Keith E. Wilson

Overview of the Ground-to-Orbit Lasercom Demonstration (GOLD)

The ground-to-orbit Lasercom Demonstration conducted between the ETS-VI spacecraft and the ground station at JPLs Table Mountain Facility, Wrightwood CA was the first ground-to- space two-way optical communications experiment. The demonstration was conducted over a period of seven months and required simultaneous and cooperative operations by team members in Tokyo and California. A key objective was to measure the atmospheric attenuation and seeing during the demonstration to validate the performance of the optical link. The telemetry downlinked from the laser communications equipment provided information on the in-orbit performance of the onboard laser transmitter. Downlinked PN data enabled measurement of bit error rates. BERs as low as 10-4 were measured on the uplink and 10-5 on the downlink. Measured signal powers agreed with theoretical predictions.

Overview of the Laser Communications Relay Demonstration Project

Abstract : This paper provides an overview of the Laser Communications Relay Demonstration Project (LCRD), a joint project between NASA?s Goddard Space Flight Center (GSFC), the Jet Propulsion Laboratory, California Institute of Technology (JPL), and the Massachusetts Institute of Technology Lincoln Laboratory (MIT/LL). LCRD will provide two years of continuous high data rate optical communications in an operational environment demonstrating how optical communications can meet NASA?s growing need for higher data rates, or for the same data rate provided by a comparable RF system, how it enables lower power, lower mass communications systems on user spacecraft. In addition, LCRD?s architecture will allow it to serve as a testbed in space for the development of additional symbol coding, link and network layer protocols, etc. This paper reviews the current concepts and designs for the flight and ground optical communications terminals, the critical technologies required, and the concept of operations. It reports preliminary conclusions from several trade studies conducted at GSFC, JPL, and MIT/LL. The flight optical communications terminals will be flown on a commercial communications satellite in geosynchronous orbit to be launched no earlier than December 2016, and will demonstrate a technology critical for NASA?s Next Generation Tracking and Data Relay Satellite.

Preliminary results of the Ground/Orbiter Lasercom Demonstration experiment between Table Mountain and the ETS-VI satellite

The Ground/Orbiter Lasercomm Demonstration (GOLD) is an optical communications demonstration between the Japanese engineering test satellite (ETS-VI) and an optical ground transmitting and receiving station at the Table Mountain Facility in Wrightwood, California. Laser transmissions to the satellite are performed approximately four hours every third night when the satellite is at apogee above Table Mountain. The experiment required the coordination of resources at CRL, JPL, NASDAs Tsukuba tracking station and NASAs Deep Space Network at Goldstone, Calif. to generate and transmit real-time commands and receive telemetry from the ETS-VI. Transmissions to the ETS-VI began in November 1995 and are scheduled to last into the middle of January 1996 when the satellite is expected to be eclipsed by the Earths shadow for a major part of its orbit. The eclipse is expected to last for about two months, and during this period there will be limited electrical power available on board the satellite. NASDA plans to restrict experiments with the ETS-VI satellite during this period, and no laser transmissions are planned. Post-eclipse experiments are currently being negotiated. GOLD is a joint NASA-CRL (Communications Research Laboratory) experiment that is being conducted by JPL in coordination with CRL and NASDA.

Comparative study of optical and rf communication systems for a Mars mission

We have performed a study on telecommunication systems for a hypothetical mission to Mars. The objective of the study was to evaluate and compare the benefits that microwave (X-band and Ka-band) and optical communications technologies afford to future missions. The telecommunication systems were required to return data after launch and in-orbit at 2.7 AU with daily data volumes of 0.1, 1, or 10 Gbits. Space-borne terminals capable of delivering each of the three data rates were proposed and characterized in terms of mass, power consumption, size, and cost. The estimated parameters for X-band, Ka-band, and optical frequencies are compared and presented here. For data volumes of 0.1 and 1 Giga-bit per day, the X-band downlink system has a mass 1.5 times that of Ka-band, and 2.5 times that of optical system. Ka-band offered about 20% power saving at 10 Gbit/day over X-band. For all data volumes, the optical communication terminals were lower in mass than the rf terminals. For data volumes of 1 and 10 Gb/day, the space-borne optical terminal also had a lower required dc power. In all three cases, optical communications had a slightly higher development cost for the space terminal.

Deep space optical communications link availability and data volume

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.

GOPEX: a laser uplink to the Galileo spacecraft on its way to Jupiter

In the Galileo Optical Experiment (GOPEX), optical transmissions were beamed to the Galileo spacecraft by Earth-based transmitters at Table Mountain Observatory (TMO), California, and Starfire Optical Range (SOR), New Mexico. The demonstration took place over an eight-day period (December 9 through December 16) as Galileo receded from Earth on its way to Jupiter. At 6 million kilometers (15 times the Earth-Moon distance), the laser beam sent from Table Mountain Observatory eight days after Earth flyby covered the longest known range for laser transmission and detection.

Data analysis results from the GOLD experiments

Analyses of uplink and downlink data from recent free-space optical communications experiments carried out between Table Mountain Facility and the Japanese ETS-VI satellite are presented. Fluctuations in signal power collected by the satellites laser communication experiment due to atmospheric scintillation and its amelioration using multiple uplink beams are analyzed and compared to experimental data. Downlink data was analyzed to determine the cause of a larger than expected variation in signal strength. In spite of the difficulty in deconvolving atmospheric effects from pointing errors and spacecraft vibration, experimental data clearly indicate significant improvement in signal reception on the uplink with multiple beams, and the need for stable pointing to establish high data rate optical communications.

Development of laser beam transmission strategies for future ground-to-space optical communications

Optical communications is a key technology to meet the bandwidth expansion required in the global information grid. High bandwidth bi-directional links between sub-orbital platforms and ground and space terminals can provide a seamless interconnectivity for rapid return of critical data to analysts. The JPL Optical Communications Telescope Laboratory (OCTL) is located in Wrightwood California at an altitude of 2.2.km. This 200 sq-m facility houses a state-of- the-art 1-m telescope and is used to develop operational strategies for ground-to-space laser beam propagation that include safe beam transmission through navigable air space, adaptive optics correction and multi-beam scintillation mitigation, and line of sight optical attenuation monitoring. JPL has received authorization from international satellite owners to transmit laser beams to more than twenty retro-reflecting satellites. This paper presents recent progress in the development of these operational strategies tested by narrow laser beam transmissions from the OCTL to retro-reflecting satellites. We present experimental results and compare our measurements with predicted performance for a variety of atmospheric conditions.

Development of a 1-m Class Telescope at TMF to Support Optical Communications Demonstrations

With the impetus towards high data rate communications in inter-satellite and space-to-ground links, the small size, low-mass, and low-power consumption of optical communications is seen as a viable alternative to radio frequency links. Recent NASA/JPL optical communications field demonstrations have shown some of the operational strategies needed for space-to-ground optical links. In preparation for the optical communications demonstrations planned for the turn of the century, NASA/JPL is building an Optical Communications Telescope Laboratory (OCTL) with a 1-m class telescope. The OCTL will be located at JPLs Table Mountain Facility complex in the San Bernadino mountains of Southern California and will be capable of supporting demonstrations with satellites from LEO to deep space ranges. In addition, it will support advanced optical communications research, astrometry and astronomy research.

Effect of aperture averaging on a 570-Mbps 42-km horizontal path optical link

Optical communications offer high data rate satellite to ground communications in a small, low mass, and low power consumption package. However, turbulence-induced scintillation degrades the link performance as the zenith angle increases. To investigate the effect of atmospheric turbulence on the optical link at high zenith angles, we performed a 570 Mbps optical communications link across a 42 km horizontal path, and have measured the effects of aperture averaging on the irradiance variance. The variance clearly showed a dependence on the aperture size, decreasing with increasing aperture size. These results were used to calculate the log-amplitude variance and the atmospheric structure constant, Cn2, across the link. The bit error rates across the link were also measured. The results show that the link performance was dominated by burst errors with error rates that ranged from 10-6 to 10-2, increasing with decreasing aperture size.