This is one of the earliest barometric altimeters.
Altimeters are used to determine the altitude of something in relation to a fixed level surface. As early as 1643, Italian scientists were using barometric pressure measurements to determine the altitude of different areas. This was the earliest form of the altimeter. Today, altimeters are used in planes and by skydivers. There are different types of altimeters, all of which find the altitude in different ways.
Barometric Altimeter
Barometric altimeters use air pressure to determine the altitude. They measure the barometric pressure around them and then calculate altitude by comparing that to the barometric pressure at sea level or ground level, depending on the setting. One of the drawbacks to this kind of altimeter is it can be effected by outside weather conditions, which can make its readings unreliable.
Types of Barometric Altimeter
There are two types of barometric altimeters: the counter pointer altimeter and the counter drum pointer altimeter. The counter pointer has a two-digit counter display that indicates altitude in 1000-foot increments. The pointer counts off the hundreds of feet between each 1000 feet. This type of altimeter can be tricky to read, because the counter switches over to 1000 feet while the pointer is still in between 900 and 1000 feet. This is why the counter drum pointer altimeter was introduced. The drum next to the counter keeps track of the hundred-foot measurements as a backup to the pointer.
Radio Altimeter
A radio altimeter uses radar to determine altitude. It sends out radio waves to a fixed point on the ground and then determines the altitude by the time it takes the wave to bounce back. Modern systems use the change of phase between the transmitted and reflected signal to determine altitude. Unlike the barometric altimeter, a radio altimeter won't be affected by the weather. However, a radio altimeter is usually only used when the plane descends below 2500 feet.
GPS Altimeters
A global positioning system (GPS) uses satellites to determine position anywhere on the globe. GPS can also use satellites to determine an object's altitude. GPS altimeters work in much the same way as radio altimeters, in that they use radio waves to determine position. However, these radio waves are sent from the GPS unit to satellites, which then send them to fixed points on the ground.
Tags: determine altitude, 1000 feet, barometric pressure, pointer altimeter, radio waves