INS Initialisation

[P2T2]

Just wandering if there is anyone out there who can explain (in laymans terms) how an INS aligns itself with True North during initialisation and why they are apparently not usefull at the poles. The books are little confusing on these two.

Cheers and Beers...

[togaroo]

The INS needs to know where it is located initially so that "calibrate" its location. So for mechanical type INS its really like lining up your DI with your compass. You are telling the INS something that you know for sure. So the indication that you are given is then always referenced to the distance & position that you move away from your original set position. At each airbridge gate the lat and long are usually printed on the stand position so that you can either check or realign the INS BEFORE departure and not along the taxiway....

So to get back to your question about aligning to true north, the INS does not actually do this (correct me if I am wrong) rather it sets its self up in known space and then the minute deviations are calculated to represent a real change in LAT and LONG, so there is no actual magnetic influence on the system.

And for further explination (much better than my dribble) see the other topic posted on this forum called Inertial Reference System A320 in Tech Log, Luftwaffle & HPSOV has all the answers for you...

[This message has been edited by togaroo (edited 21 September 2000).]

[reynoldsno1]

In a gyro stabilised platform, alignment with True North will occur when the East gyro cannot sense any component of earth rate along its sensitive axis. Any component of earth rate causes an apparent wander of the East gyro. The output of this wander causes the platform to tilt about East resulting in the North accelerometer detecting a component of gravity. The accelerometer output is used to torque the azimuth gyro and the platform until the output from the East gyro is zero. The platform is then aligned. The accelerometer output is also fed to the levelling gyros and motors to return the platform to the level.

This principle still applies to an IRS with a Ring Laser Gyro, but the platform doesn't physically move - the corrections are digitally computed and applied.

Within the value of earth rate affecting the E/W accelerometer is a component dependent on the cos.lat. Therefore, for an aircraft at very high latitudes, this component gets very close to zero and makes accurate alignment to True North virtually impossible. However, there are INS around now with GPS integration modules that allow in-flight alignment, and I believe that these work well at all latitudes, provided the GPS is also working well!

[Nightstop]

Any of you B733 guys operating above North 60 degrees use the high latitude alignment technique (i.e. select OFF, Align, enter gate position, wait 17 minutes, select NAV)? Or isn't it really necessary, doesn't seem to make any difference ex HEL.

[CaptainSquelch]

Never did it in HEL and always found my way back home. (So far)