Three instruments compose the core payload of the mission: a microwave imager, a microwave water vapor sounder, a radiative budget radiometer. Preliminary studies have defined the main characteristics of these instruments. Their overall spectral characteristics are given below:
MADRAS (Microwave Analysis and Detection of Rain and Atmospheric Systems) is a microwave imager, with conical scanning (incidence angle 56°), close from the SSM/I and TMI concepts. The main aim of the mission being the study of cloud systems, a frequency has been added (157 Ghz) in order to study the high level ice clouds associated with the convective systems, and to serve as a window channel relative to the sounding instrument at 183 GHz.
Table 1 : main characteristics of the MADRAS channels
The main uses given here are only indicative, most of the products being extracted from algorithms combining the different channels information. The resolutions are those expected in the different channels, accounting for the specification of 10 km given for the 89 Ghz channel.
SAPHIR (Sondeur Atmosphérique du Profil d'Humidité Intertropicale par Radiométrie) is a sounding instrument with 6 channels near the absorption band of water vapor at 183 Ghz. These channels provide relatively narrow weighting functions from the surface to about 10 km, allowing retrieving water vapor profiles in the cloud free troposphere. The scanning is cross-track, up to an incidence angle of 50°. The resolution at nadir is of 10 km.
Figure 1 : The 6 channels of SAPHIR positioned versus the water vapour absorption line at 183,31 Ghz.
ScaRaB (Scanner for Radiation Budget) is an optical scanning radiometer devoted to the measurements of radiative fluxes at the top-of-atmosphere (TOA) in the shortwave and longwave domain.
The optical radiometer is composed of 4 parallel and independent telescopes focusing the reflected solar and emitted thermal radiation of the earth atmosphere on 4 detection channels with high absolute accuracy (1%-2%) owing to onboard calibration modules.
Channel 2 and channel 3 are considered as the main channels, channel 2 providing directly the solar energy reflected by the earth-atmosphere, channel 3 measuring the total energy (solar and thermal). During night-time, the longwave radiance is directly given by the channel 3. During daylight, however, the longwave radiance is given by a difference between the channel 3 and the channel 2 radiance measurements. Since the ScaRaB 2 and 3 channels have very similar spectral response in the shortwave spectral domain, no additional spectral correction is necessary to determine the longwave radiance from such a difference. However, as for the previous ERBE (Earth Radiation Budget Experiments) an excellent cross-calibration between the shortwave (channel 2) and the total (channel 3) channel is required.
Channel 1 and channel 4 are narrow band channels used for scene identification in the visible (channel 1) and in the Infrared (channel 4) domains.
The main channels characteristics are listed in the table hereafter:
At the altitude of 866 km, the instantaneous ScaRaB field of view (FOV) of 48 mili-radians corresponds to a geographical footprint of approximately 40 km squared at nadir to 200 km on the edge.
To ensure a swath of about 2200km, a cross track scanning of the instrument is performed. The cross track scanning is obtained by the rotation of the telescopes and associated detectors in the nadir plane, which is perpendicular to the satellite speed vector. The procedures of calibration and processing of the data in order to derive fluxes from the original radiances have been set up and tested by CNES and LMD.
|Last Updated ( Tuesday, 13 September 2011 )|