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Boeing
BSS 601 DirecTV 3 Experiences SCP Glitch
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DirecTV
reports that a spacecraft control processor (SCP) on DirecTV 3
switched off on May 4 and is disabled. The satellite automatically
switched to the spare SCP and the spacecraft is currently
operating normally. DirecTV is a Boeing BSS 601 bus, which has
experienced similar problems in the past on other satellites.
DirecTV 3 is part of a run of about 25 Boeing BSS (HS) 601
spacecraft built prior to mid-1997 that are prone to failure of
their spacecraft control processors.
DirecTV
has begun shifting traffic from DirecTV 3 to other DirecTV
satellites, also located at 101°W, as a precautionary measure.
DirecTV 3 will become an on-orbit spare. At no time did
DirecTV’s direct broadcast subscribers experience any loss of
service. With the recent launch of DirecTV 5, DirecTV Inc will
have four satellites in its primary slot at 101°W, one satellite
at 110°W, and two satellites at 119°W. DirecTV 7S is scheduled
for launch in 2003. DirecTV 3 was launched on June 15, 1995.
 Several
other Boeing 601 satellites have experienced the same failure,
some of which include: DirecTV 1 (DBS 1) experienced the failure
of its primary SCP on 4 July 1998. Galaxy 3R reported an SCP
failure on 21 April 2001. Galaxy 4 experienced total failure on 19
May 1998 when the backup SCP failed, the primary SCP having failed
earlier. Galaxy 5 experienced the failure of its primary SCP on 13
June 1998, with the secondary SCP failing on 22 November 2000,
resulting in total failure of the spacecraft. Solidaridad 1 lost
its secondary SCP on 27 August 2000, resulting in total failure of
the spacecraft; the primary SCP had failed on 28 April 1999. PAS 4
is reported to have had its primary SCP fail.
Earlier
BSS (HS) 601 processor defects could be the problem. Previous
failures resulted from electrical shorts caused by internal
tin-plated relay latching switches that act as on/off switches
within the Spacecraft Control Processor. Boeing Satellite Systems
(then Hughes) found that under certain conditions, a tiny,
crystalline structure, less than the width of a hair, can grow and
bridge a relay terminal to its case, causing an electrical short.
Boeing now uses nickel-plated switches and improved processes,
which appear to have corrected the processor defect on newer
satellites.
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