In October 2011, an Indo-French scientific satellite, Megha-Tropiques, designed to measure rainfall over the tropical regions of the world, took to the skies. A continuation of that collaborative effort has now produced another scientific mission, this time to monitor the oceans. The ‘Satellite for ARgos and ALtiKa’ (Saral) is to be launched by India’s Polar Satellite Launch Vehicle today.
Ties between the Indian Space Research Organisation (ISRO) and its French counterpart, Centre National d’Etudes Spatiales (CNES), go back five decades to the early years of India’s space programme.
“We started to think about increasing and reinforcing the long-term cooperation in the late 1990s by developing together … a scientific satellite,” according to Sylvie Callari, head of International Relations at CNES. Finding that they were pursuing similar scientific objectives in terms of earth observation, the two space agencies joined hands to develop the Megha-Tropiques satellite. “Very quickly we started to think about doing another one … and we ended up with this altimetry programme for the study of oceans,” she told this correspondent.
A satellite altimeter works on the principle of the radar, emitting microwave pulses and picking up signals that bounce back. The time taken for the signal to return provides a measure of the distance between the satellite and the surface of an ocean. By establishing the satellite’s position in orbit very precisely, the sea surface height can then be determined. The returning signals can also be used to estimate wave heights and winds over the ocean.
France flew its first altimeter on the Topex/Poseidon, a spacecraft built jointly with the U.S, which was launched in 1992 and worked till 2006. French altimeters also went on Jason-1 and Jason-2 spacecraft that the two countries sent up in 2001 and 2008 respectively.
Advanced instrument
The French ‘AltiKa’ altimeter on the Saral will operate in a higher frequency band (known as Ka) than previous satellite altimeters. Use of a higher frequency, along with correction for atmospheric delays, will allow this altimeter to determine sea surface height with greater precision. In addition, its higher spatial resolution confers the ability to gather data closer to the seashore than before, and also supply more accurate information about inland water bodies, like rivers and lakes.
The suggestion for an altimeter-carrying satellite came up at a meeting of the Indo-French Joint Working Group about a decade back, according to persons who occupied senior positions in Indian space programme at the time. For ISRO, which has not made an altimeter, the collaboration offered a way to gain experience in operating and using such a satellite.
Moreover, the proposal also fitted in with the space agency’s own ideas for creating a small satellite weighing about 400 kg at launch. Typically, instruments are embedded in various parts of a satellite during its assembly. But a modular approach was evolved for the Saral. Its instruments came from France as a single module. The Indian side developed the basic satellite structure, known as the ‘bus’, which provides basic housekeeping functions. The instrument module was attached to the bus with a few bolts and an electrical connector.
The same bus could be used again in future, reducing the time needed to put together satellites carrying other instrument modules, observed a retired ISRO scientist.
Saral’s altimeter will help scientists watch the world’s oceans and its shrinking ice sheets. As the climate has warmed, global sea levels have risen. “A significant fraction of the world population is settled along coastlines, often in large cities with extensive infrastructure, making sea-level rise potentially one of the most severe long-term impacts of climate change,” pointed out a World-Bank-sponsored study last year.
National security perspective
“It is good for India, which has long coastline, to get into sea-level monitoring,” remarked Raghu Murtugudde, professor of atmospheric and oceanic sciences at the University of Maryland in the U.S. From a national security perspective, India also needed to worry about countries in its neighbourhood that might get swamped by rising seas, like Bangladesh or the Maldives.
“Sea surface height is actually a very good indicator of how much heat there is in the underlying ocean,” said Krishna AchutaRao of IIT Delhi, one of the principal investigators on the Saral/AltiKa science team. Nearly 90 per cent of the additional heat trapped by greenhouse gases ended up in the oceans. Data from Saral’s altimeter, along with other information, would be useful for understanding how the long-term heat content of the oceans has been changing, especially that of the Indian Ocean.
“On a global scale, how the ocean takes up and distributes heat affects the atmospheric temperature change we experience under warming,” he pointed out. “Regionally, the availability of heat affects many weather and climate phenomena, including the monsoon.”
At the Indian National Centre for Ocean Information Services (Incois) based in Hyderabad, Saral’s altimeter data would go into high-resolution ocean state forecasting models that are being set up for the Indian coast. Such predictions of sea conditions would help fishermen, the shipping industry, oil and gas companies as well as the Coast Guard and Navy, according to its director, Satheesh Shenoi.
Saral will also carry an Argos payload for relaying data from transmitters that provide worldwide environmental monitoring and tracking. Thousands of such transmitters have been deployed on ocean buoys as well as various other platforms on land and sea. They have even been attached to birds and animals to track migration over long distances.
