Authors:
Dr. Paul Rosen | Jet Propulsion Laboratory, California Institue of Technology | United States
Dr. Gerald W. Bawden | NASA Headquarters | United States
Dr. Raj Kumar | NRSC/ISRO | India
The NASA-ISRO Synthetic Aperture Radar (NISAR) mission will use synthetic aperture radar to map Earth’s surface every 12 days, persistently on ascending and descending portions of the orbit, over all land and ice-covered surfaces. The mission’s primary objectives will be to study Earth land and ice deformation, and ecosystems, in areas of common interest to the US and Indian science communities. This single observatory solution with an L-band (24 cm wavelength) and S-band (10 cm wavelength) radar has a swath of over 240 km at fine resolution, and will operate primarily in a dual-polarimetric mode in an exact repeat orbit.
NISAR will characterize long-term and local surface deformation on active faults, volcanoes, potential and extant landslides, subsidence and uplift associated with changes in aquifers and subsurface hydrocarbon reservoirs, and other deforming surfaces. These measurements will be used to model the physics of the subsurface, potential hazards associated with the deformation, and associated risks. The variable and largely unpredictable nature of these phenomena lead to a systematic collection strategy to capture as many signals as possible. Surface deformation measurements will be validated over globally distributed GPS networks in a variety of environmental settings.
NISAR will determine changes in carbon storage and uptake resulting from disturbance and subsequent regrowth of global woody vegetation, by regularly measuring the amount of woody biomass and its change in the most dynamic regions of the world. NISAR also has objectives in characterizing changes in the extent of active crops to aid in crop assessments and forecasting, as well as changes in wetlands extent, freeze/thaw state and permafrost degradation. The ecosystems data sets will be validated through in situ measurements at dozens of sites around the world in partnership with other missions and organizations.
NISAR will investigate the nature and causes of changes to Earth’s ice sheets and sea ice cover in relation to the atmospheric and ocean forces that act upon them, through systematic deformation measurements of Greenland’s and Antarctica’s ice sheets, seasonal dynamics of highly mobile and variable sea ice, and inventory the variability of key mountain glaciers which are retreating in many places at a record pace. Validation of deformation on the ice sheets will employ bare-rock references and cross-over analysis, as well as some deployed GPS stations on the flowing ice. The sea ice community will compare sea ice motion to in situ buoy data.
In addition, NISAR will be operated to observe potential hazards and disasters on a best-efforts basis to demonstrate rapid assessments in urgent events such as earthquakes, volcanic eruptions, floods, and severe storms. These data will support research into effective rescue and recovery activities, system integrity, lifelines, levee stability, urban infrastructure, and environment quality. The mission team has implemented an urgent-response tasking system that will combine automated triggers for earthquakes, volcanoes, and fires with manual requests by certified users.
The joint science team at NASA and ISRO has created a stable, joint science and observation plan, robust calibration and validation plan. The team also has defined a suite of science products, including raw data, complex images at full resolution in both natural radar coordinates and in an orthorectified form, and lower resolution polarimetric and interferometric products also in radar and ortho-rectified coordinates. These products will be organized in frames roughly 240 km x 240 km in size, and will be available at the Alaska Satellite Facility Distributed Active Archive Center under NASA’s full and open data policy.
The science team has developed the observation plan prioritizing continuity of time series measurements. To that end, the polar measurements prioritize the South Pole, creating a coverage gap north of 77.5 degrees of latitude, due to the inclined orbit and consistent southward off-nadir pointing of the radars. The radar instruments have many possible modes, operated at L-band globally, and jointly with S-band regionally over India and select other locations around the world. The NISAR science observation plan is designed to tackle the science questions posed by persistent and consistent imaging of Earth’s land and ice surfaces throughout the life of the mission, delivering time series of approximately 30 images per year from ascending and descending vantage points.
The cadence of science operations is expected to be highly routine. The initial observation plan will be in place pre-launch, and it is anticipated that there will only be minor adjustments to the plan once in orbit. The project has defined a six-month replanning cycle, whereby scientists identify changes they would like to see based on the data previously acquired and analyzed science data, the science and project mission planning teams evaluate the impact of those changes on resources and the ability to meet science requirements, and if acceptable, the observation plan is revised. Each of these steps is allocated roughly 2 months. Given that the goal of NISAR is to create regular, easy-to-use, time series of Earth change, the project expects that any adopted changes will not break the time series.
The science team is developing algorithms to produce higher level products for validation purposes. These products will be over local or regional validation sites, with sufficient to demonstrate that the required accuracies can be achieved over the Earth. The algorithms for producing these products will be made available to scientists interested in using NISAR data. They are being developed in the form of jupyter notebooks, serving the purpose of processing, algorithmic description and documentation, and instruction. Sample data sets will be available prior to launch to prepare the community for the data products and the algorithmic workflows to produce higher level products.
NISAR is in its third phase of integration and test, when all the components of the instrument payload, including the L- and S-band radar electronics, the solid-state recorder, GPS, and engineering payload, and the mechanical boom and reflector systems are assembled and tested in environments at NASA’s Jet Propulsion Laboratory. This completed payload will be subsequently shipped to India for the final phase of integration, test and launch, currently planned for 2023. The mission systems have been built and are in extended operational testing.