Ground Receiver Unit for Optical Communication between LADEE Spacecraft and ESA Ground Station

NASA's Lunar Laser Communication Demonstration (LLCD) demonstrates optical communication between ground stations on Earth and NASA's LADEE spacecraft orbiting the Moon. As a contribution to the LLCD experiment, the European Space Agency (ESA) prepares its existing optical ground station to serve as a complementary ground communication terminal. A ground receiver unit was developed for this purpose. It comprises optical detectors and electronics to recover the frames, decode the data and store them in a large data buffer. The overall ground receiver unit design is presented and explained together with ground test activities. Different optical detectors had been evaluated and two detector types were chosen for implementation. A multimode fiber is used to couple the optical signal from the existing telescope into the detectors. The 16ary-Pulse Position Modulation (PPM) signal is then processed in the electronics. Clock recovery and frame synchronization could be shown to work reliably at low power and under severe power fluctuations. For error correction, a powerful 1/2-rate Serially Concatenated PPM (SCPPM) decoder is applied. The user data is decoded at a throughput of 39Mbps with up to 20 turbo iterations. With the developed ground receiver unit an average signal power of only a few hundred picowatts is required to yield a frame error rate smaller than 10(-5). The ground receiver unit was tested for compatibility with NASA's ground support equipment and installed in ESA's Optical Ground Station on Tenerife. It was used in actual optical downlinks in October and November 2013. LLCD downlink results are presented and analyzed.