The effect of refractivity variations in the atmosphere is a known problem to spacegeodetic observations such as SLR, VLBI, radar altimetry, and GPS. Nevertheless, its influence on radar interferometry has been ignored for several years, largely due to the assumptions of the all-weather capability of radar, which has been one of the main arguments for using radar instead of optical remote sensing from the beginning.
A specific class of radar systems are the imaging radars, such as Side-Looking (Airborne) Radar (SLR or SLAR) and later Synthetic Aperture Radar (SAR). The side-looking geometry of a radar mounted on an aircraft or satellite provided range sensitivity, while avoiding ambiguous reflections.
Very-long-baseline interferometry (VLBI) is a type of astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescopes on Earth or in space. The distance between the radio telescopes is then calculated using the time difference between the arrivals of the radio signal at different telescopes.
A radar altimeter (RA), radio altimeter (RALT), electronic altimeter, or reflection altimeter measures altitude above the terrain presently beneath an aircraft or spacecraft by timing how long it takes a beam of radio waves to travel to ground, reflect, and return to the craft.
The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force.