Insights into Editorial: Eye in the sky: on RISAT-2B
Context: Radar imaging satellite RISAT-2B:
RISAT-2B, the country’s newest microwave Earth observation satellite, rode to its orbit 557 km above the ground.
With the successful pre-dawn launch of RISAT-2B satellite, the Indian Space Research Organisation (ISRO) has added another feather to its cap.
Data that will come from the all-weather day-and-night satellite are considered to be vital for the Armed Forces as also agriculture forecasters and disaster relief agencies.
Dubbed as a ‘spy’ satellite, RISAT-2B (Radar Imaging Satellite-2B) will replace its predecessor RISAT-2 which has been actively used by India to monitor activities in terror camps across the border in Pakistan to thwart infiltration bids by terrorists.
Radar imaging satellite RISAT-2B:
- Radar imaging is important for surveillance applications, as it does not require sunlight or clear skies to be able to observe its target.
- Optical imaging satellites are only able to see points of interest when they are illuminated by the sun and not hidden by cloud, whereas a spacecraft equipped with SAR can still observe at night and its radio waves can propagate through cloudy skies.
- The satellite aboard launch will be a replacement for RISAT-2. The original RISAT-2 was built for ISRO Israel Aerospace Industries (IAI), and is based on the Polaris satellite (known as TecSAR or Ofeq 8) which ISRO had launched for Israel in 2008.
- The RISAT-2B satellite uses X-band synthetic aperture radar for the first time; the synthetic aperture radar was developed indigenously.
- Its X-band synthetic aperture radar can give added details such as size of objects on Earth, structures, movement and change.
- 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. 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.
- With ISRO planning to launch four more such radar imaging satellites in a year, its ability to monitor crops and floods as well as engage in military surveillance will be greatly enhanced.
RISAT-2B will use Microwave radiation:
- Unlike visible light, microwaves have longer wavelength and so will not be susceptible to atmospheric scattering.
- Microwave radiation can thus 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, it 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 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 between the transmitted and reflected signals will help determine the distance to the object.
RISAT-2B Equipped with SAR (Synthetic Aperture Radar):
A Synthetic Aperture Radar (SAR) or SAR, is a coherent mostly airborne or spaceborne side looking radar system which utilizes the flight path of the platform to simulate an extremely large antenna, and that generates high-resolution remote sensing imagery.
Over time, individual transmit/receive cycles (PRT’s) are completed with the data from each cycle being stored electronically.
The signal processing uses magnitude and phase of the received signals over successive pulses from elements of a synthetic aperture.
After a given number of cycles, the stored data is recombined to create a high-resolution image of the terrain being over flown.
Countries using Radar Imaging Satellites:
India is not the only country to use radar imaging for military reconnaissance – other systems currently in operation include the United States’ TOPAZ constellation, Japan’s IGS Radar and Italy’s COSMO-SkyMed.
SAR has also been used on civilian scientific and commercial satellites and on interplanetary probes.
India’s partnership with Israel on this satellite boosted its radar imaging technology, and provided an initial capability ahead of the launch of India’s indigenously-developed RISAT-1. Newer satellites, including RISAT-2B, have been developed by ISRO.
The information will complement data from the normal optical remote-sensing satellites. Such data are useful for agencies that need ground imageries during cloud, rain and in the dark.
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.
The biggest advantage is that ground imageries can be collected during rains and despite dust, clouds or darkness and during all seasons, thus ensuring continuous and reliable data.
ISRO is now gearing up for the launch of Chandrayaan-2 onboard GSLV MkIII during the window of July 2019, with an expected Moon landing on September 2019.