Very low frequency because VLF waves are reflected from the bottom of the ionosphere, while higher frequency shortwave signals are returned to Earth from higher layers in the ionosphere, the F1 and F2 layers, by a refraction process, and spend most of their journey in the ionosphere, so they are much more affected by ionization gradients and turbulence. Therefore, VLF transmissions are very stable and reliable, and are used for long distance communication. Propagation distances of 5000 to 20000 km have been realized. However, atmospheric noise (sferics) is high in the band, including such phenomena as "whistlers", caused by lightning. VLF waves can
Radio occultation amounts to an almost instantaneous depiction of the atmospheric state. The relative position between the GPS satellite and the low-Earth orbit satellite changes over time, allowing for a vertical scanning of successive layers of the atmosphere. GPSRO observations can also be conducted from aircraft. or on high mountaintops. Current missions include: Radio occultation Radio occultation (RO) is a remote sensing technique used for measuring the physical properties of a planetary atmosphere or ring system. Atmospheric radio occultation relies on the detection of a change in a radio signal as it passes through a planet's atmosphere, i.e. as it is occulted