Daniel E. Raible

A prototype high-speed optically-steered X-band phased array antenna

We develop a prototype of optically-steered X-band phased array antenna with capabilities of multi-band and multi-beam operations. It exploits high-speed wavelength tunable lasers for optical true-time delays over a dispersive optical fiber link, enabling agile, broadband and vibration-free RF beam steering with large angle.

Optical frequency optimization of a high intensity laser power beaming system utilizing VMJ photovoltaic cells

An effective form of wireless power transmission (WPT) has been developed to enable extended mission durations, increased coverage and added capabilities for both space and terrestrial applications that may benefit from optically delivered electrical energy. The high intensity laser power beaming (HILPB) system enables long range optical ‘refueling’ of electric platforms such as micro unmanned aerial vehicles (MUAV), airships, robotic exploration missions and spacecraft platforms. To further advance the HILPB technology, the focus of this investigation is to determine the optimal laser wavelength to be used with the HILPB receiver, which utilizes vertical multi-junction (VMJ) photovoltaic cells. Frequency optimization of the laser system is necessary in order to maximize the conversion efficiency at continuous high intensities, and thus increase the delivered power density of the HILPB system. Initial spectral characterizations of the device performed at the NASA Glenn Research Center (GRC) indicate the approximate range of peak optical-to-electrical conversion efficiencies, but these data sets represent transient conditions under lower levels of illumination. Extending these results to high levels of steady state illumination, with attention given to the compatibility of available commercial off-the-shelf semiconductor laser sources and atmospheric transmission constraints is the primary focus of this paper. Experimental hardware results utilizing high power continuous wave (CW) semiconductor lasers at four different operational frequencies near the indicated band gap of the photovoltaic VMJ cells are presented and discussed. In addition, the highest receiver power density achieved to date is demonstrated using a single photovoltaic VMJ cell, which provided an exceptionally high electrical output of 13.6 W/cm2 at an optical-to-electrical conversion efficiency of 24 %. These results are very promising and scalable, as a potential 1.0 m2 HILPB receiver of similar construction would be able to generate 136 kW of electrical power under similar conditions.

On the Physical Realizability of Hybrid RF and Optical Communications Platforms for Deep Space Applications

There are hefty constraints levied on all space systems, and some of the most fundamental of them are purely physical. Much of the literature published on optical communications for space data systems focuses on the increased capabilities that optical communication systems have to offer, but ignores the practical infusion strategies of the technology into an operable network. Significant resources have been spent over the better half of the last century across multiple governments to establish robust radio frequency (RF) infrastructure around the world, and any new spacecraft will include provisions to utilize that investment. Therefore, every optical communication system flown will be a functional subset of a larger communication architecture dominated by RF. The key is to optimize the architecture and integrated components into a system that can be used to support hybridized RF and optical communications within the same asset, throughout diverse atmospheric (weather) and in-space conditions. The focus of this paper is on the physical realizability of a hybrid optical and RF communications system, and its ability to increase mission capability without imparting extra burden to the host spacecraft in the form of either increased physical mass or power requirements or additional operational requirements. The space applications may feature a hybrid RF/optical aperture this conjunction of a telescope and an antenna has been dubbed the teletenna. The teletenna is a single co-boresighted aperture that combines the radio and optical beams together into one physical package. The tradeoffs associated with the teletenna optimization are discussed, as well as the pointing requirements and strategies. Initial optical terminals on spacecraft have utilized Earth-based beacons Aerospace Technologist in Telecommunications, Optics and Photonics Branch (LCP), 21000 Brookpark Road/Mail Stop 54-1, AIAA and Technical Committee Member Aerospace Technologist in Telecommunications, High Frequency Branch (LCH), 21000 Brookpark Road/Mail Stop 54-1 Senior Aerospace Technologist in Telecommunications, Architectures, Networks and System Integration Branch, (LCA), 21000 Brookpark Road/Mail Stop 54-1 Aerospace Technologist in Data Systems, Information and Signal Processing Branch (LCI), 21000 Brookpark Road/Mail Stop 54-1 Research Aerospace Technologist in Data Systems, Networking and Architectures Branch (LCA), 21000 Brookpark Road/Mail Stop 54-1 Research Aerospace Technologist in Telecommunications, Information and Signal Processing Branch (LCI), 21000 Brookpark Road/Mail Stop 54-1 Aerospace Technologist in Mechanical Components, (LMT), 21000 Brookpark Road/Mail Stop 86-2 to establish beam tracking, but eventually beacon-less pointing will be developed. Investments in technologies to implement autonomous on-board navigation (and maneuvering) will permit a reduction in dependence on ground-based tracking, ranging, trajectory/orbit/attitude determination and maneuver planning support functions resulting in increased intelligence and autonomy of the complete payload. Innovative approaches include exploiting the optical communications terminal to perform navigational measurements such as star sighting, or a star tracker technology developed to process images at very high data rates. This paper discusses how such a beaconless strategy may be incorporated to work with the proposed hybrid communications system, and highlights the synergistic benefit to both communication and navigation functions for the purpose of decreased size, mass and power burden to users. A common constraint between both deep space and near Earth scenarios is mass. As elegant as a proposed optical communications solution may be, it must be shown that the final product is not so cumbersome as to outweigh the benefits, and that includes the necessary RF technology to complete the full communications payload. We establish a stringent goal of compressing the aforementioned highly capable hybrid RF/optical system into a package which installs within a comparable size, mass, and power envelope as the telecommunications subsystem of Mars Reconnaissance Orbiter (MRO). The goal of this paper is to demonstrate that with modern technology our goal is feasible and therefore has a place in today’s space data systems as well as building a bridge to tomorrow’s.

Dual-pulse pulse position modulation (DPPM) for deep-space optical communications: Performance and practicality analysis

Due to its simplicity and robustness against wave-front distortion, pulse position modulation (PPM) with photon counting detector has been seriously considered for long-haul optical wireless systems. This paper evaluates the dual-pulse case and compares it with the conventional single-pulse case. Analytical expressions for symbol error rate and bit error rate are first derived and numerically evaluated, for the strong, negative-exponential turbulent atmosphere. The capacity of the turbulent FSO channel modeled as a Z-channel is evaluated, and throughput, bandwidth efficiency and energy efficiency of Dual-pulse and single-pulse PPM are subsequently assessed. It is shown that, under a set of practical constraints including pulse width and pulse repetition frequency (PRF), dual-pulse PPM enables a better channel utilization and hence a higher throughput than its single-pulse counterpart. This result is new and different from the previous idealistic studies that showed multi-pulse PPM provided no essential information-theoretic gains over single-pulse PPM.

Concurrent system engineering and risk reduction for dual-band (RF/optical) spacecraft communications

This paper describes a system engineering approach to examining the potential for combining elements of a deep-space RF and optical communications payload, for the purpose of reducing the size, weight and power burden on the spacecraft and the mission. Figures of merit and analytical methodologies are discussed to conduct trade studies, and several potential technology integration strategies are presented. Finally, the NASA Integrated Radio and Optical Communications (iROC) project is described, which directly addresses the combined RF and optical approach.

Integrated RF/Optical Interplanetary Networking Preliminary Explorations and Empirical Results

Abstract Over the last decade interplanetary telecommunication capabilities have been significantly expanded—specifically in support of the Mars exploration rover and lander missions. NASA is continuing to drive advances in new, high payoff optical communications technologies to enhance the network to Gbps performance from Mars, and the transition from technology demonstration to operational system is examined through a hybrid RF/optical approach. Such a system combines the best features of RF and optical communications considering availability and performance to realize a dual band trunk line operating within characteristic constraints. Disconnection due to planetary obscuration and solar conjunction, link delays, timing, ground terminal mission congestion and scheduling policy along with space and atmospheric weather disruptions all imply the need for network protocol solutions to ultimately manage the physical layer in a transparent manner to the end user. Delay Tolerant Networking (DTN) is an approach under evaluation which addresses these challenges. A multi-hop multi-path hybrid RF and optical test bed has been constructed to emulate the integrated deep space network and to support protocol and hardware refinement. Initial experimental results characterize several of these challenges and evaluate the effectiveness of DTN as a solution to mitigate them.

On Applications of Disruption Tolerant Networking to Optical Networking in Space

Abstr act - The integration of optical communication links into space networks via Disruption Tolerant Networking (DTN) is a largely unexplored area of research. Building on successful foundational work accomplished at JPL, we discuss a multihop multi-path network featuring optical links. The experimental test bed is constructed at the NASA Glenn Research Center featuring multiple Ethernet-tofiber converters coupled with free space optical (FSO) communication channels. The test bed architecture models communication paths from deployed Mars assets to the deep space network (DSN) and finally to the mission operations center (MOC). Reliable versus unreliable communication methods are investigated and discussed; including reliable transport protocols, custody transfer, and fragmentation. Potential commercial applications may include an optical communications infrastructure deployment to support developing nations and remote areas, which are unburdened with supporting an existing heritage means of telecommunications. Narrow laser beam widths and control of polarization states offer inherent physical layer security benefits with optical communications over RF solutions. This paper explores whether or not DTN is appropriate for space-based optical networks, optimal payload sizes, reliability, and a discussion on security.

Networked Operations of Hybrid Radio Optical Communications Satellites

In order to address the increasing communications needs of modern equipment in space, and to address the increasing number of objects in space, NASA is demonstrating the potential capability of optical communications for both deep space and near-Earth applications. The Integrated Radio Optical Communications (iROC) is a hybrid communications system that capitalizes on the best of both the optical and RF domains while using each technology to compensate for the others shortcomings. Specifically, the data rates of the optical links can be higher than their RF counterparts, whereas the RF links have greater link availability. The focus of this paper is twofold: to consider the operations of one or more iROC nodes from a networking point of view, and to suggest specific areas of research to further the field. We consider the utility of Disruption Tolerant Networking (DTN) and the Virtual Mission Operation Center (VMOC) model.

High Data Rate Architecture (HiDRA)

One of the greatest challenges in developing new space technology is in navigating the transition from ground based laboratory demonstration at Technology Readiness Level 6 (TRL-6) to conducting a prototype demonstration in space (TRL-7). This challenge is com- pounded by the relatively low availability of new spacecraft missions when compared with aeronautical craft to bridge this gap, leading to the general adoption of a low-risk stance by mission management to accept new, unproven technologies into the system. Also in consideration of risk, the limited selection and availability of proven space-grade components imparts a severe limitation on achieving high performance systems by current terrestrial technology standards. Finally from a space communications point of view the long duration characteristic of most missions imparts a major constraint on the entire space and ground network architecture, since any new technologies introduced into the system would have to be compliant with the duration of the currently deployed operational technologies, and in some cases may be limited by surrounding legacy capabilities. Beyond ensuring that the new technology is verified to function correctly and validated to meet the needs of the end users the formidable challenge then grows to additionally include: carefully timing the maturity path of the new technology to coincide with a feasible and accepting future mission so it flies before its relevancy has passed, utilizing a limited catalog of available components to their maximum potential to create meaningful and unprecedented new capabilities, designing and ensuring interoperability with aging space and ground infrastructures while simultaneously providing a growth path to the future. The International Space Station (ISS) is approaching 20 years of age. To keep the ISS relevant, technology upgrades are continuously taking place. Regarding communications, the state-of-the-art communication system upgrades underway include high-rate laser terminals. These must interface with the existing, aging data infrastructure. The High Data Rate Architecture (HiDRA) project is designed to provide networked store, carry, and forward capability to optimize data flow through both the existing radio frequency (RF) and new laser communications terminal. The networking capability is realized through the Delay Tolerant Networking (DTN) protocol, and is used for scheduling data movement as well as optimizing the performance of existing RF channels. HiDRA is realized as a distributed FPGA memory and interface controller that is itself controlled by a local computer running DTN software. Thus HiDRA is applicable to other arenas seeking to employ next-generation communications technologies, e.g. deep space. In this paper, we describe HiDRA and its far-reaching research implications.

Comparison of square and radial geometries for high intensity laser power beaming receivers

In an effort to further advance a realizable form of wireless power transmission (WPT), high intensity laser power beaming (HILPB) has been developed for both space and terrestrial applications. Unique optical-to-electrical receivers are employed with near infrared (IR-A) continuous-wave (CW) semiconductor lasers to experimentally investigate the HILPB system. In this paper, parasitic feedback, uneven illumination and the implications of receiver array geometries are considered and experimental hardware results for HILPB are presented. The TEM00 Gaussian energy profile of the laser beam presents a challenge to the effectiveness of the receiver to perform efficient photoelectric conversion, due to the resulting non-uniform illumination of the photovoltaic cell arrays. In this investigation, the geometry of the receiver is considered as a technique to tailor the receiver design to accommodate the Gaussian beam profile, and in doing so it is demonstrated that such a methodology is successful in generating bulk receiver output power levels reaching 25 W from 7.2 cm2 of photovoltaic cells. These results are scalable, and may be realized by implementing receiver arraying and utilizing higher power source lasers to achieve a 1.0 m2 receiver capable of generating over 30 kW of electrical power. This type of system would enable long range optical ‘refueling’ of electric platforms, such as MUAVs, airships, robotic exploration missions and provide power to spacecraft platforms which may utilize it to drive electric means of propulsion. In addition, a smaller HILPB receiver aperture size could be utilized to establish a robust optical communications link within environments containing high levels of background radiance, to achieve high signal to noise ratios.