3 ms·

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,

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.

magicalhippo 2y ago
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 https://www.youtube.com/watch?v=pAPt5vbr-YU (don't recall timestamp, sorry)
beerandt 2y ago
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.
scottshambaugh 2y ago
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.
- londons_explore 2y ago
  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.
- michaelt 2y ago
  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/S0263224118300873 https://www.sciencedirect.com/science/article/abs/pii/S02632...
stavros 2y ago
What's absolute position? Isn't all position relative?
- westmeal 2y ago
  https://www.youtube.com/watch?v=bZe5J8SVCYQ https://www.youtube.com/watch?v=bZe5J8SVCYQ
  - Maxatar 2y ago
    Either I'm very stupid or that video is some kind of epic trolling.
    - mianosm 2y ago
      Epic trolling, you're sane.
- adastra22 2y ago
  Absolute position is global lat/long coordinates. Relative position is “I’m 0.5cm from the middle of the peg.”
  - willglynn 2y ago
    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 2y ago
      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.
bmicraft 2y ago
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 2y ago
  I'd love to hear about 2cm accuracy uncorrected. Does it have dual GPS units?
  - bmicraft 2y ago
    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 2y ago
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.

magicalhippo 2y ago

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

beerandt 2y ago

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.

scottshambaugh 2y ago

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.

londons_explore 2y ago
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.
michaelt 2y ago
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/S0263224118300873 https://www.sciencedirect.com/science/article/abs/pii/S02632...

stavros 2y ago

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

westmeal 2y ago
https://www.youtube.com/watch?v=bZe5J8SVCYQ https://www.youtube.com/watch?v=bZe5J8SVCYQ
- Maxatar 2y ago
  Either I'm very stupid or that video is some kind of epic trolling.
  - mianosm 2y ago
    Epic trolling, you're sane.
adastra22 2y ago
Absolute position is global lat/long coordinates. Relative position is “I’m 0.5cm from the middle of the peg.”
- willglynn 2y ago
  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 2y ago
    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.

bmicraft 2y ago

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

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.