ABSTRACT
The MODerate-resolution Imaging Spectroradiometer (MODIS) is a premier Earth observing sensor of the early 21st Century, flying on-board the Terra (T) and Aqua (A) spacecrafts. Both instruments far exceeded their 6 year design life and continue to operate satisfactorily for more than 15 and 13 years, respectively. The MODIS instrument is designed to make observations at nearly a 100% duty cycle covering the entire Earth in less than 2 days. The MODIS sensor characteristics include a spectral coverage from 0.41 µm - 14.4 µm, of which those wavelengths ranging from 3.7 µm - 14. 4 µm cover the thermal infrared region which is interspaced in 16 Thermal Emissive Bands (TEB). Each of the TEB contains 10 detectors which record samples at a spatial resolution of 1 km. In order to ensure a high level of accuracy for the TEB measured Top Of Atmosphere (TOA) radiances, an onboard BlackBody (BB) is used as the calibration source. This paper reports the noise characterization and performance of the TEB on various counts. First, the stability of the onboard BB is evaluated to understand the effectiveness of the calibration source. Next, key noise metrics such as the Noise Equivalent Temperature difference (NEdT) and the Noise Equivalent dn difference (NEdN) for the various TEB are determined from multiple temperature sources. These sources include the nominally controlled BB temperature of 290 K for T-MODIS and 285 K for A-MODIS, as well as a BB Warm Up - Cool Down (WUCD) cycle that is performed over a temperature range from roughly 270 K - 315 K. The Space View (SV) port that measures the background signal serves as a viable cold temperature source for measuring noise. In addition, a well characterized Earth View (EV) Target, the Dome C site located in the Antarctic plateau, is used for characterizing the stability of the sensor, indirectly providing a measure of the NEdN. Based on this rigorous characterization, a list of the noisy and inoperable detectors for the TEB for both instruments is reported to provide the science user communities quality control of the MODIS Level 1B calibrated product.
ABSTRACT
The VIIRS instrument on board the S-NPP spacecraft has successfully operated for more than four years since its launch in October, 2011. Many VIIRS environmental data records (EDR) have been continuously generated from its sensor data records (SDR) with improved quality, enabling a wide range of applications in support of users in both the operational and research communities. This paper provides a brief review of sensor on-orbit calibration methodologies for both the reflective solar bands (RSB) and the thermal emissive bands (TEB) and an overall assessment of their on-orbit radiometric performance using measurements from instrument on-board calibrators (OBC) as well as regularly scheduled lunar observations. It describes and illustrates changes made and to be made for calibration and data quality improvements. Throughout the mission, all of the OBC have continued to operate and function normally, allowing critical calibration parameters used in the data production systems to be derived and updated. The temperatures of the on-board blackbody (BB) and the cold focal plane assemblies are controlled with excellent stability. Despite large optical throughput degradation discovered shortly after launch in several near and short-wave infrared spectral bands and strong wavelength dependent solar diffuser degradation, the VIIRS overall performance has continued to meet its design requirements. Also discussed in this paper are challenging issues identified and efforts to be made to further enhance the sensor calibration and characterization, thereby maintaining or improving data quality.
