Machine Learning Image Processing Algorithms Onboard OPS-SAT

We discuss the deployment of image processing algorithms developed for BeaverCube-2, a project under development between the MIT Space Telecommunications, Astronomy, Radiation (STAR) Lab and the Northrop Grumman Corporation. The algorithms were uploaded to and executed on OPS-SAT, a 3U CubeSat owned and operated by ESA with a processing payload that allows rapid prototyping, testing, and validation of software and firmware experiments in space at no cost to the experimenter. Testing these algorithms onboard OPS-SAT significantly reduces risk for future on-orbit image processing missions such as BeaverCube-2. We focus on four image processing algorithms used for cloud detection: a luminosity-thresholding method, a random forest method, an U-Net based deep learning method — all developed by STAR Lab for BeaverCube-2 — and a k-means clustering deep learning method implemented by the OPS-SAT Flight Control Team (FCT). We evaluate each method in terms of in terms of overall accuracy, power draw, and temperature rise on-orbit, and discuss the challenges of implementing these methods on embedded hardware and the lessons learned for BeaverCube-2

Satellite for Estimating Aquatic Salinity and Temperature (SEASALT) a Payload and Instrumentation Overview

The Satellite for Estimating Aquatic Salinity and Temperature, or SEASALT, is a 6U CubeSat designed to acquire coastal images to measure Sea Surface Temperature (SST) and to develop and utilize an algorithm to estimate Sea Surface Salinity (SSS). SSS can be retrieved in coastal zones by utilizing atmospherically corrected optical images to retrieve remote sensing reflectance (Rrs). Rrs and SSS can then be empirically related through algorithms specific to different aquatic bodies. Current satellite instruments used for SSS calculations, such as MODIS and VIIRS, have limited revisit times and low spatial resolutions that make it challenging to implement SSS retrieval algorithms. The Planet constellation imagers have lower revisit times and higher spatial resolution than MODIS and VIIRS, but lack the optical bands to enable retrieval of SSS. SEASALT is designed to address both of these limits. SEASALT utilizes bands centered at 412 nm, 470 nm, 540 nm, and 625 nm in the visible (VIS), and 746 nm, and 865 nm in the Near Infra-Red (NIR) to provide accurate atmospheric corrections related to aerosols. A constellation of SEASALT instruments would be feasible to launch and operate, allowing for SSS to be retrieved frequently on a global scale. The SEASALT mission requires a two-year development phase from its current post-instrument PDR state. The SEASALT instrument design has multiple detectors and corresponding optical paths to capture the science bands. The instrument has a large primary catadioptric Ritchey-Chrétien based telescope covering the 412 nm, 746 nm, and 865 nm bands, with the RGB and LWIR cameras each on their own optical paths. The instrument has two custom-designed calibrators, one for the 412, 746, and 865 nm wavelength cameras, which have both a light source and a shutter mechanism. The payload assembly also integrates an additional calibrator for the LWIR camera. Finally, a dual-redundant Raspberry Pi flight computer, based on the MIT DeMi and BeaverCube missions, monitors and controls all payload operations. In this work, we discuss design trades for payload and instrumentation, covering overall optical design, telescope design, electronic interfaces, and structural design requirements for fitting in a 6U Cubesat and performing its mission. We also present a detailed radiometric performance analysis of the optical path to determine each band’s signal-to-noise ratio (SNR) and ensure it will meet mission SSS retrieval requirements

Satellite for Estimating Aquatic Salinity and Temperature (SEASALT) - A Scientific Overview

SEASALT is a small satellite mission designed to explore the estimation of salinity in coastal environments using ocean color. A SEASALT constellation would fill the coastal gap by providing coastal SSS observations with much higher spatial resolution (30m) and much shorter revisit times (less than 1 day) on a global scale. Planet’s nanosatellites currently provide daily monitoring of the earth’s surface, as well as coastal locations, at 3-meter resolution. However, they do not have the required bands needed in the near infrared (NIR) for atmospheric correction (they only possess 1 NIR band), thus making atmospheric correction over water very challenging. Accurate atmospheric corrections are fundamental to reliably retrieving salinity from ocean color. SEASALT has these required bands by design. Planet’s nanosatellites also do not have a 412nm band to monitor CDOM and create optimized salinity products. SEASALT has bands centered at 412nm, 470nm, 540nm, 625nm, 746nm, 865nm, and 12013nm. A SEASALT constellation has the potential to monitor coastal regions consistently on a global scale as locally-optimized salinity retrieval algorithms can be developed. Besides retrieving SSS with a high temporal and spatial resolution, SEASALT will retrieve concurrent sea surface temperature (SST)

Folded Lightweight Actuator Positioning System (FLAPS)

Precision actuation of mechanical structures on small spacecraft is challenging. Current solutions include single-use actuators, which rely on pyrotechnics and springs, and multiple-use actuators, which typically consume more size, weight, and power than available on CubeSats. The Folded Lightweight Actuated Positioning System (FLAPS) demonstrates the use of a simple rotary shape memory alloy (SMA) actuator in a bending architecture, along with a feedback control loop for repeatable and precise deployment. The FLAPS mechanism consists of a pair of SMA strips mounted to a hinge assembly, with one side attached to the CubeSat bus and the other to the deployable element. A custom actuator shape was manufactured using oven annealing. SMA actuation is achieved using joule heating. Feedback control is provided by a closed-loop PID control scheme, feedback sensor, and controller board. The FLAPS actuator is currently being developed for CubeSat solar panel positioning and drag control. Other potential FLAPS applications include aperture repositioning, deployable radiators, and steerable antennas. The FLAPS team will validate the actuator system in a microgravity environment on a parabolic fight in late 2019

Optical Performance and Prototyping of a Liquid Lens Laser Communications Transceiver

Laser communications can enable more efficient and higher bandwidth communications than conventional radio frequency (RF) systems. Free-space optical communications systems' beams are typically narrower in divergence and require precise pointing, acquisition, and tracking (PAT) systems to establish and maintain links. Several technologies for beam steering exist, including MEMS fast-steering mirrors (FSMs), gimbals, and photonic integrated circuit (PIC) devices. However, these may not meet steering, aperture, power handling, or size, weight, and power (SWaP) requirements for small spacecraft. The Miniature Optical Steered Antenna for Intersatellite Communications (MOSAIC) aims to utilize liquid lenses to provide miniaturized non-mechanical beam steering, allowing wide field-of-view communications and multiple access capabilities. MOSAIC uses three liquid total liquid lenses: one lens is on-axis to provide divergence control, whereas the other two are offset in +x and +y respectively to provide steering. Previous work has focused on qualifying liquid lenses for the space environment, showing a clear path for evaluating their optical performance and constructing prototypes. Lenses from Corning Varioptic (France) and Optotune (Switzerland) are both considered in this work. The liquid lenses undergo environmental testing for liquid lenses, including microgravity testing, radiation exposure, and quantifying how much power the lenses can effectively couple. An analytic formulation of beam steering is presented, which can be used as a feedforward controller and to optimize system size to fit in constrained spaces, such as CubeSats. Simulation work in Zemax is presented to characterize the transmit and receive gain parameters, and a complete optical link budget is constructed from simulation results. Simulation results are validated with experiments showing beam profiles. An evaluation of diffusers is made, to evaluate the trade in increasing numerical aperture (NA) at the expense of beam quality. Transmit and receive capability is also demonstrated experimentally using two laboratory prototypes. Transceiver architecture trades are discussed, introducing the baseline design, strategies to incorporate diffusers, the benefits of apodization, optimizing the receive path, and strategies to beacon using nutation. Characterization of prototype 1550 nm (optical C-band) optimized lenses from Corning Varioptic are also characterized. Preliminary simulation results for steering multiple beam using a single optical train of variable focuses lenses is also presented. The liquid lenses from both Corning Varioptic and Optotune show excellent power handling capabilities, with no visible damage to either lens with input powers of up to 2 W continuous wave (CW) at 1550 nm. Both sets of do not show increased degradation rate due to radiation exposure. The visible-spectrum Corning Varioptic and Optotune lenses decrease in transmission from 100% to 91% (Corning Varioptic) and 85% (Optotune), after exposure to radiation equivalent to 10 years in low Earth orbit (LEO) with 0.5 mm aluminum shielding. Additional tip/tilt and coma aberration is measured on the lenses in gravitational environments due to the optical fluid sagging. Tip/tilt changes by 0.74 mrad and 4.05 mrad for the Corning Varioptic A-39N0 and Optotune EL-16-40-TC lenses, respectively. Beam quality significantly improves in microgravity for the Optotune EL-16-40-TC lenses, with significantly decreased coma aberration. The Corning Varioptic A-39N0 lenses maintain excellent beam quality throughout gravitational regimes and show a slight decrease in coma aberration in microgravity. Extended environmental testing qualifies these lenses to a TRL 5-6 on NASA's Technology Readiness Level (TRL) scale. Optical link budgets show that with a reference system, the Corning A-39 and Optotune EL-16-40-TC lenses can maintain a 25 Mbps 16-PPM link with 4 W of input power at 1550 nm with hemispherical steering up to 40 km (Corning Varioptic) and 220 km (Optotune).S.M