The future demand for enhanced telecommunication capacity required to support human and robotic exploration from deep-space has motivated the advancement of free-space laser communication technologies for the past few decades. Steady advances in these technologies, validated through space-to-ground demonstrations, have resulted in incremental advances with the deep-space optical communications (DSOC) technology demonstration being one of the next milestones on NASA’s roadmap. NASA’s Psyche Mission to launch early next decade plans to host a DSOC flight laser transceiver for link demonstrations extending from 0.1 to farther than 2 astronomical units (AU). The capabilities validated though this demonstration, we expect, could spur the use of optical communications infrastructure around Mars in the next few decades. In this paper we summarize ongoing activities underway at the Jet Propulsion Laboratory in preparation for the DSOC technology demonstration and go on to present discussions on the drivers for developing a robust deep space laser communications operational capability.
A multi-beam beacon was transmitted from the Optical Communication Telescope Laboratory (OCTL) located at Table Mountain, CA to the Lunar Laser Space Terminal (LLST), on-board the Lunar Atmospheric Dust and Environment Explorer (LADEE) spacecraft, during NASA’s recent Lunar Laser Communication Demonstration (LLCD). The laser beacon (1568±0.1 nm) was square wave modulated and sensed by a quadrant sensor on LLST. While link acquisition and tracking proceeded with the sensed signal, on-board processing extracted power incident on the quadrant sensor and telemetered it down over the optical downlink. Subsequently, post-processing of the codewords received at OCTL retrieved the power time series recorded at LLST. Analysis comparing measured and predicted mean irradiance delivered to LLST consistently agreed to within < 1 decibel (dB). Irradiance fluctuations detected at LLST were reconciled with an uplink wave-propagation simulation model using Kolmogorov phase screens.
The Laser Communication Relay Demonstration is NASA’s multi-year demonstration of laser communication to a geosynchronous satellite. We are currently assembling the optical system for the first of the two baseline ground stations. The optical system consists of an adaptive optics system, the transmit system and a camera for target acquisition. The adaptive optics system is responsible for compensating the downlink beam for atmospheric turbulence and coupling it into the modem’s single mode fiber. The adaptive optics system is a woofer/tweeter design, with one deformable mirror correcting for low spatial frequencies with large amplitude and a second deformable mirror correcting for high spatial frequencies with small amplitude. The system uses a Shack- Hartmann wavefront sensor. The transmit system relays four beacon beams and one communication laser to the telescope for propagation to the space terminal. Both the uplink and downlink beams are centered at 1.55 microns. We present an overview of the design of the system as well as performance predictions including time series of coupling efficiency and expected uplink beam quality.
Optical Ground Station 1 (OGS1) is the first of a new breed of dedicated ground terminals to support NASA’s developing space-based optical communications infrastructure. It is based at NASA’s Optical Communications Telescope Laboratory (OCTL) at the Table Mountain Observatory near Wrightwood, CA. The system will serve as the primary ground station for NASA’s Laser Communications Relay Demonstration (LCRD) experiment. This paper presents an overview of the OCTL telescope facility, the OGS1 ground-based optical communications systems, and the networking and control infrastructure currently under development. The OGS1 laser safety systems and atmospheric monitoring systems are also briefly described.
A number of laser communication link demonstrations from near Earth distances extending out to lunar ranges have been remarkably successful, demonstrating the augmented channel capacity that is accessible with the use of lasers for communications. The next hurdle on the path to extending laser communication and its benefits throughout the solar system and beyond is to demonstrate deep-space laser communication links. In this paper, concepts and technology development being advanced at the Jet Propulsion Laboratory (JPL) in order to enable deep-space link demonstrations to ranges of approximately 3 AU in the next decade, will be discussed.
The Laser Communications Relay Demonstration (LCRD) will implement an optical communications link between
a pair of Earth terminals via an Earth-orbiting satellite relay. Optical turbulence over the communication
paths will cause random
uctuations, or fading, in the received signal irradiance. In this paper we characterize
losses due to fading caused by optical turbulence. We illustrate the performance of a representative relay link,
utilizing a channel interleaver and error-correction-code to mitigate fading, and provide a method to quickly
determine the link performance.
Ground-based, low-cost, uncooled infrared imagers are specially calibrated and deployed for long-term measurements of spatial and temporal cloud statistics. Measurements of cloud optical depth are shown for thin clouds, and validated with a dual-polarization cloud lidar. Good comparisons are achieved for thin clouds having 550-nm optical depth of 3 or less.
A number of space agencies, including NASA, are considering free-space laser communications as a means for returning
higher data-rates from future space missions. In this paper, potential deep-space missions are evaluated to show that
with optical communication a 10× increase relative to state-of-the art telecommunication systems could be achieved.
The maximum deep-space distance where ground transmitted laser beacons could assist acquisition and tracking; and
operating points where optical communication performance degrades faster than the inverse square distance are also
Previous research at Montana State University led to the development of the Infrared Cloud Imager (ICI) for measuring
downwelling cloud and sky thermal emission for producing cloud coverage statistics using radiometrically calibrated
images of the sky. This technique, that was developed primarily for detection of clouds for studies of arctic climate,
provides benefits over commonly used systems by producing localized high resolution data in comparison to satellites
images, and, in contrast to visible systems, provides continuous day and night operation. As a continuation of the first
effort, in collaboration with the Optical Communications Group at the NASA's Jet Propulsion Laboratory (JPL), here we
present a new generation of the ICI that can be used to monitor the cloud coverage of a site that can house a ground
telescope dedicated to Earth-space optical communication paths. This new instrument, based around the FLIR Photon
camera, expands the field of view (FOV) from 20° to 50° (up to 100° in the latest version), reduces instrument size,
reduces instrument cost, and extends the time between calibrations to hours instead of minutes. This has been
accomplished by characterizing the changes in the output data for changes in the camera's internal temperature while
viewing a constant source. Deployment of this instrument has taken place at JPL's Table Mountain facility, CA, and
The requirements and design concepts for a ground-based laser assembly for transmitting an uplink beacon to a Mars
bound spacecraft, carrying a laser communications terminal, are reported. The effects of the atmosphere are analyzed
and drive the multi-beam design.
NASA’s upcoming Mars Laser Communication Demonstration (MLCD) scheduled for the 2010-2011 time-frame is planning to use the Hale telescope at Palomar Mountain, California to receive the downlink. The optical links will be demonstrated in the presence of daytime sky backgrounds with the characteristic faint laser signal associated with transmission from deep space. A system level description for acquiring and tracking the laser downlink signal in order to achieve the desired communications performance is presented.
Optical links from a spacecraft at planetary distance to a ground-based receiver presume a cloud free line of site (CFLOS). Future ground-based optical receiving networks, should they be implemented, will rely on site diversity of cloud cover to increase link availability. Recent analysis shows that at least 90% and as high as 96% CFLOS availability can be realized from a cluster comprised of 3-4 nodes. During CFLOS availability variations of atmospheric parameters such as attenuation, sky radiance and “seeing” will determine the link performance. However, it is the statistical distributions of these parameters at any given node that will ultimately determine the data volumes that can be realized. This involves a complex interaction of site-specific atmospheric parameters. In the present work a simplified approach toward addressing this problem is presented. The worst-case link conditions for a spacecraft orbiting Mars, namely, maximum range (2.38 AU) and minimum sun-Earth-probe (SEP) angle of 3-10° is considered. A lower bound of ~100 Gbits/day under the most stressing link conditions is estimated possible.
Cloud opacity is one of the main atmospheric physical phenomena that can jeopardize the successful completion of an optical link between a spacecraft and a ground station. Hence, the site location chosen for a telescope used for optical communications must rely on knowledge of weather and cloud cover statistics for the geographical area where the telescope itself is located. In this work, the effects of cloud cover on an optical link are statistically described, considering ten observation sites at locations in the southwestern United States, From California to Texas. The data used for the preparation of this work are surface observation data provided by the National Climatic Data Center (NCDC). NCDC provides hourly information on the cloud coverage of an observation site. Using proper algorithms, these data give a statistical description of link blockage over the ten selected observations sites. Statistics averaged over a number of years for each observation site are presented. Cloud coverage statistics for two and three site diversity are also given for a ground network of optical telescopes. Finally, it is shown quantitatively how the use of two or three telescopes can improve the probability of completion of an optical link and how to select the right locations for a ground network of telescopes in the southwestern United States.