The S-Net cubesat constellation. Credit: DLR

PARIS — The German Aerospace Center DLR is funding a series of nanosat projects aimed at demonstrating the potential of inter-satellite communications in space.

The center’s most recent endeavor, S-NET, is qualifying a group of four identical nanosats aimed at developing a low-cost, autonomous narrow-band satellite network that could support IoT and machine-to-machine applications.

Working with IQ Spacecom GmbH, the Technical University of Berlin (TUB) — which has a long history in smallsats — developed an S-band transceiver (SLink) with 100 kbps crosslink and 1 mbps downlink suitable for the nanosat form factor. In addition to SLink, the quartet of S-Net satellites also carries a laser reflector for high-precision position measurement.

Launched in early 2018 on a Soyuz 2-1a Fregat-M, S-NET marks Germany’s first satellite constellation. Circling at roughly 580 km altitude, the 9kg nanosats have a design life of one year, though they remain operational nearly two years after launch.

During this time, S-NET’s S-band transceiver has demonstrated stable inter-satellite links over various separation distances up to 250 km under different in-orbit scenarios, including passive orbital control and applying orbital drag, according to Katharina Borm of the DLR’s space administration office.

Borm presented S-NET and DLR’s cubesat program here Sept. 10 during a meeting of the Commercial Smallsat Spectrum Management Association (CSSMA).

The mission’s S-band radio offers two selectable operating modes: An uplink/downlink mode, and an inter-satellite-link (ISL) mode. The downlink is at 1-Mbps at 1.4 MHz, the uplink is 32 kbps, and the ISL is a nominal 100 kbps. Both ISL and the uplink have a symbol rate of 80 kHz. The ground link is designed for a 3-meter ground station reflector at a minimal elevation angle of 10 degrees.

To date, S-NET has tested its link budget using varied misalignments of its transceivers. For example, with the spacecraft at 10 km and 65 km apart, the antennas were tested at a misalignment of 45 degrees; At 148 km distance, they were trialed with the antennas off by 30 degrees.

The S-band links were also tested successfully at 25 km separation with the nanosats in tumble mode.

The S-NET satellites during testing. Credit: DLR

At greater distances, “if you adjust the misalignment a little bit, you still have a stable link,” Borm said. “It shows the success of the mission that even with big misalignments over big distances, we can still have a stable inter-satellite link.”

Although the initial aim of S-NET was to test SLink with the satellites positioned up to 400 km apart, after nearly two years in orbit, the four spacecraft have not drifted as planned.

“At the beginning we were thinking due to the ejection of the satellites from the rocket they would drift apart naturally,” she said. “But we figured out that even today after 750 days in orbit we still have separation of 250 km. Right now we’re still even lower than that, so we can even test inter-satellite links and do analysis longer than we were expecting.”

Borm said the integration of a small satellite network into an M2M network holds promise for supporting missions including maritime monitoring and disaster assistance.

Established satellite service providers, such as Argos, Iridium, and Orbcomm, already provide IoT services that relay ground sensor data to users by satellite. More recently, a handful of commercial satellite IoT initiatives, including Hiber, Diamond and Kepler, launched their first nanosatellites, demonstrating the technology and feasibility for satellite machine-to-machine communications.

With this in mind, DLR is planning the next steps beyond S-NET. The agency has some 20 nanosatellite missions in development and expected to launch in the coming year. Though most involve Earth observation and spectrum analysis missions, two feature inter-satellite link trials.

The first, known as XLink, is an X-band successor to S-NET. Led by TUB, the X-band receiver for inter-satellite connections aims to demonstrate higher data-rate communications in space.

A second project is a joint effort between DLR and payload manufacturer Tesat-Spacecom. It will culminate in a laser communication terminal for nanosatellites. Known as the CubeLCT, a 360-gram optical data transmitter capable of transferring data at 100 mbps from LEO satellites to the ground. Measuring 9.5 x 9.5 x 3.25 cm, it consumes 8W of power, transmits 100mW and is designed to operate for five years in orbit.

In August, Tesat delivered its first CubeLCT to an undisclosed U.S. customer, marking the start of a new product family focused on applications for small satellites and cubesats.

“Due to physical properties of a laser beam, the technology is unaffected by ITU regulations and independent of frequency bands while enabling enhanced security aspects: optical links are interference-free and resilient to eavesdropping by technology and are the medium for quantum key distribution technology,” the company said in a statement.

Tesat hired GomSpace of Sweden and Denmark to develop a platform for the terminal under a $605,000 contract signed in August 2017. The first CubeLCT is expected to launch this year, according to Borm.

Tesat has said the CubeLTC customer is a U.S.-based entity that it declined to identify.

Tesat, Airbus Defence and Space in 2018 signed an agreement to launch the Tesat-built T-Osiris lasercom terminal on Europe’s Bartolomeo platform for the International Space Station. That launch is now scheduled for 2020.

DLR has been developing space-based laser communication terminals for more than 35 years. The U.S. Missile Defense Agency has used Tesat’s laser communication terminals to provide links between the agency’s NFIRE satellite and the German TerraSAR-X Earth observation satellite. Tesat also collaborates with General Atomics on UAV-based laser communication applications, though the company views the cubesat market as its biggest near-term opportunity.

DLR boasts the largest number of nanosatellites launched in Europe, with 26 orbited in June and another eight in July this year.

With an annual budget of $345 million, DLR funds 75% of all nanosat missions, most of which are designed, built and operated by the nation’s technical universities.

Borm said her office within DLR employs 22 people and spends roughly $40 million annually on satellite communications, including payload and platform technologies, ground-segment and antenna technologies, services and applications for satellite communication and frequency management. 

Amy Svitak is a Space Intel Report contributing editor.