Radar systems often assembled with synchronized transmitter and receiver and often send radio wave and detect their reflections from objects, surfaces or sub-surface structures. They are largely used for aviation, navigation, space and meteorological applications. Commercial airliners rigged with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. Planes can land in fog at airports equipped with radar-assisted ground-controlled approach (GCA) systems, in which the plane's flight is observed on radar screens while operator's radio landing directions to the pilot. They can also measure distances and map geographical areas and navigate and fix positions at sea. Meteorologists use radar to monitor precipitation forecasting short-term weather conditions and also used to watch severe weather such as thunderstorms and tornado. Radar can be used to study the planets and the solar ionosphere and to trace solar flares and other moving particles in outer space. Systems like radar tracking and surveillance systems are used for scientific study and defense.