Performance Stability Evaluation of Clouds and Earth's Radiant Energy System (CERES) Instruments on EOS-Terra, EOS-Aqua and S-NPP Spacecraft

Clouds and the Earth's Radiant Energy System (CERES) instruments were designed to measure the reflected shortwave and emitted longwave radiances of the Earth's radiation budget and to investigate the cloud interactions with global radiances for the long-term monitoring of Earth's climate. The three scanning thermistor bolom¬eter sensors on CERES measure broadband radiances in the shortwave (0.3 to 5.0 micrometer), total (0.3 to >100 micrometer) and in 8 - 12 micrometer water vapor window regions. Four of the CERES instruments (Flight Models1 through 4) fly aboard Earth Observing System (EOS) Terra and Aqua platforms with two instruments aboard each spacecraft, in 705 KM sun-synchronous orbits of 10:30 AM and 1:30 PM equatorial crossing time. A fifth CERES instrument (Flight Model 5) is aboard the Suomi – NPP spacecraft flying at 824 KM sun-synchronous orbit of 1:30 PM equatorial crossing time. The last of the CERES instrument, Flight Model 6 (FM6) will be flying on the JPSS-1 spacecraft. The CERES data products consist of geolocated instantaneous unfiltered radiances through temporally and spatially averaged Top of the Atmosphere (TOA) and Surface fluxes. These CERES data products have achieved a higher level of calibration accuracy and stability than the previous Earth Radiation Budget Energy (ERBE) products. This improvement was attained through the development of a rigorous and comprehensive radiometric validation protocol comprising of studies covering different spatial, spectral and temporal time scales. The in-flight calibration of CERES sensors are carried out using the internal calibration module (ICM) comprising of blackbody sources and tungsten lamp, and a solar diffuser plate known as the Mirror Attenuator Mosaic (MAM). The ICM calibration results are instrumental in understanding the ground to flight shift and in-flight drifts in CERES sensors' responses in both longwave and shortwave regions. The MAM calibrations with the diffused solar energy provide insight into the visible and near infrared spectral region of shortwave and total sensor responses. In addition, several validation studies utilizing the inter-comparisons of measurements from two CERES instruments on the same spacecraft and CERES instruments on different spacecraft, as well as cross-comparisons of measurements between the sensors on a single instrument are conducted to monitor the behavior of the sensors in each instrument at various spectral regions. Measurements from the targets such as deep convective clouds and tropical ocean are used to evaluate the three sensors' stability within an instrument. Tropical ocean measurement called the Tropical Mean study is also used as a inter-comparison metric between the Total sensors on various CERES instruments. Having two CERES instruments on the same spacecraft for the Terra and Aqua missions, inter comparison of similar sensor measurements through coincident earth observations are conducted. This study enabled the team to evaluate any instrument operational dependency on the measurement as well as sensor measurement differences for various scene types covering different spectral wavelength regions. Another inter-comparison evaluation between the CERES instruments on different spacecraft is performed with observations of targeted locations in special scan orientation at near time over-fly passes. These measurements allow a good assessment of similar sensor outputs from different CERES instruments and to bring the measurements from instruments on different spacecraft to a common radiometric scale. The results from these individual studies have collectively given an understanding of each CERES sensor's behavior in different spectral regions. These studies have enabled the team to achieve the stability of the CERES radiation budget data products to better than 0.2 percent. This paper discusses the various studies and summarizes the status of the radiometric accuracy and stability of the CERES data products.