Small satellite optical communication receiver for

simultaneous spatial tracking and data demodulationby

Jessica S. ChangS.B., Massachusetts Institute of Technology (2018)

Submitted to the Department of Electrical Engineeringand Computer Science

in partial fulfillment of the requirements for the degree ofMaster of Engineering in Electrical Engineering and Computer Science

at theMASSACHUSETTS INSTITUTE OF TECHNOLOGY

June 2019c○ Massachusetts Institute of Technology 2019. All rights reserved.

Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Department of Electrical Engineering

and Computer ScienceMay 24, 2019

Certified by. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .David L. Trumper

Professor of Mechanical EngineeringThesis Supervisor

Certified by. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Bryan S. Robinson

Associate Group Leader, Lincoln LaboratoryThesis Supervisor

Certified by. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Curt M. Schieler

Technical Staff, Lincoln LaboratoryThesis Supervisor

Accepted by . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Katrina LaCurts

Chairman, Master of Engineering Thesis Committee

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Small satellite optical communication receiver for

simultaneous spatial tracking and data demodulation

by

Jessica S. Chang

Submitted to the Department of Electrical Engineeringand Computer Science

on May 24, 2019, in partial fulfillment of therequirements for the degree of

Master of Engineering in Electrical Engineering and Computer Science

Abstract

Free-space optical communications in space offer many benefits over established radiofrequency based communication links; in particular, high beam directivity results inefficient power usage. Such a reduced power requirement is particularly appealingto small satellites with strict size, weight and power (SWaP) requirements. In thecase of free-space optical communication, precise pointing, acquisition and tracking(PAT) of the incoming beam is necessary to close the communication link. Due tothe narrow beam of the laser, the critical task of accomplishing PAT becomes increas-ingly arduous and often requires complex systems of optical and processing hardwareto account for relative movement of the terminals. Recent developments in bodypointing mechanisms have allowed small satellites to point with greater precision.In this thesis, an approach to a low-complexity PAT system that utilizes a singlequad-cell photodetector as an optical spatial sensor is presented in the context ofa system which exploits the body pointing capabilities of the spacecraft to performtracking maneuvers, eschewing the need for additional dedicated optical hardware.The design and validation of this approach is presented, and preliminary results re-garding the implementation of this system are discussed. In particular, we examinethe implementation of the system on NASA’s TeraByte InfraRed Delivery (TBIRD)demonstration.

Thesis Supervisor: David L. TrumperTitle: Professor of Mechanical Engineering

Thesis Supervisor: Bryan S. RobinsonTitle: Associate Group Leader, Lincoln Laboratory

Thesis Supervisor: Curt M. SchielerTitle: Technical Staff, Lincoln Laboratory

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Acknowledgments

This thesis could not have been written without the friends and communities that

have supported me through all the ups and downs of research and writing. Though

it is impossible to individually identify every person that has had a hand in getting

me through to this point, there are a few that I would like to expressly acknowledge.

First and foremost, I would like to thank the folks in Group 67 at MIT Lincoln

Laboratory for introducing me to the world of free-space optical communications and

providing an incredibly supportive community in which I have felt comfortable to learn

and grow. In particular, I would like to thank Bryan Robinson and Curt Schieler,

my thesis supervisors, whose patience and suggestions helped pave the path of my

work at Lincoln Laboratory, and especially all of the "just popping in" conversations

and discussions I have had with Curt over the last few years. I am grateful for Kat

Riesing’s work in creating the spacecraft attitude dynamics simulation and her help

and mentorship wrangling with paper editing while preparing madly for a conference

deadline. Steve Constantine was an incredible resource when it came to working in

the lab and building the test setup. His constant reassurance that everything would

work out, freely provided life advice and stories often brightened my days in the

blackout-curtain clad lab room. I am grateful for Ken Aquino’s work designing and

realizing the quad circuit, as well as his dedication and ability to find time and come

to the rescue whenever I was struggling to resolve a frustrating bug. I would like to

thank the rest of the TBIRD team for their help and understanding from the very

first day I started out in the Lab. I look forward to continuing my work with you all

in the coming years.

I would also like to thank Professor David Trumper for supporting me over the

last year in this endeavor. He has been a friendly and helpful mentor since the first

time I set foot in his Feedback Controls classroom several years ago.

I would like to thank David, who has been there for me through thick and thin,

and patiently listened to my rants about "getting something working soon" while

consuming the products of my stress baking.

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Finally, I would like to thank my parents, HsiaoLung and ChinHuei and my

brother, Jason, for their support to this day.

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DISTRIBUTION STATEMENT A. Approved for public release. Distribution is

unlimited.

This material is based upon work supported by the National Aeronautics and

Space Administration under Air Force Contract No. FA8702-15-D-0001. Any opin-

ions, findings, conclusions or recommendations expressed in this material are those

of the author(s) and do not necessarily reflect the views of the National Aeronautics

and Space Administration.

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Contents

1 Introduction 17

1.1 Outline and Objectives of this Thesis . . . . . . . . . . . . . . . . . . 19

2 Background 21

2.1 Optical Communications in Space . . . . . . . . . . . . . . . . . . . . 21

2.2 TeraByte InfraRed Delivery Demonstration System Overview . . . . . 22

2.2.1 Automatic Repeat Requests . . . . . . . . . . . . . . . . . . . 25

2.3 Pointing, Acquisition and Tracking . . . . . . . . . . . . . . . . . . . 25

2.3.1 Reaction-Wheel Based Three-Axis Attitude Control . . . . . . 26

3 Signal Design 29

3.1 Uplink Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

3.2 Design of a tracking signal . . . . . . . . . . . . . . . . . . . . . . . . 31

3.3 Design of a communication signal . . . . . . . . . . . . . . . . . . . . 32

3.4 Design of a simultaneous tracking and communication signal . . . . . 36

3.4.1 Pulse Shaping . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.5 Extraction of information from uplink signal . . . . . . . . . . . . . . 37

3.5.1 Tracking information . . . . . . . . . . . . . . . . . . . . . . . 38

3.5.2 Demodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4 Hardware 43

4.1 Uplink Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

4.1.1 Sensor Noise Characteristics . . . . . . . . . . . . . . . . . . . 46

9

4.1.2 Quad Sensor Circuit . . . . . . . . . . . . . . . . . . . . . . . 47

4.2 Receiver Computation . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5 Simulation 51

5.1 Simulation of TBIRD Uplink . . . . . . . . . . . . . . . . . . . . . . . 51

5.2 Simulation of Spacecraft Pointing Performance . . . . . . . . . . . . . 52

5.3 Simulation performance . . . . . . . . . . . . . . . . . . . . . . . . . 53

5.3.1 Simulation Pointing Loss . . . . . . . . . . . . . . . . . . . . . 53

6 Experimental Setup and Results 55

6.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.2 Testbench characterization . . . . . . . . . . . . . . . . . . . . . . . . 57

6.2.1 Direct-drive laser . . . . . . . . . . . . . . . . . . . . . . . . . 58

6.2.2 Beam Profile and Defocusing . . . . . . . . . . . . . . . . . . 60

6.2.3 FSM Calibration . . . . . . . . . . . . . . . . . . . . . . . . . 61

6.2.4 Power Calibration . . . . . . . . . . . . . . . . . . . . . . . . . 62

6.2.5 Beam Alignment . . . . . . . . . . . . . . . . . . . . . . . . . 63

6.3 Tracking performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

6.3.1 Uncalibrated Results . . . . . . . . . . . . . . . . . . . . . . . 65

6.3.2 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

7 Summary and Future Work 69

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List of Figures

2-1 Overview schematic of the TBIRD demonstration. The space terminal

is a CubeSat in low-Earth orbit (altitude less than 2000 km) which

communicates with a ground terminal via a high rate downlink (solid

red arrow) and a low rate uplink (dashed red arrow). Not to scale. . . 24

2-2 Block diagram of TBIRD system. The space terminal is divided into

payload (blue) and CubeSat bus (green). Attitude feedback inputs into

spacecraft controller can come from either an onboard star tracker or

the payload optical tracker/receiver. . . . . . . . . . . . . . . . . . . 24

3-1 Example of M-ary PPM signal. A symbol is divided into 𝑀 slots, one

of which contains a pulse. . . . . . . . . . . . . . . . . . . . . . . . . 35

3-2 Uplink modulation scheme. Insertion of dead time in a binary PPM

waveform with a 10 kHz slot rate introduces discrete tone at 10 kHz

for tracking when sampling rate is 100 kHz. . . . . . . . . . . . . . . 37

3-3 The quad sensor is located 𝑓𝑐 away from the lens. As the azimuth

and elevation angles of incidence 𝜃𝑥, 𝜃𝑦 of the planar wave changes,

the location 𝑢𝑥, 𝑢𝑦 of the spot on the quad sensor changes. When the

planar field arrives normal to the lens, the spot is centered. . . . . . . 39

3-4 Example signals from the four quadrants of the sensor, with the BPPM

signal visible in the centered and off-centered cases. The red spots

depict the location of the full-width half-max spot impinging upon the

sensor. When no pulse is present, no magnitude difference between

quadrant signals is apparent. . . . . . . . . . . . . . . . . . . . . . . . 40

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4-1 Block diagram of optical and digital components. The receive optics

are shown on the left side of the diagram in blue, while the digital

components are shown on the right side in black. The green high-

lighted area comprises the quad sensor circuit. The circuit amplifies

and digitizes the quad sensor signals for processing. . . . . . . . . . . 45

4-2 Quadrant sensor with amplification and digital conversion circuit. . . 46

5-1 Block diagram of spacecraft pointing simulation. . . . . . . . . . . . . 52

5-2 Simulated spacecraft pointing error for a 160 second pass from 30∘

elevation through a maximum of 75∘ elevation. Spacecraft bus pointing

without payload feedback (blue) and with payload feedback (orange)

are shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5-3 Downlink pointing loss with payload feedback. The median loss is

0.2 dB, and the 99th percentile is 1.3 dB. There is an initial transient

period of increased loss. . . . . . . . . . . . . . . . . . . . . . . . . . 54

6-1 Block diagram of test bench. . . . . . . . . . . . . . . . . . . . . . . . 56

6-2 Experimental setup. Uplink beam path indicated by arrows.

Components legend: A: Fast-steering mirror, B: Receive aperture lens,

C: Quadrant sensor circuit, D: Autocollimator, E: Fiber collimator . 57

6-3 Test set up to evaluate direct-drive performance of the laser source. . 59

6-4 Resulting waveform of directly driven laser with 10 kHz square wave

source. The purple waveform is the source, and the yellow waveform

is the resulting optical output as measured with a PIN photodetector.

The rise time is much faster than the 10 kHz dynamics. . . . . . . . . 59

6-5 Camera image of spot intensity, showing symmetric Airy diffraction

pattern. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

6-6 Beam alignment setup. Quad mounted on 3-axis stage is centered (X

and Y axes) and in the focal plane (Z axis) of the aperture. Analog

outputs from the quad are monitored in order to verify alignment. . . 63

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6-7 Experimental result. Tracking disturbances introduced with a fast

steering mirror (blue) compared to quad sensor measured disturbances

(orange). Two-axis error: 5.7 𝜇rad RMS . . . . . . . . . . . . . . . . 66

6-8 A raster scanning pattern across the quad sensor area can be used for

calibration of the quadrant sensor. . . . . . . . . . . . . . . . . . . . . 67

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List of Tables

2.1 Summary of recent selected optical communications demonstrations in

Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

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Chapter 1

Introduction

Optical communication in space is attractive due to several benefits over established

radio frequency based communication links. By using lasers, a much wider band

of modulation frequencies is available, opening up the possibility for high rate data

communications using the largely unregulated frequencies in the terahertz spectrum.

In addition, laser beams have high directivity and narrow beams, and thus are par-

ticularly desirable due to efficient power delivery. In small satellites, where low size,

weight and power (SWaP) is critical, optical communication offers clear advantages.

However, the cost of the narrow laser beam is an increased pointing and tracking

performance requirement. Optical communication requires line of sight in order to

close the communication link; pointing and tracking ensures that adequate power

is received at the terminal during the communication period. Power loss at the

receiving terminal results in poor signal to noise ratio and reduces the achievable

communication rate. Typically, a system must be able to point to within a fraction

of the beamwidth to close the link. For optical communication systems, beamwidths

of ∼10 to 1000 𝜇rad are typical.

Such space-based optical communication links have been successfully demonstrated

before, such as in the Lunar Laser Communication Demonstration (LLCD) in 2013,

with a downlink data rate of up to 622 Mbps[4] from the Moon and the Aerospace

Corporation’s Optical Communication and Sensor Demonstration (OCSD) in 2017,

with a downlink data rate of 100 Mbps from a 450km low-Earth orbit [16]. However,

17

LLCD, and most space-based optical communications systems since then were unable

to fully take advantage of the high data rates possible with laser-based communication

due to long link distances and other design constraints. At the same time, the long

link distances imposed strict pointing requirements, and drove the implementation

of designs using complex steering optics to ensure the pointing requirements could

be met. Generally, components such as gimbals and other fast steering actuators

have been used to achieve fine pointing performance. However, additional actuators

further strain the power and weight constraints of missions.

Currently, science missions in space face a bottleneck in data transfer from LEO

orbit to ground, where scientists can process and interpret data gathered. Current

sensors are capable of generating terabytes of data in a short period of time, but com-

munication links have not kept up with the rapid advancement of sensing technology,

rendering the use of high resolution and high rate sensing systems more challenging

to realize, since the data cannot be easily recovered. The development of a low com-

plexity and accessible solution would be useful for the science community and could

enable novel research in space.

NASA’s TeraByte InfraRed Delivery (TBIRD) system is one such solution cur-

rently in development. TBIRD seeks to demonstrate high downlink data rate transfer

at 200 Gbps from a CubeSat in low-Earth orbit (LEO) to a ground terminal while

minimizing complexity, in part by utilizing commercial off-the-shelf (COTS) parts[15].

Furthermore, TBIRD uses only the spacecraft bus reaction-wheel-based actuation for

pointing. The geometry of orbits in LEO result in short passes over a ground terminal

several times per day, during which line of sight may be achieved. Though the passes

may be on the order of a few minutes in duration, over the course of 24 hours the short

bursts of communication can result in delivery of many terabytes per day from a small

space terminal in LEO to a small ground terminal [15]. This direct-to-Earth concept

is well suited for science missions, where latency in data transmission is acceptable.

In this work, we present a simultaneous spatial tracking and communication sys-

tem using a quadrant photodetector sensor, to be used as the uplink receiver on the

TBIRD payload [15]. The quadrant sensor uses the optical uplink to provide the

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spacecraft bus with pointing feedback for fine attitude control. The dual-purpose

uplink receiver reduces SWaP while improving pointing performance.

1.1 Outline and Objectives of this Thesis

While TBIRD is a complex demonstration with many subsystems, this thesis will

focus solely on the components which relate to the uplink receiver design. In order

to maintain a clear focus, a key objective of this thesis aims to provide the reader

with an in depth understanding of the design and verification to date of the uplink

receiver subsystem of TBIRD. As such, this thesis will be structured as follows.

Chapter 2 will first provide some background information regarding the state of

space-based optical communications today and motivate the goal of a dual-purpose

uplink design for TBIRD. In addition, a brief description of the pointing, acquisition

and tracking problem will be given. The goal of this chapter is to equip the reader

with a big picture view of why the dual-purpose uplink approach is considered, and

how the uplink receiver fits in to the overall system design.

The next two chapters are focused on the design of the uplink system.

Chapter 3 dives into the design of the uplink signal, and how the needs between

the tracking and communication components of the uplink are balanced in choosing

the modulation scheme used for transmissions between the ground station and the

spacecraft. First, the design considerations for the tracking and communication are

presented separately, followed by the changes made in order to accommodate both

modes. Finally, a brief description of the computations required to then extract the

relevant information from such a signal at the spacecraft receiver is given.

Chapter 4 then explores the choice of a quadrant sensor for the uplink detector

and the choice of a microcontroller for uplink-related computations in depth.

The following two chapters shift the focus from the design of the system to the

preliminaries of implementation.

Chapter 5 presents the results of a simulation of the uplink system as designed

in Chapters 3 and 4, and additionally considers the spacecraft pointing dynamics in

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order to validate the previous design choices.

Finally, Chapter 6 delves into the experimental test setup that was created to

verify the performance of the system, with the goal of creating an environment that

would be as true to the on-orbit conditions of the demonstration as possible. The

setup, which is meant to be integrated into the overall TBIRD test bench in the

future is validated in depth; the validation checks are documented in detail in this

chapter, and meant to ensure reproducible results. Preliminary results of the tracking

performance are also given in this chapter, as well as a notional calibration procedure

that will be conducted in order to improve the performance.

Concluding this thesis is Chapter 7, which summarizes the results of this work

and explores the next steps towards a fully realized flight demonstration.

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Chapter 2

Background

This chapter aims to provide relevant information regarding the current state of opti-

cal communications technology, and motivate the work done in this thesis, specifically

the design of a dual-purpose uplink receiver for the TBIRD demonstration.

First, a brief history of optical communications missions in space will be presented,

followed by an overview of the TBIRD demonstration system and the importance of

pointing, acquisition in tracking.

2.1 Optical Communications in Space

Optical communications have been for many years a promising method for the trans-

fer of information at high rates. Some of the driving reasons for looking to laser-

based systems include not only the higher theoretical data rates possible by virtue

of the shorter wavelengths in comparison to established RF technologies, but also

the promise of improved power consumption and weight. Development of laser-based

communication systems has been ongoing since the 1970s and 80s, though it was only

in the 2001 that the first space-based optical link was demonstrated by the European

Space Agency’s Advanced Relay and Technology Mission (ARTEMIS)[2].

In recent years, laser communication systems have been designed and investigated

by development programs around the world, including NASA in the United States and

the ESA in Europe. These systems have demonstrated the viability of space-based

21

optical links under a variety of conditions and over a number of link distances. In the

future, a well developed network of optical terminals in space could be the answer to

connecting remote locations on Earth to a worldwide information network. Further-

more, as sensor technology has improved, science missions in Space have encountered

a bottleneck in transferring data back to Earch for for analysis; existing technologies

can be expensive in terms of size, weight and power, and can be constrained by lower

data rates which result in smaller data volume capacity.

A recent noteworthy mission was NASA’s Lunar Laser Communication Demon-

stration (LLCD) which successfully demonstrated a downlink data rate of up to

622 Mbps from the Lunar Atmospheric Dust and Environment Explorer (LADEE)

spacecraft [4]. Of all successful demonstrations to date, LLCD featured the longest

link distance. Table 2.1 provides a brief summary of past and ongoing laser com-

munications missions in Space. As the technology for laser communications systems

improves, missions are able to continue to push link distances ever farther and data

rates ever higher.

2.2 TeraByte InfraRed Delivery Demonstration Sys-

tem Overview

The TBIRD demonstration consists of a free-space laser communications link between

a CubeSat in LEO and a ground terminal. LEO orbits are close to the Earth, with

altitudes generally less than 2000 km, in contrast to geo-synchronous orbits (GEO),

which have altitudes around 35,786 km above the Earth’s surface. CubeSats, which

are small satellites with standardized dimensions designed primarily for space research

have become a convenient platform for science and research based missions. CubeSats

are typically made of 10x10x10 cm units, with a maximum of 1.33 kg per unit.

The TBIRD payload features a high rate downlink of 200 Gbps and a low rate up-

link of 10 kbps. Though a main draw of laser communication systems is the promise

of high data rates and wide spectrum availability, prior laser communication demon-

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Program Year Link Type Downlink Data Rate

Advanced Relay andTechnology Mission(ARTEMIS)[2]

2001 GEO-Ground 2.048 Mbps

Lunar LaserComm. Demonstration

(LLCD)[4]2013 Lunar-Ground 622 Mbps

Optical PAyload forLasercomm Science

(OPALS)[1]2014 ISS-Ground 50 Mbps

Optical Comm. andSensor Demonstration

(OCSD)[17]2017 LEO-Ground 100 Mbps

Laser CommunicationsRelay Demonstration

(LCRD)[8]planned GEO relay 1.244 Gbps

Deep SpaceOptical Communication

(DSOC) [3]

planned2022

Deep Space-Earth132 Mbps at 0.25 AU,14 Mbps at 1 AU,2 Mbps at 2 AU

Table 2.1: Summary of recent selected optical communications demonstrations inSpace

strations have not yet been able to make use of much of this available spectrum, being

constrained by various other factors. As TBIRD is to be located in LEO, the space-

craft will make short passes transmitting in short bursts of high data rates, which

could amount to over 50 Tb delivered per day to a single ground station[15].

TBIRD utilizes commercial off the shelf (COTS) parts, reflecting the goal of creat-

ing a laser communication system with minimal complexity. Furthermore, the small

payload volume of 2.4U highlights the minimal impact that such a laser communica-

tion link can have if integrated with a larger system.

In this thesis, the design of a dual-purpose uplink receiver is presented. The

TBIRD uplink system is motivated by a desire to minimize complexity, size, weight

and power by combining the requirements of two distinct subsystems - uplink com-

munications and pointing, acquisition and tracking - into a single uplink receiver.

Figure 2-2 presents a block diagram of the TBIRD system. The bidirectional link

between the space terminal and ground terminal are shown, and the dual-purpose

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Figure 2-1: Overview schematic of the TBIRD demonstration. The space terminal isa CubeSat in low-Earth orbit (altitude less than 2000 km) which communicates witha ground terminal via a high rate downlink (solid red arrow) and a low rate uplink(dashed red arrow). Not to scale.

uplink receiver presented in this thesis is shown in the payload portion of the space

terminal, with two paths emerging from the receiver block. The communications

path creates a communication feedback loop within the TBIRD payload and ground

terminal, while the pointing, acquisition and tracking path leads to the CubeSat

bus attitude determination and control. Section 2.2.1 will motivate the needs of the

communication feedback loop, while Section 2.3 will discuss the challenge of pointing,

acquisition and tracking in laser communication systems.

Figure 2-2: Block diagram of TBIRD system. The space terminal is divided intopayload (blue) and CubeSat bus (green). Attitude feedback inputs into space-craft controller can come from either an onboard star tracker or the payload opticaltracker/receiver.

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2.2.1 Automatic Repeat Requests

TBIRD includes an uplink communication link between the ground terminal and the

spacecraft. In order to achieve the reliability of a high data rate downlink at 200

Gbps in the presence of atmospheric fading, TBIRD implements an automatic repeat

request (ARQ) protocol. The ARQ commands are transmitted from the ground

terminal to the spacecraft on the uplink when the ground terminal detects corruption

or loss of a downlink data frame[18]. Although the downlink data rate is necessarily

very high, the uplink data rate carrying ARQ commands is not bound to the same

requirements and can in fact be much lower; TBIRD utilizes a target uplink rate of

5 kbps, which is sufficient to avoid throughput penalties associated with the ARQ

protocol [19].

The uplink together with the downlink form a communication feedback loop,

as shown in the upper portion of Figure 2-2, between the space terminal payload

and the ground terminal. The receiver architecture handling the uplink data must

have sufficient bandwidth to accommodate the frequency content and modulation of

the communication signal. In addition, the optical receiver sensor must be sensitive

enough to detect signals at weak power levels, with a favorable signal to noise ratio

since the uplink signal will experience atmospheric fading effects.

2.3 Pointing, Acquisition and Tracking

Pointing, acquisition and tracking (PAT) refers to the process by which the two

terminals in an optical link make contact, detect the respective beams and begin

to track. A critical requirement of optical communications systems is the ability

to accurately point the optical beam on the receiver; without line of sight, the link

cannot be closed, and in the case of inaccurate pointing, power losses can result in

high bit error rates due to low signal to noise ratio.

Because pointing performance is so tightly coupled with the downlink communi-

cation performance, PAT systems are generally designed to be able to point to within

a fraction of the communication beamwidth. In order to point, a number of actuation

25

strategies are employed. Some larger systems, such as LLCD have used gimbals for

coarse pointing, coupled with piezoelectric actuators or other devices such as fast

steering mirrors to attain mechanical alignment for fine tracking[4]. Other, smaller

systems, like NASA’s Optical Communications and Sensor Demonstration have used

a combination of steam thrusters and reaction wheels[10].

2.3.1 Reaction-Wheel Based Three-Axis Attitude Control

Because TBIRD seeks to reduce complexity by eliminating excess actuation elements

such as fast steering mirrors on the payload, it relies on the CubeSat bus’s reaction

wheel-based attitude control for fine pointing and tracking. Reaction wheels generate

a torque when spun, which can control the orientation of a spacecraft in space when

applied. For small satellites with limited mass, reaction wheels can be particularly

efficient. The strategy of controlling the entire orientation of the spacecraft body is

known as body pointing.

Three-axis attitude control of small satellites has improved significantly due to

miniaturization of star trackers and reaction wheels. Recently, the Jet Propulsion

Laboratory’s ASTERIA mission demonstrated 2.5 𝜇rad RMS pointing accuracy with

a two-stage control system[14]. The Aerospace Corporation’s Optical Communica-

tion and Sensor Demonstration (OCSD) mission showed that a lasercom link can be

established on a small satellite using open-loop body pointing. The beam widths

used on OCSD were 0.05∘ (873 𝜇rad) FWHM and the spacecraft achieved about

±0.01∘ (175 𝜇rad) pointing accuracy[16]. In comparison, to achieve high burst rates

on TBIRD, a narrow downlink beam of about 130 𝜇rad is necessary, which places an

even tighter pointing requirement on the spacecraft.

On TBIRD, a quad sensor and ground beacon are utilized to improve body point-

ing with closed-loop feedback. For acquisition, the spacecraft must first open-loop

point using information from the onboard star tracker accurately enough that the

ground beacon is within the field of view of the quad sensor, approximately 20 mrad.

Once the beacon is acquired, closed-loop feedback is provided by the payload to the

bus at a rate of 30 Hz to improve pointing to a fraction of the downlink beamwidth.

26

However, TBIRD couples the pointing and communication subsystems together

through a common uplink receiver. Spatial tracking feedback is generated from a

communication signal, rather than from a CW or modulated beacon. A single quad

sensor on the spacecraft is used to receive the uplink signal. The sum of the quadrants’

received power yields the communication signal, and comparison of the quadrants’

received power yields the pointing feedback that is later provided to the bus for

closed-loop pointing.

27

28

Chapter 3

Signal Design

In this chapter, the design of the uplink signal to suit the needs of the dual-purpose

architecture will be presented. While considering the system, we must take into

account the possible noise and corruptions to the signal in the communication channel

such as atmospheric fading and power fluctuations, which can particularly impact the

performance of the receiver in situations where the signal to noise ratio is poor. When

received power decreases, the noise can overpower the uplink receiver and hinder signal

processing attempts to recover pointing feedback and communication signals. Thus,

it is important to understand the conditions during which the signal may deteriorate

and how we can design the signal to work well in those situations. Therefore, the first

section of this chapter will describe the communication channel.

In addition to considering the channel through which the signal will be propa-

gated, we must also consider the system requirements. For TBIRD, the uplink signal

must be thoughtfully designed with both the requirements of the tracking and com-

munications requirements in mind. The following sections in this chapter will then

present the process of choosing an appropriate uplink signal waveform, first by consid-

ering the tracking and communication requirements individually, and then balancing

the relevant characteristics to design a signal whose performance is simultaneously

appropriate for both. Finally, the extraction of information from the designed uplink

signal will be discussed.

29

3.1 Uplink Channel

As the uplink optical signal travels between the transmitter on the ground and the

receiver in LEO, several effects need to be acknowledged - atmospheric fading and

background light. This section will briefly describe the concerns that arise, but will

not delve into the physics of the phenomenon. Instead, the aim is to point out the

reasons why we cannot assume perfect signal propagation through the channel and

to provide a basis for the design choices made later in this chapter and in Chapter 4.

The atmospheric channel is first and foremost, an imperfect medium for laser com-

munications signals. The obvious impeding factor is the presence of clouds. But more

broadly, the aerosols and other molecules in the atmosphere can result in scattering

and absorption effects, while scintillation effects constituting variances in amplitude

and phase of the signal occur when there is locally variant indices of refraction of the

air, as described in [11].

Though these are general concerns regarding atmospheric links, in the uplink

case,where the turbulent layer of the atmosphere is far from the receiver, we care

primarily about the potential attenuation of the signal, which dominates the atmo-

spheric effects on the signal [9]. With the effects of atmospheric fading, the amplitude

of the optical signal at the receiver can vary with time. As the signal power decreases,

the SNR deteriorates, making it more difficult to interpret the received signal.

The incorporation of fading effects of the signal as considered in this work can be

considered as a multiplicative factor as follows:

𝑃𝑑𝑒𝑡(𝑡) = 𝑠(𝑡)𝑓(𝑡)

Where 𝑃𝑑𝑒𝑡(𝑡) is the received power at the detector, 𝑠(𝑡) is the signal, and 𝑓(𝑡) is the

time-variant fading parameter. The simulation of the fading effect will be discussed

with more detail in Section 5.1, however for this chapter, it is sufficient to understand

that atmospheric conditions result in power fades and surges, and thus the uplink

signal and receiver ultimately need to be robust to these effects.

Another component of the uplink channel to consider is the presence of background

30

light. Since the uplink receiver is located on the spacecraft and is pointed at the

Earth, background radiation from the Earth including solar reflections off of the

Earth’s surface and black body radiation can be detected at the receiver. Therefore,

it is important the uplink signal can be successfully extracted even in the presence of

other light. While strategies for this can include receive sensor choices as discussed

in Section 4.1, as well the use of filters in the optical (or other) domains, this chapter

will discuss robustness to background in the context of signal design.

3.2 Design of a tracking signal

This section will present and discuss the design of a spatial tracking signal. In optical

communications systems, it is necessary to accurately point and align the arriving

optical field with the receiver lens in order to maximize the power received and there-

fore the performance of the communication link. A tracking signal is thus used to

provide pointing feedback for the system. Specifically, for TBIRD the uplink tracking

signal provides pointing feedback to the spacecraft bus attitude determination and

control system (ADCS). The pointing feedback takes the form of azimuth and eleva-

tion measurements of the spacecraft orientation relative to the ground station, and

enables the ADCS to close the pointing loop and improve PAT performance. Thus,

the accuracy of the measured azimuth and elevation in particular is critical. Proper

design of the tracking signal can help ensure accurate pointing feedback.

Typically, a beacon can be used to illuminate the receiver for spatial acquisition

and tracking. In a general sense, the beacon is different from the communication

signal because it does not necessarily carry data. Rather, the pointing feedback that

is obtained from the beacon is solely determined as a function of how the optical

wavefront impinges upon the chosen detector. These beacons also typically have a

wider beam width compared to the communication beams to aid in spatial acquisition.

In the case of two way communications, such a beacon could be differentiated

from the communication signal by utilizing a different wavelength [1, 6]. However,

for TBIRD, since the tracking and communication are coupled and need to operate

31

simultaneously, we cannot consider differentiation via different wavelengths. Thus,

the tracking signal must be designed knowing that it must work in tandem with the

communication signal as discussed in Section 3.3.

In some optical communication systems, such as LLCD, an uplink beacon is mod-

ulated in order to aid in background rejection [6, 5]. Without modulation, stray light

can impede beacon detection, as described in Section 3.1. For example, the Optical

PAyload for Lasercomm Science (OPALS) demonstration in 2014 reported challenges

in determining appropriate beacon detection thresholds due to background distri-

bution uncertainties [1]. Beacon modulation at a known frequency helps mitigate

concerns about background light, since filtering for the modulated frequency can im-

prove detection. However, the frequency spectrum of a communication signal may

not have any discrete tones to track on. Therefore, careful choice of modulation for

the uplink signal is required.

The chosen frequency for a modulated uplink beacon must also respect the band-

width requirements of the attitude control system. The uplink beacon must provide

enough information for the system to lock and track in a timely manner, thus there is

a practical lower bound for the modulation frequency. As the modulation frequency

decreases, the cycle time increases. If the frequency is too low, the time required to

calculate the pointing feedback can rise to an unaccepable level. In addition, the bea-

con frequency must be higher than the atmospheric fading power spectrum. Since the

pointing bandwidth of TBIRD’s attitude control system is assumed to be around 1 Hz

and the atmospheric fading spectrum is typically below 1 Hz, a choice of modulation

frequency greater than 1 Hz is sufficient.

3.3 Design of a communication signal

In this section, the design of an optical communication signal will be briefly discussed.

The optical uplink communication signal affects a number of system properties, in-

cluding the data rate and bit error rate of the overall communication system. In

particular, the merits of pulse-position modulation (PPM) will be presented.

32

Historically, there have been a variety of communication modulation techniques

implemented in optical systems. Typically, the technique used depends on the char-

acteristics of the available laser transmitter and receiver technology.

The modulation techniques used can be grouped by detection method - direct

detection and coherent detection. In general, direct detection waveforms are more

straightforward to implement, because they only require detection of the optical in-

tensity and impose fewer requirements on the receiver thresholding. Thus, here we

exclude coherent detection waveforms from the discussion. Instead, we will limit this

section to presenting on-off keying and pulse position modulation.

On-off keying (OOK) was used for the NASA’s Optical Communications Sensor

Demonstration (OCSD)[17]. This modulation technique simply encodes two levels

of laser power as binary "0" (low signal power) and binary "1" (high signal power).

With OOK, each symbol encodes one bit of information, and requires a lower signal

bandwidth relative to the data rate.

Though detection and demodulation can be as simple as setting a threshold to dis-

tinguish between the two intended levels (typically the time-averaged signal power),

choosing the appropriate threshold can be difficult, considering the time-varying at-

mospheric fading effects that may be present in the channel, as well as data which

may have disproportionate occurrences of "0" or "1" symbols, though efforts can be

made to encode the data to avoid this case.

Another modulation technique is pulse position modulation (PPM). In contrast

to OOK, pulse position modulation (PPM) can encode multiple bits of information

in a single pulse. M-ary PPM divides symbols into M slots, with one slot per symbol

containing a pulse. Thus, demodulation of a symbol can be done by comparing the

relative signal levels within a symbol.

The relative signal power can be described by the extinction ratio 𝑟𝑒, defined as

𝑟𝑒 =𝑃𝑜𝑛

𝑃𝑜𝑓𝑓

When the extinction ratio is low, the bit error rate suffers, since it becomes more

33

difficult to tell which slots have a pulse. Furthermore, decoding a PPM signal requires

slot and symbol synchronization, or the determination of the boundaries between

slots, as well as between consecutive symbols.

In addition, the existence of a single pulse within a number of slots improves the

power efficiency, as high peak-to-average power laser sources can be used[13]. For

a given data rate, as M increases, the average power required decreases. However,

larger values of M result in narrower pulses, requiring higher bandwidth capabilities

of the detector and thus complicating detection and demodulation.

An example of a M-ary PPM signal is shown in Figure 3-1. Since it is the relative

position of a pulse within a symbol frame that encodes data, it is possible for a single

pulse to encode multiple bits of information, which is advantageous for systems with

average-power limited transmitters. For instance, the case where M = 8, a single

symbol could encode 3 bits of information, since there are eight possible slots where

pulses could occur. Specifically, each symbol encodes 𝑙𝑜𝑔2𝑀 bits.

PPM has been used in several optical communication systems to date. His-

torically, 2-PPM was used in the European Space Agency’s ARTEMIS [2], while

variable-rate 16-PPM and 4-PPM were used for NASA’s LLCD downlinks and up-

links, respectively[6, 4]. NASA’s ongoing Deep Space Optical Communications (DSOC)

project is also using PPM [20]. The use of PPM in multiple programs past and present

is a testament to its suitability for space-based optical communications. However, it

is important to recognize that PPM is not always the ideal modulation strategy for

all situations.

PPM also provides advantages in demodulation over OOK since it does not rely

on a set threshold. Rather it is self-thresholding in the sense that the value of a

particular symbol can be determined solely by comparing the relative power between

the slots of a symbol, which is more immune to time-varying link conditions compared

to the result from comparison to a single predetermined threshold. As a result, bit-

error rates (BER) for PPM can be lower than those for OOK at the same data rate.

However, this comes at a cost, as PPM requires double the bandwidth compared to

OOK for the same data rate.

34

Figure 3-1: Example of M-ary PPM signal. A symbol is divided into 𝑀 slots, one ofwhich contains a pulse.

BER is important for the TBIRD uplink because the communication bursts are

short, on the order of several minutes, yet target uplink data rate is relatively low, on

the order of 10kbps in order to achieve the desired downlink performance [19, 18, 15].

As described in the system overview section of Chapter 2, the uplink communication

is used for the ARQ; uplink communication performance is thus instrumental to

ensuring the integrity of the downlink communications. Thus, it is critical that the

BER is as low as possible, because any errors are expensive time-wise, and could

impede downlink retransmissions, ultimately being detrimental to the target downlink

communication rate.

For TBIRD, the use of PPM for the uplink communication signal is also desirable

because it allows for direct detection at the uplink receiver, which is discussed further

in Chapter 4. In addition, generating the signal at the ground station transmitter is

relatively straightforward. This choice is in line with the stated goal of minimizing

the complexity of the system, though requiring additional consideration towards the

synchronization of the signal at the spacecraft.

35

3.4 Design of a simultaneous tracking and commu-

nication signal

In the previous two sections, the relevant parameters for the tracking and commu-

nication uplink requirements are considered independently. In this section, we will

balance those parameters to design a signal which is appropriate for the TBIRD

uplink as a whole.

The tracking function imposes a requirement on the uplink signal for a discrete

tone with a minimum frequency to form a beacon that can be identified at the receiver.

The communication function in turn, leads us towards the usage of PPM data

modulation for the communications aspect of TBIRD, with a symbol frequency which

respects the 5 kbps data rate required to support the ARQ function.

However, we recognize that a pulse position modulation scheme does not inher-

ently contain any discrete tone. Thus, we introduce the concept of pulse shaping.

3.4.1 Pulse Shaping

In order to accommodate the tracking’s need for a known frequency content, a 2-PPM,

or binary PPM (BPPM) scheme with pulse shaping is used. As in traditionally defined

binary PPM, each symbol is divided into two slots, wherein the location of the slot

in which the pulse is located determines the value of the symbol.

However, instead of assuming a rectangular pulse in the occupied slot, we choose a

different profile for the pulse. Dead time is inserted on either side of the pulse within

the slot, which further increases the peak power of the received signal, and introduces

a discrete tone which can later be isolated through a narrow bandpass filter, as shown

in Figure 3-2.

Because the communication aspect of the signal requires much greater bandwidth

(5 kbps data rate) while the pointing aspect of the signal requires much lower band-

width (30 Hz updates) owing to mechanical actuation constraints and the response

time of the attitude control, it is possible to track on the pulse-shaped signal.

36

Since the uplink data rate for TBIRD is determined to be roughly 5 kbps, and

we choose the number of slots per symbol to be 2, a nominal slot rate of 10 kHz is

imposed.

Figure 3-2: Uplink modulation scheme. Insertion of dead time in a binary PPMwaveform with a 10 kHz slot rate introduces discrete tone at 10 kHz for trackingwhen sampling rate is 100 kHz.

3.5 Extraction of information from uplink signal

Given the uplink signal design as described previously, this section aims to present

an overview of the methods used to extract the dual-purpose information. First, the

extraction of the spatial tracking information will be presented. Then, the demodu-

lation of the communication signal will be briefly described.

For both tracking and communication, it is important to remind the reader that

in the TBIRD uplink receiver architecture, a single detector is used and thus, the

differentiation between the tracking and communication signals does not occur until

after detection and digitization.

37

3.5.1 Tracking information

In Sections 3.2 and 3.4.1 a case is made to introduce dead time in the BPPM pulses

to introduce a discrete tone for tracking.

The spatial tracking information is not directly encoded in the signal, since the

relative orientation and movement of the ground station transmitter and space ter-

minal receiver is solely a function of the link conditions. However, it is the use of a

quad cell photodetector which provides four spatially related signals from a 2x2 array

of PIN photodetectors that provides the spatial tracking information. Section 4.1

details the choice of the quad cell as the uplink receiver sensor; this section will only

discuss the processing of the resulting signals.

The four quadrants of the sensor each yield a signal proportional to the optical

power incident on that quadrant. In the ideal case, a lens with focal length 𝑓𝑐 focuses

the optical signal on the sensor, producing an Airy disc in the focal plane with in-

tensity as given in [9]. In the non ideal case, which might arise due to defocus, the

intensity distribution can instead be generally described as 𝐼(𝑥, 𝑦) where 𝑥, 𝑦 describe

the plane of interest of the intensity. As the angle of incidence on the lens varies, the

spot shifts on the sensor, resulting in unequal power levels on each of the quadrants.

In the situation where the Airy spot is moving away from the center into a particular

quadrant, the signal increases, saturating when the Airy pattern is encompassed in

the quadrant and then decreasing to zero as the spot leaves the detector entirely.

The relationship between the azimuth and elevation incidence angles 𝜃𝑥, 𝜃𝑦 and spot

location, 𝑢𝑥, 𝑢𝑦 is shown in Figure 3-3.

Figure 3-4 shows a set of example signals in the noise-less scenario from each quad-

rant as the spot moves. The signal magnitude increases with the amount of optical

power incident on a given quadrant. A perfectly centered spot results in all signal

magnitudes being equal. However, when there is no pulse present, in the absence

of noise we see that the signal magnitudes are all zero; here no useful information

regarding the spatial location of the spot can be obtained.

A discriminant calculation based on the magnitudes of the four signals yields the

38

Figure 3-3: The quad sensor is located 𝑓𝑐 away from the lens. As the azimuth andelevation angles of incidence 𝜃𝑥, 𝜃𝑦 of the planar wave changes, the location 𝑢𝑥, 𝑢𝑦 ofthe spot on the quad sensor changes. When the planar field arrives normal to thelens, the spot is centered.

relative location of the spot on the quad. The discriminant values 𝑥 and 𝑦 are related

to the spot locations 𝑢𝑥 and 𝑢𝑦 as shown in Figure 3-3 as a function of the intensity

of the optical power in the plane of the sensor.

𝑦 =(𝐴+𝐵)− (𝐶 +𝐷)

𝐴+𝐵 + 𝐶 +𝐷

𝑥 =(𝐴+ 𝐶)− (𝐵 +𝐷)

𝐴+𝐵 + 𝐶 +𝐷

𝑥, 𝑦 = 𝑓(𝑢𝑥,𝑦, 𝐼(𝑥, 𝑦))

If the focal length 𝑓𝑐 of the lens is known, and small angles of incidence 𝜃 are

assumed, the location of the spot 𝑢 on the spot can be determined.

𝑢 = 𝜃𝑓𝑐

Though the discriminant is a measure of spot location, in practice, the value

of the discriminant must still be experimentally mapped to the angle of incidence

of the uplink optical wavefront upon the receiver aperture, because the relationship

between 𝜃 and 𝑢 may not be linear, especially for larger values of 𝜃. Furthermore, it is

39

(a) Centered quad signals. (b) Off centered quad signals.

(c) Centered spot on quad.

Equal power on each quadrant.

(d) Off centered spot on quad.

Increased power on quadrant B

relative to centered case.

Figure 3-4: Example signals from the four quadrants of the sensor, with the BPPMsignal visible in the centered and off-centered cases. The red spots depict the locationof the full-width half-max spot impinging upon the sensor.When no pulse is present, no magnitude difference between quadrant signals is ap-parent.

important to recognize that the accuracy of the discriminant relies upon the following

conditions.

First, the gap size between the four quadrants impacts accuracy by impeding

detection of all of optical power. There should be minimal gaps between the four

quadrant sensors of the quad detector, in order to maximize the amount of power

detected. If power is lost between the gaps of the quadrant sensor, then the relative

power levels will no longer be accurate.

Second, the quad sensor must be aligned with the boresight of the lens in order

to ensure that there is no spatial offset.

Finally, the level of defocus and quality of the spot intensity pattern can impact

accuracy by spreading the optical power across the sensor. If the spot intensity

40

pattern is not radially symmetric due to aberrations or astigmatism of the lens, there is

potential for spatial offset of the calculated discriminant. The discriminant determines

a centered spot in a given axis as one where the total power on one side of the axis

is equal to the total power on the other size of the axis. However, if the intensity

distribution of the spot is greater on one side, then the center as determined by

the discriminant will have some offset. For pointing, a small offset is not inherently

detrimental to the performance of the link, since a maximum amount of power is still

centered. However, when combined with a potential defocus, the performance may

suffer as the discriminant becomes less responsive.

Ideally, the receiver is located in the focal plane of the lens and optical power is

concentrated in a very small spot. In this ideal case, the power is concentrated and

centered at the intersection of the four quadrants. Small changes in the incident angle

will result in large changes in the discriminant value.

As the spot is defocused, the power is spread radially, and the difference between

relative power levels among the four quadrants decreases . In the extreme case, if the

spot is extremely defocused, then the power is spread such that the responsivity of

the discriminant is poor; equal power blankets each of the quadrants and the value

of the discriminant changes only for large changes in wavefront incidence angle.

The discrete tone that is included provides the receiver with a known frequency to

lock on to in order to isolate the uplink signal from background and communication

signaling. Thus, a bandpass filter is implemented to isolate the tone.

The calculation of the discriminant is most useful in cases with relatively large

differences in signal magnitude. In addition, as observed in Figure 3-4, where there

is no pulse, the discriminant by itself does not provide a useful measure of the spatial

location of the spot, therefore, a phase-locked loop (PLL) is used to lock on to the

tone and obtain an appropriate discriminant.

3.5.2 Demodulation

In order to extract the communication signal, due to the intensity modulating BPPM

signal chosen, only the total received power needs to be considered. As a result, the

41

four signals from the quad sensor are summed in order to determine the total received

power.

As described earlier when presenting the characteristics of PPM, the chosen modu-

lation technique requires synchronization of both slots and symbols for demodulation.

Slot synchonization can be determined by using the PLL used for the extraction of

tracking information, since the PLL locks on the the pulses of the signal.

However, symbol synchronization, which determines which slots belong in to which

symbol is still necessary. Since there is no shared clock between the receiver and the

transmitter, and since there is no guarantee of where in a symbol a transmission

might lock on and begin, symbol boundaries cannot be directly determined.

Methods for symbol synchronization include the use of a repeating known sequence

which the receiver can identify at either the beginning of a transmission or interpersed

between data symbols.

Once slot and symbol synchronization is achieved, demodulation of the signal can

be achieved in a straightforward manner due to the construction of the BPPM signal,

which is self thresholding, as mentioned in Section 3.3. The relative power levels

between the slots in a symbol can be compared in order to determine the symbol

value.

42

Chapter 4

Hardware

The choice of hardware for a given system drives not only the potential performance

of the system, but also reflects the implementation approach. Because a goal of the

TBIRD demonstration is to illustrate the potential for low complexity implementa-

tions of high data rate laser communication systems, the hardware for the TBIRD

uplink receiver was carefully considered and critical to the development of this the-

sis. Furthermore, due to the dual-purpose design of the receiver, it was important to

balance both the uplink communication and pointing feedback requirements in order

to achieve satisfactory performance. In this section, two key hardware choices of the

TBIRD uplink receiver will be highlighted: the sensor and computation component.

Fundamentally, the priorities of the two subsystems are different; while the com-

munication aspect prioritizes high sampling bandwidth to enable higher possible data

rates, the attitude determination goal of the pointing feedback aspect prioritizes min-

imization of background radiation and increased field of view, since the spacecraft

jitter and attitude control bandwidth is slow relative to the communication band-

width. COTS hardware is desired that will not only serve both subsystems, but is

also reasonable to implement. As such, the following sections will touch upon the

performance of the chosen hardware component with regards to the aforementioned

metrics. The first section in this chapter will discuss the choice of sensor, while the

second section will discuss the computation aspect of the receiver.

43

4.1 Uplink Detector

This section will focus on the uplink receiver sensor. First, a description of the role of

the uplink receiver sensor will be given. For TBIRD, a quadrant photodetector sensor

was chosen. A brief introduction to the quad sensor will follow, and the section will

conclude by detailing the work done to integrate the quad sensor into the TBIRD

architecture.

The receiver sensor plays an important role in the design of this system, as the

sole provider of information to the uplink receiver. The detector converts the optical

signal at the spacecraft into another signal domain for further processing. The design

of a detection system depends upon the signal to be detected; modulated signals that

require mixing of the optical carrier to some frequency result in coherent detection

systems, while systems that do not require mixing at the receiver may utilize direct

detection. Direct detection is more straightforward to implement, since the optical

signal is converted directly to an electrical signal, which corresponds to the received

intensity of the optical power [12, 13]. As discussed in Chapter 3, TBIRD uses

a pulse-position modulation scheme. In this scheme the intensity of the uplink is

modulated over time, without any mixing frequencies; thus a direct-detection design

is appropriate.

Common detectors for laser communication include avalanche (APD) and p-intrinsic

(PIN) photodiodes. PIN photodiodes generate a current which is proportional to the

optical power incident on the detector, whose response is typically described as a

function of the responsivity, 𝑅 of the detector as follows, where 𝑖 is the current gen-

erated and 𝑃𝑜 is the optical power. There is no internal gain available with PIN

photodiodes.

𝑅 =𝑖

𝑃𝑜

In contrast, APDs have increased photosensitivity, because they have internal gain

resulting from the exploitation of the avalanche process in which collisions occur from

carriers within the photodiode being ionized[12]. Due to this internal gain, APDs also

44

have increased noise, and are less suited for cases where background light is present,

since the noise arising from background has the potential to overpower the signal.

Typically, high rate applications use PIN photodiodes, as APD photodiodes trade

increased sensitivity for lower electrical bandwidth[7].

Multiple photodiodes can be arranged to form an array. A camera consists of many

photodiodes put together, with each photodiode forming a pixel. Design complexity

scales up with the increase in array size; more sensors means more signals to read

out and process; the read out rate of a detector with many pixels is often limited in

practice. For communications applications, we care about data rate of the detector,

and camera frame rates are not sufficient in many circumstances. We also care about

the noise characteristics of our detectors and thus consider the signal-to-noise ratio

(SNR) of our receiver design.

Figure 4-1: Block diagram of optical and digital components. The receive optics areshown on the left side of the diagram in blue, while the digital components are shownon the right side in black. The green highlighted area comprises the quad sensorcircuit. The circuit amplifies and digitizes the quad sensor signals for processing.

In this application a quadrant PIN photodetector sensor, an array of four pixels

was chosen as the exclusive sensor for the uplink receiver. A block diagram detailing

the uplink receiver components and architecture is shown in Figure 4-1. The quad-

rant sensor has four photodetectors arranged in a 2x2 grid. Each quadrant produces

an analog output commensurate with the amount of power received on the quadrant.

In order to maximize the detected signal, a sensor with minimal gaps between pho-

45

todiodes was chosen. An amplification circuit amplifies the signals and an analog

to digital converter (ADC) digitizes the signals to be processed. The sensor, along

with the amplification and digitization circuits which are discussed further in Sec-

tion 4.1.2 are pictured in Figure 4-2. The small footprint of the sensor and circuit is

well suited for TBIRD’s small payload volume. Figure 4-2b additionally shows the

relative positioning of the receive aperture and the quad sensor itself.

(a) Size relative to U.S. quarter(b) Mounted in front of receive aper-

ture.

Figure 4-2: Quadrant sensor with amplification and digital conversion circuit.

The quadrant detector was chosen over a larger focal plane array primarily due

to the low implementation complexity and straightforward interpretation of the four

signals. A larger focal plane array would suit the uplink tracking by improving back-

ground rejection, but it has a much higher interfacing and computational complexity

due to the high frame rate that is required to support 5 kbps comm. Per Nyquist, the

sampling frequency must be twice the highest bandwidth of the signal; in the case

the 10 kHz slot rate of the PPM format chosen implies that the sampling frequency

is at least 20 kHz. The overall throughput of the read out scales with this worst-case

sampling frequency requirement 𝑛𝑓𝑛, where 𝑛 is the number of pixels, and 𝑓𝑛 is the

minimum sampling frequency, 20 kHz.

4.1.1 Sensor Noise Characteristics

As discussed earlier in this chapter, we care about the SNR of the chosen sensor. In

PIN photodetectors, there is both background noise and process noise to consider.

Background noise arises from stray light impinging on the sensor. From the previous

46

discussion regarding signal design in Chapter 3, we chose to introduce a discrete tone

into the signal so that the tone may be filtered and background rejected. Furthermore,

an optical filter is used in front of the sensor as shown in Figure 4-1 to isolate the

transmitted signal wavelength.

Process noise includes both dark noise, as well as shot noise. Dark noise is the

amount of noise in the detected signal when the sensor is not illuminated, an intrinsic

characteristic of the photodetection process and the sensor itself, depending on both

the temperature and the size of the sensing area. Shot noise arises as a result of

the random fluction of current from carrier interactions within the photodetector.

Typically, shot noise increases with signal current. For PIN photodetectors, shot

noise usually dominates.

4.1.2 Quad Sensor Circuit

A circuit was designed to amplify, filter and digitize the quad circuit signals. Am-

plification serves two purposes, allowing us to condition the signals such that they

are appropriate for digitization, but also to enable us to observe slight changes in

received intensity. As such, the quad sensor circuit was designed to have two analog

gain stages. Performing the amplification in the analog domain is desirable because

amplification of electrical signals is well understood and easily implemented, without

requiring additional computation resources. A high gain amplifier is used in low SNR

situations, while a low gain amplifier is used for higher SNR situations. In addition,

an analog band-pass filter is included. A single 8-channel ADC is used to digitize the

low and high gain analog signals. The ADC has 12 bits of resolution, and when com-

bined with the amplification coefficients, can distinguish between a received power

difference of only 1.2 pW. The ADC is able to interface serially with the receiver

microcontroller using a SPI interface.

47

4.2 Receiver Computation

Because the uplink receiver must interface with both the communications system

(internal to the payload) and the tracking system (external to the payload) while

only using a single sensor, signal processing must occur to obtain the data for each

subsystem; the communication data must be demodulated, and the meaningful point-

ing feedback measurements must be determined. To minimize complexity, the signal

processing is performed in the digital domain.

A COTS radiation hardened microcontroller was chosen to perform the tracking

part of the signal processing for the digitized output of the quadrant sensor. Al-

though microcontrollers are not as computationally efficient as a field-programmable

gate array (FPGA), and the chosen microcontroller is not specifically optimized for

digital signal processing (DSP) applications, they require much less power. However,

microcontrollers are not capable of executing parallel computations, require several

clock cycles to perform computations or read/write to memory, which makes the effi-

cient design and implementation of DSP a challenge. Here, the sensibility of choosing

a quad sensor is reinforced, because the computational resources required to interface

with sensor is still within the capabilities of the chosen microcontroller.

The signals from the quadrant sensor are processed through the circuit and the

digital outputs are sent to the microcontroller over an SPI interface with four wires.

Within the SPI interface, the microcontroller acts as the master, and requests 12-

bit samples from one channel at a time. The microcontroller decides whether to

sample from the high gain or low gain channels, depending on link conditions. Ulti-

mately a sampling rate consistent with 10 samples per slot are desired for processing

of the uplink communication signal, resulting in a desired ensemble sample rate of

100 kilo samples per second for each of the four channels. Because the samples must

be obtained serially, the microcontroller must be capable of sampling from the quad

circuit at a minimum of 400 kilo samples per second, with extra clock cycles in order

to perform online signal processing. Therefore, the microcontroller’s maximum SPI

clock rate of 16 MHz, which is adequate for this application is used.

48

Furthermore, since memory in the chosen microcontroller is 8-bit aligned, packing

of the 12-bit data from the quad sensor circuit ensures efficient memory allocation.

In the TBIRD architecture, it is desirable to reduce the amount of time that the

high power components are powered up, due to thermal considerations. Excess heat

generated by components such as an existing onboard FPGA limits the duration of

the downlink transmissions. Thus, the design of the TBIRD space terminal utilizes

the microcontroller to perform those signal processing operations associated with ac-

quisition and tracking, which occurs prior to downlink transmissions, and the onboard

FPGA to demodulate the uplink data. As a result, it is possible to perform tracking

without powering on the FPGA.

49

50

Chapter 5

Simulation

In order to validate the tracking component of the dual-purpose uplink receiver ap-

proach, a simulation of the system was created using MATLAB and Simulink. A key

goal of the simulation is to tie together the PAT performance with the system per-

formance as a whole. The simulation encompasses both the spacecraft bus pointing

dynamics, as well as the payload uplink, including transmission at the ground termi-

nal and channel properties. Proof that the pointing error provided by the payload to

the spacecraft bus improves the pointing performance is desired.

This chapter will present the components of the simulation, as well as results of

the pointing performance validation. The first section will lay out the TBIRD-specific

elements, while the second section will present the payload pointing elements. The

final section will present the results of the simulation.

5.1 Simulation of TBIRD Uplink

The simulation of the TBIRD uplink includes both the signal generation and trans-

mission from the ground station to the spacecraft. The signal generation takes a

pseudorandom binary sequence and modulates it according to the BPPM scheme

with dead time as described in Chapter 3. Atmospheric fading effects are introduced

using a fading profile which characterizes the power losses incurred in the communi-

cation channel.

51

At the spacecraft receiver, sensor noise is introduced as an additive white Gaussian

process on each of the quadrant photodetector’s four sensing quadrants. The quad

sensor circuit filters are also included in the simulation. A phase-locked loop is then

used to generate an angular pointing error, to be provided to the spacecraft.

5.2 Simulation of Spacecraft Pointing Performance

A time-domain simulation was developed in Simulink to characterize the spacecraft

pointing performance. A block diagram of the major components is shown in Fig-

ure 5-1. A 6-U CubeSat is modeled in a 400 km orbit. A reference trajectory is

generated for a ground station pass with a maximum elevation of 75∘. The spacecraft

is assumed to have one star tracker and gyroscopes in three axes, as well as three

reaction wheels. These sensors and actuators are modeled based on existing COTS

components. Attitude estimation is performed with a 6-state extended Kalman filter

(EKF) and a simple proportional-derivative controller generates reaction wheel com-

mands. The control gains are selected to set the closed-loop control bandwidth at

0.5 Hz with no payload feedback and 1 Hz with payload feedback. Sources of error in

the simulation include uncertainty in inertial properties, reaction wheel misalignment,

sensor noise, sensor misalignment, gyro drift, and onboard timing error.

Figure 5-1: Block diagram of spacecraft pointing simulation.

Feedback from the communications payload is provided to the bus at a rate of

30 Hz. A binary PPM waveform with a 10 kHz slot rate and with 50% dead time

52

per slot is generated to simulate the uplink. The dead time introduces a 10 kHz tone

that is used for background rejection. An atmospheric fading profile is applied to the

waveform. On the spacecraft, a phase-locked loop is used to demodulate the uplink

in software and generate an angular attitude measurement. The measured angular

error is fed directly into the controller and the EKF provides an error estimate for

the remaining axis.

5.3 Simulation performance

Results from a simulated spacecraft pass are shown in Figure 5-2. If no feedback from

the payload is provided, the root-mean-square pointing error of the bus is 62 𝜇rad.

With payload feedback, the pointing error reduces to 22 𝜇rad RMS. The payload

feedback greatly reduces low-frequency bias since the quad cell is rigidly coupled with

the downlink transmitter.

This result highlights a key advantage to the single uplink receiver design. Because

the pointing error is determined directly from the uplink receiver which is in turn

rigidly coupled with the downlink transmitter, the pointing acquisition and tracking

system is robust to potential misalignment between the payload and the star tracker

on the spacecraft bus. Although misalignment is not included in the simulation at this

time, it is reasonable to predict that with misalignment, the pointing performance

with the payload sensor would not significantly differ from the aligned case, while the

pointing performance without the payload sensor (relying only on the spacecraft bus

pointing capabilities) could be potentially seriously degraded.

5.3.1 Simulation Pointing Loss

Ultimately, an understanding of how the pointing feedback from the payload affects

the communication performance is desired. As mentioned previously, loss of optical

signal power on the receiver negatively impacts communication, which is the reason

proper pointing and alignment is so critical.

Figure 5-3 relates these pointing errors back to the communications link losses.

53

Figure 5-2: Simulated spacecraft pointing error for a 160 second pass from 30∘ eleva-tion through a maximum of 75∘ elevation. Spacecraft bus pointing without payloadfeedback (blue) and with payload feedback (orange) are shown.

The losses in Fig. 5-3 correspond with the pointing errors with payload feedback in

Fig. 5-2. There is a large initial transient as the filters converge. The median pointing

loss is 0.2 dB and losses are less than 1.3 dB 99% of the pass. These pointing losses

are low enough to support a 200 Gbps burst rate.

Figure 5-3: Downlink pointing loss with payload feedback. The median loss is 0.2 dB,and the 99th percentile is 1.3 dB. There is an initial transient period of increased loss.

54

Chapter 6

Experimental Setup and Results

An experimental setup was created for the testing and verification of the TBIRD

uplink receiver, from receiving the optical signal at the quad cell sensor to the spatial

tracking and communication data at the microcontroller. A key goal of the experimen-

tal setup was to create a setting which would reasonably reflect on-orbit conditions.

As such, extensive steps were taken to verify and validate the setup in order to be

certain that any results would be faithful to the conditions which will ultimately be

experienced by the receiver. Thus, in addition to the main testing of the uplink

receiver, there was additional, isolated testing of the sub-components involved. In

addition, the test bench was designed to allow for future integration with other parts

of the TBIRD system.

This chapter will first detail the design and validation of the experimental set up,

then present the testing procedure and preliminary results.

6.1 Experimental Setup

The goal of the experimental setup is to generate an optical signal for detection with

the quad cell sensor and processing with the microcontroller. In addition, the ex-

perimental setup includes a method to introduce tracking disturbances to the system

in order to validate the uplink tracking performance. As such, the test bench can

be roughly divided into three parts consisting of optical, mechanical and electrical

55

components.

The optical part of the test bench consists of those components which generate

and manipulate the optical signal, while the mechanical part introduces tracking

disturbances into the system. The electrical part begins at the quad cell sensor,

where the optical signal is converted into an electrical signal, and includes the quad

circuit and microcontroller. A block diagram of the test bench is shown in Figure 6-

1. The optical signals are shown with red solid and dotted lines, while the electrical

signals are shown with double blue lines.

Figure 6-1: Block diagram of test bench.

The experimental setup, pictured in Figure 6-2 shows the mounted quadrant sensor

circuit and microcontroller for testing. A fiber collimator for the laser was sized such

that the beam blanketed a 2 cm aperture lens, which will serve as TBIRD’s space

terminal receive aperture. In addition, a fast steering mirror (FSM) was included

in order to introduce tracking disturbances to the system. An autocollimator was

inserted into the system to measure the tracking disturbances induced by the FSM.

Because angular disturbances at the FSM result in large linear displacements at

the receiver aperture, and because it is desired for the entirety of the spot to blanket

the aperture, care was taken to minimize the beam path between the FSM and the

56

receiver aperture as much as possible to avoid pointing the beam off of the aperture.

Figure 6-2: Experimental setup. Uplink beam path indicated by arrows.Components legend: A: Fast-steering mirror, B: Receive aperture lens, C: Quadrantsensor circuit, D: Autocollimator, E: Fiber collimator

The testing inputs, which consisted of the uplink signal waveform as well as the

tracking disturbances were controlled with a Speedgoat Real-Time target. This al-

lowed for a centralized testing interface. The received signal at the quadrant sensor

was amplified and digitized on the sensor circuit, and the resulting digitized values

sampled by the microcontroller, as described in 4.2.

In the work presented in this thesis, values sampled at 1.85 kHz were then sent to a

host computer for offline signal processing. Though this received data rate was slower

than the ultimate goal, it was constrained by the need for offline signal processing.

In the future, the signal processing required for tracking will be fully implemented on

the microcontroller, and will attain faster sampling rates.

6.2 Testbench characterization

The components of the test bench were separately tested and qualified before exper-

imental testing to validate the test accuracy to on-orbit conditions. This section will

present several of the validation checks that were performed, including measurements

for the extinction ratio of the modulated laser, images taken of the beam profile of the

received optical signal at the quad sensor, calibration of the FSM and measurement

57

of the received power. Furthermore, this section will present the procedure used to

ensure alignment of the beam.

6.2.1 Direct-drive laser

In this test setup, the laser must be modulated in order to emulate the uplink signal,

as discussed previously in Chapter 3. The chosen BPPM modulation with shaped

pulses requires intensity modulation between high and low power settings with sharp

transitions. Ultimately, the ground station for TBIRD will be generating this uplink

signal.

Instead, a direct-driving method is chosen to modulate the intensity of the laser

by modulating the current source which drives the laser. An initial concern is with

the bandwidth of the laser source, since sharp transitions are desired. For example,

a perfect square wave contains infinite frequency content which is impractical to

generate. As the bandwidth of the laser source increases, the sharper the edges, but

as the bandwidth decreases, the edges will slow and eventually take on a sawtooth-like

pattern.

Degradation of the optical signal impacts the performance of the uplink receiver’s

ability to recover the desired spatial tracking and communication signals.

In order to evaluate the laser intensity waveform, a 10 kHz square wave source

was used to modulate the laser, so chosen to match the target uplink bandwidth.

The fiber coupled laser was then provided as an input to a separate PIN photodiode,

whose electrical output signal was then displayed on an oscilloscope, as shown in the

block diagram in Figure 6-3.

The resulting waveform, as shown in Figure 6-4 depicts sharp transitions which

are appropriate for the uplink optical signal. The 10-90 % rise time, averaged over

20 cycles was ≈ 600 ns, which is much faster than the other dynamics in the system.

In addition to the shape of the laser intensity waveform, it is also important to

evaluate the extinction ratio of the modulated laser. The extinction ratio, 𝑟𝑒 measures

the ratio between the high and low optical power levels of the laser, and is typically

measured in dB. Poor modulation extinction results in power losses at the transmitter,

58

Figure 6-3: Test set up to evaluate direct-drive performance of the laser source.

Figure 6-4: Resulting waveform of directly driven laser with 10 kHz square wavesource. The purple waveform is the source, and the yellow waveform is the resultingoptical output as measured with a PIN photodetector. The rise time is much fasterthan the 10 kHz dynamics.

and adversely affects the probability of bit errors as described in [7] and [6].

𝑟𝑒[𝑑𝐵] = 10𝑙𝑜𝑔10(𝑃′1′

𝑃′0′)

In the ideal case, a high extinction ratio is attained by maximizing the power in

the pulses of the PPM signal and minimizing the power where no pulse is present.

This minimizes the loss of power at the transmitter, and furthermore ensures that

future consistency of results from uplink receiver testing.

The extinction ratio was measured by taking the level of the logic ’0’ and logic

’1’ values as measured by the oscilloscope, respectively, and was determined to be 30

59

dB. The measured 𝑟𝑒 is appropriate for the uplink receiver testing, and can be used

as a consistency check for later experimental procedures.

6.2.2 Beam Profile and Defocusing

In Section 3.5.1, the impact of the quality of the spot intensity pattern on the tracking

performance is presented. It is important to verify the spot intensity in order to ensure

that the received spot on the quad is sized appropriately for the application, and that

the power is not spread excessively across the four quadrants. In the ideal case, since

a uniformly illuminated circular aperture is used, the intensity of the spot incident

on the quad would be in an Airy pattern, which appears as a spot with concentric

symmetric bright rings. The majority of the optical power should be located in the

center spot, whose diameter can be described by

2𝜔0 u 1.22𝜆𝑓

𝑑

where 𝜔0 is the radius of the first null , 𝜆 is the wavelength of the impinging beam and

𝑓, 𝑑 are the focal length and aperture diameter, respectively. With the wavelength

being 1550 nm, and the lens having focal length 4 cm and diameter 2 cm, the diameter

of the spot is expected to be approximately 6.4 𝜇m.

The setup was verified with a scanning slit laser beam profiler, which checked that

the beam was appropriately sized and reasonably suited our expectations, as well as

with a camera. Because the slit of the beam profiler was larger than the expected

spot size, a 40x magnification lens was used. The resulting measured spot diameter

was 12 𝜇m, larger than the expected ideal value, but acceptable for the experimental

setup, especially since it may be desired to intentionally introduce a small amount

of defocus into the system to spread the power for improved tracking performance.

Possible reasons for the discrepancy include the challenge of aligning the beam profiler

with the focal plane, as well as imperfections in the lens used. Furthermore, the shape

of the spot intensity was imaged with a camera, as shown in Figure 6-5 which showed

that the rings associated with the Airy diffraction pattern were indeed symmetric and

60

concentric.

Figure 6-5: Camera image of spot intensity, showing symmetric Airy diffraction pat-tern.

Though the beam profile was verified to be appropriate for the testing set up at

the focal plane, the placement of the quad in the setup in the focal plane is still

challenging. While it can be verified experimentally that the quad is in the focal

plane during the beam alignment process described in Section 6.2.5, the lateral pitch

of the mounted quad must be adjusted by hand and is prone to small errors.

6.2.3 FSM Calibration

The testing of spatial tracking performance of the uplink receiver requires the ability

to inject spatial disturbances in the system. In this test bench, spatial disturbances

are introduced by using a FSM to aim the beam at different locations.

It is desired to calibrate the FSM in order to ensure that the magnitude of the

disturbances are appropriate and accurate, and that the throw and resolution of the

FSM are adequate for the testing scenario. Linearity of the FSM actuation is also

desired. In addition, calibrating the FSM aids in the beam alignment procedure

in Section 6.2.5 by allowing for precise and repeatable adjustment of the beam, in

61

contrast to manual adjustment using micrometer knobs and a three-axis stage.

The FSM used for this testbed is driven with two analog voltages for the x and

y axes respectively provided to a controller. The input analog voltages were in turn

generated by the Speedgoat real-time target, and can also be manually set through

a graphical user interface designed for the Speedgoat. Calibration involved mapping

the Speedgoat-generated input analog voltages to angular displacements of the mir-

ror. An autocollimator, positioned in front of the FSM was used to determine this

mapping. The autocollimator was very sensitive to angular disturbances to a fraction

of a microradian.

A sine wave was used to drive the each of the mirror’s two axes, and the dis-

placements were measured by the autocollimator. Comparison between the sine wave

and autocollimator measurement confirmed the FSM to have very linear performance

and a scale factor relating the input voltage and output angular displacement was

calculated for each axis. Furthermore, it was found that the FSM had more than

adequate throw for the application. Limits in the Speedgoat controller were added to

ensure that the FSM would not point the beam off of the receive lens.

6.2.4 Power Calibration

The performance of the uplink receiver is highly dependent upon the received power at

the sensor. Increased power results in higher SNR and better performance and is one

of the critical metrics in assessing the performance of the uplink receiver. Therefore,

it is important to accurately determine and control the received power at the sensor.

In order to control the optical power level, a digital variable optical attenuator

(VOA) is used. The VOA was first verified for linearity across its’ entire range [0dB, -

60dB] by measuring the power at the output port with a fiber-coupled power meter

and stepping the attenuator setting. However, this does not take into account any

losses incurred between the output of the attenuator and the uplink sensor. Therefore,

a free space power meter is used near focus of the uplink receive aperture. Because

the free space power meter is not linear past u -8 dBm, and the expected on-orbit

received power is between -60 and -80 dBm, it is necessary to measure the power

62

at the receiver with no attenuation and use the knowledge of attenuator linearity to

infer the lower power levels. The power near focus of the uplink receive aperture was

measured to be -4.2 dBm, meaning that with attenuator, the power at the sensor can

be set with a range from -64.2 dBm to -4.2 dBm. When additional attenuation is

needed, a fixed attenuator can be introduced to the system to achieve power levels

down to the target of -80 dBm.

6.2.5 Beam Alignment

Beam alignment with the center of the sensor precludes accurate testing results.

Therefore, careful alignment is necessary. Four analog outputs from the quad sensor

circuit (one per quadrant) are used in the alignment process. A modulated laser with a

7 kHz sine wave was used to produce a spot on the quad, which is mounted on a 3-axis

stage, and the analog outputs were observed. The displacement of the 3-axis stage is

controlled by three micrometers, which each have a resolution of 2.54 𝜇m. Although

the location of the spot relative to the sensor can also be controlled by adjusting the

tip and tilt of the FSM, thereby altering the angle of incidence upon the lens, for the

beam alignment, initial manual adjustment by micrometer is preferred.

Figure 6-6: Beam alignment setup. Quad mounted on 3-axis stage is centered (X andY axes) and in the focal plane (Z axis) of the aperture. Analog outputs from thequad are monitored in order to verify alignment.

When the quad is located in the focal plane, the spot incident on the quad has

63

a minimum diameter, and thus any perturbations of the axes will result in large

observed amplitude changes of the analog output, which can be described has having

high sensitivity.

In order to find the focal plane and align the spot to the center of the quad sensor,

an iterative procedure consisting of two steps is performed. First, the closest focus is

found by perturbing the Z and X axes of the stage. As the Z axis is moved in and

out of the focal plane, the sensitivity of the X axis will decrease due to the spot size

increasing. When the highest sensitivity is achieved, it is appropriate to move on to

the centering step.

The centering step consists of adjustment of the X and Y axes until the output

amplitudes are equal; there is equal power on each quadrant. One method of achieving

alignment in this manner is to adjust the axes until two of the four outputs exhibit

the same amplitude. At this point, the spot is straddling the line between two of

the quadrants. From here, it is possible to walk the spot down the line between the

quadrants to the center of the quad by perturbing one axis until the two signals are

no longer aligned and using the other axis to realign the two amplitudes until the

goal of observing equal amplitudes across all four amplitudes is attained.

When the beam is centered and in focus, small disturbances in the micrometer

will result in large observed amplitude changes of the analog outputs; a light touch

to the micrometer is enough to push the quad out of alignment.

The process described in this section is followed prior to experimental testing in

order to certify that the spot is centered on the quad, and as close to the focal plane

as possible. By always starting with the quad in the focal plane, any defocusing that

is later introduced to the system can be calibrated to the focused setting and thus

reproducible.

6.3 Tracking performance

Previously, the methods for qualifying the test bench set up were described. In this

section, the experimental procedure for the evaluation of the tracking performance of

64

the TBIRD dual-purpose uplink receiver will be discussed.

Performance evaluation involves a comparison between the output of the uplink

receiver and a ground truth of the position disturbances introduced by the FSM. As

described in Section 6.1, a Speedgoat real-time target is used to control the entire

test set up through a graphical user interface designed in MATLAB.

Analog outputs from the Speedgoat are used to drive the tip and tilt of the FSM,

respectively. In each axis, a driving pattern can be set to either a settable constant

value or a sinusoid with tunable frequency, phase, amplitude and offset. Analog

outputs from an autocollimator positioned parallel to the face of the FSM is used to

measure a ground truth of the actual disturbance. A separate analog output is used

to modulate the laser, which can be set to either a square wave or a BPPM signal

with dead time. The percentage of dead time in a slot can be set. All values are

logged by the Speedgoat for future analysis.

The modulated laser beam is collimated and propagates through free space, re-

flecting off the disturbance-injecting FSM, through the receiver aperture and focusing

on the quad sensor. For the tracking performance test, the signal from the quad cir-

cuit is sampled by the microcontroller. Because the microcontroller implementation

of the signal processing has not been completed, the microcontroller then passes the

samples to a host computer for offline processing.

6.3.1 Uncalibrated Results

A preliminary test was conducted in which the FSM was driven in a Lissajous pattern

in a high signal to noise ratio scenario. The total received power on the quadrant

sensor was -58 dBm. The result from the offline signal processing is plotted in Fig-

ure 6-7.

Although the system has not undergone full calibration, the results, which show

the agreement of the quad sensor measurement and the true disturbance are promis-

ing. The two-axis error is 5.7 𝜇rad RMS. After calibration, it is expected that the

RMS error will be reduced.

65

Figure 6-7: Experimental result. Tracking disturbances introduced with a fast steer-ing mirror (blue) compared to quad sensor measured disturbances (orange). Two-axiserror: 5.7 𝜇rad RMS

6.3.2 Calibration

The calibration process serves two purposes. First, the calibration generates a map-

ping between the quad determinant and the location of the spot on the quad sensor,

with the byproduct of establishing the boundaries of the linear region of the quad as

well as verifying the gaps between quadrant sensing areas. Second, calibration allows

for the quantification of the relationship sensitivity of the tracking performance and

the level of defocus of the beam, and confirmation of the spot size on the sensor.

In order to calibrate the tracking portion of the receiver, the Speedgoat interface

is used to set the position of the FSM to a set of discrete values, from which the

expected beam displacement can be determined. These settings could be scanned

in a raster pattern across the sensor area, as in Figure 6-8. The raw determinant

from the sampled sensor outputs is recorded and plotted against the FSM settings.

From this , the regions where a nonlinear or discontinuous relationship exists can

be observed, and those samples used to establish the boundary of the linear region.

A transfer matrix to characterize the remaining samples’ relationship between the

66

discriminant and the displacement can be extracted through a linear regression.

Figure 6-8: A raster scanning pattern across the quad sensor area can be used forcalibration of the quadrant sensor.

The sensitivity of the tracking performance with regards to the level of defocus

can then be determined by repeating the above calibration with the quad displaced

out of the focal plane and comparing the resulting transfer matrices. It is expected

that the sensitivity of the tracking performance will decrease as the spot is defocused.

However, the range of the linear regime will increase. An increased linear regime is

desired for tracking since it allows for tracking of spots in a larger field-of-view.

Finally, confirmation of the spot size of the sensor arises from the knowledge of the

sensor area combined with displacing the spot and observing where the spot power

disappears from the sensor.

67

68

Chapter 7

Summary and Future Work

This thesis introduced the dual-purpose uplink receiver architecture for NASA’s Ter-

aByte InfraRed Delivery demonstration. The uplink receiver combines both the up-

link communication subsystem with the pointing, acquisition and tracking subsystem.

By using a single sensor and utilizing a microcontroller for the tracking signal pro-

cessing, size, weight and power are conserved.

Chapter 2 provided context and motivated the design of such a dual-purpose

optical communications receiver. The next two chapters, Chapter 3 and 4 presented

the relevant design regarding the uplink as a whole, from choice of a BPPM scheme

with dead time for the uplink signal modulation to the use of a microcontroller for the

signal processing aboard the spacecraft. Chapter 5 presented a simulation including

payload and spacecraft dynamics which predicted the performance of the system as a

whole. The result of the simulation showed that for a simulated pass, payload pointing

feedback was able to reduce the pointing error from 62𝜇rad RMS to 22𝜇rad RMS, with

a median pointing loss of 0.2 dB which is adequate to support a 200 Gbps burst rate.

Finally, Chapter 6 presented the design of an experimental setup for the validation

of the dual-purpose uplink receiver design. The test setup was rigorously tested

and validated, and ultimately results showing the uncalibrated performance of the

quad sensor tracking disturbance measurement were presented. Without calibration,

pointing feedback was able to measure disturbances introduced by a FSM with a

two-axis error of 5.7𝜇rad RMS.

69

Ongoing and future work will include the necessary calibration of the quad sensor,

as well as the implementation of the tracking signal processing routine on the micro-

controller. An accurate mapping between the quadrant sensor discriminant and spot

location will be performed. The gaps of the quadrant sensor may also be measured.

In continuing development of the communications component of the system, uplink

coding will be designed and validated on the experimental test bench as presented in

this work. Methods for efficient slot and symbol synchronization at the spacecraft,

as well as symbol demodulation will be part of the ongoing work. Such methods will

seek to maximize the use of the onboard microcontroller, along with the tracking

signal processing that is already being performed there.

Later, a test bench integration with the downlink communication system will be

completed and allow for testing of the entire payload as a whole. The Speedgoat inter-

face will be altered such that BER measurements for both the uplink and downlink in

aggregate can be automatically measured and characterized. Thorough performance

characterization will confirm the dual-purpose uplink system behavior.

In addition, the spacecraft dynamics simulation presented in Chapter 5 will be

incorporated into the test bench. Ultimately, the future work regarding this project

will culminate with the integration process into the TBIRD payload and finally, the

spacecraft bus, at which point the performance of the system in orbit will be evaluated

in its entirety.

70

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