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ESTEC, Noordwijk The Netherlands 6-th March 2003 HYPER Industrial Feasibility S

ESTEC, Noordwijk The Netherlands 6-th March 2003 HYPER Industrial Feasibility Study Final Presentation Precision Star Tracker Activity 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 3 6-th March 2003, ESTEC, Noordwijk, The Netherlands Introduction 1/2 PST purpose To allow the measurement of the Lense-Thirring effect – This measurement is performed as relative measurement between the Precision Star Tracker (PST) giving angles between a guide star, fixed in inertial space and an atomic gyroscope direction, which has an extremely high short time sensitivity for rotation rates (angular rates). – The PST is directed to far-distant guide stars, which are not affected by the Lense-Thirring effect. They represent a reference for the measurement and 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 a series of laser beams, whose orientation is rigidly linked to the PST boresight 4 6-th March 2003, ESTEC, Noordwijk, The Netherlands Introduction 2/2 Today GA has the capability to provide Star Sensors for a wide variety of mission requirements and applications, ranging from high accuracy pointing of scientific instruments and platform, to medium FOV sensors with AAD capability The required accuracy of the HYPER PST is about 1000 times more severe than most accurate GA star trackers (ISO, XMM). This makes this study very challenging 5 6-th March 2003, ESTEC, Noordwijk, The Netherlands 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 frequency range 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 frequency range 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 frequency range 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 frequency range below 3.5*10-5 Hz. < 1.2 * 10-9 rad (< 0.25 * 10-3 arcsec) R1-PST-05 Timing/Jitter 1 ms 6 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Requirements 2/4 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 on Star 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) 7 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Requirements 3/4 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) 8 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Requirements 4/4 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 9 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST CCD Characteristics 1/1 To obtain typical value the following characteristics of the ATMEL TH7890 (used by GA ASTR) have been taken into account. Its main characteristics are: Full Well Capacity 2*105 electrons (17 micron pixel size) dark current 15 pA/cm2 Quantum Efficiency See figure 10 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST System Trade-off 1/3 To identify the baseline PST configuration the following guidelines have 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 Centroid and NEA: this can be obtained by a longer focal length. INCREASE OF CCD FWC (Pixel size). In order to avoid CCD saturation CONSIDER LARGE TRACKING MATRIXES. In order to match large PSF produced by high F number (considered odd rows/columns matrixes from 3x3 up to 25x25 pixels) and reduce Centroid error in terms of fraction of pixel Use of GA simulator to evaluate PST performance, especially in terms of Centroid error and NEA 11 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST System Trade-off 2/3 Used Centroid algorithm: N i i N i i i C Col Col w y 1 1 ' * ' N i i N i i i C Row Row w z 1 1 ' * ' 1,2,...N i ) 1 ( * ) 1 ( * 9 ' m w m i w i i 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 and then the rounding effect introduced by the physical pixel dimension is reduced Increasing N gives rise to greater sensitivity to CCD non-uniformities and noise – 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 the fraction of the star spot energy collected by the tracking matrix #* * 44 . 2 min f d 12 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST System Trade-off 3/3 To identify the best N value the following simulation steps have been 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 have been 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 . 2 min f d 13 6-th March 2003, ESTEC, Noordwijk, The Netherlands 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 14 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Optics 1/5 The principal aim of the optics study has been to find a configuration with the smallest number of elements, reducing as small as possible the 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 15 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Optics 2/5 16 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Optics 3/5 17 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Optics 4/5 18 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Optics 5/5 19 6-th March 2003, ESTEC, Noordwijk, The Netherlands PST Internal Baffle 1/2 20 6-th March 2003, ESTEC, Noordwijk, uploads/s1/ galileo-avionica-precision-star-tracker.pdf