I've noticed during SpaceX streams (this could happen with other LSPs but I've only noticed it with SpaceX) that shortly after the vehicle goes to internal power the controller says "Vehicle is in self align". What exactly does this mean? My best guess is it has to do with the flight computer being ready for launch but not 100% certain.


Up until that point in time, ground control has been regularly telling the vehicle where it is in inertial space relative to the center of the Earth, and in which direction it is pointing in inertial space. "Vehicle is in self align(ment)" represents a mode change where the launch vehicle's flight software begins to use the vehicle's onboard navigation sensors (gyroscopes, accelerometers, GPS, and later, star trackers) to navigate itself. The vehicle begins to be on its own.

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    $\begingroup$ Similarly with Apollo: "T-15 seconds, guidance is internal." nasa.gov/62282main_countdown_launch.wav $\endgroup$ Oct 14 '19 at 2:10
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    $\begingroup$ I don't know what SpaceX is using the term for, but in aircraft avoinics "self align" means the aircraft is stationary and is using it's internal gyros to measure the rotation of the earth to get Latitude and platform orientation to a high degree of precision. This takes about 7-15 minutes and is done just after power is applied. Side note, the current ring laser gyros used are sufficiently precise that the oscillation of the aircraft due to the inverted pendulum with the CG of the earth can be measured in flight, and using the period of oscillation altitude can be calculated. $\endgroup$
    – mfarver
    Oct 14 '19 at 2:29
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    $\begingroup$ @mfarver - extraordinary! as an interested novice - can you point me to a book or article or something that discusses the engineering of the altitude measurement you mentioned? $\endgroup$
    – davidbak
    Oct 14 '19 at 16:41
  • $\begingroup$ I don't know if a paper was ever written but a friend that was on a team of engineers tasked with removing sources of error from the inertial reference mentioned that it had taken them a while to figure out this slight "wobbling" in flight once every source of error they could imagine was removed. As I recall he said there period was about 15 minutes and the offset from the expected course was about 5 feet to either side. One of the engineers did the math and figured it out, and eventually showed that it could be used to estimate altitude. I doubt the discovery was ever put to use. $\endgroup$
    – mfarver
    Oct 17 '19 at 19:02

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