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On Random and Systematic Errors of a Star Tracker

Maxim Tuchin , Anton Biryukov , Mikhail Prokhorov, Andrey Zakharov Space Project Laboratory, Sternberg Astronomical Institute, Lomonosov Moscow State University. On Random and Systematic Errors of a Star Tracker. Comparing real and estimated random errors.

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On Random and Systematic Errors of a Star Tracker

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  1. Maxim Tuchin, Anton Biryukov,Mikhail Prokhorov, AndreyZakharov Space Project Laboratory,Sternberg Astronomical Institute,Lomonosov Moscow State University On Random and Systematic Errors of a Star Tracker

  2. Comparing real and estimated random errors

  3. The effect of shot noise of star fluxdecreases with the increase of signal from star ~ Shot noise of background (light scattering) Random dark signal componentcan be reduced by cooling the sensor Readout noise of the sensorreading out with a lower frequency Estimation of random error Random error sources

  4. Image pixelization Limitation of the analyzed area while measuring coordinates Different sensitivity of the sensor elements (PRNU) Inhomogeneous sensitivity within a sensor element Dark current non-uniformity (DCNU) Effect of "hot" pixels Displacing effect of electron traps Various aberrations Differences in the spectral sensitivity of elements (SRNU) Non-uniformity of gains, bias Degradation of sensor and optics Etc. Systematic error sources

  5. Using integrated signal from pixels rather then signal distribution function across focal plane Using limited area instead of whole focal plane i.e. using instead of Pixelization and bounded area errors

  6. Error of star image position depending on image size Using PSF , where S – signal from the star, – radius with 80% of signal within

  7. Shift of measured coordinatesdepending on position within pixel Large size star images Small size star images Bounded area effect Pixelization effect

  8. Sensor elements have different levels of thermo generation rate of electrons Non-uniformity is high (up to 20-30%) It leads to increased errors Dark current non-uniformity

  9. Effect of DCNU on the position error Note: SNR is signal-to-noise ratio with DCNU=0 and reflects the signal’s strength

  10. Effect of DCNU on the star image detection Note: SNR is signal-to-noise ratio with DCNU=0 and reflects signal’s strength

  11. DCNU changes with Total Dose Irradiation and after annealing High Accuracy Star Tracker (HAS)Version 2 CMOS Active Pixel image Sensor (CMOS APS) Source: HAS2 Detailed Specification - ICD

  12. Cooling the sensor • We can forget about DS (except “hot” pixels) • Additional elements to design • More power consumption • Heat dumping problem • Increased effects of electron traps • Dark signal estimation • Increasing requirements to calculation capabilities • Less accuracy due to additional noise • Recalibrations are highly recommended Solutions to DCNU problem

  13. To achieve the high accuracy one should • Do ground-based calibrations • Store fair-sized calibration data (like bias or sensitivity maps) onboard • Provide for various corrections during data reduction • Cool the sensor or calculate dark signal • Do some space-based calibrations Conclusions

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