SOHO gets new, more robust computer software
28 September 1999Just like personal computers on Earth, ESA's Solar and Heliospheric Observatory is getting a software upgrade of its own. After SOHO vanished in space in June 1998, engineers on two continents struggled for several weeks to regain control of the spacecraft. In December1998, the loss of the last working gyroscope caused major orientation problems and rapid fuel depletion. But once again, engineers and ground controllers put the satellite back in working order. By February 1999, an unprecedented solution - emergency software rushed up to SOHO - allowed it to reorient itself. Yet that fix, which has been working perfectly ever since, was only meant to be temporary.
"It was the equivalent of an emergency-room procedure to stabilize a patient," said Michel Verdant, ESA's SOHO Programme Manager. "But now we have new software. This is the medicine we need to cure the patient once and for all."
Indeed, the new software will make SOHO much more robust and reliable in case orientation problems ever happen again, Verdant said. On 27 September, ground controllers beamed the new software up to SOHO. Both software and spacecraft were developed mainly by prime contractor Matra Marconi Space. Now even if SOHO loses sight of the guide star it needs to orient itself to remain stable on all three axes, it won't have to resort to the Emergency Sun eacquisition mode(ESR), which up to now automatically meant halting all scientific observations.
"It's a major accomplishment to be out of this situation," said Bernhard Fleck, ESA's SOHO Project Scientist. "Before this new software, losing the guide star would have meant that we had no way of measuring the spacecraft's roll rate or of controlling it."
In case of trouble, the new software will now allow engineers to determine SOHO's drift by measuring the slight changes in the speed of the so-called momentum wheels. In a way, these wheels, which are normally used to impart the necessary force to point the spacecraft, will be used as if they were gyroscopes and to return SOHO to its correct orientation.
"It is very similar to using the gyroscopes, but here we are using the whole spacecraft as a gyroscope," said Verdant. And this sets a real space record. For the first time, a spacecraft is operating in space without the gyroscopes it was designed to rely upon.
To avoid possible problems with other programs already installed, the software was uploaded to a part of the spacecraft memory never before used. "The software is something we must always respect," said Francis Vandenbussche, SOHO System Engineering Manager. "I am very confident everything will work fine."
"This new software will provide an extra safety net to the satellite." In fact, if SOHO now lost its guide star, the spacecraft would automatically switch to others loaded in its memory. Should that method fail, control of the spacecraft would be regained by using the momentum wheels as reference. And that would still allow the satellite to continue its observations.
Finally, there will always be ESR mode, as a last resort. Besides halting all observations, it takes several days to bring the spacecraft back to top gear. "With this new software, we'll sleep much better," said Verdant.
About a week of so-called flight commissioning will be required before scientific observations - which had been partly suspended - can resume in full.
"What really amazes me is the competence and skills of these engineers who are capable of coming up with solutions to solve incredible problems," said Fleck.
Barring unforseen problems, SOHO should continue its mission until at least 2003, seven years after its launch, Fleck said. "But what I would really like to see is the spacecraft carry out observations for a full solar cycle of 11 years."
SOHO, the most sophisticated satellite ever to study the Sun, is part of a joint programme between ESA and NASA.