Atmosphere of Earth they penetrate more deeply. The various layers of Earth's ionosphere, important to HF radio propagation, begin below 100 km and extend beyond 500 km. By comparison, the International Space Station and Space Shuttle typically orbit at 350–400 km, within the F-layer of the ionosphere where they encounter enough atmospheric drag to require reboosts every few months. Depending on solar activity, satellites can experience noticeable atmospheric drag at altitudes as high as 700–800 km. The division of the atmosphere into layers mostly by reference to temperature is discussed above. Temperature decreases with altitude starting at sea level, but variations in this
International Union of Radio Science Geophysical Union) as well as the "Journal of Atmospheric and Solar-Terrestrial Physics". URSI's original objective (to encourage "scientific studies of radiotelegraphy, especially those which require international cooperation") has been broadened to include all radio science, from telecommunications to radio astronomy, acquisition of radar information about distant passive objects, studies of the radiation stimulated or spontaneously emitted by these objects, biological effects of electromagnetic radiation and active modification of objects by radio waves, within the spectrum from extremely low frequency to the optical domain. A few Commissions are engaged with international projects in cooperation with other international bodies, for example with