[AR] Re: Star Trackers, was Re: Re: Spin stabilized rocket
- From: "Monroe L. King Jr." <monroe@xxxxxxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 25 Dec 2018 14:42:35 -0700
"although getting a full understanding of the details -- so you know
what
to compute -- can certainly be a headache."
This is like a major understatement. lol
Thanks for some of those answers.
if a star tracker can be used from earth (this is why I'm supposing
they can't work from earth) Why don't telescope tracking systems use it?
This question was only because I have a great interest in telescopes and
enjoy astronomy.
I'm also forever looking to improve things I enjoy.
Never looked at them, but one thing that's likely to be an issue is that
solving the "lost in space" problem quickly, with a star catalog of
reasonable size, typically requires a rather wider field of view than the
usual astronomical telescope.
I think the star tracker could be faster and more accurate technology
for this. I have an all-sky camera perhaps someday these 2 things will
combine?
The most pertinent question however is:
My last question at the moment is: how difficult is it really to go from
an earth frame of reference to an inertial, body fixed from an earth
fixed frame?
Because If I can grasp this I can rewrite the Ardupilot code to work on
orbit in a small sat.
I've been told that an earth frame of reference won't get me to orbit.
I've used the Ardupilot to fly back from 100kft which is damn near the
objective and it works quite well at that altitude.
I think with the right explanation I can make progress.
We did get ardupilot to work with reaction wheels in Gazebo simulation
amd we can do RCS with thrusters. But this is not enough I'm told.
Because of this frame of reference.
I live on the earth and I have a great grasp of that but I have a mental
block when it comes to other frames of reference and how they relate to
software.
I really need help here.
I've also been talking about this for a long time. So you guys must know
what I'm talking about. I just get blank stares everywhere I go. People
that do earthbound guidance just don't seem to want to even try. If I
could grasp it I could direct the people I know that can write the code.
I have books on the subject! I just have a block that won't let go so I
get stuck right away.
I'm sure a person with understanding can help me remove this block.
Seeing the world the way I do limits me I know this. The mind has no
limits I believe but I do have limits imposed by my own view. Please
widen my view so I can progress.
I'm 56 years old right now. I've been working on this since 2007 and I'm
really happy with my progress on most fronts. I can do this.
I've devoted my entire life to it and I've done this full time since
2007. I didn't know jack when I started. I've been called genius and
rocket scientist smart ect... None of these things are true. I'm just a
guy on a mission. I did solve how to devote all my time to this project
and that was the major factor and that is all it is.
So someone explain frames of reference and how one might convert an
earth based frame to a space based frame?
And help me work threw this block?
Monroe
-------- Original Message --------
Subject: [AR] Re: Star Trackers, was Re: Re: Spin stabilized rocket
From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
Date: Tue, December 25, 2018 1:42 pm
To: Arocket List <arocket@xxxxxxxxxxxxx>
On Tue, 25 Dec 2018, Monroe L. King Jr. wrote:
My first question is can you test a Star Tracker from earth?
Yes. We've tested star trackers outside on a clear night.
Can it get a position from a single frame?
Yes. The Sinclair star trackers, in particular, solve the "lost in space"
problem -- no idea what the attitude is, figure it out from the image --
every time. Some other designs do that the first time, and then do faster
or more precise updates using a different algorithm that starts from a
known approximate position.
I also wonder if the spacecraft is tumbling can it get a position? What
roll rate and tumble can a star tracker handle?
That's part of the star-tracker specs. They all have an attitude-rate
limit -- as do most other attitude-determination methods -- it's just a
question of how high it is. Spacecraft can generally live with fairly low
limits, because they don't often get into *fast* spins (and if they do,
generally they can use some less-precise sensor, like a MEMS gyro, to
first bring the spin rate down to something the star tracker can handle).
The camera would matter in this right?
For sure. One important question is, how long an exposure does it need to
get clear star images in the worst case? (There are some areas of the
sky, especially the southern-hemisphere sky, that don't have much in the
way of bright stars.) This tends to set the attitude-rate limit -- when
the motion is fast enough to turn star images into trails, longer
exposures don't help, because they don't make the trails brighter, just
longer.
This also means that when designing the camera, more pixels -- which
usually means smaller pixels -- is not necessarily better. The CCDs in
the Clementine star trackers were 576x384.
...if a star tracker can be used from earth (this is why I'm supposing
they can't work from earth) Why don't telescope tracking systems use it?
Never looked at them, but one thing that's likely to be an issue is that
solving the "lost in space" problem quickly, with a star catalog of
reasonable size, typically requires a rather wider field of view than the
usual astronomical telescope.
(Another noteworthy issue, by the way, is that errors in star catalogs are
not uncommon. Yes, including the Hubble Guide Star Catalog.)
...What kind of math does a star tracker use?
Depends somewhat on what algorithm it uses; there's more than one answer.
My last question at the moment is: how difficult is it really to go from
an earth frame of reference to an inertial, body fixed from an earth
fixed frame?
Actually calculating coordinate transformations is pretty straightforward,
although getting a full understanding of the details -- so you know what
to compute -- can certainly be a headache.
Henry
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