A CubeSat-Based Optical Communication Network for Low Earth Orbit

A CubeSat-Based Optical Communication Network for Low Earth Orbit Richard Welle, Alexander Utter, Todd Rose, Jerry Fuller, Kristin Gates, Benjamin Oakes, and Siegfried Janson The Aerospace Corporation 10 August 2017 2017 The Aerospace Corporation

Why Laser Communication? Potential to reduce complexity and mass of space-based communication networks Previous demonstrations of space-based laser communication used terminals with a mass of ~30 kg and cost in excess of $20M per terminal Much of the cost is in the two-axis pointing system Far too massive and expensive for high-density LEO constellation Current optical systems rely on GEO relay to get signal to ground Enabler for CubeSat-scale, low-cost, high-density LEO relay network Requires massive/expensive terminal on LEO satellites to reach GEO Body pointing presents a simple, low-cost, alternative to two-axis gimbal systems OCSD CubeSat laser terminal OPAL laser terminal NASA Photo 15 cm 2

OCSD Retroreflector LED Beacon Funded by NASA s Small Satellite Technology Program Goals: Star Tracker 2-Axis Sun Sensor Demonstrate optical communications from a CubeSat to a 30-cm diameter ground station from low Earth orbit (LEO) at rates between 5 and 50 Mb/s Demonstrate tracking of a nearby cooperative spacecraft using a commercial, off-the-shelf (COTS) laser rangefinder Demonstrate attitude determination using a sub-cubic-inch star tracker. Demonstrate orbit control using variable drag Demonstrate propulsive orbit control using a steam thruster 3 Pathfinder spacecraft, OCSD-A, launched October 8, 2015 Two flight units scheduled for launch in November 2017 Hi-Res Camera GPS Antenna RF Comm. Antenna Laser Rangefinder Stowed Wing 2-Axis Sun Sensor Earth Horizon Sensor (4 x 16 array)

R-Cubed (AeroCube-11) R3 will demonstrate CubeSat-based remote sensing activities analogous to Landsat 8 s Operational Land Imager (OLI) instrument Custom-designed refractive telescope High-framerate commercial CMOS focal plane pushbroom mode imager filter block identical to those flown on Landsat 8 Six of the nine Landsat 8 OLI bands will be read individual frames will be downlinked time-delay integration will be performed on the ground Space-based vicarious calibration will be tied to OLI R3 is expected to launch in mid 2018 Optical communications will provide the necessary data downlink capacity Downlink laser 4

Relay Networks Space-based relay network can provide continuous downlink capability GEO Relay satellite GEO-based relays Three relay satellites can cover all of LEO space Typical link range is 40,000 km RF link to ground avoids cloud issues Optical link RF or optical link All-LEO network Fifty to 100 satellites required to cover all of LEO space Typical link range is under 5,000 km Multiple paths to ground for optical downlink EO satellite 600-km circular orbit 5

Maximum possible crosslink range as a function of orbit altitude and minimum tangent height T L h Earth LEO Orbit 6

Laser Communication Data Rates Data rates vary inversely with the square of the range A 4000-km (LEO) link can be two orders of magnitude faster than a 40,000-km (GEO) link operating with similar hardware 4-W laser 10-cm-diameter receiver Range 7

LEO Optical Network Applications High-volume, short-range download from satellites in LEO Low-latency download from satellites in LEO Low-latency Earth-to-spaceto-Earth data transfers Relay satellite EO satellite Optical links Relay satellite Requirements Lasers Detectors Pointing and tracking Cloud Ground station Ground station Data handling/management Earth 8

Pointing and Tracking Laser communication data rates scale with inverse square of pointing accuracy Most laser communication systems use a two-axis gimbal for steering the transmitter and receiver OCSD dispenses with the gimbal and instead uses body pointing The laser is hard-mounted to the spacecraft and pointed using the spacecraft Attitude- Control System (ACS) With current ACS capabilities, data rates are limited to about 200 Mb/s for downlink from LEO Reasonable extrapolation of ACS capabilities indicates data rates for body pointing can reach well into the Gb/s range in the near future Body pointing does not support simultaneous receive and transmit as required for a relay 2500 2000 Peak Data Rate (Mb/s) 1500 1000 500 Initial OCSD goal 9 0 0 0.05 0.1 0.15 0.2 0.25 0.3 Pointing Accuracy (deg)

Attitude Determination and Control System The Attitude Control System is designed to point the downlink laser to within 0.07 Degrees (3s) of the optical ground station Sun Sensor Quad Cell A combination of custom-designed attitude sensors (sun and earth) and star trackers are used to meet stringent power, size and performance requirements Miniature Reaction Wheels and Torque Rods are used for actuation and momentum control OCSD minimum anticipated pointing performance Star Tracker Reaction Wheels Error Sources Pointing Error 3σ (Deg) Real-time Clock Drift 0.002 Orbit Determination / Ephemeris Error 0.003 3 cm Attitude Determination Error 0.030 Attitude Control Error 0.054 Total (RMS) 0.062 10

Small Satellite Pointing Accuracy Forecast in 2015 AeroCube-4A AeroCube OCSD Pathfinder AeroCube OCSD-B&C 2016 2018 11 From: Small Spacecraft Technology: State of the Art, by C. Frost, E. Agasid, et al., p.61, NASA Technical Report TP-2014-216648/REV1, NASA-Ames Research Center, 2014

Small Satellite Pointing Accuracy Status in 2017 AeroCube-4A AeroCube OCSD Pathfinder AeroCube OCSD-B&C 2016 2018 12 From: Small Spacecraft Technology: State of the Art, by C. Frost, E. Agasid, et al., p.61, NASA Technical Report TP-2014-216648/REV1, NASA-Ames Research Center, 2014

Small Satellite Pointing Accuracy Status in 2017 AeroCube-4A AeroCube OCSD Pathfinder AeroCube OCSD-B&C Blue Canyon 2016 2018 13 From: Small Spacecraft Technology: State of the Art, by C. Frost, E. Agasid, et al., p.61, NASA Technical Report TP-2014-216648/REV1, NASA-Ames Research Center, 2014

Laser Communication Data Rates Data rates vary inversely with the square of the range A 4000-km (LEO) link can be two orders of magnitude faster than a 40,000-km (GEO) link operating with similar hardware 4-W laser 10-cm-diameter receiver Range 14

LEO Optical Nodes Options for optical nodes without twoaxis gimbals: Co-orbiting relay EO satellite Receiver satellite Two-satellite node Operate in store-and-forward mode Point first at source, then at destination Client Relay Optical links RF link Transmitter satellite Two-satellite node Dedicated receive and transmit satellites point respectively at source and destination Communication between them through short-range omnidirectional link Single-axis gimbal combined with body rotation of satellite about receive axis (next slide) Cloud Ground station Ground station Earth 15

Relay Node with single-axis gimbal Receive reflector Receive beacon camera Gimbal Receive beacon Receiver Transmit beacon Transmit laser output Transmit beacon camera Mirror rotation Axis Transmit mirror 16

Relay node with single-axis gimbal Incoming Laser Ɵ Mirror Rotation Axis Ɵ = 0 o Mirror Rotation Axis Body Rotation Axis Incoming Laser Outgoing Laser Ɵ = 4 o Ɵ = 0 o 135 o q 0 o Body Rotation Axis 17

Relay node with single-axis gimbal Available transmit directions F = 0 o Mirror Rotation Axis Incoming Laser Ɵ Outgoing Laser F = 0 o Ɵ = 0 o Exclusion cone Outgoing Laser Body Rotation Axis F F = 180 o 18

Summary OCSD will demonstrate CubeSat-scale laser downlink at up to 200 Mb/s with no secondary pointing system Laser communication is designed into upcoming Aerospace satellites for operational service Laser relay nodes can be built on the CubeSat scale, without the use of complex two-axis gimbals There are no unmanageable technical barriers to CubeSat-scale LEO optical networks Rapid improvements in CubeSat-scale attitude control will quickly drive data rates well into the Gb/s range 19