Altimeter

An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.

Pressure altimeter
Altitude can be determined based on the measurement of atmospheric pressure. The greater the altitude the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate altitude, the instrument is called a pressure altimeter or barometric altimeter. 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.

The calibration of an altimeter is of the form
 * $$z=c\;T\;\log(P_o/P),$$

where c is a constant, T is the absolute temperature, P is the pressure at altitude z, and Po is the pressure at sea level. The constant c depends on the acceleration of gravity and the molar mass of the air. Although one must be aware that this type of altimeter relies on "Density Altitude" and can shift reading by hundreds of feet due to a sudden change in air pressure such as from a cold front without any actual change in attitude.

Use in hiking and climbing
A barometric 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; GPS altimeters may be unavailable, for example, when one is deep in a canyon, or may give wildly inaccurate altitudes when all available satellites are near the horizon. 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.

Use in aircraft
In it, an aneroid barometer measures the atmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase of altitude--approximately 100 hectopascals per 800 meters or one inch of mercury per 1000 feet near sea level.

The aneroid altimeter is calibrated to show the pressure directly as an altitude above mean sea level, in accordance with a mathematical model defined by the International Standard Atmosphere (ISA). Older aircraft used a simple aneroid barometer where the needle made less than one revolution around the face from zero to full scale. This design evolved to the drum-type altimeter, where each revolution of a single needle accounted for 1,000 feet, and with thousand foot increments recorded on a numerical odometer-type drum. To determine altitude, a pilot first had to read the drum to get the thousands of feet, then look at the needle for the hundreds of feet. Modern aircraft use a "sensitive altimeter," which has a primary needle, and one or more secondary needles that show the number of revolutions, similar to a clock face. In other words, each needle points to a different digit of the current altitude measurement. On a sensitive altimeter, the sea level reference pressure can be adjusted by a setting knob. The reference pressure, in inches of mercury in Canada and the US and hectopascals (previously millibars) elsewhere, is displayed in the small Kollsman window, on the face of the aircraft altimeter. This is necessary, since sea level reference atmospheric pressure at a given location varies over time with temperature and the movement of pressure systems in the atmosphere.

In aviation terminology, the regional or local air pressure at mean sea level (MSL) is called the QNH or "altimeter setting", and the pressure that will calibrate the altimeter to show the height above ground at a given airfield is called the QFE of the field. An altimeter cannot, however, be adjusted for variations in air temperature. Differences in temperature from the ISA model will, therefore, cause errors in indicated altitude.

Sonic altimeter
In 1931, the US Army Air Corps and General Electric 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 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
A 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.

Global Positioning System
Global Positioning System (GPS) receivers can also determine altitude by trilateration with four or more satellites. In aircraft, altitude determined using autonomous GPS is not precise or accurate enough to supersede the pressure altimeter without using some method of augmentation. In hiking and climbing, it is not uncommon to find that the altitude measured by GPS is off by as much as a thousand meters, if all the available satellites happen to be close to the horizon.

Other modes of transport
The altimeter is an instrument optional in off-road vehicles to aid in navigation. Some high-performance luxury cars that were never intended to leave paved roads, such as the Duesenberg in the 1930s, have also been equipped with altimeters.

Hikers and mountaineers use hand-held or wrist-mounted barometric altimeters, as do skydivers.

Diesel submarines have barometers installed on them to monitor vacuum being pulled in the event that the snorkel closes while the diesels are running and, as a consequence, sucking the air out of the boat.

Satellites


A number of satellites (see links) use advanced dual-band radar altimeters to measure height from a spacecraft. That measurement, coupled with orbital elements (possibly augmented by GPS), enables determination of the terrain. The two different wavelengths of radio waves used permit the altimeter to automatically correct for varying delays in the ionosphere.

Spaceborne radar altimeters have proven to be superb tools for mapping ocean-surface topography, the hills and valleys of the sea surface. These instruments send a microwave pulse to the ocean’s surface and time how long it takes to return. A microwave radiometer corrects any delay that may be caused by water vapor in the atmosphere. Other corrections are also required to account for the influence of electrons in the ionosphere and the dry air mass of the atmosphere. Combining these data with the precise location of the spacecraft makes it possible to determine sea-surface height to within a few centimetres (about one inch). The strength and shape of the returning signal also provides information on wind speed and the height of ocean waves. These data are used in ocean models to calculate the speed and direction of ocean currents and the amount and location of heat stored in the ocean, which, in turn, reveals global climate variations.