Navigation Compasses

Gyro Theory A Gryro compass is a North Seeking Gyroscope A spinning wheel held by two GIMBALS Has 3 axies of angular freedom Spin Axis Horizontal Axis Vertical Axis

Gyrocompass Gyroscope 3 axes Gyrorepeaters Spin Torque Precession Precession axis Spin axis The Gyrocompass: The gyrocompass is essentially a north-seeking gyroscope. It is encased in a housing fitted within various electronic components that keep the spin axis of the gyro aligned with terrestrial meridians, and sense the angle between the ship’s head and the gyro spin axis. The gyrocompass has three axes: the spin axis, torque axis, and precession axis. As centrifugal force of the earth’s rotation (tangential velocity), acts upon the gyro, the torque and precession axis will react, and keep the spin axis oriented to a terrestrial meridian. The tangential velocity is maximized at the equator, and diminishes to zero at the poles. Consequently, the gyrocompass must be checked constantly for error beyond 70 degrees north or south latitude. Most shipboard compasses become useless at latitudes beyond 85 degrees. The Gyrocompass is usually located well down in the interior of the hull in order to minimize the effects of pitch and roll. The gyrocompass is connected by cables to gyrocompass repeaters located throughout the ship. These repeaters use electronic servo-mechanisms to reproduce the master gyrocompass readings at remote locations. Torque axis

Compasses - Abbreviations Gyrocompass PGC Magnetic Compass Steering Compass PSTC Standard Compass PSC Compasses Most oceangoing vessels, including all navy warships, have at least one gyro compass installed The magnetic compass is used as a backup in case of gyro failure, and as a primary means of checking gyrocompass accuracy while underway. Older Navy warships usually have two magnetic compasses. One located near the helmsman called the steering compass and a second located at a secondary conning station called the standard compass. Newer Navy warships have only one magnetic compass because they have two independent gyrocompass systems for which the probability of simultaneous failure is extremely remote.

Gyro Theory Physics – A Gyro Scope will always point in one direction (in this case TRUE NORTH) Precision – Apply force to the spin axis and the gyro scope will rotate, not in the direction in which the force was applied but 90 degrees from that force vector.

Gyro Compasses Ships normally have 2 gyro compasses Main – Orientated N/S used for Navigation Auxiliary – Oriented E/W for combat systems or as a backup for Navigation. Located deep within the ship On the center line

Gyro Uses Gyro Repeaters Located on the bridge wings, Navigation Table, Dead reckoning plot Are used for Fire Control for gunnery systems – Pitch, yaw, and Roll Centerline Pelorus – Center of Bridge used as the most accurate reference

Gyro Error Mechanical Device – has some inherent error – THE KEY IS TO KNOW WHAT IT IS AND HOW TO CORRECT FOR IT Sources of error Mechanical Friction Ship’s motion Electronic malfunctions Power Fluctuations

Determining Error Visual Range Triangulation Adjusting three or more lines of position to find location and bearing . Compare course with a known true course Entering and exiting port or in a channel Celestial Azmiuths and Amplitudes Comparison with a gyro of known error

Gyro Error Gyro Least, error East Gyro Best, error West Error – the difference between the true bearing and the Gyro reading Expressed in degrees East or degrees West Gyro Least, error East Gyro Best, error West Gyro Bearing + East Error = True Bearing Methods of determining gyrocompass error: Although the gyrocompass is a very accurate instrument and normally has a very small error associated with its readings (less than .10 to .20), the navigator is required to determine gyro error at least once a day. Gyrocompass error like magnetic compass error, is measured in degrees east or west:. If the gyrocompass bearing is lower than the actual bearing, the the error is east: Hobbs has “Compass best, error west”--- same same talking about the Gyro-Compass If the gyrocompass bearing is higher than the actual bearing, the error is west:.

Error Example GYRO = 175 True = 173 GYRO is BEST ERROR is WEST You are dead center on a Range Bearing 173 Degrees True and your Gyro reads 175 Degrees PGC what is your Error? GYRO = 175 True = 173 GYRO is BEST ERROR is WEST 175 – 173 = 2 degrees West

Shipboard Compasses Three principle references of direction Ships longitudinal axis RELATIVE BEARING (R) Local magnetic meridian MAGNETIC BEARING (M) True meridian TRUE BEARING (T) Onboard ship, there are three principal references for direction: the ship’s longitudinal axis, the magnetic meridian, and the true or geographic meridian. Bearing: The horizontal direction of one terrestrial point from another, expressed as an angle from 0000 clockwise to 3600 . Relative bearings (abbreviated with an R following the bearing): Bearings measured with reference to the ship’s longitudinal axis. Magnetic bearings (abbreviated with an M following the bearing): Bearings measured with respect to magnetic north. They are measured with a magnetic compass. True bearings (abbreviated with a T following the bearing): Bearings that are measured with respect to true or geographic north. They are measured with a gyrocompass of known error. Ship’s head, or heading: A special bearing denoting the direction in which the ship is pointing. It can be be expressed with reference to magnetic or true north.

Relative Bearing + Ship’s Head = True Bearing 090 + 090 = 180 Relative Vs. True Relative bearings Can NOT be plotted so must converted Relative Bearing + Ship’s Head = True Bearing 090 + 090 = 180

Magnetic The Earth is a core of Iron which acts as a large magnet with its poles aligned with the earth’s axis NOT located at 090 North Latitude Difference is VARIATION Magnetic Meridians - Skewed due to irregularities in the Earth’s core

Applying Variation Variation is found by referencing the compass rose closest to the ship’s position

Deviation Each ship is made of Steel and Iron It has its own magnetic field that can effect changes in the ship’s magnetic compass Determined by angle in which the Keel is laid during initial construction Equipment – electronics can also cause deviation Expressed in Degrees East and West (rounded to nearest .5 Degree) Changes with the ship’s heading Deviation: The angle between the magnetic meridian and the north axis of the compass card. Deviation, like variation, is expressed in degrees east or west to indicate on which side of the magnetic meridian the compass card north lies. Deviation exists because of the interaction of the ship’s metal structure and electrical currents with the earth’s magnetic lines of force and the compass magnets. Deviation varies with ship’s heading. Deviation can change as large metal objects are moved around the ship. Semipermanent magnetism induced by long periods spent pierside (such as during overhaul) can affect deviation. A Navy ship’s degaussing system -- a set of wire coils installed around the underside of the ship’s hull in order to reduce its magnetic “signature”-will also have an effect on deviation. Deviation can be compensated for, but never eliminated. Therefore, some correction must be applied to the compass in order to determine a true heading or bearing. A table of deviation for every 15 degrees of the ship’s head magnetic, starting with 0000M, provides correcting values to be added or subtracted from the compass heading. The table is based on ship’s head magnetic. The data on the table is obtained by a process called swinging ship. This process entails turning the ship 15 degrees at a time and recording the magnetic compass error. The data obtained from swinging ship is displayed in two columns, one that provides corrections for deviation with degaussing on (DG ON) and one that provides corrections for deviation with degaussing off (DG OFF).

Degaussing Degaussing system - electrical installation designed to protect ships against magnetic mines and torpedoes Purpose – counteract the ship’s magnetic field and establish a condition such that the magnetic field near the ship is, as nearly as possible, just the same as if the ship were not there Degaussing installation consists of permanently installed degaussing coils wrapped around ship on underside of hull, control unit to control the coil current, and compass compensating equipment to prevent disturbances to mag compasses Coil is a large diameter electrical wire A, F, L, M, Q Coils

Compass Error Deviation + Variation = Compass Error Observed Bearing Least = Error East Observed Bearing Best = Error West Ex. Deviation = 3 West Variation = 13 West 3 + 13 = 16 West

Example USS Princeton is on course 125 degrees true when you lose your gyro scope. The Navigation gives you the following: Variation = 8 degrees West Deviation = 2.5 degrees East

Example Truly – True Course = 125 Valiant – Variation = 8 W Marines - Magnetic Course = 133 Don’t - Deviation = 2.5 E Cry – Compass – 133 –2.5 = 130.5 Because of ADD WEST

Application of Compass Error Can Dead Men Vote Twice At Elections Compass Head What the mag compass reads Deviation Magnetic Head What the magnetic heading is Variation True Head What the true heading is Add East

Sample Problem #1 Degaussing is ON Ship’s Magnetic Compass reads 030° PSTC Var is 11° W Find ship’s true head

Interpolation Bill and Jorge decide to split a pizza. The pizza costs $9.00 and has 6 slices. Bill eats 5 slices Jorge eats 1 slice How much should Bill pay?

Interpolation Eating all the pizza slices costs $9.00 Eating none of the pizza costs $0.00 So Bill has eaten 5/6 of $9.00 5/6 times $9.00 = $7.50 This is interpolation - calculation of an internal value by assuming a linear relationship with surrounding data.

Sample Problem #2 C D M V T AE CDMVT 030 + (-3) = 027 Reinterpolate 027 - 11 = 016 degrees true

Sample Problem #2 C D M V T AE 030

Sample Problem #2 C D M V T AE 030 (-3)

Sample Problem #2 C D M V T AE 030 (-3) 027

Sample Problem #2 C D M V T AE 030 (-3) 027 030 (-3) 027 Reinterpolate - We assumed that the magnetic head was 030, we now see it was 027

Sample Problem #2 Reinterpolate - We assumed that the magnetic head was 030, we now see it was 027. Value for 030 = 3.0W Value for 015 = 3.5W 3.0W-3.5W = -0.5W

Sample Problem #2 030 - 015 = 15 degrees 027 - 015 = 12 degrees So we are 12/15 of the way from 015-030 12/15 times -0.5 = -0.4 BUT we round to the nearest .5 so it is -0.5 3.5W - 0.5W = 3.0W Or no change (in this case)

Sample Problem #2 C D M V T AE 030 (-3) 027 Reinterpolate

Sample Problem #2 C D M V T AE 030 (3) 027 Reinterpolate (11)

Sample Problem #2 C D M V T AE 030 (3) 027 Reinterpolate (11) 016

Questions? Homework – Read Chapter 8 pg. 129-147 1st Homework due Tuesday 2nd due Next Thurs