Magnetic compass correction

EdwART. Explanatory Naval Dictionary, 2010

See what “Magnetic compass correction” is in other dictionaries:

COMPASS CORRECTION- The angle between the true and compass directions, which is the algebraic sum of magnetic declination and deviation. It is expressed by the difference between the compass and true (taken from the map) directions to any lighthouse, navigation sign, etc... Marine encyclopedic reference book

A device for determining horizontal directions on the ground. Used to determine the direction in which a ship, aircraft, or ground vehicle is moving; the direction in which the pedestrian is walking; directions to some... ... Collier's Encyclopedia

A device for determining the sides of the horizon and measuring magnetic azimuths on the ground, for example. when moving along the route. Basic parts of the compass - a magnetic needle, freely rotating on the tip of a needle, a dial with degree divisions, and also (in some... ... Geographical encyclopedia

The Compass request is redirected here; see also other meanings. Brunton mountain compass, precision instrument ... Wikipedia

A device indicating direction on the earth's surface; it includes one or more gyroscopes. Used almost universally in navigation and control systems of large sea vessels; unlike a magnetic compass, its readings... ... Collier's Encyclopedia

A science that studies the shape, size and gravitational field of the Earth, as well as technical means and methods of measurements on the ground. Geodesy originated in the countries of the Ancient East and in Egypt, where long before Christ. e. methods for measuring land were known... ... Geographical encyclopedia

1. Determining the correction of a magnetic compass and monitoring its operation at sea 1. 1. General provisions The magnetic compass is simple in design, it is autonomous and reliable. The main disadvantage is the low accuracy in determining directions. Errors reach 2–4°, especially when pitching. Sources of errors: magnetic declination, deviation, inertia and insufficient sensitivity of the magnetic needle system to the Earth's magnetic field. The magnetic compass card arrives at the meridian 3–4 minutes after maneuvering.

An accurate knowledge of the deviation of the magnetic compass is important in navigation. Deviation is destroyed at least once a year using the methods studied in the course “Technical means of navigation”. The residual deviation is determined by navigation methods and should not exceed several degrees. In accordance with good maritime practice, magnetic compass deviation is determined: – at least once a year; – after repair, docking, demagnetization of the vessel, as well as after loading and unloading cargo that changes the ship’s magnetic field; – with a significant change in magnetic latitude; – when the tabulated deviation diverges from the actual one by more than 1° for main compasses and 2° for way compasses; - before a long flight.

All methods for determining deviation are based on the use of formula (4. 6) MP = CP + δ → δ = MP – CP Deviation depends on the ship’s course, so it is usually determined on 8 equally spaced compass courses, and intermediate values ​​are found by linear interpolation. Usually these are courses corresponding to the main and quarter directions, i.e. courses 0, 45, 90, 135, 180, 225, 270, 315 degrees

It is assumed that the ship's magnetic field is symmetrical relative to the ship's DP, i.e., the deviation is symmetrical relative to the magnetic meridian, therefore the average value of the compass bearing to a distant object taken to a distant landmark on equally spaced courses can be taken as an estimate of the magnetic bearing. The formula will be as follows: MP = ∑KPi /8 + A (5. 1) Where A is some correction for systematic error (constant deviation), which is determined for a specific compass during the destruction of deviation.

1. 2. Methods for determining deviation 1. 2. 1. Along the target The vessel crosses the target on 8 equally spaced compass courses, and the navigator takes KPi. Compass courses are 45 degrees apart. Magnetic bearing is calculated using the formula MP = IP – d (5.2)

The value of IP and magnetic declination is taken from the map. Magnetic declination leads to the year of voyage. When maneuvering in the vicinity of the target, you should take into account the inertial characteristics of the magnetic compass. If the magnetic declination is unknown, then use formula (5. 1) - then calculate the deviation on each heading: δi = MP - KPi (5. 3) and draw up a table or graph of deviation as a function of the compass heading. The table is compiled in 10 degrees according to the compass heading.

1. 2. 2. By a distant landmark The vessel circles at a distance D from the navigational landmark and takes bearings on 8 equally spaced compass courses. Determine δ using formula (5.3). Magnetic bearing can be calculated using formula (5.2) or using IP and d taken from the map.

It should be borne in mind that the distance to the landmark is selected taking into account the accuracy of determining the bearing and can be determined by the formula: D = r/sinά (5. 4) If the water area is limited for maneuvering, the vessel is anchored or barreled and turned around by tug.

1. 2. 3. According to the bearing of the star The method is similar to that described above. At eight compass courses, the compass bearings of the luminary are determined. Then its azimuth (AI) is calculated using astronomical formulas, and, knowing the declination (from the map), the MF is obtained. To calculate the deviation, formula (5.3) is used. To increase the accuracy of direction finding, choose a luminary located at a low altitude (no more than 30 degrees). A significant advantage of the method is that due to the great distance of the luminary, the accuracy does not depend on the coordinates of the vessel, i.e. there is more space for maneuvering.

1. 2. 4. By comparison with another magnetic compass or gyrocompass. Comparing compasses means simultaneously noticing their readings. Comparisons are made between the traveling compass and the main or gyrocompass. It is usually produced in 8 equally spaced courses. Deviation is determined by taking into account the equality of magnetic courses obtained from different course indicators. For example, when comparing a travel compass with the main one or when comparing it with a gyrocompass, the following expressions are correct: KPp + δp = KKgl + δgl (5. 5) KPp + δp = GKK + ΔGK – d (5, 6) From these relations the unknown is calculated

1. 2. 5. By the method of mutual bearings (in extreme situations) A magnetic compass installed on the shore or on a non-metallic boat is taken from the ship, and a compass installed on the ship is synchronously taken from the shore or from the boat. It is clear that the MP is taken from the compass on the shore or on the boat. Deviation is determined: δi = (180 o + MP i) - CP i (5. 7)

2. Determining the correction of the gyrocompass and monitoring its operation at sea 2. 1. General provisions Gyrocompasses generate a heading with an accuracy of 0.5° (with a probability of 95%) with a constant heading and pitching of no more than 2 degrees. With increased pitching and intense maneuvering, the gyrocompass error can reach 4°. Due to inertial errors, the highest measurement accuracy can be achieved 30 -40 minutes after the end of the maneuver. The gyrocompass has its own systematic errors, which must be compensated for by corrections. Well-known formulas are used as formulas for calculation: ΔK = IR – KK (5.8) ΔK = IP – KP (5.9) Where ΔK, KP are general designations for compass correction, compass course and compass bearing, measured using magnetic or gyrocompass.

The task comes down to determining the true directions, which are usually taken from the map or calculated by celestial navigation methods if direction finding of luminaries is carried out.

2. 2. Methods for determining the gyrocompass correction 2. 2. 1. By the bearing of a distant object The method is used if the ship is moored. Determine the exact compass coordinates on a map or plan and take an IP at a remote known navigation landmark.

For about an hour and a half, they take a bearing to this landmark after 1015 minutes, find the correction using the formula (5.9) for each direction finding, and then display its average value (line aa"). This will be the so-called constant correction of the main code. This operation is also should always be carried out after a new start of the GK, when it has entered the meridian ΔGK a a" ΔGKi ΔGKsr ΔGKi t

2. 2. 2. According to the bearing of the navigation alignment, the IP alignment is indicated on the map. Having taken the direction of the target and compared the IP with our GKP, we obtain ΔGK (formula (5.9)). With this method, you can use not only artificial, but also natural alignments. 2. 2. 3. Using the true bearing of the luminary To do this, it is necessary to take the bearing of the luminary, calculate its azimuth (A), and this is the same as the IP. Comparing GKP and A, we obtain ΔGK. The most common way to determine ΔGK is by the bearing of sunrise and sunset, by the bearing of the North Star. The determination of ΔGK by celestial bodies is studied in more detail in the course “Nautical Astronomy”.

2. 2. 4. By comparison with the heading indicator, the correction of which is known. In this case, use the formulas obtained by equating IC: KKp + ΔMKp = KKgl + ΔMKl (5.10) KKp + ΔMKp = GKK + ΔGK (5.11) Equation (5.10), (5.11) are solved with respect to the unknown correction . The indicated formulas are used when switching from one compass to another if any of them fails.

If the instantaneous correction of the gyrocompass, determined by one of the methods at sea, differs by more than 1° from the constant correction determined in the port, then the alarm should be sounded. Determining the compass correction is one of the most important responsibilities of a boatmaster. Navigation service rules require that compass correction be determined whenever possible. Comparison of the gyroscopic and magnetic compasses is carried out once during the watch (4 hours), if the course does not change and with each change of course. This is necessary in order to know the current compass correction in the event of a gyrocompass failure.

Conclusions: 1. 2. 3. Determining corrections for direction indicators is one of the most important responsibilities of a navigator. It is necessary to determine the compass correction whenever possible. The methods used to determine the correction of the magnetic compass and gyrocompass are the same, namely: along the alignment, using the bearing of the luminary, by the bearing of a distant landmark, by comparison with a heading indicator, the correction of which is known. But it should be remembered that the correction for the gyrocompass determined in this way is constant on all courses. For a magnetic compass, this correction is valid only on this course. Comparison of the gyroscopic and magnetic compasses is carried out once during the watch (4 hours), if the course does not change and with each change of course.

Contents of issues discussed at the seminar 1. Basic points, lines and planes for orientation on the earth's surface. 2. Horizon division systems: rhumb, circular, semicircular and quarter 3. Directions relative to the plane of the true meridian and the center plane of the vessel 4. Use of a magnetic compass, magnetic declination, magnetic compass deviation, magnetic compass correction. 5. The relationship between compass and true directions 6. Methods for determining direction indicator corrections 7. Calculation of directions using a gyrocompass and a magnetic compass

Sometimes, when interviewing 3rd mates, I jokingly ask: “How does the morning begin for the 3rd mate and for the captain?”

The young guys are confused and try to come up with something to answer my unexpected question.

I explain to them all that the captain’s morning begins with a cup of aromatic coffee, and for the 3rd mate, the morning begins with adjusting the compass. A joke of course, but with a grain of truth. This is what I want to talk about.

All boatmasters know that the compass correction must be determined every watch. How to do this?

In coastal navigation, when there are coastal landmarks, this is very simple and takes a few minutes. What to do if the ship is on the open ocean? There is nothing around, only the sky, the ocean, seagulls and the captain, who is watching with interest how the 3rd mate will solve the task. He probably considers you “GPS generation”. As they say, everything ingenious is simple.

There is a quick and easy way to determine the compass correction based on the lower or upper edge of the Sun. To do this, you need very little - install a direction finder on board where the Sun sets, and at the moment when the last segment disappears behind the horizon. After this, you should take a bearing, note the time, latitude, longitude and enter the data into the Navimate or Skymate computer program. If you don’t want to blush in front of the captain, or at some inspection, then show your class and calculate the correction manually.

For this we need a manual called Nautical Almanac.

So, we take a bearing on the Sun, record the current time and coordinates, record the course using the gyro and magnetic compass.

Example:

Date: 03/19/2013 LMT(UTC+2): 17:46:30 Lat: 35-12.3 N Long: 35-55.0 E

Gyro bearing: 270.6 Heading 005 Magnetic heading 000

We adjust the time to Greenwich Mean Time (2nd time zone) GMT 15:46:30

Finding GHA (Greenwich Hour Angle)

Finding DEC (declension)

To find them, go to the main table of the Almanac and find the current date. We write out GHA and DEC for the current hour, and also write out correction d for the Sun (bottom right of the table). In our case it is equal to 1.0.

Then you need to correct the Greenwich hour angle and declination by adjustments to minutes and seconds.

This data can be found at the end of the book. The pages are headed by minutes and a GHA correction is provided for each second. There is also a correction for declination on the right side, which is selected according to d.

M’S” = 11-37.5 corr = 0-00.8

Now we adjust the Greenwich hour angle to the local time zone. To do this, we add (if E) or subtract (if W) our longitude:

GHA = 54-42.5 + Long 35-55.0

LHA = 90-37.5

Go to the Sight reduction table and select the values ​​A, B, Z1:

A = 55.0 B = 0 Z1 = 0

For the second entry in the table we need F and A.

To get F you just add B and DEC (+/-).

Our DEC is positive if the sign of declination and latitude coincides (N and N/S and S).

If our declination and latitude are different, then DEC is negative.

B=0

DEC=0-20.6S

F = 359 39.4 (rounded to 360)

Now having F and A, we enter the same table for the second and last time, and write out the second component of the azimuth Z2:

Z2 = 90

Then we add Z1 and Z2 and get the semicircular azimuth Z:

Z = 0 + 90 = 90

We convert semicircular azimuth to circular using the rule:

For northern latitude, if LHA is greater than 180: Zn = Z, if LHA is less than 180: Zn = 360 Z

For Southern latitude, if LHA is greater than 180: Zn = 180 – Z, if LHA is less than 180: Zn = 180 + Z

In our case Zn = 360 – 90 = 270

The desired bearing has been found. We take away our compass bearing 270 – 270.6 = - 0.6W

In order not to get confused in the order of calculations, I present the algorithm:

1. We make calculations, record bearing, position, time, and course.
2. We convert local time to Greenwich Mean Time.
3. We select the value of LHA and Dec from the tables.
4. We correct them by adjusting them for minutes and seconds.
5. Select the values ​​A, B, Z1 from the table.
6. We calculate F and select Z2 from the table.
7. We find the azimuth and convert it to circular.
8. We find the compass correction (true bearing minus compass bearing).
9. WE HANG A LARGE ASTRONOMICAL MEDAL ON OUR CHEST.

At first glance, everything looks cumbersome and unclear. But after a couple of practical calculations, everything will fall into place.

By the way, by adjusting your compass as the sun sets, you will have a unique chance to see the green beam. The fact is that at sunset, at the moment when the Sun disappears behind the horizon, due to refraction and refraction of color, it is very rare, but you can observe a green ray for several seconds. This mysterious, enigmatic and very rare phenomenon is reflected in numerous legends of different peoples, and is overgrown with legends and predictions.

For example, according to one legend, the one who saw the green ray will receive a promotion, prosperity, and will be able to meet the one with whom he will meet his happiness.

And this is not a story, since the Captain, having seen and appreciated the efforts, as well as the competence of the young navigator, will, of course, recommend him for promotion.

So determining the compass correction based on sunset is a direct path to promotion and, as a result, to well-being and happiness.

I wish all young navigators calm seas, career advancement, and a return to their native shores. May the green ray bring you happiness in your life.

Lecture 5

Topic: Methods for determining compass corrections

1. Determining the correction of the magnetic compass and monitoring its operation at sea

2. Determination of the gyrocompass correction and control of its operation at sea

As already mentioned, the main advantages of a magnetic compass are the simplicity of its design, autonomy and reliability. The main disadvantage is the low accuracy in determining directions. Errors reach 2–4 o, especially when pitching. Sources of errors: magnetic declination, deviation, inertia and insufficient sensitivity of the magnetic needle system to the Earth's magnetic field. The magnetic compass card arrives at the meridian 3–4 minutes after maneuvering.

Two magnetic compasses are usually installed on a ship. One, the main one, serves to determine the location of the vessel. It is installed in the ship's DP in a place that provides all-round visibility and protection from the effects of the ship's magnetic fields. As a rule, this is the upper bridge of the ship (direction-finding deck). The navigation compass is used to steer the vessel and is installed in the area of ​​the steering station. Although, taking into account that the magnetic compass is a backup device, at present, very often one main compass is installed on the direction-finding deck. The course from it is transmitted to the ship's control station using a periscope.

An accurate knowledge of the deviation of the magnetic compass is important in navigation. Deviation is destroyed at least once a year using the methods studied in the course “Technical means of navigation”. The residual deviation is determined by navigation methods and should not exceed several degrees.

According to good nautical practice, magnetic compass deviation is determined by:

A. at least once a year;

b. after repair, docking, demagnetization of the vessel, as well as after loading and unloading cargo that changes the ship’s magnetic field;

V. with a significant change in magnetic latitude;

d. when the tabulated deviation diverges from the actual one by more than 1 o for the main compasses and 2 o for the way compasses;

before a long flight.

All methods for determining deviation are based on the use of formula (4.6)

MP = CP + δ → δ = MP – CP

Since deviation depends on the ship's heading, it is usually determined on 8 equally spaced compass courses, and intermediate values ​​are found by linear interpolation. Usually these are courses corresponding to the main and quarter directions, i.e. courses 0, 45, 90, 135, 180, 225, 270, 315 degrees.

It is assumed that the ship's magnetic field is symmetrical relative to the ship's DP, i.e. the deviation is symmetrical relative to the magnetic meridian, therefore the average value of the compass bearing to a distant object taken to a distant landmark on equally spaced courses can be taken as an estimate of the magnetic bearing. The formula will look like this:

Where A is a certain correction for systematic error (constant deviation), which is determined for a specific compass during the destruction of deviation.

Basic methods for determining deviation:

1.1. On target(Fig.1)

§ 17. Magnetic and compass points, courses and bearings. General compass correction

The direction at sea is determined not only relative to the true meridian, but also relative to the magnetic and compass meridians, calling them in the general case magnetic compass points.

Rice. 21.

Let us depict three meridians on the plane of the true horizon (Fig. 21): true NiSi, magnetic MMSM and compass NKSK, the direction of the center plane OD and the direction from the ship to the coastal landmark OM. In the drawing, the angle N and OD is the true heading of the ship, and the angle N and O M is the true bearing. By analogy, it is believed that the NMOD angle is the magnetic heading (MC), the NKOD angle is the compass heading (CC), the NMOD angle is the magnetic bearing (MP), and the NKOM angle is the compass bearing (CP).

Thus, Magnetic course The vessel is called the angle at the center of the compass, measured from the northern part of the magnetic meridian to the direction of the bow of the ship's center plane clockwise from 0 to 360°. Compass heading- the angle at the center of the compass, measured from the northern part of the compass meridian to the direction of the bow of the centerline plane of the ship clockwise from 0 to 360°. Magnetic bearing an object is called the angle at the center of the compass, measured from the northern part of the magnetic meridian to the direction towards the object clockwise from 0 to 360°. Compass bearing an object is called the angle at the center of the compass, measured from the northern part of the compass meridian to the direction towards the object clockwise from 0 to 360°.

True courses and bearings are related to the magnetic following algebraic relations:

Example 19. I K = 355°, d = 11°5W.

Solution(formulas 19)

Example 20. MP = 132°, d = 5° O st .

Solution(formulas 20)

Magnetic courses and bearings are related to the compass by the following algebraic relationships:
Example 21. CC = 357°; 5 = 5°O st .

Solution(formulas 21)

Example 22. MP = 4°: CP = 358°

Solution(formulas 22)

The combined action of the forces of earthly magnetism and the magnetic field of the ship leads to the fact that the magnetic needle deviates from the true meridian by a certain total angle, called general compass correction. It is designated by the symbol AK.

The general correction is called the truss or leading and is given a “plus” or “minus” sign depending on whether the northern part of the compass meridian is deviated towards the truss or the leading from the northern part of the true meridian. For example:

AK = +3° or AK = -10°.

The general compass correction, declination and deviation are related by the following algebraic relationships.