Introduction to INS

Introduction to INS
A technology known as INS, or Inertial Navigation System originally developed in the mid 60s for Missile Guidence systems has undergone an extensive evolutionary process and has now been introduced into Civil Aviation. Aircraft such as the Boeing 747 are now fitted with INS systems. INS coupled with GPS (Global Positioning Systems) can be used to navigate by global Lattitude and Longitudinal co-ordinates and also allows for real-time calibration of the INS.


How does INS work?
Gimballed INS
Early IN systems used gravity and momentum. Gyros and accelorometers were mounted on rotating plates known as Gimballed Inertial Platforms, these gyros and motors can measure the change in angle on various axis,

Strapdown INS The 70s introduced Strapdown INS, which was similar in principal to the Early IN systems but had no moving parts, infact the gyros and accelerometers were fixed i.e strapped down, to the chassis/cicuitry of the equipment. The strapdown systems suffered from a major flaw in that power consumption was so high, thermal problems were introduced, making it unreliable.

and hence using electronic circuitry can calculate the position relative to the start, thus they cannot determine their initial location, just the change relative to it. The early IN systems relied heavily on a precision engineered mechanism. Left: Simplified Schematic Diagram of a Gimballed INS Mechanism. © GEC Marconi Above: Photo of Internals of a Gimballed Inertial  Platform INS Mechanism. © GEC Marconi

Ring Laser Gyro INS
RLG INS (Ring Laser Gyro INS), is another form of Strapdown INS (i.e has no reliance on a physical mechanism as such). RLG uses a solid state glass block with three drilled tubes, mirrors placed at each coner act as optical resonators and reflect the beam.

The tubes are filled with a Helium/Neon Halogen mixture and a High voltage (about 1Kv) is applied, just like a Tube-light or Cathode Ray tube in a Telivision set. RLG systems use two counter beams around the tube initially at the same frequency, as the unit is moved the distance each photon (sub atomic particle responsible for light) has to travel (to the next mirror) changes, resulting in a subtle change in Frequency of that beam. The frequency of the beam can be measured then the change in angle be calculated from Delta

Above: Simplified Schematic Diagram of a RLG INS System. © GEC Marconi

Frequency  (in this case the change in Frequency from before and after the unit was moved).

Left: Internals of the RLG System. Above: Housing and Circuitry of the RLG INS unit. © GEC Marconi

Current INS in Civil Aircraft
In the recent 10 years or so, precision engineering of mechanical parts has
improved dramatically making it possible to create accurate gimaballed
IN systems, which are currently being used in Civil Aviation Applications.
The computing power behind that of this INS is equivelant to the 68040
or 80486 (486) processor, and is far less than that of todays standard
home PC! - Bear that in mind next time your flying by 747!!

References / Acknowledgements
Special thanks to Anthony D. King of GEC Marconi for his excellent White paper and to the IEE (Institute of Electrical Engineers).