How does a Falcon 9 booster know how far away the ground is?

Question

In the latest landing video, it seems the Falcon 9 just hovered for a moment above the ship. That being said, it begs the question, how does the booster know where the ground is? Does it purely work off of GPS, a 1 way radar/beacon type system from the landing zone, radar from the rocket, or some means that I'm not aware of?

A laser based system is probably out, it wouldn't work through the rocket's exhaust — GdD, Jun 17 '16 at 16:06
@DanPichelman Something like that might work for the final touchdown, but not likely to work, say, 10m high. — PearsonArtPhoto, Jun 17 '16 at 16:08
@PearsonArtPhoto That's why I added the smiley. I suspect the actual solution probably works from even higher than 10m. A radar altimeter might be a possibility, but that's just an uninformed guess. — Dan Pichelman, Jun 17 '16 at 16:22
I think it knows it generally from the system of locating the barge in 3D, same as horizontal offsets. Now how does it know these... — SF., Jun 17 '16 at 18:35
If it's GPS, even the best GPS systems are off by a few meters sometimes, and a drop from 2m leads to a huge impact... I'd hope there was something better than that. — PearsonArtPhoto, Jun 17 '16 at 18:37
@PearsonArtPhoto: Differential GPS is pretty solid. You'd need a GPS reference on the barge for that, but you probably wanted that anyway ;) — MSalters, Jun 18 '16 at 00:47
"That’s it! That’s a good name – ground!
I wonder if it will be friends with me?" — Keith Thompson, Jun 18 '16 at 19:12
2m drop is much, but if you reduce your descent speed to what your landing gear is designed to withstand, you can just keep descending at constant rate until you find the ground. — SF., Jun 18 '16 at 20:33
The booster can't hover, or so we've been told. So you can't have a constant rate of decent... — PearsonArtPhoto, Jun 19 '16 at 02:51
@PearsonArtPhoto: And yet... how would you describe what happens here: https://www.youtube.com/watch?v=AYRZTERK5-g ? Or here: https://www.youtube.com/watch?v=QyaUjOCxOX0 ? — SF., Jun 19 '16 at 08:38
Does this answer your question? How does the Falcon 9 know where the landing barge/pad is? — Christoph, Aug 26 '20 at 09:50

score 7 · Answer 1 · answered Jun 18 '16 at 13:14

7

When commenting on one of the early water landings, Elon Musk mentioned there's a radar altimeter (which was calibrated for flat, hard surfaces and might have trouble detecting the water surface as a result).
Can't find a primary source for this, though.

answered Jun 18 '16 at 13:14

Hobbes

127,529
4
396
565

As of your own answer on basically the same question the source is the post launch interview of Elon Musk from the Falcon Heavy launch. – Christoph Aug 26 '20 at 09:52

How does a Falcon 9 booster know how far away the ground is?

1 Answers1

Linked

Related