An IBM rides with Pathfinder

The flight computer has a single-board IBM RS/6000 reduced-instruction-set-computing
processor, the same commercial chip that IBM Corp. puts into its RS/6000 workstations,
radiation-shielded for Mars.


The 32-bit processor is software-controlled to operate at four different clock speeds.
At its fastest 20-MHz rate, the flight computer can process 22 million instructions per
second.


After controlling the spacecraft's cruise, the flight computer will control the entry,
descent and landing on July 4. It will stay especially busy during the five-minute entry
into the Martian atmosphere, said Lloyd Keith, an engineer at NASA's Jet Propulsion
Laboratory (JPL) in Pasadena, Calif., because it must precisely position the spacecraft's
heat shield and target a 25-kilometer entry corridor.


"If you hit too shallow, you skip out of the atmosphere and into space like a rock
on a lake," Keith said.


Once on the Martian surface, the IBM computer will direct a panoramic camera to
photograph the landscape and establish two-way communications between the lander vehicle
and earth-bound flight engineers.


To make sure the computer survives the hazards of arrival and harsh conditions at the
surface, JPL engineers designed aluminum thermal plates as backing for the main VME system
board and subsystem boards. "When you're in a vacuum, you can't just flip a fan
switch and blow air across," Keith said.


The added support of the aluminum plates will help the wiring and boards withstand
vibration. Under worst-case conditions, the lander might bounce as high as a 10-story
building, he said.


The Pathfinder's 40-ampere-hour potassium hydroxide batteries will deteriorate over
time and become unrechargeable, but they are expected to hold enough power to operate each
board in the flight computer at 20 MHz for four to six hours, Keith said. At 20 MHz, the
RISC processor consumes about 10 watts.


"Eventually we'll run off solar power alone-the batteries will not be there,"
Keith said. Then JPL scientists will just have to sit and wait for the sun to come up and
energize the solar arrays on the lander vehicle.


The Mars flight computer has no hard drive. It stores applications and a real-time
operating system in electronically erasable programmable ROM (EEPROM). All operations are
executed in the computer's 128M RAM. The 6M EEPROM also has backup copies of the operating
system and application software "in case there's a problem with radiation or
corruption," Keith said.


The flight computer runs the VxWorks real-time operating system from Wind River Systems
Inc. of Alameda, Calif., ported to the RS/6000. A team of eight C programmers at JPL has
written 150,000 lines of real-time application code for the flight computer.


Severe limits on space and power forced them to code more efficiently than for
earthbound computers. "If you have a hard disk, you can store data in areas already
preinitialized on the disk. We don't have that luxury," Keith said.


All operations are preprogrammed up to the point where the flight computer establishes
a two-way, line-of-sight communications link to JPL's Deep Space Network, which has
70-meter antennas at Goldstone, Calif., Canberra, Australia, and Madrid, Spain. Over that
network, ground crews will send new instructions to the computer.


The Pathfinder lander will open up and release Sojourner, a 22-pound roving vehicle
capable of communicating with the flight computer by low-voltage, ultra-high-frequency
radio signals.


The rover has a radiation-shielded, 8-bit Intel 80C85 processor and scientific
instruments to study the composition of Martian rocks and photograph surface features.


NASA has no plans to bring back the Pathfinder lander or rover after the
meteorological, geological and soil studies are complete. But future missions might
retrieve
the lander and rover to study the long-term effects of the Martian atmosphere, Keith said.


JPL will use copies of components designed for Mars Pathfinder in other space programs.
For example, RAD-6000-SC-the radiation-shielded IBM RISC chip produced on a manufacturing
line at Lockheed Martin Federal Systems in Manassas, Va.-will power the computer used in
JPL's SeaWinds and New Millennium Deep Space I programs.


The Mars Pathfinder mission, which started in 1993, has a cost cap of $150 million.
"We're going to Mars for less than it took to make the motion picture 'Titanic,'
" Keith said.


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