Altimeter

An altimeter is an instrument used to measure the altitude of an object relative to a fixed reference level, most commonly mean sea level or ground level. The science of measuring altitude is known as altimetry, a term conceptually related to bathymetry, which concerns the measurement of underwater depth. Altimeters are essential in aviation, surveying, mountaineering, meteorology and various scientific and navigational applications, providing height data necessary for safety, navigation and environmental study.
Altimeters operate using different physical principles, ranging from atmospheric pressure to reflected sound, radio waves, lasers and satellite signals. Each method has characteristic advantages, limitations and typical applications.

Pressure altimeter

The traditional altimeter used in aviation is the pressure altimeter, which operates on the principle that atmospheric pressure decreases predictably with altitude. The instrument senses static air pressure from the aircraft’s pitot–static system and converts it into an altitude reading based on the International Standard Atmosphere (ISA) model.
A pressure altimeter includes a Kollsman window, in which the pilot sets the current reference pressure (for example, 29.92 inHg or 1013 hPa). Adjusting this reference allows the instrument to display accurate altitude above mean sea level according to local atmospheric conditions. Pressure altimeters are widely used because they are lightweight, reliable and require no external signals. However, their accuracy is affected by changes in temperature, non-standard pressure conditions and rapid weather variations.

Sonic altimeter

Sonic, or ultrasonic, altimeters measure altitude by emitting high-frequency sound waves towards the surface and timing their return. One of the earliest experimental systems was tested jointly by the United States Army Air Corps and General Electric in 1931. This device was intended to provide greater reliability in conditions of heavy fog or rain, situations in which pressure altimeters offered reduced situational precision. The sonic altimeter worked by mimicking natural echolocation: pulses of sound emitted from the aircraft were reflected from the ground and converted into altitude readings displayed in the cockpit.
While these systems improved low-altitude accuracy, limitations included sensitivity to atmospheric conditions, variations in air density and interference from environmental noise. Sonic altimeters were largely superseded by more advanced radar and laser systems but remain conceptually important in the evolution of altimetry.

Radar altimeter

The radar altimeter (or radio altimeter) measures altitude by determining the time taken for radio waves to travel from an aircraft to the terrain and back. This form of active sensing provides a direct measurement of height above ground level (AGL) rather than height above sea level, making it vital during low-altitude manoeuvres, landing operations and approach procedures.
Two principal radar techniques are used:

• Pulsed radar, which sends discrete bursts of radio energy and measures the return time.
• Frequency-Modulated Continuous-Wave (FMCW) radar, which transmits a continuously varying signal and measures the frequency shift of the return. FMCW systems are capable of significantly higher accuracy for similar equipment cost and have become the industry standard.

Radar altimeters form a core part of terrain avoidance warning systems, issuing alerts when an aircraft approaches rising ground or descends below safe margins. In military aviation, radar altimeters underpin terrain-following radar, enabling low-level flight for stealth and ground-hugging manoeuvres. Research has shown that phase radio-altimeters are particularly suitable for ground-effect vehicles, offering robust performance compared with ultrasonic or laser methods.

Laser altimeter

Laser altimeters operate using Lidar (Light Detection and Ranging) technology. They emit focused laser pulses towards the surface and measure the time-of-flight of the reflected light. The extremely short wavelength of laser light allows exceptionally high resolution and precise surface mapping.
Laser altimetry has become critical in modern aerospace and planetary exploration. A prominent application is the Mars Ingenuity helicopter, which uses a downward-facing laser altimeter to maintain stable flight over varying Martian terrain. Laser altimeters are also widely used in topographic mapping, forestry, autonomous vehicles and robotic navigation.

Satellite and GPS-based altimeters

Global Positioning System (GPS) receivers can determine altitude by trilateration using signals from at least four satellites. While GPS provides useful height information for many consumer and recreational activities, its stand-alone accuracy is insufficient for aviation as a substitute for pressure altimetry without GNSS augmentation systems. GPS-derived altitude is sensitive to satellite geometry and signal quality; errors of several tens of metres are common in hiking, climbing and handheld navigation.
Satellite missions equipped with altimeters, such as Jason-2 and those involved in ocean surface topography, measure sea surface height to improve understanding of ocean circulation, climate change and global sea-level variations. Spaceborne radar or laser altimeters offer global coverage and enable long-term environmental monitoring.

Additional instruments related to altitude measurement

In aviation, several instruments complement the altimeter:

• A rate-of-climb indicator (or vertical speed indicator) measures the rate at which altitude changes, assisting pilots in maintaining stable ascent and descent.
• Integrated systems combine pressure, radar and GPS data to provide redundancy and reduce error margins.
• Altimeter settings and procedures are central to flight safety; misreading or malfunctioning altimeters have contributed to past aircraft accidents, underscoring the importance of correct calibration and instrument cross-checking.