Altimeter

Types

Pressure altimeter

Sonic altimeter

In 1931, the US Army Air corps and General Electric together tested a sonic altimeter for aircraft, which was considered more reliable and accurate than one that relied on air pressure when heavy fog or rain was present. The new altimeter used a series of high-pitched sounds like those made by a bat to measure the distance from the aircraft to the surface, which on return to the aircraft was converted to feet shown on a gauge inside the aircraft cockpit.

Radar altimeter

Main article: Radar altimeter

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. Nebylov, Prof. Alexander and Sharan Sukrit. "Comparative Analysis Of Design Variants For Low Altitude Flight Parameters Measuring System". 17th IFAC Symposium for Automatic Control.

Laser altimeter

Lidar technology is used to 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

Satellite navigation receivers like those used with the Global Positioning System (GPS) can also determine altitude by trilateration with four or more satellites. In aircraft, altitude determined using autonomous GPS is not reliable enough to supersede the pressure altimeter without using some method of augmentation. In hiking and climbing, it is common to find that the altitude measured by GPS is off by as much as 400 ft depending on satellite orientation.

References

References

1. Wragg, David W.. (1973). "A Dictionary of Aviation". Osprey.
2. [https://books.google.com/books?id=9ycDAAAAMBAJ&pg=PA35 "Meter Gives Elevation"], ''Popular Science'', March 1931
3. (17 February 2021). "How NASA Designed a Helicopter That Could Fly Autonomously on Mars". IEEE Spectrum.
4. (2017-08-16). "Accuracy of Flight Altitude Measured with Low-Cost GNSS, Radar and Barometer Sensors: Implications for Airborne Radiometric Surveys". Sensors.
5. "Understanding the Accuracy of the GPS Elevation Reading". Garmin.