Aircraft navigation is the science about the means and tools of controlling aircrafts while flying from one point on earth’s surface to another according to the trajectory selected in time and space. (Continue, the bigining No.20 )
1.7. Equipment for location determination
Controlling the aircraft, its angular positions are continuously measured – inclination, heeling and flight course. Also measured is spatial velocity (change rate), and in some cases – even acceleration. Inclination, heeling and their accelerations are measured with gyroscopic instruments – an aviahorizon, gyrovertical and direction indicators. In the larger aircrafts such devices are duplicated by the inertial navigation systems. Flight courses are measured by magnetic compasses, gyroscopic course gauges and various magnetic, satellite and inertial systems.
Gyroscopic devices and systems
Such devices and systems are called gyroscopic, because their main component is a gyroscope. With a constant gyroscope signal during the flight it is possible automatically stabilize the angular positions of the aircraft or to make exact changes. Gyroscope stabilization systems, which can not only determine the position of the aircraft, but stabilize other aircraft equipment – antennae, cameras, and thermal imaging cameras, in military aircrafts – weapons, are used more and more increasingly.
One of the advance features of a gyroscope is that when rotor is rotating, the main axis of the gyroscope does not change its position in space, even if one tries to push it. Gyroscopes with main axes oriented horizontally or vertically are used for measuring the angular position of the aircraft in space. Measuring of the aircraft turns to the left or to the right, the main axis of the gyroscope must be horizontal. Aircraft inclination or heeling is measured based on vertical axis of the gyroscope.
Not changing the angle between the horizontal axis of the gyroscope and the main longitudinal axis of the aircraft, one can fly in a straight line, without departing to the sides. It is possible turn the aircraft at exact angle and then it continues to flight at the straight line again. The disadvantage is that the gyroscope can not set one of its main axes itself, as does the magnetic compass, which arrow always turns to the north magnetic pole. In order to fly in the right direction, it is necessary first set the longitudinal axis of the aircraft with flight direction by other means, and then one can fly by gyroscope. Over time, the main axis of the gyroscope itself changes its position in space due to some manufacturing inaccuracies and friction in the axle bearings; it is called spontaneous gyroscope precession. Precession of precisely machined gyroscope is about a few tenths of degrees per hour. When Earth is turning, it seems that the main axis of the gyroscope is drifting; it is called the observational precession. Its size is proportional to the speed of rotation of the Earth and the geographic latitude, at which the gyroscope is located. Applying external force for a longer time to the frame of gyroscope, the main axis also drifts; it is called a forced precession. The longer the external force is applied and the bigger it is, the sooner main axis starts to rotate around the other axis.
Forced precession of gyroscopes is carried out in order to keep their main axis horizontal or vertical, because only then one can measure the inclination, tilt and yaw. With inductive sensor signal one can control the main axis orientation by matching it with the magnetic meridian. Then gyroscope is able to measure the magnetic course and accurately shows course of the aircraft during maneuvers. Forced precession is carried out with DC electric motor on the frame of the main axis. Changing the current, this force can be adjusted.
Main gyroscope axis always needs to be horizontal, in gyroverticals and aviahorizon – vertical. An aircraft must have a device that can accurately determine the vertical. Such a device is the level sensor (Figure 20). . It consists of sealed container (1), partially filled with electrically conductive fluid (2), and the remainder (4) is filled with an inert gas. In the middle and at the ends it has electrical contacts (3, 5 and 6). When the container is horizontal, electrolyte covers contacts 3 and 5 similarly and resistance between middle and two rear terminals is the same.
When tilting level sensor from the horizontal position, more liquid is collected in the lower end of the container, and the resistance becomes unequal. Signal that is proportional to resistance is sent to the corresponding adjustment electric motor. Motor has two control coils VA. To one of them a signal proportional to the resistance of the sensor of one side is sent, while to the other coil – from other side.
Course gyroscope measures aircraft course with respect to its main axis. Course gyroscope is used to precisely rotate the aircraft at the right angle (change course). Main axis of the gyroscope remains unchanged, as, the aircraft turns itself to adjust to this axis. If the aircraft continues to fly without changing course, the angle between the longitudinal axis and the main axis of the gyroscope does not change.
Course gyroscope or gyrocompass is a gyroscope, which external axis is vertical, and the main axis – horizontal and maintained in this position by correction system (Figure 21). . The main axis of the gyroscope, being horizontal, can be tilted to any angle to the longitudinal axis of the aircraft. As an example, before aircraft take-off, it can be perpendicular to the axis of the aircraft.
Course gyroscope is a Cardan suspension consisting of two frames – external and internal. The external frame axis Z is vertical and can rotate on the device housing with bearings. Internal frame axis Y is horizontal and can rotate on bearings mounted on the external frame. The internal frame is attached to level sensor, which is sensitive to the deviation of main axis from the horizontal position.
Sensor signals are sent to the horizontal leveling correction motor which implements gyroscope correction, maintaining its main axis in the horizontal position.
If resolver stator is fixed in the body of the aircraft and resolver rotor (course signal sensor) is placed on the external frame axis Z, it is possible to measure changes in the course of the aircraft, changing the direction of flight, or fly at the same course.
If aircraft is turned (changing course), the main axis of the gyroscope does not change. The external frame does not rotate, and the aircraft turns to the main axis of the gyroscope. If aircraft is turned, resolver stator rotates in respect to the rotor, so the resolver signal is- proportional to the turn. This signal can be sent remotely to the cockpit indicator on automatic control systems, or other devices. Course gyroscope can be found in older aircrafts, which do not have advanced navigation systems.
Course gyroscope, unlike induction compass, can not display the magnetic course (angle to the magnetic meridian). Its main axis remains in the position in which it was when gyroscope was turned on. Such gyroscope is used when course of the aircraft in the flight, or at least its initial course is measured by other means. Using a course gyroscope one can only turn the aircraft to the desired angle and fly in the chosen direction. Such a gyroscope is also called gyrocompass.
Main reasons for course gyroscope errors (major axis deviation from the set position) are:
– Earth’s rotation about its axis;
– Main axis self deviation from the horizontal position.
When making course gyroscope adjustments (forced main axis turn, equal to the size of the deviation, but in the opposite direction), gyroscope can be used as an independent device to measure the course during few hours. As course sensors, such gyroscopes are used in course systems. When aircraft turns, gyroscope control is turned off with correction switches, but resolvers’ signals remain unchanged and pilots can accurately rotate the aircraft (change course) at any angle.
Course gyroscope errors occur due to the Earth’s rotation so that the gyroscope does not change its position in relation to the universe. Figure 22 shows several positions of the gyroscope on Earth. Earth angular velocity is Ω = 15° / h. Gyroscope B with a major axis oriented at the equator by the true meridian , will not change its position relative to the Earth. Gyroscope C vertical axis in six hours will flip to horizontal and in a day it flips over once more. Gyroscope D at pole with vertical axis will not change position. Gyroscope E horizontal axis will have angular velocity ΩE = Ω sinφ = 15°/ h .
Let’s put gyroscope A in any place on Earth, for example, in Vilnius, in an initial position:
– The main axis – horizontal and pointing to the north;
– Internal frame axis – horizontal and pointing to the east;
– External frame axis – vertical.
After some time of observation it will be seen that the main axis that was directed toward the north, gradually is turning to the east. The angular speed of the axis is ΩA = Ω sinφ , where φ – latitude. For example, in latitude 30º gyroscope main axis in the horizontal plane will rotate 7.5 ° per hour. It will also be apparent that the northern end of the main axis tilts upwards. If such a gyroscope (B) would be put at the Earth’s equator, its main axis will not rotate to the east and not tilt upwards. Course gyroscope yaw rate and direction depends on the orientation, geographical latitude, the aircraft flight direction and speed. Course gyroscope on board can be used only with two compulsory precessions:
– turning back axis of gyroscope proportionally to geographical latitude. This is done by supplying the appropriate voltage to azimuth correction motor from the cockpit panel. Then axis orientation does not change;
– allowing axis to deviate from the horizontal position. This is done by level sensor signal supplied to a leveling correction motor.
If we can make timely and accurate gyroscope bearing and leveling adjustments, it could be used in flying in the ortodrome, even at the poles.
Ortodrom course often is called gyrocompass course (due to measurement method). It is different from the gyroscope course only in that gyroscope axis deviation due to Earth’s rotation is compensated, by making latitude correction. Horizontal axis is maintained with a leveling adjustment. During the first hour of the flight course error of about 1°, but with longer flight error increases rapidly.
Azimuth correction is done automatically or by manually setting new values for latitude (which changes with the aircraft in flight) by means of control knob in the panel. Knob changes voltage of electrical motor. Voltage is proportional to the geographical latitude. Flying exactly to the east or west, longitude does not change.
Then the latitude value is set, and it does not need to be changed during the flight…
When flying in the north- south direction, latitude is changing rapidly. Then, the knob should be set to the new latitude value, after it changes by more than 1°.
Course gyroscope or gyrocompass is one of the oldest aircraft navigation instruments operating in all geographical latitudes.
Where the aircraft decline, incline or yaw, course gyroscope keeps its original direction from which course is measured. Course gyroscope is quite uncomfortable to pilots because with it can not be determined reference direction relative to magnetic or true meridian. During the flight azimuth and leveling adjustments are needed. Manual input of changes in the geographical latitude of the aircraft is need, if it is not done automatically by the flight control system. With flight time accuracy deteriorates.
Inductive magnetic compass measures magnetic course, but its readings are inaccurate when aircraft is maneuvering. Such compass does not work if latitude is exceeding 70°. When course is corrected by the signal of inductive sensor signal, it is possible to measure magnetic course, and such compass called gyromagnetical.
The compass, which is the main part of the gyroscope, has two modes of operation: magnetic correction and half-gyrocompass. Magnetic correction of gyroscope main axis (by the magnetic meridian orientation) is performed when there is an induction transmitter, latitude signal is not required. Closer to the Earth’s poles half-gyrocompass mode is used (device acts as a course gyroscope, the azimuth adjustment is made by latitude signal).
Gyromagnetic compass consists of a magnetic induction course sensor, gyroscope with correction mechanisms, and the cockpit indicator. Course signal to the cockpit indicators and other aircraft systems is sent via resolvers: on the outer gyroscope axis frame is mounted resolver rotor sensor and stator is mounted inside gyroscope housing.
Gyrocompass combines positive features of inductive sensor and course gyroscope (half-gyrocompass). Inductive sensor is able to choose only one direction – the magnetic meridian: a magnetic sensor measures magnetical course. When aircraft makes inclinations, yaws, inductive sensor oscillates, it is affected by centrifugal force. Short-term changes in the inductive sensor signal fails to change the main gyroscope axis position. Even a sudden change in the induction sensor signal, gyroscope „absorbs” such deviations and they are not shown in the cockpit instruments and other systems. Gyromagnetic compass shows average fluctuations. Such a course called gyromagnetical.
When aircraft makes longer maneuver, inaccurate inductive sensor signal, albeit slowly, may start forced gyroscope azimuth precession. To prevent this, inductive sensor signal is temporarily disabled by the correction breaker
Aviahorizon and gyroverticales
Gyromagnetic compass quite accurately measure the magnetic course and may act as a course gyroscope (in half-gyrocompass mode) , but can not measure the aircraft inclinations, yaws. These aircraft movements are measured by aviahorizons and gyroverticales. The main part of such device is the gyroscope, which rotor axis oriented vertically. In this position, the axis supports two level sensors: one signal is used of axis inclination compensation, other – tilt compensation. Aviahorizon can be installed in cockpit panel or its gyroscope – in the technical section, and inclinations, yaws indicator – in the cockpit (Figure 23.).
When the aviahorizon indicator is the cockpit instrument panel, its scale is directly connected to the gyroscope frame. Inclination is measured from the aircraft symbol (in degrees) and tilt – from index top. Larger aircraft aviahorizon indicator shows more navigation elements. Remote controlled aviahorizon is managed by selsyns.
Gyroscopes of aviahorizons are not very accurate, their scales are small. Gyroverticales are more accurate as kinetic moment of their gyroscope is higher. They are established in the center of mass of the aircraft, to reduce effects of centrifugal forces. Automatic flight path control systems require much more precise measurement of the aircraft position, they receive signals from gyroverticales. The latter do no have the cabin indicator, they only send inclination and tilt signals to other devices. Three similar gyroverticales are used, self- monitoring system compares the signals with each of the other two. Failed gyrovertical signal will be different from the other two, then the message is displayed to the pilots about fault of the gyroverticale.
Inertial navigation systems
Inertial navigation systems are autonomous navigation tools. They do not need land-based radio stations and the Earth magnetic field. During the navigation they are able to use data from other aircraft systems, such as the DME hardware, GPS receivers or computers. Then navigational calculations are more accurate.
Navigation system sends signals to the cockpit instruments and other aircraft systems. Main user of inertial data is the FMS (flight management system). Accurate navigation data is obtained only by initial adjustment of the system, prior to flight inputting coordinates of the aircraft (local latitude and longitude coordinates – original location), the direction of true north and the local vertical (initial course, inclination and tilt).
After loading the system information on the initial position of the aircraft , during the flight it continues to keep supplying accurate information about the actual location of the aircraft , its position in the space ( trajectory ) , the magnetic or true course and measuring the cruise and vertical velocity, wind direction and speed of the aircraft drift , etc. ( Figure 24). . This is done by measuring the accelerations of the aircraft and its turns in respect to coordinate axes and processing of measured data in the computer.
Inertial systems determine aircraft position and velocity vectors in three-dimensional coordinate system, thus they have three accelerometers and three gyroscopes. All of these six components of the system and the computer are on the same module, which is usually called the inertial course and positioning device IRU (Inertial reference unit).
Accelerometers are oriented according to the three aircraft axes X, Y and Z, so one accelerometer measures accelerations of north- south direction, the other – the east -west direction, and the third – the vertical accelerations. Gyroscopes, in the newer systems are laser-based, monitor aircraft rotations about these axes. Accelerometers signals are amplified and sent to the integrator. After the first integration of the received signals, signal corresponding to the speed of the aircraft in three directions is received. After next integration signal corresponding to the traveled distance in these directions is received.
For example, the third signal of accelerometer after one integration is proportional to the vertical velocity of the aircraft (no pressure variometer is needed), and integrating second time – proportional to the height of an aircraft from the original location.
Flight direction from the initial direction is measured with three gyroscopes. After aircraft turned, each time in the amount of turn in space is measured. Accelerometers and gyroscopes (IRU) are close to the center of mass of the aircraft that they were less affected by centrifugal force.
The main requirement is that the inertial system to function well, is its precise adjustment before the flight, when the aircraft is stationary. During adjustment vertical direction and the true north are set. All gyros and acidometers are adjusted, later are set to zero. During adjustment aircraft moves in conjunction with the Earth, no other aircraft movement should be present.
Inertial systems accurately measure all flight control and navigation elements, and their electric gyroscopes together with acidometers (IRU Module) are mounted on a horizontally stabilized platform that measurement errors are smaller. If gyroscopes are laser-based, the IRU is directly attached to the hull of the aircraft, and then computer corrects the errors of the measurement.
When the INS is turned on, it requires about 10 minutes of the gyroscope and accelerometer reconciliation (vertical setting, gyroscope axes orientation to the north and east, and the reset of accelerometers signals). Then IRU computer starts to calculate all the control and navigation elements, for example, after 1 hour of flight deviation from the chosen path can not be greater than 2 nautical miles. If radio signals or satellite navigation provide more accurate data, IRU computer calculations can adjust the navigation signals.