Satellites measure clouds, moisture, temperature and winds over the land and oceans around India, filling the gaps between ground stations that forecasts depend on.
Much of the monsoon's story unfolds over the Arabian Sea and the Bay of Bengal, where weather stations are few. To follow it, forecasters need to see moisture, clouds and winds over the sea as well as the land. Satellites can see all of these.
INSAT-3DS and its sister satellites sit in geostationary orbit and watch the same view of India and the surrounding seas all the time. Their imagers show clouds, and their sounders measure temperature and water-vapour profiles. ISRO says INSAT-3DS data serve the India Meteorological Department, the National Centre for Medium Range Weather Forecasting and INCOIS, for weather forecasting and disaster warning.
India and France built Megha-Tropiques to study the water cycle and energy exchanges in the tropics, and the life cycle of the storm systems that drive tropical weather. It was launched on 12 October 2011 on PSLV-C18.
Its orbit was unusual. Instead of passing over the poles, it flew at 867 km on an orbit tilted only 20° to the equator. That kept it over the tropics on every orbit, so it revisited the monsoon belt far more often than a polar satellite could. Its instruments included MADRAS, a microwave imaging radiometer; SAPHIR, a humidity sounder; ScaRaB, which measured the Earth's radiation budget; and ROSA, a radio occultation sensor.
After more than eleven years in orbit, ISRO brought it down in a controlled re-entry over the Pacific Ocean on 7 March 2023.
Ocean satellites add winds. EOS-06 (Oceansat-3) carries a Ku-band scatterometer to continue the wind-vector data first provided by Oceansat-2, along with an ocean colour monitor and a sea surface temperature monitor.
Find the INSAT satellites over the equator: their fixed view covers the whole monsoon region from the Arabian Sea to the Bay of Bengal. See India's weather satellites
Why fly Megha-Tropiques at 20° instead of over the poles? A satellite's ground track can never reach a latitude higher than its inclination. At 20°, every orbit is spent between 20° N and 20° S, which is where tropical convection happens.
Its period follows from Kepler's third law:
r = 6,378 + 867 = 7,245 km T = 2π × √(r³ / GM) = 2π × √(7,245³ / 398,600) 7,245³ = 3.803 × 10¹¹ km³, and 3.803 × 10¹¹ / 398,600 = 954,100 s² √954,100 ≈ 976.8 s, so T ≈ 2π × 976.8 ≈ 6,137 s ≈ 102 minutes
That is 1,440 / 102 ≈ 14 orbits per day. A polar satellite also makes about 14 orbits a day, but spends most of each one outside the tropics; NASA notes that a polar orbiter sees most of the Earth only about twice a day, once in daylight and once at night. Megha-Tropiques, by contrast, crossed the tropics on every orbit, so it could revisit a tropical location several times a day.
The trade-off: India's northern states lie beyond 20° N, so Megha-Tropiques looked at them only at an angle near the edge of its view, while the oceans and the southern peninsula were well covered.
Geostationary satellites complement this: they see the whole region continuously, but from 35,786 km, so their pictures are much coarser than those from 867 km.
To keep it over the tropics on every orbit, so it could revisit monsoon and tropical storm regions many times a day.
Any two of MADRAS (microwave imager), SAPHIR (humidity sounder), ScaRaB (radiation budget) and ROSA (radio occultation).
20° north and 20° south.