Hack in Space

MelloDawg writes: "From the press release: NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft, which some had given up for dead in December after critical guidance components failed, was returned to full operations when the team developed an innovative new guidance system. The system uses a complex new set of procedures that lets controllers use electromagnets in the satellite to push and pull on the Earth's magnetic field. Details of the mission are online."

Comments

The Real Deal

[cybermage]

Pretty clever hack. Every time I read about real scientists pulling off neat things like this, I get more and more disgusted with people who simply reboot a server whenever it hangs because it's easier than actually thinking about what the problem is and how to fix it.

Re:The Real Deal

[fahrvergnugen]

Don't be ridiculous. I can tell you that in IT, this line of thinking will get you fired quickly.

Most often, if a system breaks in a production environment, it's imperative to get a it working again as fast as possible. Treating the symptom is paramount in a high availability situation, and while finding and resolving the cause is of course important, it takes an immediate back seat to getting the system back into production. If you can do both at once then that's grand. If not, then you restart the daemons or reboot the server once you've deemed it safe to do so.

Then, armed with log files and information gathered while the system was on error, you can go back through and trace the cause. Even better, you can duplicate the issue on a staging server. What you do NOT do is leave the system down for any reason one second longer than you have to, no matter how much the urge to tinker grabs you.

On the other side of the coin, diagnosing a problem on trivial or near-trivial system is a waste of your valuable time. Why the hell, except for fun, would you diagnose a BSOD issue on a secretary's computer when you could just back up her home directory, restore an image, then restore her personal files?

Obviously, every situation is different, but there are times when inexperienced techs will spend a day searching for the cause of a trivial problem instead of getting back to work.

Taking the time to root out a deep problem instead of just hitting the reset button is most often a luxury.

Re:NASA these days

[PhoenixK7]

Everyone seems to think that since many of the outward looking missions are having trouble that NASA isn't really doing much, check out all the reasearch thats being done on our actual planet. NASA has been collecting huge amounts information about Earth's atmosphere, oceans and land, and they've been doing important analyses. I'd suggest you check out places like:

earth.nasa.gov
earthobservatory.nasa.gov
terra.nasa.gov
data assimilation office

and for image products:

visibleearth.nasa.gov

NASA does alot of interesting earth science too!

Where to put angular momentum

[Animats]

This is indeed neat.

A basic problem in satellite stabilization is how to get rid of unwanted angular momentum. There are a few options.

You can throw away something. Usually this is reaction mass from a small rocket, or just compressed gas. Weights on the ends of cables that unwind and break free have been used to despin satellites.

You can store angular momentum in an inertia wheel, which is a flywheel on a motor. This doesn't get rid of angular momentum; it just stores it as long as you keep the wheel spinning. Eventually, you hit the maximum motor speed and can't do anything more in that axis. So it's also necessary to have some way to drain off angular momentum, even if very slowly.

You can couple to a gravity gradient. This is done with a long pole aimed towards a nearby planet. The difference between the gravity at the ends of the pole is tiny, but enough that if you get the thing pointed down and stable, it usually stays that way. Only good for one axis, of course.

You can couple to a planetary magnetic field, like these guys are doing. Again, only good for one axis, but it's a different one than the gravity gradient. It's a weak effect, but stronger than the gravity gradient.

You can put out sails and get reaction forces from solar energy. This gets talked about a lot, but isn't done much.

All of these are known techniques. It sounds like this satellite had four inertia wheels and an electromagnet for torquing against a planetary magnetic field. The plan was presumably to maneuver with the inertia wheels, and slowly drain off unwanted angular momentum with the magnetic torquer.

With two inertia wheels down, there are still three torquing devices available, so control of orientation is theoretically possible. Tough, but possible. It's impressive that they made it work.

Re:Where to put angular momentum

[fwc]

This technology is also being used (with great success) on the Amateur Radio satellite AO-40 which was on Slashdot a while back.

Specifically, it was used to de-spin the satellite from almost 18 RPM down to the desired 5 RPM.

Some more relevant quotes I found while looking around on the web about AO-40's system:

From http://www.amsat-dl.org/journal/adlj40ge.htm

Magnetorquer In the satellite, several electro-magnets, also named magnetorquer, are distributes that can be used in the interplay with the Earth's magnetic field close to perigee for the attitude-control of the satellite. The satellite acts as the rotor of an electric motor while the magnet-field of the Earth forms the stator. The process of this movement is named as magnetorquing. With the magnetorquing, the flight-attitude of the satellite and the spin-speed can be changed during perigee-passes.

From http://www.rac.ca/spacenws.htm:

The onboard magnetorquing system--which consists of solenoid coils--makes use of Earth's magnetic field to control the spacecraft's spin and orientation. Magnetorquing is most effective when Earth's magnetic field is strongest, so it typically only takes place at perigee--when the satellite is closest to the Earth. Ground controllers have been making incremental adjustments during each perigee.

I also remember someone saying that this was somewhat "experimental" on AO-40. I can't find a quote though...

I agree fully that it's good to see the NASA engineers thinking "Well it's broke, we can't send someone up to fix it, so what can we do to make it work?" What I would like to know is who came up with the original idea (pre AO-40, or this satellite). It sure doesn't seem like the type of thing which I would have thought about when trying to figure out how to control the attitude of a spacecraft.

Re:Where to put angular momentum

[Gino]

All of these are known techniques. It sounds like this satellite had four inertia wheels and an electromagnet for torquing against a planetary magnetic field. The plan was presumably to maneuver with the inertia wheels, and slowly drain off unwanted angular momentum with the magnetic torquer.

In most cases at least 3 single axis magneto-torquers are provided, one per axis and normally one extra for redundancy. They are indeed used to reduce body angular rates and to control the wheel angular momentum. The reaction wheels (or inertia wheels) of course are limited to a maximum speed and to avoid wheel saturation the magneto-torquers are used to 'dump' some excess angular momentum.

With two inertia wheels down, there are still three torquing devices available, so control of orientation is theoretically possible. Tough, but possible. It's impressive that they made it work.

Actually, in smaller LEO (Low Earth Orbit) satellites this technique has been used for years to facilitate orientation control. It is normally used to reduce the spacecraft angular rates directly after separation from the launch vehicle. Using the magneto-torquers and a magnetometer monitoring the earth's magnetic field, the spacecraft can be controlled into a stable state with the minimum of hardware and with relatively simple control algorithms.

The idea is to get the spacecraft into a stable, known state (either earth pointing or aligned with the earth's magnetic field) before the more complex systems are powered and tested. You need to be sure your star camera, reaction wheels, propulsion systems and all the other cool hardware is in a nominal state before you enable the complex control algorithms required for more accurate orientation control.

The first satellite I've worked on we could not afford the more expensive magneto-torquers (compact electro magnetic rods) and instead opted for a self made magnetic coil without the assistance of ferrite material. Basically just a very long copper wire rolled into a coil and fixed to the frame of the solar panel (one coil per axis). Crude, but it had the desired effect!

What is really impressive is that they've managed to achieve this level of pointing accuracy with a system intended to achieve only basic orientation control and for desaturation of the reaction wheels.

Re:Where to put angular momentum

[Gino]

You have to know that not all satellites have their own dedicated launch vehicle with a perfectly controlled release mechanism - first by the last stage of the launch vehicle and then by the S/C itself.

In fact, the majority of satellites share a ride with other satellites. The satellites are in fact clamped to the last stage of the launch vehicle. At the moment of separation so called piro bolt cutters (in fact an anvil propelled by a small amount of explosive to cut through the bolts) will cut through the bolts fixing the satellite to it and a passive spring loaded mechanism (in most cases) will slowly and safely push the satellite on its way.

...once things have settled down after release of the satellite...

It is this initial release from the last stage that causes the initial angular momentum and there is nothing but these control mechanisms on the satellite to help 'things to settle down'.

Re:Where to put angular momentum

[Skuld-Chan]

Some sattelites you don't want to de-spin - like phase 2 and phase 3 sattelites which spin with the earth (or at least they seem to). Of recent concern is AO-10 which apparently is de-spinning by itself. Actually I don't think anyone is too worried about AO-10 anymore since it is a sattelite literally plagued with problems - but it still works! (when its in sunlight that is). But you read about what guys did to get AO-10 and AO-40 running again its pretty amazing nasa like stuff.

Read all about it - here

Re:Where to put angular momentum

[heikkile]

Magnetorquer [...]for the attitude-control of the satellite.

Do they work on girlfriends as well?

Re:Where to put angular momentum

[Alsee]

Magnetorquer [...]for the attitude-control of the satellite.
Do they work on girlfriends as well?

This has been an extremely dificult question to answer. It could only be tested by someone who has one of each.

-

NASA's troubles

[colmore]

First off bravo to the NASA geeks. I always find it truly heartening when they pull these last minute hacks off. They don't really need to. I hadn't even heard about this one breaking, it wasn't that much of a PR problem. I think the only reason they really bothered is because, like any good geeks, this is their toy, and they love it too much not to try.

Second, to people bemoaning the absense of mars missions and moon bases. Why go? Can we learn so much more by sending people that it justifies the risk and cost of doing so? These are the questions being asked. And if you think this means our motives for going to space aren't pure, think about this: would we have gone to the moon had the Russians not been trying to beat us there?

I think the best way to get American astronauts on Mars today would be to convince Bin Ladin to start a space program.

I for one wonder if NASA has perhaps outlived its usefulness. Could it perhaps persist as a regulatory body, overseeing commercial space ventures, and allowing all-to-scarce public research money to go to other areas?

The mission to Mars sounds even less appealing once you consider how much cancer / AIDS / environmental / fusion / fuel cell / quantum computing / immortality / (name your favorite project) research it would replace.

Obviously the answer is more funding for public research, but then, does anyone really see that happening?

Re:NASA's troubles

[Rogerborg]

• to people bemoaning the absense of mars missions and moon bases. Why go?

We choose to go to the moon. We choose to go to the moon. . . . Not because it is easy, but because it is hard. . . . Because there is new knowledge to be gained. We shall send to the moon--240,000 miles away--from the control station in Houston, a giant rocket more than 300 feet tall, made of new metal alloys, some of which have not yet been invented . . . on an untried mission . . . on the greatest adventure.

Now, I'm a pretty cynical old bastard, but those words, nearly forty years on, still choke me up. To me, the defining quality of humanity is that our reach can exceed our grasp.

It's impossible (I think) to justify space exploration in any rational or economic terms. But if "because it's there" isn't justification enough, then that's a sad indicator we have become society of navel gazers and bean counters. And history shows us that societies only go one way once they've reached that stage.

You're right that the space race was based on competition. But I believe that still applies, and if we no longer want to compete, sooner or later we will be superceded by a society that does.

Re:NASA's troubles

[Shivetya]

"The mission to Mars sounds even less appealing once you consider how much cancer / AIDS / environmental / fusion / fuel cell / quantum computing / immortality / (name your favorite project) research it would replace."

Actually its more appealing because of just how much it could benefit these areas of research. Don't forget that the first American space program that eventually sent man to the moon was the basis for many technologies we take for granted today.

There is reason to believe that the obstacles overcome to support man living on the moon or venturing to Mars would also produce such technological findings that many seemingly unrealated areas would benefit.

I thing power production, the environment, and manfucturing would be the 3 key areas of benefit, and from them others areas would benefit as well.

Magnetic stabilization only good for one axis?

[XNormal]

You can couple to a planetary magnetic field, like these guys are doing. Again, only good for one axis

Not really. Full 3-axis stabilization can be implemented using the Earth's magnetic field. Unlike a passive pole the magnetotorquers are active elements and the magnetic field doesn't always point down so at different parts of an orbit it can be used to control all 3 axes.

Design and Experimental Test of Magnetic-Torquer-Based 3-Axis Satellite Attitude Controllers

Yet Another Star Trek Moment

[Ecyrd]

Quote from the press release:

To make up for the missing reaction wheels, scientists and engineers developed new sets of procedures and software that let them use equipment known as magnetic torquer bars in a new way. Controllers can generate local magnetic fields by running electric current through FUSE's three torquer bars, and can flip the polarity of these fields by changing the direction of current flow.

So, reversing the polarity really DOES work! :-)

The remaining wheels are still used

[Gino]

At first I was quite surprised at the quoted pointing accuracy achieved (1/4000th of a degree) by using only the magneto-torquers - as implied by the article. The original intended use for these torquer rods are clearly explained in other comments, so I will not repeat them here.

What the article omits is that the fine pointing accuracy is achieved using the magneto-torquers in combination with the two still operational reaction wheels. From the original NASA press release (dated 14 Dec 01) where the anomalies with the two reaction wheels were reported the strategy was laid out to rescue the science mission:

One of the new control mode concepts being investigated is to use the two operational reaction wheels in conjunction with the satellite's magnetic torquer bars to provide control in all three axes. The magnetic torquer bars are presently used to manage the momentum of the reaction wheels by applying a torque on the satellite against the Earth's magnetic field. The torques necessary to make up for the failed wheel would be in addition to that required for momentum management. This is well within the capability of the magnetic torquer bars.

What they probably managed to do is to use the two remaining wheels to do the fine pointing but the satellite will tend to slowly spin 'of course' lacking the two other wheels to compensate. By bringing the torquers into the loop they cancel the spin and attain the fine pointing.

It is nowhere mentioned but I wonder if they can maintain the pointing accuracy long enough to get maximum exposure time. Since they've said that the spacecraft has been restored to full operations I guess it is not a problem.

I'm not trying to take anything away of what they achieved, only to clarify what is omitted from the article (possibly not to make it too technical I guess). It still is an impressive feat and indeed a super hack!

Re:The remaining wheels are still used

[FUSENerd]

You're right, the two remaining wheels are still used.

What they probably managed to do is to use the two remaining wheels to do the fine pointing but the satellite will tend to slowly spin ?of course? lacking the two other wheels to compensate. By bringing the torquers into the loop they cancel the spin and attain the fine pointing.

Actually, not quite. The roll and "skew" wheels are the ones that still work fine. The new design allows the torquer bars to correct an axis that is basically perpendicular to the roll and skew directions, which is sort of diagonal across the science apertures.

It turns out that although the torquer bars are not designed for fine control (their control bandwidth is 15 times lower than the reaction wheels), the satellite is also heavy compared to the MTB's torque. So, the pointing stability is in fact quite good, and the jitter is not much worse than what we had before... as long as the satellite is not pointed in a part of the sky where gravity gradient disturbances are strong.

My favorite space hack

[Thagg]

About five years ago, Hughes launched a communication satellite. To be useful, these have to reach a geosynchronous orbit. It turns out that the most efficient way to get to from a low-earth orbit to a geosynchronous orbit is to fire a rocket twice [or so it was thought.] The first firing raises the apogee of the orbit to the geosynchronous altitude; so that the satellite is in a very elliptical orbit. Then, when the satellite is at apogee, you fire the rocket again to circularize the orbit. Usually this same motor changes the plane of the orbit as well. Most satellites are launched into orbits inclined to the equator somewhat, and geosynchronous satellites have to be over the equator. It is most efficient to make the orbital plane change at apogee, too.

There was a rash of apogee kick motor failures, and in this particular satellite the motor failed, leaving the satellite in a uselessly ellipitical orbit. There were small thrusters on the satellite which were to be used for station-keeping (small orbital adjustments) but it didn't have nearly enough propellant to raise the perigee. Hughes finally abandoned the satellite.

But one engineer refused to give up. It turns out that the transfer orbit paradigm above is the probably most efficient path in a single-planet system, but Earth has this anomolously large, close, Moon. And while there wasn't nearly enough fuel to get raise the perigee to geosynchronous altitude, there was more than enough fuel to raise the apogee out to lunar orbit. He was given permission to try to rescue the satellite.

In the end, two passes by the moon were made, each raising the perigee somewhat and lowering the inclination of the orbit. The remaining fuel in the satellite was used to lower the apogee back to the geosynchronous orbit altitude, but unfortunately the inclination couldn't be brought down quite to zero, so the satellite isn't in its desired orbit even today. Still, it's in an orbit where some use can be derived from it.

The satisfying conclusion to this story would be that all geosynchronous satellites are launched this way, now. Unfortunately, you can't mess with the status quo to that extent; and satellites are still, in the main, launched the old transfer-orbit way.

thad