Tim Furniss/LONDON

The Boeing Satellite Systems (formerly Hughes Space and Communications)-built Galaxy VII communications satellite operated by PanAmSat has stopped functioning in geostationary orbit, following a spacecraft control processor (SCP) fault that has hit several sister satellites.

The second SCP on the Boeing 601 model Galaxy VII stopped working on 25 November and the spacecraft lost attitude control, causing the solar arrays to lose track of the sun, draining its electrical power. The spacecraft lost its first SCP in June 1998.

PanAmSat has filed an insurance claim for the loss of the satellite's remaining predicted operational lifetime. Galaxy VII was launched in October 1992 with an expected 12-year lifespan, providing television and other communications services to the USA and Caribbean. It was recently moved to a back-up role and replaced by Galaxy XI, which means that no customers were affected by the failure. PanAmSat is continuing to operate its 21-satellite fleet of Boeing 601 and 702 spacecraft.

Galaxy VII's sister craft Galaxy IV ceased operations in 1998 after a second SCP failure, wiping out communications and pager services across the USA. Mexico's Solidaridad 1 went out of service in August after losing its second SCP. PanAmSat 4 and DirecTV's DBS 1 have each lost one SCP. Another 19 Boeing 601 satellites launched before 1997 could also be susceptible to the problem, but have not yet experienced anomalies.

The fault is attributed to electrical shorts caused by internal tin-plated relay latching switches that act as on/off switches within the processors. Under certain conditions, tiny crystalline structures or "whiskers" can grow and bridge the relay terminal to its case, causing an electrical short.

"Customers know about it, the insurance community understands the nature and scope of the problem, but nonetheless it is unfortunate," says Boeing, adding that PanAmSat 4 and DBS 1 still have functional SCPs.

Hughes is believed to have identified the potential problem in 1995 from analysis of telemetry data. After the Galaxy IV failure in 1998, Hughes revealed that it had already begun incorporating improved relay switches and related processes on its satellites "several years ago".

It is believed that Hughes switched to nickel plating on the SCPs instead of tin in 1997.

The problem relates only to the Boeing 601 "classic" model and not the new HS-601 High Power or the Medium Earth Orbit buses, says the company. =

Source: Flight International