Altimeters measure altitude above a reference point, which makes it an essential navigational tool for aircraft, its most common application. Other applications include skydiving, hiking, climbing, and search and rescue.
Altimeters rely on barometric pressure, radar, LiDAR, and GPS to complete altitude measurements. Barometric pressure altimeters have the advantages of being self-contained and cheaper to implement, but also run into problems with differing air temperatures and moisture content levels. Barometric altimeters work by measuring current air pressure and comparing that to the pressure at sea level. The difference will determine the current altitude.
Radar and LiDAR are accurate and can scan the ground to provide information about ground topography and current altitude, but radar requires special licensing and use must comply with FCC guidelines. Radar and LiDAR work by sending out electromagnetic waves. Radio frequency waves are used for radar and LiDAR uses waves in the ultraviolet, visible or near IR parts of the spectrum. The waves will reflect back to the system that will interoperate the time it takes for the wave to return into current altitude.
GPS altimetry is also easy to implement due to advanced technology but is not always the most accurate, is not self-contained as it relies on government satellites, and can be expensive with all the required electronics. GPS altimetry works by using the geometric method of trilateration, which uses signals from four or more orbiting satellites.
- Analog three-pointer altimeter: This type of altimeter will have three separate pointers on a circular dial. One pointer will represent hundred feet increments, another represents thousand feet increments, and the last ten thousand feet increments.
- Analog drum pointer altimeter: This type of altimeter will have a single pointer and a two-way incrementing number drum similar to automobile odometers. The pointer will display 0 to 99 feet, where the drum will display the hundreds and thousands of feet.
- Encoding altimeter: This altimeter will encode the measured altitude into an 11 digit binary code, called Gillham code, that the aircraft transponder sends to ground control.
- Blind encoder: A blind encoder is an encoding altimeter with no visible altitude indication. A second non-encoding altimeter must be used for visible indication for the pilot.
- Digital altimeter: A digital altimeter will have a digital display with no dials. Digital altimeters used in modern aircraft will be a graphical display that shows information on a single screen for the pilot to view.
Altimeters used in aircraft are mounted within the aircraft’s instrumentation panel. These altimeters can provide an analog dial or digital display. Altimeters for individuals in non-aviation related usages can be worn on the wrist (such as for sky divers), placed in a pocket, or have a clip that enables it to be attached to hiking gear or clothing. Personal altimeters typically have a digital display.