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

  1. Coarse alignment
  2. 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.

  • $\begingroup$ Welcome to Av.SE $\endgroup$ – Ralph J Jan 23 '19 at 14:48
  • $\begingroup$ INS alignment is a convergent/asymptotical process with no completion. The more you wait, the higher the precision, see Why does IRS alignment take so much time? $\endgroup$ – mins Jan 25 '19 at 21:57
  • $\begingroup$ @mins Actually it is not asymptotic. As you integrate for longer periods, errors in the integration process add up and cause drift, that is, random walks rarely end up at the starting point. Same effect as clocks: the longer you wait after setting a clock, the more wrong it gets. The measure is known as Allan Variance, see section 5, for example. $\endgroup$ – user71659 Feb 23 '19 at 3:46
  • $\begingroup$ @user71659: Seems convergent. $\endgroup$ – mins Feb 23 '19 at 13:00
  • $\begingroup$ @mins Look at fig 11 in my linked document. At 1000+ seconds for that model, error starts to trend upwards. It's intuitive when you think about it, the longer you run an IRS, the more it drifts. $\endgroup$ – user71659 Feb 23 '19 at 23:53

I was a Doppler Radar & 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.

  • 1
    $\begingroup$ This answer does not seem to address the questions. It is tangential or informational at best. $\endgroup$ – aerobot Oct 21 '19 at 16:18

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