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Solar radio waves could help monitor glacier thickness

28 Jul 2021
Photo of instruments sitting on an ice sheet with the Sun bright in a blue sky above
Solar signal Store Glacier, Greenland, where researchers showed that a battery-powered radio receiver with an antenna placed on the ice can use the Sun's radio waves to measure the glacier's thickness. (Courtesy: Sean Peters)

Radio signals from the Sun could be used to monitor changes to ice sheets, researchers from the US have demonstrated. This new, passive radar system could offer a cheaper, lower power and more easily scalable method to gather long-term data on the melting of ice sheets and glaciers due to climate change, say the researchers.

The melting of land ice is one of the principal drivers of sea level rise, threatening coastal and low-lying communities around the globe. At present, the main way of measuring the extent of these ice sheets is to make ice-penetrating radar measurements from aircraft, using an active system to transmit a radar signal down through the ice sheet and measuring the radio waves reflected back. Airborne radar has its limitations, however, being both resource-intensive and able to provide only a snapshot of ice conditions at the time of the flight.

Low-energy monitoring

The alternative, proposed by radar engineer Sean Peters of the Massachusetts Institute of Technology (MIT) Lincoln Laboratory in the US and colleagues, instead uses naturally-occurring radio waves emitted by the Sun. The team’s sensors isolate a snippet of solar radiation in the 200–400 MHz band, then listen for that same signature in the echo created when the Sun’s radio waves bounce off the base of the underlying ice sheet – taking advantage of the randomness of the Sun’s emissions, which are, the team explain, somewhat like a song that never repeats. As with airborne radar, the delay between the original signal and the echo can then be used to calculate the distance between the surface sensor and the base.

Without the need to transmit its own signal, the team’s system is considerably less energy-intensive than its active counterparts. It could even run off batteries, potentially facilitating the creation of practical, continually-operating ice monitoring networks if the hardware can be suitably miniaturized, the team say.

“Our goal is to chart a course for the development of low-resource sensor networks that can monitor subsurface conditions on a really wide scale,” explains Peters, who conducted research for the study during his time as a graduate student at Stanford University in California, US. Such a network, he added, “could be challenging with active sensors, but this passive technique gives us the opportunity to really take advantage of low-resource implementations”.

Glacier tests

The researchers tested their approach on Store Glacier in western Greenland. There, the prototype sensor recorded an echo delay time of around 11 microseconds, which equates to an ice thickness of about 900 m — the same value recorded by ground-based and airborne active radar systems.

The concept does have limitations. One is that the level of solar radiation is relatively low, especially at the Earth’s poles. However, Hugh Griffiths, an electrical engineer from University College London, UK, who was not involved in the research, says that integrating the signal for long periods should help overcome this barrier. “The solar radiation is broadband, and in polar regions the likelihood of any interference is practically zero,” Griffiths adds.

Another drawback is that, just as active airborne systems only work during a fly-over, the passive sensor concept only works when the Sun is positioned above the horizon. The team is presently exploring whether it might be possible to harness other naturally-occurring or human-made radio sources to overcome this restriction.

Icy moon measurements

Terrestrial ice monitoring is not the only potential application for this passive radar system. The concept was originally conceived by team member and astronomer Andrew Romero-Wolf of NASA’s Jet Propulsion Laboratory as a means of probing Jupiter’s icy moons. In that application, the need for a passive system arose after it became clear that radio waves from Jupiter itself would interfere with active radar systems aboard a spacecraft — but also that the very same waves could be used to solve the problem they created.

“Monitoring ice sheets under climate change and exploring icy moons at the outer planets are both extremely low-resource environments where you really need to design elegant sensors that don’t require a lot of power,” explains team member and Stanford geophysicist Dustin Schroeder.

Electrical engineer Chris Allen of the University of Kansas, US, calls the design a “significant accomplishment that will spark other innovations in passive radar applications”. Allen, who was not involved in the work, adds: “There is no doubt that with the advent of future generations of software-defined radar technologies, the realizable spatial, spectral, and temporal resolutions will likewise improve.”

The study is described in Geophysical Research Letters.

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