Proceedings Article | 12 July 2023
KEYWORDS: Light emitting diodes, Equipment, Calibration, Sun, Lamps, Light sources, Black bodies, Diffusers, Astronomical imaging, Ultraviolet radiation
In-orbit white light sources (WLS) have been used in many in-flight calibration units of optical space instruments. Usually, a QTH lamp (Quartz Tungsten Halogen, broad band light source) is used on board for in-flight characterisation and health checks of the instruments. The major issue of the QTH lamps used are their limited lifetime, reliability and recently their obsolescence and lack of suppliers. This study addresses these issues and looks for alternatives to replace in-orbit WLS as QTH with the best equivalents today available on the market. The objective of the activity is to identify an alternative broad band (in-flight) WLS to replace and outperform the QTH. Here, alternative light sources are proposed, their concept and relevant use cases for their implementation are described. The study explores the feasibility of the alternatives proposed and establishes trade-off criteria based on use case requirements. Finally, the future potential usage of the alternative WLS and further steps for breadboard manufacturing, testing and space qualification are presented. Preliminary requirements for the innovative WLS (iWLS) are collected, partly based on requirements of the imaging spectrometers OMI, SCIAMACHY, GOME-2, TROPOMI and Sentinel 5, and on typical use cases. Typical use cases cover on-ground and in-orbit instrument characterisation and monitoring, in particular of detector properties such as electronic offset, electronic gain, noise properties, linearity, and pixel response non-uniformity (PRNU). Ideally, the iWLS can be used to transfer the radiometric calibration of the instrument from ground to orbit and monitor it for the entire mission duration. We identify a list of alternatives for the on-board WLS. For a selection of alternatives, we perform a trade-off based – where available – on performance. The trade-off addresses technology readiness level (TRL) for use on ground and space, market availability for use on ground and space, radiometric stability, radiometric level, spectral coverage, power usage, heat dissipation, mass, size and volume. In addition, the applicability of the sources to the required use cases is addressed in the trade-off: PRNU, pixel map, relative radiance, absolute radiance, non-linearity, on-ground usage, and potential new usages (extended functionality). Light sources selected for the final trade-off are: Sun irradiance, Earth radiance, Moon irradiance, QTH lamp (for comparison), single narrowband LED, single white light LED, multiple narrowband LEDs, multiple white LEDs, and white laser. The trade-off results in multiple narrowband LEDs as the optimal solution using scores of the aspects mentioned above and are presented in this paper. The QTH lamp and natural sources also rank high, confirming, as expected, the validity of the trade-off. A conceptual breadboard implementation of a multiple narrowband LED source is finally proposed. An analytical approach for performance estimation is also presented in support of the proposed breadboard concept and compared to the required classical QTH WLS performance for Sentinel 5.
