NISAR is the first satellite mission to collect radar data in two microwave bandwidth regions, called the L-band and the S-band, to measure changes in our planet's surface less than a centimeter 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.
NISAR uses a sophisticated information-processing technique known as synthetic aperture radar to produce extremely high-resolution images. Radar penetrates clouds and darkness, enabling NISAR to collect data day and night in any weather. The instrument's imaging swath — the width of the strip of data collected along the length of the orbit track — is greater than 150 miles (240 kilometers), which allows it to image the entire Earth in 12 days.
Over the course of multiple orbits, the radar images will allow users to track changes in croplands and hazard sites, as well as to monitor ongoing crises such as volcanic eruptions. The images will be detailed enough to show local changes and broad enough to measure regional trends. As the mission continues for years, the data will allow for better understanding of the causes and consequences of land surface changes, increasing our ability to manage resources and prepare for and cope with global change.
NISAR is planned to launch in 2022 from India’s Satish Dhawan Space Center in Sriharikota, India, into a near-polar orbit. NASA requires a minimum of three years of global science operations with the L-band radar, and ISRO requires five years of operations with the S-band radar over specified target areas in India and the Southern Ocean.
All NISAR science data, L-band and S-band, will be freely available and open to the public, consistent with the long-standing NASA Earth Science open data policy. NASA has chosen the Alaska Satellite Facility Distributed Active Archive Center (DAAC) to host the mission’s data and products.
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| NISAR |
Mission Requirements
Level 1 Science Requirements Level 2 Science Requirements
The science team has worked with the NISAR project team, ISRO and NASA to develop a set of achievable science requirements. The Level 1 Science Requirements define the specific science measurements that NISAR must perform to satisfy NASA's and most of ISRO's science goals. In addition to these joint requirements, ISRO has identified a number of additional Level 1 science requirements that are to be satisfied by the L-band radar instrument. These requirements then flow down to lower-level science and mission requirements that define the scope of the mission development and operations.
Baseline Requirements
The Baseline Level 1 Science requirements define the capabilities that NISAR is designed to achieve on orbit.
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| NASA-ISRO |
Joint NASA-ISRO Requirements
For a minimum of 3 years, with accuracy specified as 1-sigma:
The NISAR mission shall measure at least two components of the point-to-point vector displacements with a sampling interval of 12 days or shorter over at least 80% of 12-day or shorter intervals, and a maximum time gap in a sampling of 60 days, over specified regions of Earth’s land surface. Accuracy shall be 3.5 (1+L1/2) millimeters or better, over length scales greater than 0.1 kilometers and less than 50 kilometers, with a resolution of 100 meters, over at least 70% of the specified regions. (These include all land areas predicted to have relative velocities faster than 1 millimeter per year over 50 kilometers; Earth's known volcanoes above sea level; areas of rapid glacial mass changes; representative aquifers, hydrocarbon, and geothermal reservoirs, CO2 sequestration sites, and landslide-prone areas; and the sites of earthquakes and other disasters.)
The NISAR mission shall measure time-varying displacements over 90% or more of Earth's ice-covered surfaces with an average sampling capability of 6 days or shorter at a scale of 100 meters; displacement accuracy shall be 100 mm or better over at least 70% of 12-day or shorter intervals.
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The NISAR mission shall measure both Arctic and Antarctic sea ice velocities on a 5 km grid with an average sampling capability of three days; velocity accuracy shall be 100 m/day or better over at least 70% of the sea ice area.
The NISAR mission shall measure above-ground woody vegetation and its disturbance and recovery globally at the hectare scale; accuracy shall be 20 metric tons (Mg) per hectare (ha) or better for areas of woody biomass less than 100 Mg/ha over at least 80% of these areas.
The NISAR mission shall measure seasonally global cropland and inundated areas; classification accuracy shall be 80% or better at the scale of 1 ha.
In support of response to major natural or anthropogenic disasters, the mission system shall be capable of providing revised scheduling for new acquisitions within 24 hours of an event or an event forecast notification and delivering data within 5 hours of being collected and shall exercise this capability on a best efforts basis.
Additional ISRO Requirements
The NISAR mission shall collect the L-band SAR data to support the following ISRO baseline science goals.
During science operations, with accuracy specified as 1-sigma:
The NISAR mission will measure coastal wind velocity on a 1 km grid with an average sampling capability of 6 days, with an accuracy goal of 2 m/s over at least 80% of oceans within 200 km of India's coast.
The NISAR mission will measure bathymetry every six months on a 100 m grid from India’s coast to an offshore distance where the depth of the ocean is 20 m or less, with an accuracy goal of 20 cm over at least 80% of the coverage area.
The NISAR mission will measure the position of India’s coastal shorelines at 10 m resolution at an average sampling interval of 12 days, with an accuracy goal of 5 m over 80% of India’s shorelines.
The NISAR mission will image geological features over selected regions of India at 10 m resolution at an average sampling interval of 90 days with at least two viewing geometries. The regions include paleochannels in Rajasthan, linear features, and structural studies in the Himalayas and on the Deccan plateau.
The NISAR mission will observe sea ice characteristics over the seas surrounding India’s Arctic and Antarctic polar stations at 10 m resolution at an average sampling interval of 12 days.
Threshold Requirements
The Threshold Level 1 Science Requirements define the minimum capability NISAR must be able to achieve on orbit.
Joint NASA-ISRO Requirements
For a minimum of two years, with accuracy specified as 1-sigma:
The NISAR mission shall measure at least two components of the point-to-point vector displacements with a sampling interval of 18 days or shorter over at least 80% of 18-day or shorter intervals, and a maximum gap in a temporal sampling of 60 days, over specified high-priority regions of Earth’s land surface. Accuracy shall be 3.5 (1.5 + L1/2) millimeters or better, over length scales greater than 0.1 kilometers and less than 50 kilometers, with a resolution of 100 meters, over at least 70% of the specified regions. High-priority regions include seismically active zones with relative velocities greater than 2 mm/yr. over 50 km and a limited selection of high-priority aquifers, reservoirs, and landslides.
The NISAR mission shall measure time-varying displacements over high-priority ice-covered surfaces with an average sampling capability of 9 days or shorter at 100-m scale; displacement accuracy shall be 100 mm or better over at least 70% of 18-day or shorter intervals. High-priority surfaces include all coastal ice sheets and tidewater glaciers.
The NISAR mission shall measure sea ice velocities on a 5 km grid in the western Arctic (0 to 180 degrees west extending from land to the North Pole); velocity accuracy shall be 100 m/day or better over 50% or greater of these sea ice areas.
The NISAR mission shall measure the areal extent of disturbance and recovery in above-ground biomass around the globe at the hectare scale annually; classification accuracy shall be 80% or better for changes greater than 50% in woody canopy cover.
The NISAR mission shall measure seasonally global cropland and inundated areas; classification accuracy shall be 80% or better at the hectare scale.
In support of responses to major natural or anthropogenic disasters, the mission system shall be capable of providing revised scheduling for new acquisitions within 36 hours of an event or an event forecast notification and delivering data within 9 hours of being collected and shall exercise this capability on a best efforts basis.
NISAR Observatory
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 SweepSAR scan-on-receive wide swath mapping systems. The spacecraft will launch on an ISRO GSLV-II launch vehicle into a polar sun-synchronous dawn-dusk orbit.
NASA's contributions include the L-band SAR instrument, including the 12-m diameter deployable mesh reflector and 9-m deployable boom and the entire octagonal instrument structure. In addition, NASA is providing a high capacity solid-state recorder (approximately 9 Tbits at end of life), GPS, 3.5 Gbps Ka-band telecom system, and an engineering payload to coordinate
command and data handling with the ISRO spacecraft control systems. ISRO is providing the spacecraft and launch vehicle, as well as the S-band SAR electronics to be mounted on the instrument structure.
The NISAR system comprises a dual-frequency, fully polarimetric radar, with an imaging swath greater than 150 miles (240 km). This design permits complete global coverage every 12-days, allowing researchers to create time-series interferometric imagery and systematically map the changing surface of Earth. The satellite will be three-axis stabilized, that is, using reaction wheels that rotate to keep it correctly oriented to Earth and Sun. It will be launched into a polar Sun-synchronous dawn-dusk orbit (crossing the poles, trailing Earth's shadow to remain in a perpetual sunrise or sunset).
After a 90-day commissioning period, the mission will conduct a minimum of three full years of science operations with the L-band radar to satisfy NASA’s requirements, while ISRO requires five years of operations with the S-band radar. If the system does not use all its fuel reserves during the mission, operations may be extended further for either radar instrument.
NISAR Planned Launch Date: 2022
The NASA-ISRO SAR (NISAR) Mission will measure Earth’s changing ecosystems, dynamic surfaces, and ice masses providing information about biomass, natural hazards, sea-level rise, and groundwater, and will support a host of other applications.
NISAR will observe Earth’s land and ice-covered surfaces globally with 12-day regularity on ascending and descending passes, sampling Earth on average every 6 days for a baseline 3-year mission.
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Mission Characteristics
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| GSLV-MK2 |
- Orbit Altitude 747 km
- Orbit Inclination 98.4°
- Repeat Cycle 12 days
- Time of Nodal Crossing 6 AM/ 6 PM
- Orbit Control < 500 m
- Pointing Control < 273 arcsec
- Pointing Left (south)
- L/S Duty Cycle > 50%/10%
- Baseline Mission Duration 3 years
- Consumables 5 years
- Data and Product Access Free & open
- Wavelength L-band: 24 cm
- S-band: 9 cm
- SAR Resolution 3–10 m mode-dependent
Observation Strategy
The NISAR mission has to meet numerous science requirements set by NASA and ISRO. These requirements include global coverage of land where biomass (organic matter) exists, nearly full coverage of all land surface properties, sea ice at both poles and mountain glaciers, and frequent coverage of land areas that are deforming rapidly (earthquake faults, volcanic activity, landslide-prone areas, etc.).
NISAR’s L-band and S-band instruments permit many different radar observation modes. The NISAR Science Team has identified which mode will work best for each type of desired observation of land and ice areas across the globe. For a discussion of the SAR polarization modes detailed in the following diagrams, visit the SAR page on this site.
The Observation Plan
Because of the broad science goals of the mission, and the wide variety of radar modes that could be employed over any given area, there is a great potential for complexity in the observation plane. The NISAR science team asked users in each of the three main disciplines that NISAR serves — solid Earth, cryosphere, and ecosystems — to create a set of geographic science targets and observational radar modes to optimize their science. The science team then combined these lists, eliminating observational conflicts to produce a simplified target strategy.
The targeting strategy assigns a single radar mode to a given area on Earth. Where target areas overlap, the modes are compatible so that no science discipline loses information. The specific modes assigned to these geographic targets are shown in the combined mode tables below.
This set of global target types and associated radar modes will provide each of the individual disciplines the data they need for their science. The observation plan calls for nearly continuous global coverage over land and ice.
India has planned specific radar modes to fulfill ISRO’s science requirements for the mission. For the rest of the globe, the most inclusive radar mode was chosen where conflicting science discipline needs were identified. The east coast of Antarctica has been singled out for categorization of sea ice type beyond the basic sea ice data that will be taken over the surrounding Southern Ocean (light pink). Most of the world’s landmass will be observed in the “Background Land” mode (bright green), except for North America, which will be observed in more detail.
As NISAR will be only left-looking – a change in observation tactics since the early planning stages – the mission will rely on data from the international constellation of SAR satellites to supplement its coverage around the Arctic pole.
Overview
Synthetic aperture radar (SAR) refers to a technique for producing fine-resolution images from a resolution-limited radar system. It requires that the radar be moving in a straight line, either on an airplane or, as in the case of NISAR, orbiting in space.
The basic principle of any imaging radar is to emit an electromagnetic signal (which travels at the speed of light) toward a surface and record the amount of signal that bounces/echoes back, or “backscatters,” and its time delay. The resulting radar imagery is built up from the strength and time delay of the returned signal, which depends primarily on the roughness and electrical conducting properties of the observed surface and its distance from the orbiting radar.
The wavelengths that remote sensing radars use to observe Earth’s surface are microwaves, typically in the range of a few to tens of centimeters. Because the radar signal loses energy as it travels – at a rate equivalent to the beam width (wavelength/antenna size) – by the time it hits the surface, the beam has spread dramatically. For example, with a signal wavelength of 10 centimeters and an antenna of 10 meters in diameter, the beam width is 1/100 radians (0.6 degrees). From an altitude of 1,000 kilometers, the resulting beamwidth on the ground becomes a very large 10 km, producing an image resolution that is insufficient for most applications. SAR is the solution to this dilemma as it can vastly improve the resolution.
SAR techniques take advantage of the fact that the radar is moving in orbit to synthesize a virtual 10-km-long antenna from the physical 10-m antenna in the direction of flight. As the radar moves along its path, it sweeps the antenna’s footprint across the ground while continuously transmitting pulses – short signal bursts separated by time – and receiving the echoes of the returned pulses.
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Radar in motion diagram
Configuration of the radar in motion to enable synthetic aperture radar imaging. The radar antenna illuminates an area on the ground determined by its wavelength and antenna dimension. Pulses are sent and received continuously such that any point on the ground is sampled numerous times.
The radar images every point on the ground in the path of its “swath” many times. The distance from a specific point to the radar constantly and predictably changes as the radar passes overhead. This change in distance is precisely encoded in the received pulse’s phase (the alignment of the wavelength compared to the original) as a “phase history.” By compensating for the phase history of each pulse recorded for a point, it is possible to focus the signal through computer processing – creating a "synthetic aperture" instead of a limited real aperture. The resulting image resolution can then be improved to a theoretical one-half of the antenna’s diameter, or 5 m to continue the above example.
This SAR process improves the resolution in the "along-track" or "azimuth" direction, which corresponds to the direction of flight. At a right angle to the spacecraft’s flight path is the "cross-track" or "range" direction, which is the direction the radar actually faces. Here the size of the antenna isn’t the constraining factor, rather the width of the transmitted pulse dictates the resolution of cross-track imagery. The pulse intersects surface elements at different parts of its individual waveforms’ crests and troughs, reflecting back the pulse-echo in smaller pieces. After a two-way trip of a transmitted pulse from radar to the ground and back, two objects can be distinguished if they are separated on the ground by more than half the pulse width. Wider bandwidth signals generate finer resolution images in range.
For most purposes, the transmitted signal can be thought of as a single frequency sinusoid wave (S-shaped) with a well-defined amplitude (height) and phase. SAR processing provides a complex image: a pixel with associated amplitude and phase. Once calibrated, the pixel’s amplitude is proportional to the reflectance of the surface. The phase is proportional to the distance the wave traveled between the radar and the ground, any delays due to traveling through the atmosphere, and any phase contribution imparted by the reflectance from the surface.
The NISAR spacecraft will accommodate two fully capable synthetic aperture radar instruments: NASA’s 24 cm wavelength L-band Synthetic Aperture Radar (L-SAR) and a 10 cm wavelength S-band Synthetic Aperture Radar (S-SAR) provided by ISRO. NISAR has a ~ 240 km swath, 7 m resolution along the track and 2-8 m resolution cross-track (depending on model).
In this way, SAR beats the resolution limits of what can physically be put in space to provide images and science of much higher quality than would be possible if the antenna size was used as-is.
Isla Isabella radar image
An example of a radar image showing part of Isla Isabella in the western Galapagos Islands. This image was taken by the L-band radar in HH polarization from the Spaceborne Imaging Radar C/X-Band Synthetic Aperture Radar on the 40th orbit of the space shuttle Endeavour in 1996. Credit: NASA
Interpreting Radar Images
Source: Alaska Satellite Facility (ASF)
The interpretation of synthetic aperture radar (SAR) images is not straightforward. The reasons include the non-intuitive, side-looking geometry. Here are some general rules of thumb:
Regions of calm water and other smooth surfaces appear black because the radar pulse reflects away from the spacecraft.
Rough surfaces appear brighter, as they reflect the radar in all directions, and more of the energy is scattered back to the antenna. Rough surface backscatter even more brightly when it is wet.
Any slopes lead to geometric distortions. Steeper angles lead to more extreme layover, in which the signals from the tops of mountains or other tall objects “layover” on top of other signals, effectively creating foreshortening. Mountaintops always appear to tip towards the sensor.
Layover is highlighted by bright pixel values. The various combinations of the polarization for the transmitted and received signals have a large impact on the backscattering of the signal. The right choice of polarization can help emphasize particular topographic features.
In urban areas, it is at times challenging to determine the orbit direction. All buildings that are perfectly perpendicularly aligned to the flight direction show very bright returns.
Surface variations near the size of the radar's wavelength cause strong backscattering. If the wavelength is a few centimeters long, dirt clods and leaves might backscatter brightly.
A longer wavelength would be more likely to scatter off boulders than dirt clods, or tree trunks rather than leaves.
Wind-roughened water can backscatter brightly when the resulting waves are close in size to the incident radar's wavelength.
Hills and other large-scale surface variations tend to appear bright on one side and dim on the other. (The side that appears bright was facing the SAR.)
Due to the reflectivity and angular structure of buildings, bridges and other human-made objects, these targets tend to behave as corner reflectors which are used for calibrating NISAR instruments (see photo) and show up as bright spots in a SAR image. A particularly strong response — for example, from a corner reflector or ASF's receiving antenna — can look like a bright cross in a processed SAR image.
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