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How does Inmarsat manage its geostationary satellites?

  • Written by Steve Nichols and originally published in ADS Advance magazine.
Inmarsat's Mark Dickinson.

Inmarsat’s Mark Dickinson.

How do you look after and manage 13 different communications satellites, some which weighed around six tonnes at launch, and all hurtling through space at more than 9,000 kilometres per hour? They are also nearly 36,000 kilometres away, being blasted by cosmic rays and solar particles, and yet are expected to transmit and receive gigabytes of important data each and every day faultlessly.

If those are the kind of questions that would keep you awake at night, spare a thought for Mark Dickinson, Inmarsat’s Senior Director, Satellite Operations.

Mark, who has a Ph. D in astrophysics, is responsible for keeping all of the company’s 10 satellites in place, and the company maintains the Hylas-1 and Hylas-2 satellites for Avanti. His team of 27 is also preparing to support four more satellites that will be launched over the next two years.

Inmarsat has a whole host of satellites in geostationary orbit, ranging from between five to 23 years old. Geostationary means they appear to stay in the same spot in the sky, day in, day out. But if only life were that simple.

“Despite the term, a geostationary satellite is constantly trying to move position,” says Mark, who is based at the company’s imposing City Road, London, headquarters. “Due to asymmetries in the Earth’s gravitational field a geostationary satellite is always trying to move to one of two points or “gravitational wells” as they are called.

“If your satellite is positioned at one these points all well and good, but most aren’t.

“Sunlight also exerts a force as a result of photons striking the satellite,” he said. “And if that wasn’t bad enough the gravitational effects of the Sun and Moon causes the satellites to try to head north and south at the same time – some of our older satellites actually describe a figure of eight in space every 24 hours.”

Adjustments
For this reason Inmarsat constantly has to make orbital adjustments so that the satellites stay well separated from their neighbours, normally a minimum of 0.1 degree apart, with equates to a virtual box in space about 150km across.

An Inmarsat I-5 satellite for its Ka-band GX Aviation service.

An Inmarsat I-5 satellite for its Ka-band GX Aviation service.

But Inmarsat’s three next-generation Ka-band satellites called Inmarsat 5s (the first of which was launched in late 2013)  use much higher frequency frequencies with 89 pencil-thin Ka-band beams, operating at around 20-30GHz. These will need more management to keep them locked more tightly into their orbital positions.

So how do you manoeuvre a three-tonne satellite from the ground? The answer used to lie with small rocket thrusters, which are used exclusively on the Inmarsat 2 and 3 satellites. But the Inmarsat 4 and I-5 satellites also use small plasma ion thrusters. More efficient than chemical rockets, these use ionised xenon gas, which is then accelerated electrically and propelled out of the spacecraft.

Newton’s third law of motion says that the spacecraft will move in the opposite direction.

Mark says the acceleration the satellite gets with xenon is small, compared with chemical rockets, but over a two-hour ion thrust period you get the same net effect and you don’t use much fuel in the process.

“With plasma thrusters you typically have to make orbital adjustments about six days out of seven,” says Mark. Inmarsat’s new Boeing 702HP I-5 satellites have a design life of about 15 years. That means they have to carry enough xenon “fuel” to push them up into their final orbit, to see them though their design life and still have enough left over to transfer them up into a graveyard orbit when they are no longer of use, which is about 300km above the geostationary band. As a result Inmarsat’s new I-5 satellites will carry about 200kg of xenon gas.

“It’s an international requirement that we move redundant satellites into a graveyard orbit,” said Mark.

Viable
Apparently a 20-year-old satellite has reduced commercial value so it isn’t economically viable to refuel them a bit like trying to justify having a 20-year-old television repaired. But this TV is 36,000 kilometres away.

So are there any other hazards for satellites? Apparently quite a few.

We also have to plan for eclipses,” said Mark. “This is where the sun disappears behind the earth and stops illuminating the satellite’s solar panels. An eclipse can last up to 72 minutes and we have to ensure the satellite has enough battery power to maintain itself during that time the eclipse periods occur in the spring and autumn around the March and September equinoxes.”

Then there are rogue satellites and debris that can drift toward our orbital locations, the risk of satellite sensors being blinded by the sun, plus the effects of intergalactic cosmic rays and solar storms, which are commonly known as space weather events.

“These latter two can cause what we call ‘single event upsets’, which is when ions or electro-magnetic radiation strikes a sensitive part of a micro-electronic device. Basically, the satellite’s electronics get confused, but the satellite can automatically switch to backup equipment.

“These always seem to happen in the middle of the night!” Mark joked, which is why the Inmarsat control centre in City Road is manned 24/7 and he is always on call.

“We now have around 180 satellite-years of operation and have had no significant payload outage or equipment failure caused by a space weather event,” he said.

“The Inmarsat 4 satellites are also constantly sending down around 19,000 discrete items of telemetry data. If our systems detect that something isn’t quite right it sounds an alarm and we have to work out what it is, what’s caused it and what we have to do to fix the problem.

“And the new Alphasat is even more complex with double the amount of data points,” Mark said.

Los Angeles

An Inmarsat I-5 satellite being built at Boeing’s El Segundo facility near Los Angeles.

An Inmarsat I-5 satellite being built at Boeing’s El Segundo facility near Los Angeles.

Boeing is currently building the second and third Inmarsat-5 satellites for its Global Xpress service in El Segundo, just south of Los Angeles, with the first having been launched late last year. While Inmarsat’s I-4 satellites can currently transmit and receive data at hundreds of kilobits per second, the higher frequency Ka-band satellites are able to handle data at up to 50Mbps (more than 100 times faster) for thousands of users on the land, at sea and in the air.

As Mark alluded, Inmarsat has also launched another L-band satellite. Called Alphasat, it was produced in conjunction with ESA and offers an additional 7 MHz of L-band spectrum over Europe, Africa and the Middle East. It is initially co-located with an Inmarsat I-4 satellite at 25 degrees East, which also has a navigational transponder providing GPS augmentation signals via the European EGNOS system.

“Users will hardly notice the changeover when we switch from the I-4 to Alphasat,” said Mark. “It will be an almost seamless transition.”

Inmarsat’s radio signals go and to from a number of dedicated satellite access stations (SASs) around the globe. The main ones are located at Burum in the Netherlands, Fucino in Italy and Paumalu in Hawaii. Inmarsat also uses a number of additional sites in Auckland, Canada and Beijing to control the satellites.

Mark admits he was always interested in science and space as a youngster, hence his first degree in physics and astrophysics at the University of Leeds and his subsequent Ph. D looking at gamma ray production around black holes. Mark then joined a software company working on defence and space computer simulations, before joining Inmarsat 13 years ago to help develop the software that now control’s Inmarsat’s satellites. This has been so successful that the same software is also used by the satellite manufacturer Astrium for the UK MoD’s Skynet satellites and also by Abu Dhabi-based Yahsat for its Ka-band satellites.

“This job offers the best of both worlds,” says Mark. “It has the science, but also the commercial aspects of making sure we continue to offer an excellent service to our end users.

“It’s complex, it’s challenging, but ultimately very rewarding,” Mark concluded.

 

 

How does Inmarsat manage its geostationary satellites? was last modified: March 19th, 2017 by Steve Nichols
Filed in: Features Tags: , ,

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