Space Communications

The Advanced Communications Technology Satellite (ACTS) program was slated for decommissioning in October 2000 as was announced at the 6th Ka‐band Utilization Conference in May 2000. Quite a celebration was had at that event too, centering on the decommissioning of this very successful technology program [1]. With plans in place to move the spacecraft to an orbital graveyard and then shut the system down, NASA was challenged to consider the feasibility of extending operations for education and research purposes, provided that an academic organization would be willing to cover operations costs. Continuing operations of the system was determined viable and in the fall of 2000, an announcement was made by NASA to consider extending operations. Plans are now in place to continue the operations of ACTS through a university‐based consortium led by Ohio University, Athens, Ohio. Initial plans are for two more years of operations, with options to extend up to a total of four years.

This paper will present the change in plans to continue operations of ACTS. A description of the multi‐month transition of the spacecraft to its new and final orbital location is provided. With the spacecraft at this new location, an update on its performance is presented as well as estimates of long‐term performance. The consortium development will be presented along with its organization, membership and operations plans for using ACTS.

Ka‐band offers the promise of very low‐cost broadband terminals deployed in quantities an order of magnitude greater than the total number of all the two‐way C‐ and Ku‐band terminals installed since the birth of satellite communications. For small terminals, installation and commissioning costs can easily overwhelm equipment cost unless the terminal is pre‐qualified for interference‐limiting performance without on‐site testing. The satellite industry has addressed this issue for C‐ and Ku‐band with mutually‐recognized type approval procedures, but operation at 20 and 30 GHz gives rise to special considerations in equipment design, qualification, and testing. Work is beginning to adapt existing type approval procedures for Ka‐band and to review performance standards for discrepancies, with the ultimate goal of universal type approval procedures and standards recognized not only by satellite operators but also by government administrations.

For Ka‐band satellite systems to compete successfully in the broadband marketplace, two fundamental challenges must be addressed: cost, and the quality and performance of the offered services/applications. Specific issues related to these challenges are discussed and a set of solutions is proposed. The solutions range from the development of common standards to specific techniques to mitigate the effect of rain fades in Ka‐band frequencies and the propagation delay of the geosynchronous satellite. Finally, dynamic resource allocation and congestion control techniques are described which exploit the key characteristics of Ka‐band satellite systems and maximize the availability of satellite capacity to user applications.

In 1993, a proposal at the Japan‐US Science, Technology, and Space Applications Program (JUSTSAP) workshop led to a subsequent series of satellite communications experiments and demonstrations, under the title of Trans‐Pacific High Data Rate Satellite Communications Experiments. The first phase of which was a joint collaboration between government and industry teams in the United States and Japan that successfully demonstrated distributed high definition video (HDV) post‐production on a global scale using a combination of high data rate satellites and terrestrial fiber optic asynchronous transfer mode (ATM) networks. This was followed by the Phase‐2 Internet Protocol (IP) based experiments and demonstrations [4–6] in tele‐medicine and distance education, using another combination of two high data rate satellites and terrestrial fiber optic networks.

The Visible Human tele‐medicine and Remote Astronomy distance education demonstrations and their use of distributed systems technologies afforded an opportunity for people around the world to work together as a virtual team under one roof, using resources thousands of miles away as if they were next to each other. The visible human activity demonstrated global‐scale interactive biomedical image segmentation, labeling, classification, and indexing using large images; the remote astronomy activity demonstrated collaborative observation and distance education at multiple locations around the globe and the transparent operations of distributed systems technologies over a combination of broadband satellites and terrestrial networks.

The use of Internet Protocol related technologies allowed the general public to be an integral part of the exciting activities, helped to examine issues in constructing a global information infrastructure with broadband satellites, and afforded an opportunity to tap the research results from the (reliable) multicast and distributed systems communities. This paper summarizes the Phase‐2 of Trans‐Pacific series of experiments and demonstrations by an international team in Canada, Japan, and the United States.

In response to the demands for broadband satellite systems and services, TRW initiated the Gen*Star processing payload in 1995. The Gen*Star system objectives of profitable broadband connectivity with seamless interfaces to a terrestrial infrastructure imply highly reliable network nodes in space. To achieve these objectives, robust communications links and sophisticated onboard signal and data processing are required. The Gen*Star satellite payload provides those communication links and processing functions in a package suitable for launch into space, using technologies designed to operate for years in space.

The TRW Gen*Star payload development incorporated business analysis, network simulation and analysis, payload technologies, network operations, and terminal concepts. Since initiating the effort, TRW has completed simulations, a functional hardware prototype of the payload, and a complete engineering model payload. The end‐to‐end engineering model of a broadband payload operating at Ka‐band features engineering model hardware built to flight production standards and tested to flight environmental conditions.

This presentation and paper will summarize features and capabilities of the TRW broadband engineering model payload and the payload test results.

Phased Array Antennas (PAAs) using patch‐radiating elements are projected to transmit data at rates several orders of magnitude higher than currently offered with reflector‐based systems. However, there are a number of potential sources of degradation in the Bit Error Rate (BER) performance of the communications link that are unique to PAA‐based links. Short spacing of radiating elements can induce mutual coupling between radiating elements, long spacing can induce grating lobes, modulo 2π phase errors can add to Inter Symbol Interference (ISI), phase shifters and power divider network introduce losses into the system. This paper describes efforts underway to test and evaluate the effects of the performance degrading features of phased‐array antennas when used in a high data rate modulation link. The tests and evaluations described here uncover the interaction between the electrical characteristics of a PAA and the BER performance of a communication link.

Weather radars have been routinely used for investigating propagation phenomena which affect satellite communication links. Weather radar returns can be used to estimate both attenuation and depolarization produced by hydrometeors. Some of the early radar measurements resulted in the discovery of high altitude ice particles as a potential source for depolarization and the development of models for the melting layer or the radar bright band. The radar return from precipitation particles is proportional to the number density of particles in the radar pulse volume. The reflectivity can be converted to an equivalent rain rate or signal attenuation through appropriate assumptions on the particle size distribution. If the radar is capable of measuring reflectivity in two orthogonal polarizations, the difference between the two reflectivity measurements is a direct estimate of the anisotropy of the particulate medium. Differential reflectivity can be used to detect regions containing highly non‐spherical particles such as the melting layer and high altitude ice particles. Results of an experiment involving a dual polarized radar to estimate Ka‐band path attenuation at a tropical location are presented.

The ITALSAT propagation experiment started in early 1991 and ended in January 2001. A receiving station for the radio‐wave propagation experiment at 18.7, 39.6 and 49.5 GHz was installed in 1992 at Spino d'Adda (45.4 N, 9.5 E), near Milano, Northern Italy. In addition to the beacon receiver, which records every second the attenuation measured at the 3 frequencies along a slant path of 37.8° elevation angle, the earth station was equipped with a tipping bucket rain‐gauge and meteorological instruments recording temperature, humidity and pressure.

This paper presents 8 years (1993–2000) of copolar attenuation statistics. The cumulative distribution functions (c.d.f.) of attenuation due to rain for 1993 and total attenuation (gas, clouds, turbulence and rain effects) for the period 1994–2000 are presented along with the rain intensity c.d.f. for the period 1992–2000.

The total attenuation c.d.f., averaged over 7 years, is compared with predictions obtained using ITU‐R Rec. P.618‐7 as for gas, clouds and scintillation contributions and ITU‐R Rec. P.618‐7 and EXCELL model for rain contribution. The measured long‐term cumulative distribution of rain intensity, averaged over the period 1992–2000, is used as input for rain attenuation models.

The ratio between standard deviation and mean value of total attenuation calculated at various time percentages for the 7 years is used to characterize the year‐to‐year variability. C.d.f. of total attenuation conditioned to the season and the hour of the day are also calculated.

For the past decade numerous companies have been advocating Ka‐band satellites for new broadband services. Only a few of the proposed systems are currently deployed or under construction (Spaceway, WildBlue, EuroSkyWay, StarBand, DirectWay, SES ASTRA, EUTELSAT, and, perhaps, INTELSAT). Many concepts have disappeared because they lacked business merit, including some with strong corporate backing.

It is appropriate to examine the current situation and consider why the implementation has been slower than initially expected. One of the best ways to determine the success of a new business is to examine closely related industries. In order to predict the growth of broadband satellite services, this paper examines the development of VSAT services and early Internet access initiatives. We expect that the VSAT business will transform itself over the next few years into the broadband services business. We have some data to track growth in that industry. If this growth trend continues, the number of broadband users could be as small as 1.3 million in 2010. However, a surge in demand for high speed Internet‐access could increase growth rates in the stagnant VSAT industry. We include an updated demand estimate that is based on the number of paying users of high data rate services.

This paper discusses:

1. Required conditions for financial success of broadband satellites.

2. Several technical challenges that must be overcome.

3. How Ka‐band systems can avoid the business difficulties of the Big LEO systems that were technically functional but too expensive to attract a sufficient number of subscribers.

Site diversity is an important means of improving availability for Earth‐space systems with low propagation margins. Current site diversity models, such as the ITU‐R model, are empirically derived from diversity gain measurements. This paper presents an improved site diversity model that allows for unbalanced margins between each site, longer baseline distances, and the direct incorporation of local statistics through the statistical parameters of a two‐component model.

This paper presents the design, manufacture, integration and measured performance of a redundancy and reconfiguration matrix developed for the transmit section of the EuroSkyWay multimedia satellite. The matrix includes the RF section and the control section with the telemetry/telecommand functions.

The EQM equipment includes two functions; a 12‐ for‐10 redundancy switch matrix (RSM) including the possibility to switch each redundant path to any of the outputs and a 3‐to‐9 configuration switch matrix (CSM) including the possibility to switch any input to any output. The used switch is a GaAs MMIC SP3T reflective type.

Trade‐offs have been made in order to select the best electrical solution and mechanical structure in terms of minimum losses, high isolation, minimum mass and size, and low cost.

This paper presents an overview of the architecture for a broadband, high‐speed packet switch processor used in the TRW Gen*Star system. The first application of the Gen*Star design is the Astrolink program. TRW is currently in production of the 4th generation of digital communications processors. Features and benefits of several key capabilities of the processor design for space applications are presented in this paper. The high‐speed packet switch processor uses standard asynchronous transfer mode (ATM) cell structure, which conveys all the Quality of Service (QoS) characteristics of ATM to the Gen*Star network. A non‐blocking crossbar switch with overspeed and input arbitration optimizes switch performance and alleviates output port contention. The downlink has output priority queues with programmable downlink scheduling and adaptive coding that provides maximum flexibility for traffic control and QoS. The resource control function is a distributed architecture using a two‐layer approach that maximizes performance vs. weight and power.

This paper describes the design and development of a Ka‐band Modulator Assembly which is currently in EM/EQM production phase at Bosch SatCom. The assembly is part of a complete transmit section for the European multimedia satellite system EuroSkyWay (ESW) planned by Alenia Spazio and developed in the frame of the ARTES 3 line, which is a co‐funded programme between ESA and the industry.

The modulator assembly receives the digital data streams, performs the appropriate formatting and baseband processing according to the DVB‐S standard and directly modulates the Ka‐band carriers.

The modulator assembly incorporates all functions to be autonomously operated on a spacecraft platform. Each function has been realized fully redundant and consists of DC supply, carrier frequency generation, carrier distribution and switching, direct modulation, a static baseband switching, the TM/TC interface and the signal distribution. Up to 10 active Ka‐band channels operating with three different carriers can be supported.

To deal with the complex signal distribution the assembly is constructed using a common baseplate containing the central infrastructure as well as all external DC, TM/TC and Data interfaces. With this approach signal integrity as well as spurious emission/susceptibility are tightly controlled. The RF functions and the DC/DC converters are realized as plug‐in modules which gives flexibility w.r.t. the number of communication channels and carriers. Furthermore testability and integration procedures are improved by this plug‐in module concept. Mechanical stiffness and the heat transfer has been optimized to comply with the S/C thermal and mechanical interfaces.

The assembly is capable to cover typical multi‐media applications and the results prove that highly integrated multi‐functional assemblies are ready to use.

The high data rate and global satellite communication systems are expected to be an important technology to the social and economic activities in this century. The experimental high data rate satellite communications more than 1.2 Gbps are required for the future multi‐media applications. For these communication systems, a Ka‐band Scanning Spot Beam Antenna (SSBA) which realizes multiple beam forming and operates a number of beams is required. For the full range connectivity all over the world, the Active Phased Array Antenna (APAA) is applied for the SSBA as one of the most important and challenging technology. It is required to realize high EIRP and G/T, and have the capability of changing on‐board the locations of the user spot beams to match the varying broadband traffic and to provide coverage contingency for other satellites. We had developed a Ka‐band APAA and small‐sized modules using Monolithic Microwave Integrated Circuits (MMICs) with wide frequency bandwidth and evaluated the transmission performance for use of multiple carriers.

Communications systems have always been a critical element in aviation. Until recently, nearly all communications between the ground and aircraft have been based on analog voice technology. But the future of global aviation requires a more sophisticated ‘information infrastructure’ which not only provides more and better communications, but integrates the key information functions (communications, navigation and surveillance) into a modern, network‐based infrastructure. Satellite communications will play an increasing role in providing information infrastructure solutions for aviation. Developing and adapting satellite communications technologies for aviation use is now receiving increased attention as the urgency to develop information infrastructure solutions grows. The NASA Glenn Research Center is actively involved in research and development activities for aeronautical satellite communications, with a key emphasis on air traffic management communications needs. This paper describes the recent results and status of NASA Glenn's research program.