SAPHIR
Sounder for Probing Vertical Profiles of HumiditySAPHIR is a scientific instrument on-board the French-Indian satellite Megha-Tropiques mainly dedicated to the probing of vertical profiles of humidity in the intertropical belt. It was designed and developed by CNES.
The SAPHIR instrument is a 6 channel passive microwave humidity sounder. Its channels are located close to the 183.31 GHz water vapour absorption line. Its spatial resolution is 10 km at nadir and its swath width is 1700 km.
The following table gives the characteristics of the different channels:
| Channel | Center frequency (GHz) | Bandwith (MHz) | Polarization | Radiometric accuracy (NeDT) | Interchannel calibration |
| S1 | 183.31±0.20 | 200 | H | 2.35 K | 0.5 K |
| S2 | 183.31±1.10 | 350 | H | 1.45 K | 0.5 K |
| S3 | 183.31±2.80 | 500 | H | 1.36 K | 0.5 K |
| S4 | 183.31±4.20 | 700 | H | 1.38 K | 0.5 K |
| S5 | 183.31±6.80 | 1200 | H | 1.03 K | 0.5 K |
| S6 | 183.31±11.0 | 2000 | H | 1.10 K | 0.5 K |
The atmospheric opacity spectrum shows a water vapour absorption line centred at 183.31 GHz. This line is high enough to enable sounding the atmosphere between 2 and 12 km. 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. The selection of channels has been performed by simulations in order to optimize the humidity profile inversion.
Acquisition principle:
The instrument performs a continuous cross-track scanning while the satellite is moving along-track. Every scan period, the antenna reflector performs a complete rotation. Part of the period is devoted to the collection of Earth atmosphere temperature data. During the scan period, acquisition of temperature measurements of cold sky and of a hot target are done. The horn focuses the radiation collected by the antenna reflector. The signal is then separated into the 6 channels.
Products:
The AERIS ICARE Data and Service Center archives and distributes SAPHIR Level 1 and Level 2 products.
The Level 1 are processed by ISRO while the Level 2 scientific processing is performed by ICARE.
Level 1 products are available segment wise or orbit wise.
Level-2 are ‘water vapour’ products, obtained for clear and cloudy but non-precipitating scenes, over land and ocean. They include the relative humidity profiles and the upper tropospheric humidity.
SAPHIR products available at ICARE:
- Level 1A – Brightness temperature in the scan geometry
- Level 1A2 – Brightness temperature resampled in a fix grid
- Level 2 – ‘Water vapour’ product at 10-km resolution
- Level 2B – ‘Water vapour’ product at 1°
The products available in the catalog start from November 3, 2011.
At the end of 2018, the satellite suffered a degradation of the data management subsystem. Data availability is significantly reduced for the last quarter of 2018 and no product is available between December 2018 and April 2019. From April 2019, to overcome the problems of thermal sensitivity of the memory, it was decided to switch off data management (cycle 90 min ON/300 min OFF) reducing data availability to less than 25%.
SAPHIR level 1 data are also available in NRT via EUMETCast and assimilated for weather forecasting by several Meteorological Agencies.
The data are available in AERIS until the 14 of March 2022 (end of Meghatropiques mission).
Data access:
https://www.icare.univ-lille.fr/data-access/distribution-catalogue/
Contact:
Point of contact
Rémy Roca (Laboratoire d’Etudes en Géophysique et Océanographie Spatiales) – Project scientist
remy.roca@legos.obs-mip.fr
Michel Dejus – CNES project manager
michel.dejus@cnes.fr