Compass (navigation)

See also: Compass

The compass is a Instrument of navigation which gives a reference of direction (the northern ) on the horizontal level and thus allows the measurement of horizontal angles compared to this direction. The compass is graduated of 0° (northern) with 359° in the direction of the needles of a watch (direction retrogresses). The compasses used by the armies use also other systems of graduation (see rank, also called gon , Thousandth)

There exist three kinds of compass:

  • the magnetic compass , whose principle of operation is, like a Boussole, orientation of a needle magnetized in the field of the terrestrial magnetism;
  • the gyroscopic compass , of which the principle is the gyroscopic effect, orientation of the axis of rotation of a spinning top (free on the 3 axes) in the direction of that of the Earth.
  • the satellite compass , which exploits the difference of the signals received by two antennas to calculate the orientation of the segment uniting these two antennas in the geodetic Système associated with the Système of positioning by satellites used.

Aeronautics

There exists for the aircraft several types of compass:
  • an electromechanical compass (in bottom in the center among the Instruments)
  • a standby compass of the conventional type (sphere turning freely in a liquid) or electromechanics (graduated ribbon ravelling in a window letting appear a segment of the circle)
  • of the various information of direction presented on screen in the modern Cockpit S (see Panel instrument any screen.

The navigation of the aircraft (Plane S and Helicopter S) is carried out for more precision according to radioelectric signals emitted by beacons on the ground. The angles between these beacons and the aircraft are indicated to the pilot using an instrument called Radiocompas provided with several needles for each beacon.

North compass, magnetic north and true north

It is necessary to distinguish:

  • the northern compass ( Nc ), the indication of north by the compass.

  • the magnetic north (Nm) , indicated on the charts, but not used in navigation.
  • the true north or northern truth (Nv) , the reference on the chart.

    • the angular difference between north of the compass ( Nc ) and the magnetic north ( Nm ) is called deviation (δ) .

    • This deviation varies with each compass embarked on each ship and is function of the Compass heading ( DC ).

Nm=Nc+δ (algebraically) of the same Cm=Cc+δ (the magnetic course is equal to the course compass + the deviation)

δ is positive if it is " est" , negative if it is " ouest".

  • the northern truth (Nv) or true north.

  • the angular difference between true north and the magnetic north is called variation (D) . It is the horizontal component of the magnetic declination. Indeed, the magnetic north indicates the magnetic pole which moves in time more or less around the geographical pole. The sea charts, some is their type of projection, always indicate the true north as well as the magnetic variation and its annual variation. The lines of equal variation are called isogonic lines.
  • There exist zones where the magnetic compasses are deviated by the surrounding geological masses, these local variations are also indicated on the charts.

Nv=Nm+D (algebraically)

Cv=Cm+D (the true course is equal to the magnetic course + the variation)

D is positive if it is " est" and negative if it is " ouest".

one defines the variation: W= (δ+D)

Nc+ (δ+D)=Nv

Cc+W=Cv (the true course is equal to the course compass + the variation)

the variation is the correction to be brought to the course compass to obtain the true course and to thus be able to defer it on a chart. It geographically includes/understands a local element (D) and a correction which belongs to the compass, with the ship and which moreover is function of the Compass heading: (δ)

Gyroscopic compass

The use of gyroscopic compasses makes it possible to be freed from these difficulties due to the terrestrial magnetism. The angular difference between true north and north compass is called variation (W) . This variation is generally weak (0,5° with 1°). But, the gyroscopic compasses can not be perfectly regulated or be put out of order. The variation is measured (or checks itself) regularly by aiming at stars to the rising or sleeping, and in coastal navigation while concerned with known alignments. (see also: Gyroscope).

Satellite compass

August 1st

Magnetic compass marine

Summary diagram:

Some magnetic needles (magnetic crew) related to a mobile crown (pink) provided with a float; the resting whole, in a liquid mixture composed of water and alcohol, on a pivot.

The terrestrial magnetic field being very weak, it was obligatorily necessary to decrease to the maximum frictions of the pink on the pivot (by the addition of a float inter alia). The use on a ship also asked for the installation of a system Cardan joint.

The basin of the compass is fixed on the internal crown of the Cardan joint, thus giving to the compass more possibility of being able to keep horizontal the with the sea.

  • the compass is placed in a cockpit made up of wood and/or non-magnetic materials.
Soft irons (spheres of compensations and Flinders bars), and fine-grained iron (loving longitudinal, side corrective measures and magnet of band) are used for the compensation.
  • the soft irons compensate for the induced magnetic fields.
  • fine-grained iron compensate for the permanent magnetic fields.
The cockpit, placed if possible in the central axis of the ship, is always distant as much as possible from possible magnetic disturbances (satellite antennas, radios). It can comprise a system of mirrors of optical reference of reading of the compass for the cox in the footbridge of navigation, but this system is replaced more and more by a system of reading remotely electronic (sensor placed under the basin). The magnet of band not represented on the diagram is used for the compensation for possible a lodging permanent (band), this last is adjustable vertically by a chain.

  • Errors of the magnetic compass:

The magnetic compass is subject to the influence of the horizontal component of the Terrestrial magnetic field D (variation magnetic) and also the influence of magnetic fields induced by the close ship and its electrical equipment and equipment, posts with quay, lines high voltage, etc the reading of the quay compass is often not representative.

  • the operation consisting with to minimize the deviation is called the compensation of the compass , it is made regularly by a specialist. Following the compensation the deviation should not exceed 8° if work is correctly carried out.

  • the operation consisting with to measure the deviation with the various courses is called the regulation , it is made by the crew or by a technician, the curve which results from this must be recalled annually and permanently controlled by comparative observations (observation of azimuths of stars, terrestrial alignments, etc).

  • the approximate value of the magnetic deviation according to DC (course compass) is given by the formula of Sir Archibald Smith (1813-1872), British mathematician.

\ delta = A+ B \ sin Cc+ C \ cos Cc+ D \ sin 2Cc + E \ cos 2Cc \,

  • the ship out of steel acquires with its construction an induced magnetism, a part will become permanent, but because of its voyages, it will undergo permanently the effects of magnetic fields, terrestrial on the one hand, but also external. Its magnetism will vary in time and the place, one can already say that the compensation of a compass is valid at a time T in a given place, and that it will be necessary to think of remaking it periodically.

Regulation of the magnetic compass

Operation consisting in measuring the deviations with the various cardinal courses and inter cardinals, and deducing the curve from it from deviation. Be generally carried out on open roadstead while using as bases an alignment. Can be also carried out with the sea while basing itself on the gyroscopic course, but it will be necessary to consider the variation of the gyroscopic compass.

1 Some controls and measurements of good sense:

  • It should not have there bubbles D “air in the basin.
  • the compass must be well aligned in the axis of the ship.
  • the axes of Cardan joint must be free and lubricated.
  • the speaker should have on him of metal mass neither of magnetic field (electronic device, high speaker of radio, iron tool, etc)
  • When the compass is deviated temporarily by a magnet, it must take again its alignment origin according to minimal standards of time (neither too length, nor too short).
  • the soft irons should not have remanent magnetism (to avoid rust on the spheres).
2 To operate so as to cut alignment on the way to the various cardinal courses and inter-cardinals, to measure
  • the Zc of alignment.
  • known Zv (chart)
  • D known (chart)
  • Zm = Zv - D

Correction of compass or compass

The compass of a boat must be corrected errors, called deviation of compass, caused by iron and steel present in its structure and its equipment. One then makes turn the boat around a fixed point while the courses indicated by the compass are noted according to its alignment on fixed benchmarks (bitter) on the shore. One thus obtains a chart of deviation of compass allowing the navigator to rectify the directions indicated by the compass according to the magnetic directions. The deviation is positive when it varies towards the east, and negative in the other direction (mnémo: is this more western less?).

A compass or a compass can be corrected in three manners. Initially the line of oaf must be adjusted so that it is aligned on the direction in which the boat moves. Then, the effects of the permanent magnets can be corrected by small magnets inserted in the box of the compass. The influence of nonferromagnetic materials can be compensated by two assembled iron balls on each side of the framework of the compass. The graphics of the deviation of compass can be included/understood by using the series of Fourier. The coefficient a0 representing the error in the line of oaf, while a1, b1 represents the ferromagnetic effects and a2, b2 the nonferromagnetic component.

The compasses with Flux valve (electronic) are gauged automatically according to the ambient magnetic field and can also take account of the correct local variation of magnetism so that the compass indicates the true course.

Balancing of compass or compass

The terrestrial attraction varying according to the latitude, the compasses are often balanced during their manufacture. This prevents the needle too strongly plunging to the bottom and from remaining blocked by the friction on the dial. The majority of the manufacturers rebalance the needle of their compasses according to one of five zones of the world. Zone 1 covers the major part of the northern hemisphere, zone 5 covering Australia and the oceans of the southern hemisphere.

Uses

In aerial navigation or maritime, the compass has two principal uses:

  • the compass of road makes it possible to the navigator to hold a course (course compass).
  • the compass of ''' raising ''' makes it possible to carry out a raising, i.e. by aiming at an object (another mobile, a star or a Amer) to measure the angle between its direction and north (direction called Azimut).
This last measurement allows:
* to make the not either, for coast, by successive aimings of three land-marks or, by aiming at a star, to contribute to the calculation of an astronomical point;
* to ensure its safety by supervising the other mobiles, a mobile in constant raising being on the way of collision;
* on a warship, or a combat aircraft, to indicate an objective with a system of weapon;
* to check the variation of its compass by aiming at a Alignment known or a star to the rising or sleeping.

Related articles

  • Gyroscope
  • Cape (navigation)
  • Compass

Simple: Compass

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