SPEXone is a compact five–angle spectropolarimeter that is being developed as a contributed payload for the NASA Plankton, Aerosol, Cloud and ocean Ecosystem (PACE) observatory, to be launched in 2022. SPEXone will provide accurate atmospheric aerosol characterization from space for climate research, as well as for light path correction in support of the main Ocean Color Instrument. SPEXone employs dual beam spectral polarization modulation, in which the state of linear polarization is encoded in a spectrum as a periodic variation of the intensity. This technique enables high polarimetric accuracies in operational environments, since it provides snapshot acquisition of both radiance and polarization without moving parts. This paper presents the polarimetric error analysis and budget for SPEXone in terms of polarimetric precision and polarimetric accuracy. We consider factors that contribute to instrumental polarization and modulation efficiency, which will be calibrated on-ground with high, but finite accuracy. The sensitivity to dynamic systematic effects in a space environment, such as degradation and ageing of components and small variations in the temperature and thermal gradients is addressed and quantified. Finally, the impact of scene dependent error sources, mainly resulting from stray light, are assessed and the total polarimetric error budget is presented. We show that SPEXone complies with the radiometric SNR requirement of 300, yielding a minimum polarimetric precision of 200 (fully polarized light) to 300 (unpolarized light) over the full spectral range for dark ocean scenes at high solar zenith angle. Assuming a stray light correction factor of 5 and considering a moderate contrast scene, the expected in-flight polarimetric accuracy of SPEXone is 1.5 · 10<sup>−3</sup> for unpolarized scenes and 2.9 · 10<sup>−3</sup> for highly polarized scenes, compliant with the polarimetric accuracy requirement. This performance should enable SPEXone to deliver the data quality that enables unprecedented aerosol characterization from space on the NASA PACE mission.
We have developed a 6 dm<sup>3</sup>-sized optical instrument to characterize the microphysical properties of fine particulate matter or aerosol in the Earth atmosphere from low Earth orbit. Our instrument can provide detailed and worldwide knowledge of aerosol amount, type and properties. This is important for climate and ecosystem science and human health [1, 2]. Therefore, NASA, ESA and the European Commission study the application of aerosol instruments for planned or future missions. We distinguish molecular Rayleigh scattering from aerosol Mie-type scattering by analyzing multi-angle observations of radiance and the polarization state of sun light that is scattered in the Earth atmosphere . We measure across the visible wavelength spectrum and in five distinct viewing angles between -50° and +50°. Such analysis has been traditionally done by rotating polarizers and band-filters in front of an Earth observing wide-angle imager. In contrast, we adopt a means to map the linear polarization state on the spectrum using passive optical components . Thereby we can characterize the full linear polarization state for a scene instantaneously. This improves the polarimetric accuracy, which is critical for aerosol characterization, enabling us to distinguish for example anthropogenic from natural aerosol types. Moreover, the absence of moving parts simplifies the instrument, and makes it more robust and reliable. We have demonstrated this method in an airborne instrument called SPEX airborne [5, 6] in the recent ACEPOL campaign together with a suite of state-of-the art and innovative active and passive aerosol sensors on the NASA ER-2 high-altitude research platform . An earlier report on the SPEX development roadmap was given in . In this contribution we introduce SPEXone, a compact space instrument that has a new telescope that projects the five viewing angles onto a single polarization modulation unit and the subsequent reflective spectrometer. The novel telescope allows the observation of five scenes with one spectrometer, hence the name. We describe the optical layout of the telescope, polarization modulation optics, and spectrometer and discuss the manufacturability and tolerances involved. We will also discuss the modelled instrument performance and show preliminary results from optical breadboards of the telescope and polarization modulation optics. With SPEXone we present a strong and new tool for climate research and air quality monitoring. It can be used to study the effect of atmospheric aerosol on the heating/cooling of the Earth and on air quality. Also, SPEXone can improve the accuracy of satellite measurements of greenhouse gas concentrations and ocean color that rely on molecular absorption of reflected sunlight by providing detailed knowledge of the aerosol properties, required to accurately trace the light path in presence of scattering.
SPEXone is developed in a partnership between SRON Netherlands Institute for Space Research and Airbus Defence and Space Netherlands with support from the Netherlands Organisation for Applied Scientific Research (TNO) as a Dutch contribution to the NASA PACE observatory launching in 2022.