Definitions

Inertial Navigation

a form of “Dead-Reckoning” that relies on accelerometers and gyroscopes to detect acceleration and velocity respectively along 3 perpendicular axes. An approximate 2 or 3 dimensional position can be constantly determined in relation to a known starting point, velocity and orientation. also a term used to refer to the whole subject: theory, design, technology, and application.

Dead Reckoning

a type of navigation from a known starting point and then by using vector information (direction and speed) against a clock, an estimate of the current position can be made. An INS will calculate a continuous dead-reckoning position. Position accuracy is dependent on the accuracy of vector information and the time since the last “known” position.

Inertial Platform

refers either to the self-referencing gyro-stabilised platform on which the accelerometers are mounted, or
refers to the aircraft, or the neutral aircraft position (pitch, roll and yaw axes), to which a strap-down inertial navigation system is firmly attached.

Inertial Reference System (IRS)

refers to a solid-state unit of three Ring Laser Gyros detecting accelerations in 3 dimensions; they may also contain quartz accelerometers.

Inertial Reference Unit (IRU)

refers to a computer that integrates IRS outputs and provides inertial reference outputs for use by other navigation and flight control systems, including the Flight Management System (FMS).

Gyroscope/Gyro

has traditionally referred to mechanical spinning gyros set within gimbals and frames allowing a platform to be stabilised in space regardless of the motion of the aircraft.

nowadays the term Gyro usually refers to a solid-state Ring Laser Gyro (RLG), which is predominantly used in modern aircraft INS.

several other types of Gyro are available, such as:

• Hemispherical Resonating Gyros (Wine Glass Gyros), which use hollowed out quartz resonators in which harmonic standing wave rotations are measured.
• Tuning Fork Gyros use electrically driven quartz tuning forks on a silicon chip; movement will twist the forks and this changes the capacitance between the tines proportional to angular movement, which can be measured.
• MEMS (MicroElectricalMechanical) Gyro.
• Vibrating Wheel Gyro.
• Cylindrical Vibratory Gyro.
• Piezoelectric Gyro.

Inertial Navigation System

traditionally (1960’s and 70’s) referred to a self-contained navigation system utilising a gyro-stabilised platform for dead-reckoning, and with a pilot interface allowing a limited number of waypoints to be entered and basic navigation information to be displayed.

then (1980’s) referred to a similar system as above but with the additional capability of feeding inertial data to other aircraft systems such as the Flight Director and Autopilot.

nowadays is predominantly used to describe an Inertial Platform, Inertial Reference System (IRS) and Inertial Reference Unit (IRU); and often used to describe all three of these units (as one system) typically installed on a single aircraft.

INS Position

refers to the latitude and longitude, and altitude, being fed from the IRU to other systems for reference, or as selected for display by a pilot on the Electronic Flight Instrument System (EFIS).

There are many other interchangeable terms in use such as: Inertial Guidance System, Air Data Inertial Reference Units and Systems, Inertial Measurement Unit etc, which can all be used when referring to INS. For the purposes of this Article, the definition of INS (what it is and what it does) that is likely to be most commonly used in the aviation community is as follows:

Regardless of the technology used and the configuration of individual components, an Inertial Navigation System (INS) refers to the Inertial Platform, Reference System and Unit, and associated Inertial Outputs. Its purpose is to provide a continuous dead-reckoning position, altitude and speeds to the FMS, where this data is compared with inputs from other systems, such as ground-based and space-based navigation systems. This other navigation data can be used to update the INS. Data from the INS can also be used for input to flight control and guidance systems.

Alignment

Vertical

acceleration due to gravity is always perpendicular to the Earth’s surface and this provides the vertical reference, allowing output of attitude.

Position

the INS can detect Latitude during the alignment, however it will require and accurate Longitude input. Usually the system memory will contain the position where the INS was last shut down, and as long as the aircraft has not been moved this position can be used to speed up the alignment. Once the vertical reference is attained, and the aircraft remains stationary, the only movement to be detected is the Earth’s rotation (roughly 15 degrees per hour). If the heading is known, then the velocity detected by the INS will determine the local Latitude.

True North

Technology

Of the many different designs of INS, each with different performance characteristics, there are two main categories used in aircraft: stabilised platform and strap-down.

Stabilised Platform INS

Strap-Down INS

Accuracy and Limitations

INS accuracy will drift with time; this can occur for many different reasons. Older stabilised platform INS (1970’s and early 1980’s) may have position errors of 2 nautical miles (nm) per hour. Modern strap-down LRG INS tend to have error rates of 0.6 nm/hr. An INS is, generally, not the sole means of navigation on commercial airliners. INS position errors can be reduced by frequent updates of position from GPS and ground-based navigation aids. GPS tends to be given primacy over INS, by the FMS, in determining the aircraft position. However, when GPS signals are lost, or become unreliable, then INS can retain useful FMS position accuracy until the GPS reception improves. Also, it is common nowadays for GPS data to be used by INS for correction of some of the errors described below.

Drift

is mostly associated with imperfections. In stabilised platform INS, these imperfections exist within gyro bearings and mass imbalances. In strap-down RLG devices, the cause is derived almost entirely from imperfections in the mirrors and their coatings. Small imperfections in accelerometers cause small errors in acceleration sensing and these are compounded by the integration of these signals, first to velocity and then to distance. This is known as Integration Drift.

Transport Wander

a gyro will attempt to maintain it’s original alignment with the Earth. When the aircraft moves across the globe, the local horizontal reference changes and this will create an angle with a gyro. This error is known as Transport Wander and also Apparent Drift; it is usually corrected by signals derived from the INS Latitude and Longitude.

Schuler Loop

as the inertial platform accelerates in any one plane, it not only detects this linear acceleration, but (like somotogravic illusions in the inner ear) the platform will tilt and detect a component of Gravity, which increases the sensed linear acceleration. The opposite occurs during decelerations. Also the INS will try to compensate for this additional tilt in the same way as it compensates for Transport Wander. The system never catches up with itself and is always “chasing” itself. This loop is known as the Schuler Loop, and anywhere on the Earth the period of this loop is 84.4 minutes. What this means is that all things being equal and in a perfect INS with no drift, the position error would be greatest after 42.2 mins, and zero after 84.4 mins. In reality, the position error grows with each cycle of the Schuler Loop.

Earth Rotation

in the same way that a stationary INS detects the Earth’s Rotation to determine its Latitude and Orientation, it also detects this rotation when moving. The final movement detected will be a combination of both the aircraft’s movement and the Earth’s rotation. These errors are small and can be compensated for.

Coriolis

as an aircraft travels from A to B around the globe it actually will follow a curved path (or a series of shorter curved paths). An INS will detect this as turning (a bit like a pendulum swinging out in a turn) and an error can be introduced.

Inputs to an INS

Inertial Outputs

Systems and Avionics utilising Inertial Outputs