Hacker News Reader

Comment by mbell | Hacker News Reader

Armchair aerospaceing here, but it feels like he's a whole class of positioning sensors in this analysis. It seems to be you only need GPS and related absolute positioning systems to get you close to the tower. At that point, what you care about is the relative positioning of the tower and the booster. I would think this can be done very accurately with a host of options: cameras, radar, lasers, ILS style systems, etc, etc.

View on HN

Right? They land fighter jets on carriers with a light signal that projects out from the ship at a particular angle. It seems very easy to do something like this with some form of electromagnetic radiation. Or have some way for the tower to detect the exact position and communicate with the rocket.

I understand engineering is complicated but this honestly seems like the easiest part of the problem to solve.

It's more likely that SpaceX determined they didn't need super tight tolerances and called it a day.

> SpaceX determined they didn't need super tight tolerances and called it a day

Yup! This is my conclusion in the article - the landing box for the Super Heavy booster is 5x13x18 meters on each side, with 5-15 degrees of angular tolerance in each of the vehicle axes. So the margins are big enough that you don't need millimeter level precision for the rocket position.

> Could you use other real-time distance measurements like laser rangefinding or visual processing? I don’t think so

This is the part I question though. Seems like an org as well motivated as SpaceX could easily solve that if it was necessary.

My take is that it would probably be possible with enough effort, but there isn't an easy solution. And if you don't need it then the best part is no part. :)

Carrier landings are accomplished using a combination of indications, and the meatball is only one of three primary tools. If you are not flying an on-speed angle of attack AND lined up on the centerline of the landing area, the meatball position is invalid to a degree proportional to the degree to which those other inputs are off.

The meatball Fresnel lens is canted slightly side-to-side, and only places the hook in the right spot at a given angle of attack. Which is a design compromise necessitated by having to allow multiple types of aircraft with multiple hook-to-eye distances to land on the same aircraft carrier while using a visual input in one location (the cockpit) to properly place a device in another location (the hook point) with high precision.

Source: I've done it.

So just as it is not "very easy" to trap on board the boat with "just" a light signal, I would assume landing a building-sized booster has a similar if not bigger list of potential "gotchas."

Very easy compared to all the other hard problems SpaceX has to solve, yes.

I suspect the real problem is wind. A last moment gust could push the booster far enough away that it cannot recover.

This is fixable by setting wind and gust limits for recovery, just like every other aircraft on the face of the earth.

I'm sure they've done that. But the wind is a chaotic system, and once the booster has begun its descent it's committed regardless of wind changes.

There have been many airplane crashes because of sudden unexpected wind changes while landing.

Does SpaceX even use traditional GPS? I'd assume with something like Starlink they would be able to employ something more precise/fit for purpose.

GNSS RTK is incredibly accurate these days. By the time that you're close enough to the landing zone, you're close enough to get positioning down to centimeters on consumer grade hardware, which the article points out.

The actual question is literal: Can SpaceX land a rocket with sub 1 cm (1/2 cm) accuracy? GNSS RTK can get you down to a couple of centimeters, but getting more granular resolution than this isn't reliably possible with current professional grade technologies.

I'm personally unsure if the military has greater resolution than what's possible with RTK or w.r.t. military use GPS, but I would not be surprised if they did. If that's the case, NASA would most likely have access to it, I would assume. But the article specifically calls this out saying that it's not accurate enough to surpass the resolution of using RTK.

What's really cool about these questions is that the same problem space is applicable to self-driving cars and SLAM, if you're into that sort of thing. Lane detection, etc.

>you're close enough to get positioning down to centimeters on consumer grade hardware

But in realtime? (single-digit second latency, at least)

Yes, for the purposes of landing speeds. In fact, at vertical aircraft landing speeds, your time-step to position Δ is more accurate than automotive SLAM.

Nice, TIL. Thanks!

I mean rtk accuracy is considering rtk in isolation. You can get better accuracy if you combine rtk with other methods such as an IMU.

Wow, I can’t believe I never realized that SpaceX could sell access to a positioning system far better than GPS…

Depends on what you define as 'traditional': basically if you want positioning information via GNSS, the techniques for getting a better reading a fairly well understood, and it doesn't really matter how good your satellites are, the atmospheric distortion is the issue and you need to model and compensate for it by measuring it with a near-enough base station, using multiple frequencies and constellations, and if you're moving, an IMU to constrain your motion over as long a period as you can to effectively average out the other noise sources. Half a centimeter, given all of the above, is better than what I've seen quoted in the space, but not utterly crazily so.

Not really, the Gyros described in the post are also essential. If you don’t know the attitude of the vehicle, you can’t point the engines in the direction your control algorithm says you should to hit the target.

Edit: I think I misunderstood the comment. Yes, you can use the absolute methods for rough guidance and then use relative positioning for the final approach. The article has a line about why the author doesn’t think that’s likely though.

Reply on news.ycombinator.com