SAPHIR Instrument on MEGHA-TROPIQUES

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.

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 increase absorption by the atmosphere with frequency. The first water vapour line is too low to permit 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 3 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 clouds 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 drive 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, the intermediate frequency processor and manages the RF signal acquisition.

Saphir instrument

Antenna Unit

Every scan period, the antenna reflector will perform a complete rotation. 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, acquisition of cold sky temperature measurements will be done. During part of the scan period, acquisition of hot target temperature measurements will also be done. 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, analog power detection of the signal is performed for each channel. The IFP unit includes sampling and integration of radiometer data: The 6 video data flows will be sampled and integrated using analog to digital converters.

Latest Update 09/10/2006