1153365364 <= HIP identifier <= 115438+2.81 <= V <= +3.99+2.21 <= m_PST <= +3.95+17.147 <= R.A. [deg.] <= +350.743-28.135 <= DEC. [deg.] <= +9.892+0.15 <= B-V <= +1.673216 <= Teff [K] <= 9733Maximum �(R.A.) = 26.40 [deg.] (Req. <= 30 deg.)



entry n. HIP Ra [deg.] Dec [deg.] V mPST B-V spectr.class

5360 5364 +17.146932 -10.181928 +3.46 +3.07 +1.16 K2III6532 6537 +21.006047 -8.182754 +3.60 +3.30 +1.06 K0III8097 8102 +26.021364 -15.939556 +3.49 +3.44 +0.73 G8V8639 8645 +27.865044 -10.334945 +3.74 +3.38 +1.14 K2III13689 13701 +44.106682 -8.897610 +3.89 +3.57 +1.09 K1III-IV15886 15900 +51.203490 +9.029065 +3.61 +3.45 +0.89 G8III17364 17378 +55.812317 -9.765199 +3.52 +3.34 +0.92 K0IV22432 22449 +72.458909 +6.961247 +3.19 +3.24 +0.48 F6V

… … … … … … … …

� Final results (continued)



� Final results (continued)� Guide star co-ordinates



� Final results (continued)� Guide star catalogue histograms

– Number of catalogue stars vs. Teff and instrumental magnitude


PST Conclusions 1/1

� This study has shown the feasibility of HYPER PST within therequirements:� The CCD selection is not critical� The Optics design is effective, reliable and simple� The baffle system shows very good straylight performance� The star centre determination algorithm (centroid) is derived from well

known GA algorithms� The sky coverage is guaranteed in each period of the year

Galileo Avionica- Precision Star Tracker

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