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NASA experience has indicated the need for uniform criteria for the design of space vehicles. Accordingly, criteria are being developed for the following areas of spacecraft technology: Environment Structure Guidance and Control Chemical Propulsion Individual components of this work will be issued as separate monographs as soon as they are completed. This document, “Spacecraft Star Trackers,” is one such monograph. A list of‘ all published monographs in the series can be found in the back of this document. These monographs are to be regarded as guides to design and not as NASA requirements, except as may be specified in formal project specifications. It is expected, however, that the criteria sections of these monographs, revised as experience may dictate, eventually will become uniformly applied to the design of NASA space vehicles. This monograph was prepared by the Kollsman Instrument Corp. under the cognizance of the NASA Electronics Research Center. The effort was guided by an advisory panel chaired by Louis E. Sharpe of Kollsman Instrument Corp. The following individuals participated in the advisory panel activities: J. Bebris NASA Electronics Research Center R. F. Bohling NASA Office of Advanced Research and Technology F. J. Carroll NASA Electronics Research Center R. L. Cleavinger Ball Brothers Research Corp. T. P. Dixon ITT Aerospace C. D. Engel NASA Langley Research Center F. F. Forbes University of Arizona K. V. Knight Litton Systems, Inc. A. R. Leslie Kollsman Instrument Corp. F. D. MacKenzie NASA Electronics Research Center J. M. McLauchlan Jet Propulsion Laboratory T. S. Michaels NASA Office of Advanced Research and Technology G. R. Quasius General Electric Corp. Comments concerning the technical contents of these monographs will be welcomed by the National Aeronautics and Space Administration, Office of Advanced Research and Technology (Code RVA), Washington, D.C. 20546. July, 1970 i For sale by the Clearinghouse for Federal Scientific and Technical Information Springfield, Virginia 22151 - Price $3.00 Page 1. INTRODUCTION ........................................ 1' 2 . STATEOFTHEART ...................................... 2 2.1 Evolution ........................................ 2 2.2 Basic Principles of the System .............................. 3 2.3 Comparison of Recent Tracking Systems ........................ 4 2.4 CanopusTracker ..................................... 5 2.4.1 Mariner ...................................... 5 2.4.2 Surveyor ..................................... 8 2.4.3 Lunar Orbiter .................................. 9 2.5 Rocket-Borne Trackers .................................. 9 2.6 Gimbaled Star Trackers ................................. 10 2.7 Fine-Guidance Trackers .................................. 11 2.8 Summary ......................................... 12 3. CRITERIA ............................................ 13 3.1 InputPhenomena ..................................... 13 3.1.1 Stars to be Tracked ............................... 13 3.1.2 Starsignal .................................... 14 3.1.3 RandomNoise .................................. 14 3.1.4 FalseSignals ................................... 14 3.1.5 Interference Sources ................................ 14 3.2 Performance ........................................ 15 3.2.1 Acquisition .................................... 15 3.2.2 Tracking ..................................... 15 3.2.3 IndexError ................................... 16 3.3 Design Considerations .................................. 16 3.3.1 Photodetector .................................. 16 iii 3.3.1.1 Photomultipliers ............................. 16 3.3.1.2 Image Dissectors ............................. 17 3.3.1.3 Vidicons ................................. 17 3.3.1.4 Solid-state Detectors .......................... 17 3.3.2 Optics ...................................... 18 3.3.2.1 Image Formation ............................ 18 3.3.2.2 Shielding ................................ 18 3.3.3 Mechanical Design ................................ 19 3.3.3.1 Structure and Materials ......................... 19 3.3.3.2 Thermal Considerations ......................... 19 3.3.3.3 Lubrication ............................... 19 3.3.4 Electronics Design ................................ 19 3.3.4.1 Circuitry ................................. 19 3.3.4.2 Corona Suppression ........................... 20 3.4 Alinement. Calibration. and Test ............................. 20 3.4.1 Internal Alinement ................................ 20 3.4.2 Calibration .................................... 20 3.4.3 Test with Spacecraft ............................... 20 4 . RECOMMENDED PRACTICES ................................. 21 4.1 Input Phenomena .................................... 21 4.1.1 Stars ....................................... 21 4.1.1 -1 Star Availability ............................. 21 4.1.1.2 Star Signal ................................ 22 4.1.2 RandomNoise .................................. 23 4.1.3 False Signal ................................... 25 4.1.3.1 Nonuniform Illumination ........................ 25 4.1.3.2 Uniform Illumination .......................... 27 4.1.3.3 Particles ................................. 27 4.1.4 Miscellaneous Interference ............................ 27 4.1.4.1 Microphonics .............................. 28 iv 4.1.4.2 Radiofrequency Interference ...................... 28 4.1.4.3 Electrostatic-Charge Leakage ...................... 28 4.2 Performance ....................................... 28 4.2.1 Acquisition .................................... 28 4.2.1.1 Search Program ............................. 28 4.2.1.2 Acquisition Probability ......................... 29 4.2.1.3 Threshold Logic ............................. 31 4.2.2 Tracking ..................................... 31 4.2.2.1 Tracking in Noise ............................ 32 . 4.2.2.2 Tracking with False Signal ........................ 33 4.2.2.3 Gain Control .............................. 33 4.2.2.4 Tracking Mode for Scan Systems .................... 33 4.2.3 Index Error ................................... 34 4.2.3.1 Mechanical Modulation Stability .................... 35 4.2.3.2 Electronic Scan Stability ........................ 35 4.2.3.3 Optical Stability ............................. 36 4.3 Design Considerations .................................. 36 4.3.1 Photodetector .................................. 36 4.3.1.1 Photomultipliers ............................. 37 4.3.1.2 Image Dissectors ............................. 38 4.3.1.3 Vidicons ................................. 38 4.3.1.4 Solid-state Detectors ........................... 39 4.3.2 Optics ...................................... 40 4.3.2.1 Image Formation ............................ 40 4.3.2.2 Shielding ................................ 41 4.3.3 Mechanical Design ................................ 42 4.3.3.1 Structure and Materials .......................... 42 4.3.3.2 Thermal Considerations ......................... 43 4.3.3.3 Lubrication ............................... 43 4.3.4 Electronics Design ................................ 44 4.3.4.1 Circuitry ................................. 44 4.3.4.2 Corona Suppression ........................... 44 V 4.4 Alinement. Calibration. and Test ............................. 45 4.4.1 Internal Alinement ................................ 45 4.4.2 Star Sensitivity Calibration ............................ 45 4.4.3 Test with Spacecraft ............................... 46 REFERENCES .............................................. 47 APPENDIX A-GLOSSARY ...................................... 51 APPENDIX B-STELLAR PHOTOMETRIC DATA FOR VARIOUS PHOTOCATHODE MATERIALS ...................................... 55 .APPENDIX C-SIGNAL AND NOISE EQUATIONS .......................... 61 NASA SPACE VEHICLE DESIGN CRITERIA MONOGRAPHS ISSUED TO DATE .......... 63 vi Star sensing and tracking devices have been developed for a variety of spacecraft. applications in which the determination of a known reference direction is required for vehicle attitude control and for the generation of navigation and guidance data. Star-tracker system design is dominated by the low power level of starlight in the presence of extraneous light sources and system noise. Other factors influencing design are requirements for high accuracy, low weight, low power drain, and survival during launch and in the space environment. The preferred approach to the design of a spaceborne star tracker is one that efficiently concentrates the star energy into a small image and, by preferential sensing, recognizes and registers the presence of such an image relative to a central point in the field of observation. The resulting measures of presence and pointing error are separated electronically, the first being used for star verification (and often for automatic gain control (AGC)) and the second for tracking control. In equipment design, lightweight and compact optical systems, field scan systems that minimize mechanical friction, and solid-state circuitry generally are preferred. Degraded performance or equipment failure can result from designs based on an incorrect appraisal of mission and vehicle constraints or from an incomplete knowledge of the operational environment. Examples of difficulties encountered include the tracking of stray light, arcing of high-voltage circuits, and development of index error from internal mechanical, electronic, and magnetic shifts. The scope of this monograph is limited to star trackers. It considers that part of the tracking system
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