Clouds and the Earth's Radiant Energy System

Clouds and the Earth's Radiant Energy System (CERES) is NASA climatological experiment from Earth orbit. The CERES are scientific satellite instruments, part of the NASA's Earth Observing System (EOS), designed to measure both solar-reflected and Earth-emitted radiation from the top of the atmosphere (TOA) to the Earth's surface. Cloud properties are determined using simultaneous measurements by other EOS instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS). Results from the CERES and other NASA missions, such as the Earth Radiation Budget Experiment (ERBE), could lead to a better understanding of the role of clouds and the energy cycle in global climate change.



Scientific goals
CERES experiment has four main objectives:


 * Continuation of the ERBE record of radiative fluxes at the top of the atmosphere (TOA) for climate change analysis.
 * Doubling the accuracy of estimates of radiative fluxes at TOA and the Earth's surface.
 * Provide the first long-term global estimates of the radiative fluxes within the Earth's atmosphere.
 * Provide cloud property estimates that are consistent with the radiative fluxes from surface to TOA.

Each CERES instrument is a radiometer which has three channels - a shortwave channel to measure reflected sunlight in 0.3 - 5 µm region, a channel to measure Earth-emitted thermal radiation in the 8-12 µm "window" region, and a total channel to measure entire spectrum of outgoing Earth's radiation. The CERES instrument was based on the successful Earth Radiation Budget Experiment which used three satellites to provide global energy budget measurements from 1984 to 1993.

CERES spatial resolution at nadir view (equivalent diameter of the footprint) is 10 km for CERES on TRMM, and 20 km for CERES on Terra and Aqua satellites. Onboard calibration sources for channels measuring reflected sunlight include solar diffusers and tungsten lamps. A pair of blackbody cavities that can be controlled at different temperatures are used for the Total and WN channels. Cold space observations and internal calibration are performed during normal Earth scans.

First Launch
The first CERES instrument (PFM) was launched aboard the NASA Tropical Rainfall Measuring Mission (TRMM) in November 1997 from Japan. However, this instrument failed to operate after 8 months due to an on-board circuit failure.

CERES on the EOS Mission Satellites
An additional four CERES instruments were launched on the Earth Observing System Terra satellite in December 1999 (FM1 and FM2) and on EOS Aqua satellite in May 2002 (FM3 and FM4). A fifth instrument is on the Suomi NPP satellite (FM5). Currently operational are all CERES instruments on Terra and Aqua satellites, and the one on Suomi NPP. The FM6 instrument is to be launched on the JPSS-1 spacecraft in November 2016.

Operating modes
CERES operates in three scanning modes: across the satellite ground track (cross-track), along the direction of the satellite ground track (along-track), and in a Rotating Azimuth Plane (RAP). In RAP mode, the radiometers scan in elevation as they rotate in azimuth, thus acquiring radiance measurement from a wide range of viewing angles. Until February 2005, on Terra and Aqua satellites one of 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. However the multi-angular CERES data allowed to derive new models which account for anisotropy of the viewed scene, and allow TOA radiative flux retrieval with enhanced precision.