Whether it’s a parched field or a boggy marsh, the ground naturally emits microwave energy. Not much energy – but enough that NASA’s newest, more technologically advanced radiometer instrument can detect it from space, allowing scientists to study how much water is in the soil.
Soil moisture is an important measurement for weather forecasting, drought and flood predictions, agriculture, and more. All types of soil emit microwave radiation, but the amount of water changes how much of this energy is emitted. The drier the soil, the more microwave energy; the wetter the soil, the less energy. Radiometers measure this radiation, and scientists use the data to calculate water content.
This week a new radiometer launches into orbit aboard the Soil Moisture Active Passive (SMAP) satellite. SMAP carries two instruments to measure how much water is in the soil. In addition to the radiometer, which detects naturally emitted energy, a microwave radar will send a signal to the ground that will bounce back to the satellite with information after it encounters and interacts with the soil. To collect signals from the surface for both the radiometer and radar, SMAP has a 20-foot-wide mesh antenna that rotates 14 times per minute – the largest such spinning antenna in space. A receiver then interprets both sets of signals.
"The receiver is very, very sensitive," said Jeff Piepmeier, radiometer instrument scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "If we could put a cell phone on the moon, working at the same frequency as the receiver, we could see the thing turn on and off."
Similar to how night-vision goggles detect infrared wavelengths from heat, the radiometer measures the "brightness temperature" of the soil, a combination of surface temperature and soil moisture. By factoring out the temperature, determined by computer models, scientists can calculate the wetness of the soil. Scientists have worked for decades on interpreting radiometer data in terms of soil moisture, and, the results are very accurate.
[image-69][image-96]But, SMAP’s radiometer takes measurements over relatively large areas, with a 25-mile resolution. To make that measurement accurate on the scale of agriculture practices, scientists will integrate data from SMAP’s radar instrument, developed and built at NASA’s Jet Propulsion Laboratory in Pasadena, California. The radar, which sends out a signal and listens for the return, has a resolution of about 0.6 to 1.9 miles. But because the radar is more sensitive to vegetation and other features on top of the soil, it’s not as accurate as the radiometer.
So the two instruments complement each other: the radiometer provides an accurate measurement of a large block of land, while the radar provides finer detail of the soil moisture in smaller parcels.
"Combine the two together, use the best of both, and you come up with a pretty accurate soil moisture product at a spatial resolution of 6 miles," said Peggy O’Neill, SMAP deputy project scientist.
The high quality signal detected by the radiometer also comes with noise. Radio-frequency interference, or RFI, is what happens when technology, like air traffic control radars or closed circuit televisions, broadcast at the same or neighboring frequencies. The noise bleeds over into the frequency that the radiometer is tuned into, corrupting the data. It’s like on radio, when snippets from a music radio station break into a news radio broadcast just a few clicks down the dial.
SMAP operates at a frequency of 1.4 gigahertz, which is set aside for scientific instruments listening in on Earth and space. But earlier instruments listening in to that frequency have run into radio-frequency interference signals. To quiet that din, SMAP’s radiometer has new anti-RFI enhancements, O'Neill said.
"Because we knew RFI was a problem, engineers at Goddard said we’ve got to design a way to detect it, and if possible throw out the bad data and leave enough good data behind," O’Neill said. "It’s a new and unique system."
The technique involves separating data coming from the satellite into different bins, based on sub-frequency and time. If there are outliers, that only appear at one time or a narrow frequency band, computer programs can throw those data out to isolate the natural signals from the soil, which will be more constant and from a wider frequency band.
The Goddard radiometer team worked with scientists and engineers at universities, Piepmeier said, to determine how bad the RFI was, and its potential impact on science measurements.
"We discovered that without this technology, SMAP wouldn’t meet its science requirements," he said.
SMAP will turn on the radiometer approximately 11 days after the satellite launches – even before the mesh antenna is deployed to feed it signals from Earth. Instead, in those first few days, the radiometer will look out into space. The goal is to get calibration measurements to "tune" the instrument. Radio astronomers have detailed maps of the cold microwave temperatures in space, so SMAP will take readings to ensure the radiometer is in agreement and make adjustments if necessary.
As the mission progresses, the satellite will periodically flip over from its normal Earth-looking mode to take calibration measurements pointing to deep space, which will be used to keep the radiometer’s accuracy consistent over time. With these accurate measurements, scientists hope to get a better view of the state of the soil, which could help farmers, emergency managers, weather forecasters and more.