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Data, transmitted in the form of hundreds of millions of short pulses of light every second, will be sent by the Lunar Lasercomm Space Terminal (LLST), aboard the LADEE spacecraft.
The LLST, developed by MIT Lincoln Laboratory, is comprised of three modules: an Optical Module (OM), a Modem Module (MM), and a Controller Electronics (CE) module. The optical module is situated on the exterior of the LADEE spacecraft payload module, while the Modem and Controller Electronics are mounted on the interior of the spacecraft. The LLST payload weighs about 65 pounds (30 kg.)
The OM consists of a four-inch diameter (10-cm) telescope used to transmit and receive a modulated laser beam to and from the Earth ground station, respectively. Vibrations on the LADEE spacecraft are measured and compensated by the OM, providing the stabilization necessary for accurate beam pointing to the ground terminal. The OM is mounted on a 2-axis gimbal for coarse pointing, contains an acquisition detector, and is fiber-coupled to the Modem Module.
The MM resides within the LADEE spacecraft and contains the optical transmitter and receiver. The MM is mounted inside LADEE and contains the 0.5-watt infrared laser transmitter that will transmit data at a rate of 622 Mbps from the Moon to Earth. The MM also contains the highly sensitive receiver that deciphers the light pulses of data sent from the ground telescopes at up to 20 Mbps. It comprises four slices: a power slice that provides power conditioning for some of the major functions, a digital slice containing high-speed digital components, an analog slice that includes the analog signal processing used in receiving and demodulating the uplink signals, and an electro-optic slice that includes a 0.5-watt power amplifier for the modulated downlink signal plus an amplifier and detector for receiving the uplink data signal, as well as the electro-optics to create the downlink signal and also demodulate the optical uplink tracking signal. Lastly, the MM provides a high-speed data interface with the LADEE spacecraft for downlinking the spacecraft telemetry as well as instrument science data.
Controller Electronics Module
The CE resides within an adjacent compartment inside the LADEE spacecraft, and is connected to both modules as well as the spacecraft. The CE includes interfaces to the sensors and mechanisms in the OM and performs the control algorithms to sequence OM operations and stabilize the OM and its beams. It also configures and passes commands to the MM. The CE interfaces with LADEE to pass received uplink commands, telemetry to be downlinked, and other information from the spacecraft such as time and spacecraft attitude
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