How Inertial Nav Systems work

[180ToTheMarker]

Neil Boortz' explanation:

Aircraft Inertial Guidance Systems
The aircraft knows where it is at all times. It knows this because it knows where it isn't. By subtracting where it is from where it isn't, or where it isn't from where it is (whichever is the greater), it obtains a difference, or deviation.

The Inertial Guidance System uses deviations to generate error signal commands which instruct the aircraft to move from a position where it is to a position where it isn't, arriving at a position where it wasn't, or now is. Consequently, the position where it is, is now the position where it wasn't; thus, it follows logically that the position where it was is the position where it isn't.

In the event that the position where the aircraft now is, is not the position where it wasn't, the Inertial Guidance System has acquired a variation. Variations are caused by external factors, the discussions of which are beyond the scope of this report.

A variation is the difference between where the aircraft is and where the aircraft wasn't. If the variation is considered to be a factor of significant magnitude, a correction may be applied by the use of the autopilot system. However, use of this correction requires that the aircraft now knows where it was because the variation has modified some of the information which the aircraft has, so it is sure where it isn't.

Nevertheless, the aircraft is sure where it isn't (within reason) and it knows where it was. It now subtracts where it should be from where it isn't, where it ought to be from where it wasn't (or vice versa) and integrates the difference with the product of where it shouldn't be and where it was; thus obtaining the difference between its deviation and its variation, which is variable constant called "error".

 

[Swede]

Huh? Lotsa gibberish to explain something that's pretty simple. Inside a sealed box are 3 gyros. Tell the box where it is, then stand back. Any acceleration in any direction is detected, and given an acceleration vector, some simple Newtonian math formulas derive velocity. They'll always drift over time, but a good INS is very good indeed.

Inertial systems are what guided (and probably still do) our ICBM's in the days before GPS. What makes it so effective is the fact that no external input or signal is required, beyond the initial position, for the remainder of the flight.

 

[airksk]

Nice technical description of a joke! I hope you realize that Boortz isn't an aviation writer.

 

[DL_Infidel]

Tell the box where it is, then stand back.

Actually, on some aircraft you don't even have to do that. The 757/767 "finds itself" by using the earth's gravity and rotation during a 10 minute initial alignment. Pretty sweet!

 

[Checknsix]

>>Actually, on some aircraft you don't even have to do that. The 757/767 "finds itself" by using the earth's gravity and rotation during a 10 minute initial alignment. Pretty sweet!<<

Well...not exactly. Airplanes with IRS's (new versions of INS systems), do sense the Earth's rotation and true north. With this information, the flight management system can locate which latitude it is on...but not which longitude. Hence the reason you enter your location (or accept the GPS input for location). With that information, the FMS now has longitude data. INS's in general, used gyros. IRS's use Ring laser gyros, which detect acceleration in red or blue shift in the light spectrum. Very accurate with none of the gyro precess issues. 777's have 6 of these, each with its own position. The positions of all IRS's are then averaged together for aircraft position.

The refinement on IRS's, before GPS became part of the FMS position data, was extremely good. With input of individual gate lat/longs, and ground based nav aid position updates, I believe the 747-400 had something like a 18 hour limit on over-water operations (read as, without ground updates) for navigation. Very impressive.

In reality though, with GPS now being part of these systems, GPS data is weighed very heavily in the computation of position...even if the GPS data is degraded. IRS data becomes secondary for position, but remains primary for attitude. This nice thing about the IRS position backup, is that bad people that want to block GPS signals locally, will have little or no effect on modern airplanes ability to navigate.

Sorry if this is more than you ever wanted to know about IRS's and FMS's.
- Checknsix

 

[180ToTheMarker]

I thought my post was a joke....did not know we would get so technical with our replies

 

[SWAdude]

Fellow Aviators,

What I find to be interesting is the fact that all the techno gurus forgot the most important element of the system.

If the flux capacitor does NOT produce 1.21 jigawatts of power, the whole thing will implode.

May the force be with you and live long and prosper OB1.

SWAdude

 

[mar]

Funny as hell.

Roflmao!

But I thought jigawatt was spelled with a 'g'.

Or does a jigawatt measure the power of a jigger?

 

[Falconjet]

Simple math

Come on Mar, everybody knows that 1 jigawatt equals 100 gigawatts!

FJ

 

[mar]

Dang it.

I was never any good at the Metric System.