Sun-synchronous Low Earth Orbit Research Articles

SPHEREx payload thermal architecture and analysis

The SPHEREx (Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) is an all-sky spectral survey in a Sun-synchronous low earth orbit. It measures spectrum in the near infrared band from 0.75 µm to 5 µm. To achieve this wavelength coverage, the instrument has two focal plane arrays, both passively cooled at <55 K (mid-wave infrared) and <80 K (short-wave infrared). Additionally, the instrument requires no moving parts for spectral resolution, relying instead on spacecraft-controlled pointing. A thermally isolating structure and a set of V-groove radiators reduce the heat flow conducted and radiated from the spacecraft bus. The thermal system also blocks the incident solar flux and earth radiation from reaching the sensitive cryogenic instrument while still allowing substantial pointing flexibility. The combination of passive cryogenic cooling and survey requirements in a sun exposed low earth orbit environment presents a challenge for the thermal design. This paper will discuss the SPHEREx payload overall thermal architecture, bounding thermal environments, the component thermal design, and analysis of on-flight performance.

Parameterizing spectral surface reflectance relationships for the Dark Target aerosol algorithm applied to a geostationary imager

Abstract. Originally developed for the moderate resolution imaging spectroradiometer (MODIS) in polar, sun-synchronous low earth orbit (LEO), the Dark Target (DT) aerosol retrieval algorithm relies on the assumption of a surface reflectance parameterization (SRP) over land surfaces. Specifically for vegetated and dark-soiled surfaces, values of surface reflectance in blue and red visible-wavelength bands are assumed to be nearly linearly related to each other and to the value in a shortwave infrared (SWIR) wavelength band. This SRP also includes dependencies on scattering angle and a normalized difference vegetation index computed from two SWIR bands (NDVISWIR). As the DT retrieval algorithm is being ported to new sensors to continue and expand the aerosol data record, we assess whether the MODIS-assumed SRP can be used for these sensors. Here, we specifically assess SRP for the Advanced Baseline Imager (ABI) aboard the Geostationary Operational Environmental Satellite (GOES)-16/East (ABIE). First, we find that using MODIS-based SRP leads to higher biases and artificial diurnal signatures in aerosol optical depth (AOD) retrievals from ABIE. The primary reason appears to be that the geostationary orbit (GEO) encounters an entirely different set of observation geometry than does LEO, primarily with regard to solar angles coupled with fixed-view angles. Therefore, we have developed a new SRP for GEO that draws the angular shape of the surface bidirectional reflectance. We also introduce modifications to the parameterization of both red–SWIR and blue–red spectral relationships to include additional information. The revised red–SWIR SRP includes the solar zenith angle, NDVISWIR, and land-type percentage from an ancillary database. The blue–red SRP adds dependencies on the scattering angle and NDVISWIR. The new SRPs improve the AOD retrieval of ABIE in terms of overall less bias and mitigation of the overestimation around local noon. The average bias of the DT AOD compared to the Aerosol Robotic Network (AERONET) AOD shows a reduction from 0.08 to 0.03, while the bias of local solar noon decreases from 0.12 to 0.03. The agreement between the DT and AERONET AOD is established through a regression slope of 1.06 and a y intercept of 0.01 with a correlation coefficient of 0.74. By using the new SRP, the percentage of data falling within the expected error range (±0.05 % + 15 %) is notably increased from 54 % to 78 %.

A SmallSat Concept to Resolve Diurnal and Vertical Variations of Aerosols, Clouds, and Boundary Layer Height

Abstract A SmallSat mission concept is formulated here to carry out Time-varying Optical Measurements of Clouds and Aerosol Transport (TOMCAT) from space while embracing low-cost opportunities enabled by the revolution in Earth science observation technologies. TOMCAT’s “around-the-clock” measurements will provide needed insights and strong synergy with existing Earth observation satellites to 1) statistically resolve diurnal and vertical variation of cirrus cloud properties (key to Earth’s radiation budget), 2) determine the impacts of regional and seasonal planetary boundary layer (PBL) diurnal variation on surface air quality and low-level cloud distributions, and 3) characterize smoke and dust emission processes impacting their long-range transport on the subseasonal to seasonal time scales. Clouds, aerosol particles, and the PBL play critical roles in Earth’s climate system at multiple spatiotemporal scales. Yet their vertical variations as a function of local time are poorly measured from space. Active sensors for profiling the atmosphere typically utilize sun-synchronous low-Earth orbits (LEO) with rather limited temporal and spatial coverage, inhibiting the characterization of spatiotemporal variability. Pairing compact active lidar and passive multiangle remote sensing technologies from an inclined LEO platform enables measurements of the diurnal and vertical variability of aerosols, clouds, and aerosol-mixing-layer (or PBL) height in tropical-to-midlatitude regions where most of the world’s population resides. TOMCAT is conceived to bring potential societal benefits by delivering its data products in near–real time and offering on-demand hazard-monitoring capabilities to profile fire injection of smoke particles, the frontal lofting of dust particles, and the eruptive rise of volcanic plumes.

TSC-1 Optical Payload Hyperspectral Imager Preliminary Design and Performance Estimation

The Thai Space Consortium aims at building capacities in space technologies and industries with the objective to develop satellites in Thailand. In this framework, the first Earth Observation satellite that will be developed by this consortium is called TSC-1. This satellite comprises a hyperspectral imager orbiting in a Sun-Synchronous Low-Earth Orbit at the altitude equal to 630 km. The optical payload is specified to provide data cubes with a Ground Sample Distance equal to 30 m, a swath equal to 30 km, a spectral resolution equal to 10 nm over the spectral domain from 400 nm to 1000 nm with a Signal-to-Noise Ratio (SNR) higher than 100. Firstly, we present the trade-off performed to select the design of the Front Telescope and the Spectrometer. Secondly, we describe the payload design and present the image quality, Modulation Transfer Function and distortion. Next, we establish the tolerance budget to estimate the performance of the optical system including manufacturing errors, assembly errors and stability of the mechanical structure. After that, we calculate the instrument’s spatial and spectral response functions and the contamination of the adjacent pixels due to the straylight. Finally, we estimate radiometric performance in both nadir pointing mode and forward motion compensation mode.

SUNSTORM 1/X-ray Flux Monitor for CubeSats (XFM-CS): Instrument characterization and first results

SUNSTORM 1 CubeSat was launched to Sun-synchronous low Earth orbit on August 17 2021. The primary purpose of the mission is an in-orbit demonstration of X-ray Flux Monitor (XFM) instrument. XFM is an innovative solar X-ray spectrometer for measuring and characterizing solar flares, which are known to be linked to a variety of space weather phenomena. XFM represents a next generation of solar X-ray flux monitors. It is based on silicon drift detector technology, which provides several notable performance improvements over its predecessors, which are based on Si PIN detectors. Transversal electric field and lower output capacitance allow operation at much faster pulse processing shaping times, allowing the system to achieve about 10 times higher throughput without saturation while also making it less sensitive to the increase of leakage current due to high temperature and/or radiation damage. Thus, XFM instruments can cover a very wide dynamic range of solar X-ray emission from the most quiescent conditions to the strongest X-class solar flares, while maintaining good spectral resolution (<200 eV at 6 keV). The instruments are thus well suited for both X-ray flux monitoring and spectroscopic studies of the solar corona.In this paper we describe the ground characterization and evaluation of the scientific performance of the first operational XFM instrument, a CubeSat version XFM-CS on-board SUNSTORM 1, which is a satellite built in the framework of the FLY element of ESA’s General Support Technology Programme, dedicated to the early space testing of promising new technologies. We also provide an analysis of the first results obtained during the in-orbit commissioning and early operations of SUNSTORM 1. We show the comparison of the observed scientific performance of XFM-CS with the expected performance based on theoretical modeling and ground calibration results. Finally, we describe the cross calibration of the X-ray flux data from XFM-CS with the new GOES-R series GOES X-ray sensors.

Space applications of GAGG:Ce scintillators: a study of afterglow emission by proton irradiation

We discuss the results of a proton irradiation campaign of a GAGG:Ce (Cerium-doped Gadolinium Aluminum Gallium Garnet) scintillation crystal, carried out in the framework of the HERMES-TP/SP (High Energy Rapid Modular Ensemble of Satellites — Technological and Scientific Pathfinder) mission. A scintillator sample was irradiated with 70 MeV protons, at levels equivalent to those expected in equatorial and sun-synchronous low-Earth orbits over orbital periods spanning 6 months to 10 years. The data we acquired are used to introduce an original model of GAGG:Ce afterglow emission. Results from this model are applied to the HERMES-TP/SP scenario, aiming at an upper-bound estimate of the detector performance degradation resulting from afterglow emission.

LEO Satellites Constellation-to-Ground QKD Links: Greek Quantum Communication Infrastructure Paradigm

Quantum key distribution (QKD) has gained a lot of attention over the past few years, but the implementation of quantum security applications is still challenging to accomplish with the current technology. Towards a global-scale quantum-secured network, satellite communications seem to be a promising candidate to successfully support the quantum communication infrastructure (QCI) by delivering quantum keys to optical ground terminals. In this research, we examined the feasibility of satellite-to-ground QKD under daylight and nighttime conditions using the decoy-state BB84 QKD protocol. We evaluated its performance on a hypothetical constellation with 10 satellites in sun-synchronous Low Earth Orbit (LEO) that are assumed to communicate over a period of one year with three optical ground stations (OGSs) located in Greece. By taking into account the atmospheric effects of turbulence as well as the background solar radiance, we showed that positive normalized secure key rates (SKRs) up to 3.9×10−4 (bps/pulse) can be obtained, which implies that satellite-to-ground QKD can be feasible for various conditions, under realistic assumptions in an existing infrastructure.

Heat Load Calculations on an HTS Coil Integrated into a Small Satellite during a Sun-Synchronous Low Earth Orbit

High-temperature superconductivity (HTS) has the potential to be a useful technology for space applications, allowing for high current densities and magnetic field generation in compact devices. However, HTS requires cryogenic temperatures and it is not well understood how this can best be achieved in a space environment. Using a modelling approach, the expected heat load on a hypothetical 3U CubeSat with an HTS coil during a sun-synchronous low Earth orbit was predicted. The direction and magnitude of solar, albedo and infrared incident radiation toward the satellite was calculated for each orbital position of a circular 732 km, Ω = 0° longitude of ascending node orbit. Using a finite element approach, the surface radiosity and temperature of the CubeSat was predicted and validated. Finally, the instantaneous heat load on the HTS magnet, which generates a 1T magnetic field, was calculated as a function of orbital position. This study provides technical information about the characteristics of the refrigeration device required to maintain cryogenic temperatures for an HTS coil on a space faring vessel, such as a portable cryocooler. Selection and design of a satellite cooling system must be optimised according to the calculated heat load and available solar power.

THERMAL DESIGN, ANALYSIS AND TEST VALIDATION OF TURKSAT-3USAT SATELLITE

When CubeSat projects are a useful means by which universities can engage their students in space-related activities. TURKSAT-3USAT is a three-unit amateur radio CubeSat jointly developed by the Space Systems Design and Test Laboratory and the Radio Frequency Electronics Laboratory of Istanbul Technical University (ITU), in collaboration with TURKSAT, A.S. company as well as the Turkish Amateur Technology Organization. It was launched on April 26, 2013 as a secondary payload on a CZ-2D rocket from China’s Jiuquan Space Center to an altitude of approximately 680 km. The mission of the satellite has two primary goals: (1) to voice communication at Low Earth Orbit (LEO) and (2) to educate students by providing hands-on experience. TURKSAT-3USAT was designed to sustain a circular, near sun-synchronous LEO, and has dimensions of 10 x 10 x 34 cm3. Within the course of this paper, TURKSAT-3USAT’s thermal control will be addressed. TURKSAT-3USAT’s thermal control model was developed using ThermXL and ESATAN-TMS software. Using this model, temperature distributions of the CubeSat when subjected to various experimental conditions of interest were computed. Using a thermal vacuum chamber (TVAC), thermal cycling and bake-out testing were carried out on the flight model to verify the thermal design performance and check the mathematical model. Based on thermal analysis results, the temperature of equipment was within the allowable temperature range except for the batteries that were between 42.56 oC and -20.31 oC. Heaters were used for the batteries in order to maintain the batteries’ temperature within the allowable temperature range.

The potential of a constellation of low earth orbit satellite imagers to monitor worldwide fossil fuel CO2 emissions from large cities and point sources

BackgroundSatellite imagery will offer unparalleled global spatial coverage at high-resolution for long term cost-effective monitoring of CO2 concentration plumes generated by emission hotspots. CO2 emissions can then be estimated from the magnitude of these plumes. In this paper, we assimilate pseudo-observations in a global atmospheric inversion system to assess the performance of a constellation of one to four sun-synchronous low-Earth orbit (LEO) imagers to monitor anthropogenic CO2 emissions. The constellation of imagers follows the specifications from the European Spatial Agency (ESA) for the Copernicus Anthropogenic Carbon Dioxide Monitoring (CO2M) concept for a future operational mission dedicated to the monitoring of anthropogenic CO2 emissions. This study assesses the uncertainties in the inversion estimates of emissions (“posterior uncertainties”).ResultsThe posterior uncertainties of emissions for individual cities and power plants are estimated for the 3 h before satellite overpasses, and extrapolated at annual scale assuming temporal auto-correlations in the uncertainties in the emission products that are used as a prior knowledge on the emissions by the Bayesian framework of the inversion. The results indicate that (i) the number of satellites has a proportional impact on the number of 3 h time windows for which emissions are constrained to better than 20%, but it has a small impact on the posterior uncertainties in annual emissions; (ii) having one satellite with wide swath would provide full images of the XCO2 plumes, and is more beneficial than having two satellites with half the width of reference swath; and (iii) an increase in the precision of XCO2 retrievals from 0.7 ppm to 0.35 ppm has a marginal impact on the emission monitoring performance.ConclusionsFor all constellation configurations, only the cities and power plants with an annual emission higher than 0.5 MtC per year can have at least one 8:30–11:30 time window during one year when the emissions can be constrained to better than 20%. The potential of satellite imagers to constrain annual emissions not only depend on the design of the imagers, but also strongly depend on the temporal error structure in the prior uncertainties, which is needed to be objectively assessed in the bottom-up emission maps.

Simulation of the electrostatic charging of the MetOp-SG satellites in the polar auroral zone

The ability to withstand the harsh space environment is one of the key challenges in the design and construction of satellites. One of these special characteristics of space is the plasma environment encountered in orbit. In this paper, the system-level charging analysis and simulations for the METOP-SG satellites are presented. The METOP-SG satellites will be launched into a sun-synchronous low Earth orbit (LEO) with an altitude of 835 km. In this orbit, the local plasma parameters in the so-called auroral zone close to the poles can lead to critical charging of the satellites [1]. The simulations are performed using a three-dimensional satellite model, where the main elements and surfaces are included. The tool for the simulations is the spacecraft plasma interaction software (SPIS) which has been developed under ESA contract [2]. The calculated surface potentials on the satellite are critically reviewed, the possibility for electrostatic discharges (ESD) is analysed, and protective measures for the minimization of the risks to the satellite mission are discussed.

Timepix in LEO Orbit onboard the VZLUSAT-1 Nanosatellite: 1-year of Space Radiation Dosimetry Measurements

The VZLUSAT-1 satellite, the first Czech CubeSat, was successfully launched on June 23, 2017, to a 510 km Sun-synchronous low-Earth orbit. It carries several scientific payloads including a Timepix detector as focal plane imager for the X-Ray telescope onboard. The Timepix detector contributes significantly to the satellite data collection, with more than 25 000 sampling acquisitions in the first year of deployment. Despite limitations of the satellite attitude control system, necessary for capturing X-Ray images of the Sun, the Timepix detector allows measuring the space radiation environment along the satellite orbit. As of September 2018, we conducted 33 whole-Earth mappings, recording radiation doses around the planet. Further, we show data from scans of the South Atlantic Anomaly and polar radiation horns, where the location and acquisition time were tailored to minimize event pile-up and particle track overlap. Since October 2017, the optics segment of the onboard X-Ray telescope was deployed, which exposed the Timepix detector unshielded to free open space. This change produced entirely new observations namely of low energy charged particles and a significant increase of measured particle flux. We also registered the effects of exposing the sensor to direct intense sunlight. We will summarize on the actual performance of the custom readout interface, which exceeds expectations in the constrained environment of the low-cost and low-powered CubeSat nanosatellite.

Single Event Upset rate determination for 65 nm SRAM bit-cell in LEO radiation environments

The degradation of SRAM bit-cells designed in a 65nm bulk CMOS technology in a Sun-Synchronous Low Earth Orbit (LEO) ionizing radiation environment is analyzed. We propose an inflight SEU rate estimation approach based on a modeled heavy ion cross section as opposed to the standard experimental characterization. Effects of irradiation with estimated LET spectrum in SRAM bit cell, i.e. the location of sensitive regions, its tendency to cause upset, magnitude and duration of transient current as well as voltage pulses were determined. It was found with SEU map that 65nm SRAM bit-cell can flip even if high LET particle strikes in close proximity of bit-cell outside the SRAM bit-cell area. The SEU sensitive parameters required to predict SEU rate of on-board target device, i.e., 65nm SRAM were calculated with typical aluminum spot shielding using fully physical mechanism simulation. In order to characterize the robustness of scaled CMOS devices, state of the art simulation tools such as Visual TCAD/Genius, GSEAT/Visual Particle, runSEU, were utilized whereas LEO radiation environment assessment, upset rate prediction was accomplished with the help of OMERE-TRAD software.

Satellite Solar Array with Silicon Laser Downlink to Denver and the Arctic

Gerard O’Neill's outstanding satellite solar array concepts are updated from geostationary systems to include sun synchronous low earth orbits and Brandon Molniya orbits. The low earth orbits would offer low cost and Brandon orbits would offer the convenience of stationary ground antennas. Jalali's ten micron silicon laser links are studied for high reliability and low cloud attenuation.

End of Life and Beginning of Life Thermal Analysis of a Micro Satellite

Satellites with passive thermal control system use thermal coatings, secondary and first surface mirrors and paints to maintain the temperatures of their electronic equipment within operating temperatures. Satellite coatings are exposed to harsh space environments like ultraviolet radiation (UV) and atomic oxygen (AO) that cause thermal degradation. As nature of the space environment, degradation of the surface paints and coatings cause increase in temperatures of the equipment in time. Thermal designer must consider the thermal degradation of the coatings and paints and optimize the radiator sizes of the satellite at Beginning of Life (BOL) and End of Life (EOL) of the satellite in order to maintain the temperatures of equipment within their safe operation limits. For this analysis, a micro-class satellite using passive thermal control with surface paints and interface conductance within each equipment has been studied. The satellite top surface (+Z) faces the earth and bottom surface (-Z) faces deep space. The lateral sides of the satellite are covered with honeycomb solar panels and top and bottom surfaces are covered with white paint which act as satellite radiator. The satellite orbit has been considered as 700 km Sun-Synchronous Low Earth Orbit. In this analysis BOL and EOL thermo optical properties have been used to predict the satellite temperatures before and after degradation of paints. Thermal analysis have been performed and predicted temperatures obtained by using THERMICA thermal analysis software.

Infrared optical coatings for the EarthCARE Multispectral Imager.

The Earth Cloud, Aerosol and Radiation Explorer mission (EarthCARE) Multispectral Imager (MSI) is a radiometric instrument designed to provide the imaging of the atmospheric cloud cover and the cloud top surface temperature from a sun-synchronous low Earth orbit. The MSI forms part of a suite of four instruments destined to support the European Space Agency Living Planet mission on-board the EarthCARE satellite payload to be launched in 2016, whose synergy will be used to construct three-dimensional scenes, textures, and temperatures of atmospheric clouds and aerosols. The MSI instrument contains seven channels: four solar channels to measure visible and short-wave infrared wavelengths, and three channels to measure infrared thermal emission. In this paper, we describe the optical layout of the infrared instrument channels, thin-film multilayer designs, the coating deposition method, and the spectral system throughput for the bandpass interference filters, dichroic beam splitters, lenses, and mirror coatings to discriminate wavelengths at 8.8, 10.8, and 12.0μm. The rationale for the selection of thin-film materials, spectral measurement technique, and environmental testing performance are also presented.

Space base laser torque applied on LEO satellites of various geometries at satellite’s closest approach

In light of using laser power in space applications, the motivation of this paper is to use a space based solar pumped laser to produce a torque on LEO satellites of various shapes. It is assumed that there is a space station that fires laser beam toward the satellite so the beam spreading due to diffraction is considered to be the dominant effect on the laser beam propagation. The laser torque is calculated at the point of closest approach between the space station and some sun synchronous low Earth orbit cubesats. The numerical application shows that space based laser torque has a significant contribution on the LEO cubesats. It has a maximum value in the order of 10−8Nm which is comparable with the residual magnetic moment. However, it has a minimum value in the order 10−11Nm which is comparable with the aerodynamic and gravity gradient torque. Consequently, space based laser torque can be used as an active attitude control system.

Ground Base Laser Torque Applied on LEO Satellites of Various Geometries

This paper is devoted to investigate the feasibility of using a medium power ground-based laser to produce a torque on LEO satellites of various shapes. The laser intensity delivered to a satellite is calculated using a simple model of laser propagation in which a standard atmospheric condition and linear atmospheric interaction mechanism is assumed. The laser force is formulated using a geocentric equatorial system in which the Earth is an oblate spheroid. The torque is formulated for a cylindrical satellite, spherical satellites and for satellites of complex shape. The torque algorithm is implemented for some sun synchronous low Earth orbit cubesats. Based on satellites perigee height, the results demonstrate that laser torque affecting on a cubesat has a maximum value in the order of 10 -9 which is comparable with that of solar radiation. However, it has a minimum value in the order of 10 -10 which is comparable with that of gravity gradient. Moreover, the results clarify the dependency of the laser torque on the orbital eccentricity. As the orbit becomes more circular it will experience less torque. So, we can conclude that the ground based laser torque has a significant contribution on the low Earth orbit cubesats. It can be adjusted to obtain the required control torque and it can be used as an active attitude control system for cubesats.

Analysis on the Relationship between Burst Errors and Tracking Control Frequency in Satellite-to-Ground Laser Communications

In this paper, burst errors in uplink communication due to lags of the tracking system control in Satellite to Ground optical link was analyzed in detail. In order to make sure how about these bursts in uplink communication are affected by control frequency can, system model of the satellite terminal was founded, parameters of a satellite in sun synchronous Low Earth Orbit and detectors were set to initialize the simulations. The zenith angle of the link was set in the range of [0°, 70°]. The simulation results showed that the bursts due to lags of the tracking system control were different for communication links, but the bursts can be avoided by raising the control frequency to a certain level if it accessible.

Cubesat solar sail 3-axis stabilization using panel translation and magnetic torquing

A Cubesat mission with a deployable solar sail of 5 meter by 5 meter in a sun-synchronous low earth orbit is analyzed to demonstrate solar sailing using active attitude stabilization of the sail panel. The sail panel is kept parallel to the orbital plane to minimize aerodynamic drag and optimize the orbit inclination change caused by the solar pressure force normal to the sail surface. A practical control system is proposed, using a combination of small 2-dimensional translation of the sail panel and 3-axis magnetic torquing which is proved to have sufficient control authority over the gravity gradient and aerodynamic disturbance torques. Miniaturized CMOS cameras are used as sun and nadir vector attitude sensors and a robust Kalman filter is used to accurately estimate the inertially referenced body rates from only the sun vector measurements. It is shown through realistic simulation tests that the proposed control system, although inactive during eclipse, will be able to stabilize the sail panel to within ± 2 ° in all attitude angles during the sunlit part of the orbit, when solar sailing is possible.