GB2221037A - Measurement of a ship's magnetic field - Google Patents
Measurement of a ship's magnetic field Download PDFInfo
- Publication number
- GB2221037A GB2221037A GB8912344A GB8912344A GB2221037A GB 2221037 A GB2221037 A GB 2221037A GB 8912344 A GB8912344 A GB 8912344A GB 8912344 A GB8912344 A GB 8912344A GB 2221037 A GB2221037 A GB 2221037A
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- GB
- United Kingdom
- Prior art keywords
- ship
- magnetic field
- induced
- measuring
- measurements
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
- B63G—OFFENSIVE OR DEFENSIVE ARRANGEMENTS ON VESSELS; MINE-LAYING; MINE-SWEEPING; SUBMARINES; AIRCRAFT CARRIERS
- B63G9/00—Other offensive or defensive arrangements on vessels against submarines, torpedoes, or mines
- B63G9/06—Other offensive or defensive arrangements on vessels against submarines, torpedoes, or mines for degaussing vessels
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- Physics & Mathematics (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Engineering & Computer Science (AREA)
- Aviation & Aerospace Engineering (AREA)
- Measuring Magnetic Variables (AREA)
Abstract
Correct degaussing of a ship depends on a knowledge of the ratio of permanent to induced magnetisation and this, instead of as is usual being determined by magnetic ranging at different magnetic latitudes, is determined by mounting a single axis vertical field magnetometer (41) at the masthead (40) and measuring the field strength whilst sailing the ship on northerly, southerly, easterly and westerly courses respectively. Simultaneously with the magnetic field measurements the roll stabilisation system of the ship is used to drive the ship into a rolling mode when roll angles phi are measured by means of the ship's navigation system. A data logger is used to simultaneously record the magnetic field measurements and the roll motion and these measurements are then correlated. From the correlation the induced and permanent field components can be deduced. <IMAGE>
Description
Measurement D;f ~ Ship's Magnetic Afield The invention relates to a method and apparatus for determining the ratio of permanent to induced magnetisation in a ship's magnetic field.
At any time, although a ship has only one distribution of magnetisation, the magnetisation is conventionally thought of as consisting of two parts: a permanent and an induced field. The induced field is considered to depend only on the current magnetisation and direction of the Earth's magnetic field and the orientation of the vessel. Other terms contributing to the magnetic field are considered together as "permanent". The permanent magnetisation of a ship also varies, but with a much longer time constant; with its value depending on the previous magnetic environment (history). When a ship is moved fran one geographical location to another the value of permanent magnetisation will slowly reach an equilibrium state.Its value depends on terms including stress factors as caused by the buffeting of waves as well as the integrated effect of the magnitude and direction of the Earth's field through which the ship has passed. For correct operation of a ship's degaussing system worldwide there is a need for the ratio of the permanent to induced magnetisation of the ship to be known. A ship's magnetic field is measured on a range and degaussing fields are then determined to counter the ship's own field. Such degaussing arrangements will be effective for equivalent magnetic latitudes but correction will be needed at other magnetic latitudes where the induced components of the ship's field will differ because of the Girrerins local Earth's magnetic field.Various empirical ratios of permanent to induced magnetic field have been used in the past.
Techniques previously proposed for measurement of the ratio of permanent to induced magnetisation include the following: a) measurement of magnetisation at two different locationshaving at least a 10% difference in vertical component of Earths magnetic field; b) listing the ship by about 10-15 and then carrying out a magnetic ranging; and c) producing a local inducing magnetic field with a seabed coil and then observing the ship's magnetisation.
The above techniques are not easy and make no allowance for the fact that the measured values can change during a voyage.
The object of the invention is to provide a method whereby the ratio of induced to permanent magnetisation of a ship can be determined.
The invention provides a method for measuring the induced components of magnetic field in a ship comprising the steps of: a) providing on the ship a sensor for measuring the magnetic field; b) computing the longitudinal component of the induced magnetic field fron the difference in sensor measurements taken while the ship is on a northerly run and while on a southerly run; c) computing the athwartships component of the induced magnetic field fram the difference in sensor measurements taken while the ship is on an easterly run and while on a westerly run; d) measuring the roll angle of the ship simultaneously with magnetic field measurements from the sensor; and e) correlating the measured roll angles and the magnetic field measurements to determine the vertical component of the induced magnetic field.
Advantageously the magnetic field sensor is a single axis magnetaneter arranged to measure the vertical component of the field. Preferably the magnetaneter is sited at a masthead, and as conveniently close to the centre of the ship as possible.
Conveniently a data logger is used to record magnetic field and roll angle measurements. In the preferred arrangement the simLltaneous measurements of magnetic field and roll angle are carried out on a calm day with little sea movement and the ship's roll stabilisation system is used to drive the ship into a roll oscillation mode while the ship is on an easterly run.
The invention allows the ratio of the permanent and induced terms in the magnetisation to be measured conveniently while the ship' s degaussing syster is switcilea off and because a correlation tecbnique is loyed there is not a need for the ship to be oscillated through angles as large as 100.
The invention will now be described by way of example only with reference to the accompanying Drawings of which:
Figure 1 is a graphical illustration of the variation of the vertical magnetic field due to permanent and induced vertical magnetisation along the length of a ship with a vertical inducing field;
Figure 2 is a graphical illustration of the variation of the vertical magnetic field due to permanent and induced longitudinal magnetisation along the length of a ship with a longitudinal inducing field;
Figure 3 is a graphical illustration of the variation of the vertical magnetic field due to permanent and induced athwartships magnetisation across the width of a ship with a transverse inducing field; and
Figure 4 shows a ship carrying a magnetometer and travelling on an easterly course for the purpose of illustrating the invention.
A ship's magnetic field is made up of two components: one which changes fairly rapidly with the local magnetic field - the induced magnetisation, and one which is relatively constant - the permanent magnetisation. As a ship moves through the Earth's magnetic field the induced magnetic field in the ship' s hull, and the permanent field to a lesser extent, depends upon the strength and direction of the magnetic field in relation to the direction of the ship's movement. Figures 1 to 3 show the variation of the vertical component of magnetisation associated with various types of hull magnetisation.Figure 4 illustrates a single axis masnetometer 41 mounted on a masthead 40 for measuring the vertical field The mast position is snowy in Figures 1-3 by the respective dashed lines 10, 20 and 30. For simplicity the following terms are define Permanent vertical magnetisation Permanent longitudinal magnetisation PRSi Permanent athwartships magnetisation rDs Induced vertical magnetisation I;; ; Induced longituainal magnetisation 1VW1 Induced athwartships magnetisation
For greater accuracy of the invention the mast head masnetometer position would be located at position (21, 31) over the zero PLM, ILM and zero PP22 IAM positions. This is not essential however.
From Figure 2 it can be seen that the induced longitudinal magnetisation (ILM) changes sign depending on whether the ship is travelling north or south, while the permanent longitudinal magnetisation (PLM) is direction invariant.
Hence at the position of the magnetoneter the signal for a north run will be:
MG1=IVM+PVM+ILM+PAM and for south run:
MG2=IVM+PVM-ILM+PAM
ILM=0.5*(MGI-MG2) (1)
IVM+PVM+PAM+PLM=0.5*(MGI=MG2) (2) Fron Figure 3:
For an East run:
MS3=PVM+PLM+PAM+IVM+IAM
For a Nest run:
MG4=PVM+PAM+PLM+IVM rIAI-t Hence:
IAM=0.5*(MG3-MG4) (3)
Now consider the ship to be rolled through an angle phi while it is travelling on an easterly course, as depicted in Fig 4 and the local value of the earth' s magnetic field (He) and dip (PSI) angle are known.
The induced components IVM,ILM, and ISiare dependent upon the magnitude of their inducing fiela. When the ship has zero pitch and roll the inducing components are:
Vertical He*sin(PSI) (FOR IVM)
Horizontal He*cos(psi) (for IAM and I The inducing fields when a ship is travelling on an Easterly course and has a roll angle of phi are given by:
Ship's Vertical He*sin(psi+phi)
Ship's Horizontal He*cos(psi+phi)
The magnetometer signal is given by:
MG5=PLM+PAM+PVM perm terms +IVM*He*sin (psi+phi)/He*sin(psi) )
+IAM*He*cos(psi+phi)/He*cos(psi) ) Induced terms
+ILM*He*cos(psi+phi)/He*cos(psi) (psi)
+He*sin(psi+phi) - Earth's field
This signal has two components a steady state term and an oscillatory term.
The oscillatory term MS5OSC is given by:
MG5Osc=(ILM+IAM) *sin(psi+phi)/cos(psi) +IVM*cos(psi+phi)/sin(psi) +He*sin (psi+phi) Hence if we know the magnitude of this component we can determine I\Th'j. We are then left with the three values of permanent magnetism to determine.
Provided the ship has recently been measured on a Degaussing range (DG) by means of a modern three axis range which has the ability to predict the ship' s magnetic field anywhere is space, the values of PAM and PLM can be deduced.
However, say measure of these values can be made from a single axis DG ranging.
In practice the operation of the method would be as follows:
A ship recently measured on a Degaussing range is sailed on an easterly course in a calm sea, so as to minimise pitching motion. The roll stabilisation system is used to drive the ship into a roll oscillation mode. A data logger system is provided to simultaneously record the roll angle from the ship's navigation system and the signal from the mast head magnetometer. The recorded data is then analysed on a computer system using a correlation technique to determine the magnitude of the oscillatory component and from this IVM and PVM can be determined. If measurements are done in a rough sea a 3-axis l:iagnetoneter would be required and the effects of yaw and pitch would need to be taken into account.
Claims (7)
1. A method for measuring the induced components of magnetic field in a ship
comprising the steps of: a) providing on the ship a sensor for measuring the magnetic field; b) computing the longitudinal component of the induced magnetic field from the difference in sensor measurements taken while the ship is on a northerly run and while on a southerly run; c) computing the athwartships component of the induced magnetic field from the difference in sensor measurements taken while the ship is on an easterly run and while on a westerly run; d) measuring the roll angle of the ship simultaneously with magnetic field measurements from the sensor; and e) correlating the measured roll angles and the magnetic field measurements to determine the vertical component of the induced magnetic field.
2. A method for measuring the induced components of magnetic field in a ship ship as claimed in claim 1 wherein the magnetic field sensor is a single axis magnetometer arranged to measure the vertical component of the field.
3. A method formeasuringtheinducedcomponents of magnetic field in a shipe as claimed in claim 1 or 2 wherein the magnetometer is sited at a masthead.
4. A method for measuring the induced components of magnetic field in a ship as claimed in claim 3 wherein a mast is selected as conveniently close to the centre of the ship as possible.
5. A method for measuring the induced components of magnetic field in a ship as claimed in any one preceding claim wherein a data logger is used to record magnetic field and roll angle measurements.
6. A method for measuring the induced components of magnetic field in a ship as claimed in any one preceding claim wherein the simultaneous measurements of magnetic field and roll angle are carried out on a calla day with little sea msvement and the ship's roll stabilisation system is used to drive the ship into a roll oscillation mode while the ship is on an easterly run.
7. Method for measuring the induced copoonents of magnetic field ina ship substantially as described with reference to the zcssmpanying Drawings.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB888817277A GB8817277D0 (en) | 1988-07-20 | 1988-07-20 | Measurement of ship's magnetic field |
Publications (3)
| Publication Number | Publication Date |
|---|---|
| GB8912344D0 GB8912344D0 (en) | 1989-07-12 |
| GB2221037A true GB2221037A (en) | 1990-01-24 |
| GB2221037B GB2221037B (en) | 1992-06-10 |
Family
ID=10640790
Family Applications (2)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB888817277A Pending GB8817277D0 (en) | 1988-07-20 | 1988-07-20 | Measurement of ship's magnetic field |
| GB8912344A Expired - Lifetime GB2221037B (en) | 1988-07-20 | 1989-05-30 | Measurement of a ship's magnetic field |
Family Applications Before (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB888817277A Pending GB8817277D0 (en) | 1988-07-20 | 1988-07-20 | Measurement of ship's magnetic field |
Country Status (1)
| Country | Link |
|---|---|
| GB (2) | GB8817277D0 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2698698A1 (en) * | 1992-11-27 | 1994-06-03 | Thomson Csf | Magnetic measuring station for naval vessels. |
| FR2704065A1 (en) * | 1991-12-26 | 1994-10-21 | Thomson Csf | Device for measuring the magnetic signature of a naval vessel and its application to setting up the magnetic immunisation |
| RU2466903C2 (en) * | 2010-12-22 | 2012-11-20 | Федеральное государственное унитарное предприятие "Центральный научно-исследовательский институт имени академика А.Н. Крылова" (ФГУП "ЦНИИ им. акад. А.Н. Крылова") | Method of isolating signal caused by influence of vertical component of terrestrial magnetic field in ship onboard magnetic field multi-transducer control system |
-
1988
- 1988-07-20 GB GB888817277A patent/GB8817277D0/en active Pending
-
1989
- 1989-05-30 GB GB8912344A patent/GB2221037B/en not_active Expired - Lifetime
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2704065A1 (en) * | 1991-12-26 | 1994-10-21 | Thomson Csf | Device for measuring the magnetic signature of a naval vessel and its application to setting up the magnetic immunisation |
| FR2698698A1 (en) * | 1992-11-27 | 1994-06-03 | Thomson Csf | Magnetic measuring station for naval vessels. |
| WO1994012890A1 (en) * | 1992-11-27 | 1994-06-09 | Thomson-Csf | Magnetic measurement station for naval vessels |
| AU678790B2 (en) * | 1992-11-27 | 1997-06-12 | Thomson-Csf | Magnetic measurement station for naval vessels |
| RU2466903C2 (en) * | 2010-12-22 | 2012-11-20 | Федеральное государственное унитарное предприятие "Центральный научно-исследовательский институт имени академика А.Н. Крылова" (ФГУП "ЦНИИ им. акад. А.Н. Крылова") | Method of isolating signal caused by influence of vertical component of terrestrial magnetic field in ship onboard magnetic field multi-transducer control system |
Also Published As
| Publication number | Publication date |
|---|---|
| GB8912344D0 (en) | 1989-07-12 |
| GB2221037B (en) | 1992-06-10 |
| GB8817277D0 (en) | 1988-08-24 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PCNP | Patent ceased through non-payment of renewal fee |
Effective date: 19940530 |