Navigation Theory

Airspeed indicator

Diagram showing the face of a true airspeed indicator typical for a faster single engine aircraft

Principle of operation Airspeed indicators work by measuring the difference between static pressure, captured through one or more static ports; and stagnation pressure due to "ram air", captured through a pitot tube. This difference in pressure due to ram air is called impact pressure.

Altimeter

Diagram showing the face of the "three-pointer" sensitive aircraft altimeter displaying an altitude of 10,180 feet.

Pressure altimeter

Digital barometric pressure sensor for altitude measurement in consumer electronic applications A pressure altimeter (also called barometric altimeter) is the altimeter found in most aircraft. In it, an aneroid barometer measures the atmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase of altitude—approximately 100 mill bars per 800 meters or one inch of mercury per 1000 feet near sea level. The altimeter is calibrated to show the pressure directly as an altitude above mean sea level, in accordance with a mathematical model defined by the International Standard Atmosphere (ISA). Older aircraft used a simple aneroid barometer where the needle made less than one revolution around the face from zero to full scale. Modern aircraft use a "sensitive altimeter" which has a primary needle that makes multiple revolutions, and one or more secondary needles that show the number of revolutions, similar to a clock face. In other words, each needle points to a different digit of the current altitude measurement. The calibration formula for an altimeter, up to 36,090 feet (11,000 m), can be written as:

where h is the indicated altitude in feet, P is the static pressure and Pref is the reference pressure (use same units for both). This is derived from the barometric formula using the scale height for the troposphere.

Radar altimeter A radar altimeter measures altitude more directly, using the time taken for a radio signal to reflect from the surface back to the aircraft. The radar altimeter is used to measure height above ground level during landing in commercial and military aircraft. Radar altimeters are also a component of terrain avoidance warning systems, warning the pilot if the aircraft is flying too low, or if there is rising terrain ahead. Radar altimeter technology is also used in terrain-following radar allowing fighter aircraft to fly at very low altitude.

Attitude indicator An attitude indicator (ADI), also known as gyro horizon or artificial horizon, is an instrument used in an aircraft to inform the pilot of the orientation of the aircraft relative to earth. It indicates pitch (fore and aft tilt) and bank or roll (side to side tilt) and is a primary instrument for flight in instrument meteorological conditions. Attitude indicators also have significant application under visual flight rules, though some light aircraft do not have them installed.

Attitude indicator (with integrated localizer and glide slope indicators)

Principle of operation Attitude indicators use a gyroscope (powered via vacuum pump or electrical motor) to establish an inertial platform. The gyroscope is geared to a display that has two dimensions of freedom, simultaneously displaying pitch and bank. The display may be colored to indicate the horizon as the division between the two colored segments

(typically blue for sky and brown for ground), and is intended to be intuitive to use. The actual bank angle is calibrated around the circumference of the instrument. The pitch angle is indicated by a series of calibration lines, each representing 5° or 10° of pitch depending on design.

Schematics drawing of the insides of a classic attitude

Fluxgate compass Principle The basic fluxgate compass is a simple electromagnetic device that employs two or more small coils of wire around a core of highly permeable magnetic material, to directly sense the direction of the horizontal component of the earth's magnetic field. The advantages of this mechanism over a magnetic compass are that the reading is in electronic form and can be digitised and transmitted easily, displayed remotely, and used by an electronic autopilot for course correction. To avoid inaccuracies created by the vertical component of the field, the fluxgate array must be kept as flat as possible by mounting it on gimbals or using a fluid suspension system. All the same, inertial errors are inevitable when the vessel is turning sharply or being tossed about by rough seas. To ensure directional readings that are adequately stable, marine fluxgate compasses always incorporate either fluid or electronic damping. An alternative is to use a 3-axis fluxgate magnetometer to provide a 3D flux vector, and the magnetic heading is derived from the flux on a plane perpendicular to gravity, thus providing immunity from pitching, and rolling.

Gyroscope Principle

A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum. The device is a spinning wheel or disk whose axle is free to take any orientation. This orientation changes much less in response to a given external torque than it would without the large angular momentum associated with the gyroscope's high rate of spin. Since external torque is minimized by mounting the device in gimbals, its orientation remains nearly fixed, regardless of any motion of the platform on which it is mounted.

Inertial reference unit Principle An inertial reference unit (IRU) is a type of inertial sensor which uses only gyroscopes to determine a moving aircraft’s or spacecraft’s change in angular direction (referred to as "delta-theta" or Δθ) over a period of time. Unlike the inertial measurement unit, IRUs are generally not equipped with accelerometers, which measure acceleration forces. IRUs are used for attitude control and navigation of vehicles with relatively constant acceleration rates, such as geosynchronous satellites and deep space probes.

MHD sensor MHD sensors are used for precision measurements of angular velocities in inertial navigation systems (i.e., aerospace engineering). The principle of an MHD (magneto hydrodynamic) sensor is shown in the picture. The accuracy improves with the size of the sensor.

fig: Principle of MHD sensor for angular velocity measurement

Ring laser gyroscope A ring laser gyroscope (RLG) uses interference of laser light within an optical ring to detect changes in orientation and spin. It is an example of a Sagnac interferometer.

fig: Schematic representation of a ring laser setup. At the beam sampling location, a fraction of each of the counter propagating beams exits the laser cavity. A related device is the fiber optic gyroscope which operates similarly to the ring gyro, but typically has a longer optical circuit with fewer mirrors or prisms, because the transmission paths are within a coiled optical fiber. A fiber gyroscope doesn't necessarily use fiber laser technology; the laser action may be located outside a passive ring on a beam splitter port. Primary applications include navigation systems on commercial airliners, ships and spacecraft, where RLGs are often referred to as Air Data Inertial Reference Units. In these applications, it has replaced its mechanical counterpart, the Inertial guidance system.

Turn coordinator The turn coordinator (TC) is a flight instrument which displays to pilot information about the rate of yaw (turn), roll, and the "quality" or "coordination" of the turn. The turn coordinator was developed to replace the older turn and bank indicator, which displayed rate and quality of turn but not rate of yaw.

fig: Image showing the face of a turn coordinator during a standard rate coordinated right turn.

Operational principle

Graphic of a turn and bank indicator and a turn coordinator The turn coordinator is, like the turn and bank instrument it replaced, a gyroscopic instrument. An internal gyroscope, typically electrically driven (although some turn coordinator gyros are dual-powered and can be driven by either air or electricity), spins at approximately 20,000 rpm with the spin axis perpendicular to the longitudinal axis of the airplane and the free axis tilted up 30° from it. As the aircraft rotates about the yaw or roll axis, the principle of gyroscopic inertia causes the gyro to "resist" the change in its rotational axis about the free axis. This resisting force works against a spring; thus, a slow rate of turn deflects the gyro slightly while a higher rate of roll or yaw deflects it more. The gimbals’ movements are linked to the indicator dial on which is the rear view of a symbolic aircraft. The quality of turn is indicated by an coordination ball, which works on the same principle as an inclinometer. This is a glass tube mounted on the face of the instrument, below the symbolic airplane. It is actually a completely separate instrument. The

inclinometer consists of a glass tube filled with kerosene, and a steel ball. The tube is curved such that its center is the lowest point, and each end is higher. Normally, the ball will then sit in the center position of the tube, which represents a 'coordinated' turn. This position is marked by two vertical wires on the tube. The ball is said to be 'centered' when it sits perfectly evenly between the two wires.

Variometer A variometer (also known as a rate-of-climb indicator, a vertical speed indicator (VSI), or a vertical velocity indicator (VVI)) is an instrument in an aircraft used to inform the pilot of the instantaneous rate of descent or climb. It can be calibrated in knots, feet per minute (101.333 ft/min = 1 kn) or meters per second, depending on country and type of aircraft.

fig: Variometer for Para gliders, Hang Gliders and Ballooners

principle Variometers measure the rate of change of altitude by detecting the change in air pressure (static pressure) as altitude changes.

fig: Schematic drawing of the internals of a classic aircraft variometer

A simple variometer can be constructed by adding a large reservoir (a thermos bottle) to augment the storage capacity of a common aircraft rate-of-climb instrument. In its simplest electronic form, the instrument consists of an air bottle connected to the external atmosphere through a sensitive air flow meter. As the aircraft changes altitude, the atmospheric pressure outside the aircraft changes and air flows into or out of the air bottle to equalize the pressure inside the bottle and outside the aircraft. The rate and direction of flowing air is measured by the cooling of one of two self-heating thermistors and the difference between the thermistor resistances will cause a voltage difference; this is amplified and displayed to the pilot. The faster the aircraft is ascending (or descending), the faster the air flows. Air flowing out of the bottle indicates that the altitude of the aircraft is increasing. Air flowing into the bottle indicates that the aircraft is descending.

Vibrating structure gyroscope In science, a vibrating structure gyroscope is a type of gyroscope that functions much like the halteres of insects. Miniaturized devices on this principle can be used as a relatively inexpensive type of attitude indicator.

Principle The physical principle is very simple: a vibrating object tends to keep vibrating in the same plane as its support is rotated. It is therefore much simpler and cheaper than is a conventional rotating gyroscope of similar accuracy. In the engineering literature, this type of device is also known as a Coriolis vibratory gyro because as the plane of oscillation is rotated, the response detected by the transducer results from the coriolis term in its equations of motion ("Coriolis force").

Yaw rate sensor A yaw rate sensor is a gyroscopic device that measures a vehicle’s angular velocity around its vertical axis. The output is usually in degrees per second or radians per second. The angle between the vehicle's heading and vehicle actual movement direction is called slip angle, which is related to the yaw rate. The measurement is based on the Coriolis effect.