A schematic diagram of GPS meteorology: showing GPS signals passing from GPS satellites, orbiting at heights of ~20,000 km, through the intervening Earth's atmosphere to Earth-based receivers and to a low earth orbit (LEO) satellite. The delay of signals arriving at Earth-based receivers is proportional to the intervening integrated water vapor. Signals propagating from the GPS satellite to the LEO satellite, at a height of about 775 km, are refracted by the Earth's atmosphere during the ~60 second period before the signal is occulted by the Earth, providing a refractivity profile of the atmosphere.