Inertial Navigation System (INS)

• IRS stands for Inertial Reference Systems.
• INS stands for Inertial Navigation System.
• INS was originally made for Missile Guidance systems in the mid-60s.
• INS has been introduced into Civil Aviation
• INS coupled with GPS can be used to navigate by global latitude and longitudinal co-ordinates and allows for real-time calibration of the INS

Gimbaled INS

• Typical inertial navigation systems incorporate a stable platform as the inertial measurement unit.
• Gyros and accelerometers are mounted on a Gimbaled Inertial Platform.
• Gyro outputs command torque motors keep the platform steady and parallel with the earth's surface.
• Accelerometers don't produce erroneous outputs due to gravity when tilted.
• Accelerometers are the primary detectors in an INS and detect and measure any change in velocity, then integrated with time and calculates the exact aircraft position.
• Gyro outputs maintain the platform stable and give an indication of aircraft altitude.
• INS is normally the primary altitude reference in addition to providing a navigation reference.
• The inertial navigation system is a totally self-contained navigation system, comprised of gyros, accelerometers, and a navigation computer.
• INS provide aircraft position and navigation information resulting from inertial effects on system components, and does not require information from external references.
• INS is aligned with accurate position information prior to departure, and calculates its position as it progresses to the destination.
• By programming a series of waypoints, the system will navigate along a predetermined track, or insert new waypoints if a revised routing is desired.
• INS accuracy is very high initially following alignment, and decays with time at the rate of about 1-2 nautical miles per hour.
• Position update alignment can be accomplished in flight using ground-based references.
• Many INS systems have sophisticated automatic update using dual DME and or VOR inputs.
• INS may be approved as the sole means of navigation or may be used in combination with other systems.

Strapdown INS Overview

• Gimbaled INS gave way to the strapdown INS during the 1970s.
• This corresponds to the ring laser gyro and a sufficiently powerful avionics computer that could perform the numerically intensive high rate strapdown navigation computations.
• Accelerometers are fixed or 'strapped down' to the chassis of the equipment or the body of the aircraft and has no moving parts.
• Accelerometers are mounted on a platform and gimbaled gyros allow it to move independently of aircraft movement within a gimbal system.
• A body's actual spatial behavior / movement can be described with six parameters: three translatory (x-, y-, z-acceleration) and three rotatory components (x-, y-, z-angular velocity).
• Three acceleration sensors and three gyros have to be put together on a platform to define the movement of the body, in an orthogonal system.
• The distance laid back and the angle the body has actually rotated can be obtained by integration of the individual translatory and rotatory components.
• Performing these calculations accurately and periodically enables the system to trace its movement and to indicate its current position and heading.
• Early inertial navigation systems were built with a stabilised gimbaled platform by using gyroscopes on the platform to drive the gimbal motors in a feedback loop.
• Platform-mounted accelerometers could then be individually double integrated (velocity and distance) to obtain position updating in each direction.
• Most recent systems are of a different type, called strapdown INS, which eliminates mechanical gimbals and measures a craft's orientation by integrating three orthogonal angular-rate gyroscopes strapped down to the craft's frame.
• To get position, three linear accelerometers measure the acceleration, which is then integrated with the gyroscope outputs of angular velocity and converted into dead-reckoning navigation coordinates.
• Total number of sensors in a single IRS is six, there are three gyros and three accelerometers.
• In tri-redundant systems incorporating 3 IRS there are therefore 18 sensors, which consists of 9 gyros and 9 accelerometers.
• In a tri-redundant system, multiple failures must occur before navigation is affected.
• Single gyro and accelerometer failures are of little consequence during flight.
• Remaining serviceable systems continue to provide accurate navigation data.

Ring Laser Gyro

• Laser gyros are now widely used in aircraft navigation applications, providing accurate, independent navigational data with high accuracy and reliability.
• Laser gyros are a solid-state device, and are less susceptible to malfunctions, compared to gyro stabilised platform systems and extremely accurate.
• Laser gyros are still a dead reckoning system and require no external inputs to function.
• Laser ring gyro inertial units are referred to as strapdown systems because it does not require a gyro stabilised platform as described in a conventional INU.
• Pitch and roll movements which normally introduce errors in an accelerometer, are provided to the computer.
• Accelerometer outputs are modified electronically to compensate for attitude changes.
• This form of Inertial Reference Unit normally provides primary attitude information, and can also measure altitude (inertially), rate of ascent and descent and groundspeed.
• Outputs from an IRU are typically distributed over a digital data bus to flight control computers, navigation computers, multi-function displays, etc.

Fundamentals of Laser Operation

• LASER is an acronym for Light Amplification by Stimulated Emission of Radiation, discovered in 1960.
• The first step in producing Laser is the ionization of a gas which may be helium, argon, krypton, neon or xenon producing a different colour (and wavelength) of light.
• A mixture of helium and neon is used in ring laser gyros, held at low pressure inside a sealed tube exposed to an anode and cathode plate.
• When a high voltage is applied across these plates the gases ionise, producing a glow discharge similar to fluorescent tubes.
• In laser gyroscopes the applied voltage is around 3000 volts.
• White light is a mixture of many wavelengths, laser light is a single wavelength which depends on the type of gas used.
• Ordinary light is scattered in all directions, but laser light is a parallel beam.
• Laser is termed coherent light which means it is of a specific wavelength.

Laser-Beam Splitting

• A laser beam can be split into two or more beams by the use of various optical elements, such as precisely ground and polished glass or plastic plates, optical prisms, and semi transmissive mirrors.
• Splitting of the beam by means of a glass or plastic plate is accomplished by positioning the plate at a 45° angle with respect to the beam axis; although other angles are also possible.
• In the case of a cube splitter, the cube is formed by two right-angle prisms with their bases cemented together with an optically transparent balsam.
• The beam is projected perpendicularly to one of the short sides of the prism and at a 45° angle to the bases of the prisms.
• If the thickness of the balsam cement is regulated so that it is 50% transmissive, the beam is able to be split.

Laser Diode - LED

• A type of laser device is a semiconductor diode.
• Built with a PN junction at which the laser radiation is produced when a forward current is passed across the junction.
• There are several types of semiconductor laser diodes, the most common of which is the gallium arsenide (GaAs) diode.

Laser Rod - Ruby

• The rod reflects light between ends and the material sets the colour.
• Excitation happens by voltage or light.
• End surfaces are precisely parallel to each other.
• Emission of pumped photons is amplified by the oscillation of the photons between the reflecting surfaces (mirrors).
• One end of 'the rod is semitransparent, controlled by the thickness of the coating; the other end is 100% reflective by increasing the amount of coating.
• An RLG uses a gas discharge to generate a monochromatic radiation in two directions.
• The prisms or mirrors are used to reflect each beam around the enclosed area which produces a laser in a ring configuration.
• Physical rotation about the plane of travel will cause the two beams to have different distances to travel.
• The difference makes a 'fringe pattern' that is the output from the photo diodes.
• Ring laser gyros are not gyroscopes in the sense that we know them.
• They are simply two beams of laser light rotating in opposite directions engineered to detect motion and behave like a gyro.
• Laser ring gyros are constructed so that two laser beams are reflected around a triangle causing the light to travel in an enclosed loop.
• The light travels in both directions at the same time, creating a clockwise beam and a counterclockwise beam.
• A similar phenomenon takes place in our laser gyro.
• If the gyro is turned clockwise (CW), the CW beam completes the journey in a shorter time and in order to complete the journey in the same number of cycles the beam wavelength must be compressed
• The counterclockwise (CCW) beam wavelength must increase.
• Laser gyros used in modern navigation equipment are manufactured from a solid triangular block of temperature-stable glass.
• Accurate holes are drilled through the glass to create the triangular path and mirrors are installed at each corner.
• Cavity is filled with a neon-helium gas mixture.
• Each mirror is different in construction.
  • One mirror is fixed
  • Next mirror can be adjusted by a servo motor and is used to tune the path length
  • Third mirror is partially transparent allowing some laser light to reach the two photocell detectors
• Two laser beams of identical frequency are reflected around a closed loop in opposite directions at the same time.
• The laser gyro detects motion by measuring the difference in beam frequencies.
• The laser light frequency is around 4700 Tera Hertz (4 700 000 000 000 Hz).
• The gyro can sense frequency differences of only a few hertz and sense the earth's rotation rate of 15° per hour at the equator.
• A laser beam frequency change of only 4 Hz, to achieve this degree of sensitivity, the beams are directed to a pair of photocell detectors through the partially transparent mirror.
• The frequency (wavelength) difference is quantified as the movement of the laser ring gyro assembly.
• Laser gyros have some limitations, including:
  • lock-in
  • path length
  • random drift
• Lock-in is the major problem associated with laser gyros, happening at very low rotation rates.
• Mirrors are not perfect and produce miniscule amounts of backscatter, which couples energy between the two beams.
• This coupling of energy between two very high Q oscillators can cause the frequencies to lock together.
• The dither motor applies a very small oscillatory rotation (about 1 arc minute peak, at about 400 Hz) to the entire block.
• Accuracy increases exponentially with path length and modern laser gyros are triangular in shape to maximise path length in a minimal area.
• Optimum path length is one that equates to an even multiple of the desired wavelength.
• The system are subjected to random drift.
• Caused mainly by defects in mirror manufacture, particularly imperfections in mirror shape and coating material.

Readout Technique

• Readout is accomplished with photocells and beam-combining optics.
• The outputs from the CW and the CCW laser beams are combined so that they are nearly parallel.
• The wave fronts of these beams will interfere with each other, alternately cancelling and reinforcing, forming a fringe pattern.
• When the CW and CCW frequencies are different the fringe pattern will move in a direction determined by the sense of rotation and the direction of rotation as well as the rate of rotation can be determined.
• The output from the photocell activates a pulse which triggers an up-down logic and used in attitude or navigation control systems to accurately measure inertial angles and accelerations.
• Lock-in is the major problem associated with laser gyros and can be eliminated by inducing a small oscillating rotational force (i.e. vibration) to the gyro.
• The term is dithering, in the laser gyro a piezoelectric motor applies a vibration to the gyro eliminating the frequency hang-up.
• The better and accuracy increases exponentially with path length and modern laser gyros triangular to get maximum path length (and better accuracy) in a minimal area.
• Laser gyros randomly drift over time due to minute changes in mirror shape and position.
• Inertial reference systems employ self-calibrating systems which check gyro accuracy after each flight.
• An RLG IRU is two to three times smaller, lighter and cheaper than a gimballed INU with similar performance.
• A strapdown Inertial Reference System (IRS) replaces a lot of the equipment used in a gimballed INU equipped aircraft including:
  • vertical gyroscopes
  • remote-sensing compass systems (directional gyros, flux valves, amplifiers)
  • rate gyroscopes
• Systems are capable of providing some data previously only available from the air data systems, such as instantaneous vertical speed (IVSI) and altitude.

GPS and IRU integration

• Many strapdown IRUs today contain an embedded GPS receiver module.
• GPS provides attitude and positional data.
• GPS solves the problem of 'calibrating' the instrument errors in a strapdown system, providing a means of 'inflight alignment' and removes the need for the aircraft to be held stationary for up to 5 minutes, prior to flight.
• The IRU provides a seamless fill-in for GPS outages resulting from jamming or obscuration, and a means of smoothing the noisy velocity outputs from the GPS, in addition to providing a continuous high bandwidth measurement of position and velocity.
• Electronics will evolve to the stage where it becomes an insignificant part of an IRUs cost, size, weight and power.
• Inertial Reference Unit (IRS) is based upon the Inertial Navigational System (INS).
• The IRS provides basic and complex navigational, attitude, and velocity data for its own calculations and for other systems in the aircraft.
• IRS dispenses with the use of the complex accelerometer / gyroscope-based system of the INS, and instead uses a series of strap-down laser gyroscopes to provide detailed acceleration velocities.
• IRS are capable of providing some data previously only available from the air data systems, such as instantaneous vertical speed (IVSI) and altitude.

Boeing IRS Mode Selector Panel

• The Mode Selector Unit (MSU) is used to select the mode of operation of the IRS.
• A selector switch is fitted for each of the IRUs in the aircraft.
  • DC Fail Lights indicate that DC power is not available to the associated IRU.
  • Fault Lights Indicate a Fault in the associated IRU which could cause erroneous outputs.
  • OFF - Removes power from the associated IRS and Alignment is lost.
  • ALIGN - Initiates alignment of the associated IRS.
  • The aircraft must be stationary during alignment.
  • NAV - Allows associated IRS to enter the navigation mode when alignment is complete.
  • ATT - IRS enters Attitude mode.
• Some mode selector/control panels incorporate display panels used to view navigational information, a display selector switch and a keyboard.
• ON BAT Illuminates Amber when one ADIRU is supplied by battery only.
• IR OFF: Inertial data output disconnected.
• FAULT Light: Illuminates Amber associated with an ECAM caution if a Fault is detected in the inertial reference part.
• IR Mode OFF: The ADIRU is not energised, NAV is for normal mode of operation.
• ADR OFF: Air Data output disconnected.

Triple IRS System

• Power requirements are generally lower which utilise the aircraft emergency power supply as a back-up source instead of a dedicated battery unit.
• The IRU is the heart of the IRS system and the mode selector unit is used to select the mode of operation of the IRS with a selector switch for each IRU.
• The IRS is able to provide inputs to the aircraft's autopilot and Flight Management Computer (FMC) to control the aircraft.
• This allows the IRS to navigate the aircraft without needing for ground based navigational aids.
• IRS provides position data for FMC, magnetic heading, ground track Vector, and aircraft attitude.
• Outputs from the IRU Attitude, heading and navigation data to EHSI and EADI, attitude data to weather radar.
• Heading data to the Ground Proximity Warning System (GPWS) or attitude, altitude and acceleration data to the thrust management system.

IRS Operation

• Flight crews typically navigate from departure to destination via waypoints using the Inertial System Display Unit (ISDU).
• Navigation from the departure point is calculated by constantly summing accelerometer outputs and angular rate of change outputs from gyros.

Corrections to IRS

• IRS monitors movement about all 3 axes.
• Computer constantly calculates direction of travel relative to the earth's surface.
• Data is correlated against calculated velocity and distances travelled about lateral, longitudinal and vertical axes.
• Modern IRS incorporates an automatic calibration programme that estimates drift and applies corrections to outputs from the gyros and accelerometers.
• The computer uses a previously calculated gyro error bias factor to compute a gyro error correction model.
• Accelerometer errors are monitored each time the aircraft taxis.

Initialising the Inertial Reference System (IRS)

• Before the IRS can navigate it must be initialised.
  • Apply power to the system
  • Allow the lasers to commence lasing and warm up
  • Initiate microprocessor built-in testing sequence (BITE)
  • Align with current aircraft position and attitude
• The Mode Selector Unit is used to initialise and commence alignment.
• Move MSU selector switch to ALIGN.
• During first 30 seconds after selecting ALIGN the IRU conducts a BITE check and goes to the alignment mode.
• This is indicated by illumination of the ALIGN annunciator
• The IRU can estimate the aircraft latitude, true North and aircraft heading, however, the system relies on an input of latitude and longitude for an accurate initial present position.
• If the present position entered by the pilot is displaced more than one degree the MSU selector switch is move to NAV and the system is ready for navigation.
• The system can determine aircraft latitude and heading using earth rate sensing, as it relies on pilot input of the aircraft's initial present position for accuracy.
• Each waypoint latitude and longitude is entered using Inertial System Display Unit.
• HSI and ADI are supplied with attitude and navigation data required for tracking between waypoints.
• Selecting ATT on MSU places IRS into attitude mode.
• In ATT, no navigational data is computed/calculated as the system provides basic pitch and roll data.
• If ATT is selected during flight, all navigational data is lost, and is only required if the IRS is experiencing a navigation system failure.

System Fault Indications

• The INU provides warnings to users, advising of data deficiencies.
• These signals can be indicated by the 28 VDC flag or by a warning signal referred to as flag signals.
• Many aircraft have two or even three inertial reference systems installed.
• In these cases, the present position data is constantly being compared and if a significant discrepancy exists between systems, a warning signal is illuminated.
• During alignment the IRS compares the longitude entered with the longitude stored in memory of the last position.
• If they differ by more than 1º the ALIGN light flashes immediately.
• Entering the longitude for a second time forces the system to accept it.
• System Failure Indications are displayed in RH display the select switch must be in HDG/STS.
• The component associated with IRU FAIL or ADC FAULT is a function of the SYS DSPL switch position. IRS faults include;
  • ISDU FAIL
  • IRU FAIL
  • EXCESSIVE MOTION IN ALIGN MODE
  • ALIGN FAULT
  • DAA FAULT L
  • DAA FAULT R
  • ADC FAULT
  • ENTER PRESENT POSITION
  • ENTER HEADING
  • ISDU POWER LOSS