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February 2 nd , 2013 University of Colorado Aerospace Engineering Sciences

DayStar Daytime Star Tracker for Balloon-borne Attitude Determination AAS GN&C Conference. February 2 nd , 2013 University of Colorado Aerospace Engineering Sciences. Subaru Telescope (8 m). High Altitude Balloon (2.5 m). $10 billion. Hubble Space Telescope (2.5 m).

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February 2 nd , 2013 University of Colorado Aerospace Engineering Sciences

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  1. DayStar Daytime Star Tracker for Balloon-borne Attitude Determination AAS GN&C Conference February 2nd, 2013 University of Colorado Aerospace Engineering Sciences

  2. Subaru Telescope (8 m) High Altitude Balloon (2.5 m) $10 billion Hubble Space Telescope (2.5 m) $15 million $300 million

  3. Diffraction Limit Pointing Accuracy Hubble: Diffraction Limited Performance Diffraction limit: 40-100 milli-arcseconds Pointing errors: 2-5 milli-arcseconds

  4. Diffraction limit: 40-100 milli-arcseconds ST5000 Current standard for balloon navigation Needed Higher Accuracy Daytime Performance Diffraction Limit ST5000Performance Pointing errors: >500 milli-arcseconds Saturates during daytime

  5. Eventual goal: Operate a telescope from a balloon Problem: Pointing knowledge and controlis difficult in the stratosphere Problem: Pointing knowledgeand control is difficult in the stratosphere Wallops Arc-Second Pointing System (WASP)

  6. Project Definition • Project Definition • Modeling • Modeling • System Design • System Design • Testing • Testing • Results • Results DayStar Overview Aug. 2011 • What is DayStar? • University of Colorado at Boulder aerospace senior project • Daytime star tracker for balloons • $20,000 budget • Rapid 1 year design and development • Customer: Dr. Eliot Young, SwRI • Project Goals • Nighttime tracking: 0.1 arcseconds • Daytime tracking: 1.0 arcseconds Sep. 2011 Dec. 2011 May 2012 • How well did DayStar perform on flight? Sep. 2012 Jan. 2013

  7. Performance Modeling

  8. Project Definition • Modeling • System Design • Testing • Results Nighttime vs. Daytime We mitigate daylight by: - Filtering out everything below 620 nm. - Using a red-sensitive CMOS camera. At night: - Stars are visible throughout the spectrum. - Background light is negligible. During the day: - Daylight dominates much of the spectrum. - Stars visibility is comparable to dusk. Many stars become visible during the day. Star trackers are dominated by algorithm performance. Star trackers are dominated by image quality. Background Star

  9. Project Definition • Modeling • System Design • Testing • Results Star Visibility Created a model to quantify number of visible stars (SNR > 6). Without saturating Studied visibility vs. filter wavelength for different exposures Number of Visible Stars Saturation Maximizes for low exposures Optimal filter: 620 nm Exposure Time (ms) Filter Wavelength (nm)

  10. Project Definition • Modeling • System Design • Testing • Results Estimated Attitude Performance Monte Carlo simulation of accuracy vs. number of stars Centroid Accuracy Model results Nighttime: 30-45 stars Daytime: 5-10 stars 0.1” Requirement Day Performance Nighttime: 0.06” Daytime: 0.15” Estimated Centroid Accuracy Night

  11. System Design

  12. Key Power Data Light System Diagram Optics Structure Power Computer Interfaces SSD Processor Intel I3 AVR Atmega128 Serial Port (RS232) Memory Frame Grabber Matrox Solios 2x ADC Current 2x ADC Voltage Power Supply (12V, 5V, 3.3V, -12V) Camera Clock 85 MHz Rx Tx Level Shift CMOS Sensor (Fairchild sCMOS) External Baffle Filter (620 nm) COTS Lens LDO A A MOSFET MOSFET 8x 7x

  13. Test Flight

  14. Project Definition • Modeling • System Design • Testing • Results Preparation Development and prototyping in Boulder, CO Thermal vacuum test in Palestine, TX

  15. Project Definition • Modeling • System Design • Testing • Results Integration Launch! Integration with WASP in Ft. Sumner, NM

  16. Project Definition • Modeling • System Design • Testing • Results Flight Overview • Flew from CSBF in Fort Sumner, NM on September 22, 2012 • Launched ~8:30 AM • Terminated ~10:00 PM

  17. Project Definition • Modeling • System Design • Testing • Results Test Flight Launch Ascent Daytime Float Nighttime Float Descent

  18. Results

  19. Project Definition • Modeling • System Design • Testing • Results Data Overview • Daystar operated from 3:00 – 10:00 PM • Took ~500 GB of images • 10 Hz • 500 frame bursts • Image quality suffered from noise due to design flaws Increased contrast for visibility

  20. Project Definition • Modeling • System Design • Testing • Results Flatfield Normalization • Flatfield Normalization removes artifacts inherent in the camera • Process normalizes column mean relative to image robust mean

  21. Project Definition • Modeling • System Design • Testing • Results Star Finding • Calculate robust mean of background • Use depth-first search to find pixel blobs above the background • Blob is considered a star if it is sufficiently: • Round • Bright in the center • Large/small • Centroid star using intensity2 weighted center of gravity method • Calculate robust mean of background • Use depth-first search to find pixel blobs above the background • Blob is considered a star if it is sufficiently: • Round • Bright in the center • Large/small • Centroid star using intensity2 weighted center of gravity method • Calculate robust mean of background • Use depth-first search to find pixel blobs above the background • Blob is considered a star if it is sufficiently: • Round • Bright in the center • Large/small • Centroid star using intensity2 weighted center of gravity method • Calculate robust mean of background • Use depth-first search to find pixel blobs above the background • Blob is considered a star if it is sufficiently: • Round • Bright in the center • Large/small • Centroid star using intensity2 weighted center of gravity method

  22. Project Definition • Modeling • System Design • Testing • Results Star Finding 9:00PM 50 second burst Identified stars over 500 frames Matched stars over 500 frames ~21 star average False identification

  23. Project Definition • Modeling • System Design • Testing • Results Attitude Determination • Convert star centroids to 3D boresight vectors • Use q-method to find least squares rotation between images • Add rotations to find motion relative to first image • Convert star centroids to 3D boresight vectors • Use q-method to find least squares rotation between images • Add rotations to find motion relative to first image • Convert star centroids to 3D boresight vectors • Use q-method to find least squares rotation between images • Add rotations to find motion relative to first image

  24. Project Definition • Modeling • System Design • Testing • Results Tracking +

  25. Project Definition • Modeling • System Design • Testing • Results High Pass Filtering Power Spectrum Filtered below 3.5 Hz Gondola Motion

  26. Project Definition • Modeling • System Design • Testing • Results 2D Projection Star Tracking Error Total Motion - = Balloon Motion

  27. Project Definition • Modeling • System Design • Testing • Results Nighttime Results RMS Centroid Error

  28. Project Definition • Modeling • System Design • Testing • Results Daytime Results 5:12 PM Burst Predicted 5-10 visible stars Poor image quality Low SNR Only identified 3 10 C

  29. Project Definition • Modeling • System Design • Testing • Results Conclusion 1) Nighttime performance below 0.1”, as modeled, in the best case 2) Proved feasibility of daytime star tracker

  30. Project Definition • Modeling • System Design • Testing • Results Conclusion What is next? 1) Improve electrical design on camera to reduce noise 2) Implement real-time tracking with lost-in-space solution 3) Future test flights

  31. Questions?

  32. Project Definition • Modeling • System Design • Testing • Results Pitch Motion Power Spectrum Filtered below 3.5 Hz Gondola Motion

  33. Project Definition • Modeling • System Design • Testing • Results Roll Motion Power Spectrum Filtered below 3.5 Hz Gondola Motion

  34. Project Definition • Modeling • System Design • Testing • Conclusion Modeling Flow Diagram

  35. Project Definition • Modeling • System Design • Testing • Conclusion MODTRAN Model • Sky background brightness vs. • Altitude • Wavelength • Angle from sun • Units: • photon/s/cm2/sr • Total Flux: • Integrated curve • Scales w/aperture, solid angle viewed and exposure time Photo Credit: Dr. Eliot Young, SwRI

  36. Project Definition • Modeling • System Design • Testing • Conclusion Star Modeling • Inputs (star catalog) • and • Bolometric magnitude • Temperature correction • Planck’s Law • Blackbody curve based on temperature • Total flux • Integrated curve • Scales w/aperture, exposure time

  37. Project Definition • Modeling • System Design • Testing • Conclusion Corrected Star Model Input: T, mv Apply Bolometric Correction Calculate Expected Flux Create Blackbody (Plank) Scale for correct flux Output: Flux(λ)

  38. Project Definition • Modeling • System Design • Testing • Conclusion Visibility – SNR of Single Star • Flux from background • Scales with aperture, solid angle viewed • Flux from star • Depends on and • Scales with aperture • Signal to noise ratio: • Incorporates camera noise Star Sky Background Dark Current Read Noise

  39. Project Definition • Modeling • System Design • Testing • Conclusion Star Population Number of stars per 10,000 square degrees • Stars vary by: • Magnitude • Temperature • Galactic latitude (GL) • Table used to analyze average star field • Assume worst case GL • Scale to size of FOV • DayStar ‘typical’ star field • Roughly 100 stars total • > 50% are ‘red’ stars Photo Credit: Kapteyn Astronomical Laboratory

  40. Project Definition • Modeling • System Design • Testing • Conclusion Image Quality Generic Image Data • Image quality: • Cannot saturate • Individual star or entire image • Signal-to-Noise (SNR) • Star is visible if SNR exceeds a threshold • Extra noise is created by a bright background Visible star (high SNR) Unusable stars (low SNR) Photoelectron Count • Number of stars • Drives attitude accuracy • Improves with square root of star count Pixels Photo Credit: Douglas Bennet, Stony Brook University

  41. Project Definition • Modeling • System Design • Testing • Conclusion Visibility – Total Number of Stars • The process for a single star is applied to all in the ‘typical’ star field • In order to be visible: • SNR > threshold • Total photoelectrons < well depth • The total number of visible stars determines attitude accuracy

  42. System Parameters

  43. MODTRAN Model • Sky background brightness vs. • Altitude • Wavelength • Assumptions • Zenith: >60° • Azimuth: >90° from sun • Units: • photon/s/cm2/sr • Flux in photons:

  44. Star Modeling • Must know flux as function of wavelength • Critical for seeing stars during daytime • Drives longpass filter cutoff and CMOS QE • Given values • Visual magnitude: m = 4 to 8 • Problem: • Visual magnitude is a photometric measure, not radiometric • NOT defined for all wavelengths

  45. Flux vs. Temperature • Visual magnitude • Flux in visual band only • Actual flux • Spread across spectrum • Changes with temperature

  46. Radiometric Equations • Flux depends on temperature • Plank’s Law • Total flux derives from magnitude, solar reference • W/m2 • Now we just need a radiometric magnitude

  47. Quantum Efficiency • CMOS • Gaussian QE • Center: 590 nm • FWHM: 450 nm • Filter • Cutoff: 620 nm • Total QE • Superposition of both

  48. Project Goals and Requirements • The DayStar team will develop a prototype star tracking system capable of providing pointing knowledge to a diurnal, lighter-than-air platform. DayStar will improve on current attitude determination devices used on balloon payloads by providing daytime operational capabilities and an improved nighttime accuracy of 0.1 arcseconds RMS. Nighttime: Ambient sky brightness ≤ 2 kilo-Rayleighs Daytime: Ambient sky brightness ≤ 86,000 kilo-Rayleighs

  49. Key Power Data Light Functional Block Diagram Optics Structure Power Computer Interfaces SSD Processor Intel I3 AVR Atmega128 Serial Port (RS232) Memory Frame Grabber Matrox Solios 2x ADC Current 2x ADC Voltage Power Supply (12V, 5V, 3.3V, -12V) Camera Clock 85 MHz Rx Tx Level Shift CMOS Sensor (Fairchild sCMOS) External Baffle Reimaging System Objective Lens Filter (620 nm) Field Stop LDO A A MOSFET MOSFET 8x 7x

  50. Feasibility Through Analysis Requirement Attitude Accuracy Centroiding Accuracy Star Detection 0.1” RMS attitude accuracy with 20 stars Worst case attitude of 0.05” accuracy with 20 stars Centroid stars up to 0.5” accuracy Detect 100% of stars up to 8th magnitude > × × Nighttime Centroid stars up to 0.5” accuracy 1.0” RMS attitude accuracy with 8 stars Detect 90% of stars up to 7th magnitude Worst case attitude of 0.1” accuracy with 8 stars × > × Daytime “At a given SNR, how well do we know our attitude?” “How well do we know our attitude from our centroids?” “How well can we locate what we detect?” “What SNR stars can we detect?” = × ×

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