Most of this article feels like it's discussing irrelevant methods, you only need GPS to get it close (well for what they're doing they don't need GPS at all, though I'm sure it's used), we have much much more accurate ways of measuring the positions of things from a fixed reference point, 0.5cm deviation on your positional measurement is trivially achievable with optical systems. Why is the author spending paragraphs discussing IMU accuracy when we're trying to line up a rocket with a tower. You care about the rocket's relative position to the tower, you can put your measurement equipment on the tower, you don't need to worry about how accurate your accelerometers are.

I assume they are doing something much more clever/hardened, but you can trivially achieve much greater spatial accuracy with a Vive Tracking Puck for like $100.

> Bill likely misspoke or was talking about control error.

Mixing up control errors with absolute errors is a very common form of miscommunication in robotics.

I work with relatively big robots and often my colleagues would say something like this "During the test we had 0.5m cross track error, so we did X, Y, Z ...".

And I always ask them for clarification. Were they looking at the robot and seeing that it is half a meter off where it should be, or were they looking at a screen and seeing that the robot thinks it is half a meter off from where it wants to be? Because those are two very different situations. And both can be described with the same words. (And sometimes it can be both, or just one of them.)

The article discusses the absolute error coming from RTK systems and claims that it won't be as low as 0.5cm, but surely the relevant metric is relative error, and I can see commercial systems advertising that level of precision.

i.e. the booster doesn't know it's actual position to within 0.5cm but it knows it's position relative to a buoy or the catch arms to that precision.

Just a few years ago, it was a huge leap just have a rocket return and land, now we're pushing the actual accuracy of how well it can land?

Anybody else thinking this quite the time to be alive?

I found this video from the perspective of a landing pin interesting, especially when played back at low speed: https://youtu.be/ExV6PHRM8eI?t=17

You can see the arm comes in, then there's some side-to-side bounce (not sure how much is the rocket bouncing off vs. the arm fine-tuning its position). Just after contact seems to be made, and before the shock absorbing (or yaw-correcting) pistons drop much, there's a large flash from the engine. Is that a characteristic of engine shutoff, or was there a last-second "hover" push just before shutoff and drop? I wonder how much force the arms felt.

Another perspective showing both arms, and (as mentioned in the article) how the left one adjusted more significantly at first: https://youtu.be/JlcrNakUGVs?t=3

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.

Maybe if the rocket knows where it is because it knows where it isn't.

That is what they told us in missile maintenance school.

And gyros have gotten a lot better. Especially if you're throwing money at the issue like you know those folks are.

0.5cm isn't even well defined, considering that the rocket itself is probably out of round by larger tolerances, not to mention issues of thermal expansion.

A lot of people think it landed on the large grid fins, this is not true it actually landed on much smaller landing pegs

> I think < 10 cm accuracy is achievable

If you don't know how precise GPS receivers can get with dead reckoning techniques, this demo of someone "drawing" onto a map of their driveway using a GPS receiver is very impressive: https://youtu.be/3tQjIHFcJVg?t=245

It looks like they're getting measurements that are only a few inches away of the module's real position, although of course the conditions seem favorable with an unobstructed sky and consistent alignment.

The module they use is a ZED-F9P by u-blox. I've used ~$50 u-blox GPS modules in DIY electronic projects before since they're often the brand you'll get when buying GPS modules, but this particular type with dead reckoning is much more expensive. Sparkfun has it for $275 for example: https://www.sparkfun.com/products/16481.

You can do even better with radar. If you place a set of radar reflectors around the tower at known locations, then you can detect them from the booster and triangulate the distances to a precise position. Plus, radar gives you relative velocities, so your speed and roll rate estimatimates get even more precise. I bet you could get down to millimeters with a setup like this.

As the article explains, with a well designed procedure, the required navigation accuracy is quite modest. Even the latest consumer IMUs and GPS would do, and SpaceX is using even slightly more accurate "tactical grade" units, typical for all launch vehicles.

Good article. It is nice how it goes through all the points systematically.

> Half a centimeter landing accuracy is not possible, and Bill likely misspoke or was talking about control error.

Maybe the 1/2 cm accuracy refers to the final position of the booster's catch points on the arms after they've closed, after the booster's engines are off, and after the booster settled, and maybe they mean lateral accuracy. I would forgive them for that because that's the accuracy that actually matters here.

If the catch points were off then that might spell disaster, so the catch points' landing accuracy including the help of the catch arms is what matters.

Rocket alone need not be this accurate as grabber arms should also do some maneuvering to get the final accuracy.

I would have thought the vibration from the engines would produce error of greater that 1/2 cm. Still, it seamed to have worked well. So, there you go.

I feel like he's limiting himself to just 2 positioning methods.

There are so many other methods that the lander can use to know where the tower is.

I vaguely remember Musk talking about reflective paint for some band on the landing pad of Falcon, a long time ago.

> At the most precise, an RTK positioning system could lower position accuracy all the way down to 2.5 cm (+1cm per km of distance). If SpaceX put a receiver on the launch tower or the ocean buoys, then the landing position could be incredibly accurate. But even the most advance positioning tech won’t guarantee it down to 0.5 cm. And RTK does rely on being able to acquire and maintain a link between the booster and ground for this precision.

I don't understand this last sentence. Afaik RTK correction only requires receiving correction frames on the booster's side, which can be distributed via l-band just like GPS. I suspect the latency constraints are also quite low as the conditions aren't going to change quickly near the tower with the kind of good weather they choose for launch.

If it's positioning relative to the chopsticks, I'm sure it's possible to know where you are within a centimeter, even with all the rocket exhaust flying around. That's what DGPS is all about.

It's still wildly un-nerving to me that there's no publicly stated option other than the chopsticks for landing(edit: some future passenger craft). Imagine if you've got enough fuel to avoid slamming into the ground, and a nice big ocean, or a lake sufficiently deep... couldn't a water landing happen and let future passengers survive?

I think they were simply speaking of the final pads the rocket rests on top of the chopsticks have 5cm of error either direction.

But judging from the bouncing the rocket did when in the chopsticks the error for positioning into the initial catch position is much larger in all directions. The chopsticks coming closed around the rocket do the heavy lifting for final alignment to that 5cm I imagine.

And here is me trying to find sheet metal fab that could make a simple enclosure that matches the design and is not warped or scratched.

Impossible apparently.

I wonder how much of a problem crosswinds are. Not a lot of mass, and that big can has a lot of sail area.

> And RTK does rely on being able to acquire and maintain a link between the booster and ground for this precision.

There are cameras and other electronics sending telemetry (via at least Starlink) on the things.

The "acquire and maintain a link" seems to be a very solved problem.

The designers of the raptor engine should get a Nobel prize in chemistry for combustion physics.

The accuracy that counts is for the gap between the arms and the booster, which could well be 1cm since they can measure that with sensors as the booster descends between the arms, and the arms can be controlled accurately.

I wonder if they will paint pin alignment marks on the grabber arms?

Not a US taxpayer, so I don't really have a stake in this, but I'm curious as to where SpaceX spending is compared to their NASA contract milestones.

Given the size of the booster, what corrections would placing GNSS sensors in multiple places provide to increase positional accuracy?

Eg if you space sensors 20m’s apart.

An item the analysis didn't include is using Starlink signals for location, posture, and rotation. I have no idea if they do...

Love the analysis.

Could use more simulation data with NVIDIA Omniverse and thrust vector control

Um, what about using starlink to measure position as option 3!