[sebzim4500]

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.

[beerandt]

Rtk already is 'relative' error- it requires one or more base stations (with either known absolute location or assumed one for relative positioning).

But survey grade gnss is a web of rabbit holes, if you want to get into it.

And there are ways to get sub mm accuracy both relative and absolute, but idk of one that would be quick enough for the required reaction time of dynamic landing via 'catching'.

But multi-centimeter (4-5) that's really easily doable is probably good enough for other systems to take over from.

[magicalhippo]

When Tim of EverydayAstronaut quoted this[1], as I recall the quote was within 0.5cm of the target landing site. So I assumed that to be relative accuracy and not absolute.

[1]: https://www.youtube.com/watch?v=pAPt5vbr-YU (don't recall timestamp, sorry)

[bmicraft]

My lawnmower (openmower) can do <2cm accuracy over GNSS. It is absolutely believable they could achieve 0.5cm on GNSS (plus rtk correction data from a fixed base station nearby) alone without measuring any relative distance using other systems.

[snypher]

I'd love to hear about 2cm accuracy uncorrected. Does it have dual GPS units?

[bmicraft]

Well, it isn't "uncorrected". It's just that you don't need any additional hardware other than a second gnss receiver on the base station and some kind of link between them.

GNSS is more than accurate enough once you know all slight errors in satellite orbits and the atmospheric distortions currently affecting the area near the base station and can correct for them.

[cubefox]

It's significantly more difficult to actually land a jumbo jet sized rocket booster with that precision than to measure its own relative position. Gerstenmaier was talking about landing accuracy. My guess is that measurement accuracy is a red herring. More likely it was a slip of the tongue (the good man is 70 years old) and he meant to say it landed with a 0.5 meter, not centimeter, accuracy relative to the buoy.

[scottshambaugh]

I'd be very interested in the systems advertising that! I have not seen that even for stationary surveying equipment. I think it's also important to distinguish between RMS error which is often the better topline spec that companies give you, vs the 95% confidence error which is the more relevant one for flight reliability.

[michaelt]

In the specific case of a docking-type manoeuvre presumably you only need the highest accuracy when you're getting very close to the target.

No reason you couldn't use RTK GPS for <10cm accuracy for most of the flight, then in the last few meters of landing switch over to to high-precision, short-range tracking - like optically tracking a marker on the grabbing arm.

For other specific cases - like bridge monitoring - there are reports of 2–3 mm precision [1]. Of course, bridge monitoring has quite distinctive requirements; a 5Hz vibration component and a 0.0001 Hz thermal expansion component. So there's a lot of potential to average over lots of readings to reduce noise.

[1] https://www.sciencedirect.com/science/article/abs/pii/S02632...

[londons_explore]

If you want to land 99.9% of the time, it really is te 99.9% circle you should be looking at...

Or in fact, you need even better than that, since you don't want your whole error budget used up by the GNSS system.

[stavros]

What's absolute position? Isn't all position relative?

[westmeal]

https://www.youtube.com/watch?v=bZe5J8SVCYQ

[Maxatar]

Either I'm very stupid or that video is some kind of epic trolling.

[mianosm]

Epic trolling, you're sane.

[adastra22]

Absolute position is global lat/long coordinates. Relative position is “I’m 0.5cm from the middle of the peg.”

[willglynn]

Global lat/long coordinates are defined in terms of coordinate systems like WGS84 or ITRF2020, which are themselves the result of relative measurements between reference stations.

The earth's crust floats on top of liquid rock. This matters at relevant length and time scales; in most places, these effects alone are on the order of millimeters per year. One reason why it's better to use NAD83 over WGS84 in North America is that NAD83 latitudes and longitudes move with the North American plate.

Positions _are_ relative, and the closer you can put your datum, the less drift you'll accumulate.

[adastra22]

There is a literal, autistic sense in which you are correct. But there is a practical, pragmatic distinction between measurements that we call absolute versus those we call relative, and pedantic correctness misses the point.