How does a spacecraft know it's in orbit?

[Jackissimus]

What do you think I just said. Nothing of what you said contradicts what I said. "If you're in freefall and not in orbit" was meant to imply that orbit is a form of freefall (and that if you're in any other form of freefall than orbit with a space rocket, things are probably going less well than expected). When you're on your way up, the accelerometer has a reading. When you reach apoapsis, your reading goes away. Then you make your burn, and because you've been calculating the right time to burn for days/weeks/months beforehand tada, you're in orbit. If your meter remains at 0 and doesn't get violently destroyed then you know you're in orbit. Orbit being freefall is the entire reason my proposal works. Of course I was also forgetting that "on the way up" is freefall too. So maybe it doesn't work. I'm not really bothered since it was only a side idea, I don't think that's how they do it anyway.

Also, saying an accelerometer doesn't measure gravity is like saying a clock doesn't measure time. Freefall is 0G.

This sounds about right ...... . . . .

[Person012345]

This sounds about right ...... . . . .

I was having a derp moment because "freefall" doesn't immediately sound applicable when you're going up, but of course it is.

[thescientist]

Sure, an accelerometer, anything that can measure the gravitation/acceleration on your craft. An accelerometer does measure gravity (albeit maybe not "directly").

If your rocket is in freefall and not in orbit then I imagine something has probably gone wrong. Once the meter has no reading, you'll know you're at apogee and you can make your pre-calculated burn. You'll know you're in orbit because you worked it all out beforehand.

Or something.

remember that an object in orbit IS IN FREEFALL, but his tangential speed generate a virtual force (centrifugal force) that keep it on his trajectory

[Person012345]

remember that an object in orbit IS IN FREEFALL, but his tangential speed generate a virtual force (centrifugal force) that keep it on his trajectory

I know. I explained what I meant by that in subsequent posts.

[PakledHostage]

So instead of time stamps, satellite broadcasts some predetermined f(t)? Does the later have an easy-to-compute inverse, then? I would imagine that this is only an advantage when the clean signal can be interpreted without having to wait for multiple time stamps. Otherwise, error-correction encoding would make more sense. Or am I misunderstanding that completely?

My understanding is that the f(t) is hard coded in the system, and only certain parameters are broadcast. They are downlinked as part of the almanac data when the satellite is first acquired by your receiver. I do not know how the actual pattern matching / time delay is computed at the low level or how tolerant it is of loss of signal. Most receivers only update at a rate of between 0.5Hz and 5Hz, so maybe short signal drop outs during the update cycle can be tolerated? It is a good question though. I will do some digging...

Edited August 21, 2013 by PakledHostage

[Diche Bach]

Awright I'm gonna just admit that my mind is a bit blown to learn that relativistic time dilation effects are being measured by freaking GPS devices. I have a GPS in my phone!

The fact that its all relative simultaneously does and does not make sense. I guess that makes sense . . .

There are many galaxies visible in telescopes with red shift numbers of 1.4 or higher. All of these are currently traveling away from us at speeds greater than the speed of light. Because the Hubble parameter is decreasing with time, there can actually be cases where a galaxy that is receding from us faster than light does manage to emit a signal which reaches us eventually.[18][19] However, because the expansion of the universe is accelerating, it is projected that most galaxies will eventually cross a type of cosmological event horizon where any light they emit past that point will never be able to reach us at any time in the infinite future,[20] because the light never reaches a point where its "peculiar velocity" towards us exceeds the expansion velocity away from us (these two notions of velocity are also discussed in Comoving distance#Uses of the proper distance). The current distance to this cosmological event horizon is about 16 billion light-years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light-years away, but the signal would never reach us if the event was more than 16 billion light-years away.[19]

Edited August 21, 2013 by Diche Bach

[K^2]

Not only does it have to deal with time dilation due to satellite's velocity, but also gravitational time dilation due to Earth's gravity. If you ever need to convince someone that General Relativity works, GPS is the most evident proof.

By the way, because here on Earth we experience only the gravitational time dilation, there exists an orbit on which time flows at the same rate as it does on the surface.

[Apature rocket science]

They probably see if the craft is on fire if it is they are not in orbit.

[dharak1]

Hmm I was watching a video from on the ISS and it looked like the laptop had windows on it. I guess I was wrong.

[Nibb31]

They recently switched all the laptops to Linux. They used to be on Windows.

[Apature rocket science]

If they continued to use windows on the iss it would decently get a virus and cause it to be the first space station to be taken down by a virus.

[Nibb31]

Man, you're so funny.

[Thaniel]

None of the laptops with windows on them where set to run any critical systems anyway. That´s carried out by computers running custom software. The laptops (previously running windows, now linux) is typicaly used for things like experiments, referencing, day to day high-level communication (video calls, mails, etc.) entertainment, schedules and some planning and such.

Fun fact, when they first brought up laptops to the ISS, they quickly discovered that ordinary harddrives wheren´t meant for use in 0 gravity, and they had to be replaced. Considering that ISS have so many different systems running, some hardcoded, some not, some completely self sufficient and some not, it would be somewhat risky to change everything to run on a compltely different OS, while it´s in orbit. Also, it wouldn´t realy serve any purpose, as long as what´s custom made to run the thing works, there´s no need to fix it. An excelent place to aply the "If it ain´t broken, don´t fix it" axiom.

So worstcase for a virus on the ISS would be a few experiments would be hard to follow, some camera feeds and videocalls would have to be postponed, and schedules would have to be tracked by ground control. And ofcourse, the person bringing the offennding virus onboard would get a yelling at.

[Nibb31]

This post from an IBM engineer on the NSF forums is really interesting:

http://forum.nasaspaceflight.com/index.php?topic=27043.0