Altimeters

An altimeter is an instrument used to measure altitude — the height of an object or point in relation to a specific reference level, usually mean sea level or ground level. Altimeters are essential tools in aviation, meteorology, mountaineering, space exploration, and various engineering applications where determining elevation or vertical position is crucial.

Principles and Measurement

Altimeters operate on different physical principles depending on their design and purpose. The most common types measure altitude based on changes in air pressure, radio waves, or satellite signals.

• Pressure Altimeters: These devices function on the principle that atmospheric pressure decreases with increasing height. A pressure altimeter measures the surrounding air pressure using an aneroid barometer and converts it into an altitude reading based on the International Standard Atmosphere (ISA) model, where pressure at sea level is defined as 1013.25 hPa (hectopascals).
• Radar Altimeters: Also known as radio altimeters, these emit radio waves towards the ground and calculate altitude by measuring the time it takes for the signal to reflect back. This type provides the precise height above the terrain or water surface, rather than above sea level.
• GPS Altimeters: These use signals from Global Positioning System (GPS) satellites to compute the altitude of a point relative to a geodetic reference ellipsoid. GPS altimeters are widely used in modern aircraft, smartphones, and fitness devices.
• Laser Altimeters: Used in spacecraft and surveying, these instruments employ laser beams to determine altitude or surface topography with high precision by measuring the time-of-flight of laser pulses.

Each type of altimeter serves distinct purposes and has advantages depending on the required accuracy, environment, and application.

History and Development

The concept of altitude measurement developed alongside advances in atmospheric science and aviation. The barometric altimeter was first introduced in the early twentieth century and became a standard component in aircraft during the 1920s and 1930s. The invention revolutionised flight navigation by allowing pilots to determine their altitude without visual reference to the ground.
Radar altimeters were developed later, during the 1930s and 1940s, initially for military aviation. They became crucial during World War II, especially for low-altitude bombing and night flying operations. With the rise of satellite navigation technology in the late twentieth century, GPS-based altimetry further enhanced precision, reliability, and accessibility across both professional and consumer applications.

Types and Their Applications

Altimeters can be classified into several types according to their operational principle and usage domain.

• Barometric Altimeter:
  • Commonly used in aircraft, meteorological stations, and hiking instruments.
  • Provides altitude readings based on ambient air pressure changes.
  • Requires periodic calibration to local pressure conditions to ensure accuracy.
• Radar Altimeter:
  • Used in aircraft and spacecraft for determining the precise height above terrain or ocean surfaces.
  • Plays a vital role in automatic landing systems and terrain-following radar for military aircraft.
  • Used in oceanographic studies and remote sensing from satellites.
• GPS Altimeter:
  • Found in navigation devices, smartphones, and outdoor sports watches.
  • Provides global altitude data independent of atmospheric pressure.
  • Offers real-time updates and high vertical accuracy when combined with barometric data.
• Laser Altimeter:
  • Employed in space exploration missions, planetary mapping, and Earth observation.
  • Used by NASA’s LIDAR (Light Detection and Ranging) systems for measuring terrain elevation, vegetation height, and ice-sheet thickness.
  • Extremely accurate but limited to line-of-sight measurements.

Altimeter Use in Aviation

In aviation, the altimeter is one of the primary flight instruments, critical for maintaining safe flight levels and preventing collisions with terrain or other aircraft. A typical aircraft cockpit includes a pressure altimeter, calibrated to display altitude in feet or metres.
A pilot sets the barometric pressure reference (QNH or QFE) in the altimeter to correspond with local sea-level or field pressure:

• QNH: Altitude above mean sea level.
• QFE: Altitude above a specific airfield or ground level.
• Standard pressure (1013.25 hPa): Used when flying at standard flight levels above the transition altitude.

Modern aircraft also utilise radio altimeters for approach and landing phases, particularly in low-visibility conditions. Autopilot systems rely on radar or laser altimeter data for precision landings and terrain avoidance.

Use in Meteorology and Environmental Studies

Altimeters are used in meteorology to determine pressure altitude, which helps calculate atmospheric parameters such as temperature lapse rate and air density. They also support weather balloon measurements, allowing vertical profiles of pressure and temperature in the atmosphere.
In environmental monitoring, altimeters mounted on satellites or aircraft measure ocean surface height, glacier thickness, and topographical variations. These data are crucial for climate studies, sea-level rise analysis, and resource management.

Applications in Exploration and Outdoor Activities

Altimeters are vital in mountaineering, trekking, skiing, and paragliding, helping users monitor elevation changes and navigate terrain safely. Portable devices often combine barometric and GPS altimetry for accuracy in dynamic weather conditions. Hikers use such instruments to track ascent rates, summit elevations, and environmental pressure changes that may indicate shifting weather patterns.

Accuracy, Calibration, and Limitations

The accuracy of an altimeter depends on its type and environmental conditions:

• Pressure altimeters can be affected by temperature variations and pressure changes unrelated to altitude. Regular calibration using local barometric data is essential.
• Radar altimeters may give inconsistent readings over uneven terrain or water waves.
• GPS altimeters depend on satellite signal strength and may show reduced accuracy in deep valleys, urban areas, or dense forests.
• Laser altimeters provide extremely accurate data but are generally limited to specialised equipment and applications.

Advances in Digital and Smart Altimetry

Modern technology has integrated altimeters into smart devices, including smartphones, smartwatches, and drones. Digital altimeters often combine barometric, GPS, and accelerometer inputs to enhance precision and filter out environmental noise. In drones and autonomous vehicles, altimeters aid in flight stabilisation, altitude holding, and obstacle detection.
Space exploration missions use high-resolution laser and radar altimeters to map planetary surfaces. Instruments such as the Mars Orbiter Laser Altimeter (MOLA) and Lunar Orbiter Laser Altimeter (LOLA) have produced detailed topographic maps of Mars and the Moon, contributing significantly to planetary science.