Surveillance from the sky using RISAT-2B satellite

ISRO-Optimized
RISAT-2B satellite will be able to image under almost all weather and environmental conditions.

RISAT-2B satellite launched on May 22 will use microwave radiation for crop monitoring, forestry and flood mapping. The satellite can image during day and night and under almost all weather conditions including cloud cover. It can detect objects with dimensions of as little as a metre, making it ideal for military surveillance.

With the successful pre-dawn launch of RISAT-2B satellite on May 22, the Indian Space Research Organisation (ISRO) has added another feather to its cap. The satellite will enhance India’s capability in crop monitoring during the monsoon season, forestry mapping for forest fires and deforestation, and flood mapping as part of the national disaster management programme. Given that overcast skies are a constant during the monsoon season and during times of flood, the ability to penetrate the cloud cover is essential. While optical remote sensing that relies on visible light for imaging gets obstructed by clouds, RISAT-2B will not. The reason: much like the RISAT-1 satellite that was launched by ISRO in April 2012, RISAT-2B will also use microwave radiation.

Unlike visible light, microwaves have a longer wavelength and so will not be susceptible to atmospheric scattering. As a result, microwave radiation can easily pass through the cloud cover, haze and dust, and image the ground. Hence, RISAT-2B satellite will be able to image under almost all weather and environmental conditions. Since it does not rely on visible light for imaging, RISAT-2B satellite will be able to image the ground during both day and night.

The satellite does not have passive microwave sensors that detect the radiation naturally emitted by the atmosphere or reflected by objects on the ground. Instead, RISAT-2B will be transmitting hundreds of microwave pulses each second towards the ground and receiving and detecting the signals reflected by the objects using radar. The moisture and texture of the object will determine the strength of the microwave signal that gets reflected. While the strength of the reflected signal will help determine different targets, the time delay between the transmitted and reflected signals will help determine the distance to the object.

The RISAT-2B satellite uses X-band synthetic aperture radar for the first time; the synthetic aperture radar was developed indigenously. Unlike the C-band that was used by RISAT-1, the shorter wavelength of the X-band allows for higher resolution imagery for target identification and discrimination. Since it has high resolution, the satellite will be able to detect objects with dimensions of as little as a metre. This capacity to study small objects and also movement could be useful for surveillance. As K. Sivan, ISRO Chairman and Secretary, Department of Space, had said last month, the satellite could be used for civil and strategic purposes.

RISAT-2B will have an inclined orbit of 37 degrees, which will allow more frequent observations over the Indian subcontinent; the time to complete one orbit was 25 days in the case of RISAT-1.  With ISRO planning to launch four more such radar imaging satellites in a year, India’s ability to monitor agriculture and flood as well as military surveillance will be greatly enhanced. RISAT-2B launch was originally slated only next year but was advanced owing to “demand”.

Published in The Hindu on May 23, 2019

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