If all went according to plan, on 31 March six microsatellites were carried into space stacked aboard a single U.S. Air Force Minotaur rocket. Each satellite is equipped with a radio receiver designed at the California Institute of Technology's Jet Propulsion Laboratory, in Pasadena, which will pick up signals from the U.S. Global Positioning System (GPS). Together, the six satellites make up a system called COSMIC, for Constellation Observing Systems for Meteorology, Ionosphere & Climate, whose mission is to take measurements of temperature and humidity in the atmosphere by means of a technique called radio occultation.

Conceived by Richard Anthes, president of the University Corporation for Atmospheric Research (UCAR) in Boulder, Colo., and several colleagues in Taiwan and the United States, the project is unusual and unprecedented in that the six satellites were assembled in Taiwan and largely paid for by the government of Taiwan. The system's method of sounding the atmosphere's temperature, pressure, and moisture and using the data for weather prediction is also without precedent. "It is the first mission to demonstrate the use of GPS radio occultation soundings for weather prediction in near real time," says Anthes.

The radio occultation technique works like this: each of the six COSMIC satellites in low-earth orbits will retrieve radio signals from the 28 civilian GPS satellites in higher orbits [see diagram, "Occultation"]. As the COSMIC and GPS satellites rise above the horizon or set relative to each other, the receivers will measure the changes in frequency of the radio000 signals as the signals pass through different layers and densities of Earth's atmosphere. From these frequency measurements, it is possible to compute the refraction angles of the radio waves, and from those bending angles, vertical profiles of the atmosphere's temperature, pressure, and humidity can be extracted.

Each of the COSMIC microsatellites is only 1 meter in diameter and weighs less than 70 kilograms. Once they are in space, the satellites will slowly drift apart over the course of 13 months; through a series of adjustments using four small thrusters, they will be placed at equal distances from each other, at an 800-kilometer altitude, with an inclination of 72 degrees relative to Earth's surface.

Traditional weather satellites rely on radiometers to take readings of both infrared and microwave radiation in the atmosphere. But as these satellites look down, they cannot distinguish clearly between thin atmospheric layers. What is more, traditional radiometers cannot see through clouds and are also subjected to various kinds of radiation from the sun and other parts of the universe, which can cause "instrument drift" to develop, affecting accuracy and sensitivity.