Small satellites, particularly CubeSats, have become
popular platforms for a wide variety of scientific, commercial and
military remote sensing applications. Inexpensive commercial o the
shelf (COTS) hardware and relatively low launch costs make these
platforms candidates for deployment in large constellations that
can offer unprecedented temporal and geospatial sampling of the
entire planet. However, productivity for both individual and
constellations of CubeSats in low earth orbit (LEO) is limited by
the capabilities of the communications subsystem. Generally, these
constraints stem from limited available electrical power, low-gain
antennas and the general scarcity of available radio spectrum. In
this thesis, we assess the ability of free space optical
communication (lasercom) to address these limitations, identify key
technology developments that enable its application in small
satellites, and develop a functional prototype that demonstrates
predicted performance. We first establish design goals for a
lasercom payload archi- tecture that offers performance
improvements (joules-per-bit) over radio-frequency (RF) solutions,
yet is compatible with the severe size, weight and power (SWaP)
constraints common to CubeSats. The key design goal is direct
LEO-to-ground downlink capability with data rates exceeding 10
Mbps, an order of magnitude better than COTS radio solutions
available today, within typical CubeSat SWaP constraints on the
space terminal, and with similar COTS and low-complexity
constraints on the ground terminal. After defining the goals for
this architecture, we identify gaps in previous implementations
that limit their performance: the lack of compact, power-efficient
optical transmitters and the need for pointing capability on small
satellites to be as much as a factor of ten better than what is
commonly achieved today. One approach is to address these
shortcomings using low-cost COTS components that are compatible
with CubeSat budgets and development schedules. In design trade
studies we identify potential solutions for the transmitter and
pointing implementation gaps. Two distinct transmitter
architectures, one based on a high-power laser diode and another
using an optical amplifier, are considered. Analysis shows that
both configurations meet system requirements, however, the optical
amplifier offers better scalability to higher data rates. To
address platform pointing limitations, we dene a staged control
framework incorporating a COTS optical steering mechanism that is
used to manage pointing errors from the coarse stage (host
satellite body-pointing). A variety of ne steering solutions are
considered, and microelectromechanical systems (MEMS) tip-tilt
mirrors are selected due to their advantage in size, weight and
power. We experimentally validate the designs resulting from the
trade studies for these key subsystems. We construct a prototype
transmitter using a modified COTS fiber amplifier and a
directly-modulated seed laser capable of producing a 200mW average
power, pulse position modulated…

Optical communications for small satellites

NASA's Space Communications and Navigation (SCaN) Program at NASA Headquarters is pursuing a vibrant and wide-ranging optical communications program for future near-Earth missions such as the Laser Communications Relay Demonstration and the Orion Exploration Mission-2 (EM-2) lunar mission, and planetary missions such as the Deep Space Optical Communications system on the Discovery mission to the asteroid Psyche. This presentation will also address NASA's plans to infuse this new technology into NASA's existing communications networks.

/pdf/nasa-s-optical-communications-program-for-2017-and-beyond-54xs662ckj.pdf

NASA's optical communications program for 2017 and beyond

We describe a flexible high-sensitivity laser communication transceiver design that can significantly benefit performance
and cost of NASA's satellite-based Laser Communications Relay Demonstration Optical communications using
differential phase shift keying, widely deployed for use in long-haul fiber-optic networks, is well known for its superior
sensitivity and link performance over on-off keying, while maintaining a relatively straightforward design However,
unlike fiber-optic links, free-space applications often require operation over a wide dynamic range of power due to
variations in link distance and channel conditions, which can include rapid kHz-class fading when operating through the
turbulent atmosphere Here we discuss the implementation of a robust, near-quantum-limited multi-rate DPSK
transceiver, co-located transmitter and receiver subsystems that can operate efficiently over the highly-variable free-space
channel Key performance features will be presented on the master oscillator power amplifier (MOPA) based TX,
including a wavelength-stabilized master laser, high-extinction-ratio burst-mode modulator, and 05 W single
polarization power amplifier, as well as low-noise optically preamplified DSPK receiver and built-in test capabilities

Multi-rate DPSK optical transceivers for free-space applications

The multi-rate DPSK format, which enables efficient free-space laser communications over a wide range of data rates, is
finding applications in NASA’s Laser Communications Relay Demonstration We discuss the design and testing of an
efficient and robust multi-rate DPSK modem, including aspects of the electrical, mechanical, thermal, and optical
design The modem includes an optically preamplified receiver, an 05-W average power transmitter, a LEON3 rad-hard
microcontroller that provides the command and telemetry interface and supervisory control, and a Xilinx Virtex-5 radhard
reprogrammable FPGA that both supports the high-speed data flow to and from the modem and controls the
modem’s analog and digital subsystems For additional flexibility, the transmitter and receiver can be configured to
support operation with multi-rate PPM waveforms

A multi-rate DPSK modem for free-space laser communications

NASA is presently developing first all optical high data rate satellite relay system, LCRD. To be flown on commercial geosynchronous satellite, it will communicate at DPSK and PPM modulation formats up to 1.244 Gbps. LCRD flight payload is being developed by NASA's Goddard Space Flight Center. The two ground stations, one on Table Mountain in CA, developed by NASA's Jet Propulsion Laboratory and another on Hawaiian island will enable bi-directional relay operation and ground sites diversity experiments. In this paper we will report on the current state of LCRD system development, planned operational scenarios and expected system performance.

/pdf/overview-and-status-of-the-laser-communication-relay-4fga7ip5br.pdf

Overview and Status of the Laser Communication Relay Demonstration

Recently, we demonstrated a multi-rate DPSK modem with high-sensitivity over a wide dynamic range, which can
significantly benefit performance and cost of NASA’s Laser Communication Relay Demonstration. This increased
flexibility, combined with the need to verify robust operation under challenging free-space environmental conditions,
results in a large number of operational states which must be accurately and thoroughly tested. To support this, we
developed test and diagnostic capabilities that can be easily reconfigured to assess modem performance across a wide
range of data rates and operational modes. These capabilities include internal self-test modes in which test waveforms
can be directed from the transmitter into the receiver to determine modem communications performance. We used these
self-test capabilities to demonstrate robust performance in realistic environments during thermal-vacuum,
shock/vibration, and EMI/EMC testing.

Performance and qualification of a multi-rate DPSK modem

We have designed and experimentally demonstrated a radiation-hardened modem suitable for NASA’s Laser
Communications Relay Demonstration. The modem supports free-space DPSK communication over a wide range of
channel rates, from 72 Mb/s up to 2.88 Gb/s. The modem transmitter electronics generate a bursty DPSK waveform,
such that only one optical modulator is required. The receiver clock recovery is capable of operating over all channel
rates at average optical signal levels below -70 dBm. The modem incorporates a radiation-hardened Xilinx Virtex 5
FPGA and a radiation-hardened Aeroflex UT699 CPU. The design leverages unique capabilities of each device, such as
the FPGA’s multi-gigabit transceivers. The modem scrubs itself against radiation events, but does not require pervasive
triple-mode redundant logic. The modem electronics include automatic stabilization functions for its optical
components, and software to control its initialization and operation. The design allows the modem to be put into a low-power standby mode.

J. J. Fitzgerald

Papers

Multi-rate DPSK optical transceivers for free-space applications

A multi-rate DPSK modem for free-space laser communications

Performance and qualification of a multi-rate DPSK modem

Electronics design of a multi-rate DPSK modem for free-space optical communications