Clouds and the Earth's Radiant Energy System (CERES) is an ongoing[update]NASA climatological experiment fromEarth orbit.[1][2] The CERES are scientific satellite instruments, part of NASA'sEarth Observing System (EOS), designed to measure solar-reflected and Earth-emitted radiation from the top of theatmosphere (TOA) to the Earth's surface. Cloud properties are determined using simultaneous measurements by other EOS instruments such as theModerate Resolution Imaging Spectroradiometer (MODIS).[3] Results from the CERES and other NASA missions, such as theEarth Radiation Budget Experiment (ERBE),[4] could enable near-real-time tracking ofEarth's energy imbalance (EEI) and better understanding of the role of clouds in globalclimate change.[1][5]
The CERES experiment has four main objectives:
Each CERES instrument is aradiometer which has three channels – a shortwave (SW) channel to measure reflected sunlight in 0.2–5μm region, a channel to measure Earth-emitted thermal radiation in the 8–12μm "window" or "WN" region, and a Total channel to measure entire spectrum of outgoing Earth's radiation (>0.2μm). The CERES instrument was based on the successfulEarth Radiation Budget Experiment, which used three satellites to provide global energy budget measurements from 1984 to 1993.[6]
The first CERES instrument Proto-Flight Module (PFM) was launched aboard theNASATropical Rainfall Measuring Mission (TRMM) in November 1997 fromJapan. However, this instrument failed to operate after 8 months due to an onboard circuit failure.
Six more CERES instruments were launched on theEarth Observing System and theJoint Polar Satellite System. The Terra satellite, launched in December 1999, carried two (Flight Module 1 (FM1) and FM2), and the Aqua satellite, launched in May 2002, carried two more (FM3 and FM4). A fifth instrument (FM5) was launched on theSuomi NPP satellite in October 2011 and a sixth (FM6) onNOAA-20 in November 2017.[7][8]
The mission lifetime of each FM1-FM6 CERES instrument was originally planned for a nominal five years.[9] With the failure of the PFM on TRMM and the 2005 loss of the SW channel of FM4 on Aqua, five of the CERES Flight Modules remained fully operational as of 2017. In 2023 the Aqua and Terra missions were extended into 2026 and 2027 respectively. FM2 was turned off in January 2025 due to degradation of Terra's solar power generation.[9]
CERES operates in three scanning modes: across the satelliteground track (cross-track), along the direction of the satellite ground track (along-track), and in a Rotating Azimuth Plane (RAP). In RAP mode, theradiometers scan in elevation as they rotate inazimuth, thus acquiringradiance measurements from a wide range of viewing angles. Until February 2005, onTerra andAqua satellites, one of the CERES instruments scanned in cross-track mode while the other was in RAP or along-track mode. The instrument operating in RAP scanning mode took two days of along-track data every month. The multi-angular CERES data is used derive new models which account for theanisotropy of the viewed scene and enable TOA radiative flux retrieval with enhanced precision.[10]
All CERES instruments are inSun-synchronous orbit. Comparablegeostationary data between 60°S and 60°N are also applied within "balanced and filled" data products to provide adiurnally complete representation of the radiation budget and to account for cloud changes between CERES observation times.[11]
The CERES instruments were designed to provide enhanced measurement stability andprecision; however, achieving and ensuring absoluteaccuracy over time was also known to remain an ongoing challenge.[12] Despite the more advanced capability of CERES to monitor Earth's TOA radiative fluxes globally and with relative accuracy, the only practical way to estimate the absolute magnitude of EEI (as of 2020) is through an inventory of the energy change in the climate system.[13] Consequently, an important constraint within CERES data products has been the anchoring of EEI at one point in time to a value which corresponds to several years ofARGO data.[11]
For a climate data record (CDR) mission like CERES, accuracy is highly important and achieved for pure infrared nighttime measurements using a ground laboratory SI traceable blackbody to determine total and WN channel radiometric gains. This, however, was not the case for CERES solar channels such as SW and the solar portion of the Total telescope, which have no direct unbroken chain to SI traceability. This is because CERES solar responses were measured on the ground using lamps whose output energy was estimated by a cryo-cavity reference detector, which used a silver Cassegrain telescope identical to CERES devices to match the satellite instrument's field of view. The reflectivity of this telescope, built and used since the mid-1990s, was never actually measured; it was estimated[14] only based on witness samples (see slide 9 of Priestley et al. (2014)[15]). Such difficulties in ground calibration, combined with suspected on-ground contamination events[16] have resulted in the need to make unexplained ground to flight changes in SW detector gains as big as 8%,[17] to make the ERB data seem somewhat reasonable to climate science (note that CERES currently claims[12] a one sigma SW absolute accuracy of 0.9%).
CERES spatial resolution at nadir view (equivalent footprint diameter) is 10 km for CERES on TRMM, and 20 km for CERES onTerra andAqua satellites. Perhaps of greater importance for missions such as CERES is calibration stability, or the ability to track and partition instrumental changes from Earth data so it tracks true climate change with confidence. CERES onboardcalibration sources intended to achieve this for channels measuring reflected sunlight include solar diffusers and tungsten lamps. However, the lamps have very little output in the important ultraviolet wavelength region where degradation is greatest, and they have been seen to drift in energy by over 1.4% in ground tests, without a capability to monitor them on-orbit (Priestley et al. (2001)[18]). The solar diffusers have also degraded greatly in orbit, that they have been declared unusable by Priestley et al. (2011).[19] A pair ofblack body cavities that can be controlled at different temperatures are used for the Total and WN channels, but these have not been proved stable to better than 0.5%/decade.[16] Cold space observations and internal calibration are performed during normal Earth scans.
Data is compared between CERES instruments on different mission satellites, as well as compared to scan reference data from accompanyingspectroradiometers (e.g., MODIS on Aqua). The plannedCLARREO Pathfinder mission aims to provide a state-of-the-art reference standard for several existing EOS instruments, including CERES.[12]
A study of annual changes to Earth's energy imbalance (EEI) spanning 2005-2019 showed good agreement between the CERES observation and EEI inferred from in-situ measurements ofocean heat uptake by the Argo float network.[20] A concurrent pair of studies measuring global ocean heat uptake,ice melting, andsea level rise with a combination ofspace altimetry and gravimetry suggested similar agreements.[21][22]
The 40+ year TOA radiation data record begun with ERBE in 1984 is at risk of a gap at the end of CERES missions.[23]
The measurements of the CERES instruments were to be furthered by theRadiation Budget Instrument (RBI) to be launched onJoint Polar Satellite System-2 (JPSS-2) in 2021, JPSS-3 in 2026, and JPSS-4 in 2031.[8] The project was cancelled on January 26, 2018;NASA cited technical, cost, and schedule issues and the impact of anticipated RBI cost growth on other programs.[27]
NASA announced in February 2020 its selection of the Libera instrument to launch on JPSS-3 by the end of 2027.[28] Libera is planned to provide data continuity and updated capabilities.LASP is the lead instrument developer.[29]
NASA also began in 2020 a project to develop ERB measurements integrated onto aSmallSat utilizing an ~10x reduced instrument size.[30][31] The Athena EPIC demonstration launched in July 2025.[32]
The imbalance is derived from the CERES-EBAF Edition 4.2.1 data set.
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