True north and IRS

[M2dude]

If you go back to the good old Delco Carousel 4 INS, true north was established as follows. During alignment the X & Y axis gyros would sense the error generated as the earth spin was detected and tilt the stabilised platform in order to keep their position in inertial space. Due to the earth’s spin axis, the platform would always dip in the east, making the establishment of true north automatic, the cardinal points were now locked in, until the system was shut down after flight. Once alignment was complete and NAV was either pre-selected or manually selected when alignment was complete the platform was then aligned to perfect horizontal, the gyros being torqued to retain their true horizontal also. As the aircraft moved along, the platform was Schuler tuned with aircraft acceleration in order to guarantee exact horizontality. An INS (or strap down IRS) can calculate latitude during alignment, by measuring the degree of platform tilt (or in the case of a modern IRS, the total component of all of the three RLG outputs). Longitude however can not in any way be calculated, and so the present position co-ordinates must be manually input.
As far as racking problems go, yes it is vital, repeat VITAL that the INS/IRS is fitted securely to the rack, but all such units have a central locking pin to ensure positive location with the rack, you should never be able to get this wrong. If someone says to you that they've fixed inertial attitude errors by re-racking the unit, then it was never located correctly in the first place. (The rack alignment is set up during aircraft manufacture and this alignment is totally critical, and must never be altered).

Dude