Dave Steitz
NASA Headquarters
(Phone: 202/358-1730)

Jim Sahli/Mark Hess
Goddard Space Flight Center
(Phone: 301/286-8955)

Dec. 29, 1999 — 4:00 p.m. EST


Spacecraft operations under the control of the Spacecraft Controls Computer (SCC1) resumed yesterday. All subsystems
on-board the spacecraft continue to operate extremely well and as expected.

Flight software engineers were successful in recreating the scenario that resulted in the entrance into “safe-hold” on the fourth
day of the mission. The spacecraft simulator (SSIM) was loaded with the identical conditions that were present on-orbit when
the spacecraft controls computer initialized and handed control over to the safe-hold processor. The simulator duplicated the
safe-hold entrance at the exact second in which it occurred on-orbit.

“Interestingly enough, the time was within one minute of the time in which the winter solstice occurred,” said Kevin Grady, Terra
Project Manager at NASA’s Goddard Space Flight Center, Greenbelt, Md. “When the computer dump was analyzed, the
navigation software was identified as having an anomaly.”

Grady said analysis of the data revealed that at the winter solstice, the navigation software attempted to perform a mathematical
function (arcsine of a number slightly less than -1.0) which it was incapable of doing. This occurred as a result of the
combination of the unique location of the Sun at the winter solstice and the manner in which mathematical computations are
handled. A simple software fix will correct this problem which can potentially occur twice during the solar cycle (winter and
summer solstice).

“A simple flight software fix has been developed and tested, and will be loaded after the New Year,” Grady said.

A small piece of software logic, known as a telemetry monitor, has been developed to deal with the occasional high gain
antenna gimbal safings that occur in the South Atlantic Anomaly (SAA).

In its normal operation, Terra’s flight computer commands the high gain antenna to point at one of four on-orbit TDRSS
spacecraft. TDRSS is used to relay commands and data to and from Terra. Grady indicated that on several orbits, as the
spacecraft was passing over the SAA, the high gain antenna program mode stopped and the antenna put itself into a gimbal
safing mode. He said this telemetry monitor has been uplinked and activated. The high gain antenna will now be automatically
restarted if a safing occurs over the SAA.

“An interesting feature of this behavior so far is that the resets only occur during the nighttime or ascending passes through the
South Atlantic Anomaly,” said Grady. He added that Terra Project Officials will meet with space radiation physicists at
Goddard after the holidays to more fully investigate this phenomena.

“The Terra Operations Team has done an outstanding job resolving these two anomalies while dealing with the nuances of a
transition from a pre-launch operation to one which is operational in nature,” said Kevin Grady, Terra Project Manager.
“Science data is just around the corner.” After several weeks of outgassing, the instruments will be turned on in the later part of

Terra is managed by NASA’s Goddard Space Flight Center, Greenbelt, Md., for NASA’s Office of Earth Science,
Washington, DC. A goal of the Earth Science Enterprise is to expand knowledge of the Earth System, from the unique vantage
point of space. Earth Science Enterprise data, which will be distributed to researchers worldwide at the cost of reproduction, is
essential to people making informed decisions about their environment.

More information on Terra also is available via the Internet at