In watching today's BulgariaSat-1 broadcast:

It was mentioned that the drone ship had acquired the 1st stage as it was returning. I had always been under the impression that the landing was "controlled" by computers on the 1st stage rocket as it was re-entering, but based on the video, it seemed to indicate that it was the drone ship that was doing the controlling? So, where is the autonomous landing actually controlled from? The 1st stage, or the landing platform (whether on a barge or on land)?

My guess is that the control is done from the rocket side, but with information provided to the rocket from the ground in regards to positioning, distance, wind speed, etc., but I am curious to the actual role each "side" has in the landing procedure.



2 Answers 2


I don't have any citation, but the first stage is doing all its own guidance and control; it would be insane to do it any other way.

The "acquisition" called out at about 80km altitude is acquisition of signal, not of control. What exactly is being signalled and how the ASDS makes use of it is unclear.

I've heard (again, without citation) that both the ASDS and returning stage are trying to navigate to the same GPS coordinates simultaneously; it's possible that the stage is telling the ASDS what its current trajectory error is, but the stage is so much more maneuverable than the ASDS that I can't believe the latter would try and move underneath it.

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    $\begingroup$ There is some discussion of the role of relative GPS associated with this and this question. In relative GPS the two objects exchange more information about in the signals each is receiving from the GPS satellites. This way they can get a better determination on their relative positions and velocities than by just subtracting their absolute positions and velocities. $\endgroup$
    – uhoh
    Commented Jun 24, 2017 at 3:51
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    $\begingroup$ What are the other instances where AOS is called - ground stations. Iirc Ariane calls out AOS for multiple ships stationed along the ground track of their launch. These receive and retransmit telemetry from the rocket. I believe the ASDS works as a ground station too, relaying the telemetry back to land. $\endgroup$
    – jkavalik
    Commented Jun 24, 2017 at 6:10
  • $\begingroup$ I was a bit tired when I watched the webcast, so I might be misremembering, but I'm pretty sure the announcer explained it in the very next sentence: that they were talking only about acquiring the data downlink, and that the ASDS is staying entirely passive in the landing. $\endgroup$ Commented Jun 26, 2017 at 10:50
  • $\begingroup$ I would just like to add reality is not KSP $\endgroup$
    – Raze
    Commented Jun 27, 2017 at 8:29

The drone ship acts as a relay for telemetry from the rocket to ground control, that's the signal being acquired.

It probably also feeds location and weather information to the rocket for last minute adjustments (I'd do that were I designing such a system, and I'm not as smart as the guys at SpaceX).

  • $\begingroup$ Actually, I don't think they do. But they are keeping lots of things secret, so who knows. The publicly available comments made during the broadcasts indicate that the ASDS holds at a fixed position that is determined well before launch, and that's it. $\endgroup$ Commented Jun 26, 2017 at 10:54
  • $\begingroup$ @JörgWMittag fixed position is good, but it's on the high seas where fixed can mean several meters fluctuation. Having constant precision updates would surely help with accuracy. $\endgroup$
    – jwenting
    Commented Jun 26, 2017 at 11:16

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