Improving accuracy of the satellite-based Earth Observing System is a crucial objective for climate science. Climate quality measurements require sophisticated satellite instrumentation to monitor the onboard calibration, currently available on very few sensors. Inter-calibration is the process of comparing co-located measurements with matched viewing geometries from different sensors on separate spacecraft to improve target instrument calibration and remove temporal calibration trends.
Inter-calibration ties target instrument calibrations on one spacecraft to highly accurate, preferably SI-traceable, reference instruments on another spacecraft. Low-Earth orbit and geostationary-Earth orbit instruments with similar spectral responses are compared by analyzing co-located pixels using a line-of-sight matching technique near orbit intersections. Error analyses (not part of this proposal) specify achievable inter-calibration accuracies for various instruments and sampling criteria in time/space/angle dimensions.
Two international organizations are trying to establish best inter-calibration practices for geostationary-Earth orbit and low-Earth orbit sensors. The Committee on Earth Observation Satellites Working Group on Calibration and Validation promotes high quality calibration and validation of infrared and visible optical data. The World Meteorological Organization Global Space based Inter-Calibration System community analyzes methods and algorithms to improve the calibration of operational instruments in support of climate monitoring.
The development of a common framework to improve access to co-located data with matching viewing geometries from multiple instruments will enable independent calibration groups to advance the inter-calibration of current sensors such as geostationary-Earth orbit, Advanced Very High Resolution Radiometer (AVHRR), Moderate Resolution Imaging Spectroradiometer (MODIS), and Visible Infrared Imaging Radiometer Suite (VIIRS). Furthermore, this framework will be designed to be readily adapted to handle the increased data volume from future passive remote sensing Decadal Survey and international missions, including hyperspectral reference instruments such as Climate Absolute Radiance and Refractivity Observatory (CLARREO) or Traceable Radiometry Underpinning Terrestrial and Helio studies (TRUTHS).
The Multi-Instrument Inter-Calibration (MIIC) Framework provides a real world solution to instrument teams responsible for calibration and validation of target instrument data. The MIIC Framework allows efficient access to reference data from existing Distributed Active Archive Center (DAAC) servers. Inter-calibration (IC) events from multiple spacecraft for a given time period (typically 1 month) are automatically predicted from the specified sampling criteria and orbit crossings. The event specifications are inserted into an eXtensible Markup Language inter-calibration plan. For each event inter-calibration algorithms are executed on remote servers using Open-source Project for Network Access Protocol server-side functions prior to delivery of the data to the instrument teams for further analysis.
Algorithms such as convolution over sensor point spread functions and spectral response functions are run on remote data servers. This capability saves months of time downloading extraneous data and reduces local processing and disk storage resources typically required by instrument teams to perform these functions. In addition, this framework will help science teams distinguish trends from calibration artifacts in retrieved parameters such as cloud properties. The goal of this effort is to re-use existing science algorithms and information technology software to increase the interconnectedness of existing distributed information systems and the quality of datasets.