Originally Posted by
ElNull
this might be dumb question but at ground why the IRS would need to sense the earth rotation when it already have position from the GPS? Or is it just need it for a comparison?
The IRS determines the direction of True North by measuring the direction of apparent wander of its “giros” in 3 directions, due to Earth’s rotation. The rate of apparent wander enables it to determine the Latitude. When we insert present position, it compares the value to its calculated Latitude, but it needs Longitude because that can’t be determined.