Resourcesat-2 was launched into space from Sathish Dhawan Space Centre, Sriharikota, on April 20.ISRO referred to the "rough mountainous Himalayan terrain covered with thick forested area, simultaneous persistent cloudy conditions and incessant bad weather" for its limited use of optical remote sensing data in locating the ill-fated chopper.
Though the space agency used microwave remote sensing data, "which can penetrate clouds and quickly image large areas in all weather conditions" to locate the missing helicopter, the target size in this case was expected to be of 2-3 metres only, thus requiring data of 1m spatial resolution, an ISRO release said.
"Since microwave sensors are side looking, the grazing angle and slope produce layovers, foreshortening and shadow effects causing distortions in hilly terrain hampering the interpretation and microwave data of short wavelength (X, C bands) do not penetrate dense vegetation," the release said.
Despite the complexities associated with microwave data especially in the mountainous regions, efforts were made to identify objects with "bright signatures", it said.
Based on decision criteria, probable locations for search operations were identified and communicated to Arunachal Pradesh government.
Detailed base information like roads and drainage network, spot heights and settlement areas along with a virtual media clip showing the terrain conditions were also provided to the ground team.
National Remote Sensing Centre (NRSC) of ISRO provided the Arunachal Pradesh government with high resolution satellite images acquired from its satellites -- Resouresat-1 and 2, RISAT-2, Cartosat and foreign satellite (Radarsat-2) missions prior to and after the incident, it added.
The B3 single-engine four-seater helicopter with Khandu and four others went missing after taking off from the Tawang helipad on Saturday last and crashed in the mountains.
The bodies and the wreckage of the chopper were located after a five-day search at Kyela, situated at an altitude of 4,500 metres, on May 4 in Tawang district.
