help navigate the helicopter Ingenuity on its record-setting flights over the terrain of Mars by means of a downward-facing Lidar altimeter. Satellite navigation An altimeter, or altitude meter, is an instrument used to measure the altitude of an object relative to a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. An altimeter, or altitude meter, is an instrument used to measure the altitude of an object relative to a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water. A radar altimeter measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to the aircraft. Alternatively, Frequency Modulated Continuous-wave radar can be used. The greater the frequency shift the further the distance travelled. This method can achieve much better accuracy than the pulsed radar for the same outlay and radar altimeters that use frequency modulation are industry standard. The radar altimeter is used to measure height above ground level during landing in commercial and military aircraft. Radar altimeters are also a component of terrain avoidance warning systems, warning the pilot if the aircraft is flying too low, or if there is rising terrain ahead. Radar altimeter technology is also used in terrain-following radar allowing combat aircraft to fly at very low height above the terrain. After extensive research and experimentation, it has been shown that "phase radio-altimeters" are most suitable for ground effect vehicles, as compared to laser, isotropic or ultrasonic altimeters.