The Megha-Tropiques Satellite Explained

Megha-Tropiques is a joint Indian-French satellite, built by the Indian Space Research Organisation (ISRO) and the French space agency CNES (Centre National d’Etudes Spatiales), that was launched in 2011 to study the water cycle and energy exchanges in the tropical atmosphere, especially over the Indian monsoon region. Its name combines “Megha”, the Sanskrit word for cloud, with “Tropiques”, the French word for the tropics.

The Megha-Tropiques satellite flew in a low-inclination orbit that let it observe the same tropical regions many times a day, something that most weather satellites in polar orbits cannot do. This article explains what Megha-Tropiques is, what its four instruments measured, how its orbit worked, what it contributed to monsoon and cyclone science, and how its mission ended with a controlled re-entry in 2023.

Key Takeaways

• Megha-Tropiques is a joint India-France satellite launched in 2011 to study the water cycle and energy exchanges of the tropical atmosphere.
• The Megha-Tropiques satellite carried four instruments: MADRAS, SAPHIR, ScaRaB and ROSA.
• Megha-Tropiques flew in an orbit inclined at about 20 degrees to the equator, which allowed it to pass over tropical regions several times each day.
• The satellite’s data helped scientists study the Indian monsoon, tropical convection, cloud systems and the structure of tropical cyclones.
• Megha-Tropiques was designed for three years of operation but continued to provide data for much longer than planned.
• ISRO brought Megha-Tropiques out of orbit in a controlled re-entry over the Pacific Ocean in March 2023, in line with international space-debris guidelines.

What Is Megha-Tropiques?

Megha-Tropiques is an Indo-French earth-observation satellite dedicated to studying how water and energy move through the atmosphere of the tropics. It was the result of long cooperation between ISRO and CNES, two space agencies that have worked together on rockets, satellites and science missions for decades.

The mission was conceived to fill a gap. The tropical belt receives most of the Sun’s energy, drives the world’s largest rainfall systems and shapes weather far beyond its borders. Yet before Megha-Tropiques, tropical clouds, humidity and radiation were often observed only a few times a day by satellites in polar orbits. Scientists wanted a mission that returned to the same tropical locations repeatedly to follow the life cycle of cloud systems.

Megha-Tropiques was built on an Indian satellite platform, with several instruments supplied by France and one by Italy. ISRO handled the satellite bus, launch and operations, while CNES led the development of key instruments and supported data processing.

Why Was Megha-Tropiques Built?

Megha-Tropiques was built to improve understanding of the tropical water cycle, in particular the processes that produce monsoon rainfall and tropical cyclones. A better grasp of these processes helps forecasters and climate scientists predict rainfall and storms more accurately.

The central scientific questions of the mission included:

• How does water vapour in the tropical atmosphere condense into clouds and rain?
• How much energy is absorbed, reflected and radiated back to space by tropical clouds and the surface?
• How do convective cloud systems form, grow and decay over the tropical oceans and land?
• What are the typical amounts of atmospheric moisture before and during monsoon rainfall and cyclone development?

For India, which depends heavily on the southwest monsoon for agriculture, water supply and the economy, these questions have practical importance. Better data on monsoon processes helps weather prediction models used by the India Meteorological Department (IMD) and by research institutions.

What Instruments Does Megha-Tropiques Carry?

Megha-Tropiques carries four scientific instruments: MADRAS, SAPHIR, ScaRaB and ROSA, each measuring a different aspect of the tropical atmosphere. Together they were intended to measure rainfall, humidity, radiation and temperature profiles from a single platform.

MADRAS

MADRAS stands for Microwave Analysis and Detection of Rain and Atmospheric Structures. It was a passive microwave imager that measured radiation at several frequencies to estimate rainfall, cloud water content and ice in clouds. Its design combined work from both agencies. MADRAS stopped operating relatively early in the mission, after a problem with its scanning mechanism, so most of the long-term data came from the other instruments.

SAPHIR

SAPHIR is the French acronym for Sondeur Atmospherique du Profil d’Humidite Intertropicale par Radiometrie. It was a microwave humidity sounder that measured water vapour at several heights in the atmosphere by using channels near a strong water-vapour absorption frequency of 183 GHz. SAPHIR could observe humidity through many types of cloud, and it became the most widely used Megha-Tropiques instrument.

ScaRaB

ScaRaB stands for Scanner for Radiation Budget. It measured the sunlight reflected by Earth and the infrared energy emitted by Earth, which together define the planet’s radiation budget. In the tropics, clouds strongly influence this balance, so the data was used to study how cloud systems warm or cool the climate.

ROSA

ROSA means Radio Occultation Sensor for Atmosphere. Supplied through Italian collaboration, it received signals from navigation satellites such as GPS as they passed through the atmosphere, and used the bending of those signals to compute temperature and humidity profiles with good vertical detail.

What Orbit Did Megha-Tropiques Fly In?

Megha-Tropiques flew in a nearly circular orbit at an altitude of roughly 870 km, tilted at about 20 degrees to the equator. This low inclination meant that the satellite stayed within the tropical belt and never travelled to high latitudes.

The choice of orbit was the key design idea of the mission. A satellite in a polar sun-synchronous orbit passes over a given tropical location at about the same local time each day, usually only once or twice. A satellite in a low-inclination orbit drifts through different local times over days and weeks, so it samples the whole daily cycle of tropical clouds and rainfall. Over the tropics, Megha-Tropiques could visit the same area several times a day, depending on location.

Many tropical rain systems develop in the afternoon over land and at night or early morning over oceans. Frequent revisits at changing times of day help capture these cycles, which are hard to observe with a single morning or evening pass.

When and How Was Megha-Tropiques Launched?

Megha-Tropiques was launched in October 2011 by a PSLV rocket from the Satish Dhawan Space Centre at Sriharikota, India. The PSLV placed it directly into its intended low-inclination orbit, a more demanding launch than the polar missions for which the rocket is usually used.

The PSLV carried small passenger satellites along with Megha-Tropiques, including university and research nanosatellites. The launch was a visible example of Indo-French cooperation, which also includes later missions such as SARAL/AltiKa for ocean altimetry and joint work on the Indian ocean and climate observation.

How Did Megha-Tropiques Contribute to Monsoon and Cyclone Science?

Megha-Tropiques contributed to monsoon and cyclone science by supplying frequent measurements of tropical humidity, clouds and radiation that were used in research and in forecasting models. Its humidity profiles from SAPHIR in particular were used by weather centres to improve the starting conditions for numerical forecasts.

Key contributions include:

• Monsoon research: Scientists used the data to examine how moisture builds up over the Indian Ocean and the Indian subcontinent before and during monsoon rainfall, and how clouds and radiation vary within monsoon spells.
• Cyclone studies: Humidity and cloud data helped researchers examine the structure of tropical cyclones over the Bay of Bengal and Arabian Sea, including how moisture surrounds and fuels a developing storm.
• Weather forecasting: Several meteorological centres incorporated the satellite’s humidity data into their models, which can improve forecasts of rainfall and storm tracks.
• Climate research: Radiation budget measurements contributed to studies of how tropical cloud systems affect the planet’s energy balance.
• Rainfall estimation: Megha-Tropiques became part of the international network of satellites that estimate global precipitation, alongside missions such as the Global Precipitation Measurement mission.

Because the data was shared with the global scientific community, its value extended well beyond India and France.

Who Uses Megha-Tropiques Data?

Megha-Tropiques data is used by meteorological agencies, universities and climate research groups in India, France and many other countries. Within India, data was made available through ISRO’s Meteorological and Oceanographic Satellite Data Archival Centre (MOSDAC) at the Space Applications Centre in Ahmedabad. In France, the data was distributed through CNES-supported data centres.

Typical user groups include:

• National weather services, which use humidity information in forecast models.
• Research institutes studying the monsoon, convection and cyclone intensity.
• Climate scientists building long records of tropical humidity and radiation.
• Students and academic groups working on atmospheric science.

How Long Did Megha-Tropiques Operate?

Megha-Tropiques was planned for a three-year mission, but the satellite continued to provide data for well over a decade, with some instruments working much longer than others. This extended life gave scientists a longer time series than expected, which is valuable for studying year-to-year variations in the monsoon.

Over the years, individual instruments reached the end of their useful life. The early loss of the MADRAS imager was a setback for rainfall studies, while SAPHIR and some other instruments continued to deliver usable data for a considerable period. By the early 2020s, the satellite was approaching the end of its operational life, and ISRO began planning its safe disposal.

How Did the Megha-Tropiques Mission End?

The Megha-Tropiques mission ended in March 2023, when ISRO guided the satellite through a controlled re-entry into the Earth’s atmosphere over a remote region of the Pacific Ocean. The operation was planned so that any fragments surviving re-entry would fall into the sea, far from land and shipping routes.

Before re-entry, the satellite still held a significant amount of fuel. Leaving fuel on board an inactive satellite can pose an explosion risk, and an uncontrolled fall could endanger people. ISRO therefore carried out a series of manoeuvres using the satellite’s thrusters to lower its orbit step by step, finishing with final burns that brought it down on a planned path. The satellite was not designed for deorbiting, so the plan required careful analysis of its propulsion system and battery safety.

The exercise followed the Inter-Agency Space Debris Coordination Committee (IADC) guidelines, which recommend that satellites in low Earth orbit be removed from orbit within a limited time after the end of mission. ISRO has since highlighted such planned disposals as part of its commitment to space sustainability.

How Does Megha-Tropiques Reflect Indo-French Space Cooperation?

Megha-Tropiques reflects Indo-French space cooperation by combining Indian launch and satellite capabilities with French scientific instruments and expertise, within a partnership between ISRO and CNES that goes back several decades. The two agencies share an interest in climate and ocean science and have jointly developed other missions beyond Megha-Tropiques.

In this mission, the roles were divided clearly. ISRO built the spacecraft, arranged the launch from Sriharikota and ran the satellite from its control centre. CNES led the development of important scientific instruments and supported data processing and scientific validation, while Italian partners contributed the radio occultation receiver. Scientists from both countries analysed the data and published results together.

This kind of partnership allows each agency to spread cost and risk while giving its scientists access to a larger dataset than either could build alone.

What Is the Legacy of Megha-Tropiques?

The legacy of Megha-Tropiques is a long and openly shared dataset on tropical humidity and radiation, together with a model of Indo-French scientific cooperation and responsible end-of-life satellite management. The mission showed that a small satellite in a well-chosen orbit can answer questions that larger polar-orbiting missions cannot.

Its observations remain useful to researchers who study the monsoon and the tropical climate, and its data continues to appear in scientific papers and reanalysis studies. The controlled re-entry also provided ISRO with experience in deorbiting a satellite not originally designed for it, which is useful for future missions.

Bottom Line

Megha-Tropiques was a joint ISRO-CNES satellite launched in 2011 to study the tropical water cycle, monsoon and cyclones from a low-inclination orbit using the MADRAS, SAPHIR, ScaRaB and ROSA instruments. It outlived its planned three-year life and ended with a carefully controlled re-entry over the Pacific Ocean in 2023, leaving behind valuable data and a template for responsible satellite disposal.

Frequently Asked Questions

What is Megha-Tropiques?

Megha-Tropiques is a joint Indian-French satellite built by ISRO and the French space agency CNES to study the water cycle and energy exchanges of the tropical atmosphere. It was launched in 2011 and was particularly aimed at improving understanding of the Indian monsoon and tropical cyclones.

What does the name Megha-Tropiques mean?

The name combines ‘Megha’, the Sanskrit word for cloud, with ‘Tropiques’, the French word for the tropics. Together the name reflects the mission’s focus on clouds and moisture in the tropical belt and its joint Indian and French origin.

What instruments are on the Megha-Tropiques satellite?

The Megha-Tropiques satellite carries four instruments: MADRAS, SAPHIR, ScaRaB and ROSA. MADRAS is a microwave imager for rain and clouds, SAPHIR is a humidity sounder, ScaRaB measures the radiation budget, and ROSA is a GPS radio occultation receiver that gives temperature and humidity profiles.

Why does Megha-Tropiques use a low-inclination orbit?

Megha-Tropiques uses an orbit inclined at about 20 degrees to the equator so that it stays over the tropics and revisits the same places several times a day. This repeated sampling at changing times of day helps scientists follow the daily and life cycles of tropical clouds and rain systems.

How did the Megha-Tropiques mission end?

The Megha-Tropiques mission ended in March 2023 with a controlled re-entry over a remote part of the Pacific Ocean. ISRO lowered the orbit through a series of thruster manoeuvres so that any surviving fragments would fall into the sea, following international space-debris guidelines.

How does Megha-Tropiques help in understanding the monsoon?

Megha-Tropiques helps in understanding the monsoon by providing frequent measurements of tropical humidity, clouds and radiation. Scientists and forecasters use this information to study how moisture builds up and produces rainfall, and weather centres use it to improve forecast models.

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