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That was really amazing. I've been following it closely on YouTube but to see it actually fly - mind boggling! Not quite clean room facilities. The nose blew ov

That was really amazing. I've been following it closely on YouTube but to see it actually fly - mind boggling! Not quite clean room facilities. The nose blew over in a storm. Locals streaming video from cameras mounted on telegraph poles. When it landed various parts came shooting off spraying gas as they tumbled away. Full-flow engine - first to ever fly. And boy did it fly! I love it. Back yard hacking hardware. Summary - SpaceX are the first people to land rockets for reuse. Then they go and do the same thing with a different engine design, different fuel, in a water tower controlled with Falcon 9 reaction control system. Quick question for the experts: 1 - how do they measure altitude? GPS is notoriously poor at this. 2 - any idea of how the control software was written for a flying water tower? Could they use the Falcon 9 software?

cryptoz 7y ago
I'm no rocket expert, but I have some ideas about altitude. > 1 - how do they measure altitude? GPS is notoriously poor at this. GPS is indeed notoriously terrible. They might have better luck with it though, given they can buy high-quality receivers and always have a good view of the sky. However I think I read (or hope at least) that they use barometers. Even cheap smartphones have barometers for relative altitudes these days, and while the barometer data is influenced by the atmosphere and weather of the day (the focus of like all my startups [1]), the rocket could easily know about the differences in the weather and create MSLP (mean sea level pressure) numbers that would allow for excellent precision at landing. Your phone barometer can tell the difference in altitude between your head and your feet. [1] My latest go at this is https://www.allclearweather.com https://www.allclearweather.com (US, Android) with a goal of eliminating the altitude part of barometer data so that the remaining data can be QC'd, summarized with statistics and eventually hopefully included in weather models for increased forecast accuracy
- bdamm 7y ago
  I think they do also have a radar for distance to pad information but I couldn't cite that. I believe they have a form of instrument landing system, similar to how it is done at major airports but oriented for the rocket of course, and the distance calculating "radar" could be part of that (using signal timing) but I am not sure exactly how they did it.
  - cryptoz 7y ago
    Yes I think that radar idea is quite correct. I now recall Elon/SpaceX explaining that this rocket (BFR or whatever) should be able to land on any solid body in the solar system. This would eliminate barometers and GPS for long-term solutions, but keeping radar/lidar and other light-based imagery in the running.
  - jccooper 7y ago
    Falcon 9 is known to have a radar altimeter; it's quite likely the Starhopper does too.
WatchDog 7y ago
> 1 - how do they measure altitude? GPS is notoriously poor at this. I would imagine they would use a number of different sensors, they probably have barometric altimeters, but probably rely on some kind of radar altimeter for low altitude maneuvers.
- monk_e_boy 7y ago
  Oh yeah - radar! Duh, I was thinking lazers or something.
njoubert 7y ago
Modern GNSS can reliably measure altitude at <50cm 2-sigma accuracy using techniques such as RTK and PPP-AR. Modern GNSS is a better source of altitude data than barometers. Likely SpaceX is using a combination of radar altimeters and GNSS. Source: I do engineering on modern GNSS
- cryptoz 7y ago
  Super interesting info. Sorry for my annoying questions. Does modern GNSS receive that level of accuracy in cities among buildings, or is that an ideal case like what SpaceX has, being outside? Also, how expensive is a modern GNSS receiver that can get that level of altitude accuracy? Is this something that could make it into phones some day?
  - walrus01 7y ago
    To add to the parent comment, super high precision gnss/rtk is often paired with multiple inertial measurement units, not very dissimilar from what you find in a smartphone. Also used in COTS drone flight controllers. Software can combine data from an imu that some vertical acceleration had taken place, with data from gnss sensors that altitude is now 2m higher, for additional level of confidence in what the gnss sensors are reporting.
  - njoubert 7y ago
    Not an annoying question whatsoever. Dense urban environments are challenging. The error distribution is more complex there, its hard to definitively say. The numbers I'm quoting is for open sky environments where you have plenty of satellites and little multipath. For the average end-user, these are $500 GPS units (this is the one I work on: https://www.swiftnav.com/piksi-multi https://www.swiftnav.com/piksi-multi). The technology is already moving down the supply chain to ~$10 GNSS units from Broadcom and ST (we're doing this: https://www.swiftnav.com/news/swift-%E2%80%8B%E2%80%8Bnavigation-%E2%80%8B%E2%80%8Bintroduces-starling-gnss-positioning-engine-and-interoperability https://www.swiftnav.com/news/swift-%E2%80%8B%E2%80%8Bnaviga...) The issue with phones is mainly an antenna issue. Your phone antenna is tiny and squeezed in alongside half a dozen other antennas and radios. This degrades the GNSS signal quality a lot. A stopgap here is external GNSS antennas for your phone: you might plug your phone into your car and get lane-accuracy turn-by-turn directions, but it's unclear when we'll have high accuracy GNSS inside phones, but most major phonemakers are indeed working on this.
leecb 7y ago
In similar systems, the position is generally tracked with the combination of interial sensors (accelerometer / gyro / angular rate sensors) and external references like GPS and star trackers. Inertial sensors are able to provide high frequency data on change in position and orientation. Higher frequency means the software can make much more rapid decisions to steer the rocket, on the order of 1000 a second or so, much more often than can be achieved with GPS. The fact that inertial sensors don't rely on external signals (like GPS does) means that there is some degree of robustness- for instance, if there is a temporary disruption in GPS signal reception, the rocket will still have some idea of its position. Over time, integration and measurement error accumulate from the inertial sensors. (Remember that they generally measure changes in position / orientation, not absolute position or orientation). For this reason, it is usually necessary to use external position and orientation references to correct the error that accumulates over time. GPS is used for this, and in some applications, star trackers can be used as an absolute orientation reference. On the algorithm side, a Kalman filter combines measurements from all of the position and orientation sensors to generate a prediction of the current position / orientation / velocity / acceleration etc.
- Symmetry 7y ago
  A minor addendum, you should feed your Kalman filter the gimbal and thrust of the rocket too, it'll happily chow down and combine that with everything else to further improve its estimate.
  - tlb 7y ago
    It's dangerous to throw extra sensors into a Kalman filter, because if the sensor goes bad it'll corrupt the output. Angular rate sensors are extremely accurate, far more accurate than a thrust sensor could be, so it probably wouldn't improve the overall accuracy anyway.
    - Symmetry 7y ago
      Ah, I guess I'm used to wheeled robots where odometers are often your most accurate gauges of position.
phkahler 7y ago
>> any idea of how the control software was written for a flying water tower? Could they use the Falcon 9 software? https://www.semanticscholar.org/paper/Lossless-Convexification-of-Nonconvex-Control-Bound-A%C3%A7ikmese-Carson/9209221aa6936426627bcd39b4ad0604940a51f9 https://www.semanticscholar.org/paper/Lossless-Convexificati... Reddit blurb about that paper: https://www.reddit.com/r/spacex/comments/7t2tb2/a_paper_by_lars_blackmore_of_spacex_on_soft/ https://www.reddit.com/r/spacex/comments/7t2tb2/a_paper_by_l...

cryptoz 7y ago

I'm no rocket expert, but I have some ideas about altitude. > 1 - how do they measure altitude? GPS is notoriously poor at this. GPS is indeed notoriously terrible. They might have better luck with it though, given they can buy high-quality receivers and always have a good view of the sky. However I think I read (or hope at least) that they use barometers. Even cheap smartphones have barometers for relative altitudes these days, and while the barometer data is influenced by the atmosphere and weather of the day (the focus of like all my startups [1]), the rocket could easily know about the differences in the weather and create MSLP (mean sea level pressure) numbers that would allow for excellent precision at landing. Your phone barometer can tell the difference in altitude between your head and your feet. [1] My latest go at this is https://www.allclearweather.com https://www.allclearweather.com (US, Android) with a goal of eliminating the altitude part of barometer data so that the remaining data can be QC'd, summarized with statistics and eventually hopefully included in weather models for increased forecast accuracy

bdamm 7y ago
I think they do also have a radar for distance to pad information but I couldn't cite that. I believe they have a form of instrument landing system, similar to how it is done at major airports but oriented for the rocket of course, and the distance calculating "radar" could be part of that (using signal timing) but I am not sure exactly how they did it.
- cryptoz 7y ago
  Yes I think that radar idea is quite correct. I now recall Elon/SpaceX explaining that this rocket (BFR or whatever) should be able to land on any solid body in the solar system. This would eliminate barometers and GPS for long-term solutions, but keeping radar/lidar and other light-based imagery in the running.
- jccooper 7y ago
  Falcon 9 is known to have a radar altimeter; it's quite likely the Starhopper does too.

WatchDog 7y ago

> 1 - how do they measure altitude? GPS is notoriously poor at this. I would imagine they would use a number of different sensors, they probably have barometric altimeters, but probably rely on some kind of radar altimeter for low altitude maneuvers.

monk_e_boy 7y ago
Oh yeah - radar! Duh, I was thinking lazers or something.

njoubert 7y ago

Modern GNSS can reliably measure altitude at <50cm 2-sigma accuracy using techniques such as RTK and PPP-AR. Modern GNSS is a better source of altitude data than barometers. Likely SpaceX is using a combination of radar altimeters and GNSS. Source: I do engineering on modern GNSS

cryptoz 7y ago
Super interesting info. Sorry for my annoying questions. Does modern GNSS receive that level of accuracy in cities among buildings, or is that an ideal case like what SpaceX has, being outside? Also, how expensive is a modern GNSS receiver that can get that level of altitude accuracy? Is this something that could make it into phones some day?
- walrus01 7y ago
  To add to the parent comment, super high precision gnss/rtk is often paired with multiple inertial measurement units, not very dissimilar from what you find in a smartphone. Also used in COTS drone flight controllers. Software can combine data from an imu that some vertical acceleration had taken place, with data from gnss sensors that altitude is now 2m higher, for additional level of confidence in what the gnss sensors are reporting.
- njoubert 7y ago
  Not an annoying question whatsoever. Dense urban environments are challenging. The error distribution is more complex there, its hard to definitively say. The numbers I'm quoting is for open sky environments where you have plenty of satellites and little multipath. For the average end-user, these are $500 GPS units (this is the one I work on: https://www.swiftnav.com/piksi-multi https://www.swiftnav.com/piksi-multi). The technology is already moving down the supply chain to ~$10 GNSS units from Broadcom and ST (we're doing this: https://www.swiftnav.com/news/swift-%E2%80%8B%E2%80%8Bnavigation-%E2%80%8B%E2%80%8Bintroduces-starling-gnss-positioning-engine-and-interoperability https://www.swiftnav.com/news/swift-%E2%80%8B%E2%80%8Bnaviga...) The issue with phones is mainly an antenna issue. Your phone antenna is tiny and squeezed in alongside half a dozen other antennas and radios. This degrades the GNSS signal quality a lot. A stopgap here is external GNSS antennas for your phone: you might plug your phone into your car and get lane-accuracy turn-by-turn directions, but it's unclear when we'll have high accuracy GNSS inside phones, but most major phonemakers are indeed working on this.

leecb 7y ago

In similar systems, the position is generally tracked with the combination of interial sensors (accelerometer / gyro / angular rate sensors) and external references like GPS and star trackers. Inertial sensors are able to provide high frequency data on change in position and orientation. Higher frequency means the software can make much more rapid decisions to steer the rocket, on the order of 1000 a second or so, much more often than can be achieved with GPS. The fact that inertial sensors don't rely on external signals (like GPS does) means that there is some degree of robustness- for instance, if there is a temporary disruption in GPS signal reception, the rocket will still have some idea of its position. Over time, integration and measurement error accumulate from the inertial sensors. (Remember that they generally measure changes in position / orientation, not absolute position or orientation). For this reason, it is usually necessary to use external position and orientation references to correct the error that accumulates over time. GPS is used for this, and in some applications, star trackers can be used as an absolute orientation reference. On the algorithm side, a Kalman filter combines measurements from all of the position and orientation sensors to generate a prediction of the current position / orientation / velocity / acceleration etc.

Symmetry 7y ago
A minor addendum, you should feed your Kalman filter the gimbal and thrust of the rocket too, it'll happily chow down and combine that with everything else to further improve its estimate.
- tlb 7y ago
  It's dangerous to throw extra sensors into a Kalman filter, because if the sensor goes bad it'll corrupt the output. Angular rate sensors are extremely accurate, far more accurate than a thrust sensor could be, so it probably wouldn't improve the overall accuracy anyway.
  - Symmetry 7y ago
    Ah, I guess I'm used to wheeled robots where odometers are often your most accurate gauges of position.

phkahler 7y ago

>> any idea of how the control software was written for a flying water tower? Could they use the Falcon 9 software? https://www.semanticscholar.org/paper/Lossless-Convexification-of-Nonconvex-Control-Bound-A%C3%A7ikmese-Carson/9209221aa6936426627bcd39b4ad0604940a51f9 https://www.semanticscholar.org/paper/Lossless-Convexificati... Reddit blurb about that paper: https://www.reddit.com/r/spacex/comments/7t2tb2/a_paper_by_lars_blackmore_of_spacex_on_soft/ https://www.reddit.com/r/spacex/comments/7t2tb2/a_paper_by_l...