Development of a Pointing, Acquisition, and Tracking System for a Nanosatellite Laser ...

129 Pages·2016·19.7 MB·English

Development of a Pointing, Acquisition, and Tracking System
for a Nanosatellite Laser Communications Module

Kathleen Riesing, Kerri Cahoy

September 2015 SSL # 19-15

Development of a Pointing, Acquisition, and Tracking System
for a Nanosatellite Laser Communications Module

Kathleen Riesing, Kerri Cahoy

September 2015 SSL # 19-15

This work is based on the unaltered text of the thesis by Kathleen Riesing submitted to the
Department of Aeronautics and Astronautics in partial fulfillment of the requirements for the degree
of Master of Science at the Massachusetts Institute of Technology.

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Development of a Pointing, Acquisition, and Tracking System
for a Nanosatellite Laser Communications Module
by
Kathleen Michelle Riesing
Submitted to the Department of Aeronautics and Astronautics
on August 20, 2015, in partial fulfillment of the
requirements for the degree of
Master of Science in Aeronautics and Astronautics
Abstract
Launch opportunities for small satellites are rapidly growing and their technical capa-
bilities are improving. Several commercial constellations of small satellites for Earth
imagingandscientificobservationaremakingtheirwayontoorbit,increasingtheneed
for high bandwidth data downlink. Obtaining regulatory licensing for current radio
frequency (RF) communications systems is difficult, and state of the art nanosatel-
lite RF systems struggle to keep up with the higher demand. Laser communications
(lasercom) has the potential to achieve high bandwidth with a reduction in power and
size compared to RF, while simultaneously avoiding the significant regulatory burden
of RF spectrum allocation.
Duetonarrowbeamwidths, theprimarychallengeoflasercomisthehigh-precision
pointing required to align the transmitter and receiver. While lasercom has been suc-
cessfully demonstrated on multiple spacecraft platforms, it has not yet been demon-
strated on a scale small enough to meet the size, weight, and power constraints for
nanosatellites. The Nanosatellite Optical Downlink Experiment (NODE) developed
at MIT is designed to achieve a lasercom downlink of 10 to 100 Mbps within the
constraints of a typical 3-U CubeSat.
This thesis focuses on the development of the pointing, acquisition, and tracking
system for NODE. The key to achieving a high bandwidth downlink is to bridge the
gapbetween existing CubeSat attitude determination andcontrolcapabilities andthe
narrow beamwidths of lasercom. We present a two-stage pointing control system to
achieve this. An uplink beacon and detector provide fine attitude feedback to enable
precision pointing, and CubeSat body pointing is augmented with a fine steering
mechanism.
The architecture of the pointing, acquisition, and tracking system is presented,
followed by the in-depth design and hardware selection. A detailed simulation of
the ground tracking performance is developed, including novel on-orbit calibration
algorithmstoeliminatemisalignmentbetweenthetransmitterandreceiver. Atestbed
is developed to characterize the selected fine steering mechanism for performance
and thermal stability. The proposed system is capable of achieving at least two
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orders of magnitude better pointing than existing CubeSats to enable high bandwidth
nanosatellite downlinks.
Thesis Supervisor: Kerri L. Cahoy
Title: Assistant Professor of Aeronautics and Astronautics
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Acknowledgements

This work was performed primarily under contract #NNX14AL61H with NASA as part of the
NASA Space Technology Research Fellowship (NSTRF) program.
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Contents
1 Introduction 21
1.1 Motivation for a Nanosatellite Laser Communications System . . . . 21
1.2 Laser Communications Background & Challenges . . . . . . . . . . . 24
1.2.1 Pointing, Acquisition & Tracking Subsystem . . . . . . . . . . 25
1.2.2 Overview of Prior Missions . . . . . . . . . . . . . . . . . . . . 29
1.3 Nanosatellite Attitude Determination & Control Background . . . . . 33
1.3.1 Three-axis-stabilized CubeSat Missions . . . . . . . . . . . . . 33
1.3.2 Commercial-Off-The-Shelf Hardware . . . . . . . . . . . . . . 35
1.4 Nanosatellite Laser Communications . . . . . . . . . . . . . . . . . . 37
1.4.1 Key Challenges & Existing Efforts . . . . . . . . . . . . . . . . 37
1.4.2 NODE Concept of Operations . . . . . . . . . . . . . . . . . . 39
1.5 Thesis Objective & Roadmap . . . . . . . . . . . . . . . . . . . . . . 40
2 Pointing, Acquisition & Tracking Approach 43
2.1 NODE System Overview . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.1.1 Design Approach & Key Requirements . . . . . . . . . . . . . 43
2.1.2 Summary of Key Parameters . . . . . . . . . . . . . . . . . . . 46
2.2 Pointing, Acquisition, & Tracking Architecture . . . . . . . . . . . . . 47
2.2.1 Concept of Operations . . . . . . . . . . . . . . . . . . . . . . 48
2.2.2 Single-Stage vs. Two-Stage Design . . . . . . . . . . . . . . . 49
2.2.3 Monostatic vs. Bistatic Design . . . . . . . . . . . . . . . . . . 53
2.2.4 Hybrid Laser & Radio Calibration Method . . . . . . . . . . . 55
2.3 Derivation of Requirements . . . . . . . . . . . . . . . . . . . . . . . 56
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2.3.1 Host Spacecraft Performance . . . . . . . . . . . . . . . . . . . 57
2.3.2 Fine Steering Requirements . . . . . . . . . . . . . . . . . . . 57
2.3.3 Beacon Detector Requirements . . . . . . . . . . . . . . . . . 58
2.3.4 On-orbit Calibration Requirements . . . . . . . . . . . . . . . 59
2.4 Fine Stage Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
2.4.1 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
2.4.2 Beacon Detector . . . . . . . . . . . . . . . . . . . . . . . . . 61
2.4.3 Fast Steering Mirror . . . . . . . . . . . . . . . . . . . . . . . 61
3 Simulation Analysis & Results 65
3.1 Ground Tracking Simulation . . . . . . . . . . . . . . . . . . . . . . . 65
3.1.1 Motivation & Overview . . . . . . . . . . . . . . . . . . . . . . 66
3.1.2 Dynamic Models . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.1.3 Sensor Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
3.1.4 Actuator Models . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.1.5 Control & Estimation Models . . . . . . . . . . . . . . . . . . 77
3.1.6 Pointing & Tracking Results . . . . . . . . . . . . . . . . . . . 82
3.2 Post-Acquisition Calibration Simulation . . . . . . . . . . . . . . . . 85
3.2.1 Motivation & Overview . . . . . . . . . . . . . . . . . . . . . . 86
3.2.2 Description of Algorithms . . . . . . . . . . . . . . . . . . . . 87
3.2.3 Noise & Error Models . . . . . . . . . . . . . . . . . . . . . . 89
3.2.4 Calibration Results . . . . . . . . . . . . . . . . . . . . . . . . 93
4 Fast Steering Mirror Characterization & Results 97
4.1 Fast-Steering Mirror Testbed . . . . . . . . . . . . . . . . . . . . . . . 97
4.1.1 Components & Layout . . . . . . . . . . . . . . . . . . . . . . 98
4.1.2 Thermal Test Environment . . . . . . . . . . . . . . . . . . . . 99
4.2 Description of Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.2.1 Comparison to Fixed Mirror . . . . . . . . . . . . . . . . . . . 101
4.2.2 Response to High Voltage Enable . . . . . . . . . . . . . . . . 101
4.2.3 Voltage Sweeps . . . . . . . . . . . . . . . . . . . . . . . . . . 101
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