NASA not only peers up to gather information about space; it also peers down to gather information about Earth. As part of the Science Mission Directorate, NASA’s Earth Science Program aims to improve predictions about climate, weather, and natural hazards by understanding Earth’s response to natural and human-induced changes. One way scientists are tracking these changes is by monitoring the Earth’s soil moisture and ocean salinity.
As part of the water cycle, soil moisture and ocean salinity are interconnected with Earth’s energy and biogeochemical cycles (carbon, nitrogen, and others). Together, these natural cycles play a role in moderating the overall environment. For example, ice can melt into the ocean to dilute ocean salinity, which affects ocean circulation, which affects an area’s climate.
In 2015, NASA plans to embark on a mission called Soil Moisture Active and Passive (SMAP) to provide global measurements of soil moisture and its freeze and thaw states. Measurements from the SMAP spacecraft will lead to a better understanding of the processes that link the water, energy, and carbon cycles, and will significantly inform Earth system science, water resource assessment, and natural hazards mitigation.
The best way to measure surface conditions such as soil moisture and ocean salinity is through microwave remote sensing at long wavelengths, such as at L-band. However, receiving long wavelengths from Earth requires large antennas in space. To prepare for microwave remote sensing at long wavelengths, NASA has supported the development of smaller, lighter, more compact devices to uncover fluctuations in soil moisture and ocean salinity.
As a result of Small Business Innovation Research (SBIR) funding from Goddard Space Flight Center in 2000, ProSensing Inc., of Amherst, Massachusetts, developed a compact, ultrastable radiometer for sea surface salinity and soil moisture mapping. Taking advantage of the rapid advances in telecommunications at the time, ProSensing incorporated small, low-cost, high-performance elements into just a few circuit boards. By using two or more radiometers (called arraying) and combining the signals they receive, the radiometers act as small pieces of one large system.
“It was important to make them small enough to hold in the palm of your hand. Previous radiometers were about 10 times the size. If we built radiometers the old way, it would take a lot more time, money, and effort. By using cell phone technology, it cost 50-percent less and gave us a competitive advantage,” says James Mead, president of ProSensing.
By 2005, ProSensing had delivered 35 units to Goddard for use in a research instrument called 2D-STAR (Two Dimensional Steered Thin Array Radiometer). Flown on an aircraft, 2D-STAR was developed to demonstrate interferometric technology (arraying radiometers to act as one large radiometer) for remote sensing of soil moisture.
David M. Le Vine, a scientist at Goddard Space Flight Center, says, “2D-STAR achieves the results of a large antenna without putting a massive structure in space. We were able to show the technology could work from airplanes, with the idea of going from airplanes to space.”
During its work with Goddard, ProSensing received a request from the University of Melbourne, Australia, to build six ultrastable radiometer modules for airborne remote sensing at a new facility being developed through the support of the Australian Research Council, University of Melbourne, University of Newcastle, James Cook University, and Airborne Research Australia at Flinders University. By 2006, ProSensing had built and delivered a polarimetric L-band microwave radiometer (PLMR) using the radiometer modules.