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Climate Satellite Terra Fixed, Again
By Andrew Bridges
Chief Pasadena Correspondent
posted: 04:50 pm ET
30 December 1999
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terra_fixed_991230NASAs $1.3- billion climate satellite Terra is back on track since engineers created a pair of software patches to correct for glitches that marred the spacecrafts first days in orbit. Terra spent a week orbiting in "safe-hold" when its main control computer shut down because a unique math problem crippled the satellite. Kevin Grady, the Terra project manager at NASAs Goddard Space Flight Center in Greenbelt, Md., said engineers created a software patch to correct for the glitch that cropped up at the precise moment of the recent winter solstice. During the solstice, the satellites onboard navigational system attempted to take the arc sine of a number less than negative one while calculating the position of the sun. In trigonometry, such a number can only be between negative one and positive one. The mathematical misstep prompted the computer to temporarily shut down, forcing a backup computer to fill its shoes. If left unremedied, Terra would experience a similar glitch during every winter and summer solstice. Grady said the new software would be uplinked to the satellite after Jan. 1. Later that month, engineers will raise Terra to its final 437-mile (705-kilometer) orbit and begin to turn on its suite of five sophisticated science instruments. The instruments will spend the duration of the six-year mission measuring everything from the Earths oceans, landmasses, atmosphere and ice and how they work in concert as a global climate system. NASA engineers have also devised new software to prevent proton radiation from prompting glitches in Terras high-gain antenna. When the satellite passes through the South Atlantic anomaly, an area where the Earths magnetic field varies, stray protons were causing false signals in a portion of the antenna, again causing unexpected glitches. "Weve had a couple of problems to work through, but now the spaceship is really doing well," Grady said. NASA intends the satellite to be the flagship of its Earth Observing System. Terra was launched Dec. 18 from Vandenberg Air Force Base, Calif.
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