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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 – Spin – Torque – Precession Precession axis Spin axis Torque axis • Gyrorepeaters Compasses - Abbreviations • Gyrocompass • Magnetic Compass PGC – Steering Compass – Standard Compass PSTC PSC 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 • 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 Error Example 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) 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 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 Deviation Magnetic Head Variation True Head Add East What the mag compass reads What the magnetic heading is What the true heading is 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 Sample Problem #2 C D 030 M V T AE Sample Problem #2 C D M 030 (-3) V T AE Sample Problem #2 C D M V 030 (-3) 027 T AE Sample Problem #2 C D M V T AE 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 030 (-3) 027 Reinterpolate 027 AE Sample Problem #2 C D 030 (3) M V T 027 Reinterpolate 027 (11) AE Sample Problem #2 C D 030 (3) M V T 027 Reinterpolate 027 (11) 016 AE Questions? • Homework – Read Chapter 8 pg. 129-147 • 1st Homework due Tuesday • 2nd due Next Thurs