Tumbling satellite no problem: DST


Equipment failure on SA’s Sumbandila satellite, launched 18 months ago, has set the craft to tumbling in its orbit, but scientists have found a unique way round the problem, Parliament’s science and technology portfolio committee heard yesterday.
“There is a problem with the stabilisation of the satellite… [it] is tumbling. What has happened is [one set] of reaction wheels has packed up,” science and technology deputy director-general Val Munsami told MPs. The “reaction wheels” referred to correct the stability of the satellite in three axes, ensuring it keeps its on-board camera directed towards earth, the South African Press Association reported.

Munsami said most satellites had built-in redundancy in the form of back-up working parts, but not Sumbandila, which was built at a cost of R26-million, a relatively low price for a satellite. “Sumbandila doesn’t have that. In fact, there’s a single line, so if there’s a failure, and it’s on the critical path, the satellite is gone. Fortunately a lot of the failures have not been on the critical path. So the satellite is still functioning, but limping,” he said.

The satellite’s controllers had come up with a method to manage the problem. “It tumbles, but as it passes over the area where they want to take the images, they slow the satellite down. It takes an image… and it carries on. This is the first time this has ever been done; it’s very innovative,” he said.

The earth-observation satellite – launched in September 2009, from the Baikonur Cosmodrome in Kazakhstan atop a Russian Soyuz rocket – was still managing to snap two images a day.

Speaking to SAPA after the briefing, Munsami said the problem had started about eight months after the launch, which had placed the tiny craft in a polar orbit. He also confirmed the 81kg satellite had lost a battery, one of two on board, which was damaged after the craft passed through a belt of high radiation over the South Atlantic. Asked how long Sumbandila would continue to operate, Munsami said this was more a function of the fuel it carried than the remaining battery. The fuel was needed to maintain the satellite’s 500 km-high orbit.
“Once we lose all the fuel, the satellite’s orbit will degrade and it will fall to earth and burn out in the atmosphere… We can probably go for another two years,” he said. Sumbandila means “lead the way” in the Venda language. It is South Africa’s second home-built satellite. The first, SUNSAT, was launched in the United States 12 years ago.