View Full Version : inertial nav

17th Sep 2004, 02:26
Hello all,
Here is a question about inertial nav initial alignment: It is my understanding that the IRU, when it is initially aligned at rest on the ground actually finds true north thru a gyro-compassing technique (I understand the basic idea here, it is pretty neat). I can see how this works for the old style spinning gyro's. But apparently a similar thing happens for fiber-optic gyros and ring laser gyros (Sagnac effect, etc). Does anyone have better insight on the alignment technique, or a good engineering reference?
As an aside there was a recent Scientific American issue about Einstein and his legacy. He was apparently very involved in the early patent fight over the gyro-compassing technique. It was also mentioned that GPS position would not be more accurate than about 30 meters if relativisitic affects are not accounted for (not considering a differential GPS system).

17th Sep 2004, 07:47
The ring laser gyro keeps track of the rotation in three different planes. Summed up, this becomes one single rotation about one axis. When parked on the ground, this axis is the rotational axis of the earth. Thus, unless parked at either pole, you can use this axis to determine (true) north. Just project it onto the ground and you have it.

Did that answer your question or did I misunderstand?


17th Sep 2004, 15:03
Yes, that helps. It is easy to see the mechanical gyroscope, along with the torques induced by earth rotation, and the associated precessional affects. I guess for the RLG, one simply takes the rotational component about each axis (determined by phase shifts, not mechanical torques), and sums them up to get the resultant vector, as you say. It is still impressive that there is enough sensitivity to detect the earths rotation, although I guess these devices are incredibly accurate in measuring phase shifts.