Advanced Communications Technology Satellite Research Articles

Probability distributions of rain attenuation obtainable with linear combining techniques in space‐to‐Earth links using time diversity

SummaryThe purpose of this paper is to show how the complementary probability distribution of rain attenuation is drastically changed in the lower rain attenuation range by applying linear combining techniques, namely, equal‐gain combining and the maximal‐ratio combining, discussed in the historical paper by Brennan in 1959. These combing techniques can also be applied to the Automatic Repeat Request techniques. Defined the instantaneous processing gain and the equivalent attenuation in the 3 cases, we show examples of time series of the various parameters, based on the experimental rain attenuation time series recorded with the ITALSAT 18.7 GHz beacon, in a 37.8° slant path in Spino d'Adda (Italy). Then, we report long‐term complementary probability distribution functions of the instantaneous gain and equivalent attenuation, by simulating rain attenuation time series at 19.7 and 39.4 GHz, path elevation angle 35.5°, with the Synthetic Storm Technique, using on‐site measured rain rate time series of 10 years, by simulating the ALPHASAT link at Spino d'Adda. Similar results are also found at different frequencies and elevation angles in Tampa (Advanced Communications Technology Satellite, ACTS result test), the Isle of Guam, and Prague. The main conclusions are as follows: (1) As expected, the instantaneous time diversity gain can be large when the delay time is large and rain attenuation is large; (2) scintillation affects time diversity links as the direct links; (3) equal‐gain and maximal‐ratio combining can add up to 3 dB to the selection diversity gain when the time diversity gain is very small; and (4) equal‐gain and maximal‐ratio combining reduce the fraction of time of rain attenuation in an average year to a value less than the probability of exceeding 3 dB in the link without diversity.

A short note on the variation of path loss in the atmosphere

Diurnal variations of received satellite signal in Ka-band, and from co-located radiometer data were investigated using data collected from the propagation measurement campaign conducted by the National Aeronautics and Space Administration (NASA) Advanced Communications Technology Satellite (ACTS). The database contains a maximum of five consecutive years of data collected from seven sites in North America that were receiving signals from a geostationary satellite, using similar on-site equipment and data processing tools. The investigation emphasis is on simultaneous diurnal variation seen in the beacon attenuation as well as attenuation inferred from the co-located radiometer. Evidence showed that the diurnal variations observed on satellite beacon experiments are an atmospheric effect, unconnected with the satellite or the satellite motion. The spectral content of the beacon attenuation data as well as from attenuation inferred from the radiometer showed an element of a solar day periodicity with ‘side-bands’ of sidereal and anti-sidereal periodicities.

Communications Research and Development at NASA Glenn Research Center

AbstractOver the last several decades, the National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC), formerly Lewis Research Center (LeRC) has performed research and technology development of aeronautic- and space-based communications in support of NASA and the nation. In the 1970s, GRC partnered with the Canadian Department of Communications through the Communications Technology Satellite (CTS) Project, in which GRC researchers were responsible for the development of critical technology components, such as the high-power, traveling-wave tube amplifier (TWTA), thereby pioneering the surge of television channels via satellite. For its efforts, LeRC was awarded an Emmy by the television industry. The decade of the 1980s served as a period for technology development that culminated in the launch of the Advanced Communications Technology Satellite (ACTS) in 1993. The ACTS demonstration of spot beam antenna technology resulted in an overall increase of efficiency in satellite communicat...

Fade Slope Analysis of Ka-Band Earth-LEO Satellite Links Using a Synthetic Rain Field Model

Because the motion of a low Earth orbit (LEO) satellite across the sky causes the Earth-space path to very quickly pass through any rain cells in the vicinity, the degree of rain fading on such paths changes more rapidly and leads to steeper fade slopes than in the geostationary case. Because comprehensive measurement data have not yet been compiled for fading on LEO links in the Ka-band, we have used simulations based on Goldhirsh's method for determining the key parameters of the well-known EXCELL model of a horizontal rain structure from long-term global rain statistics to obtain plausible estimates of the fade slope distributions for selected scenarios. The results that we obtained for geostationary satellites closely match those observed at selected sites during the Advanced Communications Technology Satellite program. The results that we obtained for LEO satellites show how fade slopes will steepen as 1) the altitude of the satellite decreases; 2) the frequency band of operation increases; and 3) the average rain rate increases. Furthermore, they suggest that, at a given probability level, the fade slopes could be between two and ten times greater than those for geostationary satellites and that mobile terminals with a clear view of the sky will experience fade slopes that are similar to those encountered by fixed or transportable terminals. These results have important implications for the design of power control algorithms and other fade-mitigation techniques.

Protocol Testing of SCPS-TP over NASA's ACTS Asymmetric Links

As the transport layer protocol of the Space Communication Protocol Standard (SCPS) suite, SCPS-Transport Protocol (SCPS-TP) is actually transmission control protocol (TCP), with a set of extensions aimed at improving TCP performance in space communications. Some work has been done on the performance evaluation of SCPS-TP in comparison with TCP. However, those works have been done by testing SCPS-TP via emulation test-beds. We present the file transfer experimental work of testing the SCPS protocol stack over NASA's geostationary orbit (GEO) Advanced Communications Technology Satellite (ACTS). Considering that space communication channels in practice are frequently asymmetric in terms of channel bandwidth, our work is concentrated on the evaluation of the effectiveness of SCPS-TP in coping with channel rate asymmetry. We conclude that SCPS-TP is better suited than TCP over long-delayed, asymmetric, satellite channels with a throughput advantage of over 30 Kbyte/s for the transmission of a 10 Mbyte file. A hybrid of the channel rate control and acknowledgment frequency reduction mechanisms makes significant contributions to the performance enhancement of SCPS-TP over asymmetric space channels.

Analysis of the effects of water on the ACTS propagation terminal antenna

The NASA advanced communications technology satellite (ACTS) propagation experiment was designed to observe the attenuation produced by rain on Earth-satellite paths operating in the Ka-band. Unwanted effects of water on the antenna reflector surface were noted. Wet-antenna attenuation could be attributed to the combined effect of a water layer on the reflector surface and water wetting the feed window surface. A model was developed to calculate the antenna reflector and feed surface water layer thickness values as a function of position on each surface. The thickness values were used to calculate the additional attenuation produced by the water layers as a function of rain rate on the antenna. The wet-antenna-attenuation prediction model was verified by sprayer tests. The goal of the ACTS propagation experiment was to obtain path attenuation statistics, statistics that represent the effects of rain on the Earth-satellite path but not on the antenna itself. The wet-antenna attenuation prediction model was used to remove the effects of water on the antenna from the combined antenna-plus-path attenuation statistics produced by the experiment. The overall efficacy of the model was demonstrated by comparing the corrected path loss statistics from two ACTS propagation experiment sites with earlier COMSTAR path loss measurements made at or near those sites. The empirical distribution functions from both the ACTS and COMSTAR experiments were identical within the expected uncertainty of an empirical annual distribution of attenuation by rain.

Characteristics of rain fading on Ka-band satellite-earth links in a Pacific maritime climate

Characteristics of rain-induced attenuation at Ka-band measured over a four-year period (1994-1997) with the advanced communications technology satellite (ACTS), Vancouver, BC, Canada, are reported. Event-based analyses of fading are performed and examples of the instantaneous 30/20-GHz frequency-scaling ratio are presented. Long-term cumulative statistics of rain rate as derived from rain-gauge measurements, along with annual attenuation statistics at 20 and 30 GHz, are provided. Finally, attenuation statistics are compared with the ITU-R model and aspects related to local climatic characteristics are considered, aiming to improve the predictions of attenuation due to rain.

NASA/DARPA Advanced Communications Technology Satellite Project for Evaluation of Telemedicine Outreach Using Next-Generation Communications Satellite Technology: Mayo Foundation Participation

To describe the development of telemedicine capabilities-application of remote consultation and diagnostic techniques-and to evaluate the feasibility and practicality of such clinical outreach to rural and underserved communities with limited telecommunications infrastructures. In 1992, Mayo Foundation (Rochester, Minn, Jacksonville, Fla, and Scottsdale, Ariz), the National Aeronautics and Space Administration, and the Defense Advanced Research Projects Agency collaborated to create a complex network of fiberoptic landlines, video recording systems, satellite terminals, and specially developed data translators linking Mayo sites with other locations in the continental United States on an on-demand basis. The purpose was to transmit data via the asynchronous transfer mode (ATM) digital communications protocol over the Advanced Communications Technology Satellite. The links were intended to provide a conduit for transmission of data for patient-specific consultations between physicians, evaluation of medical imagery, and medical education for clinical staffs at remote sites. Low-data-rate (LDR) experiments went live late in 1993. Mayo Clinic Rochester successfully provided medical consultation and services to 2 small regional medical facilities. High-data-rate (HDR) experiments included studies of remote digital echocardiography, store-and-forward telemedicine, cardiac catheterization, and teleconsultation for congenital heart disease. These studies combined landline data transmission with use of the satellite. The complexity of the routing paths and network components, immaturity of available software, and inexperience with existing telecommunications caused significant study delays. These experiments demonstrated that next-generation satellite technology can provide batch and real-time imagery for telemedicine. The first-generation of the ATM and satellite network technology used in these experiments created several technical problems and inconveniences that should be overcome as the network infrastructure matures.

Performance evaluation of TCP extensions on ATM over high bandwidth delay product networks

Practical experiments in a satellite network environment assist in the design and understanding of future global networks. This article describes the practical experiences gained from TCP/IP on ATM networks over a high-speed satellite link and presents performance comparison studies of such networks with the same host/traffic configurations over local area and wide area networks. These comparison studies on the LAN, WAN, and satellite environments increase our understanding of the behavior of high-bandwidth networks. NASA's Advanced Communications Technology Satellite (ACTS), with its special characteristics and high data rate satellite channels, and the ACTS ATM Internetwork (AAI) were used in these experiments to deliver broadband traffic. Network performance tests were carried out using application-level software (Netspec) on SONET OC-3 (155.52 Mb/s) satellite links. Finally, we experimentally study the performance, efficiency, fairness, and aggressiveness of TCP Reno, TCP New Reno, and TCP SACK end hosts on ATM networks over high BDP networks.

NASA's broadband satellite networking research

ATM is currently considered the primary WAN technology with Internet protocols providing the routing and transport requirements. Another WAN technology being considered is packet over SONET. Using the Advanced Communication Technology Satellite, NASA has demonstrated and is experimenting with these technologies. This article summarizes some of the major completed and ongoing experiments and demonstrations performed using commercial standard protocols such as ATM and TCP/IP over broadband satellite networks.

Application of the advanced communications technology satellite to teleradiology and real-time compressed ultrasound video telemedicine.

The authors have investigated the application of the NASA Advanced Communications Technology Satellite (ACTS) to teleradiology and telemedicine using the Jet Propulsion Laboratory (JPL)-developed ACTS Mobile Terminal (AMT) uplink. In this experiment, bidirectional 128, 256, and 384 kbps satellite links were established between the ACTS/AMT, the ACTS in geosynchronous orbit, and the downlink terrestrial terminal at JPL. A terrestrial Integrated Digital Services Network (ISDN) link was established from JPL to the University of Washington Department of Radiology to complete the bidirectional connection. Ultrasound video imagery was compressed in real-time using video codecs adhering to the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T) Recommendation H.261. A 16 kbps in-band audio channel was used throughout. A five-point Likert scale was used to evaluate the quality of the compressed ultrasound imagery at the three transmission bandwidths (128, 256, and 384 kbps). The central question involved determination of the bandwidth requirements to provide sufficient spatial and contrast resolution for the remote visualization of fine- and low-contrast objects. The 384 kbps bandwidth resulted in only slight tiling artifact and fuzziness owing to the quantizer step size; however, these motion artifacts were rapidly resolved in time at this bandwidth. These experiments have demonstrated that real-time compressed ultrasound video imagery can be transmitted over multiple ISDN line bandwidth links with sufficient temporal, contrast, and spatial resolution for clinical diagnosis of multiple disease and pathology states to provide subspecialty consultation and educational at a distance.

A hybrid satellite/terrestrial network linking Europe and North America

This short paper will document a demonstration given at the Third Ka-band Conference, Sorrento, Italy, September 1997. The principal objective was to illustrate interoperability over a complex hybrid satellite and terrestrial Asynchronous Transport Mode (ATM) network. The network linked Ka-band satellites in Europe and in North America, via the European JAMES Research and Development (R + D) Network, and the CANTAT-3 trans-Atlantic submarine fibre. The ISABEL application facilitated Computer Supported Collaborative Workspace (CSCW) among locations in Canada, Italy and the U.S.A. The demonstration consisted of the presentation of one full conference session from Italy to observers in Canada and the U.S.A., with the opportunity for the remote, as well as the local audiences, to interact with presenters during the 'Question and Answer' periods. There were also short presentations from the Communications Research Centre (Canada), and NASA Lewis Research Centre (U.S.A.). The ISABEL application required two virtual networks, one to handle unicast signals for control and monitoring, etc., the other to distribute multicast traffic. They shared a common star topology over point to point symmetrical Virtual Paths (VPs) through the interconnected ATM networks. The three participating locations were connected to a Network Node at the Communications Research Centre (CRC), Ottawa, Canada over full duplex El (2.048 Mbps) circuits. The application traffic from each ISABEL workstation was directed to the Network Node, where it was combined. The Control Workstation, at the Conference Centre, Sorrento, Italy, allocated window sizes in the combined image, selected local inputs, and switched presentation material as required. The combined image was multicast from the Network Node to the three networked locations. The overall network included the Italsat-2 satellite and the U.S. Advanced Communications Technology Satellite (ACTS), the Canadian and Italian national test networks, the European R + D network, and the CANTAT-3 submarine fibre.

Advanced Communications Technology Satellite (ACTS): four-year system performance

The Advanced Communications Technology Satellite (ACTS) was conceived at the National Aeronautics and Space Administration (NASA) in the late 1970s as a follow-on program to ATS and CTS to continue NASA's long history of satellite communications projects. The ACTS project set the stage for the C-band satellites that started the industry, and later the ACTS project established the use of Ku-band for video distribution and direct-to-home broadcasting. ACTS, launched in September 1993 from the Space Shuttle, created a revolution in satellite system architecture by using digital communications techniques employing key technologies such as a fast hopping multibeam antenna, an on-board baseband processor, a wide-band microwave switch matrix, adaptive rain fade compensation, and the use of 900 MHz transponders operating at Ka-band frequencies. This paper describes the lessons learned in each of the key ACTS technology areas, as well as in propagation investigations.

Advanced Communications Technology Satellite (ACTS) fade compensation protocol impact on very small-aperture terminal bit error rate performance

The Advanced Communications Technology Satellite (ACTS) communications system operates at Ka band. ACTS uses an adaptive rain fade compensation protocol to reduce the impact of signal attenuation resulting from propagation effects. The purpose of this paper is to present the results of an analysis characterizing the improvement in VSAT performance provided by this protocol. The metric for performance is VSAT bit error rate (BER) availability. The acceptable availability defined by communication system design specifications is 99.5% for a BER of 5E-7 or better. VSAT BER availabilities with and without rain fade compensation are presented. A comparison shows the improvement in BER availability realized with rain fade compensation. Results are presented for an eight-month period and for 24 months spread over a three-year period. The two time periods represent two different configurations of the fade compensation protocol.

Four years of experimental results from the New Mexico ACTS propagation terminal at 20.185 and 27.505 GHz

The Advanced Communications Technology Satellite (ACTS) propagation experiment has collected four years of propagation data at 20.185 and 27.505 GHz. The objective of the experiment is to develop long-term statistics and modeling techniques for predicting atmospheric propagation effects in the Ka band. The experiment includes seven identical earth stations at different locations in North America. Each location is meant to characterize a unique rain region. This paper presents the data collected in White Sands, NM. The data from this site provide an excellent resource for validating rain attenuation models due to its unique arid climate with occasional high rain-rate storms. The seasonal and cumulative four-year attenuation statistics for the 20.2 and 27.5 GHz beacons are presented. The attenuation with respect to clear air (ACA) is compared to five different rain attenuation models and seven different frequency scaling models. The results illustrate how well each model predicts rain attenuation in a desert climate region.

Avanced Communications Technology Satellite (ACTS) propagation campaign in North America

Ka-band propagation measurements with the Advanced Communications Technology Satellite (ACTS) are entering their fifth year of data collection. Beacon-signal levels and radiometric sky noise are being measured at 20.2 and 27.5 GHz, with supporting meteorological information, at seven experiment sites in the US and Canada, using ACTS propagation terminals (PSTs) supplied by NASA. Uniform system-calibration, data-collection and data-processing procedures and software, prescribed and developed by the NASA Propagation Experimenters (NAPEX) Group, promote commonality of the data sets and permit comparison among results from the various sites. Goals of the program are to develop basic Ka-band propagation knowledge, and to supply information needed to improve propagation-prediction models and to develop impairment-mitigation techniques. The APT sites were selected to sample a variety of North American climate regions and their associated propagation features. Statistical results from the propagation measurements are presented and compared to the predictions of new and existing prediction models. Other aspects of the measurements, including unusually strong antenna-wetting effects observed with the APTs, plus analyses related to impairment dynamics, rain-rate modeling, and clear-air effects, are discussed.

Distributed Numerical Weather Prediction via Satellite

Abstract This paper describes a recent undertaking in distributed numerical weather prediction via high data rate networks. The governing project involved the operation of a coupled mesoscale modeling system on widely separated supercomputers, and experiments and demonstrations were performed to explore both long-distance and local-area operation of the system. Connectivity was provided either by NASA's Advanced Communications Technology Satellite (ACTS) or by a terrestrial Asynchronous Transfer Mode (ATM) network. The modeling system was built around the Fifth-Generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (PSU–NCAR MM5) and consisted of the MM5, an ocean circulation model, and a surface wave model. The primary experiments and demonstrations utilized ACTS to link supercomputers at both NCAR in Boulder, Colorado, and at the Ohio Supercomputer Center in Columbus, Ohio. Other experiments and demonstrations utilized an ATM network to link supercomputers runn...

Telemedicine and the Diagnosis of Speech and Language Disorders

To summarize results of telemedicine evaluations of speech and language disorders in patients in a small, rural hospital and in large multidisciplinary medical practices. Eight patients underwent assessment as part of experiments with the National Aeronautics and Space Administration-launched Advanced Communications Technology Satellite. A second clinician was on-site with patients to assess the reliability of satellite observations. Twenty-four previously videotaped samples of speech disorders were also transmitted to assess agreement with original face-to-face clinical diagnoses. In addition, results of 150 telemedicine evaluations among Mayo Clinic practices in Minnesota, Arizona, and Florida were examined retrospectively. Evaluations were reliable, and patient satisfaction was good. Diagnoses were consistent with lesion localization and medical diagnosis when they were known, and they frequently had implications for lesion localization and medical diagnosis and management when they were previously unknown. The frequency of uncertain diagnosis (13%) for evaluation among the Mayo practices was only slightly higher than that encountered in face-to-face practice. Face-to-face evaluations were considered necessary for only 6 of the 150 patients (4%). Telemedicine evaluations can be reliable, beneficial, and acceptable to patients with a variety of acquired speech and language disorders, both in rural settings and within large multidisciplinary medical settings.

Ka-band propagation measurements: an opportunity with the Advanced Communications Technology Satellite (ACTS)

The Advanced Communications Technology Satellite (ACTS) was conceived at the National Aeronautics and Space Administration (NASA) as a follow on program to its long history in satellite communications projects that have reduced the risk of developing new technologies that fall outside the sponsorship capability of the private sector. To counter the foreign challenge that developed in the late 1970's to the once insuperable US lead in this field, ACTS was developed to maintain the US preeminence. Launched in September 1993 from the space shuttle, key technologies on ACTS include a multibeam antenna, a baseband processor, a 900 MHz wideband microwave switch matrix, adaptive rain fade compensation techniques, and the use of Ka-band frequencies. Since this is the United States' first effort in using Ka-band for satellite communications, beacons are incorporated on the satellite, which provide an opportunity for propagation measurements. NASA is sponsoring a network of propagation experimenters using these beacons and receive-only terminals identical in design. This paper provides some history that leads to the eventual development of ACTS. Also, a system overview of the spacecraft is provided for those less familiar with it.

The application of S-band polarimetric radar measurements to Ka-band attenuation prediction

In September 1993, the National Aeronautics and Space Administration's Advanced Communications Technology Satellite (ACTS) was deployed into a geostationary orbit near 100/spl deg/ W longitude. The ACTS satellite employs two Ka-band beacons, one at 20.185 GHz and another at 27.505 GHz. Impairments due to rain attenuation and tropospheric scintillations will significantly affect new technologies for this spectrum. Heavy rain at Ka-band can easily produce 30 dB of attenuation along the propagation path. Propagation experiments being conducted in seven different climatic zones involve multiyear attenuation measurements along the satellite-Earth slant path. Measurements in the B2 climatic zone are made with an ACTS propagation terminal located in northeast Colorado. In order to gain move understanding about the physical processes that are responsible for Ka-band attenuation, the Colorado State University CHILL S-band polarimetric radar is used to take radar measurements along the slant path. The Colorado Front Range experiences a variety of weather conditions throughout the year ranging from upslope rain conditions to winter storms. Four such events measured along the slant path are illustrated in this paper. They include two convective cases and two bright-band cases. The S-band polarimetric radar data is used to initialize radar-based attenuation-prediction models, which are applied to the four precipitation events described. The comparisons of predicted attenuation to measured attenuation are quite good. It was also found during the course of the experiment that water droplets standing on the antenna surface can cause appreciable attenuation at Ka-band frequencies. That finding needs to be recognized in future model development and statistical analysis.