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ALEXIS satellite marks fifth anniversary of launch
LOS ALAMOS, N.M., April 23, 1998 -- Los Alamos National Laboratory's ALEXIS satellite marks its fifth birthday Saturday. To celebrate, the durable craft will continue circling Earth and compiling observations of celestial emissions of extreme ultraviolet light with its six coffee can-sized telescopes.
Over the years, ALEXIS has been a training ground for Los Alamos staff in operating -- and rescuing -- a satellite and a proving ground for techniques and technologies used in or proposed for subsequent space missions.
A radio experiment aboard ALEXIS has captured radio signals that led to discovery papers on mysterious atmospheric discharges and allowed a host of students to cut their teeth on a scientific project.
And the satellite has compiled more than four years of unique observations of extreme ultraviolet emissions from the sky, though a complete analysis of these data is ongoing.
Launched April 25, 1993, the $17 million Department of Energy-funded satellite has lasted well beyond its nominal one-year mission to demonstrate its telescope and radio-receiver technology for nonproliferation applications and past a three-year lifetime engineering estimates gave it. After five years it still has most of its capabilities intact, even though the satellite is starting to show signs of wear and tear.
"We're seeing normal and expected degradation at this point, but it's less than we would have originally expected after five years in orbit," said Diane Roussel-Dupré, ALEXIS project leader. "It's very satisfying to see it still working as well as it is."
ALEXIS' rechargeable batteries aren't as potent as they once were, its four solar panels don't produce quite as much energy as they used to, and a bad memory chip garbles a portion of the data the satellite regularly transmits to a ground station at Los Alamos.
The satellite's scientific packages -- the Blackbeard radio experiment and the Array of Low-Energy X-ray Imaging Sensors telescopes -- still perform as capably as when they were launched.
The Blackbeard experiment, however, has been superceded by Los Alamos' FORTÉ satellite, and only occasional experiments are planned for Blackbeard in the future.
ALEXIS had a rocky beginning. One of the satellite's solar panels tore free of its hinge attachment during launch; a subsequent analysis showed the hinge had probably been damaged during ground testing, and launch vibrations were the last straw. Only electrical cables and a guide wire kept the panel attached.
Controllers could not establish contact with the satellite, and for some time it was feared lost.
Six weeks after launch ALEXIS sent a brief radio signal, telling the ground crew it was alive. Four weeks after that satellite operators successfully communicated with the craft, starting the slow process of bringing it under control, determining its condition and readying it for scientific operations.
Project officials attributed ALEXIS' survival to the robust design of the satellite bus -- the foundation of the satellite that supports the scientific packages, communications and telemetry -- provided by AeroAstro Corp., a small space-technology company that worked closely with Los Alamos.
"A key to ALEXIS' durability has been the innovative, low-power bus design provided by AeroAstro and the company's innovations in the telemetry system, which made ALEXIS a simpler spacecraft to operate," said Jeff Bloch, one of the scientific investigators for ALEXIS. Bloch also credits the Los Alamos engineering team that integrated all the components of the satellite and prepared ALEXIS for launch and the software team that helped interpret what the satellite was doing and modify computer codes as necessary.
Another key was the dedication of the satellite operators who nursed the craft back to health. This required attending satellite passes in the middle of the night, over weekends or on holidays and sacrificing a lot of personal time. In fact, the satellite operators kept to this round-the-clock schedule until last November, when the team made automated contacts part of the operation.
The broken solar panel imparted a wobble to the spin-stabilized spacecraft that had to be precisely modeled so the photons captured by the six telescopes could be mapped back to their originating location on the sky.
The final computer software for decoding the telescope data was recently completed and used to process all 49 months worth of archived data, stored on more than 150 CDs. The latest software more precisely models the satellite's orientation and produces sharper images than earlier versions.
To know precisely the strength of the emissions from the sky and interpret the data in the all-sky map, however, the team must calibrate each telescope accurately so they can separate instrumental effects from the signal measured. Bloch estimates this task will be completed in the next six weeks.
"When the calibration watershed happens, we expect that the science papers will flow," said Sandy Fletcher, a member of the ALEXIS team.
The Blackbeard radio experiment, which doesn't care which way the satellite is pointing, early in the mission provided data that led to the first published reports of atmospheric discharges dubbed "trans-ionospheric pulse pairs," or TIPPs. No one has yet conclusively identified what atmospheric mechanism generates them.
Blackbeard demonstrated technology for studying a wide band of radio frequencies and capturing signals of interest amidst a noisy radio background, important for nonproliferation work
A more advanced version of Blackbeard's technology was launched last August on FORTÉ, or Fast On-orbit Recording of Transient Events. In addition, much learned from ALEXIS' operation has been useful to FORTÉ.
"ALEXIS has been a pathfinder for a lot of technology ported directly over to FORTÉ," Roussel-Dupré said. ALEXIS' ground station software, for example, was transferred directly to FORTÉ's operation. A ground station set up in Fairbanks, Alaska, and tested through ALEXIS contacts is now a principal link for FORTÉ. And useful tricks of the trade such as replaying recorded data to train operators and ways to use real-time state-of-health measurements also have been adopted from ALEXIS.
ALEXIS also has provided beneficial educational experience to nearly 30 students, undergraduate and graduate. For many, it was their first introduction to a research project and helped them decide on an educational or career path. Some have gone onto to jobs in the aerospace industry or onto graduate schools. "Their resumes are considerably strengthened by their experience with ALEXIS," Fletcher said.
Fletcher worked on ALEXIS while a graduate student at the University of New Hampshire, and has since joined Los Alamos' scientific staff. "One of the things that really attracted me to this project is that we talk to the satellite four times a day; it's all done here," she said. Students gain hands-on experience and carry a lot of responsibility for the project.
"The students have made significant contributions to the data analysis and code generation. They've also provided needed hands-on support for satellite control," Roussel-Dupré said.
The satellite's budget for the current fiscal year is $615,000 for scientific studies and operations. Eight people, of which only the three students are fulltime, make up the project team.
Roussel-Dupré hopes to keep ALEXIS going at least another year, into the peak of solar activity, to see the effects of increased numbers of charged particles in the orbital environment on the satellite's hardware and ability to survive given the radiation dose it has accumulated after five years.
Keeping ALEXIS going also helps gather information about long-term performance of various systems that is useful to the small satellite business. The team is learning how to coax more performance out of an aging satellite. The ALEXIS groundstation also has been studied by NASA as a possible model for controlling small satellites. And the ALEXIS experience has enabled Laboratory scientists to develop more competitive proposals for other missions.
"When DOE funded this project they had two questions: Would the telescope design work, and could Los Alamos build and operate a satellite? The ALEXIS team proves every day that the answer to both questions is, 'yes,' " Fletcher said.
Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.
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