A cubesat designed by Purdue researchers will soon test a promising new technique for gathering climate data. The SigNals Of Opportunity: P-band Investigation (SNOOPI) mission, spearheaded by Purdue professor Jim Garrison, launched to the International Space Station on March 21, aboard NASA's SpaceX CRS-30. It will soon be released from the ISS into Low Earth Orbit and begin its mission.

As weather patterns shift globally, having the data to predict floods and droughts is an increasingly important challenge. This data is important not only for early flood and drought warnings, but also crop-yield forecasts and accurate climate modeling that are not possible through current technology.

Garrison expects SNOOPI will show whether "signals of opportunity" (SoOp) can be effective alternatives to radar systems that transmit and read back their own signal. 

This proof-of-concept mission follows from decades of Garrison’s work. He performed some of the earliest research on signals of opportunity. He was also on the science team for NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission, which used this technique to measure wind speed over oceans. Garrison was also elevated to fellow of the Institute of Electrical and Electronics Engineers (EEE) for his contributions to SoOp field, and his students have earned prestigious research grants for SoOp work. 

Measuring in the root zone

SNOOPI is designed to monitor signals in P-band, around 300 MHz, which are used for basic satellite communications. The key advantage of this method is that these frequencies 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."

But there are strict regulations controlling who can transmit on those frequencies. Plus, a satellite powerful enough to do this job on its own would need a very large antenna, making for an expensive launch. The European Space Agency’s Biomass mission is estimated to cost €400 million.

SNOOPI’s method will only listen on these frequencies, meaning the team can design a much smaller satellite.

"It turns out that there’s some powerful communication satellites that are operating in P-band frequencies. So, we came up with the idea of capturing these existing P-band signals to show that we can make subsurface soil measurements. Instead of transmitting its own radio signals toward Earth and analyzing the returned signal, SNOOPI will take advantage of already-available telecommunications signals. 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,” Garrison says.

“This way, we get the source for free.”

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. This ability can provide critical information for increasingly shifting world when it comes to weather, rainfall, and climate-smart agriculture. By better understanding where water is, we can predict where it might go and how it can help various stakeholders be more climate resilient.

Garrison has worked with other Purdue faculty to pursue how SNOOPI may be able to help provide needed information, and he served as a research lead for Purdue’s Center for the Environment in remote sensing and on the advisory board for Purdue’s Climate Change Research Center (now Purdue’s Institute for a Sustainable Future). SNOOPI is an ESTO-funded collaboration that includes Purdue University, NASA Goddard Space Flight Center, NASA Jet Propulsion Laboratory and Mississippi State University.

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. Once SNOOPI is released into low-Earth orbit and commissioned, Garrison’s research team will have a 9-month demonstration cycle to collect and analyze data. NASA’s network of ground stations will be used to validate their readings and prove out this use of SoOp.

Garrison and his students visited NASA’s Goddard Space Flight Center over summer 2022 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.  "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," Garrison says.