Veteran software makes it to Titan

This month's successful landing of the Huygens probe on Saturn's moon Titan relied on precise software programmed almost a decade ago. But tiny coding errors nearly prevented the craft from transmitting any of its findings back to Earth.

Today, software is central to every space mission, from the International Space Station orbiting the Earth to probes flying to the outer reaches of the solar system.

The environment in which these applications must operate are much more demanding than many of the functions they serve on Earth: systems coded today must survive the rigours of space for several years before reaching their destination.

After a four billion kilometre journey through the Solar System that lasted almost seven years, Huygens plunged into Titan's atmosphere on 14 January, landing safely on its frozen ground at 14:45 GMT.

Cassini, Huygens' 'mother ship' passing by 60,000km above, received data from the surface of Titan for just over an hour before it disappeared over the horizon and out of radio signal range.

More than 474Mbs of data were transmitted, including some 350 pictures collected during the descent and on the ground, giving the first insights into the most distant alien environment so far visited from Earth.

But a single line of software code omitted from the craft's onboard communications system resulted in the failure of one of its two channels of communication, meaning data on only one channel was radioed back to Cassini.

Instead of having two redundant channels duplicating all the data being sent back - giving it a back-up should one fail - the European Space Agency (ESA) mission controllers decided to duplicate the science data, but not the images, to get as many pictures as possible back in the short time they had.

The software error resulted in Cassini not turning on its receiver for channel A, which meant only half the 700 received pictures were beamed back.

While the error is the subject of an ESA inquiry, another problem discovered years before - while Cassini was travelling to Saturn - nearly resulted in no data coming back at all.

Back in early 2000, ESA head of systems and projects support section Boris Smeds, acting on something of a hunch, discovered that Cassini's receiver was in danger of scrambling Huygens' data beyond recognition, because of a design flaw in the onboard telemetry system's software.

'We have a technical term for what went wrong here,' said John Zarnecki, one of Huygens' principal investigators from the UK's Open University, at the time. 'It's called a cock-up.'

The flaw centred on the radio receiver's inability to cope with distortions in the frequency - caused by the a phenomenon known as Doppler shift - with which Huygens would transmit data back to Cassini. It would have rendered the data unintelligible.

Unfortunately, unlike the descent control software for Huygens programmed by LogicaCMG, the radio receiver's software was hard-coded in the instrument's firmware and could not be changed. 'It was hard-coded, and there was no access to change this,' says Smeds.

To solve the problem, mission control eventually had to alter the planned trajectory of Cassini, manoeuvring the craft into a position that would negate the distorting effects of the Doppler shift problem - so that today we can see the first striking images from a moon millions of miles away.

But as an increasing number of functions and controls in space missions become software-controlled, ground control is gaining more and more flexibility to make changes when things do go wrong.

'You can have systems that can be reprogrammed, even in-flight. It's a good feeling to know that you can correct things, particularly if there's a long mission duration and a lot of things may be discovered after launch,' says Smeds.

'Of course you have to be very careful when you re-program a thing, to make sure that you can change it if you make an error.'

And, as the Titan mission has demonstrated, the margin for error is slim indeed.

Mission-critical software

The Huygens probe, named after the astronomer who discovered Titan, was developed by ESA and has been attached to the Cassini spacecraft during its journey through space. Its instruments were designed to gather information about what makes up the Moon's atmosphere, which, like the Earth's, is mostly made up of nitrogen, but at a frostier -180 degrees Celsius.

Two of the probe's six instruments - the surface science package and the Huygens atmospheric structure instrument - were built by UK researchers from the Open University, while the software that co-ordinated the probe's descent onto Titan was developed by LogicaCMG.

For the programmers behind the scenes, mission-critical doesn't get much more critical than this: a single-shot mission in a hostile environment without the possibility of failure recovery.

LogicaCMG business development manager for space Pat Norris says rigorous project management and testing was absolutely vital for the mission's success.

'Once these things are launched, the only thing you can change is the software. But when you change software, you may introduce errors as well as improvements, so we had to make sure we could test whatever we had to change,' he says.

'The technology may have changed from 10 years ago, but the management of such a project has not. The lessons we learned then about how to make sure a project works first time a billion miles away, which requires a certain discipline, exhaustive testing and so on, are lessons that now apply to a range of other projects.'

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