Using the Sun to monitor Earth’s melting ice sheets

Ice-penetrating radars transmit a powerful electromagnetic pulse and record the reflected echo’s delay time to measure the thickness of an ice sheet. But those measurements, which are typically taken from a plane flying overhead, are expensive when performed over multiple years and on spatial scales greater than a few kilometers. They are also informative about ice conditions only at the time of the flight. A team of glaciologists and electrical engineers led by Sean Peters, a recent Stanford University graduate now on the technical staff of MIT’s Lincoln Laboratory, has developed a novel, inexpensive, passive radar-sounding technique that replaces active radio pulses with the Sun’s own radio signals.

The theory behind the researchers’ idea was developed decades ago and led to the creation of ambient-noise correlation. Most commonly applied to passive seismic imaging, the technique remotely senses pressure fluctuations from, say, diffuse waves in the atmosphere or ocean waves crashing on a shoreline. The planet’s elastic response is determined by applying a simple processing step to the data—cross-correlation, the statistical measure of the waveforms’ similarities at different points in space as a function of the time lag applied to one of them (see the article by Roel Snieder and Kees Wapenaar, Physics Today, September 2010, page 44 ). The requirements for the team’s system are modest: a battery-powered radio receiver, electronic filters, low-noise amplifiers, and a radio antenna placed directly on the ice sheet, as seen in the image. The amplifiers and filters circumvent the main problem with using the Sun as a radio source. Its power level in the 200–400 MHz frequency band is well below that of galactic background noise and the transmission power of FM and television stations.

The receiver sits on the ice sheet’s surface and records the Sun’s direct path and its reflected path, which is delayed and attenuated as the reflected beam propagates through the ice. Treating the Sun’s signal as white noise, the cross-correlation technique extracts two distinct peaks, the Sun’s direct signal and its reflected, attenuated echo. The delay time between those peaks can then be converted to an ice thickness. Ice conditions can be monitored daily for hours, many times a year, and potentially over the spatial scale of a continent. To demonstrate the proof of concept, Peters and his colleagues measured the ice thickness on Store Glacier, Greenland. After filtering the RF interference, they measured an echo delay time of 10.8 µs. That time corresponds to an ice thickness of 1008 meters—reassuringly close to the value measured by active radar sounding systems. (S. T. Peters et al., Geophys. Res. Lett. 48, e2021GL092450, 2021 .)