SpaceX Falcon 9 v1.1 - Dragon - CRS-5/SpX-5 -Jan. 10, 2015 - DISCUSSION

[kevin-rf]

Yes, it's almost as if there was a new NASA space flight that people on NASAspaceflight.com wanted to talk about in other subforums. 

NASA fly a non spaceX rocket, surely you must be joking

[TOG]

Yes, it's almost as if there was a new NASA space flight that people on NASAspaceflight.com wanted to talk about in other subforums. 

NASA fly a non spaceX rocket, surely you must be joking

No, I'm serious...
    And stop calling me Shirley.
      (I picked a bad day to give up drinking coffee...)

[chamann]

Is there a list somewhere of all of the 256 science and research investigations this flight is supporting?

[Oersted]

any idea why things have been quite recently?

Yes, it's almost as if there was a new NASA space flight that people on NASAspaceflight.com wanted to talk about in other subforums. 

Well, you call it a new flight, but the whole concept was 40+ years old. CRS-5/SpX-5 will be truly new!

[Bogeyman]

How does the 1st stage home in onto the barge? Does it use GPS signals or is there like a beacon installed on the barge that guides the stage in like an airplane does on an ILS system?

[NovaSilisko]

How does the 1st stage home in onto the barge? Does it use GPS signals or is there like a beacon installed on the barge that guides the stage in like an airplane does on an ILS system?

My impression is that it doesn't home in on the barge, it homes in at the exact coordinates the barge is supposed to be at, while the barge will be doing its damndest to make sure that it and the returning stage are at the same place at the same time.

[Bogeyman]

You mean it's gonna land at a certain position, "hoping" the barge will be sitting there?

[Tnarg]

My impression is that it doesn't home in on the barge, it homes in at the exact coordinates the barge is supposed to be at, while the barge will be doing its damndest to make sure that it and the returning stage are at the same place at the same time.

I don't think that's true, I remember reading the the grasshopper compared the the GPS signal at the landing site to its on board one as this improved the accelerate or the system.

[rpapo]

There are two problems with homing in on a signal: (1) How good is the booster at receiving signals of any sort, especially with the engines running?  (2) Using a homing signal infringes on the one and only part of the Blue Origin patent that (in my mind) is even remotely defensible in court.

[ugordan]

You mean it's gonna land at a certain position, "hoping" the barge will be sitting there?

To be even more precise, it's gonna try to land at a certain position while the barge is trying to maintain that same position.

[MattMason]

You mean it's gonna land at a certain position, "hoping" the barge will be sitting there?

To be even more precise, it's gonna try to land at a certain position while the barge is trying to maintain that same position.

Given Musk's description of the barge's ability to stay within 3 meters of a given location, that's the most logical assumption. The booster can be told to go to a very specific location. So long as both barge and booster have this positioning down to the finest degree, any error should fall within that 3m range--I hope. I've flown model rockets only. Landing them under power was completely outside of NAR regulations. 

[Sohl]

My impression is that it doesn't home in on the barge, it homes in at the exact coordinates the barge is supposed to be at, while the barge will be doing its damndest to make sure that it and the returning stage are at the same place at the same time.

I don't think that's true, I remember reading the the grasshopper compared the the GPS signal at the landing site to its on board one as this improved the accelerate or the system.

It could be there was a _basestation_ GPS at the landing site that provides differential GPS correction data (e.g. RTCM) to the GPS unit(s) on the Grasshopper/F9R.  That approach can improve the accuracy of GPS very significantly.  I've not heard whether or not SpaceX is actually doing that. And for getting within a meter or two, it is not really necessary.  One or more good GPS receivers on the rocket using wide-area corrections from a satellite service (for example, the WAAS signal, standard stuff for higher-end GPS) is plenty good enough to home in on a big pad/barge.

[macpacheco]

WAAS typically gets position down to 1 meter lateral, 2 meter vertical. Add a semi codeless receiver (iono corrections done directly by the receiver, WAAS used for clock and ephemeris corrections) and you should get down to 1 foot error lateral and vertical. No need for specialized GPS corrections.
I think its unlikely that the stage can achieve better than around 10 meter accuracy anyways. Specially in the first landings.

[jaufgang]

The GPS would be more than adequate for steering the stage to the vicinity of the barge, but wouldn't there also be radar being used, especially during the final approach and landing? 

Keep in mind that in addition to the  barge actively keeping its station to within 3 meters, depending on the weather there may also be long wavelength swells causing some degree of uncertainty in the exact landing elevation until the last moments. 

[baldusi]

I believe most are forgetting the point that the barge will also be used to prove to the range that the stage can safely return to the land pad on land. You want to demonstrate return to a very specific coordinate. Final approach should use the radar that Elon stated was tested on Grashopper.

[laika_fr]

Let's have a look at the weather models, 6 days ahead.

the forecast wind map :



The place itself looks relatively quiet, including wind gusts up to 20 Km/h.
The waves and swell reach less than 2 meters.
 
Overall the outlook is not totally clear since some precipitations and clouds are still circling in the surroundings but for now the picture is pretty favourable.

[SoulWager]

My impression is that it doesn't home in on the barge, it homes in at the exact coordinates the barge is supposed to be at, while the barge will be doing its damndest to make sure that it and the returning stage are at the same place at the same time.

I don't think that's true, I remember reading the the grasshopper compared the the GPS signal at the landing site to its on board one as this improved the accelerate or the system.

The differential GPS improves precision relative to firm ground, but that's not really relevant here, because the barge GPS and rocket GPS would be targeting coordinates in the same frame of reference.  You only need to send ground based corrections to the rocket if the pad can't move to where the rocket thinks the landing point is.

Altitude on final descent would likely require a radar altimeter anyway, even if you used the best available GPS systems.

[ugordan]

Meanwhile... I'm really loving the pre-flight processing images.

/s

[hrissan]

My impression is that it doesn't home in on the barge, it homes in at the exact coordinates the barge is supposed to be at, while the barge will be doing its damndest to make sure that it and the returning stage are at the same place at the same time.

I don't think that's true, I remember reading the the grasshopper compared the the GPS signal at the landing site to its on board one as this improved the accelerate or the system.

The differential GPS improves precision relative to firm ground, but that's not really relevant here, because the barge GPS and rocket GPS would be targeting coordinates in the same frame of reference.  You only need to send ground based corrections to the rocket if the pad can't move to where the rocket thinks the landing point is.

Altitude on final descent would likely require a radar altimeter anyway, even if you used the best available GPS systems.

Small correction: the barge and the stage can see different sets of satellites (typical for barge to see less, but exact sets depend on antenna locations), so their position/time solutions might be different, and not be "in the same frame of reference". Example: extra single satellite just above the horizon which is visible to stage but not barge may greatly affect their GPS solutions in the direction of that satellite.

But assuming "within 3m" Elon's comment, that will likely not matter. The barge is large enough to use GPS X/Y without corrections, with radar altimeter for Z.

[mvpel]

"Within 3 meters" is "in a storm." I'm quite certain that in the upcoming weather on the 16th the Thrustmasters will do a far more accurate job than that.