The SAPHIR instrument is a multi-channel passive microwave humidity sounder. Atmospheric humidity profiles can be obtained by measuring brightness temperatures in different channels situated close to the 183.31 GHz water vapour absorption line.
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The atmospheric opacity spectrum (see below) shows a first water vapour absorption line centred at 22.235 GHz, and a second one at 183.31 GHz (pure rotation line). Between these two lines, the water vapour continuum slowly increases absorption by the atmosphere with frequency. The first water vapour line is too low for profiling, and its partial transparency is used to obtain the total columnar content. The second line is high enough to enable sounding in the first 10 - 12 km of the atmosphere. The sounding principle consists of selecting channels at different frequencies inside the absorption line, in order to obtain a maximal sensitivity to humidity at different heights. Previous microwave sounders are SSMT2 and AMSU-B, which are operational instruments, have three channels within the 183.31 GHz absorption line (at ±1, ±3 and ±7 GHz), and two window channels, at 150 and 89 GHz. These additional channels give information on the surface and near surface.
Atmospheric opacity for a US standard atmosphere
The SAPHIR sounder is based on the same general principle, measuring humidity in 6 different channels located in the 183.31 ± 12 GHz bandwidth.
Location of the 6 SAPHIR channels with respect to the centre of the absorption line
The selection of channels was performed by first building a learning data base, consisting of meteorological profiles (TIGR data base), and brightness temperatures simulated by running a radiative transfer model on the profiles, then a neural network inversion scheme was applied to retrieve the humidity profile.
Finally, vertical humidity profiles can be retrieved from brightness temperature measurements on the 6 channels, and ancillary data such as temperature profiles derived from meteorological model, water vapour total content derived from SAPHIR or MADRAS data, and cloud identification derived from the other sensors of the payload.
Example of weight functions of the 6 channels for a dry tropical atmosphere (integrated water vapour 20 Kg/m²), over sea, at nadir
The SAPHIR instrument is composed of two units:
- The electronic unit (EM): the electronic module manages the interfaces with the platform (power, telemetry, commands) and drives the RF unit for the science data acquisition.
- The radio-frequency unit (RFU): this module contains the scanning mirror protected by a shroud, the on-board calibration target, the front-end, and the the intermediate frequency processor, and manages the RF signal acquisition.
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The antenna reflector performs a complete rotation during every scan period. Part of the period will be devoted to the collection of Earth atmosphere temperature data. During the scan period, when the reflector is properly oriented, cold sky temperature measurements are acquired, as well as hot target temperature measurements during part of the scan period. The horn will focus the free space radiation collected by the antenna reflector.
Front End Unit
The millimeter Front End is composed of a local oscillator, a mixer and a low noise amplifier. The mixer associated with the local oscillator will perform the down conversion of the signal.
Intermediate Frequency Unit
The Intermediate Frequency Unit will de-multiplex the signal of the various bandwidths, perform amplification and filtering for each channel. After amplification, analogue power detection of the signal is performed for each channel. The IFP unit includes sampling and integration of radiometer data: the six-video data flows will be sampled and integrated using analogue to digital converters.