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A Method for Optical Tracking of On-Orbit Servicing Operations in Geostationary Orbit Using Speckle Interferometry

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A Method for Optical Tracking of On-Orbit Servicing Operations in Geostationary Orbit Using Speckle Interferometry A Method for Optical Tracking of On-Orbit Servicing Operations in Geostationary Orbit Using Speckle Interferometry by Robert L. Scott, M.S., P.Eng. A thesis submitted to the Faculty of Graduate and Postdoctoral Affairs in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Aerospace Engineering Carleton University Ottawa, Ontario ©Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2015 Abstract On Orbit Servicing (OOS) is a class of space missions that could potentially extend the life of orbiting satellites by fuel replenishment, repair, orbital maintenance or satellite repurposing, and possibly reduce the rate of space debris generation. OOS performed in geostationary orbit poses a unique challenge for the optical space surveillance community. OOS satellites performing close proximity operations with separations less than 1 km in geosynchronous orbit subtend an angle comparable to the angular size of atmospheric seeing (turbulence) cells as viewed by a ground-based observer. This degrades the ability of a ground-based observer from resolving a geostationary satellite pair in order to perform astrometric measurements on the objects’ relative positions. In this research, a speckle-interferometric observational approach designed to detect and estimate the relative orbits of servicing satellites about their client is described. It was found that the use of the cross-spectrum of speckle imagery and extraction of its imaginary component provides unambiguous measurables useful for relative orbit estimation for closely-spaced satellites. This approach is enabled by the availability sensitive Electron Multiplying CCD (EMCCD) cameras which permit the detection of very faint speckles from an astronomical source. This thesis describes an observational model, the speckle-imaging acquisition process, an image-processing algorithm and experimental test results pertaining to the speckle measurement of very closely-spaced satellites. Testing of the cross-spectrum technique was conducted by observing rare optical alignments (optical conjunctions) of co-located geostationary satellites using medium and small aperture telescopes configured to perform speckle interferometry. Satellite pairs of magnitudes of Mprimary = 10.3, Δm = 0.3 and Mprimary = 9.0, Δm = 1.5 were observed with the 1.6-m Mont-Mégantic telescope. Minimum detected separations between the two satellites of 1.5 arcseconds were achieved. A subsequent small aperture telescope test successfully detected a satellite pair of Mprimary = 9.67, Δm = 1.63. It was found that in- track and cross-track relative orbit positions precise to 100 meters with respect to reference ephemeris provided by a satellite operator was achieved and evidence is shown that 50-meter precision is possible with longer observational periods. ii It was also found that due to the satellites’ relatively high relative motion rates that strong fringe rotation affected the separation measurements. This issue is less likely to affect real OOS satellite cases as the relative separation distances between them are much less than co-located satellites making their relative angular velocities much smaller in comparison. A technique to handle cases when the fringes are rotating at an excessive rate was developed and was found to be a more reliable approach to detecting the separation distance of the secondary satellite. In order to use speckle interferometry to measure the relative orbits of satellites in close proximity to one another a visibility requirement must be respected. This is due to atmospheric turbulence where both satellites must fall within the isoplanatic patch (< 5 arcsecond conical radius) before speckle patterns can be reliably acquired. This angle sets an upper limit on the separation of the satellite pair of ~1 km in geosynchronous orbit. Further to this, the seeing conditions must be steady during observation in order to ensure that the fringe measurements are reliably correlated during image processing. Due to the dynamic coupling between the radial and in-track motion of a servicer about its client, the radial component requires ~3 hours of observations to reliably estimate it. This is due to a partial observability limitation where the near-alignment of the camera field of view and the radial direction of the client coordinate frame are nearly co-linear. While partial observability of the radial component of the servicer’s motion is detectable the relative orbit is overall observable due to the dynamic coupling between the in-track and radial directions. iii Acknowledgements It is a pleasure to acknowledge the tremendous support of the women and men of the Canadian Armed Forces who watch the skies to maintain Canada’s position as a leader in the emerging field of Space Situational Awareness. I am very grateful to Defense Research and Development Canada who provided funding support for this work while I worked on the development of the NEOSSat microsatellite project and the Ground Based Optical space surveillance system. Appreciation is extended to: Gary Geling, Malcom Vant, Maria Rey, Caroline Wilcox, Dr. Brad Wallace, Stefan Thorsteinson, Major Donald Bedard, Lt Colonel Aaron Spaans, Lt Colonel Pascal Tremblay, Lt Colonel Catherine Marchetti, Dr Scott McLelland, Paul Pulsifer, Brian Eatock, Chris McMillan, John Weaver who have all played a role helping me either build sensors or collect observational data when I needed it. I also wish to acknowledge the talented engineering teams at the Department of National Defence who built Canada's first space based space surveillance satellites SAPPHIRE and NEOSSat, without whom I would not have learned the technical boundaries about the field of space surveillance. It has been my privilege to play a part in the Canadian Armed Forces’ role in Space Situational Awareness. From Carleton University, I wish to acknowledge my thesis advisor Dr. Alex Ellery who also gave me the tremendous flexibility needed to accommodate both my graduate studies while working as a mission scientist on NEOSSat. Bruce Burlton provided tremendous insights into flight operations at Telesat Canada. Both of these professors’ experiences led to the combined topic of space surveillance of robotic On-Orbit Servicing in GEO. I would also like to acknowledge the talented staff of the Mont-Mégantic observatory in Mont-Mégantic, Quebec and the University of Montreal for their generous granting of observational time on the 1.6m telescope. This work would not have been possible without access to one of the highest performance EMCCD cameras available and appreciation to Nuvu Camera Inc.; Oliver Daigle, Felecien Legrand and Phillipe Richelet is extended. Tim Douglas at Telesat Canada provided tremendous support by supplying satellite orbital ephemerides, without which the experimental portions of this research would not have been possible. iv My family has been so supportive and I also wish to extend a thank you to my mother Bonnie, father Lauchie and my partner Rebecca for their continued patience and encouragement. You have all been so generous and your kindness will not be forgotten. My grandfather once said that “education doesn’t cost anything to carry around”. This wise advice has always steered my endeavors in that knowledge is the best investment one can make in oneself and in others. Per aspera ad astra….. Robert Lauchlan Scott v Table of Contents Abstract ......................................................................................................................... ii Acknowledgements ...................................................................................................... iv Table of Contents ........................................................................................................ vi List of Tables ............................................................................................................... xi List of Illustrations ..................................................................................................... xii List of Appendices ................................................................................................... xviii Chapter 1: Introduction ........................................................................................... 1 1.1 On Orbit Servicing as a space surveillance problem ...................................... 1 1.2 A short overview of On-Orbit-Servicing missions ......................................... 2 1.3 Geostationary orbit.......................................................................................... 5 1.4 Space surveillance and the problem of OOS in GEO ..................................... 7 1.5 Motivation for this research: Space surveillance of closely-spaced objects . 10 1.5.1 The OOS problem ................................................................................. 14 1.6 Thesis problem statement ............................................................................. 15 1.6.1 Initial exploration using light curve inversion ...................................... 17 1.6.2 Speckle interferometry .......................................................................... 22 1.6.3 EMCCDS .............................................................................................. 24 1.6.4 Other considered technologies: Adaptive optics ................................... 25 1.6.5 Other considered technologies: Space surveillance radars ................... 27 1.6.6 Other considered technologies:
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