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

Design and Verification of a Clock System for Orbital Radio Interferometry

Radio interferometry using multiple small satellites will enable measurements with high angular resolution for remote sensing and astronomy. The NASA sponsored Auroral Emissions Radio Explorer (AERO) and Vector Interferometry Space Technology using AERO (VISTA) CubeSats will demonstrate orbital interferometry from 0.1 MHz to 15 MHz, frequencies which are largely blocked by the ionosphere. We report on the design and testing of a clock system for radio interferometry between these orbital receivers. We discuss the clock system design up to PCB fabrication, including requirements flow and major hardware trades. The performance of the timing components has been verified using a phase noise test set with a high-quality benchtop crystal. While these results are presented for the AERO-VISTA mission payload, they are more generally applicable to any orbital interferometry platform with multiple satellites

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)

Design and Prototyping of a Nanosatellite Laser Communications Terminal for the Cubesat Laser Infrared CrosslinK (CLICK) B/C Mission

The CubeSat Laser Infrared CrossLink (CLICK) mission goal is to demonstrate a low cost, high data rate optical transceiver terminal with fine pointing and precision time transfer in aleq1.5U form factor. There are two phases to the technology demonstration for the CLICK mission: CLICK-A downlink, and then CLICK-B/C crosslink and downlink. The topic of this paper is the design and prototyping of the laser communications (lasercom) terminal for the CLICK-B/C phase. CLICK B/C consists of two identical 3U CubeSats from Blue Canyon Technologies that will be launched together in Low Earth Orbit to demonstrate crosslinks at ranges between 25 km and 580 km with a data rate of ≥20 Mbps and a ranging capability better than 0.5 m. Downlinks with data rates of ≥10 Mbps will also be demonstrated to the Portable Telescope for Lasercom (PorTeL) ground station. Link analysis using current parameters & experimental results predicts successful crosslink & downlink communications and ranging. Moreover, closed-loop 3σ fine pointing error is predicted to be less than 39.66 μrad of the 121.0 μrad 1/e² transmit laser divergence. The status of the payload EDU and recent developments of the optomechanical and thermal designs are discussed

Testing of the CubeSat Laser Infrared CrosslinK (CLICK-A) Payload

The CubeSat Laser Infrared CrosslinK (CLICK-A) is a risk-reduction mission that will demonstrate a miniaturized optical transmitter capable of ≥10 Mbps optical downlinks from a 3U CubeSat to aportable 30 cm optical ground telescope. The payload is jointly developed by MIT and NASA ARC, and is on schedule for a 2020 bus integration and 2021 launch. The mission purpose is to reduce risk to its follow-up in 2022, called CLICK-B/C, that plans to demonstrate ≥20 Mbps intersatellite optical crosslinks and precision ranging between two 3U CubeSats. The 1.4U CLICK-A payload will fly on a Blue Canyon Technologies 3U bus inserted into a 400 km orbit. The payload will demonstrate both the transmitter optoelectronics and the fine-pointing system based on a MEMS fast steering mirror, which enables precision pointing of its 1300 μrad full-width half-maximum (FWHM) downlink beam with anestimated error of 136.9 μrad (3-σ) for a pointing loss of -0.134 dB (3-σ) at the time of link closure. We present recent test results of the CLICK-A payload, including results from thermal-vacuum testing, beam characterization, functional testing of the transmitter, and thermal analyses including measurement of deformation due to the thermal loading of the MEMS FSM

Auroral arc detection using a COTS spectral photometer for the Auroral Emission Radio Explorer (AERO) CubeSat Mission

Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, May, 2020Cataloged from the official PDF of thesis.Includes bibliographical references (pages 83-85).Auroral phenomena are dynamic in nature: observed events have rich structures that are both spatially and temporally complex, with scientifically interesting features. While optical auroral observations using CCDs or all-sky cameras are common, the aurora also have interesting emission properties at radio frequencies (RF), specifically in low-frequency and high-frequency bands. The Auroral Emission Radio Observer (AERO) is a 6U CubeSat, equipped with a novel electromagnetic Vector Sensor (VS) antenna. The VS will target auroral emission in a measurement band from 100 kHz - 15 MHz, which enables the study of interesting emission types such as Auroral Kilometric Radiation (20 kHz -750 kHz), Medium Frequency Bursts (1.6 MHz - 4.4 MHz) and cyclotron emission (2.8 MHz - 3.0 MHz).The VS antenna measures 4-meters tip-to-tip once deployed from the CubeSat frame, and expands to form electric dipoles and magnetic loop antennas that are sensitive enough to probe this diverse set of science targets. Having a spacebased platform, such as AERO's vector sensor antenna, positions the detector above the ionospheric plasma frequency which would otherwise limit observations of radio emissions. Novel measurements from AERO's VS antenna require a set of contextual data to validate the fidelity of resulting data products. AERO includes a secondary payload referred to as an Auxiliary Sensor Package (ASP) that will augment VS measurements with contextual optical and magnetic data. The objective of AERO's contextual optical measurement is to detect the presence of auroral emission in multiple spectral bands, namely green-line emission at 557 nm and red-line emission at 630 nm. An AMS AG AS7262 6-channel visual band spectral photometer is selected as the optical sensor.We present a radiometric model that evaluates the AS7262 sensor's ability to measure target auroral events. We consider a number of different test scenarios, including varying parameters such as auroral source radiance in units of Rayleigh, spacecraft altitude, and others, to fully assess the sensor's ability to detect optical auroral signatures. The mission requirements include a minimum detection of 5 kR for the sensor to satisfy the optical measurement requirement. In our initial assessment, we find that the selected sensor in its current configuration may not be able to meet this requirement. In its current configuration, the sensor may be capable of detecting the presence of auroral events at high levels of intensity, in the over 100 kR range. The model developed in this work indicates that further analysis and possible modification to the front end optic or the sensor itself are needed.Though the radiometric model presented is tailored for the AS7262 sensor, it is easily adaptable to assess the performance of other auroral imagers. The contextual measurements provided by the ASP will contribute to the success of the AERO CubeSat mission in demonstrating that remote sensing techniques on CubeSat platforms can address unanswered questions about the aurora.by Cadence Payne.S.M.S.M. Massachusetts Institute of Technology, Department of Aeronautics and Astronautic

Laser Crosslink Atmospheric Sounder to Investigate the Effects of Deep Convection on Ozone

Deep convection at mid-latitudes can directly inject water vapor into the upper troposphere/lower stratosphere (UTLS). Increased water vapor creates a photochemical environment that can activate inorganic chlorine (HCl) which then catalytically destroys ozone, increasing health and environment risk [1]. The Laser Crosslink Atmospheric Sounder (LCAS) is a prototype toward a low-Earth-orbiting (LEO) constellation to concurrently measure both UTLS water vapor and temperature at high vertical resolution, improving temporal and spatial coverage. The two-part approach uses both beam pointing and intensity. First, precision pointing measures atmospheric refraction and can be used to obtain temperature profiles [2]. Second, intensity on the continuum and near absorption features of the spectrum can measure water vapor concentration. This work presents demonstration using near-infrared laser crosslinks wavelengths in ITU-S (1460 nm to 1530 nm) and ITU-C (1530 nm to 1565 nm) bands for measurement of water vapor, where commercial components already exist for applications like coarse wavelength division multiplexing (CWDM) over terrestrial optical fiber. We use MODTRAN to assess the measurability of varying concentrations of water vapor in the UTLS in these bands. We find that 1550 nm is a suitable continuum reference wavelength, and that 1504 and 1509 nm have detectable changes in transmissivity with increases in water vapor

Integration and Testing of the Nanosatellite Optical Downlink Experiment

Free space optical (FSO) communications have the potential to outperform traditional radio frequency data rates by orders of magnitude using comparable mass, volume, and power. The Nanosatellite Optical Downlink Experiment (NODE) is a 1.2U, 1 kg, 15 W, 1550 nm CubeSat downlink transmitter that uses a master-oscillator power amplifier configuration with a modest 1.3 mrad half-power beamwidth (HPBW) enabled by a microelectromechanical system (MEMS) Fast Steering Mirror (FSM) [1],[4]. NODE is designed to be compatible with the Portable Telescope for Lasercom (PorTeL) ground station [3],[6],[19], which has successfully demonstrated tracking of low Earth orbit objects to better than 5 arcseconds RMS. The flight-like opto-mechanical NODE engineering model has successfully passed vibration testing at qualification levels specified by NASA GEVS [9]. The engineering model has also passed thermal testing in vacuum under worst-case expected environmental loads, and component operational temperatures remained within limits. Tests of the opto-mechanical alignment and control algorithms meet +/- 0.05 mrad (3-sigma) for the space and ground terminals. We present results from the NODE engineering unit and flight unit development, integration, and testing, as well as interface test results with PorTeL

The CubeSat Laser Infrared CrosslinK Mission (CLICK)

© COPYRIGHT SPIE. The CubeSat Laser Infrared CrosslinK mission is a joint Massachusetts Institute of Technology (MIT), University of Florida (UF), and NASA Ames Research Center effort to develop laser communications (lasercom) transceivers. The terminals demonstrate full-duplex intersatellite communications and ranging capability using commercial components to enable future large constellations or swarms of nanosatellites as coordinated distributed sensor systems. CLICK will demonstrate a crosslink between two CubeSats that each host a < 2U lasercom payload. Range control is achieved using differential drag in Low Earth Orbit (LEO), with attitude controlled using a three-axis reaction wheel assembly and attitude sensors, including star trackers. The lasercom terminals are direct-detect and rate scalable, designed to achieve a 20 Mbps crosslink at ranges from 25 km to 580 km and operate full-duplex at 1537 nm and 1563 nm with 200 mW of transmit power and a 14.6 arcscecond (0.07 milliradian) full width half max (FWHM) beamwidth. The terminals also use a 976 nm, 500 mW, 0.75 degree FWHM beacon and a quadcell for initial acquisition, and a low-rate radio crosslink for exchanging orbit information. The payload transmitter is a master oscillator power amplifier (MOPA) with fiber Bragg grating for pulse shaping and MEMS fast steering mirror (FSM) for fine pointing, modeled after the MIT Nanosatellite Optical Downlink Experiment. The transceiver leverages UF's Miniature Optical Communications Transmitter (MOCT) including a chip-scale atomic clock (CSAC). The receiver implements both a time to digital converter (TDC) as well as pulse recovery and matched filtering for precision ranging