System Design for Small Satellites in Very Low Earth Orbit

In the recent years, the very low earth orbit (VLEO) mission concepts are getting popular owing to the advantages it brings. VLEO is considered to be below 400 km in general, and as the satellites orbit closer to the surface, the earth observation data can be more accurate than the data from LEO. The image resolution can be better for the same camera technology, the radio wave applications will experience lower ionospheric disturbances, and the scientific data collection in those altitudes can help to improve the atmospheric models. However, there a significant challenge in orbiting at such altitudes, which is the atmospheric drag. Atmospheric drag increases exponentially in the atmosphere as the altitude decreases, and in those regimes, the drag forces are significant enough to shorten the lifetime of the satellite orbit exponentially. The higher drag also challenges the attitude control systems\u27 ability to keep the satellite stable. This paper discusses the system design of small satellites for performing a VLEO mission. The discussion is drawn by comparing the satellites with various form factors and mass with different solar panel configurations. Concept design process of propulsion system, attitude control system, power system, and communication system is conducted, and trade-offs are considered to safeguard the system. Finally, three cases studies are shown for a 3 kg nanosatellite, 30 kg micro-satellite, and a 100 kg micro-satellite

Extremely Low earth orbit Imaging and Technology Explorer (ELITE): A Very Low Earth Orbit Mission

Extremely Low earth orbit Imaging and Technology Explorer (ELITE) is an experimental micro-satellite on a mission to demonstrate the very low earth orbit (VLEO) flight, high-resolution imaging, and atmospheric data collection. The spacecraft will be launched at an altitude of 550 km, and it will gradually manoeuvre its orbit into VLEO and perform sustained flights are different altitudes for data collection. The camera on-board uses time-dependant integration (TDI) technology to produce high-resolution images. Therefore, the objective is to orbit as low as possible while maintaining the attitude stability required for TDI imaging. Besides the primary imaging mission, the spacecraft also carries: 1) atomic oxygen (AO) fluence detector for characterising the changing AO field in the region of flight, and 2) an ionospheric probe for in-situ plasma density and drift velocities. To support the orbit manoeuvres and drag compensation, the spacecraft is equipped with a propulsion system. There are numerous challenges to overcome to sustain a flight in VLEO which do not occur in LEO. The atmospheric density increases exponentially with altitude, i.e. the drag increases exponentially as the orbit altitude is lowered. The propulsion system has to be sized with adequate margin for sustained operations in VLEO. The increased drag also applies additional stress on to the attitude control system, compromising the stability of the spacecraft. The power generation and ground contact will also be affected as the spacecraft shall maintain minimum drag and high stability orientation instead performing sun-tracking or ground tracking. Besides the ambient environmental challenges, the spacecraft is also subjected to surges in atmospheric density due to solar storms. The storms can increase the density by 10 or 100 times which can be catastrophic for the spacecraft. This paper discusses the mission design for ELITE mission considering the estimated launch time. Analytic results are shown for altitude profile, drag analysis, and structure optimisation. The objective is to highlight the mission design process considering the limitations and considerations of sub-systems. ELITE mission is fully funded by Singapore government and developed by Nanyang Technological University

Integration, Launch, and First Results from IDEASSat/INSPIRESat-2 - A 3U CubeSat for Ionospheric Physics and Multi-National Capacity Building

The Ionospheric Dynamics and Attitude Subsystem Satellite (IDEASSat) is a 3U CubeSat carrying a Compact Ionospheric Probe (CIP) to detect ionospheric irregularities that can impact the usability and accuracy of global satellite navigation systems (GNSS), as well as satellite and terrestrial over the horizon communications. The spacecraft was developed by National Central University (NCU) in Taiwan, with additional development and operational support from partners in the International Satellite Program in Science and Education (INSPIRE) consortium. The spacecraft system needed to accommodate these mission objectives required three axis attitude control, dual band communications capable of supporting both tracking, telemetry and command (TT&C) and science data downlink, as well as flight software and ground systems capable of supporting the autonomous operation and short contact times inherent to a low Earth orbit mission developed on a limited university budget with funding agency-imposed constraints. As the first spacecraft developed at NCU, lessons learned during the development, integration, and operation of IDEASSat have proven to be crucial to the objective of developing a sustainable small satellite program. IDEASSat was launched successfully on January 24, 2021 aboard the SpaceX Falcon 9 Transporter 1 flight. and successfully began operations, demonstrating power, thermal, and structural margins, as well as validation of uplink and downlink communications functionality, and autonomous operation. A serious anomaly occurred after 22 days on orbit when communication with the spacecraft were abruptly lost. Communication was re-established after 1.5 months for sufficient time to downlink stored flight data, which allowed the cause of the blackout to be identified to a high level of confidence and precision. In this paper, we will report on experiences and anomalies encountered during the final flight model integration and delivery, commissioning, and operations. The agile support from the international amateur radio community and INSPIRE partners were extremely helpful in this process, especially during the initial commissioning phase following launch. It is hoped that the lessons learned reported here will be helpful for other university teams working to develop spaceflight capacity

Gravity waves generated by thunderstorms

Gravity waves are considered to be one of the integral parts of the atmosphere which are responsible for energy and momentum distribution among various layers and regions in the atmosphere. These can be generated from various sources such as earthquakes, volcanoes and thunderstorms which can create disturbances in the atmosphere. Thunderstorms are one of the phenomena which occur quite frequently in the tropical region. Thunderstorms are known to generate a wide range of waves including electromagnetic, sound and gravity waves. As it is known, thunderstorms are one of the strongest disturbances in the atmosphere involving huge amount of energy, therefore the gravity wave perturbations created by this can carry large amount of energy and momentum into the middle atmosphere which can affect the circulation and constitution of the layer where the wave dissipates. This thesis is focused on the quantifying the gravity wave perturbations created by the latent heat inside a thunderstorm which is the driving force of the storm, as these relationships have not been quantified based on a large-scale statistical study. Multi-disciplinary approaches have been used in order to develop a gravity wave-latent heat model by studying the ten-year radiosonde data from Singapore. The gravity waves are detected in the stratosphere using the radiosonde profiles, and the source of these waves are traced back using ray tracing technique. The sources are classified into thunderstorms using global outgoing radiation maps and these thunderstorms sources are further analysed for the latent heat generation. Since the latent heat is not a measurable quantity, it is realised using a cloud-resolving model. The simple linear model developed is applicable to the South-East Asia region. The model is able to estimate waves phase velocities and wave amplitudes which are validated using the radiosonde in Singapore when the thunderstorm is in the close vicinity of the station. The model shows reasonable performance with mean estimation error between 14-23% and standard deviation of estimation between 11-19%. vi Since the cloud-resolving models are computationally expensive, it becomes quite impractical to perform these quantifications on a large/global-scale. Therefore, satellite-based latent heat estimations are deemed to be necessary. The latent heat estimations are developed for CloudSat’s radar and MODIS imaging spectroradiometer which can be used to develop gravity wave models on a global scale. The cloud resolving models are used to develop a database of thunderstorms valid for a region (eg. South-east Asia). The database of reflectivity profiles of thunderstorms simulated using a cloud-resolving model is used for comparing with CloudSat radar reflectivity profiles of thunderstorms using Bayesian Monte-Carlo approach to determine the vertical profiles of latent heat. The performance of the algorithm is evaluated using the simultaneous cloud-resolving model simulations near Singapore in 2016. The estimated latent heat profiles are used for training a Bayesian neural network with cloud inputs from MODIS on-board Aqua. This approach exploits the synergy between the CloudSat and Aqua satellites which are on the similar orbit with 60 s time lag. The gravity wave models are developed using satellite based latent heat estimations for South-East Asia and West Africa regions with validation performed using local radiosonde profiles. The gravity wave models developed using satellites also show reasonable performance which gives encouragement and hope for developing global relationships. The models developed here represent the interactions that affect circulation and constituency of the atmosphere which can be used for improving the weather models.Doctor of Philosoph

Inter-hemispheric coupling during sudden stratospheric warming events with elevated stratopause

Sudden stratospheric warmings (SSW) are large-scale disruptions of the wintertime state of the stratosphere that can affect the circulation at synoptic and global scales, including altitudes up to the mesopause in both winter and summer hemispheres. In this study, the response of the summer mesosphere is analyzed during the SSW in the winter stratosphere. In particular, we focus on major SSW events where the climatological stratopause disappears and subsequently reforms at higher altitude, which we refer to as “extreme SSW” in this article. The summer mesosphere response to such extreme SSW events is analyzed in three different phases: (a) stratosphere warming phase, (b) stratopause discontinuity phase, and (c) stratopause reformation phase. Composites of anomalies with respect to climatology derived from the Microwave Limb Sounder and the extended version of the Whole Atmosphere Community Climate Model with specified dynamics are analyzed. The polar summer mesosphere cools during the stratospheric warming phase and warms in subsequent phases. A detailed lag-correlation analysis shows strong negative correlation of −0.6 to −0.8 between the summer mesosphere and the winter stratosphere during the stratosphere warming phase, and a positive correlation of 0.4–0.6 in the phases thereafter. An attempt is made to explain the apparent drivers and dynamics responsible for these couplings, supported with evidence from observations and model output.Ministry of Education (MOE)Published versionThis research is supported by the Ministry of Education, Singapore, under grant AcRF Tier 1-2018-T1-002-166 (RG 196/17)

Source tracing of thunderstorm generated inertia-gravity waves observed during the RADAGAST campaign in Niamey, Niger

In recent years, the climate changes and weather have become a major concern which affects the daily life of a human being. Modelling and prediction of the complex atmospheric processes needs extensive theoretical studies and observational analyses to improve the accuracy of the prediction. The RADAGAST campaign was conducted by ARM climate research stationed at Niamey, Niger from January 2006 to January 2007, which was aimed to improve the west African climate studies have provided valuable data for research. In this paper, the characteristics and sources of inertia-gravity waves observed over Niamey during the campaign are investigated. The investigation focuses on highlighting the waves which are generated by thunderstorms which dominate the tropical region. The stratospheric energy densities spectrum is analysed for deriving the wave properties. The waves with Eulerian period from 20 to 50 h occupied most of the spectral power. It was found that the waves observed over Niamey had a dominant eastward propagation with horizontal wavelengths ranging from 350 to 1 400 km, and vertical wavelengths ranging from 0.9 to 3.6 km. GROGRAT model with ERA-Interim model data was used for establishing the background atmosphere to identify the source location of the waves. The waves generated by thunderstorms had propagation distances varying from 200 to 5 000 km and propagation duration from 2 to 4 days. The horizontal phase speeds varied from 2 to 20 m/s with wavelengths varying from 100 to 1 100 km, vertical phase speeds from 0.02 to 0.2 m/s and wavelengths from 2 to 15 km at the source point. The majority of sources were located in South Atlantic ocean and waves propagating towards northeast direction. This study demonstrated the complex large scale coupling in the atmosphere.Accepted versio

A Very Low Altitude Satellite for Equatorial Ionosphere and Atmospheric Temperature Measurements

The Satellite Research Centre at Nanyang Technological University is currently developing the Atmospheric Coupling and Dynamics Explorer (ARCADE) Mission, which is flying a hall effect thruster to progressively lower the altitude from an initial 500 km to 250 km. ARCADE is also the fourth satellite in the INSPIRE (International Satellite Program in Research and Education) satellite series with joint development from IIST, India and NCU, Taiwan. ARCADE is a 27U spacecraft carrying an ionospheric plasma payload which will make ion temperature, velocity, density and electron temperature measurements. The satellite will be launched along with six other Singaporean satellites on a Singapore dedicated PSLV in 2020 into a near equatorial orbit. Since the final altitude is expected to be 250 km, the ARCADE/INSPIRESat-4 mission provides an excellent opportunity to study the equatorial ionosphere at low altitudes where the ion and electron density are much higher. The mission is expected to provide new information on plasma irregularities along the magnetic equator. The mission is also a technology demonstration of a hall effect thruster developed by French Startup \u27Thrust Me\u27. Another addition to the mission is a Spatial Heterodyne Interferometer Infra-Red Imager for imaging the Mesosphere and Lower thermosphere region between 60-120 km. The SHI instrument will provide temperature information and help for understanding the dynamics of the equatorial MLT region. The presentation will cover the teams approaches to dealing with Very Low Earth Orbit (VLEO) and the challenges it poses in terms of thermal and atomic oxygen effects

Lessons Learned from IDEASSat: Design, Testing, on Orbit Operations, and Anomaly Analysis of a First University CubeSat Intended for Ionospheric Science

International audienceGiven the pervasive use of satellite and over the horizon wireless communication tech- nology in modern society, ionospheric disturbances that can disrupt such services are a crucial consideration. Ionospheric irregularities, plasma bubbles and other phenomena can have a great impact on satellite navigation and communications, impacting other systems reliant on such technolo- gies. The Ionospheric Dynamics and Attitude Subsystem Satellite (IDEASSat) was a 3U developed by National Central University (NCU) to measure irregularities in the ionosphere, as well as to establish spacecraft engineering and operations capacity at NCU. The onboard Compact Ionospheric Probe (CIP) could measure high-resolution plasma parameters, which can be used for identifying iono- spheric irregularities that can cause scintillation in satellite navigation and communications signals. Part of the spacecraft sub-systems were independently designed and developed by students, who were also responsible for integration, testing, and operations. IDEASSat was successfully launched into low Earth orbit on 24 January 2021, and then began mission operations. The spacecraft success- fully demonstrated three-axis attitude stabilization and control, tracking, telemetry and command (TT&C), as well as flight software and ground systems that could support autonomous operation. The spacecraft experienced a critical anomaly 22 days after launch, followed by a 1.5-month commu- nications blackout. The spacecraft briefly recovered from the blackout for long enough to replay flight data, which allowed for the cause of the blackout to be determined as an inability of the electrical power subsystem reset circuit to recover from an ionizing radiation induced single event latch-up. Although the mission was not completed, flight data obtained during the mission will help to im- prove the designs of future spacecraft in development at NCU. This paper will introduce IDEASSat’s final flight model design and implementation, integration, testing, environmental verification, and failure analysis, and will review the performance of the spacecraft during on-orbit operations. The results and experiences encountered in implementation and operations of the IDEASSat mission are presented here as a reference for other university small satellite teams