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NASA NTRS1990

SATCOM antenna siting study on a P-3C using the NEC-BSC V3.1

The location of a UHF SATCOM antenna on a P-3C aircraft is studied using the NEC-Basic Scattering Code V3.1 (NEC-BSC3). The NEC-BSC3 is a computer code based on the uniform theory of diffraction. The code is first validated for this application using scale model measurements. In general, the comparisons are good except in 10 degree regions near the nose and tail of the aircraft. Patterns for various antenna locations are analyzed to achieve a prescripted performance.

Bensman, D.↗

NASA NTRS1991

GaAs monolithic RF modules for SARSAT distress beacons

Monolithic GaAs UHF components for use in SARSAT Emergency Distress beacons are under development by Microwave Monolithics, Inc., Simi Valley, CA. The components include a bi-phase modulator, driver amplifier, and a 5 watt power amplifier.

Cauley, Michael A.↗

NASA NTRS1991

Observations of Enhanced Radar Backscatter (ERB) from Millstone Hill

Intense enhancements of the incoherent radar backscatter spectrum from the topside ionosphere were observed with the Millstone Hill UHF radar. Enhancements occurring at the local ion acoustic frequency causing large asymmetries in the measured ion line may be produced by current driven instabilities. These enhancements pose a practical problem for space surveillance systems because their cross section and spectral width are characteristic of satellites. Conversely, their hard target signature complicates the study of naturally occurring ERB events; it is nearly impossible to distinguish them from satellites based on a single measurement. Statistical comparisons of observed coherent echo distributions with predictions from a satellite catalog were used to broadly identify periods of ERB activity. A series of experiments using multiple diagnostics, including satellite instruments, for simultaneous observations have established the association of ERB with large fluxes of soft suprathermal electrons carrying field aligned currents. Zenith data are also presented which show the asymmetric growth of ion acoustic waves directly above Millstone Hill. Details of these results are presented.

Lee, M. C.↗

NASA NTRS1990

A simple low-cost array on the lunar near-side for the early lunar expeditions

A concept of a simple, low-cost array on the lunar near-side is discussed. The concept includes 19 stations laid in a T, with three arms each 36 km long, and separated by 6 km. Each station consists of two 10-m-long orthogonal dipoles, each feeding a receiver of a commercial pocket digital shortwave radio type, and a 3-m spring-loaded mast with a small UHF array at the top for receiving the timing signals and tuning commmands and for sending out the digitized data streams. Each station has 0.25 square meters of solar arrays and 20 kg of lithium batteries. At the array center there is either a digital correlator for processing the signals from each antenna pair to compact the data into a 32-kb/sec channel or a 10-cm telescope for sending back raw data to earth.

Kuiper, T. B. H.↗

NASA NTRS1991

SATCOM antenna siting study on P-3C aircraft, volume 2

This volume contains an antenna location study for the P-3C aircraft. From this location study, a determination can be made of the complete antenna system required to achieve the desired pattern and polarization coverage. The antenna used is the same Batwing airborne UHF satellite communications antenna use in volume 1. The aircraft model used in the majority of the locations studied is the simple cylindrical aircraft model defined in volume 1.

Bensman, D. A.↗

NASA NTRS1991

Advanced electromagnetic methods for aerospace vehicles

The Advanced Helicopter Electromagnetics is centered on issues that advance technology related to helicopter electromagnetics. Progress was made on three major topics: composite materials; precipitation static corona discharge; and antenna technology. In composite materials, the research has focused on the measurements of their electrical properties, and the modeling of material discontinuities and their effect on the radiation pattern of antennas mounted on or near material surfaces. The electrical properties were used to model antenna performance when mounted on composite materials. Since helicopter platforms include several antenna systems at VHF and UHF bands, measuring techniques are being explored that can be used to measure the properties at these bands. The effort on corona discharge and precipitation static was directed toward the development of a new two dimensional Voltage Finite Difference Time Domain computer program. Results indicate the feasibility of using potentials for simulating electromagnetic problems in the cases where potentials become primary sources. In antenna technology the focus was on Polarization Diverse Conformal Microstrip Antennas, Cavity Backed Slot Antennas, and Varactor Tuned Circular Patch Antennas. Numerical codes were developed for the analysis of two probe fed rectangular and circular microstrip patch antennas fed by resistive and reactive power divider networks.

Balanis, Constantine A.↗

NASA NTRS1989

Near Zone: Basic scattering code user's manual with space station applications

The Electromagnetic Code - Basic Scattering Code, Version 3, is a user oriented computer code to analyze near and far zone patterns of antennas in the presence of scattering structures, to provide coupling between antennas in a complex environment, and to determine radiation hazard calculations at UHF and above. The analysis is based on uniform asymptotic techniques formulated in terms of the Uniform Geometrical Theory of Diffraction (UTD). Complicated structures can be simulated by arbitrarily oriented flat plates and an infinite ground plane that can be perfectly conducting or dielectric. Also, perfectly conducting finite elliptic cylinder, elliptic cone frustum sections, and finite composite ellipsoids can be used to model the superstructure of a ship, the body of a truck, and airplane, a satellite, etc. This manual gives special consideration to space station modeling applications. This is a user manual designed to give an overall view of the operation of the computer code, to instruct a user in how to model structures, and to show the validity of the code by comparing various computed results against measured and alternative calculations such as method of moments whenever available.

Marhefka, R. J.↗

NASA NTRS1990

US development and commercialization of a North American mobile satellite service

U.S. policies promoting applications and commercialization of space technology for the 'benefit of mankind,' and emphasis on international competitiveness, formed the basis of NASA's Mobile Satellite (MSAT) R&D and user experiments program to develop a commercial U.S. Mobile Satellite Service. Exemplifying this philosophy, the MSAT program targets the reduction of technical, regulatory, market, and financial risks that inhibit commercialization. The program strategy includes industry and user involvement in developing and demonstrating advanced technologies, regulatory advocacy, and financial incentives to industry. Approximately two decades of NASA's satellite communications development and demonstrations have contributed to the emergence of a new multi-billion dollar industry for land, aeronautical, and maritime mobile communications via satellite. NASA's R&D efforts are now evolving from the development of 'enabling' ground technologies for VHF, UHF, and L-Band mobile terminals, to Ka-Band terminals offering additional mobility and user convenience.

Arnold, Ray J.↗

NASA NTRS1991

Channel coding for digital HDTV terrestrial broadcasting

The Federal Communications Commission of the United States has ruled that high-definition television (HDTV) will occupy no more than 6 MHz of the VHF and UHF bands now used for conventional TV. In order to transmit the HDTV signal in 6 MHz, the four United States digital HDTV proponents, the DigiCipher, DSC-HDTV, ADTV, and ATVA-P systems, are reducing the video data rate of HDTV to 15-17 Mb/s, a compression ratio of approximately 60-70 times. The high compression dictates that channel coding be used to avoid block errors and multiframe error propagation. High efficiency in channel utilization required by the 6-MHz limitation means that the channel must be properly equalized and that the multipath and interfering signals must be severely limited. The channel coding techniques used for error reduction include data interleaving, error detection and replacement, and error correction at different levels of protection for bits and blocks of unequal importance.

Beakley, Guy W.↗

NASA NTRS1992

Radar detection of centimeter-sized orbital debris - Preliminary Arecibo observations at 12.5-cm wavelength

Orbital debris objects with sizes of 6 mm and larger were detected when they passed through the main beam of the high-power, 12.6-cm wavelength radar at the Arecibo Observatory. The number of objects detected at altitudes below 1000 km in the size range of 6 mm to 2 cm agreed with orbital debris model predictions to better than a factor of two. Radar detections of larger orbital debris (10-cm diameter and above) were also compared with the objects in the U.S. Space Command catalog (the cataloged objects have UHF radar cross-sections larger than 0.01 sq m). This experiment demonstrated that the large, earth-based radar telescopes normally used for planetary studies can provide useful data about the near-earth orbital debris populations.

Thompson, T. W.↗

NASA NTRS1992

Satellite sound broadcast propagation measurements and system impairments

An evaluation of the operational characteristics of indoor portable reception of satellite sound reception has identified the nature of UHF and L-band signals' penetration loss in the cases of buildings having intricate spectral and spatial signal structures. These propagation impairments must be mitigated by resort to a combination of link margin, diversity techniques, and/or the efforts of a listener to place the radio (or its antenna) in an advantageous location. Attention is given to cost/performance tradeoffs for a mix of these measures, with a view to Direct Broadcast Satellite Radio system design.

Vogel, Wolfhard J.↗

NASA NTRS1992

IRD dropout study

This final report describes work performed by SRS Technologies for the NASA Marshall Space Flight Center under Contract NAS8-39077, entitled 'Integrated Receiver-Decoder Dropout Study'. The purpose of the study was to determine causes of signal fading effects on ultra-high-frequency (UHF) range safety transmissions to the Space Shuttle during flyout. Of particular interest were deep fades observed at the External Tank (ET) Integrated Receiver-Decoder (IRD) during the flyout interval between solid rocket booster separation and ET separation. Analytical and simulation methods were employed in this study to assess observations captured in flight telemetry data records. Conclusions based on the study are presented in this report, and recommendations are given for future experimental validation of the results.

Yalowitz, Jeffrey S.↗

NASA NTRS1991

Advanced electromagnetic methods for aerospace vehicles

During this period the research program addressed the following three topics: (1) composite materials; (2) precipitation static (P-Static); and (3) antenna technology. On the topic of Composite Materials our main efforts were directed toward making measurements on several new samples of composite materials made available to ASU by Stanford Research Institute (SRI) through the efforts of Mr. Frank Casler of AVRADA. These samples can be classified into three distinct materials with each material having its own distinct electrical properties. In addition, attempts were made to make predictions of the effects on antenna patterns by composite materials. This will take a greater emphasis in the next reporting period. In Precipitation Static (P-Static), the main effort was devoted toward developing a Voltage Finite-Difference Time-Domain computer code to account for the voltage variation on a conducting body as the primary source of corona discharge, instead of the electric field. Due to complexities stemming from the interactions between the potentials, the fields, and current sources, the decision was made to begin with a simple two-dimensional problem without the corona discharge and check our programs in a series of simple models, culminating in the full corona discharge problem. This report deals with the first stage of such development. During this reporting period, the main effort in Antenna Technology was toward the design, fabrication, and testing of a cavity-backed slot antenna using ferrite material. Using the ferrite material available to us during this period, the resonances of this antenna were around 5 and 8 GHz. Attempts will be made to model such an antenna and to lower its resonance down into the VHF and UHF bands.

Balanis, Constantine A.↗

NASA NTRS1991

GaAs monolithic R.F. modules for SARSAT distress beacons

Cauley, Michael A.↗

NASA NTRS1992

Concept for a lunar array for very low frequency radio astronomy

We discuss the design considerations relevant to a very low frequency array, to be deployed on the lunar near side during an early expedition. Such an array would operate in the frequency range 1-10 MHz, and would consist nominally of 20 antennas distributed over a region approximately 40 km in extent. Each antenna station would consist of a crossed-dipole antenna, together with a receiver, digitizer, solar cells, and batteries. In addition, the station will contain a UHF transmitter for relaying the digitized signal to a central station where it will be transmitted to Earth for subsequent processing, including cross-correlation with signals from other antennas. Each antenna station (including the central station) would be deployed as a self-contained unit, mounted on a miniature robotic vehicle. No fixed structures are required for the array.

Marsh, Kenneth A.↗

NASA NTRS1991

Natural noise above 50 MHZ from terrestrial and extraterrestrial sources

This paper offers a brief overview of natural radio noise for frequencies above 50 MHz in terms of brightness temperature as observed from two vantage points. The first is from an Earth station located at 40 degrees north latitude and observing at elevation angles from 0 to 90 degrees with an ideal antenna. The second is a satellite in geostationary orbit communicating with the Earth. Earth station noise at VHF and UHF is dominated by galactic and solar noise. Emission from the atmosphere, gases and hydrometeors, are dominant at EHF and SHF. Radiative transfer theory is invoked in the calculation of brightness temperature from the atmosphere. The situation is not vastly different from geostationary orbit if communications is with the Earth. Emission from the land and sea, even under idealized conditions, enters significantly. Land is a much more effective emitter than sea water, but at frequencies above 30 GHz the differential becomes much less due to the increasing significance of atmospheric emission.

Smith, E. K.↗

NASA NTRS1993

Structure and thermochemistry of ClO2 radicals

The structure of ClO2 has been calculated for the X 2A-double prime ground state using unrestricted Hartree-Fock (UHF), unrestricted second-order Moller-Plesset perturbation (UMP2), configuration interaction employing single and double excitation (CISD), and quadratic configuration interaction (QCI) ab initio molecular orbital methods. Calculations using UMP2 and CISD wave functions predict a ClO bond length of 1.728 +/- 0.01 A. The single-configuration-based QCI in the singles and doubles space with perturbation inclusion of triple substitutions, denoted QCISD(T), yield a ClO bond length of 2.205 A. The QCI results are consistent with results of Jensen (1990) who showed that the ClO bond length is 2.181 A using annihilated self-consistent methods (AUMP2). The thermochemistry of ClO2 radical has been calculated using MP2 and QCI methods employing an isodesmic scheme. Our scheme predicts the heat of formation for ClO2 at 0 K to be 24.6 +/- 2 kcal/mol.

Francisco, J. S.↗

NASA NTRS1985

Radar wind profilers

Continuous, automated measurement of tropospheric wind profiles with UHF and VHF Doppler radars has been demonstrated. Ground-based networks of these radars will be available as part of a global wind measurement system, and remote single stations could be built to further complement a spaceborne measurement device. A number of ground-based wind profilers will be in place by the time a space system is tested so the global wind measurement system should be designed with these ground-based profilers providing part of the picture.

Strauch, R. G.↗