# UVSQ-SAT, a Pathfinder CubeSat Mission for Observing Essential Climate Variables
## 1 Introduction  
UltraViolet and infrared Sensors at high Quantum efficiency onboard a small SATellite (UVSQ-SAT) is a Laboratoire Atmospheres, Milieux, Observations Spatiales (LATMOS) nanosatellite mission with scientific and technological goals [1] mainly for observing essential climate variables, namely shortwave and longwave radiative fluxes at the top of the atmosphere and UV solar spectral irradiance. However, the UVSQ-SAT pathfinder mission will not provide a continuity of the essential climate variables' data records since it is a demonstrator. Another objective of the UVSQ-SAT mission is to provide hands-on experience to UVSQ and Paris-Saclay University students in the requirements' definition, reliability and quality assurance, cost and risk management, design, construction, spacecraft integration and testing, mission operations, and control of complete satellite systems that will serve as the basis for a variety of future space missions for Earth observation and astronomy and astrophysics. The UVSQ-SAT nanosatellite is a cube of about 11 cm with a mass of up to 1.6 kg and a power consumption of up to 2 W. The launch of the CubeSat is currently targeted in the time frame of 2020/2021. The choice of the orbit is directly related to scientific goals while taking into account the optimization for launch opportunities as piggybacking and the rules governing the space debris mitigation. The selected orbit is a Sun-Synchronous (SSO) Low Earth Orbit (LEO) with a maximum altitude of 600 km and a Local Time at Ascending Node (LTAN) of 10:30 hours, which will lead to an atmospheric reentry of the satellite within 25 years. The operational mission lifetime will be at least of one year in orbit, including the commissioning phase, to achieve the expected UVSQ-SAT's scientific objectives. The first scientific objective of the UVSQ-SAT in orbit demonstration CubeSat is to measure the incoming solar radiation (total solar irradiance) and the outgoing terrestrial radiation (top of atmosphere outgoing longwave radiation and shortwave radiation) using twelve miniaturized Earth radiative sensors (thermopiles based on the advantages of carbon nanotubes and Qioptiq optical solar reflectors). Thus, it might be possible to constrain better the Earth's radiative balance and, more importantly, the Earth's Energy Imbalance (EEI) [2], which is defined as the difference between the incoming solar radiation and the outgoing terrestrial radiation (longwave and shortwave radiation). The EEI's direct determination is very challenging because EEI is two orders of magnitude smaller than the radiation fluxes in and out of the Earth's system. The second scientific objective is to monitor the solar spectral irradiance in the Herzberg continuum (200-242 nm) using four photodiodes, which benefit from the intrinsic advantages of Ga\({}_{2}\)O\({}_{3}\) alloy based sensors grown by pulsed laser deposition [3]. A better understanding of natural factors in climate variability is the essential motivation of the UV solar spectral irradiance measurements. The UV solar variability over time has significant implications for atmospheric chemistry and its modeling. The main goal of this paper is to present the UVSQ-SAT mission and the justifications for its existence. Section 2 describes in detail the scientific rationale of the UVSQ-SAT mission. Section 3 provides a description of the UVSQ-SAT nanosatellite and its operational requirements. Section 4 presents a preliminary analysis of the expected results using numerical simulations. The results concern a spatial reconstruction of Earth's radiation budget that might be carried out with UVSQ-SAT data for a given time period of observation. This analysis will also highlight the interest to implement a satellite constellation in order to improve the determination of EEI, which is a crucial quantity for testing climate models and for predicting the future course of global warming. Today, the implementation of an \"EEI\" constellation based on small satellites is possible.