More on Gyros

I read in the other thread that a good mechanical gyro will drift
about 0.01 degree/hour and GPS can correct for this. What happens on
say a year long (or longer) deep space mission? How do we correct the
drift then? Also, when you land the spaceship do you keep teh gyros
spinning? How else do you know where you are? I could see you may be
able to point to a star but that wouldnt be accurate.


Wang King

gyansorova@gmail.com wrote:
> I read in the other thread that a good mechanical gyro will drift
> about 0.01 degree/hour and GPS can correct for this. What happens on
> say a year long (or longer) deep space mission? How do we correct the
> drift then? Also, when you land the spaceship do you keep teh gyros
> spinning? How else do you know where you are? I could see you may be
> able to point to a star but that wouldnt be accurate.

I can't give you a direct answer, but perhaps John Doty can. (Cross 
posting to tap his expertise.) I know that at least some deep-space 
probes lock onto known stars, Canopus among them.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

gyansorova@gmail.com wrote:
> I read in the other thread that a good mechanical gyro will drift
> about 0.01 degree/hour and GPS can correct for this. What happens on
> say a year long (or longer) deep space mission? How do we correct the
> drift then? Also, when you land the spaceship do you keep teh gyros
> spinning? How else do you know where you are? I could see you may be
> able to point to a star but that wouldnt be accurate.
> 
> 
> Wang King
> 
Pointing at stars can be _very_ accurate.  It's just seriously more 
tedious than letting your GPS system do the work for you.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google?  See http://cfaj.freeshell.org/google/

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Repeating crosspost because it didn't appear in comp.lang.forth

gyansorova@gmail.com wrote:
> I read in the other thread that a good mechanical gyro will drift
> about 0.01 degree/hour and GPS can correct for this. What happens on
> say a year long (or longer) deep space mission? How do we correct the
> drift then? Also, when you land the spaceship do you keep teh gyros
> spinning? How else do you know where you are? I could see you may be
> able to point to a star but that wouldnt be accurate.

I can't give you a direct answer, but perhaps John Doty can. (Cross
posting to tap his expertise.) I know that at least some deep-space
probes lock onto known stars, Canopus among them.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

gyansorova@gmail.com wrote:
> I read in the other thread that a good mechanical gyro will drift
> about 0.01 degree/hour and GPS can correct for this. What happens on
> say a year long (or longer) deep space mission? How do we correct the
> drift then? Also, when you land the spaceship do you keep teh gyros
> spinning? How else do you know where you are? I could see you may be
> able to point to a star but that wouldnt be accurate.

I can't give you a direct answer, but perhaps John Doty can. (Cross
posting to tap his expertise.) I know that at least some deep-space
probes lock onto known stars, Canopus among them.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:
> Repeating crosspost because it didn't appear in comp.lang.forth

It appeared int comp.lamg.forth.

:-)

Andor wrote:
> Jerry Avins wrote:
>> Repeating crosspost because it didn't appear in comp.lang.forth
> 
> It appeared int comp.lamg.forth.
> 
> :-)

Yeah. (What's the emoticon for "red face"?)

Jerry
-- 
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:
> gyansorova@gmail.com wrote:
>> I read in the other thread that a good mechanical gyro will drift
>> about 0.01 degree/hour and GPS can correct for this. What happens on
>> say a year long (or longer) deep space mission? How do we correct the
>> drift then? Also, when you land the spaceship do you keep teh gyros
>> spinning? How else do you know where you are? I could see you may be
>> able to point to a star but that wouldnt be accurate.
> 
> I can't give you a direct answer, but perhaps John Doty can. (Cross
> posting to tap his expertise.) I know that at least some deep-space
> probes lock onto known stars, Canopus among them.

By coincidence, I'm working on this problem today (software in my 
favorite Forth dialect, LSE64), see:

http://space.newscientist.com/article/dn9870-satellites-could-navigate-by-xray-stars.html

And just to brag a bit, I designed the measurement chains and some of 
the algorithms that produced the pretty picture of the x-ray nebula 
around the Crab pulsar in the article.

The question confuses two problems, I think. A gyro helps you find your 
*orientation* in space ("attitude determination"), while GPS helps you 
find your *location* in space ("navigation"). Of course they're not 
completely independent problems. You can get rough attitude from 
differential GPS by comparing the locations of different places on your 
spacecraft. And knowing your attitude may be necessary for navigation.

You need more than a gyro for attitude determination, though. A gyro 
just measures your rotation rate, usually only about one axis. With 
three of them you get rates about all three axes, but that still doesn't 
tell you your orientation. One easy reference point is the Sun: it's a 
very bright object in a known place. Canopus is handy because for a 
spacecraft near the ecliptic plane the angle between it and the Sun is 
always near 90 degrees, and it's very bright, but other stars may be 
used. With two reference directions you know your attitude. With more 
you can detect errors and measure accuracy.

For interplanetary navigation, the primary tools are radio Doppler 
tracking and ranging. Doppler shift measures the velocity of the 
spacecraft in the direction away from the tracking station. Or you can 
ping the spacecraft from the tracking station and measure the time for 
the response to return, yielding the distance to the spacecraft. One 
difficulty here is that you cannot get tangential position or velocity 
this way. Gravity couples the radial acceleration to the tangential 
parameters, so with a bunch of measurements you can fit all the 
parameters of the trajectory. Another useful method for a spacecraft 
with an imager is to compare the position of the target object with 
stars as you approach.

Mechanical gyros are essentially carefully balanced electric motors: you 
keep them going with electricity. But a lander doesn't normally need a 
gyro after it has landed. Perhaps rovers use gyros (I don't know), but I 
doubt they're the same ones used in free fall.

-- 
John Doty, Noqsi Aerospace, Ltd.
--
Specialization is for robots.

On Apr 30, 7:21 pm, Tim Wescott <t...@seemywebsite.com> wrote:
> gyansor...@gmail.com wrote:
> > I read in the other thread that a good mechanical gyro will drift
> > about 0.01 degree/hour and GPS can correct for this. What happens on
> > say a year long (or longer) deep space mission? How do we correct the
> > drift then? Also, when you land the spaceship do you keep teh gyros
> > spinning? How else do you know where you are? I could see you may be
> > able to point to a star but that wouldnt be accurate.
>
> > Wang King
>
> Pointing at stars can be _very_ accurate.  It's just seriously more
> tedious than letting your GPS system do the work for you.
>
> --
>
Hello Tim et. al.,

Actually NASA has done a bunch of work on identifying "guide stars." A
large percentage of the points of light you can see in the sky are
systems of stars with more than one component.  And of course "binary
stars" move. So the whole point of finding good guide stars was
essential to interplanetary missions. Visually bright ones tend to be
close by and this magnifies their motions. For example Sirius is the
2nd brightest star in the sky, and is comprised of two stars where
their obital motion is detectable as a wobble (50 year period). Since
Sirius B is much fainter than Sirius A, we just see Sirius A wobble
back and forth. It takes quite a good 'scope to actually resolve
Sirius into its two components.

Clay

On May 2, 5:35 am, John Doty <j...@whispertel.LoseTheH.net> wrote:
.
>
> Mechanical gyros are essentially carefully balanced electric motors: you
> keep them going with electricity. But a lander doesn't normally need a
> gyro after it has landed.
How do you get home? You would need to re-align I expect after you
have blasted off but wouldnt the stars be different if you were
outside your own solar system? You would need a star map showing how
teh stars would look like from your destination surely in order to get
back.