Salute!
We may be closing in on how the doggone nav system works depending on your ride. Before waiting for questions and enjoying Easter, I put on my instructor hat and try to help. You can't "teach" anything to anybody. All you can do is help them to learn. Bear with me for 5 or 6 paragraphs and then class is dismissed.
Goldenrivett and Check Airman are homing on to the ability of modern systems to do airborne alignments of the inertial unit. Your overall nav system may only use the inertial unit for certain things, and have other subsytems provide inputs to the overall nav and guidance solution. First time I saw a change was in late 60's, and the nav system called the inertial unit "inertial measurement unit" verus inertial navigation system. In other words, our nav system used the inertial unit and other inputs to provide us a super positional and velocity output for display and other purposes. Wasn't a self-contained system as the Double Ugly had.
The biggie way back was digital computers the size of shoeboxes versus your refrigerator. And then the folks at Draper Lab and other places implemented Kalman filters or their equivalent to the nav system. Apollo and sub-launched missiles were the impetus, and I personally benefited along with many others in my profession. Our nav gear and computed weapon delivery capability skipped a generation of traditional systems development. So by 1971, I flew a neat jet with a nav system which combined various subsystems in a central digital computer to provide me position, velocity, wind data, steering to a selected point in space, etc. One unit was the inertial measurement unit.
PLZ take a look at the links I include later to help you "
Fathom" what I try to explain. BTW, sir, I completely unnerstan what an AHRS system does and how it works or I wouldn't be on the platform now.
So the short answer concerning airborne alignments is your system uses one box to sense rotation and acceleration - six degrees of freedom. Another box to integrate the data to get a velocity vector( you know, v=a*t). Given a starting position, assumed attitude ( like st and level) and a clock, it's easy to calculate where you are, how fast you are going, etc. Even wind! Using heading and groundspeed, the "wind triangle" is a no brainer. Washington Center used to routinely ask us what the wind was at FL250 as we came home from a range up north of The Beach. You don't need a "platform" stabilized in space as we did in the 60's and 70's. Given an initial attitude, the accelerometers and rate sensors can measure body rates and new attitude referenced to the initial values. The nav computer can use the data with initial inertial position in space to compute and display all kindsa good stuff. Your iPhone can "navigate" real well between position updates from its GPS. I truly believe we could have gone to the moon using one, with the IBM PI box as a backup, heh heh. The Draper folks used predicted visible posiiton of certain stars at predicted points in space and a great, small "telescope" to correct the nav solution, and they didn't have GPS back then. See how Kalman filters and their cousins work, and the airborne alignment solution is obvious, especially with frequent positional data from GPS or some external area nav system.
Some decent learning material, especially about Kalman:
https://en.wikipedia.org/wiki/Kalman_filter
https://www.kalmanfilter.net/default.aspx
https://en.wikipedia.org/wiki/Inerti...igation_system
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