The National Aeronautics and Space Administration’s (NASA) Laser Communications Relay Demonstration (LCRD) completed the first 18 months of its Experiment Program in December 2023. Geosynchronous-ground experiments to date have included demonstrations of optimetrics and of Delay/Disruption Tolerant Networking (DTN), and measurements of the effects of the atmosphere on lasercom performance and availability. Future operational scenarios have been emulated. This paper provides an overview and highlights of the first 18 months of LCRD experiments, and a preview of the upcoming experiments, including relaying data to and from the International Space Station.
In order to appropriately budget satellite resources for a new lasercom terminal, system architects must define an accurate size, mass and power (SWaP) estimate in advance. However, SWaP drivers are often tracked individually during initial design phases, when in reality these drivers are intertwined. Consequently, SWaP estimates attempted at the beginning of a build program can differ significantly from the results seen at the completion of the build. A more holistic initial estimate is needed to capture these complex relationships. A data-based model lends empirical insights into drivers for SWaP, providing a baseline reference for future lasercom terminals. Given the significant number of lasercom demonstrations reported over the last several years, it is now possible to explore a baseline model for SWaP founded on empirical data. These lasercom terminals span a wide range of designs with different SWaP to meet link requirements such as communication distance and data-rates. Here, we consider SWaP drivers such as orbit, maximum data rate × range2, and modulation format for 80 unique lasercom terminals. Through iterative analysis of cross-correlation coefficients, p-values, root mean squared errors, and R2 metrics, we establish multivariable parametric regression models as baseline SWaP references for future system design.
As lasercom transitions from individual demonstrations to wide-spread deployment, diverging standards and proprietary implementations will impede interoperability between terminals from different suppliers. In this environment, there is an urgent need for a universal lasercom translator terminal capable of supporting multiple acquisition sequences and communication waveforms in order to connect disparate networks. Development of such a generalized terminal is a key goal of the DARPA Space-Based Adaptive Communication Node (Space-BACN) program. Pointing, Acquisition, and Tracking (PAT) are crucial aspects of interoperability, and multiple styles of PAT sequences have emerged. Even within a given category of beacon generation (e.g., synthesized, in-band, out-of-band), multiple parameters must be specified to define specific entry/exit criteria for each PAT stage, requiring detailed pre-coordination to ensure that terminals from different suppliers can establish a link between them. In addition, extra hardware must be incorporated into the terminal design to accommodate a superset of PAT requirements. Finally, verification testing of multiple PAT sequences in a common testbed drives the testbed implementation requirements. Here, we examine the implications of attempting to accommodate a generalized PAT sequence with a single lasercom terminal. First, we consider the performance of different styles of PAT sequence, and the trades between pointing uncertainty, maximum acquisition range, and allowable scan time. Next, we consider potential hardware requirements for terminals that support multiple PAT sequences. Finally, we consider the testing of such a terminal, and present a testbed architecture intended to accommodate multiple PAT sequences by highly flexible emulation of the remote terminal PAT behavior.
The DARPA Space-Based Adaptive Communications Node (Space-BACN) optical terminal will be a low-cost reconfigurable optical intersatellite link (OISL) terminal capable of supporting up to 100 Gbps low-earth-orbit (LEO) links. Rapid and reliable pointing, acquisition, and tracking (PAT) is critical to OISL performance, especially in cross-plane LEO links, where contacts can be short. The Space-BACN optical terminal will demonstrate a novel reconfigurable acquisition implementation, which can be dynamically configured to operate in one of three acquisition modes: in-band, out-of-band, and synthesized beacon. Here, we review the features, implementation, performance analysis, and verification approaches for each of the three acquisition modes.
The DARPA Space-Based Adaptive Communications Node (Space-BACN) optical terminal will be a low-cost reconfigurable optical intersatellite link (OISL) terminal capable of supporting up to 100 Gbps low-earth-orbit (LEO) links. Space-BACN will enable data transport between heterogenous commercial and government LEO constellations. Key to realizing a flexible transport layer is a robust command and control mechanism to dynamically negotiate service level agreements for cross-constellation OISL access. Here, we describe the development of the hybrid adaptive management schema (HAMS) for coordinating OISL access between network operation centers for different LEO constellations. Cybersecurity by design and other system considerations are also presented.
The National Aeronautics and Space Administration’s (NASA) Laser Communications Relay Demonstration (LCRD) mission began its two-year Experiment Program in June 2022. This experimental phase includes long-term measurement of the effects of the atmosphere (turbulence, weather) on the performance and availability of lasercom. Furthermore, various future operational scenarios including robotic and exploration missions and various network service configurations are being emulated. In addition to experiments and demonstrations proposed by the LCRD Investigator Team, NASA enables individuals and groups from government agencies, academia, and industry to propose experiments under the LCRD Guest Experimenters Program. This conference paper provides highlights of the early LCRD experiments and a preview of the future experiments, including relaying data to and from the Integrated LCRD Low- Earth Orbit (LEO) User Modem and Amplifier Terminal (ILLUMA-T) on the International Space Station. The LCRD geosynchronous payload includes two laser communications terminals interconnected via an onboard electronic switch, and can relay information between two optical ground stations located in California and Hawaii. LCRD is a joint project involving NASA Goddard Space Flight Center (GSFC), the NASA Jet Propulsion Laboratory (JPL), and Massachusetts Institute of Technology Lincoln Laboratory (MIT LL).
For some free-space laser communications (lasercom) links impacted by atmospheric-induced scintillation, the principle of reciprocity provides knowledge of a remote terminal's received intensity fluctuations based on measurements of the local terminal's received intensity fluctuations. We evaluate a reciprocity-enhanced technique using optical switching between multiple spatially-diverse transmit apertures to mitigate against atmospheric-induced scintillation. Experimental bit-error rate measurements are presented to quantify the expected performance of this approach in comparison to standard static diversity approaches. We investigate system constraints of this reciprocity-enhanced transmitter diversity approach, and identify candidate optical switches and digital logic for implementation.
KEYWORDS: Receivers, Forward error correction, Transmitters, Field programmable gate arrays, Clocks, Signal attenuation, Photodetectors, Scattering, Data communications, Signal detection
We demonstrate a multi-rate burst-mode photon-counting receiver for undersea communication at data rates up to 10.416 Mb/s over a 30-foot water channel. To the best of our knowledge, this is the first demonstration of burst-mode photon-counting communication. With added attenuation, the maximum link loss is 97.1 dB at λ=517 nm. In clear ocean water, this equates to link distances up to 148 meters. For λ=470 nm, the achievable link distance in clear ocean water is 450 meters. The receiver incorporates soft-decision forward error correction (FEC) based on a product code of an inner LDPC code and an outer BCH code. The FEC supports multiple code rates to achieve error-free performance. We have selected a burst-mode receiver architecture to provide robust performance with respect to unpredictable channel obstructions. The receiver is capable of on-the-fly data rate detection and adapts to changing levels of signal and background light. The receiver updates its phase alignment and channel estimates every 1.6 ms, allowing for rapid changes in water quality as well as motion between transmitter and receiver. We demonstrate on-the-fly rate detection, channel BER within 0.2 dB of theory across all data rates, and error-free performance within 1.82 dB of soft-decision capacity across all tested code rates. All signal processing is done in FPGAs and runs continuously in real time.
NASA’s Laser Communication Relay Demonstration (LCRD) will be NASA’s first long-duration demonstration of laser communications (lasercom) in space, providing geosynchronous-satellite-hosted bidirectional relay services between two Earth ground stations. LCRD will leverage and enhance existing ground stations. Ground Station 1 (GS-1) will leverage the Optical Communications Telescope Laboratory (OCTL) built by JPL, while Ground Station 2 (GS-2) will leverage the Lunar Laser Communications Demonstration (LLCD) Ground Terminal (LLGT) built by MIT Lincoln Laboratory. While each ground system has unique telescopes and integrated optics, many of the backend subsystems (e.g., communications, environmental monitoring, control, user simulators) will be common to both terminals. Here we provide an overview of the architecture of the LCRD ground stations, and the planned enhancements to the existing facilities.
Pointing, acquisition, and tracking (PAT) systems in spaceborne optical communications terminals can exploit inertial sensors and actuators to counter platform vibrations and maintain steady beam pointing. Interferometric fiber optic gyroscopes (IFOGs) can provide sensitive angle rate measurements down to very low (sub-milliHertz) mechanical frequencies, potentially reducing the required beacon power and facilitating acquisition for a spaceborne optical communications terminals. Incoherent broadband light sources are used in IFOGs to alleviate detrimental effects of optical nonlinearities, backscattering, and polarization non-reciprocity. But incoherent broadband sources have excess noise or relative intensity noise (RIN), caused by the beating of different spectral components on the photodetector. Unless RIN noise is suppressed, IFOG performance cannot be improved once the light on the photodetector exceeds one photon per coherence time (~microWatts). We propose a simple method to dramatically suppress the RIN of an incoherent light source and thereby reduce the angle random walk (ARW) of an IFOG using such a source. We demonstrate 20 dB RIN suppression of a broadband EDFA source, which we predict could improve the angle random walk (ARW) of an IFOG using this source by 12 dB.
NASA’s Laser Communication Relay Demonstration (LCRD) aims to demonstrate a geosynchronous satellite laser
communications (lasercom) relay between two independent ground terminals. We report on the design of two
adaptive optics (AO) techniques for LCRD Ground Station #2 (GS-2). GS-2 leverages the ground terminal
developed for NASA’s Lunar Laser Communications Demonstration (LLCD). Equipping GS-2’s 40cm diameter
receive telescope with AO to mitigate atmospheric turbulence effects will enable the use of single mode, optically
preamplified receivers for high data-rate near-Earth relay applications. In this work a direct wavefront sensing AO
approach using a Shack-Hartmann sensor and a continuous facesheet micro-electro-mechanical system (MEMS)
deformable mirror (DM) was compared with an indirect sensing, hill-climbing or multidither approach using a
segmented MEMS DM. Design concepts and recent experimental progress for the two approaches are presented.
We present a design methodology for free-space laser communications systems. The first phase includes a
characterization through numerical simulations of the channel to evaluate the range of extinction and scintillation. The
second phase is the selection of fade mitigation schemes, which would incorporate pointing, acquisition, tracking, and
communication system parameters specifically tailored to the channel. Ideally, the process would include sufficient
flexibility to adapt to a wide range of channel conditions. We provide an example of the successful application of this
design approach to a recent set of field experiments.
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.
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 free-space communications community has only recently recognized the complexity of atmospheric channel
interactions, which are highly dependent on the turbulence profile, beam propagation geometry, and transceiver design.
The search for models that accurately describe link performance and overall availability is currently an active field of
research. This paper describes a method for defining link availability based on statistical channel models, which can be
derived from measured signal fluctuations during periods of stable atmospheric conditions. Measurements made during
an extended communication link experiment conducted during the summer of 2008 indicate that the intervals of channel
stability, which impact the length of link outages, can vary in duration from a few minutes to several hours.
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.
As part of a free-space optical communications experiment over a 5km horizontal path, an extensive database of
tilt-stabilized receiver data was collected for Cn2
n conditions ranging from benign to very strong. This paper focuses
on the scintillation measurements made during those tests. Ensemble probability distributions are compiled from
these results, and are subsequently compared with standard channel models such as the log-normal and gammagamma
distributions. Statistical representations of temporal behavior are also developed from this database.
Accurate statistical models of atmospheric channel effects have proved to be invaluable in the development of
high-performance free-space transceivers.
The path-integrated turbulence strength is usually thought of as a parameter that varies slowly with time. In a
recent free-space communications experiment the Cn2
n value over a 5-km horizontal path was monitored almost
continuously for a period of nearly a month. In addition to well defined and repeatable diurnal fluctuations,
strong short-term fluctuations were observed in which the turbulence strength changed by an order of magnitude
within a period of minutes. These rapid changes were independently measured by a commercial scintillometer
and the high-rate output from the communications transceiver. The characteristics and probable causes for these
dynamic atmospheric events and their impact on the design of
free-space communication systems are discussed
in this article.
High speed optical communications systems are evolving rapidly. Commercial systems achieve high aggregate data rates utilizing wavelength division multiplexing, where multiple wavelength channels carry information at electronic rates, typically 2.5 Gb/s. Data encryption in these systems will most likely be implemented electronically. However, future system may also utilize time division multiple access (TDMA) schemes and technologies for 100 Gb/s, single stream TDMA networks are currently being developed. These high speed TDMA networks will rely on all-optical switches and processors to interface the high-speed electronics in the users nodes to the ultra-high-speed optical data bus. Data encryption in these networks may need to be implemented using optical logic gates. Straightforward duplication of electronic encryption circuits using optical logic gates is not feasible because optical logic gates have low fan-out, require high optical powers, are difficult to synchronize and have high latency. In this paper, we propose a high- speed electro-optic scheme for reconfigurable feedback shift registers (RFSRs) that relies upon electronic encryption circuits to reconfigure a sequence of optical logic gates and which makes use of the latency in the optical gates as memory. We show that, for linear RFSRs, the low number of optical gates is not a drawback and that the period of the sequences is generally very large. Non-linear feedforward functions, such as all-optical bit swapping, many also be introduced to improve the pseudo-random properties of the sequences.
Optical fiber storage loops storing packets of high-speed optical return-to-zero data have been demonstrated. Storage times greater than 30 minutes have been achieved. A basic storage loop design, demonstrated at 20 Gb/s, resembles an active-passive harmonically mode-locked fiber laser. A new modulation technique, rational-harmonic modulation, has been used in a similar storage loop design to achieve 50 Gb/s storage, using a modulator with only a 13 GHz 3 dB bandwidth. A 10.6 Gb/s all-optical storage loop has also been demonstrated, in which the electro-optic modulator has been replaced with a semiconductor diode amplifier which is driven by an optical clock.
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