CHENNAI: Four days before Vardah hit Chennai, the meteorological departments – India Meteorological Department ( IMD ) and Regional Meteorological Centre, Chennai ( RMC ) -- began tracking the cyclone with images taken by satellites orbiting 36,000km above the Earth. The eye of the storm, measuring 15km, was clearly visible and the weathermen knew which way Vardah was heading. As the storm moved closer to the Tamil Nadu coast, alerts were sent to evacuate people 48 hours prior to its landfall.Meteorologists said the weather satellites that Isro sent onboard GSLVs acted as sentinels in the sky that helped them predict the intensity of the cyclone very early. Isro officials said they had sent satellites images from INSAT-3DR and Scatsat-1 to IMD to track Vardah. The satellites, launched in September 2016, have high-resolution cameras and advanced atmospheric sounders. “Once a satellite is up in its orbit, we start getting data. We send the same to IMD,” said Isro spokesperson Deviprasad Karnik.Aiding the observation were high resolution images, captured every 30 minutes, by other weather-monitoring satellites -- INSAT-3A, INSAT-3D, Kalpana-1.“With satellite images of cloud cover we can estimate the intensity of a cyclone. If the eye is clear, we can track where the system is positioned. Without satellite images, we cannot track a system in the seas,” said S Balachandran, director, Area Cyclone Warning Centre, RMC, Chennai. “Satellites supplement our observations until the storm is about 500km away from the coast after which Doppler radars take over,” he said.As per standard procedure, said IMD officials, National Remote Sensing Centre, Hyderabad collects data from the satellites and sends them to their centre in New Delhi. There a team processes the data, marks the geographical boundaries and sends them to regional centres. Images are updated on their websites for private weather agencies to access.While satellite is not the only tool for weather forecasting -- weathermen use data collected from satellites, Doppler radars and climate patterns in forecasting models – but they have improved predictions in the past three decades. After the launch of INSAT-1A in 1982, more than a dozen multipurpose satellites with better components were launched, bettering the forecast and reducing errors. “The super cyclone that hit Odisha in 1999 killed more than 15,000 people. Vardah killed 10. You can see the difference in preparedness,” chief meteorologist at Skymet Mahesh Palawat said.According to former RMC director YEA Raj, the situation today is far removed from the guess work of the 1950s when ships on the high seas would relay weather information which would then be compared with wind surface data collected from instruments installed in islands like Port Blair . Weathermen would look for swells along the coast to find if the storm was approaching. “This was the situation in the IMD from 1875 to the 1950s,” Raj said.In the 1960s, India borrowed satellite images from the US. Weathermen would acquire these by keying codes and take printouts. “But today we not only get images that have a high resolution of one to four kilometres, but our satellites can provide information on radiation and wind data,” Raj said.