Geostationary Environment Monitoring Spectrometer (GEMS) polarization characteristics and correction algorithm
Abstract. The Geostationary Environment Monitoring Spectrometer (GEMS) is the first geostationary earth orbit (GEO) environmental instrument, onboard the Geostationary Korea Multi-Purpose Satellite–2B (GEO-KOMPSAT-2B) launched on 19 February 2020, and is measuring reflected radiance from the Earth’s surface and atmosphere system in the range of 300 to 500 nm in the ultraviolet-visible (UV-Vis) region. The radiometric response of a satellite sensor that measures the UV-Vis wavelength region can depend on the polarization states of the incoming light. To reduce the sensitivity due to polarization, many current low earth orbit (LEO) satellites are equipped with a scrambler to depolarize the signals or a polarization measurement device (PMD) that simultaneously measures the polarization state of the atmosphere, then utilizes it for a polarization correction. However, a novel polarization correction algorithm is required since GEMS does not have a scrambler or a PMD. Therefore, this study aims to improve the radiometric accuracy of GEMS by developing a polarization correction algorithm optimized for GEMS that simultaneously considers the atmosphere's polarization state and the instrument's polarization sensitivity characteristics. The polarization factor and angle were derived by the preflight test on the ground as a function of wavelengths, showing a polarization sensitivity of more than 2 % at some specific wavelengths. The polarization states of the atmosphere are configured as a look-up table (LUT) using the Vector Linearized Discrete Ordinate Radiative-Transfer model (VLIDORT). Depending on the observation geometry and atmospheric conditions, the observed radiance spectrum can be included with a polarization error of up to 2 %. The performance of the proposed GEMS polarization algorithm was assessed using synthetic data, and the errors due to polarization were found to be larger in clear regions than in cloudy regions. After the polarization correction, polarization errors were reduced close to zero for almost all wavelengths, including a high peak and curvature of polarization error, which sufficiently demonstrates the effectiveness of the proposed polarization correction algorithm. From the actual observation data after the launch of GEMS, the diurnal variation for the spatial distribution of polarization error was confirmed to be minimum at noon and maximum at sunrise/sunset. This can be used to improve the quality of GEMS measurements, the first geostationary environmental satellite, and then contribute to the retrieved accuracy of various Level 2 products (hereafter, L2), such as trace gases and aerosols in the atmosphere.
Haklim Choi et al.
Status: open (until 29 Jun 2023)
- RC1: 'Comment on amt-2023-92', Anonymous Referee #1, 30 May 2023 reply
Haklim Choi et al.
Haklim Choi et al.
Viewed (geographical distribution)
The paper is very well written and organized. The results are presented clearly with a good choice of figures. The laboratory measurements are used to create an in-orbit correction for the polarization throughput sensitivity which can be larger than 2% for some viewing geometries. VLIDORT is used to create the Degree of Linearized Polarization for a correction LUT. The results with real measurement show a significant improvement over the uncorrected cases. The authors also identify areas for future improvements and deficiencies in the ground-based characterization and its implementation.
Some specific minor editorial corrections and questions:
Line 45-46 Replace
“Before reaching ... passes through …”
“Upon reaching … interacts with …”
Line 91 This needs to be rewritten. Maybe
the Stokes parameters (I, Q, and U) for various atmospheric conditions were calculated and the DoLPs are arranged in a LUT.
Line 126 The acronym SMA need to be expanded.
Is it Scan Mechanism Assembly? Scan Mirror Angle?
Also, sometimes it is used as “SMA Angle: and other times as “SMA Position” or “angle at which the SMA is located”.
Line 128 Linear Polarization Sensitivity or Polarization Factor?
The term LPS is introduced in Line 89 but is never used. The term PF is used often.
Line 296. Says that the polarization was only characterized for one North/South position at the center. Figure 1. Is the test setup used to get measurements over the in-orbit range of scan mirror viewing angles? Table 2 has 1 for SMA position. What about the East / West characterization? Was the assembly moved to vary the mirror scan angles?
Line 238 “The polarization error caused by changes in total ozone is less than those caused by other changes.”
Figure 4. Could the authors comment on why Figure 4 does not have any (or maybe very small) dependence on TOZ? I would expect that the ozone would selectively shield the shorter channels with higher ozone absorption from the surface and clouds and thus produce wavelength-dependent changes similar in magnitude to the albedo and surface pressure changes as ozone amounts increase. That is, alter the relative amounts of single scattered, multiple scattered and reflected radiances.
Figure 4 and Figure 8. Figure 4 shows errors versus SZA and SVA of 1% or more. Figure 8 does not show corrections larger than 0.1%. (Are the units in Figure 4, 8, 11 and 12 all in % error in radiances?) Was the range of cases used to construct Figure 8 much less varied than the real cases in Figures 11 and 12? Particularly in SZAs?