for retrieving atmospheric water vapor amount and cloud top height, respectively.
Measurement channels: 15 channels (three channels for each polarized band) Swath width = 1440 km x 2200 (across-track) km, ground spatial resolution of 7 km x 6 km at nadir. Data rate: 0.882 Mbit/s, 12 bit quantization. Operation requirements: global observation of the Earth with more than 15º of solar elevation, simultaneous operation with OCTS.
Figure 16: Observation concept of POLDER (image credit: CNES)
POLDER is a 2-D detector array wide-field-of-view, multi-band imaging radiometer/polarimeter. Multi-angle viewing is achieved by the along-track migration of the S/C of a quasi-square footprint. The optical detection unit of the POLDER consists of a telecentric lens, a rotating wheel supporting filters and polarizers, and a matrix CCD sensor (242 x 548 photoelements, the pixels are binned one by two, resulting in 242 x 274 sensitive areas).
The instrument has a focal length of 3.57 mm, f/4.6, FOV=±43º along-track, FOV=±51º across-track, FOV=±57º diagonal. Instrument mass= 33 kg, power=42 W.
Table 5: Spectral characteristics of POLDER
Figure 17: Optical design of the POLDER instrument (image credit: CNES)
Legend to Figure 18: The image is a blue, green and red color composite of POLDER measurements at 443 nm, 670 nm and 865 nm. Marked differences appear between the clouds (in white) and different types of surfaces: vegetation in red, soil in brown and yellow, sea in dark blue. The bright spot in the Mediterranean sea, West of Sardinia, is the sunglint pattern. More than 15000 such images are taken every day by POLDER on ADEOS providing a daily global coverage of the Earth in eight spectral channels of the visible and near infrared spectrum.
Figure 19: Illustration of POLDER Ocean Color products: from daily (Level 2) to 10-day and monthly syntheses (Level 3) over the Mediterranean basin (image credit: LOA/ LPCM/ LSCE/ CNES/JAXA)
IMG (Interferometric Monitor for Greenhouse Gases):
IMG is a MITI-funded sensor developed by JAROS (Japan Resources Observation System Organization), this is a nadir-looking Fourier transform infrared spectrometer (FTIR). Objectives: Observation of detailed spectra of IR radiation of the Earth's surface and the atmosphere, mapping greenhouse gases on a global scale (CO2, CH4, N2O, H2O and the total ozone mixing ratios of the troposphere).
MITI funded the instrument development. It selected a company developing the sensor, TOSHIBA. JAROS had supervised the sensor development. Subsequently, MITI selected the Central Research Institute of Electric Power Industry (CRIEPI) as a consignor for the IMG software system development. 38) 39)
Continuous measurements of the infrared spectrum in the range from 3.3 - 14.0 µm with a very fine spectral resolution of 0.1 cm-1 (apodized) absolute accuracy of measurement=<1 K, stability of measurement<0.1K, IFOV=10 mrad (~64 km2 footprint); vertical resolution about 2-6 km depending on species; interferogram scan time ≤ 10 s. Operation requirements: full time operation for 3 days out of 13 days. A mechanical cryogenic coolant system is used to regulate the temperature of the quantum detectors. An image motion compensation mirror is used to compensate for the satellite orbital motion. Measurements are made in 20 km swaths at 8 km resolution.
Table 6: Some characteristics of the IMG instrument
Figure 20: The observation concept of IMG (image credit: NASDA)
IMG is a Michelson-type Fourier Transform Spectrometer (FTS) with two mirrors and a beam splitter. The incident radiation is divided by the beam splitter into two paths. One mirror is moved so that the two paths produce an interference pattern when recombined. The signal measured by the detector, the interferogram, can be Fourier-transformed to obtain the incident spectrum. The diameter of the entrance aperture is 10 cm. The moving mirror is suspended on magnetic bearings and scans a 10 cm long path in 10 seconds. 40) 41)
Figure 21 shows a schematic of the IMG optical system. The heart of the IMG is the magnetically suspended moving m