A simple explanation for how an INS works out Latitude:
If I drew a quater circle on a piece of paper, then handed that paper to you with a pen, you would have no trouble completing the rest of the circle. Looking at this completed circle you could then tell me:[list=1][*]how big the circle is, and[*]where the centre of the circle is.[/list=a]That is all an INS does.
When you turn the INS on (ie place it in NAV mode) it starts tracking its movement thorough space. As it is stationary on the Earth's surface, the only movement it is tracking is the rotation of the Earth - so it starts drawing a circle. After around 10 minutes it has enough of an arc to mathematically complete the rest of the circle.
The size of that circle tells it the Latitude (how far North or South it is on the planet) while the centre of the circle is True North.
Once you tell it its Longitude it looks up a table of world wide magnetic variation to determine magnetic direction, which it then displays on the DG/Nav display.
INS aircraft don't have any magnetic sensors (like a flux gate) to sense the Earth's magnetic field, as they aren't required. The standby compass is a normal magnetic compass, however.
[This message has been edited by Checkboard (edited 18 May 2001).]
extra note : this is not how the INS/IRS actually works out latitude, but it is a nice easy way to understand the concept. See Dick's post below if you are looking at JAR exams or want a more correct technical answer!
Last edited by Checkboard; 15th April 2003 at 07:41.