Three months after landing on Mars and zipping around via radio control to explore the red planet's surface, the two Mars rovers have undergone long-distance software upgrades to resolve some problems and improve their performance.
The rovers Spirit and Opportunity, launched by NASA last June and July, are equipped with specially treated computer gear built to withstand the intense heat, cold and radiation in space. At launch, the rovers included Release 7 of their operating system software, which was upgraded to Release 8 just before they landed on Mars.
Last week, Release 9 of the software was uploaded to each rover, said Mark Adler, mission manager for the Spirit rover. "Whenever you write a big piece of software, there are going to be things you want to fix."
One flaw mission scientists wanted to correct is a heater on Opportunity that is always turned on, Adler said. The problem is the heater is a huge user of electrical power, which has to be generated by solar panels on the craft. The only way to shut off the defective heater was to turn off all power to the rover, an option frowned upon by scientists, who worried that the rover might not restart after it was turned off.
To solve the problem, mission managers decided to include a "sleep" mode in the updated operating system. Sleep mode doesn't turn off the heater, which is attached to Opportunity's robotic arm, but it puts the rover into a deep sleep so electricity can be conserved without endangering the vehicle's continued operation. In sleep mode, the rover goes into an inactive state until sunlight strikes its solar panels each morning, Adler said. "Fortunately, the sun is very reliable," he said.
Spirit, which doesn't have a problem with the heater on its robotic arm, will also benefit from the operating system upgrade, Adler said. Release 9 includes major upgrades to the mobility instructions for both rovers that will allow them to cover larger distances. The travel distance of the rovers will increase from about 43 feet a day to about 105 feet thanks to improvements in calculations and upgrades to the resolution of images the rovers transmit to scientists to show what is in their path. The average speed of the rovers is less than half an inch per second.
Upgrading an operating system from some 65 million miles away is no minor project, Adler said. Duplicates of the rovers were used as testbeds on Earth. The project also involved duplicate electronics, control systems and test uploads to be sure that the updates would load properly and work from the start.
"We did a lot of testing of all that," he said. "Once we did that, we were comfortable with it" and were ready to send the updates via radio signals. The new software was transmitted at 2Kbit/sec. over three days until all of the 8MB of files had been uploaded. The updates were onboard the Spirit as of last Monday and on Opportunity by Wednesday. They were loaded onto spare partitions on their hard drives. Once the uploads were complete, scientists booted the rovers into the Revision 9 software and went through a series of final checks to ensure the work was successful.
The rovers use an embedded VxWorks operating system from Wind River Systems Inc. in Alameda, Calif. The operating system managed the trajectory, descent and landing of the spacecraft as they approached Mars and also runs operations control, data collection and communications of the missions.
The VxWorks operating system was embedded in a specially prepared, radiation-hardened 20-MHz PowerPC CPU installed on each of the rovers, along with 128MB of RAM. The hardware was cutting-edge when it was chosen in the mid-1990s, but it had to be treated to ensure its reliability in the radiation of deep space -- a process that takes five to 10 years.
Earlier this month, NASA extended the planned three-month rover missions, giving them up to five extra months of exploratory assignments.
So far, everything is working very well on the Martian surface, Adler said. "It's a new discovery, a new place every day. It's going to be hard to turn them off if they keep working."