Boeing 601 (f.k.a. Hughes HS 601)

It is unclear how many HS 601 models are equipped with tin-plated relays, which are the cause of satellite control processor (SCP) malfunctions. Satellite manufacturer Boeing Satellite Systems (formerly Hughes Space and Communications) would not disclose the figure, saying this was "proprietary information."

The manufacturer however admitted that that SCPs on Boeing 601 spacecraft launched prior to August 1997 contain tin-plated relay switches that can experience electrical shorts when several factors are concurrently present. If this applied to all 601-model satellites launched until July 1997, at least 30 of HS 601s in orbit would be equipped with the dreaded tin-plated relays.

The SCP is the on-board computer that controls the satellite. There's also at least one backup SCP aboard every satellite.

The relay serves as an on/off switch within the SCP. Under certain conditions, tiny crystalline structures (known as "tin whiskers") can grow and bridge a relay terminal to its case, causing an electrical short which blows fuses.

Known satellites with problems related to tin-plated relays

Name Launched First SCP failure Second SCP failure
Optus B1 [Optus] 13 August 1992 21 May 2005 -
Galaxy VII [PanAmSat] 27 October 1992 13 June 1998 22 November 2000
Galaxy IV [PanAmSat] 24 June 1993 (not caused by 'tin whiskers') 19 May 1998
Solidaridad 1 [SatMex] 19 November 1993 28 April 1999 27 August 2000
DBS-1 [DirecTV] 17 December 1993 4 July 1998 -
PAS-4 [PanAmSat] 3 August 1995 3rd quarter 1998 -
Galaxy IIIR [PanAmSat] 15 December 1995 21 April 2001 15 January 2006
DirecTV 3 [DirecTV] 9 June 1995 4 May 2002 -

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