How does a spacecraft know it's in orbit?

[dharak1]

The title says it all, how do spacecraft know their orbit? I'm sure calculating velocity and distance are too much work especially for something like a Jupiter transfer. Do they have some ksp like orbit paths mapped out on their screens all done by computer? How was it done before massive computing power? what about elliptical orbit? Do they need ground distance for that?

[The Jedi Master]

Before massive computing power: Wing it, see if the thing is still broadcasting a few hours from now.

Now: Something like KSP map mode, only more advanced.

[miracmert]

I think they measure it with their current speed and distance to the ground at the moment. Then the computer calculates the rest of the orbit path by simply calculating how far the craft can rise before it starts to drop back for the periapsis or what will be the new path after it gets ejected to another orbit. I guess it just calculates SOI's and draws ellipses. These are my guesses though, somebody who studies those subjects should reply for a correct knowledge

[dharak1]

Oh god if that's the way it's done think of all the hardware and programming for stuff in space and to think the PCs on the ISS are running like windows 95.

[Nibb31]

Radars on the ground measure altitude and speed. That's enough to extrapolate the orbital parameters.

Spacecraft also use star trackers to figure out their position, similar to the way you would use a sextant.

[Person012345]

I'm not saying it's how they do it, but it would be doable with a simple gravity meter (a spring loaded pressure sensor and an object with a reasonable mass on a pendulum) I imagine. I doubt it needs super advanced computing.

Although as mentioned above it's probably done with radars or signals irl.

[PakledHostage]

Another important method is inertial navigation. The gyros and accelerometers in the IMU (Inertial Measurement Unit) "add up" all of the accelerations and rotations that have happened since the IMU was initialized, and use that to determine the orbit and position in real time. IMUs accumulate error over time, so ground radar and star tracking is used to correct accumulated error.

In the case of commercial aircraft like the 747, laser gyros are used to measure rotations. The gyros are so accurate that the aircraft can determine its latitude while sitting at the gate, just from the Earth's rotation.

[Person012345]

also remember that what lags up ksp when it's using 1/4 - 1/8th of most modern cpus is not rendering the map screen. It's calculating in real time the physical effects of all the forces on every part of your ship. Real life doesn't need help doing this (and a rocket will just follow a pre-programmed trajectory that was all calculated out in advance taking whatever time they wanted) I guess. With small adjustments if the aforementioned radar/whatever is detecting its course detects that it's a little off course.

I'm not a rocket scientist though.

[Platofan2]

Sensors can tell if it has a orbit. i think it calculates it like ksp

[Diche Bach]

I thought "orbit" was a simple matter of having a velocity within a certain mathematically defined range at a particular altitude, specified by the parent bodies mass, gravity and diameter. As such, I would think the simplest way to measure it is simply altitude and speed. If you are within the velocity window for a given altitude, then you know you are in orbit! Right?

[NGTOne]

I thought "orbit" was a simple matter of having a velocity within a certain mathematically defined range at a particular altitude, specified by the parent bodies mass, gravity and diameter. As such, I would think the simplest way to measure it is simply altitude and speed. If you are within the velocity window for a given altitude, then you know you are in orbit! Right?

The issue is, you can't easily measure velocity from your spacecraft, I think. So you need ground-based radar tracking systems.

[PakledHostage]

The issue is, you can't easily measure velocity from your spacecraft, I think. So you need ground-based radar tracking systems.

Or as I mentioned previously, an IMU. In the case of ICBMs, you need some internal reference for trajectory information. You can't rely on external references for targeting because those external references may well get destroyed. The technology is reliable in other applications because of that military background.

[miracmert]

Well, actually many things are controlled by the ground control, not by the astranauts. I mean astronauts inside the spacecraft control the phase changes, specific manuevers and such while everything from the launch till the mission goal is calculated before the mission begins. Ground control calculates what speed the craft needs at certain altitude and after how many orbits the spacecraft needs to do such manuevers. They also set the times of burns and etc. So what I mean is it's not like KSP or Orbiter, astronauts don't completely manage the craft but they trigger phases. Velocity is usually measured by star trackers and if they are at certain altitude ground control knows the spacecraft is in orbit. They don't do things we do in KSP, they measure everything and plan every manuever, calculate every step even before the launch. Other way the space history would be full of crashes.

[Jackissimus]

I'm not saying it's how they do it, but it would be doable with a simple gravity meter (a spring loaded pressure sensor and an object with a reasonable mass on a pendulum) I imagine. I doubt it needs super advanced computing.

Although as mentioned above it's probably done with radars or signals irl.

Eh ... you meant accelerometer, right? You can't really measure gravity.

[djnattyd]

...to think the PCs on the ISS are running like windows 95.

ISS runs the Debian version of Linux

[K^2]

I'm not too familiar with how things are done these days, but I know that on early flights, all of the computations for orbit transfers were done on the ground. The flight control would have tracking data, they'd use it to figure out the orbit the ship is in, find the necessary burn times and orientations and send these to the pilot.

So if you want to think of it in KSP terms, the flight control center would have equivalent of the map view in KSP and would be able to set up transfer nodes. The pilot would receive instructions equivalent to what the little blue marker on the nav ball tells you. That is, set heading for whatever it needs to be, wait for specific time, then burn for that many seconds. So in that respect, KSP is pretty similar to what actually happens, except the player has access to both sides of the mission planning and execution.

[andrewas]

I'm not saying it's how they do it, but it would be doable with a simple gravity meter (a spring loaded pressure sensor and an object with a reasonable mass on a pendulum) I imagine. I doubt it needs super advanced computing.

In freefall?

[K^2]

In freefall?

Actually, you could measure tidal forces to figure out quite a few things about your orbit assuming you know the relevant parameters for the planet you are orbiting. But yes, you can't just measure gravity directly, and that means much, much finer measurements to get any useful data.

[Person012345]

Eh ... you meant accelerometer, right? You can't really measure gravity.

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

In freefall?

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.

Edited August 21, 2013 by Person012345

[nhnifong]

I think they can "triangulate" the position of the sattelite from existing ground based radars and other sattelites using the same technology that ground based targets use to triangulate their own position from GPS sattelite signals.

In GPS, each device has a very precise clock onboard, and periodically broadcast's the time according to it's clock, accompanied by it's unique identifier. Any device attempting to determine it's location collects as may of these signals as are available. They use the unique identifier to look up the public location and orbital parameters of the sattelite, or ground station in a table. The compare the time in the message to their own clock, and correct for any relativistic effects that would be imparted by the curved spacetime between themselves and the sattelite. They then have their approximate distance between themselves and a number of known points in space-time, with which they can "triangulate" their own location in space-time. Only it's not two-dimensional so "trianglulate" is a bit of a minsomer. At least 4 or 5 points are needed, and the algorithm will really just get better with more points.

[Bunsen]

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.

Orbit is freefall. If you're in freefall while going fast enough sideways that you miss the ground, that's orbit. An accelerometer on board will read zero unless your engines are running, and KSP's gravioli detector is quite impossible in real life thanks to the equivalence principle.

[PakledHostage]

In GPS, each device has a very precise clock onboard, and periodically broadcast's the time according to it's clock, accompanied by it's unique identifier. Any device attempting to determine it's location collects as may of these signals as are available. They use the unique identifier to look up the public location and orbital parameters of the sattelite, or ground station in a table. The compare the time in the message to their own clock, and correct for any relativistic effects that would be imparted by the curved spacetime between themselves and the sattelite. They then have their approximate distance between themselves and a number of known points in space-time, with which they can "triangulate" their own location in space-time. Only it's not two-dimensional so "trianglulate" is a bit of a minsomer.

I'm going to give my mod a shameless plug here... And expand on nhnifong's post while I am at it.

The time signal is actually built in to the GPS signal. Each satellite sends a unique, time dependent pseudo-random signal. The receiver compares the expected signal for that satellite to the received signal. The time delay between when the signal was transmitted and received is evident from the difference in the pattern. The pseudo-random nature of the signal also makes it difficult to jamb and easier to pick out the very weak signal from the background noise. The clocks in the satellites and receiver are updated from atomic clocks in the network's control segment.

The time delay allows determination of the distance between the transmitter and receiver. The satellites positions are known from the orbital elements that are downlinked along with the pseudo-random signal. Trilateration is then used to calculate navigation solutions using combinations of three satellites. A minimum of four satellites are needed to obtain a fix, because each combination of three satellites generally yields two solutions and there is no way of knowing which is the correct position without more information.

If any of you are interested in learning more about how GPS works, consider setting up by your own network in the game then add the working Figaro receiver part to your air, space or ground vehicles.

Edited August 21, 2013 by PakledHostage

[K^2]

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?

[Hannu]

Coarse position and radial component of velocity detection is made with radar. By using several radar observations over time mission controller's can calculate orbit. But such accuracy is not enough to slingshot flybys or landings. When probe get closer to target, it photographs star occultation times behind target. Controllers can calculate exact position and velocity relative target from these observations. As far as I know, position resolution can be kilometers or less.

Such basic methods have been used since first lander's and slingshot probes have been launched. Apollo astronauts made occultation observations with Moon and Earth and could calculate orbits with enough accuracy. Orbit calculations does not need much computer power. They can be done with pen and paper, if there is not very time critical situation.

[Person012345]

Orbit is freefall. If you're in freefall while going fast enough sideways that you miss the ground, that's orbit. An accelerometer on board will read zero unless your engines are running, and KSP's gravioli detector is quite impossible in real life thanks to the equivalence principle.

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.

Edited August 21, 2013 by Person012345