The Earth's surface is ready for its closeup.
When the NASA-ISRO SAR satellite launches into orbit on July 30, it will ping nearly all of the planet's land and ice surfaces with a form of imaging called synthetic aperture radar twice every 12 days. The radar will capture ground movements as small as a centimeter, to create the most comprehensive picture yet of Earth's ever-changing exterior.
From forests and wetlands to mountain glaciers and sea ice, scientists will be able to better understand the causes and consequences of land surface changes, particularly as they relate to the global climate crisis.
One of the people eagerly monitoring the satellite's progress will be Rowena Lohman, professor of earth and atmospheric sciences in Cornell Engineering, who is serving on the project team and overseeing the collection of soil moisture data.
Lohman is well suited for the role thanks to her expertise with interferometric synthetic aperture radar, or InSAR, a technique that combines two images of the same area to reveal the topography or movement of the ground. At the beginning of Lohman's career, she used InSAR to study the subsurface shifts that cause earthquakes, but in recent years, she has used the same methods to examine groundwater and its extraction, which can also jostle the subsurface.
"My actual work often involves techniques to make the measurements better, dealing with noise, trying to remove the noise, so we can see smaller signals," she said. "One of the sources of noise in the data, if you're trying to observe deformation, comes from soil moisture. What's our noise is another person's signal."
The satellite, a joint effort between NASA and the Indian Space Research Organization, uses two instruments: an L-band system with a 25-centimeter wavelength and an S-band system with a 10-centimeter wavelength.
"You've got people who are using the same datasets in different ways," Lohman said. "The soil moisture people look at how bright the surface is and how that changes over time. That's their basic measurement. Whereas the ground deformation experts will look for changes in the phase of the same signal. It's been really kind of exciting, learning about all these different approaches and trying to see what we can learn from each other."
InSAR is a highly sought visualization tool because it can penetrate clouds and darkness, and the NISAR mission will result in more frequent and reliable observations, at a wavelength of light that is more stable in vegetated regions, than what scientists have obtained in the past. For soil moisture, there will also be a large step forward in spatial resolution. Most previous global estimates of soil moisture, for example, have packed many kilometers into a single pixel, but the NISAR mission will improve that to 200 meters per pixel.
The new NISAR-based soil moisture data will help everyone from climate modelers making short-term weather forecasts that rely on how moisture is transferred into the atmosphere, to scientists and climatologists tracking large-scale droughts, as well as government agencies preparing for floods.
"Let's say it's going to rain in Montana, and you're figuring out which dams downstream may reach their capacity," Lohman said. "Soil moisture is one of the things you need to know. Because if the soil is dry between, say, Montana, and the exit of the Mississippi River, then that floodwater will get absorbed into the soil. If everything is already saturated, then you'll have more flooding."
But possibly the biggest impact will be felt much farther north.
"I think we're going to be amazed by what we can see in the Arctic, things like permafrost change," Lohman said. "The Arctic has been a place where we really couldn't look before. This type of data we use is just too noisy. NISAR data, with its longer, more stable wavelength and frequent observations, is going to make it much easier to see what is going on there. That region is so dynamic. I think NISAR is going to be a game changer in terms of our understanding of what's happening in these areas that are affected really heavily by climate change."
The satellite is scheduled to launch July 30 from the Satish Dhawan Space Center in Sriharikota, India, and its mission is expected to last three years, but could extend much longer, according to Lohman.
Certain pieces of radar data will be spot-checked and validated by on-the-ground volunteers, and after about 36 hours, everything will be archived online via the Amazon cloud. Anyone from high school students to amateur meteorologists to professional scientists will be able to access the information as part of NASA's free and open-source science initiative.
As the satellite's launch date approaches, Lohman and her team are conducting operation reviews, visualization tests and calibration and validation to ensure the measurements are precise - and trying not to be nervous.
"What do I stay up at night worried about? Honestly, it's how we are going to deal with all of this data. But it's a good problem to have," Lohman said. "We're going to have petabytes of data coming from the satellite, and NASA has committed to make this available to everyone so that we can get the most value for what has been invested in this satellite. I feel like the community is so innovative, people are going to figure out all sorts of new things you can do with the data we get from NISAR."
