WASHINGTON — NASA scientists are auditioning the radar aboard a European satellite to see how well it stands in for the radar that broke down aboard the U.S. space agency’s newly launched Soil Moisture Active Passive (SMAP) satellite in July.
SMAP Project Scientist Jared Entin said in a Nov. 4 interview that the team looked at other radar satellites before concluding that, at least for now, the European Space Agency’s Sentinel-1A is the only space-based radar that makes sense to use in tandem with SMAP — an otherwise healthy spacecraft now totally dependent on its passive radiometer.
Sentinel’s C-band radar is not the only radar flying in space — or even the closest substitute for the lost SMAP L-band radar — but it is the only one that will trail SMAP closely enough to gather timely radar images of the swath of Earth that SMAP covers with its radiometer.
“It’s not coincident with SMAP [but] you’re getting many overlaps between the Sentinel and the SMAP,” Entin said, adding that the two spacecraft will occasionally observe the same spot on the ground within hours or days of one another.
That is crucial for SMAP, since observations of the same spot are not worth having unless they are taken around the same time, so that soil moisture conditions on the ground are more or less similar in both the radar and radiometer images.
SMAP was supposed to gather radiometer and radar data simultaneously and use the radar to turn 30-kilometer-per-pixel radiometer imagery into a sharper image in which one pixel represented nine kilometers. NASA says it can continue the mission with just a radiometer, but that guarantees only 30-kilometer-per-pixel soil moisture maps and severely curtails the mission’s ability to gather global data on soil freezing and thawing.
SMAP mission scientists at the Goddard Space Flight Center in Greenbelt, Maryland, are now using Sentinel radar data to refine SMAP radiometer imagery and comparing the resulting data with the two months of radar data SMAP collected before that instrument failed.
Entin estimated it will take SMAP scientists until mid-December to figure out whether Sentinel’s radar will work as a replacement. There are still many question marks, including how much time can pass between SMAP and Sentinel observations before the observations become incompatible.
“Two hours or four hours could make a little difference, or it could make a lot of difference,” Entin said. “Some of that you just can’t know until you work with the satellite data.”
Sentinel is for now the only radar satellite with which the SMAP team will work. The agency has ruled out the L-band radar aboard Japan’s Advanced Land Observing Satellite (ALOS)-2, which launched on a seven-year mission in 2014.
“The resulting overlapping area between SMAP data and ALOS-2 data is too small to constitute a global map of soil moisture on the 2-3 day timescale that SMAP produces, so the SMAP project chose to work the Sentinel data instead,” NASA spokesman Steve Cole wrote in a Nov. 11 email.
Meanwhile, space-based instruments besides radar could also help make up for the loss of the SMAP radar, Entin said.
While plugging radar data into SMAP models is the easiest thing — the mission’s software models are already configured for that — scientists could also sharpen radiometer images with help from instruments aboard NASA’s 16-year-old Terra and 13-year-old Aqua Earth observation satellites, and the 4-year-old Suomi NPP polar-orbiting weather satellite operated by the National Oceanic and Atmospheric Administration.
That is because these satellites can measure land surface temperatures, which could be an effective proxy for direct soil moisture measurements.
“Soil moisture is linked to the land surface temperature,” Entin said. “The more water that’s in the soil, the more that the sun’s energy is used to evaporate that water, which means that the temperatures in that place are going to be cooler. If the soil’s really dry, all the sun’s energy just goes into heating up the air near the ground, so those temperatures tend to be warmer.”
The Moderate Resolution Imaging Spectroradiometers on Terra and Aqua can measure land surface temperatures, as can the Visible Infrared Imaging Radiometer Suite aboard Suomi NPP, Entin said.
Meanwhile, NASA has yet to determine why SMAP’s radar failed. Engineers think something went wrong with the low-voltage power supply to the radar’s high-power amplifier. NASA Headquarters and the Jet Propulsion Laboratory are conducting concurrent failure reviews, “both of which are going to be reporting out Dec. 24, with the coal in the stocking,” Michael Freilich, director of NASA’s Earth Science Division, told members of the agency-chartered NASA Advisory Council Oct. 28.