Author Topic: SpaceX's Autonomous Spaceport Drone Ship Discussion Thread 2  (Read 2360229 times)

Online meekGee

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From 10 km to 10 m... oh dear. This is three orders of magnitude.

I don't think it's like that.

There are several stages to re-entry.  We know terminal guidance is good to a few feet, if everything is under control when you enter the terminal phase.

The previous 10 km uncertainty was because there was no good way to control the aerodynamic portion of the flight against cross winds or initial variations in the trajectory. The RCS could keep the stage stable, but could not induce large enough AoA to actually counter a persistent wind.

With the grid fins, they believe they can take care of the 10 km, and then it's back to GH land.

So it's not like some mechanism has to improve by x1000
« Last Edit: 12/17/2014 03:45 PM by meekGee »
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Offline guckyfan

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With the grid fins, they believe they can take care of the 10 km, and then it's back to GH land.

So it's not like some mechanism has to improve by x1000

I fully agree. But it would not be surprising or a problem if they need to take lessons from the first try to succeed on second try. We can certainly hope for success this time but have no reason to worry if it does not work.


Online meekGee

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With the grid fins, they believe they can take care of the 10 km, and then it's back to GH land.

So it's not like some mechanism has to improve by x1000

I fully agree. But it would not be surprising or a problem if they need to take lessons from the first try to succeed on second try. We can certainly hope for success this time but have no reason to worry if it does not work.

Yup.  My vote was that it is either a perfect landing, or not even near the barge. (>1 km)

There's a slim chance that the grid fins would get them almost, but not quite, to where the terminal landing phase can take over, and then it will be a near miss, but I think that's a low likelihood.
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Online meekGee

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Hey - can anyone here identify the comm equipment visible in the SpaceFlightNow images?

I spot a few domes that look to me like housing for radar equipment (they are also on the tubs), and one gimballed dish, (maybe 3' across?) that looks to be on the Go Quest.

Actually, the barge has two of the domes, one of each end.  Could this be related to the GPS system?  A dGPS base station?   I wonder if we can find such a dome at McGregor.
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Offline Kabloona

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Hey - can anyone here identify the comm equipment visible in the SpaceFlightNow images?

I spot a few domes that look to me like housing for radar equipment (they are also on the tubs), and one gimballed dish, (maybe 3' across?) that looks to be on the Go Quest.

Actually, the barge has two of the domes, one of each end.  Could this be related to the GPS system?  A dGPS base station?   I wonder if we can find such a dome at McGregor.

Yes, this is one of the domes on the deck of Go Quest:

http://www.cobham.com/media/967931/sailor_900_vsat_product_sheet.pdf


It's a stabilized VSAT (very small sperture terminal) antenna for Ku-band satellite broadband, with integrated GPS.

http://en.m.wikipedia.org/wiki/Very-small-aperture_terminal

And this appears to be one of the domes on the ASDS:

http://www.kvh.com/Commercial-and-OEM/Maritime-Systems/Communications/mini-VSAT-Broadband/TracPhone-V11IP.aspx

http://www.marinelink.com/news/transmit-weather-data370307.aspx

You can see the KVH VSAT dome in the SFN photo of the Elsbeth III pushing the ASDS. The dome is atop the tan container, just to the left of the Elsbeth III.
« Last Edit: 12/17/2014 05:07 PM by Kabloona »

Offline Jim

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A dGPS base station?

That only works from a fixed point. 

Offline robertross

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From spaceflightnow.com photograph. We can see one of the thrusters ?

That's affirmative. Good eye; I missed it in my overview.
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Online meekGee

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Hey - can anyone here identify the comm equipment visible in the SpaceFlightNow images?

I spot a few domes that look to me like housing for radar equipment (they are also on the tubs), and one gimballed dish, (maybe 3' across?) that looks to be on the Go Quest.

Actually, the barge has two of the domes, one of each end.  Could this be related to the GPS system?  A dGPS base station?   I wonder if we can find such a dome at McGregor.

Yes, this is one of the domes on the deck of Go Quest:

http://www.cobham.com/media/967931/sailor_900_vsat_product_sheet.pdf


It's a stabilized VSAT (very small sperture terminal) antenna for Ku-band satellite broadband, with integrated GPS.

http://en.m.wikipedia.org/wiki/Very-small-aperture_terminal

And this appears to be one of the domes on the ASDS:

http://www.kvh.com/Commercial-and-OEM/Maritime-Systems/Communications/mini-VSAT-Broadband/TracPhone-V11IP.aspx

http://www.marinelink.com/news/transmit-weather-data370307.aspx

You can see the KVH VSAT dome in the SFN photo of the Elsbeth III pushing the ASDS. The dome is atop the tan container, just to the left of the Elsbeth III.

Ok!  thanks much.  I was hoping for that.  To confirm - is this sufficient to have live telecom during descent, at least from the barge?

-------

dGPS wise, of course it only works with a stationary station on regular use, since if the platform is moving, it will not have a steady basis...  Which normally would defeat the purpose...

But can it be used to get a precise determination of the relative distance vector to the moving base station?  So it won't help you know where you are in an absolute coordinate system, but will give you a good reading of where you are in the barge's coordinate system?

*Note - this is a bit hypothetical - we don't know if there's dGPS used at all. 
« Last Edit: 12/17/2014 05:45 PM by meekGee »
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Offline Kabloona

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The Sailor 900 VSAT on board the Go Quest has an advertised max uplink rate of 256 kbps, so that is probably only good for VoIP, email, etc, not streaming video.

The KVH TracPhone on the barge has an advertised max uplink rate of 1 Mbps, so that could be adequate for low-resolution live video.

I don't think dGPS is necessary. WAAS GPS can be accurate to <1 meter, and it appears the landing zone will have good WAAS coverage.
« Last Edit: 12/17/2014 06:03 PM by Kabloona »

Online Lar

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I spot a few domes that look to me like housing for radar equipment (they are also on the tubs), and one gimballed dish, (maybe 3' across?) that looks to be on the Go Quest.
(emphasis mine) Calling the support ships tubs rather than tugs is just mean :)

Actually, the barge has two of the domes, one of each end. 

Maybe that's directionality related? If there's no high point and you want to watch the forward direction of travel maybe you have 2?
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Online meekGee

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heh, yeah, tugs.  I think various sailors might have called them worse names over the course of time though...

bit-rate wise, I think even 0.25 MBps can do wonders if you're willing to encode the file and not stream it (thus incurring a 20-30 second delay).

For that matter, drop the frame rate, drop the resolution, until it goes through.   I'm not expecting 3D HD TV in real time.   Make it B&W.  I'll still tune in...

 
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Offline Kabloona

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Maybe that's directionality related? If there's no high point and you want to watch the forward direction of travel maybe you have 2?

The domes all look like VSATs, which are not directionally fixed, because they rotate automatically to acquire their satellites. I don't think they are radars.

The other unidentified dome on the barge is probably the same KVH TracPhone unit that I was able to identify on one end of the barge. The one on the far end is too far away to positively ID, but the markings look similar from a distance, and since the barge was purpose-built it seems likely that they'd use two of the same model.

Those will both likely be streaming video from the onboard cameras back to Hawthorne.
« Last Edit: 12/17/2014 06:23 PM by Kabloona »

Online Lar

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I just meant that perhaps if you need LOS to something, having two copies of the thing might be advantageous, as other equipment might block things.
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline Jim

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But can it be used to get a precise determination of the relative distance vector to the moving base station?  So it won't help you know where you are in an absolute coordinate system, but will give you a good reading of where you are in the barge's coordinate system?
 

No, because GPS is absolute and dGPS is just a refinement of the absolute location.  The location of the dGPS transmitter is known and so it broadcasts the difference between its known position and what the GPS constellation says it is. This is then used to reduce the absolute error. 

Offline joncz

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An approach land surveying crews use that works over short time periods is to locate their GPS receiver on a well known point (a survey benchmark), allow it to get a good GPS solution, and zero out the error.  This gives them a point in time reference similar to how dGPS works.  Increasing time from when the receiver was zeroed out along with distance from the benchmark produce increasing errors in location.

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..
bit-rate wise, I think even 0.25 MBps can do wonders if you're willing to encode the file and not stream it (thus incurring a 20-30 second delay).

For that matter, drop the frame rate, drop the resolution, until it goes through.   I'm not expecting 3D HD TV in real time.   Make it B&W.  I'll still tune in...

..., and since the barge was purpose-built it seems likely that they'd use two of the same model.

Those will both likely be streaming video from the onboard cameras back to Hawthorne.
I would love to watch this event real-time or near realtime.  But I doubt they are counting on getting the maximum possible throughput from their uplinks as that would be a bad assumption.  I would dedicate redundant resources to the telemetry, not the video.  Frankly, I wouldn't dedicate anything to the video.  I'd store it on multiple SSDs on the barge.

But that's me and hopefully I'm wrong.
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Online meekGee

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But can it be used to get a precise determination of the relative distance vector to the moving base station?  So it won't help you know where you are in an absolute coordinate system, but will give you a good reading of where you are in the barge's coordinate system?
 

No, because GPS is absolute and dGPS is just a refinement of the absolute location.  The location of the dGPS transmitter is known and so it broadcasts the difference between its known position and what the GPS constellation says it is. This is then used to reduce the absolute error.
I know that.  But if you think it through, if you eliminate the bit about the base being fixed, all you have left is the relative position vector between the two receivers, which is normally useless, but in this particular case is exactly what they need.

Nobody cares if the rocket nails the landing at center barge, but the whole thing happens 10 feet to the north.
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Online meekGee

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bit-rate wise, I think even 0.25 MBps can do wonders if you're willing to encode the file and not stream it (thus incurring a 20-30 second delay).

For that matter, drop the frame rate, drop the resolution, until it goes through.   I'm not expecting 3D HD TV in real time.   Make it B&W.  I'll still tune in...

..., and since the barge was purpose-built it seems likely that they'd use two of the same model.

Those will both likely be streaming video from the onboard cameras back to Hawthorne.
I would love to watch this event real-time or near realtime.  But I doubt they are counting on getting the maximum possible throughput from their uplinks as that would be a bad assumption.  I would dedicate redundant resources to the telemetry, not the video.  Frankly, I wouldn't dedicate anything to the video.  I'd store it on multiple SSDs on the barge.

But that's me and hopefully I'm wrong.
That's true for getting data off the rocket, but unless you worry about the barge sinking, everything can be stored on the barge (including high rez video) and downloaded 5 minutes later.

Rocket-originated video, I agree.  Get the telemetry first.   But that's a direct link at close range, no satellites involved.
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Offline Kabloona

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However they do it, they have a bunch of bandwidth. ASDS appears to have two VSAT antennas each with 1 Mbps uplink, and the Go Quest appears to have multiple VSATs plus the dish antenna.

Won't need that homemade pizza pan antenna in the window of Elon's plane now.  ;D
« Last Edit: 12/17/2014 08:35 PM by Kabloona »

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I would love to watch this event real-time or near realtime.  But I doubt they are counting on getting the maximum possible throughput from their uplinks as that would be a bad assumption.  I would dedicate redundant resources to the telemetry, not the video.  Frankly, I wouldn't dedicate anything to the video.  I'd store it on multiple SSDs on the barge.

But that's me and hopefully I'm wrong.
That's true for getting data off the rocket, but unless you worry about the barge sinking, everything can be stored on the barge (including high rez video) and downloaded 5 minutes later.

Rocket-originated video, I agree.  Get the telemetry first.   But that's a direct link at close range, no satellites involved.
You're right.  There's no reason they couldn't store the data and then queue it for transmission immediately after the landing. I'm still not confident that they'll share it immediately, but you've convinced me that Hawthorne will likely have video soon after the attempt.

Great, now I'm wishing for a pony... ;)
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