INERTIAL NAVIGATION SYSTEM (INS)

With the advancement of technological development of navigational aids, specifically for military operations, the systems incorporated, needed to be self reliant, i.e. not depending upon ground based stations.

The requirement for this type of long range navigation is to determine the position of the aircraft in relation to its point of departure, and points enroute to reach a destination without depending on ground stations.

Principle: Law of InertiaTechnological

Design: Stabilized Platform

Calculation: Dead Reckoning

(process of calculating one’s current position by using a previously determined position, using estimations of speed and course over elapsed time)Inertial theory is based on Newton’s second law, i.e. velocity is the rate of change of distance with time, and acceleration is the rate of change of velocity with time.

An aircraft’s position at any time after starting to move can be determined solely from its acceleration history, and by double integration of measured accelerations the distance travelled during a given time may also be determined.

INS comprises 3 principal units:

• Mode selector unit,

• Inertial navigation unit comprising a digital computer and inertial platform

• Control display unit.

The ‘core’ of the system is the inertial platform which establishes the stable reference plane from which measurements can be computed.

It comprises of

• 3 accelerometers (Motion sensors)

• 3 gyroscopes (Rotation Sensors)

They are aligned in the 3 axes, namely:

• Longitudinal direction (North-South)

• Lateral Direction (East-West)

• Vertical direction (Up-Down)

When the aircraft starts to move, due to the law of inertia, the accelerometers registers its movement and the computer associated is able to record the acceleration history of the aircraft to determine its position in relation to its starting point.

Angles that fall between the north-south or east-west direction are calculated by the computer solving algebric vector problems.

The platform is maintained in correct azimuth alignment by mounting it in a gimbal ring system and controlling the position of the rings by3 rate-integrating gyroscopes and torque motors.

In addition, the platform must be maintained accurately horizontal in relation to the earth’s surface in order to prevent the accelerometers from sensing misleading gravity accelerations.

The inertial platform is tuned so that it accurately seeks a local vertical reference coincident with the earth’s centre.

This is achieved by applying the principle of what is known as the Schuler pendulum in such a way that by utilizing error compensations from the computer together with a mathematical model in the computer, the platform is made to behave like a pendulum

The result is that any build-up of errors in the accelerometers and gyroscopes is limited and minimised, and the platform stays parallel to the earth’s surface regardless of latitude and longitude it is put into operation.

Prior to departure, and before any movement of the aircraft takes place, the system is operated in a ‘standby‘ and an ‘align’ mode, during which the inertial platform is automatically aligned to the aircraft’s axes.

The latitude and longitude of the aircraft at its departure position are also inserted at the control display unit, and this information is integrated into a mathematical model within the computer and, by a procedure known as gyro-compassing, the system is also aligned to its north reference point.

The co-ordinates of the en-route waypoints and of the destination are also subsequently entered in the control display unit.

When the alignment sequence is completed, an annunciator light illuminates to indicate that the system is ‘ready to navigate’ and that following selection of the navigate mode on the mode selector unit, the aircraft may commence its flight.

During flight, and approximately 2 minutes before a waypoint is reached, a light on the control panel comes on, and then begins to flash on and off 30 seconds before the waypoint is crossed, thereby signalling for the aircraft to be turned.

With the automatic flight control system engaged, the INS supplies command signals to the roll control channel so that the aircraft will automatically roll into a turn towards the waypoint and will level out on the new heading from the waypoint. This is repeated at each en-route waypoint until the final destination is reached.