New positioning technique saves satellite, possibly more

Posted: October 11, 2001

  ERS 2
An illustration of ERS-2. Photo: ESA
A European Space Agency environmental satellite has been saved after all of the craft's critical positioning gyroscopes stopped functioning. The rescue was pulled off by using the ingenuity of engineers and the capabilities of the other instruments aboard the satellite.

Gyroscopes are needed to keep the ERS-2 Earth-monitoring satellite in an orientation that provides for data acquisition and images of regions around the world that are being studied by scientists.

The devices detect any positioning errors that the satellite may have. If an inaccuracy is found, spinning reaction wheels are commanded to create momentum to make up for the error.

Groups of engineers from several ESA centers and European space contractor Astrium have spent the past several years coming up with methods to continue operating the ERS-2 spacecraft once the last of its six gyroscopes failed.

Just one of the six gyroscopes was still operating in January 2000, and a failure then could have doomed the $650 million craft.

The process of inventing new space navigation techniques and writing new software to implement them was expedited with just one gyroscope still in service.

"It was risky," said Miguel Canela of the project that saved ERS-2. "We had to rearrange onboard memory to free space for the new software, written in an antiquated language from 15 years ago, when ERS-2 was first designed."

When the final gyroscope failed on January 13 of this year, engineers were poised to respond with software patches and a unique method to determine the attitude, or orientation, of the 5,535-pound spacecraft.

This ERS-2 image of Portugal shows the southern coast and the Tejo river estuary, close to Lisbon. Photo: ESA
The new ways to determine the positioning of ERS-2 include the use of the craft's Digital Earth Sensor, or DES, to use the Earth's horizon for checks on the satellite's attitude.

"After five years of use we knew we could get a lot more precise data out of the DES than just the horizon line," explained Miguel. "We cleaned up the DES signal to filter out noise, then used it to estimate pitch and roll errors."

The use of the DES left yaw discrepancies unaccounted for. But engineers recognized that they could check those errors using changes in the signal from the radar instruments aboard ERS-2.

However, analyzing those changes in the signals and turning them into useful figures could take up to three hours, hardly enough time to accurately position the satellite for science operations that could hinge on timing.

To do away with this concern, officials opted to study the behavior and pointing characteristics of ERS-2 to form a model. The model makes for faster and more precise anticipation and counteraction against any orientation errors.

"We're about to finish fine-tuning the model," Miguel said. "We operate the reaction wheels more than before, but we keep navigation errors within the limits required by all ERS-2 instruments."

"The gyro-less technique should extend the lifespan of numerous other ESA missions," ESA said in a statement.

"Most Earth or space observation satellites have their operational lifetime determined by their onboard gyroscopes. These frictionless wheels perform billions of turns. But eventually they stop or are no longer reliable," said Miguel.