Altimeters tell the aircraft how high it is. Barometric altimeters give height relative to a pressure reference, which is what air traffic uses. Radar and laser altimeters measure the actual distance to the ground below, which is what matters for terrain following, precision landing and low-level flight.
Types
Type
Measures
Strengths
Limits
Barometric
Pressure altitude
Light, cheap, always available
Drifts with weather; disturbed by rotor wash
Radar altimeter
Height above ground
Works in dust, fog, rain, night
Mass and power on larger units
Laser altimeter / rangefinder (LiDAR)
Height above ground or distance
Precise, light, narrow beam
Affected by fog, dust and some surfaces
Certified radar altimeters operate in the 4.2–4.4 GHz aviation band; compact drone radar altimeters often use other bands.
Key specifications
Range (maximum height) and minimum range for landing.
Accuracy and resolution.
Update rate.
Beam width: narrow beams measure a precise point; wide beams average over terrain.
Performance over water, vegetation and snow.
Interfaces and size, weight and power.
Certification for crewed aircraft and eVTOL.
Uses
Terrain following and constant height for agriculture and inspection.
Precision landing and auto-land.
Altitude hold close to the ground, where barometric sensors are unreliable.
What is the difference between a radar altimeter and a barometric altimeter?
A barometric altimeter infers altitude from air pressure, relative to a reference such as sea level. A radar altimeter measures the actual distance to the ground directly below the aircraft.
Can a laser rangefinder replace a radar altimeter?
For many small drones, yes: it is light and precise. Radar is preferred when the aircraft must work in fog, dust, rain or over water, where laser measurements can fail.
Why is barometric altitude unreliable close to the ground on a multirotor?
Rotor downwash and ground effect change the local pressure around the sensor, and weather changes the reference. Height-above-ground sensors are used for low-level flight and landing.
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