Winds contribute to motion in the ocean on every scale, from individual waves to currents extending thousands of miles. They affect local weather as well as large-scale, long-term climate patterns such as El Niño. Across the tropical Pacific, winds help or hinder local economies by allowing nutrient-rich water to well up from the ocean depths, nourishing marine life to the benefit of coastal fisheries, or blocking its upwelling.

In a few months, NASA will send an ocean wind-monitoring instrument to a berth on the International Space Station. That unique vantage point will give ISS-RapidScat, short for the International Space Station Rapid Scatterometer, the ability to observe daily (also called diurnal) cycles of wind created by solar heat. 

A scatterometer is a type of radar that bounces microwaves off Earth's surface and measures the strength and direction of return signals. The more uneven the surface, the stronger the return signals. On the ocean, higher winds create larger waves and therefore stronger return signals. The return signal also tells scientists the direction of the wind, because waves line up in the direction the wind is blowing.

The reason spaceborne scatterometers haven't helped much with the specific question of daily wind cycles has to do with their orbits. All modern instruments have been in sun-synchronous orbits, in which a satellite is always oriented at the same angle relative to the sun. In this type of orbit, a satellite passes over every location at the same fixed times, for example, 6 a.m. and 6 p.m. over the equator. The resulting data can't throw much light on the question of how winds develop over the course of a day.

In its berth on the space station, the two-year RapidScat mission, built and managed by NASA's Jet Propulsion Laboratory, Pasadena, California, will be the first modern spaceborne scatterometer not locked in a sun-synchronous orbit. Each time the space station passes over a spot on Earth, it's at a different time of day than on the previous visit.