The Megha-Tropiques satellite has been lauched into an orbit that has a very low inclination of 20 degrees. It is not strictly an equatorial orbit but quite nearly so. The height of the satellite is 866 km and the swath or the north-south extent of the coverage is 1700-2200 km wide. The period of revolution is 102 minutes. The repetivity is 6 times a day over a large part of the tropical belt and 4-5 times a day at higher latitudes. The satellite has four payloads that would help estimate oceanic winds, rainfall, temperature and humidity profiles, total water vapour, cloud liquid water, cloud ice and several radiation budget parameters, all from a common platform.
Since 1982, the Indian Space Research Organisation has launched a series of geostationary satellites that have provided a continuous meteorological coverage of the Indian region and the surrounding land and Indian Ocean regions. Four satellites in the INSAT-1 series, three in the INSAT-2 series, the dedicated Kalpana-1 satellite, and the current INSAT-3A satellite, have carried a total of nine VHRR instruments so far, besides the CCD cameras on the more recent satellites. The next satellite, INSAT-3D, to be launched soon, will have an advanced 6-channel imager and a 19-channel sounder. However, these satellites have primarily been designed towards meeting the requirements of operational meteorology in India.
Megha-Tropiques is the first satellite of the Indian space programme that makes a welcome departure from this philosophy. It is going to make systematic observations of parameters related to climate studies and its sensor configuration is entirely different from the INSAT payloads. Geostationary satellites, because they have to be parked at a height of 36,000 km above the earth’s surface, are not suited for microwave remote sensing as the radiance reaching them is very weak. Megha-Tropiques being in a low altitude orbit is designed to make microwave measurements and it carries two microwave payloads.
Megha-Tropiques is a joint India-France (ISRO-CNES) mission with a shared responsibility for development of payloads as well as launch. The spacecraft was launched by ISRO with its Polar Satellite Launch Vehicle (Flight PSLV-C18) from Shriharikota on 12 October 2011. The expected mission life is 3 years.
The main scientific objectives of the Megha-Tropiques mission are:
(a) To collect a long-term set of measurements with a good sampling and coverage over tropical latitudes to understand better the processes related to tropical convective systems and their life cycle.
(b) To improve the determination of atmospheric energy and water budget in the tropical region on various time and space scales.
(c) To study tropical weather and climate events like monsoon variability, droughts, floods, and tropical cyclones, and their predictabilty.
Megha-Tropiques carries a rare combination of three state-of-art payloads, MADRAS, SAPHIR and ScaRaB designed for measurements of radiative fluxes, precipitation, humidity profiles and cloud properties.
MADRAS (Microwave Analysis and Detection of Rain and Atmospheric Structures), is a passive imaging radiometer operating at five frequencies of 18.7, 23.8, 36.5, 89 and 157 GHz in both H and V polarizations except the 23.8 GHz which has only V polarization. Data from the first three channels has applications in the retrieval of rain over oceanic regions, liquid water content in clouds and vertical integrated water vapour. Their spatial resolution is 40 km. The 89 GHz channel is useful for retrieving convective rainfall over both land and ocean at a resolution of less than 10 km. The 157 GHz channel is meant for measuring the concentration of ice particles in clouds at a resolution as high as 6 km.
SAPHIR (Sounder for Atmospheric Profiling of Humidity in the Inter-tropics by Radiometry) is a microwave sounding instrument. It has six channels in the frequency region of 183 GHz, all having 10 km ground resolution. SAPHIR data will be used to retrieve atmospheric humidity profiles at six levels up to a height of 12 km. After INSAT-3D is launched, SAPHIR soundings will complement the temperature and humidity profiles that will be derived from the INSAT-3D sounder.
ScaRaB (Scanner for Radiation Budget Measurement) is the third Megha-Tropiques payload. It has four channels: Sc1 – Visible (0.5-0.7 µ), Sc2 – Solar (0.2-4.0 µ), Sc3 – Total (0.2-100 µ), and Sc4 – IR Window (10.5-12.5 µ). Sc2 and Sc3 are the main channels of the ScaRaB instrument. ScaRaB will measure fluxes at the top of the atmosphere with a ground resolution of 40 km. Longwave irradiance can be calculated from the difference between Sc3 and Sc2 measurements. Images from Sc1 and Sc4 channels will be used for scene identification and will provide the necessary compatibility with operational satellites like INSAT which have radiometers with similar spectral channels.
The MADRAS payload has been developed jointly by ISRO and CNES France, while SAPHIR and ScaRaB have been developed by CNES. ROSA (GPS Radio Occultation Sensor) is another payload that Megha-Tropiques is flying. This has been procured by ISRO from Italy. ROSA operates at L1 and L2 frequencies of 1575.42 and 1227.60 MHz and will be used for retrieving temperature and humidity profiles in the atmosphere by the GPS occultation method.
India launched the Oceansat-2 satellite on 23 September 2009, which has an on-board Ku-band scatterometer for measurement of surface winds and is already providing useful data on the global oceans. The launch of INSAT-3D with its advanced imager and sounder, is now eagerly awaited. A synergistic utilization of the data gathered from all these satellites is certainly going to lead to a great advancement of the current knowledge of the role of the tropical atmosphere and oceans in the global weather and climate.
The progress of the Megha-Tropiques mission can be seen on the web sites http://meghatropiques.ipsl.polytechnique.fr/ and http://www.isro.org/