It's crowded up there: over 2000 functioning satellites are currently orbiting the earth. And it’s getting even more crowded: certain companies want to launch tens of thousands of new satellites over the coming years. The space flight company SpaceX, which is headed by Tesla boss Elon Musk, is planning to offer broadband Internet services via over 1500 satellites by 2022. "Satellites already form part of the global Internet infrastructure," explains Sabine Klinkner, a professor at the University of Stuttgart’s Institute of Space Systems (IRS). "On the one hand, they form the backbone of intercontinental data transmission together with submarine cables, and on the other, they provide alternative broadband access to the Internet to users in regions with poor telecommunications infrastructures."
The fact that companies, such as SpaceX, now wish to launch numerous additional satellites, is directly related to the ongoing digitalization in many areas of life. "Autonomous driving or the logistical monitoring of the flow of goods around the world will generate large amounts of sensor and navigational data, which needs to be made available to users without delay via the Internet," says Klinkner.
"But, the frequency bands currently available for satellite-based data transfers are already quite busy." So not only is it crowded in the near Earth orbital zone, but also for data transfers between satellites and ground stations.
To ensure that data will not only be available in the fiber-optic and the new 5G mobile networks in future, the radio links to the satellites in question will also need to be exploited more effectively. "To do this, we have to open up new frequency bands," says the space flight engineer. She and her team are conducting research into this in the EIVE (Exploratory In-Orbit Verification of an E/W-Band Satellite Communication Link) project, which was launched in 2019.
Detection under space conditions
The EIVE project, which is funded by the German Federal Ministry for Economic Affairs and Energy, is coordinated by the Institute of Robust Power Semiconductor Systems (ILH) under the auspices of Prof. Ingmar Kallfass. The team, whose members are from the ILH and IRS is collaborating with other partners including the Fraunhofer Institute for Applied Solid State Physics (IAF) as well as RPG- Radiometer Physics and Tesat-Spacecom. The objective of the three-year research project is to build a mini-satellite to test a broadband radio link to a ground station in a previously unused frequency range of between 71 and 76 gigahertz.
"This so-called E-band lies above the established frequency ranges used in satellite communications and by the military," says Kallfass. Those frequency ranges are between one and 40 gigahertz. The E-band, on the other hand, is virgin territory. Whether or not it will be possible to transmit data at high speeds in that range needs to be investigated. "The radio signal between the ground station and the satellite has to pass through the earth's atmosphere," as Kallfass explains, "which, on a clear, sunny day, is not an issue. But, due to the many water molecules they contain, rain clouds have a major scattering effect on the signal."
However, the conditions for fast data transmissions in the E-band are not bad, because, in theory, the data rate increases with the transmission frequency due to the laws of physics. This is practical, as the ILH team has already achieved two world records in the E-band: the highest combination of data rate and distance in a radio transmission between two terrestrial sites – 6 gigabits per second over a distance of 37 kilometers and between an aircraft and a ground station at 9.8 gigabits per second. This would correspond to 120 or almost 200 DSL connections respectively at 50 megabits per second. Ultimately however, the engineering and natural sciences are not about extrapolating known data, but rather they involve the search for empirical proof – under space conditions in this specific case. The project team wants to provide this proof in the EIVE project.
Satellite the size of a shoebox
The satellite is being built by Klinkner's team. "To this end we’re using the CubeSat standard, which is now well-established for mini-satellites," says the professor. A CubeSat consists of cube-shaped units with an edge length of ten centimeters, which can be pieced together to form larger units similar to Lego bricks. Their great advantage is that virtually any commercially available rocket is able to transport these CubeSats in a device, which is also standardized, and launch them into space. And, because the satellites are so small, they can hitch a ride at a relatively low cost. This concept, which was established two decades ago, seriously reduces development and launch costs and makes small satellites affordable for universities. "Flying Laptop" was the name of the satellite developed by Klinkner and her team of students and doctoral candidates, which was launched into space in 2017 on board a Russian Soyuz rocket, although it was not based on the CubeSat concept.
The EIVE satellite will be about the size of a shoebox. It will be fitted with two exterior solar modules that can be folded out in space. As with any satellite, the inside will contain the electronics that enable the satellite to align and orient itself in orbit, as well as an energy storage system and radio technology that will enable the ground station to control it. The remaining volume, a good third, will be reserved for the payload i.e., the technology needed for the E-band tests, which basically consists of an antenna, several amplifiers and a video camera.
With any luck, we may be the first to actually succeed.Prof. Sabine Klinkner
When it gets into space in 2022, the satellite will orbit the Earth at a slightly higher altitude than the International Space Station. Klinkner's team will monitor the satellite from the ground station on the University of Stuttgart campus, whilst Kallfass' team conducts the radio experiments. They want to test the radio quality in the E-band under various weather conditions, initially using artificially generated data from the satellite before going on to transmit the uncompressed video data from the camera, which will point at the Earth, in real time and at four times full HD resolution, whereby the plan is to reach a data rate of up to 10 gigabits per second. They then plan to record camera data in different places around the world, store it temporarily on the satellite, and then retransmit it to the ground station in Stuttgart when the satellite flies overhead.
"Although the E-band is considered a promising frequency range for broadband satellite communications,” Klinkner explains, “there are only a few institutes carrying out empirical research under real conditions. "But, with any luck, we may be the first to actually succeed."
Editor: Michael Vogel