Site: NASA Lewis Research Center
21000 Brookpark Road
Cleveland, Ohio 44135
http://ctd.lerc.nasa.gov

WTEC: K. Bhasin (report author)

Hosts:

BACKGROUND

The mission of NASA's Commercial Space Communications Program conducted at the Lewis Research Center (LeRC) in Cleveland, Ohio, is to work in partnership with industry, academia, and other government agencies to enable new space communications capabilities that support NASA missions and increase U.S. industrial competitiveness, and foster the development of the National Information Infrastructure (NII) and Global Information Infrastructure (GII). The program is conducted by the following two organizations:

SPACE COMMUNICATIONS OFFICE

The Space Communications Office manages the ACTS program, carries out spectrum management for the agency, and plans and conducts collaborative experiments and projects to develop and demonstrate advanced communications technologies and services.

ACTS Experiment Office

The Advanced Communications Technology Satellite (ACTS) Experiments Office is responsible for the satellite's on-orbit operations as well as four different groups of earth terminals. The ACTS satellite has been on orbit since September 1993. This office coordinates, as well as schedules, experimenter activities. The staff is augmented by various contracts that provide 24 hour support to NASA's Master Ground Station. Tours are frequently given to members of the general public to make them knowledgeable about the ACTS Project. In addition to conducting earth station site operations, a variety of technology experiments are conducted. A sizable data processing effort is ongoing to reduce the vast amount of ACTS data in order to make it helpful to U.S. industry.

Spectrum Management Office

The NASA Spectrum Management Office is responsible for satisfying NASA program, mission, and project requirements for spectrum as well as providing technical advocacy in support of U.S. commercial aerospace industries in appropriate spectrum regulatory forums. The Spectrum Management Office is active in both national and international arenas in protecting and advocating both agency and commercial spectrum needs. These activities include providing supporting technical information and studies as well as interacting with other U.S. agencies and negotiating with foreign countries through the FCC, NTIA, Department of State, and the International Telecommunications Union.

Project Development & Integration Office

The Project Development & Integration Office (PDIO) is responsible for identifying and developing opportunities to demonstrate and insert advanced communications technologies into commercial applications and NASA missions. The PDIO is assisting the agency with the transition from its own dedicated communications infrastructure to emerging commercial communications solutions. The PDIO plans and executes projects in satellite-based aeronautical communications for air traffic management, and in direct distribution of broadband data from low earth orbiting spacecraft. In support of the ACTS program, the PDIO provides ACTS experiment support, propagation modeling, and rain attenuation experiments.

COMMUNICATIONS TECHNOLOGY DIVISION

In this division, a precompetitive space communication research and technology program is conducted in the following branches:

Satellite Networks and Architectures Branch

The branch staff members perform advanced research and development of next-generation, space-based information systems to enhance the role of satellite communications in the National and Global Information Infrastructure (NII/GII) and to meet future NASA mission communication needs. Since the realization of NII/GII is based upon global heterogeneous communication networks, standards and interoperability are an important consideration in the definition of the branch programs. The Satellite Networks and Architectures Branch carries out its mission through partnerships with the satellite communication industry and academia. The Center for Satellite and Hybrid Communication Networks at the University of Maryland, College Park, a NASA commercial space center, is an integral part of this program.

Electron Device Technology Branch

The Electron Device Technology Branch is responsible for conducting research and development in vacuum and solid state electronics in support of the commercial communications satellite industry and NASA mission needs. The work is directed primarily to increasing the efficiency, improving the reliability and reducing the mass of electron devices for space applications. A balanced program of computational and experimental research is conducted with frequent collaboration and consultation with U.S. industry, academia and other government agencies.

Applied Rf Technology Branch

The branch addresses applied radio frequency (rf) technologies, with emphasis on antennas. It conducts research and advanced development activities resulting in focused solutions for NASA enterprises and for commercial communications. It develops and demonstrates electrically scanned MMIC phased arrays, space fed active arrays, printed elements and arrays, power combining arrays, digital beam forming arrays and cryogenically cooled receivers.

Digital Communications Technology Branch

The branch works on next-generation modulation, coding, signal processing, and switching technologies. It conducts research and development activities resulting in focused solutions for commercial communications and NASA missions. It develops innovative digital subsystems, intelligent and autonomous control subsystems, and responsive computing solutions for the communications program.

RESEARCH AND DEVELOPMENT ACTIVITIES

Project Development and Integration Office—Applied Research Experiments

The effects of atmospheric propagation and rain effects must be accurately modeled and taken into consideration during the design of both the space and ground segment of high frequency communications satellites. The Project Development and Integration Office is conducting two activities in this area.

Rain Attenuation Prediction Modeling

Propagation modeling activity at LeRC has been an ongoing effort since the inception of the Advanced Communications Technology Satellite (ACTS) Project, for which such work was necessary. An enhanced rain attenuation prediction model has been constructed by Dr. Robert Manning for use in the optimal design of a communication satellite system operating at Ku-band and above (e.g., Ka-band, V-band,...) that can be applied for any location in the world. This unique model is being continuously refined and used at the request of several commercial companies for the design of future communications and direct broadcast satellite systems, and the Global Broadcasting System (GBS), which is a joint military effort. This capability is just now (9/97) gaining international recognition and it is expected that foreign concerns will also be using its novel capabilities.

Dr. Manning's pioneering mathematical modeling techniques have been incorporated into a tool that independent users confirm is more flexible and accurate than other rain attenuation prediction models in common use. Dr. Manning's model has become critical to those companies and agencies developing new communications systems in higher frequency bands, where the atmospheric effects are more pronounced, and yet the need for reliable, high-quality, efficient communications is a matter of worldwide market competitiveness and national interest.

Wet Antenna Effects Characterization

Dr. Roberto Acosta has recently discovered, and is characterizing through a set of ACTS experiments, the degradation to performance due to wet Ka-band ground based reflector antennas. The objective of his work is to experimentally characterize antenna wetting in a Ka-band ground based very small aperture terminal (VSAT) system. This process will include the theoretical explanation of physical phenomena and the development of compensation techniques for minimizing the effect. His early dissemination of findings and analyses at NASA propagation experiments (NAPEX) conferences has drawn significant attention from developers and users of satellite ground terminals. Experimental data collection was continuing into FY98, and periodic reports on results of the data analysis will be presented. The ACTS spacecraft and several VSAT's are used in the wet antenna experiments. In-house weather facilities (rain gauges) and the data acquisition network are located in the VSAT Characterization Laboratory.

The output of wet antenna research will be used to develop system performance requirements and design criteria with the revolutionary impact of potentially reducing system outages due to wet ground satellite antennas. These techniques and design criteria are being adopted by Hughes DirecTV (a Ku-band service) and by other Ka-band filers with the potential for increasing the quality of satellite systems. This research work can be easily extended to frequencies beyond Ka-band.

Project Development and Integration Office—Applied Technology Projects

The PDIO is planning and executing projects that demonstrate the application of advanced communications technologies to civil applications in air traffic management via satellite and in direct data distribution from LEO spacecraft.

Advanced Communications for Air Traffic Management (AC/ATM)

The AC/ATM Project is a sub-element of the Advanced Air Transportation Technologies (AATT) Program led by NASA Ames Research Center. AATT is an eight year program that began in 1997 and ends in 2004. The objective of the AATT Program is to develop new technologies that enable free flight; an operating system in which pilots have the freedom to select their path and speed in real time.

The objectives of LeRC's AC/ATM project are: to evaluate the technical, cost, schedule and risk characteristics of present and emerging communications, navigation and surveillance (CNS) systems, and technologies to provide the unique communications required by the Advanced ATM Concept; to define the requirements and opportunities for satellite communications in free-flight air traffic management and identify any CNS related research issues that need to be addressed to support the AATT Program; to demonstrate and evaluate emerging ATM concepts, procedures, and airborne technologies, both flight-deck and communications, through real-time simulation of the communications infrastructure and actual satellite experiments; and, to develop and demonstrate high-risk, high-pay-off advanced communications technologies required for airspace users to realize the benefits available under the future National Airspace System.

Direct Data Distribution (D3)

Raytheon TI Systems (RTIS) and LeRC are developing technologies to enable K-band direct data distribution (D3) from LEO spacecraft at 622 Mbps transmission rate to small (1.8 meter), low-cost autonomously tracking terminals. Under a 50% cost-shared cooperative agreement, RTIS is developing the first ever 19 GHz dual-beam transmit phased array antenna, while LeRC is providing a compatible high data rate digital encoder-modulator and a low-cost tracking ground terminal with terrestrial telecommunications network interfaces. Under the D3 project, a HitchHiker class Space Shuttle experiment in the 2000 time frame is planned, to demonstrate unprecedented wideband data distribution from LEO platforms, with fiber-like quality.

The LeRC D3 approach offers advantages over lower frequency (X-band), conventional mechanisms. The electronically steered (vibration free), compact array (about 8 inches in diameter and 1 inch thick), will provide an attractive alternative to much larger (about 1 meter in diameter), mechanically steered reflector antenna systems currently used on large and small spacecraft. Efficient, multichannel digital modulation will provide at least 4 times greater burst throughput and about 4 orders of magnitude better quality of service than is currently available. The D3 approach enables a new communications solution for government and commercial remote sensing satellites, the International Space Station, and near-earth science spacecraft, and applies as well to high-speed commercial satellite gateways to the terrestrial networks. The commercial space-to-earth frequency band (19 GHz) will enable NASA to explore the possibility of leveraging commercial communications assets to satisfy some of its operational needs.

Vacuum & Solid State Electronics Technologies

The work in vacuum electronics is primarily in the areas of computer modeling, computer-aided design techniques, electron emission, suppression of secondary emission, and testing of novel devices. The group claims a long list of computer modeling firsts, most recently in the area of helical TWTs and device optimization. The work in electron emission spans the range from basic research in surface physics and chemistry to life testing of cathodes, and combines both experimental and computational efforts.

The solid state research is focused on monolithic microwave integrated circuits (MMICs), materials characterization, thin film high temperature superconductor devices, device packaging, transmission lines, and thin film ferroelectric devices. The work is primarily experimental and is conducted in collaboration with U.S. industry and universities.

Digital Technologies

The work in modulation and coding is primarily for the development of power and bandwidth efficient modulation, combined modulation and coding schemes, and digital transmission techniques for application in satellite communication systems. Modulation and coding schemes are investigated, developed and optimized to meet a broad class of next generation commercial LEO, MEO and GEO satellite system requirements. Efficient digital implementations of candidate transmission schemes are developed, analyzed and validated via lab or field demonstrations.

Currently, forms of QPSK and BPSK based modulation schemes at 1 to 2 bits/Hz bandwidth efficiency and some limited use of trellis coded 8PSK (~3 bits/Hz) are the most prevalent in current systems. However, future satellite communication links will require a push to higher order modulation schemes (>4 bits/Hz bandwidth efficiency) combined with powerful coding to maintain compatibility with terrestrial data rates (100-1000 Mbps) with equal quality of service (10-9 error rate or better).

The Space Communications Technology Center (SCTC) is optimizing digital transmission techniques for various satellite rf link characterizations and terrestrial network standards by novel utilization of real time digital compression techniques combined with error correction codes to maintain robust, high speed data links.

Compatibility with commercial terrestrial and satellite communication systems will continue to play an important role as the goverment moves to augment current communication services with commercial services in order to reduce mission costs and maintain GII/NII interoperability. In addition to maintaining commercial service compatibility, the NASA user community is demanding increased data throughputs that are taxing current onboard data store and forward architectures that could be eased with high speed direct data downlink (D3) to users, central distribution, or archive sites.

The work in switching and routing is primarily for the development of highly reliable and efficient onboard processing (OBP) schemes necessary to achieve greater than 100 Gbps throughput to accommodate various traffic services including ATM, B-ISDN and SONET. In addition, high temperature superconducting (HTS) microwave components including filters and multiplexers are being developed for satellite communications. Switching and routing schemes along with miniaturized HTS/dielectric multilayer filters are being investigated to meet next generation commercial communications satellite requirements. Candidate schemes and components are being developed, tested and validated in-house and through industry and academia collaboration.

Advanced Antenna Technologies

Raytheon TI Systems (RTIS) and LeRC are developing a high gain, wide angle scanning 19 GHz MMIC transmit array under a cooperative agreement with 50/50 cost sharing. This array, with two independently steered beams, will be demonstrated in a space experiment of direct data distribution (D3) (see D3 write-up above). Under another cooperative agreement, LeRC and SS/Loral, Sanders, SRC and AFRL are developing a Ka-band active lens array with digital beam shaping. This cost shared effort will provide an engineering model of a distributed active lens capable of demonstrating simultaneous multiple beam operation with increased sensitivity and off-axis scan performance for GEO applications.

Reflect arrays providing high gain performance for space and ground applications are being investigated in-house. Innovative concepts using new approaches for element phasing and active beam steering at low cost are being developed. Another technology under investigation brings together cryocooler technology and HTS receiver device technologies in a cooled feed for reflector antennas, reducing noise temperature and thereby increasing G/T.

Other in-house investigations are focused toward advanced space-fed array designs, device integration technologies, and printed element and array designs and power combining. Definitive investigations have been performed on tapered slot antennas, a potentially low cost endfire-type configuration. Advanced metrology tools capable of characterizing array/modulator interactions for electrically large arrays under conditions of wide angle scanning have been developed.

Satellite Networks/Interoperability/System Research

LeRC is addressing an emerging need for development of architectural frameworks for the next-generation space-based global information systems. LeRC has begun to provide technical contributions to various standards-making bodies regarding architectures and reference models.

The approach is to define market trends for satellite addressable global information infrastructure markets, develop conceptual architectures for third generation space-based architectures, perform network analysis for a space-based information infrastructure, and perform technology and economic assessments.

Recently, studies have been completed on next-generation, space-based architectures for broadband services and market trends. The studies are available upon request. Technical contributions have been provided to ANSI-IISP and ITU-T Study Group 13 (GII Architectures).

Existing Internet applications are being tested to determine the effectiveness of the satellite link in the hybrid network. The satellites are being placed in networks of various sizes and topologies to fully characterize their capabilities. Telemedicine and tele-mammography applications are being tested over satellite with leading hospitals and medical schools using ACTS and other satellites.

Experiments are being carried out to evaluate proposed modifications to the TCP protocol. Extensions are being tested in an attempt to improve satellite communication. However, the extensions are also being tested in terrestrial environments. Among the extensions being tested are retransmission mechanisms based on selective acknowledgments (e.g., FACK TCP) and TCP with larger initial windows. Tests evaluate the mechanisms' performance benefits and their fairness to other traffic. LeRC participates in the Internet Engineering Task Force (IETF)'s TCP Over Satellite Working Group.

ATM testing is being performed to determine quality of service parameters that satellites must provide to remain competitive in the Global Information Infrastructure (GII) and to evaluate the effect of transmission link quality and characteristics on overall quality of service. The approach being used is to evaluate ATM over a noisy link (ATM was designed for "near" error-free channels such as fiber), evaluate digital video over satellites (digital video, particularly compressed video such as MPEG-2, is expected to require stringent quality of service), and evaluate effect of linked protocols.


Published: December 1998; WTEC Hyper-Librarian