## 2 Scientific Objectives and Requirements
### Scientific Requirements and Uncertainties on EEI Measurements  
Accurately measuring the absolute value of the EEI at the TOA and its variability over time is still a scientific and technological challenge. The relevant scientific objective is to be able to detect any long-term global trend with a high accuracy (stability of at least \(\pm 0.2\) Wm\({}^{-2}\) per decade), but also to be able to obtain fine spatial (a few tens of km) and high temporal resolutions (three hours as for weather forecasting) with an accuracy of a few Wm\({}^{-2}\). With current technologies, we do not know if all these requirements are achievable with instruments on board satellites, but they are very important to better determine the changes.  
To date, the best estimate of the EEI is obtained from the change in heat stored in the ocean, commonly referred as ocean heat content (OHC). However, the absorption of heat by the ocean acts as a \"buffer\". This causes the rate of surface warming to slow down. The OHC measurements made by the Argo automated floats are excellent, but do not provide answers to the \"short term\" dynamics. To solve this issue, we need fine spatial and temporal resolutions as previously stated. For this, we need a constellation of satellites with instruments that combine measurements with a narrow field of view (FOV) and a wide FOV. The key to solve this problem is related to the resolution of the diurnal variation, which depends on the physical parameterization of the models. The \"long term\" is associated with the convolution of quick processes and dynamic phenomena at longer temporal and spatial scales of the atmosphere-ocean system. Thus, the best strategy for measuring the EEI is to track the \"long term\" temporal evolution of the OHC since most of the excess heat (89% over 1970-2018) is absorbed by the ocean [15]. These measurements must be combined with observations at the TOA using satellite-based instruments to provide \"short term\" information. Measurements with good spatio-temporal resolutions are crucial for advancing our understanding of climate change, as the radiative balance is driven in part by the radiative impacts of aerosols and clouds, which are highly spatially and temporally variable and are still relatively poorly quantified [16]. Currently, we do not know whether cloud-related changes will amplify, mitigate, or perhaps have only  
Figure 6: Time evolution of the OLR and the EEI from CERES measurements and the ERA 5 model.  
a small effect on the increase in the Earth's global temperature due to anthropogenic radiative forcings. Furthermore, we do not know the impact of radiative forcing related to anthropogenic aerosols that alter cloud properties. Global climate models have limitations with respect to these two issues. This highlights the importance of space-based missions to make accurate measurements with excellent spatio-temporal resolutions.  
Observations are important to validate climate models and the magnitude of the energy imbalance at the TOA. Incident solar flux is an input to the model. The solar radiation reflected from the Earth or OSR and the OLR are calculated at the model level and must be compared to those measured. Table 2 summaries the expected science requirements for disruptive measurements of the energy imbalance components. In addition, some objectives of the UVSQ-SAT mission are presented. UVSQ-SAT is a science and technology demonstrator, one of whose objectives is to obtain an accurate short-term measurement of the EEI with a relative uncertainty that must be less than \(\pm 1.0\) Wm\({}^{-2}\) over the course of a year. The expected spatial resolution of the UVSQ-SAT measurements is in the order of 1000 km. Forward, the method of [PERSON] et al. (2017)[PERSON] et al. (2017) will be used to improve the accuracy of the measurements thanks to a spherical harmonic analysis.