Laboratory work № 1
-----------------------------------ALTIMETER--------------------------------Purpose of the work:
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be familiarized with the basic types of altimeter, principles of work
be familiarized with the use of altimeter simulator (Luiz Monteiro)
study the list of Q-codes for altimeter settings
Short theoretical data
1. 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.
Two types of altimeter are in common use in aircraft:
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Barometric (Pressure)Altimeter
Radio Altimeter
Diagram showing the face of the "three-pointer" sensitive aircraft altimeter displaying an
altitude of 10,180 feet.
• 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
[1]
where c is a constant, T is the absolute temperature, P is the pressure at altitude z, and P o is
the pressure at sea level. The constant c depends on the acceleration of gravity and the
molar mass of the air.
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:
Schematic of a drum-type aircraft altimeter, showing the small Kollsman windows at the
bottom left and bottom right of the face.
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,[2] 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.
Diagram showing the internal components of the sensitive aircraft altimeter.
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 highperformance 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 altimeters 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
This graph shows the rise in global sea level (in millimeters) measured by the NASA/CNES
ocean altimeter mission TOPEX/Poseidon (on the left) and its follow-on mission Jason-1.
Image credit: University of Colorado
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.
2. Q-CODES
The Q code is a standardized collection of three-letter message encodings, all of which start
with the letter “Q”.
For altimeter settings we use:
QFE - Atmospheric pressure at sea level, corrected for temperature and adjusted to a
specified datum such as airfield elevation;
QFF - Barometric pressure at a place, reduced to MSL using the actual temperature at the
time of observation as the mean temperature;
QNE - Atmospheric pressure at sea level in the International Standard Atmosphere (ISA),
equal to 1013.25 mbar or hPa and used as reference for measuring the pressure altitude.
QNH - Atmospheric pressure at mean sea level. May be either a local, measured pressure or
a regional forecast pressure
Methodical guidance
During this laboratory work you will work in Altimeter Simulator by Luiz Monteiro. The
interface of this program is shown below:
You need to input such data:
1.Type of aircraft (Aircraft Icon)
2.Indicated altitude
3.Temperature change
Indicated altitude has to be inputted for two units:
- meters
- feet
NOTE: The maximum value of temperature change is 60 degrees! If you will inpute the
number more than this you will get a mistake:
Work procedure
1.
2.
3.
4.
5.
6.
To study and write down the purpose of the work and short theoretical data.
To switch on the altimeter simulator by Luiz Monteiro.
Input all values that are indicated in Methodical Guidance above.
Be familiarized with the work of the Simulator and the results you got.
Change the values several times to see how does the simulator work.
Make a written report to your laboratory work.
The report should contain :
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Topic of the laboratory work № 1
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Purpose of the work
Short theoretical data material
The table with the columns containing such information : type of altimeter,principle
of its operation, advantages, disadvantages of work.
Data you have inputted for concrete type of an aircraft and results you have got in
Altimeter Simulator by Luiz Monteiro.
Conclusion
Questions for self control:
1.
2.
3.
4.
5.
6.
What is an altimeter? Which types of altimeter do you know?
Tell about the principle of work of pressure altimeter.
Which type of altimeter to your mind is the most suitable for use in aviation? Why?
What do you know about sonic altimeter?
Name 4 main Q-codes that are used for altimeter settings.
What does the encoding QNH mean?