Hubble Replacement?

On Thu, 21 Apr 2005 17:40:23 +0200, Jan Vorbrüggen wrote:

I've heard that the limiting factors wrt Hubble's current life span are
the batteries or the gyros. The batteries being an obvious one, while
the gyros may not be. If they are just being used for attitude
determination during maneuvers, and not the actual maneuver, it should
be possible to extend it's life so that the batteries or orbital decay
are the limiting factors.
HST doesn't "maneuver" at all. It used to rely on occasional visits
from servicing crews for periodic reboost.

Tsk, tsk, Herb - "maneuver" includes changing the pointing to look at
something interesting.

AIUI, the gyros were expected to last longer, but they have some sort of
design/manufacturing problem that makes them die much earlier than
expected. They are not only required as sensors for pointing - fine
guidance is done separately by the fine guidance sensors - but to
stabilize the whole craft against torques. Not enough gyros, not enough
stability against random torques.




Ah, ok, thanks. So, they are used for attitude control in addition to the
magnetic torquers.

Do, i have this right?:

Course Attitude determination (navigation) - Gyros as sensor

Fine Attitude determination (navigation) - optical (fine guidance sensors)

Course Attitude control (maneuvering) - Magnetic torquers as effector

Fine Attitude control (maneuvering) - Gyros as effector

So, the minimum number of gyros should be one, to look at a star. One gyro
to control body pitch and yaw, allowing for sloppy roll control. Two gyros for
proper roll control. That is, if the gyro's rotational attitude can be
moved to the proper body attitude before the exposure, and the fine
guidance sensors can get the star in the center of the cross hairs.

--
Craig Fink
Courtesy E-Mail Welcome @

On 2005-04-21, Jan Vorbrüggen wrote:
Is the raw data from "all" exposures routinely saved over the years? A
particular target that is observed this year, last year, 10 years ago?

In the case of the HST, I'm pretty sure you can get the raw data even
from the first images. At least semi-recent stuff is regularly re-processed
when, for instance, new algorithms for defect removal etc are implemented
or just the calibration constants improved. As computer storage is not a
big cost factor anymore, and the raw data is only a small fraction of all
the data in any case, I can't imagine anybody not archiving the raw data
"just in case".

Storage is cheap - and all the data is certainly archived for almost all
major telescopes as a routine matter - but it's worth noting the sheer
amount of data involved... several GB per day. Not much now, but fifteen
years back it was certainly a nontrivial investment to handle.

--
-Andrew Gray

Really this isn't quite right, one well designed gyro should give 3-axis
control as an effector. Torquing the spinning gyro gives two, the last
axis being torque that changes the angular momentum of the gyro, spinning
it faster or slower. The gyro should only become saturated when it's spin
rate reaches some minimum or maximum rate.

The Earth is one huge gyro, and it's relatively stable in all three axis.
Allowing everyone on it to accurately point in any direction.

What would the requirements for one GNU gyro look like?

Ever wonder what the GNU type concept could do for space development.

--
Craig Fink
Courtesy E-Mail Welcome @

On Thu, 21 Apr 2005 16:12:40 +0000, Craig Fink wrote:

On Thu, 21 Apr 2005 17:40:23 +0200, Jan Vorbrüggen wrote:

I've heard that the limiting factors wrt Hubble's current life span
are the batteries or the gyros. The batteries being an obvious one,
while the gyros may not be. If they are just being used for attitude
determination during maneuvers, and not the actual maneuver, it should
be possible to extend it's life so that the batteries or orbital decay
are the limiting factors.
HST doesn't "maneuver" at all. It used to rely on occasional visits
from servicing crews for periodic reboost.

Tsk, tsk, Herb - "maneuver" includes changing the pointing to look at
something interesting.

AIUI, the gyros were expected to last longer, but they have some sort
of design/manufacturing problem that makes them die much earlier than
expected. They are not only required as sensors for pointing - fine
guidance is done separately by the fine guidance sensors - but to
stabilize the whole craft against torques. Not enough gyros, not enough
stability against random torques.




Ah, ok, thanks. So, they are used for attitude control in addition to
the magnetic torquers.

Do, i have this right?:

Course Attitude determination (navigation) - Gyros as sensor

Couldn't the Course Attitude determination be replaced with a wide angle optical
sensor?

Fine Attitude determination (navigation) - optical (fine guidance
sensors)

Course Attitude control (maneuvering) - Magnetic torquers as effector

Fine Attitude control (maneuvering) - Gyros as effector

So, the minimum number of gyros should be one, to look at a star. One
gyro to control body pitch and yaw, allowing for sloppy roll control.
Two gyros for proper roll control. That is, if the gyro's rotational
attitude can be moved to the proper body attitude before the exposure,
and the fine guidance sensors can get the star in the center of the
cross hairs.

(In the "better late than never" department...)

In article ,
Craig Fink wrote:
Do, i have this right?:
Course Attitude determination (navigation) - Gyros as sensor
Fine Attitude determination (navigation) - optical (fine guidance sensors)
Course Attitude control (maneuvering) - Magnetic torquers as effector
Fine Attitude control (maneuvering) - Gyros as effector

Not quite...

First, the gyros that persist in breaking down are pure sensors. They
aren't actuators. They're used for coarse attitude sensing during
attitude changes, and also to hold a target attitude precisely enough for
the (optical) fine guidance sensors to lock onto their target stars -- the
FGS system cannot lock on in the presence of any significant motion.

Precise attitude control for imaging is normally done under the control of
the fine guidance sensors, which occupy the edges of the telescope's field
of view. (More precisely, there's an outer ring of the FOV which is
divided into four quadrants. The FGS system occupies three; the fourth
is for the WFPC camera and its successors.)

The actuators for attitude control (coarse and fine) are reaction wheels,
completely separate from the gyros. As I understand it, they've never
given any problems.

Finally, any time the reaction wheels have to fight a steady torque --
e.g., unbalanced light pressure -- they accumulate momentum. The torquers
are used to get rid of that (the buzzword is "momentum dumping"). Doing
attitude control with just torquers is tricky, because they can't apply
torque on an arbitrary axis at an arbitrary time -- they give you torque
only around an axis perpendicular to the local direction of Earth's
magnetic field -- and Hubble doesn't try.

Doing without some of the sensing gyros is possible -- it's been done
before on other astronomy spacecraft -- but tricky, given how fussy the
fine guidance sensors are.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |