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Even when the Falcon 9 "punched the barge", it did so with extreme precision. SpaceX has now had several successful landings, many of them extremely spot-on. Two questions:

  • There are obvious visual navigation markers on the barges. Is that only for show, or does the Falcon 9 use visual navigation?

  • What other techniques (if any) does the Falcon 9 use to enable those precision landings?

To be specific, I'm asking about terminal guidance. GPS alone will not do the trick. Relative GPS might do the trick.

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    $\begingroup$ I'm pretty sure I've either read or heard Elon must use the term "relative GPS" explicitly somewhere - looking for it now. I think it was on a televised interview after either the first ground or first ship landing. By the way, Elon sez we should call it a ship, not a barge :) ...also, could someone take a moment and explain what relative GPS actually means? $\endgroup$
    – uhoh
    May 9, 2016 at 12:30
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    $\begingroup$ Possible source? Sometimes you do not need to know your precise position but you do need to know your position or an object's position, relative to another object or location. $\endgroup$
    – Jerard Puckett
    May 9, 2016 at 12:48
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    $\begingroup$ @uhoh -- Relative GPS involves communicating GPS solutions amongst objects to obtain a better estimate of the relative state between the objects. Suppose object #1 uses GPS satellites A, B, C, D, and E to arrive at an estimated state, while object #2 uses GPS satellites B, C, D, E, and F. If the goal is to determine relative state as opposed to absolute state, its best to toss satellites A and F and only use the common subset, satellites B, C, D, and E in this case. $\endgroup$
    – David Hammen
    May 9, 2016 at 13:46
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    $\begingroup$ As a point of reference, the HTV, Cygnus, and Dragon switch from GPS to relative GPS and then to visual navigation as they get progressively closer and closer to berthing with the ISS. The Soyuz and Progress vehicles similarly switch from absolute navigation to Kurs to visual navigation to dock with the ISS. $\endgroup$
    – David Hammen
    May 9, 2016 at 13:50
  • $\begingroup$ I'd be very surprised if visual guidance was used at any point -- too many things could interfere. Besides GPS, I'm very curious as to whether an active radar solution is involved - like a radio beacon at the center of the ASDS. $\endgroup$
    – Russell Borogove
    May 9, 2016 at 14:54

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The rocket and ASDS use absolute GPS to share a joint location. The ASDS stays where it is supposed to be, and the rocket lands where the ASDS is supposed to be.

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  • $\begingroup$ How do you know this? $\endgroup$
    – uhoh
    Oct 8, 2018 at 16:53

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