Homodyne BPSK-based optical intersatellite communication links to be tested

Intersatellite communication links with data rates up to 10Gbps are needed, for example, in military applications. Commercial radio frequency (RF) communication systems cannot provide these rates, as their wavelengths and thus their space loss is simply too large. One promising technology is laser communication terminals, which use shorter wavelengths in the optical range. These terminals combine the advantages of high data rates and large link distance with low mass, small size, and low power consumption. Moreover, of all optical modulation schemes, homodyne binary phase shift keying (BPSK) provides the highest sensitivity for both communication and tracking. It is also the only scheme that provides full immunity against sunlight, as electrical filtering by homodyne detection performs better than optical filtering due to the varying Doppler shift. Therefore, Motorola decided to use laser communication based on homodyne BPSK for commercial programs like Celestri and Teledesic, and selected Tesat as its supplier for laser communication terminals.

In 1999, Tesat research showed that homodyne BPSK is a reliable technique for space applications. In 2005, a 152km free-space 5.625Gbps communication link between La Palma and Tenerife, two of the Canary Islands, provided evidence of the robustness of homodyne BPSK, even under severe atmospheric conditions. Soon Tesat's laser communication terminals will be verified in orbit as well, the first test of a high-data-rate optical link. A close German-US partnership will allow an optical 5.625Gbps link to be be established between the German low earth orbit (LEO) satellite TerraSAR-X and the US LEO satellite Near Field Infrared Experiment (NFIRE). The laser communication terminals have been integrated into and tested on both satellites, and are now ready for satellite launch.

Tesat's laser communication terminals are well suited for applications such as LEO, medium earth orbit (MEO) and geosynchronous earth orbit (GEO) links. The core performance data includes hemispherical tracking at the counter terminal; communication with the sun in the field of view; and duplex links with a bit error rate of 10^-8 at 10, 5, 3, and 1Gbps across 6000, 20,000, 45,000, and 72,000km, respectively.

Terminals that link distances of 6000-20,000km have a mass of 33kg and 135W power consumption (0.7W transmit optical power), whereas those that link distances of 45,000-72,000km have a mass of 45kg and 155W power consumption (2.2W transmit optical power). Both laser communication terminals are one-unit designs with electrical (external) harnesses only. The short link distance terminal has a footprint of 500×500mm and a height above the satellite's panel of 420mm and depth below of 200mm. The corresponding parameters for the long link distance terminal are 550×550, 440, and 255mm.

A current program examines the performance of an optical data relay that can be extended to LEO-to-GEO, GEO-to-GEO, and GEO-to-ground laser communication links. This short-term GEO relay mission will evaluate the performance of a real-time downlink of data from a German LEO satellite, TanDEM-X, via an optical link to a GEO satellite, such as Alphabus, and further via an RF link to the ground station. In general, this test will verify in orbit the performance of an optical, bidirectional 2.8Gbps LEO-to-GEO link based on homodyne BPSK. The system will be designed such that further GEO-to-GEO and GEO-to-ground links, the latter with adaptive optics compensating for atmospheric wavefront distortions at the ground station, can be incorporated to evaluate the performance of an entire communication network.

Tesat's homodyne BPSK-based laser communication terminals for intersatellite links result from more than two decades of development expertise in the field of free-space optical communications as well as a broad knowledge of commercial space systems production. The upcoming in-orbit tests of this technology will demonstrate its utility and suggest avenues for further research.