In a worst-case scenario, gyroscope failure could render the Landsat 7 Earth observation spacecraft useless by the end of 2007. While maintaining that the probability of such a catastrophe is unlikely, NASA and the U.S. Geological Survey (USGS) plan to begin uploading software this summer that will allow operators to retain control of Landsat 7 if the remaining gyroscopes fail, officials said.

“That is a worst-case scenario chart,” said Ray Byrnes, the USGS liaison for satellite programs. “It opened people’s eyes very wide, but it was done months ago, and there has been no degradation seen on the other gyroscopes since then.”

Landsat 7, along with the much older Landsat 5 satellite, is responsible for collecting land observation data of Earth and maintaining the continuity of the 30-year-old archive of such data. Landsat 7, launched in 1999, already is operating past its intended design life, and the USGS turned off one of the spacecraft’s three gyroscopes in May after receiving faulty readings from the hardware for nearly a year, officials said.

Gyroscopes are flywheels that stabilize a satellite’s attitude, allowing ground controllers to point the spacecraft to perform the intended mission. Satellites normally carry several gyroscopes to help maintain their position relative to the Earth and sun.

The failure assessment, completed in July, said Landsat 7 is 90 percent likely to succumb to a mission-ending gyroscope failure before 2008. The finding was presented to NASA and USGS officials in February by the Landsat Data Gap Study Team. The panel was commissioned by the agencies to recommend ways to continue the collection of Landsat data in case the two operation satellites fail before the launch of a new sensor in 2009.

Since shutting down the faulty gyroscope, the USGS has been developing software that mimics the function of mechanical gyroscopes, said Ron Smilek, the USGS flight systems manager for Landsat 5 and Landsat 7.

Officials with the USGS think the software will provide a safety net in case either of the remaining gyroscopes fails, lessening the chance of a catastrophic Landsat 7 failure.

“There is no reason to believe we will ever lose another gyroscope, but what we are undertaking is development to mitigate the risk of any potential loss,” said Steven Covington, an engineer at the Aerospace Corp. on contract to USGS for the Landsat program.

Gyroscope failure is not unique, and the USGS has consulted operators of other satellites that use software as a substitute for mechanical gyroscopes, Covington said.

The U.S.-European Solar and Heliospheric Observatory, launched in 1995, has been operating for nearly five years using software developed by Astrium SAS of Velizy, France. NASA’s Far Ultraviolet Spectroscopic Explorer lost its primary control system in 2001 and has been using similar software that allows for satellite control using three computers aboard the spacecraft to take control using stars as reference points.

The Aerospace Corp., based in El Segundo, Calif., also has created a software package that emulates the function of a hardware gyroscope. The software, developed for use on a U.S. government national security satellite and dubbed “pseudogyroscope,” was used in 1999 to extend the life of the U.S. National Reconnaissance Office’s Space Technology Experiment satellite after the craft’s primary and secondary gyroscope failed.

“Our solution for Landsat 7 is not unique,” Covington said. “The approach we’re taking will use sensor data from other devices onboard to replace what would be lost if we lost another gyroscope. This is basically the same approach used by [other operators] as risk mitigation for their missions.”

The software, which the USGS hopes to upload to Landsat 7 in June, will gather data from the spacecraft’s three magnetometer’s, said Tom Cook, lead engineer of Landsat 7 program. The magnetometers sense changes in the magnetic field around the spacecraft, and the software will substitute that data for what the gyroscopes normally provide, he said.

The first phase of the software fix, which the USGS hopes will be operational in July, will not be used to control Landsat 7 immediately, Covington said. The software will act as a backup, ready to take over control in case another gyroscope fails, he said.

However, there is still work to be done over the next year to ensure that operators will have complete control of Landsat 7.

The first phase of the software only will allow engineers to keep basic control of Landsat 7, Covington said. Additional software is still being developed will allow the USGS to continue collecting Landsat data, he said.

“Landsat has tighter pointing requirements than some of the other missions that have implemented this same type of this capability,” Covington said. “Our first responsibility is to have the ability to maintain control for the long-term safety of the spacecraft. We will add additional enhancements to the system that will allow for maneuvering, maintaining orbit and eventually precision pointing that will allow us to continue imaging.”

The USGS hopes to have all of the necessary software in place by 2006, Covington said.

The USGS development team includes Landsat 7 manufacturer Lockheed Martin Corp. of Bethesda, Md., NASA and Honeywell Technology Solutions Inc., which provides backup operations and ground system support for the Landsat 7 via its DataLynx satellite operations center in Columbia, Md.