I'm trying to use an accurate FOG gryo for tracking heading in a moving platform. The FOG gyro has two rate errors, Faraday (fixed amount regardless of alignment to the earth) and the 15.04 degrees per hour of the earths rotation (function of my alligmnet to the rotation).
I know the attitude of the gyro ring.. Roll,Pitch,Yaw and Latitude,Longitude.. If the gyro is exactly aligned with the rotation I will see all of the 15.04 deg/hr superimposed, none of it if I'm perpendicular to the rotation. Now I need mapping/function that will give me a number between -1 to 1 giving me the amount of the earths rotation imparted at a given roll,pitch and yaw on the surface of the earth.. Where roll=pitch=0 level with the surface, yaw=0 pointing true north. The gyro (yaw axis) is mounted flat in the plane of the earth.
This will allow me to remove the Faraday drift (constant) and then calculate the amount of the earths rotation projected into this gyro at this roll,pitch, yaw, latitude.
Can someone help with this transformation or point me to a document that can help?
Best Answer
Your second "drift" term is not an error term at all, it's the FOG measuring the actual sidereal rotation of the Earth. Some systems use this to get an absolute orientation reference; this is known as "gyrocompassing".
When you're working with systems that have this level of accuracy, it would be better to treat this term for what it is, and do an explicit transformation between the Earth-centered nonrotating coordinate frame (which is what the FOG is measuring) and the Earth-centered, Earth-fixed (rotating) coordinate frame you want for your answers.