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In the past few years, the SPEXone instrument has been developed, tested, calibrated and delivered to the NASA PACE (Plankton Aerosol and Ocean Ecosystems) project by a Dutch consortium consisting of SRON and Airbus Defence and Space Netherlands with support from TNO. Onboard the PACE satellite, SPEXone will fly together with the Ocean Color Instrument (OCI) and the Hyper-Angular Rainbow Polarimeter 2 (HARP2). SPEXone is a compact multi-angle channeled spectropolarimeter with five viewing angles and hyperspectral imaging of a ⇠100 km swath with a spatial resolution of 4.6 ⇥ 5.4 km in the along-track and across-track directions and a spatial oversampling ratio of 2. SPEXone has a spectral range from 385 to 770 nm and a spectral resolution slightly below 2 nm. High accuracy polarimetry is achieved by implementation of the dual-beam snapshot spectral polarization modulation concept, yielding two complementary spectrally modulated hyperspectral images of the same scene from which both radiance and state of linear polarization can be extracted. This paper presents results from the characterization and on-ground calibration campaign of SPEXone and discusses the use of the L0-L1B processor to derive the calibration key data (CKD) that is required for the operational data processing. The L0-L1B data processor has been developed in such a way that the CKD derivation is fully consistent with the L0-L1B processing steps, meaning that the data from which the CKD at a certain processing step is derived has been processed using the CKD from all previous processing steps. The processing steps that have been implemented include those related to the detector characterization (dark o↵set and dark current, non-linearity, and pixel-response non-uniformity), and those related to the instrument calibration (stray light, field-of-view, line-of-sight, wavelength, radiometric and polarimetric). We will show examples of the derived CKD and their use in the processing of measurement data. We will demonstrate the performance of the polarimetric calibration by comparing SPEXone measurement results against a well characterized polarization state generator, showing di↵erences of the order of a few times 0.001 in the degree of linear polarization. In addition, we will discuss the approach for the implementation of a stray light correction algorithm that has the potential to correct di↵use stray light with a spectral intensity and possibly size variation of the kernel, and that can correct moving ghosts with a constant kernel shape.
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