Design of an inertially stabilized telescope for the LLCD

Abstract

The Lunar Laser Communication Demonstration (LLCD) program will demonstrate the first high-bandwidth optical communication payload on a NASA space mission. The inertially stabilized 108 mm aperture telescope will fly on NASA's LADEE spacecraft and is fabricated nearly entirely of beryllium, providing a high stiffness-to-weight ratio. The telescope consists of a two-axis fine positioning stage using inertial sensors and coarse and fine optical tracking. The stabilized telescope uses a two-axis coarse positioning gimbal to provide a large field-of-regard. Inertial stabilization provides local disturbance rejection while allowing modest optical uplink power to provide an absolute pointing reference. The telescope is a three-wavelength design providing separate uplink acquisition and communication wavelengths, and a downlink communication wavelength. Acquisition and coarse tracking of the uplink beacon is via a photodiode quadrant detector, while fine tracking is via nutation tracking and piezoelectric actuation of the receive fiber. Control of the downlink point-ahead angle is via piezoelectric actuation of the transmit fiber. The telescope is thermally stabilized during normal operations. The transmit and receive beams are fiber-coupled to a separate optoelectronic module and the telescope line-of-sight will be stabilized to better than 2.5 microradians during normal operations. Provision for self-test and boresighting during on-orbit operations is provided.