10-Bands Ocean Color and
Temperature Scanner Feng Qi, Zheng Qin-Bo, Gong
Hui-Xing Abstract Shanghai Institute of Technical Physics Chinese, Academy of Science 500 Yutian Road, Shanghai 200083, P.R. China Phone & fax: 86-21-65318330 China is developing ocean observation satellite named HY-1. This satellite is scheduled to be launched in 2001 with meteorological satellite FY-1. 10-bands ocean color and temperature scanner and 4 bands CCD imager will be mounted on HY-1. In this paper we introduce the 10-bands ocean color and temperature scanner which has 10 bands from visible to thermal infrared and the ground resolution is 1.1Km at the nadir (orbit altitude =786Km). Four elements detector array is used for every band in order to improve detecting sensitivity and a "K" mirror is used to eliminate image rotating on the focal plane caused by 45o scanning mirror. Introduction HY-1 will be launched with FY-1 which altitude is 870Km and incidence is 98.8°. Because the decending crossing time of FY-1 is about 8.35 a.m., so it is not good for ocean observation. Therefore, HY-1 will change its altitude from 870Km to 798Km in order to improve the illuminance of the earth surface. HY-1 will move round the earth 14.3 cycles one day. As the reflectance of ocean is very low, four elements detector array is used to eliminate image rotating on the focal plane caused by 45o scanning mirror. There are four scan lines along the flying direction at the same time while the scanning mirror rotating one cycle. Table 1~table 2 list some important parameters of the 10-bands ocean color and temperature scanner.
Optics System There is a 45o scanning mirror (90.4 rpm) and a reflectance telescope. After the telescope there is a "K" mirror, whose rotation velocity (45.2 rpm) is half of the scanning mirror (90.4rpm) and the rotation direction is the same as the scanning mirror, to eliminate the image rotation on the focal plane. The input. The input radiance was divided into three parts, thermal infrared is reflected by the first dichroic (D1) and is focal on a 2x4 elements HgCdTe detector which covered by its filter, forming band9 and band10. The HgCdTe detector and filter are colled down by sterling cooler. The radiance pass through D1 is further divided into two parts, one is reflected by the second dichroic (D2) forming the band1~5 while the other pass through D2 forming the band6~band8. The optics aperture is 200mm and focal length for band1~band8 is nearly 650mm and 202mm for band9~band10. There is a black body over the scanning mirror used for flying calibration correction of thermal channel, its temperature is measured by Pt resistance. Polarization Sensitivity Elimination Polarization sensitivity elimination is very important for ocean color observation. Because the radiation reflected by sea surface and scattered by atmosphere has strong polarization, especially at strong wavelength. Therefore to eliminate the polarization sensitivity of the instrument used for ocean color observation is very important, otherwise it will reduce measurement accuracy of the instrument. Therefore the 10-bands ocean color uses some methods to eliminate its polarization sensitivity. First, all reflectance surfaces of mirror use Ag film, because Ag film has high reflectance and low polarization from visible to infrared band. Second, we control the polarization of two dichroics. Third, we arrange the position of two dichoroics. Third, we arrange the position of scanning mirror, "K" mirror and dichroics reasonably to avoid the polarization add in the same direction. Fourth, we use a polarization scrambler to further eliminate the polarization. As a results, the polarization sensitivity of the scanner is less than 5%. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||