Altimeter is an instrument used for altitude measurement. The measurement of altitude (height) is called altimetry. Various altimeter devices are Altimeter Watches, Digital Altimeters, Analog Altimeters, Sky Diving Altimeters. Latest cell phones such as iphone and other smartphones feature applications of altimeter that operates with GPS satellites.

Altimeter does not actually measure altitude directly. Different designs of altimeters are:

  • Pressure Altimeters or Barometric Altimeters
  • Sonic Altimeters
  • Radar Altimeters
  • GPS Altimeters

Pressure Altimeters or Barometric Altimeters

With Pressure Altimeters or Barometric Altimeters, altitude can be determined based on the measurement of atmospheric pressure. The greater the altitude, the lower the pressure. A pressure altimeter is the altimeter found in most aircraft, and skydivers use wrist-mounted versions for similar purposes. Hikers and mountain climbers use wrist-mounted or hand-held altimeters, in addition to other navigational tools such as a map, magnetic compass, or GPS receiver.

Barometric pressure altimeter, used along with a topographic map, can help to verify one’s location. It is more reliable, and often more accurate, than a GPS receiver for measuring altitude. Because the barometric pressure changes with the weather, hikers must periodically recalibrate their altimeters when they reach a known altitude, such as a trail junction or peak marked on a topographical map. Aircrafts or airplanes use sensitive altimeters which are analog altimeters with more sensitivity.

GPS Satellite Altimeters

Global Positioning System (GPS) receivers can also determine altitude by trilateration with four or more satellites. Now GPS transmitters and receivers are everywhere, most of the latest mobile phones such as iphone and smartphones, wrist watches, vehicle tracking systems. With the use available GPS hardware, it is easy to build an altimeter in hanheld electronic devices.

GPS Altimeter Altitude Accuracy

The main disadvantages of GPS altimeters are not precise and accurate enough to supersede the pressure altimeter. Many people are surprised to find that their GPS altitude often disagrees with the altimeter’s altitude by a significant amount. This effect is more pronounced at higher altitudes. It is important to understand the limitations of GPS (and barometric) altimetry before safely using either in an aircraft or in sky diving.

Sonic Altimeters and Radar Radio Altimeters

Sonic Altimeters

Sonic Altimeter is considered more reliable and accurate than one which relied on air pressure, when heavy fog or rain was present. Sonic altimeters are mostly used aircrafts, as they consume more power to measure the vertical height. Sonic altimeter uses a series of high pitched sounds like a bat to measure the distance from the aircraft to the surface, which on return to the aircraft is then converted to feet shown on a gauge inside the aircraft cockpit.

Radar Radio Altimeters

Radar altimeter measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to the aircraft. 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 fighter aircraft to fly at very low altitude.