Astronomers attempt to observe the occultation of stars by asteroids. This allows them to determine the sizes and shapes of asteroids. To observe the events astronomers have to use mobile telescopes placed in the predicted path of the shadow. The Odetics GPStar 325 GPS receiver feeds coordinates to a MicroVAX II computer that then aims the telescope at the asteroid. Timing signals from the GPS receiver are used to discipline a crystal oscillator that is used to time the event.Trueblood, Mark. "In the Shadow of the Asteroid"
GPS World 4, no. 11 (November 1993) :22-30(p)
Radio Occultation techniques allow researchers to make observations about planetary atmospheres. During the Mariner and Voyager missions, spacecraft trajectories were planned so that radio signals from the spacecraft would transect a planets atmosphere. Earth-based receivers were able to detect phase changes in the radio signals based on the refractivity of the atmosphere.By placing a GPS receiver in low earth orbit, hundred occultations of GPS signals should be observed every day. Information obtained in this fashion would include atmospheric refractivity, density, pressure temperature and humidity.
(Some work has been done. More information is available at UCAR. - ed.)
Ware, Randolph. "GPS Sounding of Earth's Atmosphere"
GPS World 3, no. 8 (September 1992) :56-57(p)