In addition to the primary gravity measurement, each day the GRACE mission will provide several hundred measurements of how the signals from Global Positioning Satellites (GPS) are affected as they travel through the atmosphere. Just as light is refracted or bends as it enters water, the GPS signals are refracted as they pass through the atmosphere. By observing the signals in the atmosphere, profiles of the pressure, temperature, and humidity can be created. These clues help the scientist predict the weather around the Earth. The GPS receivers on the GRACE satellite track refracted signals from the GPS satellites as they rise or set through the Earthís atmosphere. The scientific term for this measurement is known as occultation and will be made throughout the mission life to assist us in predicting global weather.