What Is Altimetry?

Altimetry is the science and technology of measuring altitude, or the height of an object or surface above a reference datum such as mean sea level, the geoid, or the ground directly below. Instruments that perform this measurement are called altimeters, and they operate on principles ranging from barometric pressure differences to radar pulse timing to laser ranging. Altimetry sits at the intersection of geodesy, remote sensing, atmospheric science, and navigation, contributing data to applications as varied as aircraft safety, oceanographic mapping, and the monitoring of ice sheet mass balance. In satellite form, altimetry has become one of the most powerful tools in Earth observation, enabling continuous global measurement of sea surface height with centimeter-level accuracy.

Altimetry draws its foundational methods from physics and signal processing. Radar altimeters exploit the round-trip travel time of a microwave pulse to compute range, while barometric altimeters infer height from the pressure-altitude relationship of the International Standard Atmosphere. Global Navigation Satellite System (GNSS) reflectometry, a newer technique, measures height by analyzing the delay between direct and reflected satellite signals, extending altimetric coverage to surfaces where dedicated sensors are absent.

Radar and Radio Altimetry

Radar altimetry transmits a short microwave pulse toward a surface and measures the two-way travel time of the reflected return. The range is computed as half the travel time multiplied by the speed of light, with corrections applied for atmospheric path delays through the ionosphere and troposphere. Radar altimeters aboard aircraft provide accurate terrain clearance for low-altitude operations and precision approach guidance, typically achieving accuracies of a few meters in aviation applications. In spaceborne systems, the transmitted pulse illuminates an area called the footprint, and the shape of the return waveform encodes information about surface roughness, wave height, and wind speed over the ocean, in addition to the primary range measurement. The European Space Agency's overview of radar altimetry applications describes the technique's evolution from Seasat in 1978 through subsequent missions such as TOPEX/Poseidon and Sentinel-6.

Satellite Altimetry and Atmospheric Measurements

Satellite altimetry uses radar or lidar instruments on orbiting platforms to map global surface topography with repeat coverage on timescales of days to weeks. The primary science application has been sea surface height measurement: the NOAA Laboratory for Satellite Altimetry uses altimetric data to derive mean dynamic ocean topography, geostrophic current velocities, and seafloor bathymetry. Satellite altimeters also measure the heights of inland water bodies, ice sheets, and glaciers, providing a continuous record of volume change that supports climate monitoring. Accurate altimetry over land and ice requires corrections for atmospheric effects: the dry tropospheric correction accounts for path delay due to bulk atmospheric pressure, the wet tropospheric correction addresses water vapor, and the ionospheric correction compensates for free electron density. These atmospheric measurements are thus both a prerequisite for and a byproduct of the altimetric technique, and dedicated microwave radiometers often fly alongside the radar altimeter specifically to provide the wet troposphere correction.

Sensor Technologies and Accuracy

The accuracy of altimetric measurements depends on the sensor modality, orbit determination, and the geophysical corrections applied. Conventional pulse-limited radar altimeters achieve along-track resolution of several kilometers. Synthetic aperture radar (SAR) altimeters, introduced on CryoSat-2 and Sentinel-6, apply Doppler processing to the return pulses, sharpening along-track resolution to approximately 300 meters and improving performance in coastal and inland water regions. Lidar altimeters such as NASA's ICESat-2 ATLAS instrument use photon-counting laser pulses to measure ice sheet elevation with decimeter precision. GNSS-based altimetry, which requires no dedicated transmitter, is studied as a supplement to radar systems, as reviewed in satellite altimetry research published in Remote Sensing.

Applications

Altimetry has applications in a range of fields, including:

• Aircraft navigation and terrain avoidance using onboard radar altimeters
• Oceanography, where satellite altimetry resolves mesoscale eddies and long-term sea level trends
• Cryosphere monitoring, tracking ice sheet elevation changes and glacier retreat rates
• Inland hydrology, measuring river and lake water levels from orbit for flood forecasting
• Autonomous vehicle and drone navigation using lidar-based terrain elevation maps