NASA Shuttle Atlantis Computers: Navigate Record of Reliability

The space shuttle's five general purpose computers computers, or GPCs, are slow and have little memory compared to modern home computers. On the other hand, no one straps the latest-and-greatest desktop computer inside a machine that vibrates like an old truck on a washboard road while requiring it to get a spacecraft into orbit and back safely.

In other words, when it comes to flying the shuttle, reliability means far more than performance.

"The environment of space is very harsh and unfriendly and not just space, but getting into space," said Roscoe Ferguson, a space shuttle flight software operating system engineer for the United Space Alliance.

"Something like a desktop might not even survive all the vibration. Then once you get into space you have the radiation."

Even after a major computer upgrade in 1991, the primary flight system has a storage capacity of one megabyte and runs at a speed of 1.4 million instructions per second. While this was more memory and much faster computing speed than could be achieved with the original 1970s-era Shuttle flight computers, it doesn't compare to today's desktop computers.

"The GPCs serve as the brains of the space shuttle," Ferguson said. "It’s really the heart of the control system."

The GPCs include 24 input/output links that collect the signals from the shuttle's myriad sensors and sends them to the GPCs. The computers plug the readings from the sensors into elaborate mathematical algorithms to determine when to swivel the three main engines during launch, how much to move the elevons on the wings for landing and which thrusters to fire in space to set up a rendezvous with the International Space Station, for example. That process is completed about 25 times every second.

The shuttle's computer-driven flight control system was a first for a production spacecraft. The fly-by-wire design, tested on modified research aircraft, does not have any mechanical links from the pilot to the control surfaces and thrusters. Instead, the pilot moves the control stick in the cockpit and the computers transmit signals to the control mechanisms to make them move.

The shuttle system is so dependent on computers that a fraction of a second without them could be catastrophic during the critical parts of flight.

"We have a range where if you can't control the vehicle for 120 milliseconds, you could lose the vehicle," said Andrew Klausman, the United Space Alliance technical manager for the backup flight system and multifunction electronic display subsystem. He's been working with the shuttle computers since 1986.

That's why engineers put so much time into testing and improving the system. A software change typically goes through about nine months of in-house simulator testing and then another six months of testing in a unique NASA lab before it is accepted for flight. The results of the strenuous testing regimen? Well, it has been 24 years since the last time a software problem required an on-orbit fix during a mission. In the last 12 years, only three software errors have appeared during a flight. But perhaps the most meaningful statistic is that a software error has never endangered the crew, shuttle or a mission's success.

"The current quality of this software system is really almost unimaginable," said USA's Jim Orr, who has been working with the shuttle's computer systems and software in different positions since 1978. "It's that good."

The networked computers are set up so that four are operational and one is a backup that could fly the launch and entry if the others failed. The computers receive their information from a host of sensors and actuators throughout the orbiter, external fuel tank and solid rocket boosters.

It sounds like a lot of work for any electronic device, let alone ones that are running on far less memory than a cell phone. And keep in mind that the first few dozen shuttle missions used the first-generation GPCs, which boasted memory capacities of 416 kilobytes and were a third as fast. They also weighed twice as much and it took two boxes to do the job of one of today's GPCs.

That's where the software comes in.