数据说明
HY-1B Normalized Water-Leaving Radiance at 490nm (LW3)
上传时间:2019-05-22 14:21:56 浏览次数:作者与来源:admin
LW3 is the retrieved normalized water-leaving radiance at the 490 band of the COCTS sensor onboard the Chinese ocean color satellite HY-1B.
Unit: mW/(cm2*µm*sr)
Resolution:  1.6km
Duration: 2007-2016
Source: HY-1B/COCS L1B data from the Hangzhou ocean satellite data receiving ground station.
Version: 2018
Dataset Name: SIO_HY1B_COCTS_yyyy-MM-dd-HH-mm-ss_L3A_NODEF_1KM_LWN490_HE2018
Processing:
The operational atmospheric correction algorithm developed by He et al.(2018) was applied to the HY-1B/COCTS L1B data to retrieve the normalized water-leaving radiance at different bands, as shown Fig. 1. Specifically, the algorithm for the is similar to that of the SeaWiFS and MODIS in the SeaDAS, which uses two near-infrared wavelengths to estimate aerosol scattering radiances and extrapolate these results to the visible bands. The total radiance received by COCTS can be expressed byLt(λ)=Lpath(λ)+tv(λ)Lwc(λ)+tv(λ)Lw(λ)                (1)
where Lpath is the atmosphere path radiance due to the Rayleigh scattering and aerosol scattering; Lwc is whitecap reflectance radiance; Lw is the desired water-leaving radiance, and tv is the upward atmosphere diffuse transmittance from the sea surface to the satellite.Note that the sun glint tem is ignored in Eq. (1).
The whitecap radiance can be estimated using sea surface wind speed as follows,


tv(λ)Lwc(λ)=[c(λ)×Rwc(λ)×F0(λ)×cosθ0×tv(λ)×ts(λ)]/π
Rwc(λ)=0.00000026×W3.52                                                             (2)
where c is the relative whitecap absorption coefficient; F0 is solar irradiance at the top of the atmosphere; θ0 is the solar zenith angle; tis the downward atmosphere diffuse transmittance from the Sun to the sea surface.
Using Eqs. (1) and (2), we can obtain the whitecap corrected total radiance. Moreover, we need to correct the ozone absorption (and any other atmosphere molecular absorption) as follows,
Lt(λ)=[Lt(λ)-tv(λ)Lwc(λ)]×exp{τOZ(λ)×[1/cosθ0+1/cosθv]}                 (3)
Assuming the water-leaving radiances at the two near-infrared bands (λnir1=750 nm and λnir2=865 nm) can be neglected, the ratio between Lpath and Rayleigh scattering radiance Lr can be derived as follows,


R(λnir1)= Lpathnir1)/Lrnir1)= Ltnir1)/Lrnir1)
R(λnir2)= Lpathnir2)/Lrnir2)= Ltnir2)/Lrnir2)                                 (4)
Lr can be calculated from the pre-generated Rayleigh scattering look-up tables. Based on R(λnir2), we can calculate the aerosol optical thickness at λnir2 for each aerosol model as follows,
Τanir2)×2c(λnir2)=-b(λnir2)+√{[b(λnir2)]2-4c(λnir2)[a(λnir2)-R(λnir2)]}            (5)
where the quadratic polynomial expanding coefficients a , b and c for each aerosol model can be derived from the pre-generated look-up tables for theratio. The atmosphere path radiance can be estimated by the aerosol optical thickness as follows,
R(λ)= Lpath(λ)/Lr(λ)=a(λ)+b(λ)τa(λ)+c(λ)τa(λ)2                   (6)
For each aerosol model, the aerosol optical thickness at λnir1 can be calculated as
Τanir1)= Τanir2)×cextnir1)/ cextnir2)                          (7)
where cext is the extinct attenuation coefficient on the aerosol model. Then, for each aerosol model, the ratio Rmodelnir1) between Lpath and Rayleigh scattering radiance Lr at λnir1 can be calculated from Τanir1) using Eq. (6).
Two aerosol models (mod1 and mod2) with Rmodelnir1) values closest to R(λnir1) can then be selected, and the corresponding weight can be determined using
w=[Rmod2nir1)- R(λnir1)]/[ Rmod2nir1)- Rmod1nir1)]                                (8)
Then, the aerosol optical thicknesses at the visible light bands for the two selected aerosol models (Τamod1(λ) and Τamod2(λ)) can be calculated using Eq. (7). Also, Rmod1(λ) and Rmod2(λ) can be calculated from Τamod1(λ) and Τamod2(λ), respectively based on Eq. (6). The final aerosol optical thickness Τa(λ) and R(λ) can be estimated as follows,


τa(λ)=w×τamod1(λ)+(1-w)×τamod2(λ)
R(λ)= w×Rmod1(λ)+(1-w)×Rmod2(λ)                             (9)
According to Eq. (6), Lpath(λ) can be calculated based on τa(λ) and Lr(λ).
Then, the only unknown in Eq. (1) is the atmosphere diffuse transmittance tv. For the two selected aerosol models, tvmod1(λ)  and tvmod2(λ) can be estimated from the pre-generated look-up tables. Same as Eq. (9), the final tv(λ) can be derived using the interpolation between tvmod1(λ)  and tvmod2(λ).
Finally, the normalized water-leaving radiance can be derived using
Lwn(λ)= [Lt(λ)- Lpath(λ)]/[tv(λ)ts(λ)cosθs](r/D)2                            (10)
where r and D are instantaneous and mean distances between Sun and Earth, respectively.
The key of the above algorithm is the generation of the look-up tables for the Rayleigh scattering, the ratio between path radiance and Rayleigh scattering radiance, and the atmosphere diffuse transmittance.Using the developed vector radiative transfer model PCOART and the band equivalent Rayleigh scattering optical thickness, the exact look-up tables for the Rayleigh scattering were generated for each band with the relative difference less than 0.25% compared with the results from the Rayleigh-scattering look-up tables in the SeaDAS. For the generation of the look-up tables of the R ratio and the diffuse transmittance, 20 aerosol models were established, including the oceanic aerosol with 98% relative humidity; marine aerosol with relative humidity of 98%, 90%, 80%, 70%, and 50%; coastal aerosol with relative humidity of 98%, 90%, 80%, 70%, and 50%; troposphere background aerosol with relative humidity of 98%, 90%, 80%, 70%, and 50%; and urban aerosol with relative humidity of 98%, 90%, 70%, and 50% (He et al., 2008).
The validation results using the PCOART simulated radiances at the top of the atmosphere showed that the developed atmospheric correction algorithm was accurate, with the retrieval error less than 0.0005 for the water-leaving reflectance (ΠLw/F0cosθ0), which met the requirement of the accurately atmospheric correction of ocean colour remote sensing (<0.001). Based on the retrieved normalized.
Known Issues:
Due to no orbit calibration instrument onboard the HY-1B satellite, there may have calibration issue on the HY-1B/COTS L1B data, which will propagate to the derived water-leaving radiance and further ocean color components.
Reference
Xianqiang He, Yan Bai, Delu Pan, Qiankun Zhu. The atmospheric correction HY-1B/COCTS, Proc. SPIE, 2008,7147:714717. DOI: 10.1117/12.813244.
Yan Bai, Lianghong Jiang, Xianqiang He, Vittorio Barale. An introduction to optical remote sensing of the Asian Seas: Chinese dedicated satellite s and data processing techniques. Remote Sensing of the Asian Seas, Edited by V. Barale and M. Gade, Springer, 2019,61-79. DOI:10.1007/978-3-319-94067-0_3
Acknowledgment
Please cite the above two reference if this dataset is used. Moreover, the satellite ground station and the satellite data processing & sharing center of the SOED/SIO should be acknowledged.