The STAR-CC-OGSE System for Pre-Flight Sensor Calibration

Reliable characterisation and radiometric calibration of satellite sensors are critical to their optimal performance on-orbit. Only through a robust understanding of the instrument behaviour, performance and degradation mechanisms will the significant effort and expense invested into the flight hardware be fully exploited. 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 characterisation and radiometric calibration of sensors and the facilities needed to undertake these activities.

Sensor pre-flight characterisation and calibration facilities, or optical ground support equipment (OGSE) test sensor performance over a few broad categories including: geometric performance/image quality, channel/band co-registration, spectral calibration/out-of-band rejection, radiometric calibration, polarisation sensitivity, non-linearity, non-uniformity response etc. The specific requirements of the sensor, determined by its footprint, FoV, spectral extent & resolution, nominal radiance and required sensitivity typically results in a bespoke OGSE needed to meet the specific sensor requirements. For large-scale multi-sensor series programmes, a bespoke solution may remain the preferred solution. However, for single/few unit explorer missions, commercial constellations and more agile sensor development programmes, the expense & post-use redundancy of a bespoke OGSE system may be prohibitive.

NPL has developed a universal OGSE facility, the Spectroscopically Tuneable Absolute Radiometric calibration & characterisation OGSE (STAR-CC-OGSE), a versatile facility for the radiometric calibration and characterisation of satellite sensors. The system is provided fully characterised, calibrated and performance verified, with an easy to use software interface that allows fully automated remote operation. The system can be installed at a customer cleanroom facility or operated at NPL with a customer-supplied sensor. The main components of the STAR-CC-OGSE system are:

• A large aperture integrating sphere source for radiometric calibration
• A collimated beam source, equipped with an interchangeable, position fine-tuneable feature field mask for optical performance characterisation
• A CW laser allowing monochromatic continuous tuneability from 270 nm to 2700 nm, with a broadband (white light) source extending over the same spectral extent.
• A vacuum-compatible SI-traceable radiance detector module containing both broadband photodiodes & a spectrometer, installable in TVAC at the sensor-under-test entrance aperture

The laser illumination interface to the large aperture radiance sphere, collimator beam source or direct to the feature field mask allows fully tuneable monochromatic illumination for all characterisation and calibration modes.

STAR-CC-OGSE is undergoing final performance testing, with delivery to an initial lease customer in Spring 2020. This paper will describe the STAR-CC-OGSE system, the outcome of the verification testing and system performance.