Optical space communications links demonstrated
Most space systems transmit data by radio relay satellites or to ground stations. In recent years, optical communications technology advances have made it possible to transmit larger volumes of data. Free-space optical communications systems have several advantages over radio systems, including: compact, lightweight equipment; lower power consumption; higher data rates; and limited risk of interference. The challenge of developing such a system is that the limited laser output power and the extremely narrow divergence angle of the laser beam require precise pointing control technology to maintain contact between satellites. For example, each satellite must track the other up to 40,000 kilometers away and point its laser beam with an accuracy of a few microradians. Although the distance between a satellite and a ground station usually is less than between satellites, atmospheric turbulence can introduce additional errors.
The optical inter-orbit communications engineering test satellite (OICETS, Japanese name KIRARI) was developed by the Japan Aerospace Exploration Agency (JAXA) for experiments on inter-satellite and satellite-to-ground optical communications. OICETS is a box-shaped, 0.78m×1.1m×1.5m, 570kg satellite. The laser-utilizing communications equipment (LUCE)—which has a 26cm Cassegrain telescope and a 200mW, 800nm-wavelength laser—is mounted on the satellite body. Figures 1 and 2 show OICETS and LUCE, respectively.
OICETS was launched into a sun-synchronous, low-Earth orbit at an altitude of 610km and an inclination of 97.8° on 23 August 2005.1 After launch and before the experiments, the satellite bus system and LUCE were verified for about three months. Next, acquisition and tracking sequences and optical sensors were verified by tracking stars. Then, communications experiments were started in cooperation with the European Space Agency (ESA) Advanced Relay and Technology Mission Satellite (ARTEMIS) satellite in geostationary orbit,2 and with optical ground stations (OGS) of the National Institute of Information and Communications Technology (NICT) in Tokyo, Japan, and the German Aerospace Center (DLR) in Oberpfaffenhofen, Germany.3–5
The first bidirectional inter-satellite optical communications link between OICETS and ARTEMIS was successfully established on 9 December 2005.1,5 The OICETS' optical communications scheme, which uses on-off keying and direct detection, corresponds to that of the ARTEMIS semiconductor laser inter-satellite link experiment system.1,2 The forward link from ARTEMIS to OICETS used a two-pulse-position-modulation (2PPM) format at 2.048Mbits/s, while the return link from OICETS to ARTEMIS used a non-return-to-zero format at 49.3724Mbits/s.
In the initial experiment campaign, we calibrated OICETS' pointing bias error. Figures 3 and 4 show transmission beam profiles before and after calibration, respectively. We conducted more than 100 inter-satellite experiments. Acquisition sequences, tracking performance, and bit error characteristics were measured and evaluated. The communication experiment and demonstration were successfully conducted over a period of six months from December 2005 through June 2006. The results demonstrated a greater than 90% probability of acquisition and a bit error rate of less than 10−6 without using error correction.
Next, satellite-to-ground trials with NICT were performed during March and May 2006. The uplink (OGS to OICETS) used the same format and bit rate as for the ARTEMIS-to-OICETS forward link, while the downlink (OICETS to OGS) used the same format and bit rate as for the OICETS-to-ARTEMIS return link. The first data downlink was made on 28 March at a bit error rate of 10−5. During the experiment, the optical link was repeatedly successfully established. Satellite-to-ground demonstrations with DLR were conducted in June 2006. The OGS recorded the bit error rate of 10−6 on the downlink data. September was given to more trials with NICT. The OGS and OICETS performed proper acquisition and tracking, and OICETS finally received the uplink data at the bit error rate of 10−7 on 19 September 2006.6 Figure 5 shows the OICETS fine pointing sensor error and received power on the experiment with the DLR OGS.
The results of these experiments between OICETS, and ARTEMIS, and the two ground stations should encourage the operational use of optical communications links in space. In addition, we have collected fundamental data on scintillation and other effects of atmospheric turbulence that should be of value for the development of such applications.
We express our gratitude to all members of the project and to the operations teams at ESA, DLR, and NICT for their support.
Takashi Jono earned the AB of Electrical Communications from Kumamoto National College of Technology, Kumamoto, Japan in 1990. He joined the National Space Development Agency of Japan (now JAXA) in 1990. He has worked on the OICETS project team for the development of laser communication equipment since 1993. He is now the on-orbit experiment leader with the OICETS Project Team in JAXA.
Katsuyoshi Arai earned a BS in electronic engineering from Shibaura Institute of Technology, Tokyo, Japan, in 1974. He joined the JAXA in 1974. He has worked on the OICETS project team since 2002 as a project manager.
