KEYWORDS: Navigation systems, Laser communications, Telecommunications, Data communications, Satellites, Clocks, Phase measurement, Receivers, Space operations, Global Positioning System
The Lunar Laser Communication Demonstration (LLCD) flown on the Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite achieved record uplink and downlink communication data rates between a satellite orbiting the Moon and an Earth-based ground terminal. In addition, the high-speed signals of the communication system were used to accurately measure the round-trip time-of-flight (TOF) of signals sent to the Moon and back to the Earth. The measured TOF data, sampled at a 20-kS/s rate, and converted to distance, was processed to show a Gaussian white noise floor typically less than 1 cm RMS. This resulted in a precision for relative distance measurements more than two orders-of-magnitude finer than the RF-based navigation and ranging systems used during the LADEE mission. This paper presents an overview of the LLCD TOF system, a summary of the on-orbit measurements, and an analysis of the accuracy of the measured data for the mission.
The space terminal modem for the Lunar Laser Communications Demonstration (LLCD) provides duplex
lasercom capabilities between the Earth and a satellite in lunar orbit with a 0.5-W optical transmitter
delivering downlink data rates of 39-620 Mbps and an optically-preamplified direct detection receiver
supporting uplink data rates of 10-19 Mbps. The modem consists of four subsystem modules: digital
electronics, analog electronics, power conditioning, and electro-optics. This modular approach permits
subsystems to be built and tested in parallel and provides design flexibility to address evolving
requirements. Other important design considerations for the modem include the utilization of commercial-off-
the-shelf (COTS) components to reduce delivery time, cost, minimization of size, weight, and power,
and the ability to survive launch conditions and operate over a broad temperature range in lunar orbit.
MIT Lincoln Laboratory designed and built two free-space laser communications terminals, and successfully
demonstrated error-free communication between two ground sites separated by 5.4 km in September, 2008. The primary
goal of this work was to emulate a low elevation angle air-to-ground link capable of supporting standard OTU1 (2.667
Gb/s) data formatting with standard client interfaces. Mitigation of turbulence-induced scintillation effects was
accomplished through the use of multiple small-aperture receivers and novel encoding and interleaver hardware. Data
from both the field and laboratory experiments were used to assess link performance as a function of system parameters
such as transmitted power, degree of spatial diversity, and interleaver span, with and without forward error correction.
This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002.
Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by
the United States Government.
The forward error correction (FEC) and interleaver realizations used in a 5.4 km horizontal-path link experiment
incorporated several unique elements that were specifically tailored to address turbulence-induced fading. To facilitate
optimization studies, this hardware was designed to afford a high degree of flexibility in the FEC code structure and
interleaver length. An essential aspect of this structure was the standards-compliant client interface, which provided
seamless connectivity to fiber-based terrestrial networks. Through the use of an OTU1 (2.667 Gbaud) architecture with
nonstandard interleaving, error-free transmission was achieved in the presence of strong scintillation that produced fade
events that frequently exceeded 10 ms in duration.
This work was sponsored by the Department of Defense, RRCO DDR&E, under Air Force Contract FA8721-05-C-0002.
Opinions, interpretations, conclusions and recommendations are those of the authors and are not necessarily endorsed by
the United States Government.
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