Altimeter
An altimeter is a measuring instrument that indicates the difference in altitude between the point where it is located and a reference point; it is commonly used to find the height above sea level of a point. The most common unit for calibrating altimeters around the world is hectopascals (hPa), except in North America (apart from Canada). ) and Japan, where inches of mercury (inHg) are used. To get an accurate altitude reading, either in feet or meters, the local barometric pressure must be calibrated correctly using the barometric formula.
The greatest use is made in aeronautics, as one more security element, forming part of the most important flight instruments of the plane.
In sports or activities in which large unevennesses are suffered, such as mountaineering, trekking, cycling, skiing, climbing, skydiving, trail running, etc., it is used to know the unevenness that is being overcome. As a curiosity, some of the most modern bicycle speedometers integrate an altimeter and can generate profiles of the day with the help of a computer.
History
The scientific principles on which the altimeter is based were first written down by Rev. Alexander Bryce a Scottish minister and astronomer in 1772 who realized that the principles of a barometer could be adjusted to measure height.
Barometric altimeter
It is the most common of all; its operation is based on the relationship between pressure and altitude, atmospheric pressure drops with altitude, approximately 1 hPa for every 27 feet (8.2 meters) of altitude. They take sea level as a reference base, but their operation is conditioned by weather changes, so a high-quality altimeter should make it possible to compensate for pressure variations caused by the weather. On the other hand, since the sea level is not uniform throughout the world, the basis for measuring the pressure can also vary depending on the latitudes in which we are. Or in other words, if we use the same altimeter in different countries, the results may vary if we do not adjust the base height (sea level that serves as a reference). Lastly, as the thickness of the atmosphere varies greatly according to latitude (it is much greater in the intertropical zone), the correspondence between pressure and altitude can vary.
These altimeters have an irregular operation if the change in altitude is very sudden, since they take time to respond and capture the atmospheric pressure; They also do not work well if, for example, the ascent is carried out in a car with the windows closed, since the pressure inside the car with the windows closed will be very different from that outside.
The formula for calibrating an altimeter (up to 36,090 feet) is as follows:
- h=(1− − (P0/Pref)0,19026)⋅ ⋅ 288.150,00198122{displaystyle h={frac {(1-(P_{0}/P_{ref})^{0,19026})cdot 288.15}{0,00198122}}}}}}}}}
Where h indicates the altitude at feet, P0{displaystyle P_{0}} It's static pressure and Pref{displaystyle P_{ref}} is the reference pressure (both in the same unit).
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The operation of the altimeter is based on the volume changes experienced by a closed capsule, containing gas at a certain pressure, which are measured by a mechanism that translates these changes into altitude measurements, with respect to a pressure that has been regulated Through the adjustment system used to correct the altitude measurement due to changes in atmospheric pressure (reference pressure), this adjustment data is obtained from a barometer installed at the point from which the measurement is to be made.
Aircraft use
In aircraft, an aneroid barometer measures atmospheric pressure from a pitot-static system outside the aircraft. Atmospheric pressure decreases with increasing altitude, approximately 100 hectopascals per 800 meters or one inch of mercury per 1000 feet or 1 hectopascals per 30 feet near sea level.
The aneroid altimeter is calibrated to display pressure directly as an altitude above mean sea level, according to an atmosphere model defined by the International Standard Atmosphere (ISA). Older aircraft used a simple aneroid barometer in which the needle made less than one turn around the face from zero to full scale. This design evolved into three-hand altimeters with a primary hand and one or more secondary hands showing the number of revolutions, similar to a watch face. In other words, each needle points to a different digit of the current altitude measurement. However, this design has fallen out of favor due to the risk of misreading in stressful situations. The design evolved into drum altimeters, the latest step in analog instrumentation, where each revolution of a single needle represented 1,000 feet (300 meters), with thousand-foot increments recorded on an odometer-type numerical drum. To determine altitude, the pilot had to first read the drum for thousands of feet, and then look at the needle for hundreds of feet. Modern analog altimeters on transport aircraft are usually of the drum type. The latest development in clarity is an Electronic Flight Instrument System with integrated digital altimeter displays. This technology has been trickled down from airliners and military aircraft until it is now standard on many general aviation aircraft.
Modern aircraft use a "sensitive altimeter." In a sensitive altimeter, the reference pressure at sea level can be adjusted with a trim knob. The reference pressure, in inches of mercury in Canada and the United States, and hectopascals (formerly millibars) elsewhere, is displayed in the small Kollsman window on the face of the aircraft's altimeter. This is necessary, since the reference atmospheric pressure at sea level at a given location varies over time with temperature and the movement of the pressure system in the atmosphere.
In aviation terminology, the regional or local atmospheric pressure at mean sea level (MSL) is called QNH, or "altimeter setting," and the pressure that the altimeter will calibrate to display height above ground in a certain airport is called QFE of the field. However, an altimeter cannot adjust to variations in air temperature. Temperature differences from the ISA model will consequently cause errors in the indicated altitude.
In aerospace, self-contained mechanical altimeters based on diaphragm bellows have been replaced by integrated measurement systems called air data computer (ADC). This module measures altitude, flight speed, and outside temperature to provide more accurate output data that enables automatic flight control and flight level splitting. Multiple altimeters can be used to design a pressure reference system that provides information about aircraft attitude angles to further support inertial navigation system calculations.
Skydiving
An altimeter is the most important piece of skydiving equipment, after the parachute itself. Altitude awareness is crucial at all times during the jump, and determines the appropriate response to maintain safety.
Because awareness of altitude is so important in skydiving, there are a wide variety of altimeter designs made specifically for use in this sport, and a non-student skydiver often uses two or more altimeters on a single jump:
- Hand, wrist or chest mechanical altimeters visual analogs. This is the most basic and common type, and is used by (and commonly required) virtually all parachuting students. The common design has a dial marked from 0 to 4000 m (or from 0 to 12000 feet, imitating the clock dial), in which an arrow points to the current altitude. The front plate has prominently marked sections with yellow and red, respectively, which mean the recommended deployment altitude, as well as the decision altitude of the emergency procedure (commonly known as "hard cover"). A mechanical altimeter has a command that must be adjusted manually to point to 0 on the ground before the jump, and if the landing point is not at the same altitude as the takeoff point, the user must adjust it properly. There are also some advanced electronic altimeters that make use of the well-known analog screen, despite working internally in a digital way.
- Digital visual altimetersmounted on the wrist or hand. This guy always works electronically and transmits altitude as a number, instead of a needle on a dial. Since these altimeters already contain all the electronic circuitry necessary for the calculation of the altitude, they are usually equipped with auxiliary functions such as the electronic journal, the reproduction of the jump profile in real time, the indication of the speed, the simulator mode for use in ground training, etc. An electronic altimeter is activated on the ground before the jump, and calibrated automatically to point to 0. Therefore, it is essential that the user not turn it on before necessary to avoid, for example, driving to a landing area at a different altitude of their own, which could cause a potentially fatal false reading. If the planned landing area is at an altitude different from that of the takeoff point, the user needs to enter the appropriate displacement using a designated function.
- Audible altimeter (also known as "dytters", a generic brand of the first product of this type on the market). They are inserted into the helmet and emit a tone of warning at a predefined altitude. Contemporary audibles have evolved significantly since their inceptions, and have a wide range of functions, such as multiple tones at different altitudes, multiple saved profiles that can be quickly changed, e-registration book with data transfer to a PC for further analysis, different free fall modes and canopy with different warning altitudes, approximation guide to fixed wing airplaneswoop, etc.??????? Audible devices are strictly auxiliary devices, and are not substituted, but complement a visual altimeter that remains the main tool to maintain altitude awareness. The arrival of modern parachuting disciplines such as free flight, in which the soil may not be in the field of vision for long periods of time, has made the use of audible almost universal, and practically all the parachuting helmets come with one or more ports incorporated in which audible can be placed. Audibles are not recommended and are often prohibited from using parachute students, who need to build an altitude-conscious regime for themselves.
- Auxiliary visual altimeters. These do not show precise altitude, but help to maintain a general indicator in peripheral vision. They can operate in tandem with an audible equipped with an appropriate port, in which case they emit warning flashes that complement the audible tones, or be independent and use another display mode, such as displaying a green or red light depending on the altitude.
- Blinking altimeters (also known as voice altimeters). Another type of altimeter that combines audible and visual altimeter functions. The unit has all the necessary altitudes used in parachuting and announces them as a number in the native language of the parachute. They are also inserted into the helmet (the same size as the audible), but emit voice with automatic volume adjustment depending on the speed to clear the ear. Parlating altimeters usually have a software configuration through a mobile app. The main objective of this type of altimeter is the strong security function for experienced paratroopers, so that they always know their own current position that is very useful for FS cargo organizers or AFF instructors.
The exact choice of altimeters largely depends on individual skydiver preferences, experience level, major disciplines, as well as the type of jump. At one end of the spectrum, a low-altitude demonstration jump with water landing and no free fall you could waive the mandatory use of altimeters and use none at all. Conversely, a skydiver performing free flight jumps and flying with a high performance canopy could use an analogue mechanical altimeter to facilitate free fall reference, a helmet audible for break altitude warning, additionally programmed with pitch guidance tones for canopy flight, as well as a digital altimeter on a bracelet for a quick glance at precise altitude on approach. Another skydiver doing similar types of jumps might carry a digital altimeter for her main line of sight, preferring direct altitude reading from a numeric display.
Radio altimeter
These devices are small radars that measure the distance between two air vehicles and with respect to the ground, this is used mainly in bombs and missiles. Pulse or frequency altimeters are similar to this but work by emitting other types of signals. Some of them are being mounted on satellites for scientific purposes to study the geoid, marine dynamics, sea level variations, and to analyze the topography of continental masses. Among other altimetric satellites we find the SeaSat, the TOPEX/Poseidon and the Jason-1, from the CNES/NASA collaboration, and the ERS-1, the ERS-2 and the EnviSat, from the European Spatial Agency (ESA).
The operation of the radioelectric altimeter is different from that of the barometric altimeter. They measure distance by emitting electromagnetic pulses and recording the time elapsed since the emission of the pulse, and the reception of the return echo of the signal. Since electromagnetic waves travel at the speed of light, the calculation of the distance is immediate, taking into account that the measured time is double and therefore has to be divided by 2.
- h=Δ Δ t⋅ ⋅ c2;{displaystyle h={frac {Delta tcdot c}{2}}};
Global Positioning System
Global Positioning System (GPS) receivers can also determine altitude by trilateration with four or more satellites. In aircraft, altitude determined by standalone GPS is not reliable enough to replace pressure altimeter without using some GNSS augmentation method. In hiking and climbing, it is common to find that the altitude measured by the GPS is off by up to 400 feet (122 m) depending on the orientation of the satellite.