TIM FURNISS / LONDON
Satellite's Failure to reach operational location and its current manoeuvre activity could indicate a control problem
The UK's Skynet 4F military communications satellite has so far failed to reach its planned operational location at 359°E in geostationary orbit (GEO), seven months after its launch and is in a westbound drift orbit. The satellite's manoeuvre activity could indicate a control problem.
The Astrium-built Skynet 4F satellite took a month to reach its orbital checkout location over 6íE, following its February launch, says the UK's Molniya Space Consultancy.
According to data from US Space Command, the spacecraft was taken off-station in April and drifted westbound towards its planned location. The following month it drifted back in an easterly direction, only to move westwards again in August.
"This activity is completely unlike what happened to the previous Skynets," says Molniya.
The UK Ministry of Defence (MoD) says that the spacecraft is "in the process of being commissioned", but its movements are very unusual compared with previous Skynet 4 manoeuvres. Skynet locations registered with the International Telecommunications Union are at 326°E (which equates to 34°W), 359°E, 6°E and 53°E. The 6°E location is used for in-orbit testing and checkout after which the satellite is moved to another operational location. The spacecraft are able to move to other locations if necessary.
"The satellites do not have to be accurately positioned over these locations as is the case with civil communications satellites and they do not operate north-south station keeping, but they do control east-west movements," says manufacturer Astrium.
Skynet 4B, launched in 1988, but no longer operational, reached its planned orbital position within three weeks.
Skynet 4A, launched in 1990, spent about five weeks in a checkout orbit, but was on-station within eight months. Skynet 4C, also launched in 1990, took just one month to reach an operational location. Skynet 4D and 4E, launched in 1998 and 1999 respectively, both took a month to reach a checkout location and after three months were at their operational locations.
The earlier Skynet satellites spent a long period over locations, but Skynet 4F has never been stabilised at any location. Skynet 4F was initially located over 6°E for about a month and then manoeuvred to drift to the west. When it reached 2°E, a further manoeuvre started an eastbound drift, reaching 16°E. Another manoeuvre reversed this drift and later the satellite was over 12°E, and continuing to drift westwards once more.
Source: Flight International
