Orbital Information IRL

[pandaman]

A thought occurred to me just now, after dropping a mining base on the Mun, with all the launching and orbiting shenanigans that it required.

In KSP we know our orbital velocity and whether or not we have a stable orbit and how circular it is because we have nice numbers and lines on a screen to tell us. But what about REAL spacecraft in REAL space? Especially in the 'early days'.

They can calculate what velocity they NEED to achieve an orbit at any given altitude, but how do they know how fast they are ACTUALLY going? And when a stable orbit is achieved? There is no atmosphere to get an 'air speed' reading and obviously a car speedo won't work. What do they use as a frame of reference to get a 'reading' and how accurate is it? How did they do it in the 1960s?

I know some of you guys and girls out there will have some knowledge about such things, so please enlighten me a little if you can. In the meantime I will try Google to see if that sheds any light.

Cheers.

Pandaman

[NathanKell]

* Ground stations' radar

* Inertial guidance (you know how fast you're going at the start, you have accelerometers to track all the forces applied sans gravity)

* Alignment based on reference points (horizon, fixed-stars, etc)

[KAO]

Correct me if I'm wrong here, but in the very beginning, they did indeed have difficulties and uncertainties regarding orbital speed. Such as with the launch of Yuri Gagarin, there was the possibility that he would've ended up in an orbit that would've taken more time to decay than he had supplies for.

[tntristan12]

* Ground stations' radar

* Inertial guidance (you know how fast you're going at the start, you have accelerometers to track all the forces applied sans gravity)

* Alignment based on reference points (horizon, fixed-stars, etc)

What he said. In real life, orbit determination is not a trivial task. Long range missions require very precise equipment (see Deep Space Network) and near-earth orbits can use a combination of either ground stations or GPS. Astronomical objects like planets, comets, or asteroids can be OD'd using ground stations or space based observatories.

The bottom line is that you need to determine a range from either radar ranging or parallax measurements (parallax is the difference in position observed between two concurrent measurements, which is how 3D imaging works). You also need a relative orientation, which can be determined by knowing the location of a ground station on the surface of the earth, or using the relative positions of astronomical bodies such as the sun or Polaris (the north star). Other methods include magetometry (measuring magnetic north using a digital compass) or integration of IMU output.

- - - Updated - - -

Correct me if I'm wrong here, but in the very beginning, they did indeed have difficulties and uncertainties regarding orbital speed. Such as with the launch of Yuri Gagarin, there was the possibility that he would've ended up in an orbit that would've taken more time to decay than he had supplies for.

This is correct. Orbit determination is a statistical process, meaning there are no absolute determinations. We can only say what the probability is that an object has a certain orbit.

[pandaman]

Thanks and +rep to you all.

I guess GPS makes this a lot easier nowadays. Or is it considered 'cheating' (like using mechjeb) by the traditionalist protractor and slide rule crowd?

Awesome, thanks again everyone.

[Harry Rhodan]

How did they do it in the 1960s?

https://en.wikipedia.org/wiki/Mercury-Atlas_9#Technical_problems_on_the_flight

They used a marker and a wristwatch.

[pandaman]

https://en.wikipedia.org/wiki/Mercury-Atlas_9#Technical_problems_on_the_flight

They used a marker and a wristwatch.

That's very interesting, thanks, and it shows why they wanted test pilots that can stay calm under pressure as the first astronauts.

[imaspacestation]

Orbit determination is a statistical process, meaning there are no absolute determinations. We can only say what the probability is that an object has a certain orbit.

How do they get satellites into geo-synchronous orbits, then? Doesn't that require the utmost precision?

[AbacusWizard]

Geosynchronous orbit is about 36 000 km above sea level, and the exosphere (which is hardly anything anyway) tops out at 10 000 km, so a geosynchronous satellite really shouldn't have to worry about atmospheric effects at all--allowing a much greater degree of precision.

Of course we can't get absolute accuracy, so I presume important long-term satellites are equipped with maneuvering jets and small propellant tanks so they can make minor course corrections if they start to drift away from their intended position.

- - - Updated - - -

Side note: while doing research for this post, I glanced at those numbers--36 000 and 10 000--and thought "Those are kinda small; shouldn't they be bigger? On Kerbin the atmosphere goes up to 70 000 meters, so Earth should be... OH! That's 10 000 KILOmeters! Right! Kerbin is tiny!"

[Red Iron Crown]

I guess GPS makes this a lot easier nowadays. Or is it considered 'cheating' (like using mechjeb) by the traditionalist protractor and slide rule crowd?

IRL rocket scientists cheat wherever possible. Any advantage that can be gained that increases mission success rates is used without remorse.

[cantab]

Geostationary satellites have propulsion for station-keeping. That's needed because in real life most geostationary orbits aren't stable and the satellite will slowly drift, even if it starts out in *exactly* the right orbit.

[tntristan12]

Geostationary satellites have propulsion for station-keeping. That's needed because in real life most geostationary orbits aren't stable and the satellite will slowly drift, even if it starts out in *exactly* the right orbit.

What he said. There are always perturbations from the two-body solution in orbital dynamics. This is because the Earth is not a uniform sphere, there is solar radiation pressure, gravitational pull from the sun, moon, and other planets, and even general relativistic effects must be accounted for in highly precise systems like GPS.

[AbacusWizard]

OH RIGHT. I totally forgot about n-body effects. Curse you, patched conics approximations!

[NathanKell]

Station-keeping costs in GEO are about 45m/s per year. Note that's not just from the influence of other bodies, but from the Earth's gravitation field not being uniform (it's not as bad as the moon, but it's still oblate and has various greater concentrations of mass in various places).

And yeah, why would you think the very people who designed these systems would consider using them themselves cheating? Good _lord_ the "I play pure stock with skill" stuff is infectiously toxic. :\

[Nothalogh]

https://en.wikipedia.org/wiki/Mercury-Atlas_9#Technical_problems_on_the_flight

They used a marker and a wristwatch.

Cooper had drawn lines on the window to stay aligned with constellations as he flew the craft. He later said he used his wristwatch to time the burn and his eyes to maintain attitude.

Holy Jebediah.

[Wallygator]

Let's also not forget that back then in the olden days people also used math.

[pandaman]

Let's also not forget that back then in the olden days people also used math.

True. It's becoming a bit of a dying art with all the techno gadgets we have nowadays.

And much as I enjoy doing the actual flying myself in game, and haven't installed Mechjeb, if it was RL and actually mattered I'd 'cheat' the he'll out of it and get all the help I could too.

Thanks again for all the responses guys and gals. This thread has turned put to be very informative and interesting.

Cheers

Pandaman

[Corw]

True. It's becoming a bit of a dying art with all the techno gadgets we have nowadays.

And much as I enjoy doing the actual flying myself in game, and haven't installed Mechjeb, if it was RL and actually mattered I'd 'cheat' the he'll out of it and get all the help I could too.

Thanks again for all the responses guys and gals. This thread has turned put to be very informative and interesting.

Cheers

Pandaman

Of course you would "cheat". Because IRL is about getting things done. However, game is about enjoying the experience

[NathanKell]

Going back to doing math, eh?

[Rune]

Going back to doing math, eh?

That got me back straight to childhood. A classic!

Also, regarding the OP, I was under the impression that the Doppler shift of the radio signal is one of the easiest, lowest-tech methods of working out speed, provided you know which direction the target is heading. Also the reason they have radio beacons transmitting dumb carrier beeps. Is that not so? After all, there are a lot of things there's no way in hell we can paint on radar, like NH or Cassini...

Rune. Then again, radio waves were never my thing, so I'm talking about a sibject I know next to nothing about.

[AbacusWizard]

Funny timing: although I had read Asimov's "The Feeling of Power" years (decades?) ago, just a day or two ago I discovered Arthur C. Clarke's short story "Into the Comet," which involves a similar theme but replaces "The Feeling of Power"'s cynicism and despair with heroics and hope. A spaceship with 20 scientists/engineers (and a reporter, who is the viewpoint character) is exploring the interior of a comet when the computer breaks down. Without it they can't make navigational calculations and any orbital maneuver would be useless or worse; usually they'd just radio Earth for directions but interference from the comet cuts off all communications. They're starting to feel resigned to the fact that the comet will be their grave when the reporter suddenly remembers his grandmother, years and years ago, teaching him to use... an abacus. He builds a bunch of them from spare parts, teaches the whole crew how to use them, and they all set to work on doing the navigational calculations by hand.

[ygarl]

True. It's becoming a bit of a dying art with all the techno gadgets we have nowadays.

And much as I enjoy doing the actual flying myself in game, and haven't installed Mechjeb, if it was RL and actually mattered I'd 'cheat' the he'll out of it and get all the help I could too.

Thanks again for all the responses guys and gals. This thread has turned put to be very informative and interesting.

Cheers

Pandaman

Of course, IRL - if you accidentally use the wrong conversion/forget to move the decimals the right amount, stuff like this happens:

https://en.wikipedia.org/wiki/Mars_Climate_Orbiter

Doh!

So cheating is always a good idea.

[imaspacestation]

They're starting to feel resigned to the fact that the comet will be their grave when the reporter suddenly remembers his grandmother, years and years ago, teaching him to use... an abacus.

I know what an Abacus is, and actually had a toy one when I was a child, but I've never seriously used it. What advantage does it have over using just plain pen and paper?

[Wallygator]

Going back to doing math, eh?

Well played sir! Very well played indeed!

- - - Updated - - -

Of course, IRL - if you accidentally use the wrong conversion/forget to move the decimals the right amount, stuff like this happens:

https://en.wikipedia.org/wiki/Mars_Climate_Orbiter

Doh!

So cheating is always a good idea.

That's why they had banks of guys with slide rules doing calculations in parallel.

[SAI Peregrinus]

I know what an Abacus is, and actually had a toy one when I was a child, but I've never seriously used it. What advantage does it have over using just plain pen and paper?

It's very fast (for calculations that fit within the size of the abacus, of course). Comparable to an electronic calculator once you're used to it. And it makes mental arithmetic easier once you can visualize the operations.