When a small satellite is released from the International Space Station (ISS) in early 2023, it is expected to prove out a promising new technique for measuring soil moisture and snow water levels from space. This data is important for early flood and drought warnings, crop-yield forecasts and accurate climate modeling — but it’s not currently possible to get precise global measurements with current technology, according to NASA’s Earth Science Technology Office (ESTO).

Jim Garrison, professor of aeronautics and astronautics and principal investigator on the SigNals Of Opportunity: P-band Investigation (SNoOPI) mission, expects that this technology demonstration will enable future missions to fill that important gap in climate research. It will show whether "signals of opportunity" (SoOp) can be effective alternatives to radar systems that transmit and read back their own signal.

Garrison is the first Purdue professor to serve as principal investigator on a NASA mission. SNoOPI is an ESTO-funded collaboration that includes Purdue University, NASA Goddard Space Flight Center, NASA Jet Propulsion Laboratory and Mississippi State University.

Proving out this method follows from decades of Garrison’s work. He performed some of the earliest theoretical and experimental research in the SoOp field, and he was on the science team for NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission, which used a similar technique to measure wind speed over oceans.

What makes SNoOPI unique is that it will listen on lower frequencies than CYGNSS, which will allow it to take moisture readings deeper in the soil than before.

Measuring in the root zone

SNoOPI is designed to monitor signals in P-band, around 300 MHz, which are used for satellite communications. This poses a key advantage over prior moisture-measurement methods.

"While NASA’s Soil Moisture Active Passive (SMAP) mission and CYGNSS currently gather soil moisture data, they use the higher-frequency L-band (1–2 GHz), which is sensitive to water only in the top 5 centimeters of soil on the surface of the Earth. SMAP can’t gather moisture readings at the root zone, and has trouble measuring soil moisture in more problematic terrain, such as forested and mountainous areas."

Garrison says wavelength is roughly proportional to a signal’s ability to penetrate Earth’s surface. With the P-band being approximately five times longer in wavelength, they can reach five times deeper into soil and snow.

"This allows a direct measurement of the moisture contained within the root zone, the layer of soil in which most plant roots exist to absorb the water," Garrison says. "Monitoring of this region provides an important connection between water contained within the soil and that in the atmosphere."

Although P-band measurements using conventional radar have been demonstrated using aircraft-mounted instruments, bringing those systems to space is not easy. Transmitting and receiving P-band signals from low Earth orbit requires an antenna 10 to 30 meters in diameter. Garrison says a device that large would push the mission well outside the cubesat standard, requiring a dedicated launch and driving costs into the multi-hundred million dollar range.

The European Space Agency (ESA) is attempting to do this with the Biomass mission, which has a 10-meter antenna, but it faces another problem. Because many P-band frequencies are either already allocated for telecommunications or are restricted due to potential interference with defense radar, Biomass will be prohibited from operating over North America or Europe.

The flip side is that there’s already a lot of P-band signal lighting up the Earth, just waiting to be seen.

"It turns out that there’s some powerful communication satellites that are operating these frequencies. So we came up with the idea of capturing these P-band signals to show that we can, initially, make the subsurface soil measurement," Garrison says.

Using reflective signals that already exist means it’s possible to design a small instrument, and build a satellite with low power demands. This was demonstrated on CYGNSS, a constellation of eight micro-satellites listening in on L-band GPS frequencies. SNoOPI is the size of a 6U cubesat — approximately 10 by 23 by 37 centimeters, or a modest stack of textbooks.

"Instead of generating and transmitting its own radio signals toward Earth and analyzing the returned signal, SNoOPI will take advantage of already-available telecommunications signals. This way, we get the source for free," Garrison says. "We don’t need to provide a power source for the transmitter, obtain a license or be as concerned about interference from other users in the band."

If SNoOPI succeeds, other missions could use its technology to globally monitor how much water is stored below the surface of the soil and in the snow pack. In the future, signals of opportunity could predict droughts and floods, assist with forecasting agricultural yields and even monitor trends in climate change.

Early Indicators and Mission Plan

Although P-band SoOp is promising, this new technique must be tested and proven in space before NASA will implement it in a science mission. SNoOPI is expected be released into low Earth orbit from the ISS in early 2023.

After a commissioning process, the data team will begin receiving usable transmissions for SNoOPI’s 9-month demonstration cycle. Data from a network of ground stations supporting SMAP will be used to validate SNoOPI’s readings.

Garrison and his students visited NASA’s Goddard Space Flight Center this summer for SNoOPI’s final open-sky test before it was packed up for launch. This successful test confirmed that its instrument could read P-band signals beamed down from space; whether it will effectively read signals reflected from Earth won’t be known until it’s in orbit.

The team was able to get an early look at what could be expected from these signals with help from Spire Global, a satellite services company. "They had a satellite that was reaching its end-of-life. Spire was able to reprogram the software radio for us," Garrison says. "We received about 10 seconds of data, and it was enough to show us that it was possible to read the P-band reflecting back from Earth."