Weather Forecasters Adopt NASA’s ‘Occult’ Science

Halfway through 2020, there was almost no aspect of modern life that hadn’t been affected by the COVID-19 pandemic, and weather forecasts were no exception. Weather agencies rely in part on data from atmospheric sensors on commercial aircraft, most of which were grounded. So Vienna, Virginia-based Spire Global Inc. offered weather agencies atmospheric data its constellation of small satellites was collecting by a technique, relatively new to weather forecasters, called radio occultation. The European Centre for Medium-Range Weather Forecasts accepted the offer.

“They assimilated it into their models and found that our radio occultation data was able to make up for the lack of in-situ data from airplanes,” said Vu Nguyen, radio occultation orbit scientist at Spire.

This method of atmospheric sounding wasn’t entirely new to weather scientists, though not much radio occultation data had been available previously. But it was already ancient history to researchers at NASA’s Jet Propulsion Laboratory in Southern California, where the technique was pioneered and first used to gather data on the Martian atmosphere from the Mariner spacecraft in the 1960s.

Only now, however, is the technique poised to improve accuracy and reduce costs of atmospheric modeling for weather forecasts, with further implications for other Earth sciences. Accurate weather prediction helps protect lives, health, and property and has significant benefits for industries such as agriculture, energy, construction, and transportation. Weather and atmospheric data is also foundational to understanding and predicting long-term trends in climate change.

Radio occultation is the observation of changes to a radio signal from a spacecraft as it passes behind a planetary body. As the signal’s path to a distant observer moves closer to the planetary surface, it passes through – or is “occulted” by – the planet’s atmosphere, which bends the signal. The degree of this refraction, as well as other effects on the signal, reveal information about the atmosphere’s temperature, pressure, and moisture.

After sounding the Martian atmosphere, JPL scientists used the technique to characterize the atmospheres of the rest of the solar system’s planets and a few of their moons. But observing Earth with radio occultation would be trickier. For one thing, both the transmitter and receiver have to be off the planet being observed. To take readings from other planets, the receiver could sit on Earth. But to observe our own atmosphere, the receiver would have to be in space, and receivers of the day were large and heavy. A bigger challenge was that, for the data to add to existing knowledge of Earth’s atmosphere, a large number of signals and receivers would be needed.