An altimeter is a altitude measurement device used by mountaineers, skydivers, pilots, satellites, and many others. The altimeter’s general purpose is to tell its human or machine user how elevated from sea level it is. It performs this function via one of two ways – either by measuring the local air pressure using a barometer, or by bouncing radar off a target.

It has been known for several centuries that a thin column of mercury attached to an open mercury reservoir can be used to measure atmospheric pressure. Tiny changes in atmospheric pressure are indicated by the level of the mercury column. This common instrument is called a barometer.

At altitudes relatively close to sea level, air pressure varies uniformly with altitude. Every 27 feet (8.23 m) one ascends above sea level, air pressure increases by about a millibar, a unit of pressure that can be measured in inches of mercury. Hence, a barometer can be used as an altimeter. Because it is dependent on air pressure varying a uniform amount with increasing elevation, an altimeter using mercury or even a more advanced aneroid barometer is not perfectly accurate, especially at higher altitudes. For high-altitude aerospace applications, and in any domain in which high accuracy is desired, a radar altimeter is used.

A high precision radar altimeter works by measuring the transit time of a radio wave between a target and an airplane or satellite. This allows for high altitude measurements and a degree of accuracy impossible with a barometer-based altimeter. Radar altimeters can also measure the precise heights of natural objects, like mountains or ocean waves.

Radar altimetry has been used to verify the existence of rare “freak waves”, ocean waves several times higher than other waves. They are also used in all types of airplane navigation and make it possible for pilots to land their planes smoothly. The world's tallest peaks have been precisely measured using a radar altimeter.