MISSION CONTROL CENTER STATUS REPORT #15
STS-91
TUESDAY, JUNE 9, 1998 - 6:30 p.m. CDT

Following four busy days of docked operations with the Mir station,
Discovery's crew enjoyed an afternoon of off -duty time today as they
move into the home stretch of their flight.

This morning's activities included work with the Orbiter Space Vision
System (OSVS) experiment, a system that creates a graphic digital
display of the payload bay environment on laptop computers.  The SVS
equipment will be used during the assembly of the International Space
Station to provide astronauts operating the Shuttle's mechanical arm
with precise alignment information as station components are brought
together.

The crew continued stowing items that were transferred over from the Mir
for return to Earth and monitored some of the science experiments being
carried on the flight.  Additional tests were made of the Spacehab
Universal Communications System (SHUCS), although the system is still
experiencing problems with the ability of its antenna to find and hold
communication with the satellites that support the experiment.

The Alpha Magnetic Spectrometer payload, located in Discovery's payload
bay,  was powered off for a period of time earlier today due to concerns
that the payload might be getting too warm.  After an adjustment was
made to the orientation the Shuttle was flying in, thermal conditions
improved.  The AMS was subsequently powered back up and high rate data
collection has resumed.  The 3 1/2 ton instrument is an innovative
particle collector hoped to provide scientists with insight into the
existence of dark matter and antimatter in the universe. Aboard
Discovery, astronauts Franklin Chang-Diaz and Janet Kavandi are
monitoring the experiment.

While the crew enjoyed some time off, flight controllers reviewed a
problem seen with the Shuttle's onboard computer system.

Last night while the crew was asleep, General Purpose Computer (GPC) #1
was supporting guidance, navigation and control requirements and GPC #4
was supporting systems management.  Flight controllers noted that on GPC
#1, within the software program supporting the Global Position Satellite
operation, there was a growing discrepancy between where the GPS system
was telling the Shuttle it was located and where the GPC had calculated
the Shuttle should be.  Resets of the GPS system software did not
resolve the discrepancy.  The cause of the discrepancy is not yet
understood.

A decision was made to bring up a different computer to support Shuttle
guidance and navigation with the GPS portion of the program turned off.
Since doing that, all guidance and navigation activities have been
nominal.  The GPS portion of the navigation system remains powered off.
Consideration is being given to installing a software patch for the
guidance support program that would block out the GPS portion of the
guidance software.

Discovery's crew will begin an eight-hour sleep period at 7:06 p.m. this
evening and receive a wake-up call from Mission Control  at 3:06 a.m.
Central time tomorrow to begin their ninth day in space.

Discovery is currently in a 207 by 195 nm orbit, circling the Earth once
every 91 minutes.

The next STS-91 status report will be issued at about 6 a.m. Central
time Wednesday.