# How does one actualy measure deltaV in non orbital or deep space flight? (In real life)

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With aircraft, it's easy to get true airspeed with a flow meter, and speed above ground with laser or radar (or GPS). And in orbit, you can track position over ground or use laser...but if one is traveling between planets, how do you accurately measure your speed without anything to measure against (air pressure, something for lasers to bounce off of?) I imagine a bunch of really smart people use smart people calculations to figure that a if 3 ton ship that puts out X pounds of pressure burns for 3 seconds, speed should be changed by X amount. But how do they know, other than what it *should* be? Like if that kid and his monkey that were always hiding in Speed Racer's trunk had snuck onboard and changed the weight (however slightly). How would you know the difference between what you *should* be going, or what you're actually doing?

Or if you were Voyagering your way out of the solar system, and Superman came by, grabbed ahold of your ship, and gave it a good push for 20 minutes. How would you be able to track your new speed when you don't have known burn rates and velocities to go by?

Of course these scenarios are highly implausible, so feel free to substitute a more likely occurance if that helps.

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Velocities are usually determined via doppler shift of the radio signals from the spacecraft in question, combined with its angular movement across the sky. With these, you can create a vector that tells you your velocity.

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It's also possible to measure your true absolute acceleration, no matter what's causing it. One simple example would be to attach a mass to a spring (or network of springs, if you want more than one dimension) and watch the spring(s). If there's no displacement, there's no acceleration; if the mass is displaced from equilibrium, the amount of displacement (times the spring constant divided by the mass) tells you how much acceleration you're experiencing.

(Ostensibly this is what's going on inside the Accelerometer when you right-click it and tell it to display data.)

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Edited by ArcFurnace
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In real life? Moved to Science Labs, then.

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Spacecraft since Apollo use an inertial guidance computer, which is basically a bunch of accelerometers and a gyroscopes that measure any changes in velocity and attitude. You recalibrate it from time to time based on your position and the ground data obtained from radar and doppler. Position can be calculated by using a star tracker which is a computer combined to a camera that observes the position of stars. Measure your position twice and you can extrapolate your velocity vector.

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Velocities are usually determined via doppler shift of the radio signals from the spacecraft in question, combined with its angular movement across the sky. With these, you can create a vector that tells you your velocity.

+1

The exact same technique is used by some GPS systems to calculate speed (and also from that derive acceleration). GPS: More than just a positioning system!

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Spacecraft since Apollo use an inertial guidance computer, which is basically a bunch of accelerometers and a gyroscopes that measure any changes in velocity and attitude. You recalibrate it from time to time based on your position and the ground data obtained from radar and doppler. Position can be calculated by using a star tracker which is a computer combined to a camera that observes the position of stars. Measure your position twice and you can extrapolate your velocity vector.

The star tracker is mostly for orientation, you want to point the antenna towards earth and the solar panels against the sun.

Signal delay gives distance to earth, as doppler give relative speed, guess you also calculate position change based on changes position relative to earth.

This make is easier to get the error factor early so you can do the correction burn.

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What if you're too far out to be able to bounce a radio signal off of something?

In other words, what about when you can get acceleration, time, and attitude, but not absolute position (at least, not easily)? That leaves only "dead reckoning" aka inertial guidance, right? There's bound to be some drift in any INS that will build up over time, so what can be done to correct it without a connection to earth to refer to?

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You'd have to be really far for that. We can still use radio methods to locate and recalibrate Voyager 1 and 2.

And you could still use a star tracker (which is simply a sort of computerized sextant).

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I am talking about stuff probably 10-100x as far out as Voyager 1 and/or 2. Indeed, very very far out.

And as far as I know, a star tracker is only good for getting orientation/attitude, not calculating a position fix.

Trying to use a star tracker for a position fix would mean measuring the angular velocity of several stars, while actively stabilizing the spacecraft's attitude with respect to several (likely different) stars, and I just can't figure out a way to prevent those two systems from interacting in a way that makes determining position an incredibly difficult if not impossible problem. Additionally, the angular resolution to measure such small changes would require optics that have more in common with a space telescope than a simple star tracker.

Possibly you could use the relative motion of stars to get a precise velocity with respect to the center of the galaxy, but again, it requires an incredibly stable platform and incredibly high resolving power in the tracking optics.

Seems every time I go to solve the problem, I end up with needing an incredibly stable platform and gigantic optics to even get a rough estimate of the spacecraft's position AND velocity vectors at (nearly) the same time.

Odd, it seems that I've just discovered that the Heisenberg Uncertainty principle can have large-scale effects, when I previously believed it to only need to be compensated for when dealing with atomic or quantum scale things.

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You can use redshift of the known stars for velocity, and parallax for position. If you have time, parallax can be used for velocity too.

Sure parallax measurement won't be too accurate without a proper telescope, but anything traveling that far out should have stabilization anyway.

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If you were to travel between star systems you don't quite need any corrections in the way. And upon arrival, active (ie. controller gives command, probe executes) radio contact with Earth is pretty much useless... You need an autonomous platform, so a few stiched up space telescopes and a very capable AI shouldn't hurt !

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I am talking about stuff probably 10-100x as far out as Voyager 1 and/or 2. Indeed, very very far out.

And as far as I know, a star tracker is only good for getting orientation/attitude, not calculating a position fix.

Trying to use a star tracker for a position fix would mean measuring the angular velocity of several stars, while actively stabilizing the spacecraft's attitude with respect to several (likely different) stars, and I just can't figure out a way to prevent those two systems from interacting in a way that makes determining position an incredibly difficult if not impossible problem. Additionally, the angular resolution to measure such small changes would require optics that have more in common with a space telescope than a simple star tracker.

Possibly you could use the relative motion of stars to get a precise velocity with respect to the center of the galaxy, but again, it requires an incredibly stable platform and incredibly high resolving power in the tracking optics.

Seems every time I go to solve the problem, I end up with needing an incredibly stable platform and gigantic optics to even get a rough estimate of the spacecraft's position AND velocity vectors at (nearly) the same time.

Odd, it seems that I've just discovered that the Heisenberg Uncertainty principle can have large-scale effects, when I previously believed it to only need to be compensated for when dealing with atomic or quantum scale things.

How precise are you needing to be?

There are a list of known stable pulsars that you can triangulate off of, these will be stable over geological timescales,

By seeing which ones you're pointing at you get orientation, by observing the dopplar shift of them you can get velocity, and by triangulating the angle to multiple pulsars you get high precision* location.

*When you're traveling distances measured in light years +-5km is high precision

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That's pretty impressive, actually.

For what it's worth, I'd consider +- 5km to be high precision even for sending something out to one of the outer planets, dwarf planets, KBO's or Oort cloud objects.

Of course you need more precision when it gets closer to the destination, but by that point you can use the destination object itself as a reference point.

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That's pretty impressive, actually.

For what it's worth, I'd consider +- 5km to be high precision even for sending something out to one of the outer planets, dwarf planets, KBO's or Oort cloud objects.

Of course you need more precision when it gets closer to the destination, but by that point you can use the destination object itself as a reference point.

That's for light-years distance. A KBO / Oort cloud mission can still use usual radars, although probably not for the farthest reaches of Oort cloud (which is around 1 light year). Even so, as I have stated, go farther than a time delay of a week and you better off having an autonomous platform.

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