The field of inertial navigation is a well-established field of study and yet extends. The first inertial sensors were developed and tested by rocket designers such as Robert Goddard and Wernher von Braun in the arly 1930s. Later inertial sensors technology was perfected by institutions such as Drapers Labs, creating the first Inertial Navigation Systems. Inertial navigation made possible many of the great achievements such as use with the advent of spacecraft, guided missiles, and commercial airliners.
The basic principle behind inertial navigation is straightforward.
Starting from a known point, you calculate your present position from the direction and speed traveled since starting navigation. Generally this process is known as dead reckoning.
Dead Reckoning is 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.
Therefore, inertial system is a continuously running dead reckoning.
The difference between other navigation systems and inertial systems is how it determines direction, distances, and velocities. Inertial systems come in all shapes and sizes, but one thing they have in common though is their use of multiple inertial sensors, and some form of central processing unit to keep track of the measurements coming from those sensors.
We can highlight 5 basic components of typical inertial systems:
Acceleration-measuring devices are the heart of all inertial systems. It is important that they function reliably for all manoeuvres within the capability of the aircraft.
As you can guess from their name, accelerometers measure acceleration, not velocity.
Let's assume 2 main requirements for accelerometers:
The simplest type of linear accelerometer consists of a pendulous mass that is free to rotate about a pivot axis in the instrument. There is an electrical pickoff that converts the rotation of the pendulous mass about its pivot axis into an output signal.Since the signal is proportional to the measured acceleration, it is sent to the navigation computer as an acceleration output signal.
To obtain acceleration in all directions, three accelerometers are mounted mutually perpendicular in a fixed orientation.
Accelerometers are great at measuring straight line motion, but they are no good at rotation - that's where gyroscopes come in. Gyros don't care about linear motion at all, only rotation.
Gyroscopes are used in inertial systems to measure angular acceleration and changes in orientation and heading.
Over the years of usage and research the original gimbaled gyroscopes have been replaced by newer designs. Let's briefly talk about them.
In a traditional sense, a gyroscope employs one or more spinning rotors held in a gimbal or suspended in some other system that is designed to isolate it from external torque. That type of gyroscope works because once the rotor is spinning, it wants to maintain its axis or rotation.
A gimbal uses a number of concentric rings mounted inside one another that are connected via orthogonally arranged pivots. This design allows the gyro to freely rotate in three-axes.
These gimbal-less gyroscopes consist of a ball that is suspended in a magnetic field and spun electronically. Evacuating the air in the gyro cavity further reduces friction. Optical sensors measure the ball's orientation from symbols etched on the surface of the ball. The result is a near frictionless gyro with precession rates measured in years.
Accuracy and dependability of first generation systems have greatly improved with the introduction of the ring laser gyro (RLG).
Technically, the RLG is not a gyroscope since it has no moving parts, but it gives the same information as a gyro.
A RLG is made from a single block of glass with three holes drilled through the glass to form a triangular path. Two of the openings are plugged with mirrors and the triangular tube is filled with helium, neon or other lazing gas. When the gas is charged, the lazing gas produces two counter-rotating laser beams that are reflected around the path by the mirrors. Both laser beams emerge through the third hole in the glass and are superimposed upon each other to produce an interference pattern.
As the RLG moves, one beam has a longer path to travel, the other a shorter path. This causes changes in the interference pattern that are detected by photocells. The angular rate and direction of motion are computed as accelerations.
Another recent development is the inertial sensor based on vibrating quartz crystal technology. Like the RLG, these are not true gyros. Acoustic gyros are manufactured from a single piece of microminiature quartz rate sensor. Angular accelerations affect the patterns produced by a vibrating tuning fork and result in torque on the fork proportional to the angular acceleration. These gyros appeared in inertial units in the late 1990s.
Of the many different designs of inertial systems, each with different performance characteristics, there are two main categories used in aircraft: stable platform and strap-down.
As we discussed earlier, three accelerometers are mounted on a platform and orientated (usually) north/south, east/west, and up/down. This platform is driven by gyros (two or three) to always maintain its alignment with these axes regardless of any movement of the aircraft. Analogue feeds can be taken directly from the accelerometers and gyros that are in direct proportion to acceleration, and changes in velocity and direction.
Disadvantages: because of the mechanical assembly and many moving parts, stabilised platform INS suffer from wear and tear, and friction causes the output to "drift" over time. Compensation for drift requires complex bearing assemblies and special lubricants. Maintenance of stabilised platform INS is complex, costly and time-consuming.
RLG and accelerometers are attached rigidly, or "strapped down", to the frame of the aircraft i.e. as the aircraft moves, so does the inertial system platform - exactly. Three gyroscopes sense the rate of roll, pitch, and yaw and three accelerometers detect accelerations along each aircraft axis. It integrates them to get the orientation, then mathematically calculates the acceleration of the north/south, east/west and up/down axes, as a gimballed system would.
The advantages of reduced cost, size, weight and reliability make these systems the preferred choice of inertial system for many aircrafts.
Inertial systems accuracy will drift with time. This can occur for many different reasons. Older stabilised platform inertial systems (1970's and early 1980's) may have position errors of 2 nm/hr. Modern strap-down LRG in inertial systems tend to have error rates of 0.6 nm/hr.
Inertial systems generally are not the sole means of navigation on commercial airliners. Position errors can be reduced by frequent updates of position from GPS and ground-based navigation aids.
Let's get through those typical position errors:
Although an inertial system is a "self-contained" navigation system, it does perform better when provided with some data inputs to compensate for position errors and increase accuracy. These may include:
Typical inertial system outputs fed to other aircraft avionics and systems include, but are not limited to:
Inertial systems usually require an initialisation process that establishes the relationship between the aircraft (the reference axes) and the geographic reference (position and orientation). This process is called alignment.
Alignment usually requires the aircraft to remain stationary for a period of time in order to initialise fully. It also will normally require the input of certain data from another system or from a manual entry.
Alignment can be achieved independent of any external data. This is known as self-alignment. Or the alignment process can be speeded up with data supplied from a GPS or other systems, and even manual entry.
Aircraft manufacturers usually establish an approach to interact with the onboard inertial system. Here we go through an example of a Boeing 737 NG Inertial Reference System (IRS) interface. It's located on the aft overhead panel and includes IRS Display and IRS Mode Selector Unit.
For other aircraft please refer to the appropriate flight/operational manuals.
1 - Display Selector control. Allows to select information to display:
2 - Brightness control. Adjusts brightness of the data displays.
3 - System Display Selector. Selects one of the two IRSes for the data displays.
4, 6, 7 - Enter Keyboard
5 - Data Displays Two windows display data for the IRS selected with the system display selector.
1,2,4,5 - Information lights.
3 - Inertial Reference System Mode Selector