Can you guide me about the purpose of fine alignment in inertial navigation systems? I am new in inertial navigation field. I have some confusion about fine alignment process of INS. I want to ask that what is purpose of fine alignment?

I know by process of alignment we find the initial attitude matrix from body frame to navigation frame (reference frame) which normally take place by two step


Coarse alignment
Fine alignment

a. what is major difference between both steps?

b. how fine alignment refine the output of coarse alignment? Does Fine alignment minimize the errors to get more accurate output? or Does Fine alignment re correct the initial attitude estimate by coarse alignment?

c. why sensor bias like accelerator and gyro bias and lever arm error are  estimated in Fine alignment process.
I was a Doppler Radar &amp; INS specialist for the Air Force in the 80s, and learned systems for the F4 (flying brick) and the triple carousel system for the C-5A. These systems used mechanical gyros in combination with accelerometers to establish a "Stable Platform". The carousel idea was state of the art then and rotated 1 revolution per hour to greatly reduce drift.

BATH was used for course alignment, which is an acronym for Best Available True Heading and local gravity to start level. It would then spin up and do fine alignment by sensing Earth rotation to determine True North. Typically this took about 30 minutes. Modern systems us laser gyros which aren't affected by mechanical drift.