Reliable characterization and radiometric calibration of satellite sensors are critical to their optimal performance on-orbit. The uses of satellite sensor data, with their increased use in long-term environmental monitoring and climate studies mean that the performance and data quality provided by a single sensor can no longer be considered in isolation but needs to be considered as a part of the international Earth Observation (EO) infrastructure and referenced to common standard, the SI. The drive for improved performance, together with the desire for inter-operability between sensors creates increased demands on the pre-flight characterization and radiometric calibration of sensors. Sensor pre-flight characterization and calibration facilities, or optical ground support equipment (OGSE) test sensor performance over a few broad categories including geometric performance/image quality together with spectral and radiometric calibration. The specific requirements of the sensor have historically created a drive for a bespoke OGSE. For large-scale multi-sensor series programs, a bespoke solution may remain the preferred solution. However, for single/few unit explorer missions, the expense & post-use redundancy of a bespoke OGSE system may be prohibitive. NPL together with M Squared lasers has developed a universal OGSE facility, the Spectroscopically Tunable Absolute Radiometric calibration & characterization OGSE (STAR-CC-OGSE), a versatile facility for the radiometric calibration and characterization of satellite sensors. The system is provided fully characterized, calibrated and performance verified, with an easy to use software interface that allows fully automated remote operation
Sound policymaking requires high confidence in climate predictions verified against decadal change observations with robustly known accuracy. Yet, our ability to monitor and predict the future of the climate is inadequate as we currently do not possess sufficient accuracy in our observing capability to confidently observe the small but critical climate change signals that are expected to occur over decadal time scales. These signals are fundamental to assessing the accuracy of climate change projections made by models and for the unambiguous attribution of climate change.
TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio-Studies) is a proposed small satellite mission to enable a space-based climate observing system capable of delivering data of the quality needed to provide the information needed by policy makers to make robust mitigation and adaptation decisions. This is achieved by embedding trust and confidence in the data and derived information (tied to international standards) from both its own measurements and by upgrading the performance and interoperability of other EO platforms, such as the Sentinels by in-flight reference calibration. TRUTHS would provide measurements of incoming (total and spectrally resolved) and global reflected spectrally and spatially (50 m) solar radiation at the 0.3% uncertainty level. These fundamental climate data products can be convolved into the building blocks for many ECVs and EO applications as envisaged by the 2015 ESA science strategy; in a cost effective manner. We describe the scientific drivers for the TRUTHS mission and how the requirements for the climate benchmarking and cross-calibration reference sensor are both complementary and simply implemented, with a small additional complexity on top of heritage calibration schemes. The calibration scheme components and the route to SI-traceable Earth-reflected solar spectral radiance and solar spectral irradiance are described.
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