The CYGNSS spacecraft will use a technique called "Delay Doppler Mapping". Each satellite will be equipped with a Delay Doppler Mapping Instrument (DDMI), which is capable of receiving four DDM's at once. DDM is only slightly different from standard Radar Altimetry, which measures the distance to an object by tracking how long it takes a signal bounced off the surface to return. That is the 'Delay' part of DDM. When using DDM, you 'look' at the surface of the object for much longer, allowing you to integrate over the full time. This allows the signals to be weaker, because they will be integrated over a longer amount of time. In addition, DDM differs from radar because it uses the Doppler effect to track how fast (and in what direction) something is moving. DDM can be used to calculate how fast the wind is moving at the surface (1), which is one of the main goals of the study. CYGNSS will be using special (and highly complex) algorithms (2) to retrieve the wind surface speed.
In the case of CYGNSS, the signals will originate from orbiting GPS satellites. The satellites will also beam the signal directly to CYGNSS, so that they can compare the reflected response to the emitted signal. This idea was proposed back in 1993 (3). Since CYGNSS consists of eight smallsats, each capable of measuring 4 areas, the mission will be able to reconstruct the shape of the ocean surface and windspeeds there over a significant area.
From the shape of the ocean surface, researchers will be able to infer how rough the ocean is at that particular location. While there have been attempts to create a standard parameter for roughness, there currently is not one in use (as far as I could find). Instead, researchers will rely on factors such as the wave height, spacing, speed, and steepness to determine how rough the surface is. Higher and more closely spaced waves (thus steeper waves) indicate a rougher sea than shallow, long waves.
Emily Lakdawalla does a good job of explaining how DDM is used to get the shapes of asteroids on her blog.
- Chen Li, Weimin Huang. "Sea surface wind retrieval from GNSS delay-Doppler map using two-dimension least-squares fitting" (2013). DOI: 10.1109/OCEANS-Bergen.2013.6608019
- Maria Paola Clarizia, Christopher S. Ruf. "Wind Speed Retrieval Algorithm for the Cyclone Global Navigation Satellite System (CYGNSS) Mission" (2016). DOI: 10.1109/TGRS.2016.2541343
- Daniel Pascual, et. al. "Precision Bounds in GNSS-R Ocean Altimetry" (2014). DOI: 10.1109/JSTARS.2014.2303251