From opportunity to continuity: A CubeSat implementation to enhance Earth’s radiation budget observations from space

Abstract. Earth's radiation budget (ERB) is an essential climate variable, and its continuous observation from space is critical to understanding long-term climate change. The Clouds and Earth's Radiant Energy System (CERES) has maintained the longest continuous ERB record since 2000, with its follow-on mission, Libera, launching in 2027 with a 5-year prime mission lifetime. Beyond Libera, plans for ERB continuity remain uncertain, increasing the possibility of future gaps in the record. The Compact Total Irradiance Monitor (CTIM) was a 6U CubeSat developed under the NASA In-Space Validation of Earth Science Technologies (InVEST) program to measure total solar irradiance (TSI). Launched in July 2022 and operating until November 2023, CTIM collected climate-quality science at an uncertainty of 0.017 %. During orbital eclipse, CTIM was pointed in the nadir direction to measure Earth's outgoing longwave emission, exploring the Earth-observing potential of an instrument designed for TSI. These measurements were compared to coincident CERES observations aboard Terra, Aqua, and NOAA-20. Additional adjustment factors for limb darkening were derived from radiative transfer simulations over a variety of scene types and atmospheric conditions and applied to CERES non-nadir viewing observations to better match the CTIM nadir observations and explain some of the variance exhibited between measurements. The resulting comparisons show an overall relative agreement of 1.3 %, within respective instrument uncertainties. This study demonstrates that leveraging CubeSat technology could complement heritage ERB missions and reduce the risk of future observation gaps. The routine inclusion of Earth-viewing capabilities in future TSI instrument designs represents a natural extension of this work, with the potential to meaningfully contribute to ERB observation continuity and reduce dependence on dedicated large-scale missions.