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    Re: Space sextants
    From: Don Seltzer
    Date: 2016 Jan 27, 19:40 -0500
    This seems to be similar in principle to the star tracker used in the Trident II ICBM.  Navigation is primarily by the inertial guidance system.  Before launch, the inertial platform is rotated so that an onboard telescope is pointed towards a suitable reference star.  After the boost phase, the system 'looks' at the reference star.  If the gyros have done their job perfectly, the telescope is still pointed directly at the star.  In actual practice, there are various errors so that the star is off center in the telescope's field of view.  These errors are fed back to the INS which recomputes its estimated position with an updated Kalman filter.

    Don Seltzer

    On Wed, Jan 27, 2016 at 2:34 PM, Frank Reed <NoReply_FrankReed@fer3.com> wrote:

    I wouldn't classify that as a "space sextant" since it absolutely depends on the local gravitational vertical. It won't work in freefall. This Northrop instrument is, in fact, an evolution of the old bubble sextant with three huge improvements:

    • it gets its vertical from an inertial system which means it's dramatically more accurate than any other form of terrestrial celestial navigation,
    • its optical instrument is similar to a theodolite and does not employ mirrors,
    • it's automated and can take sights more or less continuously

    The same system is supposedly installed (and still used) on the B-2 bomber, and one may "safely assume" that optical-inertial turrets like this are relatively common globally, not just on US aircraft. Because the inertial system provides a measure of the vertical accurate to better than a second of arc, the position fixes are also better than that (equivalent to accuracy better than 100 feet on the ground).

    Frank Reed
    Conanicut Island USA

    PS: That slide you included with the images of the system repeats one common error. The SR-71 was not capable of anything approaching exo-atmospheric maneuvering, and its pilots were not "classified as astronauts" (at least not "by the book" though in a classified program there's always a slim chance they were handing them out by rules in a different book). The Air Force, and by extension NASA, traditionally had a somewhat lower standard for the classification than commonly used today. Formerly in the USA astronaut wings were awarded for flights above 50 miles. Today the standard is 100 km, known as the Kármán line, which is just about 25% higher. In any case, as a true airplane --air-breathing and with entirely aerodynamic control-- the SR-71 could fly nowhere near high enough for either astronaut classification. Aircraft operate in the troposphere and the stratosphere. The SR-71 reached the upper levels of the stratosphere but no higher. By contrast over a dozen flights of the X-15 rocket plane made the cut by the old standard, while two X-15 flights (same pilot, according to Wikipedia) made it to space by the international, modern standard. By the way, both the old US standard and the Kármán line, are entirely arbitrary choices for the edge of space --and yet reasonable, too (see the Wikipedia article for more). 

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