ADV_Chapter 08 - INS & IRS -NEW

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1 Chapter 8 INS & IRS

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2 Inertial Navigation System

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3 The INS system provide navigation data and attitude data

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4 The INS sense the aircraft acceleration in North-South and East-West direction and compute the aircraft displacement by integrating acceleration with time twice. The aircraft displacement in North-South and East-West direction can be convert to latitude and longitude change.

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5 The accelerometer is basically a pendulous device. It sense the aircraft’s velocity change.

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6 The acceleration signal from the amplifier is also sent to an integrator which is a time multiplication device. It starts out with acceleration multiplied by time and the result is a velocity. In the second integrator, velocity multiplied by time and the result is a distance.

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7

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8 E -W movement ( Departure ) Departure is the distance between two meridians along a parallel of latitude. Departure (nm) =  * COS   = change of longitude ( min ) = latitude ( deg )  = Departure (nm) / COS     Departure

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9 N - S movement The distance between two Latitude along a longitude : N-S distance (nm) =  x 60  = change of latitude ( deg )  North pole N-S Distance  = N-S distance (nm) / 60

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10 The accelerometers would be mounted on a platform. One is in the North –South direction. One is in the East-West direction. One is fitted to measure vertical acceleration.

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11 The INS system is able to calculate many navigation data and display those data through the rotary switch at the bottom left of the control unit .

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12 The platform must be kept level during flight, so that the N-S , E-W accelerometer will not sense false acceleration when the airplane is pitching and rolling. If the accelerometer is not kept earth horizontal, error due to gravity effect would occur. Gravity Effects on Accelerometer

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13 The gyroscope input axis sense the gimbal tips off from level, then the gyroscope output signal to gimbal drive motor which restores the gimbal to the level position again.

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14 INS component ( Gimbal Platform ) : Gimbal drive motor receive signal from gyroscope and maintain platform level and align with true north.

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15 Three integrating gyros are mounted on the inertial platform, with their input axis mutually perpendicular. Three gimbal motors drive the platform gimbal rings about the pitch, roll and vertical axes respectively.

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16 If there is no compensation of earth rotation to the system, the platform will just remain the same attitude in space ( not parallel to the earth surface) cause the system cannot work properly. Platform Correction for EARTH ROTATION

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17 The system must compensate for earth rotation so that the platform will maintain level to the earth surfaces. Platform Correction for EARTH ROTATION

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18 Platform Correction for EARTH ROTATION The earth rotation rate is 15.04 degree / hours. Horizontal and vertical component of Earth’s rate varies with latitude. This two components will be used for earth rotation compensation for platform.

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19 Platform correction for Aircraft Movement The aircraft transport rate compensation must be made to keep the platform level to earth’s surface. It is done by torquing the gyro by the velocity divided by earth’s radius.

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20 The velocity signal 2 is also sent electronically to develop the transport rate torquing 6. The transport rate and earth rate terms are summed 9 and they are sent into the gyro torquer. This causes the rotor of the gyro 10 to tilt with respect to the case. Platform correction for Aircraft Movement

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21 When it does, an output signal 11 is generated, which is amplified and used to drive the gimbal drive motor 12 which causes the gimbal 13 to tilt in proportion to the two input terms, earth rate and transport rate torquing.

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22 B B A Earth Rotation CORIOLIS FORCE CORIOLIS FORCE Path A to B without earth rotation Path A to B with earth rotation Coriolis accelerations caused by the aircraft following a curved path in space when flying normal earth referenced flights.

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23 CENTRIPETAL FORCE Centripetal accelerations caused by platform rotation to maintain the local earth vertical.

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24 Coriolis effects and centripetal effects must be compensated for within the system.

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25 Alignment of the INS system 1. Warm up period 2. Coarse alignment - the platform is roughly levelled and aligned in azimuth, this removes gyro alignment errors and cuts the time to a minimum. 3. Fine levelling - with zero output from the accelerometers fine levelling is achieved. The process takes anything up to 1 to 1 ½ minutes, levelling the platform to within 6 seconds of arc. Gyro compassing - stabilise the platform about an East-West axis.

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26 Alignment of the INS system

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27 Schuler Period Schuler tuning provides the inertial platform of a navigation system with a feedback loop between its velocity output and its stabilizing gyros such that it behaves as though it were such a pendulum. This makes it remain vertical as the vehicle moves from place to place on the surface of the earth.

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28 If the platform is displaced from the horizontal, it would oscillate with a period of 84.4 minutes. Schuler Tuning

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Schuler Tuning

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30 Errors : Inherent Errors a.) The irregular shape and composition of the Earth. b.) The movement of the Earth in space. c.) System design, imperfections of gyro bearings and mass imbalances . Bounded Errors Errors which build up to a maximum and return to zero within 84.4 minutes Schuler cycle, are termed Bounded errors.

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31 Errors : Unbounded Errors - are either cumulative track errors or distance errors : are caused by initial azimuth misalignment, wander of the azimuth gyro, wander in the leveling gyros and integrator errors.

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32 Inertial Reference System I R S

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33 The laser gyro has caused a technological revolution in the design of inertial reference and navigation systems ( IRS ) . It eliminates the need for gimbals, bearings, torque motors, and their moving parts as in the INS system. Inertial navigation means the determination of a vehicles location without the aid of external references. Strap down inertial navigation system is a navigation system without the use of a mechanically stabilised platform.

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34 IRUs Outputs : Primary attitude……. Pitch and roll Heading……………. True, Magnetic Accelerations……… Lateral, Longitude, Normal Angular rates………. Pitch, Roll, Yaw Inertial velocity……. N/S, E/W, GS, TA, Vertical rate Position……………..Latitude, longitude, inertial altitude Wind data…………..Wind speed, wind angle, drift angle Calculated data……. Flight path angle and acceleration along and across track acceleration Inertial pitch and roll rate Vertical acceleration Potential vertical speed.

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35 Inertial information is used by: Flight management computer Flight control computer Thrust management computer Stability augmentation system Weather radar Anti skid auto brake systems Attitude direction indicator Horizontal situation indicator Vertical speed indicator Radio direction magnetic indicator Flight data recorder

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36 The primary sources of information for the IRU are its own internal sensors three laser gyros, and three inertial accelerometers.

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37 The other IRU inputs are : Initial position, Barometric altitude, ( stabilize the vertical navigation ) True Air Speed (TAS) ( allows IRU to calculate wind speed and wind direction)

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38 The laser gyro uses the characteristics of light to measure motion. This device operates based on the SAGNAC effect.

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39 If two pulses of light are sent in opposite directions around a stationary circular loop of radius R, they will traveled the same inertial distance at the same speed, so they will arrive at the end point simultaneously.

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40 No rotation

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41 If the loop is rotating, the pulse traveling in the same direction as the rotation of the loop must travel a slightly greater distance than the pulse traveling in the opposite direction. As a result, the counter-rotating pulse arrives at the "end" point slightly earlier than the co-rotating pulse.

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42 Clockwise rotation As the apparatus rotates, light in one branch travels a different distance than the other branch,

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Clockwise rotation changing its phase and resonance frequency with respect to the light travelling in the other direction,

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44 , resulting in the interference pattern beating at the detector. Clockwise rotation

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45 c = f x λ c = Speed of Light f = frequency λ= Wave length When the wavelengths change there is a concurrent change in the lights frequency.

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46 The laser gyro contains three mirrors to achieve a rotational path for two beams that are generated and sent around in a triangular path in opposite directions. If there is no movement of the device the beams cancel each other out but when movement is induced one of the beams will take longer to complete its path and the other, in opposition, a measurably shorter length of time to complete its journey. This whole process is measured by devices known as gain elements and the rate of rotation can be calculated.

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47 Two opposite laser beams are directed to the photo cell detector. Due to the interference of light wave, there is a fringe pattern on the photo cell detector. If the system has rotation, the fringe pattern will have movement. The photo cell detector can determine the rotation of the system by detecting the speed and direction of the fringe pattern. How laser gyro can determine there is the rotation of the system ?

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48 Why we use laser and not use other kind of light ? It is because Laser has pure frequency.

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49 IRS NO ROTATION λ speed of light = c = f x λ= constant Wave length f = frequency Counter clockwise laser clockwise laser

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50 IRS CLOCKWISE ROTATION c = f x λ= constant λ λ In a rotating gyro, one laser beam will exhibit an increase in frequency, whereas the other beam will exhibit a frequency decrease. f increase f decrease decrease increase counter clockwise laser clockwise laser

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51 IRS COUNTER CLOCKWISE ROTATION c = f x λ= constant λ λ In a rotating gyro, one laser beam will exhibit an increase in frequency, whereas the other beam will exhibit a frequency decrease. f decrease f increase increase decrease counter clockwise laser clockwise laser

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52 If the entire device is rotating, the beams will reach the detector at slightly different times, and slightly out of phase, producing optical interference "fringes" that can be observed and measured. Then, the rotation rate ( pitch , roll , yaw rate ) of the aircraft can be calculated. The angular rate is measured by counting the interference fringes.

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53 Interference of two circular waves

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54 What is laser lock ? At very low rotation rates of aircraft, the two light beams may synchronise each other. It causes the laser gyro unable to detect the rotation of aircraft and causes wrong computation of attitude. To prevent laser lock, Piezo Electric Dither Motor is to design for generating vibration to break the laser lock

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55 Piezo Electric Dither Motor generate vibration to prevent laser lock

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56 Strap Down System : In each IRU, there are 3 laser gyros and 3 accelerometers align with the airplane’s 3 axis. To detect the movement of airplane along with aircraft three axis. …... Laser gyros detect the angular movement of airplane around the airplane’s 3 axis. Accelerometers detect the linear movement of airplane along with airplane’s 3 axis.

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57 IRS use High speed micro processors to achieve a stable platform mathematically rather than mechanically ( as per the INS) - this results in greatly improved accuracy and reliability. The main different between IRS and INS

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58 Va N E x y -z Ve Computer converts the movements data along with aircraft axis into the movement data along with the earth’s latitude and longitude. y x -z y x -z x y

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59 Va N E x y -z Ve Computer use mathematics method to convert acceleration relate to airplane axis into acceleration related to earth co-ordinate

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60 Euler angles represent three composed rotations that move a reference frame to a given referred frame. Any orientation can be achieved by composing three rotations each around a single axis.

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61 After obtaining the airplane acceleration in earth co-ordinate( N-S, E-W), IRS use the same method as INS to compute the aircraft position.

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62 1st Stage Integration IRS use the same integration principles as the older INS system.

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63 How IRS system find the direction of True North. The IRS system, as with the INS, requires to find true north to achieve an alignment and this is achieved when the aircraft is stationary on the ground and the only rate of change is that associated with the rotation of the Earth. The vector direction of earth’s rotation is true north.

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64 For IRU initial alignment , you have to switch IRS switch to “NAV” Position. 747-400 IRS

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Then, key in the initial position on FMC “POS INIT” page

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66 Initial Alignment Process

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67 Remarks : 1. FMC position ? IRS position? 2. Map Shift !!

RNAV system structure:

68 RNAV system structure VOR/DME DME/DME GPS IRS FMS Aircraft Position

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69 FMC POSITION IRS POSITIONS

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70 IRU Drift Rate The IRU position drift rate is certified about 2 NM/HR

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71 IRS ERRORS Residual Ground Speed Replace the IRU if the residual ground speed is 21 knots or greater after one check, or 15 knots or greater after each of two consecutive checks. Radial Position Error Compare radial position errors versus the navigation time (time IRUs are in navigation mode), flight time is acceptable if navigation time is not available, to the accept/reject limits on the radial error chart .

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72 IRS POSITION ERRORS

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73 Causes of the IRU error imperfections of mirrors and their coatings . IRS ERRORS

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74 Example 1 Following a flight from New York to London the IRS showed a position of 51 º 16.5'N 00 º 8.0'W when the aircraft was stationary on the ramp at London - Gatwick. The ramp position is given as 51 º 8.5'N 00 º 2.0'W . Calculate the radial error rate of the IRS given that the IRS had been in the navigation mode for 5 hours and 24 minutes. Solution : Whilst the IRS computer bases all calculations on spherical trigonometry, the human solution of radial rate error can be achieved to a satisfactory degree of accuracy using two-dimensional trigonometry. The distance in longitude between the IRS position and the ramp position is calculated using the departure formula: RAMP POS :51º8.5'N 00º2.0'W IRS POS : 51º16.5'N 00º8.0'W

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75 Distance (nm) = d long (') x cos (mid) lat = 6 x cos 51 º 1 2.5'N = 3.75 nm Using Pythagoras distance ramp to IRS position : √( 8 2 + 3.75 2 ) = 8.8 nm the radial error rate : 8.8 nm / 5 hr 24 min = 1.63 nm/hr IRS position

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76 IRU Accuracy Improvement The longer the path available the greater the accuracy

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77 MAP SHIFT The FMC position shown on the ND Map mode is not consist with the actual position.

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78 THE END

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