NISAR is the first spacecraft mission to collect radar data in two microwave bandwidth regions, L-band and the S-band, to measure changes in our planet’s surface less than a centimetre across.
This allows the mission to observe a wide range of Earth processes, from the flow rates of glaciers and ice sheets to the dynamics of earthquakes and volcanoes.
The NISAR spacecraft will accommodate two fully capable synthetic aperture radar instruments (24 cm wavelength L-SAR and 10 cm wavelength S-SAR), each designed as array-fed reflectors to work as scan-on-receive “SweepSAR” wide swath mapping systems. The radars are designed to work independently or together and are self-contained units up to the point of radiating energy (sending a signal) to the shared reflector.
The L-SAR supplied by JPL (Jet Propulsion Laboratory of NASA) is capable of 242-km swaths with 7-m resolution along track (the direction of travel) and 2- to 8-m resolution cross-track, depending on the viewing mode. This radar comprises 24 L-band transmit/receive array elements in 2 rows, 12 per polarization.
The 12 cm wavelength S-SAR supplied by ISRO is the secondary radar and is composed of 48 S-band transmit/receive array elements in 2 rows, 24 per polarization. The S-SAR will be used to produce data over science areas of interest to India that are above and beyond the NASA requirements. The areas include coastal bathymetry (depth of water) and ocean winds, geology over India, and coastal shoreline studies. The S-band is also sensitive to light vegetation and will be used in polar regions as its signal is less sensitive to ionospheric disturbances.
The most prominent feature of NISAR is its 12 m large antenna reflector mounted on a 9 m boom. The challenge of such a large antenna is stowing it to fit within the small space in the launch vehicle fairing and deploying it in multiple steps. The radar instruments and boom supporting the reflector are mounted to an octagonal instrument structure provided by NASA.
NISAR uses sophisticated polarization, interferometry and SweepSAR processing techniques to produce extremely high-resolution images.
Specific Applications of NISAR
Addressing Natural Disasters: Earth’s surface is constantly changing as a result of both natural and human processes, and humanity’s exposure to natural hazards is increasing. Natural disasters like earthquakes, volcanic eruptions and landslides often occur abruptly. NISAR will measure changes of the Earth’s surface, from small movements of the crust up to volcanic eruptions.
Forecasting, Damage Assessment and Resource Management: NISAR’s ability to detect surface motions of our planet will help illuminate the processes occurring deep within. NISAR’s global and rapid coverage and open access will provide unprecedented opportunities to mitigate widespread damage and assess its extent.
Study of Cryosphere: NISAR will measure changes in sea ice, snow extent, permafrost, and surface melting and also ice sheets, sea ice and glaciers, which are all key indicators of climate effects. Rising sea level from melting ice sheets poses hazards to coastal areas from storm surges and erosion. Diminishing sea ice is changing shipping lanes. These measurements will provide an insight for prediction of future changes.
Improving Sea Level Projections: Flow rates and melt rates of coastal glaciers in many parts of Greenland and Antarctica have increased significantly as ice is rapidly getting lost to the sea. The current satellite missions map large-scale ice sheet changes but lack the resolution to monitor many fast-moving, rapidly thinning glaciers. NISAR will provide systematic measurements of both short-term variations and long-term trends. It will provide precise measurement of the changing position of ice sheet grounding lines. These new, high-resolution data will improve projections of sea level rise from melting glaciers and ice sheets to improve planning for mitigation strategies.
Tracking Sea Ice and Monitoring Permafrost: NISAR will provide the complete measurements of rapidly changing sea ice in both the polar regions. Understanding the causes and mechanisms of ice loss requires knowledge of ice thickness, sea ice types and sea ice dynamics. Melting permafrost releases methane to the atmosphere, erodes soil and impacts surface water distribution and the stability of infrastructure. NISAR will measure heave and thaw in the near-surface active layer of permafrost.
Ecosystems and Climate Change Projections: Landscapes are rapidly changing as forests are cut down and agricultural lands are developed. Ecosystems emit carbon to the atmosphere (carbon sources) and also remove it (carbon sinks). Living material (biomass) stores carbon, playing an important role in the carbon cycle. Land ecosystems, which act as carbon sources and sinks, must be studied to understand how they may interact with climate as atmospheric CO2 increases. NISAR will observe the distribution of vegetation and biomass for better understanding ecosystems’ responses to disturbance and recovery. NISAR will determine the contribution of Earth’s most variable biomass to the global carbon budget and characterize ecosystem disturbance and impacts on biodiversity.
Monitoring Groundwater Resources: Climate change coupled with growing populations is causing increasing stress on groundwater resources. Over-exploitation of groundwater leads to lowering of the water table and, in alluvial basins, compaction of the aquifer system. These processes cause sinking of the land surface — subsidence. Subsidence is often the first indication of changes in reservoirs or over-exploitation of aquifers. NISAR will provide systematic measurement of changes in Earth’s surface across entire aquifers. The ability to map surface deformation of a few millimetres monthly over large areas at resolutions of a few tens of meters will open new possibilities for remote monitoring of groundwater resources.
Mapping Floods: NISAR’s unique combination of radar, frequent orbit passes and polar orbit also mean that the satellite can be used to study floods, precisely measuring very small changes of water level in areas with standing vegetation. NISAR can be used to track floods, even under vegetation canopies.
NISAR is independent of light sources, capable of seeing through clouds and it does not matter how far the target area is because the spatial resolution remains the same. By virtue of the planned frequent and regular sampling of some of the world’s most hazard-prone areas, this data will be a rich source of information for proactive planning of disaster management.
Besides, study of ecosystems, cryosphere and climate change, monitoring of natural resources and infrastructure, and other value-added applications will be revolutionised by free access data of NISAR.