Graz researchers conquer W frequency band for satellite communications
On June 30, a Falcon 9 rocket carrying research satellites took off from the spaceport in Cape Canaveral (USA). One of them belongs to the Graz researchers and their international project partners: it is a "triple-cubesat", i.e. a box with a footprint of ten by ten centimeters and a height of thirty centimeters. "Packed all around it are solar cells that can generate a total of 16 watts of electrical power, and a battery is on board because it keeps flying in the Earth's shadow," Schmidt explained. Its "polar orbit" at an altitude of 540 kilometers takes it almost at right angles to the equator, passing just above the poles.
Faster than a bullet
"It flies at seven kilometers per second, which is seven times faster than a rifle bullet," the researcher said. It usually passes over and near Graz three times a day and three times at night. It sparks for five to 15 minutes each time, sleeping the rest of the time to save energy. Using the pre-calculated orbital data or with the help of a self-developed "mono-path tracking" device, which doesn't take its eyes off it once it has spotted it, the Graz researchers align their antenna with it. "That's not so easy, because the beamwidth to reach him is very narrow at 0.2 degrees," Schmidt said, "For the moon, it's two and a half times as wide by comparison."
Before the satellite was delivered to space, the researchers solicited the use of the desired frequency. Frequencies are allocated by the International Telecommunication Union (ITU). "The spectrum is finite, you can't expand it," Schmidt said, "There's a battle going on, so to speak, from zero hertz to the wavelength of light for each section to see who gets it." "But because we only want to test a single tone at 75 gigahertz with very low power, it's very harmless to potential competitors at first, and we're not interfering with anyone." Still, it took two years for approval to roll in.
Now, researchers have two to five years to demonstrate that they can do something useful with this frequency, such as accomplish Internet connectivity via feeder links for High Throughput Satellites (HTS). Then they could be used commercially with additional permissions from local authorities. "That will also be necessary in the future, because otherwise we won't achieve the necessary capacity for Internet connectivity with the high throughput satellites," he says.
The researchers received a 75 gigahertz signal from the satellite. "This was a pioneering achievement; there was nothing like it in the world before," Schmidt said. "Every frequency here has its own propagation characteristics," he explained, "At 60 gigahertz, for example, it's quite poor because the oxygen from the air dampens these oscillations almost entirely."
Focus on weather effects
So far, the 75-gigahertz frequency is known only for its attenuation in the Earth's atmosphere by measuring relatively short terrestrial links. The researchers are therefore investigating how a wide range of droplet sizes, from drizzle to heavy thunderstorms, affect the waves. In the case of a rain front, attenuation can change many times over in a second, and you need to know how to lay out ground stations to still communicate reliably with satellites, Schmidt said. The researchers get the weather data for their model calculations from the Austrian Central Institute for Meteorology and Geodynamics (ZAMG). They also measure droplet size distribution, for example, with a "distrometer" they developed themselves.
The project has received a grant of about 1 million euros from the Ministry of the Environment, with funding provided by the Austrian Research Promotion Agency (FFG) as part of an ESA project, Schmidt said.