TRACE Spacecraft Slewing Trick

This is really neat and they got it to work.  This gives us a low energy cost alternativeto the established procedures and will certainly expand mission parameters on alot of craft.

Needless to say, orbital work has been progressively improving over thedecades and here we have another important energy saver.

Bravo for a nice piece of work.

TRACE Spacecraft's New SlewingProcedure

by LoriKeesey

Greenbelt MD(SPX) Dec 28, 2010

The fastest path between Point A and Point B is a straight line. Not so fast,says a teamof scientists and engineers who recentlydisproved this commonly accepted notion using a NASA satellite that had notmoved more than 15 degrees during its 12-year mission studying the Sun.

In what may seemcounterintuitive even to engineers, a team from the Naval Postgraduate School(NPS) in Monterey, Calif., Draper Laboratory in Houston, Texas, and the NASAGoddard Space Flight Center in Greenbelt, Md., proved that the spacecraftactually rotated faster to reach a particular target in the sky when it carriedout a set of mathematically calculated movements.

These maneuvers lookedmore like the steps dancers would perform doing the tango, the foxtrot, oranother ballroom dance.

"That spacecraftwas dancing on the sky," said Osvaldo Cuevas, the mission director ofNASA's Transition Region and Coronal Explorer (TRACE), the spacecraft thatcarried the experiment before NASA decommissioned it in September. Had TRACEsported a pair of legs, its steps would have traced roughly the pattern of afive-point star.

Until the spacecraft'sdebut on NASA's version of "Dancing with the Stars," TRACE staredsteadily at the Sun producing millions of images of the corona, the Sun's outeratmosphere that extends millions of kilometers into space and is nearly 200times hotter than the Sun's visible surface.

Benefits to Current and Future Missions

The team's first-ever time-optimal slewexperiment was more than just an interesting performance or a theoreticalquestion posed in a technical journal.

The team's findingsare particularly relevant to engineers designing futureEarth-observing, astrophysics, and reconnaissancesatellites that must image one object and then quickly reorient itself toobserve another in a completely different location. Just as important, theexperiment showed that existing spacecraft can "do things that they aren'tdesigned to do," said Nazreth Bedrossian, a Draper scientist who played apivotal role in the experiment.

"The payoff is inthe pointing agility and being able to collect more science," explained Neil Dennehy, a Goddard engineerwith the NASA Engineering and Safety Center (NESC). "NASAwill benefit and so will industry." Currently, NASA engineers directspacecraft to follow a straight line when slewing to different locations in thesky. While it may be the shortest path, it is not the fastest, as theexperiment showed.

Although the findingsmight surprise some, they did not astonish scientists from NPS and Draper.Actually the discovery that a straight line is not the fastest path between twopoints was made in the early 1700s by Swiss mathematician Johann Bernoulli. Hediscovered that a sliding bead traveling from one point to another would movefaster if it followed a curved line and allowed gravity to assist in theacceleration.

The challenge then wasnot whether it was faster moving along a non-linear path, but rather what thatpath might look like. "Over the years, we forgot that the straight lineisn't the best solution because we didn't know how to calculate the fastestpath. We didn't have the tools," said Mark Karpenko, an NPS researchscientist and lead engineer in the experiment.

Similar Movements Demonstrated on Space Station

It was a conundrum that NPS Professor MichaelRoss eventually solved when he and his colleagues developed an optimal-controlsoftware package, called "DIDO," named for the ancient queen of Carthage who solved achallenging optimal-control problem even before the invention of calculus.

In fact, Ross,Bedrossian, and his colleague, Sagar Bhatt, used DIDO four years earlier to maneuverthe International Space Station 180 degrees without expending a drop of fuel.

"We became knownas the people who can take this kind of an idea and make it fly," Rosssaid. "What needs to be emphasized is that the software used forsolving the Space Station and TRACE maneuvers isexactly the same. Although the Space Station experiment demonstrated a minimum-fuel maneuver andTRACE a minimum-time maneuver - maneuvers that are quite different - themathematics are similar."

All they needed was achance to demonstrate DIDO's prowess by carrying out time-optimal maneuvers ona real satellite.

The stars had alignedin their favor. In the spring, NESC's Dennehy and Senior Engineer KennethLebsock learned that the Space Science MissionOperations Office (SSMO) at Goddard planned to decommission TRACE in September.Before doing so, SSMO management offered experimenters an opportunity to usethe spacecraft as an orbital testbed to investigate new ideas.

"I talked withthe people who worked on TRACE's design, and I asked them what they would liketo do if they could it all over again," Lebsock recalled. "The guysthought it would be neat if we could uplink maneuvering commands to see ifTRACE could carry out an optimal slew" - a job the spacecraft was neverdesigned to do, let alone quickly.

Two-Month Turnaround

NESC knew whom to call. Usually it takes atleast a year to develop a solution, Ross said. The team, however, had only twomonths to complete the job. "I called up Naz (Bedrossian) and I said, 'Iknow you made it happen with the space station. Do you think you can make ithappen this time?'"

The answer wasobvious, Bedrossian said. "When do you get an opportunity to test yourideas on an actual satellite? For engineers, a flight test is like theOlympics. It's what you train for."

While Bernouillicalculated the optimum path using gravity to its best advantage, the team hadto solve a pattern that exploited the spacecraft'smass and its four reaction wheels - a type offlywheel device that rotates spacecraft by very small amounts to keep itpointed at a star.

"We have beenworking on time-optimal maneuvers for other types of spacecraft, but never areaction wheel system," Karpenko said.

Had the team opted totake TRACE in a straight line from one point to another, for example, it wouldhave had to push one of the wheels to full saturation, with the other three notworking as hard, Lebsock explained.

That means thespacecraft could not go any faster than the speed of the one wheel. The questthen was to determine mathematically the most efficient pattern where all fourwheels worked equally hard.

By Aug. 10, the teamwas ready to begin the first of its 20 tests. Goddard engineers uploaded theteam's series of pointing commands, starting conservatively with a 10-degreeslew and then back to the starting position. By the fourth week, TRACE hadslewed over 90 degrees off the Sun line. It maintained that position for aboutsix minutes before slewing back.

"That maneuverwas interesting because it really demonstrated what we wanted to show,"Karpenko said. "We can actually reorient the spacecraft more quickly thanby using the conventional techniques."

"This was abouttaking a risk to find something and learn something new," Cuevas added."Not only were the movements faster than standard maneuvers, they alsoconsumed less than half the electrical power of a standard movement. This couldtranslate into significant savings for NASA, to say nothing of the improveddata collection."

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