# How do you tell your velocity in space? **URL:** \ **Category:** Factual Questions\ **Created:** [May 16, 2001, 9:51pm UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646 "2001-05-16T21:51:43Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![bernse](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@bernse](https://boards.straightdope.com/u/bernse)\ **Post date:** [May 16, 2001, 9:51pm UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/1 "2001-05-16T21:51:43Z")
2 part question: 1. How does the Space Shuttle tell how fast its going when in orbit? Radar? 2. How would a deep space ship do the same? Star tracking/triangulation? Some kind of device to measure hydrogen atoms? A computer that derives it from the thrust the engine produced? How would it compensate for gravity? I have been thinking about this question off and on for the past couple of days and its bugging me… I hope its not a stupid question, but I can’t think of a real simple answer since there is no air to measure (like an aircraft) or anything solid to make a “speedmeter” run against. Thanks ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [May 16, 2001, 10:06pm UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/2 "2001-05-16T22:06:20Z")
For a craft in orbit, ground-based radar tracks the ship as it passes overhead, and thus an orbital velocity is calculated. It is also possible to calculate velocity via inertial guidance…basically, a g-sensor measures g forces in three-space (X,Y, and Z coordinates, measured against time). Thus, velocity and acceleration is measured/calculated using Newtonian Physics. ---
**Author:** ![bughunter](https://avatars.discourse-cdn.com/v4/letter/b/aeb1de/32.png) [@bughunter](https://boards.straightdope.com/u/bughunter)\ **Post date:** [May 16, 2001, 11:27pm UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/3 "2001-05-16T23:27:54Z")
In earth orbit, your altitude and eccentricity determines your velocity. In a circular low earth orbit (let’s say around 600 km), the orbital velocity is around 7.5 km/sec. (I know this mostly because the earth science satellites I work with operate at this altitude.) For deep space probes, the most accurate way to measure their velocity is via doppler measurements on their radio communications. One of the things we can measure most accurately is time and frequency, down to one part in 10^12 or more! This is how JPL has discovered anomalies in the actual speeds of the Pioneer spacecraft, and the speeds at which they were expected to be travelling according to theory. Scientists still haven’t explained this divergence, either. ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [May 17, 2001, 12:54am UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/4 "2001-05-17T00:54:26Z")
Of course, if you’re talking _really_ deep space, first you have to establish what you’re measuring speed relative to. **Sound of grammar vultures descending** ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [May 17, 2001, 3:54am UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/5 "2001-05-17T03:54:36Z")
> [@](#): > > \*Originally posted by \*\*\*enolancooper \*\* > It is also possible to calculate velocity via inertial guidance…basically, a g-sensor measures g forces in three-space (X,Y, and Z coordinates, measured against time). > Thus, velocity and acceleration is measured/calculated using Newtonian Physics. A g-sensor could measure acceleration, but not velocity. There is no way to determine velocity by internal measurements since velocity must be measured relative to something external. You could use Doppler to measure your velocity relative to some other reference, but absolute velocity is explicitly denied by relativity. ---
**Author:** ![Dr.Lao](https://avatars.discourse-cdn.com/v4/letter/d/2bfe46/32.png) [@Dr.Lao](https://boards.straightdope.com/u/Dr.Lao)\ **Post date:** [May 17, 2001, 4:54am UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/6 "2001-05-17T04:54:39Z")
> [@](#): > > \*Originally posted by DrMatrix \* > \*\* > > > [@](#): > > > > \*Originally posted by \*\*\*enolancooper \*\* > > It is also possible to calculate velocity via inertial guidance…basically, a g-sensor measures g forces in three-space (X,Y, and Z coordinates, measured against time). > > Thus, velocity and acceleration is measured/calculated using Newtonian Physics. > > A g-sensor could measure acceleration, but not velocity. There is no way to determine velocity by internal measurements since velocity must be measured relative to something external.\*\* If you integrate the acceleration over time it will give you velocity. Airliners, in the days before GPS, used an inertial navigation system to determine their location on long oceanic routes. It worked remarkably well. Of course at relativistic velocities this won’t work. [This](http://www.allstar.fiu.edu/aerojava/RNAV.htm) page gives some information on aircraft navigation systems. INS is covered in section **F.** ---
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [May 17, 2001, 5:24am UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/7 "2001-05-17T05:24:58Z")
**Dr. Lao** Sorry, you still cannot tell absolute velocity. When you integrate you introduce an arbitrary constant. In order to determine the arbitrary constant, you need to supply an initial value. > [@](#): > > _from [RNAV (Area Navagation)](http://www.allstar.fiu.edu/aerojava/RNAV.htm)_ > After being supplied with initial position information, it is capable of updating with accurate displays of position, attitude, and heading. By integrating velocity, you can also determine position, but the position and velocity are _relative_ to your initial position and velocity. There is no way to determine absolute velocity or absolute position. Position and velocity and veolcity are always relative to some reference frame. ---
**Author:** ![galt](https://avatars.discourse-cdn.com/v4/letter/g/35a633/32.png) [@galt](https://boards.straightdope.com/u/galt)\ **Post date:** [May 17, 2001, 9:14am UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/8 "2001-05-17T09:14:09Z")
> [@](#): > > \*Originally posted by DrMatrix \* > Sorry, you still cannot tell absolute velocity. :rolleyes: Something tells me he knows this. The statement “if you integrate acceleration over time, it will give you velocity” is true. It gives you your velocity relative to your initial velocity. No, it’s not _absolute_ velocity, but since absolute velocity is impossible to discern, the term “velocity” is generally understood to mean velocity relative to some implied frame of reference. That is, unless you’re a pedant, in which case you assume that any use of the term “velocity” means “absolute velocity”, and consequently try to correct people who really do know what they’re talking about. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [May 17, 2001, 11:55am UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/9 "2001-05-17T11:55:27Z")
> [@](#): > > If you integrate the acceleration over time it will give you velocity. This isn’t very practical for spacecraft. While an acceleration sensor can measure acceleration due to thurust, it won’t measure gravitational acceleration. And to find out the gravitational acceleration you need to know the position and mass of all the nearby masses. It’s easy to measure the distance from a ground station (radio antenna) to the spacecraft by measuring the time delay of a signal. Like radar, except you detect re-transmitted signals instead of reflected signals so it requires much less power. Do this as the spacecraft passes overhead, repeat for a couple more orbits, and you can get enough information to determine the orbital parameters. This is how we determine the “position” of satellites and spacecraft in earth orbit. ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [May 17, 2001, 12:41pm UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/10 "2001-05-17T12:41:42Z")
In deep space could you figure velocity by observing stars as mentioned in the OP? Assume you know the distance to the star(s) you are measuring against. Figure the change in angle to the star over the course of two measurments separated by some amount of time. If done for several stars could you then figure your velocity? I am assuming we have the ability to measure the angle change even though such a change would likely be very small (correct me if I’m wrong). I am also assuming that the movement of the star itself is either not necessary to the calculation or its movement is somehow acounted for. Can do? ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [May 17, 2001, 2:04pm UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/11 "2001-05-17T14:04:34Z")
> [@](#): > > \*\*In deep space could you figure velocity by observing stars as mentioned in the OP? > > Assume you know the distance to the star(s) you are measuring against. > > Figure the change in angle to the star over the course of two measurments separated by some amount of time.\*\* It would work in theory, but in practice it’s very difficult to measure the position of a star with such accuracy. Unless you are moving VERY fast, or you make the two measurements separated by many months or years, it won’t be very practical with our current technology. If all you want is velocity (not position), it’s much simpler than that. You just observe a star’s spectrum and measure the Doppler shift. Stars’ spectra contain absorption lines at certain known wavelengths, so if the lines are shifted towards longer wavelength (red-shifted), you know you are moving away from the star. Of course, it could be the star moving towards you. To find out which, the easiest way is to look up the star’s velocity in a star catalogue. If you don’t have one handy, just observe many stars and assume that the average velocity of those stars are zero (i.e. some moving towards you, some away from you). ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [May 17, 2001, 2:21pm UTC](https://boards.straightdope.com/t/how-do-you-tell-your-velocity-in-space/65646/12 "2001-05-17T14:21:31Z")
Measuring changes in the positions of stars over time won’t give you a very sensitive measure of your speed. For a star 4 LY away, the movement of the earth from one side of the sun to the other (2.35E13 vs186E6 miles) gives a change of about 0.00046 degrees (~1.6 arcsecond) in the stars position. You might be better off looking at the shifts in the frequency of the Hydrogen absorption lines in several stars in different parts of the sky. If you knew what the frequencies were before you started accelerating, and could measure the changes accurately enough they’d allow you to determine your velocity. I’m not sure what the accuracy limits to this method are currently, but it should easily be good to within a few kilometers per second.