For nearly 20 years, Terra MODIS observations have generated a broad range of data products, enabling the remote sensing community and users worldwide for their studies of many key geophysical parameters of the Earth’s system. MODIS collects data in 36 spectral bands, covering wavelengths from 0.41 to 14.4 μm, that are calibrated by a set of on-board calibrators (OBC). Also contributed to sensor on-orbit calibration and characterization are near monthly-scheduled lunar observations and long-term trends of sensor responses over select ground targets. The quality of MODIS data products relies strongly on the dedicated efforts to the operate instrument, derive and update calibration parameters, and improve calibration strategies and algorithms in order to address on-orbit changes of sensor characteristics and its OBC functions. This paper provides an overview of Terra MODIS on-orbit operation and calibration activities over the last 20 years, including changes made to extend and preserve instrument and OBC functions and their implementation strategies. It illustrates sensor on-orbit performance using data from its OBC, lunar observations, and select ground targets and discusses major changes in sensor characteristics and corrections applied to the L1B algorithms or updates of calibration look-up tables (LUTs). Also described in this paper are lessons learned from Terra MODIS and future efforts to further extend its long-term data records.
The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on board the Terra and Aqua space- craft are equipped with several on-board calibrators (OBCs) and continue to operate normally since launch. One such calibrator is the Spectro-Radiometric Calibration Assembly (SRCA), whose regular calibrations provide accurate measurements in radiometric, spatial and spectral modes. The SRCA is able to monitor and measure the center wavelength (CW) shift, the bandwidth (BW) shift and a major portion of the relative spectral response (RSR) for each of the MODIS reflective solar bands (RSBs) while operating in spectral mode. However, there are several factors that influence the uncertainties when calculating these results. This paper provides a brief overview of the SRCA in spectral mode, along with how the CWs, BWs and RSRs of the MODIS RSBs are calculated. The operational factors that contribute to the spectral uncertainty are also discussed, including the variation of the half-included angle (β) and the grating motor offset angle (θ<sub>off</sub> ) of the monochromator. A comparison between the theoretical and on-board CW uncertainty is also provided.
The Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on-board the Terra and Aqua space- craft are equipped with several on-board calibrators (OBCs) and continue to operate normally since launch. One such calibrator is the solar diffuser (SD), which allows for the calibration of the 20 reflective solar bands (RSBs) with wavelengths ranging from 0.41 to 2.3 μm. In order to accurately characterize the RSBs on-orbit, the changes associated with the SD bi-directional reflectance factor (BRF) are tracked using a solar diffuser stability monitor (SDSM). The SDSM consists of nine detectors located within a spherical integration source (SIS) and covers wavelengths from 0.41 to 0.94 μm. During each calibration event, the SDSM alternately views sunlight through an attenuation screen and the sunlight reflected from the SD in order to accurately characterize the degradation of the SD at those nine wavelengths. This paper provides a brief overview of the SD/SDSM calibration and operation, with more emphasis on the recent performance of the SD degradation and the SDSM detectors. A methodology to compute the signal-to-noise ratio (SNR) for each of the SDSM detectors is formulated and the noise performance is tracked over the mission lifetime. The importance of the detector noise to the RSB calibration uncertainty and to other instruments, such as the VIIRS SDSM, is also discussed.
We initiated a multi-technique campaign to understand the physics and properties of the massive binary system MWC 314. Our observations included optical high-resolution spectroscopy and Johnson photometry, nearinfrared spectrophotometry, and <i>K′</i>−band long-baseline interferometry with the CHARA Array. Our results place strong constraints on the spectroscopic orbit, along with reasonable observations of the phase-locked photometric variability. Our interferometry, with input from the spectrophotometry, provides information on the geometry of the system that appears to consist of a primary star filling its Roche Lobe and loosing mass both onto a hidden companion and through the outer Lagrangian point, feeding a circumbinary disk. While the multi-faceted observing program is allowing us to place some constraints on the system, there is also a possibility that the outflow seen by CHARA is actually a jet and not a circumbinary disk.