Skynet-1A Britain's first comsat

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https://www.bbc.com/news/articles/cpwrr58801yo

Somebody moved UK's oldest satellite, and no-one knows who or why
13 hours ago
Jonathan Amos

Someone moved the UK's oldest satellite and there appears to be no record of exactly who, when or why.

Launched in 1969, just a few months after humans first set foot on the Moon, Skynet-1A was put high above Africa's east coast to relay communications for British forces.

When the spacecraft ceased working a few years later, gravity might have been expected to pull it even further to the east, out over the Indian Ocean.

But today, curiously, Skynet-1A is actually half a planet away, in a position 22,369 miles (36,000km) above the Americas.

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[StraumliBlight]

Stuart Eves' blog (referenced in the BBC article) goes into more detail. [2019]

The East-West motion is an oscillation about one of the gravitational “wells” which are located at about 75 East or 105 West on the geostationary ring. The period of this oscillation is around 800 days, and the amplitude of the “swing” is related to the original location of the satellite relative to the bottom of the well. So, if Skynet-1A had failed at its operational location of around 40 East, we would now expect it to be oscillating by +/- 35 degrees either side of 75 East.

Except that it isn’t. According to the UK Registry , Skynet-1A is currently sitting very close to the bottom of the other well at 105 West, oscillating by just a couple of degrees.  This initially looks like a case of mistaken identity – after all how could a UK satellite end up in a slot half a world away, to the west of the Americas?

The hypothesis that the object at 105 West actually is the UK’s satellite is testable, due to the North-South perturbations on its orbit. These perturbations would increase its inclination up to a value of 15 degrees and then start to bring it down again towards zero – with a complete cycle taking about 54 years. So now, nearly 54 years after its launch, we would expect Skynet-1A satellite to be back very close to an inclination of zero degrees, and somewhat surprisingly, the object at 105 West passes this test.

You might wonder why there is any doubt about the identity of the object at 105 West at all, since, if the satellite had been tracked continuously since its launch, there would clearly be no ambiguity. Unfortunately, however, this is not the case. The plot below shows all the US Space Command orbital data held by the UK MOD on this object up to 2016, and it is very clear that there are two extended periods when it was not tracked at all, (or where the data has been lost), including the crucial period, sometime in the mid-1970s, where it appears to have changed longitude from around 40 East to 105 West.

Physics doesn’t support the idea that this change in longitude could have happened as a result of natural perturbations, and this strongly implies a deliberate sequence of commands to move the satellite – one manoeuvre to initiate the drift in longitude, and another to stop it at its current location.

UK Space Registry

Date of launch: 22 November 1969
Designation: 1969-101A
Catalogue Number: 4250
Nodal period: 1,436.13 minutes
Inclination: 1.56 degrees
Apogee: 35,784 km
Perigee: 35,794 km
Disposed or decayed: Lifetime estimate (unit decay) > 1 million years
Disposal Notified to the UN: 6 December 1969
Additional information: Non-operational/not re-orbited. Now appears to be caught at 105 W stable point.

Failure cause

After approximately 18months of operation, Skynet 1A ceased to function due to its Travelling Wave Tube Amplifiers (TWTAs) failing under thermal cycling because of poorly soldered high voltage electrical joints.