COSMIC/GPS signal graph

COSMIC's low-Earth-orbiting (LEO) satellites intercept GPS radio signals to measure their bend and signal delay as they pass through the atmosphere.

Illustration courtesy Broad Reach Engineering


One of the more interesting aspects of COSMIC is the way it uses traditional GPS signals that already exist to gather information on atmospheric conditions from around 0.6 miles (1 kilometer) above the ground and higher [source: Schreiner]. Using its Radio Occultation (RO) receiver, the satellite detects a GPS signal as it starts to pass through Earth's atmosphere. Because the COSMIC satellite knows exactly where the GPS satellite really is, it can take the distortion, or refraction, caused by the atmosphere to calculate the temperature, air pressure, humidity and even electron density over a specific spot on the ground.

Each observation using this data results in a "vertical profile" over a specific spot on the ground. These observations are made up to 2,500 times per day, which over time produces a detailed three-dimensional picture of the atmosphere.

COSMIC's onboard Tiny Ionospheric Photometer (TIP) is mapping Earth's ionosphere with more precision than was available previously. It might be tiny, but it also allows continuous observation of the ionosphere at the far ultraviolet 135.6-nanometer wavelength.

Whereas the RO receiver is providing data of a vertical nature (measuring the atmosphere from the ground up three-dimensionally), the TIP instrument is mapping the ionosphere in a horizontal manner, or two-dimensionally [source: Dymond]. The TIP works only at night due to interference caused by solar ultraviolet radiation [source: Anthes].

Also mapping the ionosphere, but providing both horizontal and vertical data, is the Tri-Band Beacon (TBB). The TBB works by emitting a signal directly down from the satellite toward receiving stations, thereby determining the electron density of the ionosphere. A limited number of receiving stations have been set up along the north-south axis of the polar orbit in East Asia and North and South America [source: Anthes].

Working in conjunction with the receiving stations it passes over, and using electron density data from the other two instruments on board, the TBB provides a detailed 3-D model of the ionosphere [sources: Dymond, Bernhardt].

The six RO receivers collect up to 2,500 observations per day when all satellites are operational [source: COSMIC Web site]. The TIP and TBB are scanning constantly and provide continuous coverage.

On the next page, we will look at some of the ways in which the data collected by COSMIC is being used today, and what the future of this program might be.