Engineering topics
Publications and source records attributed to Stevens, G. H..
A rotative quadrature phase-shift keying (RQPSK) modulation scheme is proposed. By rotating the QPSK signal constellation by pi/2, either clockwise or anticlockwise, during a symbol duration, the conventional QPSK scheme can be modified to transmit 3 bits per symbol to achieve both power and bandwidth efficiency.
A comparative analysis by computer simulation of the land mobile satellite system (LMSS) is presented for the uncoded quadrature phase shift keying (QPSK), rate 2/3 8-PSK trellis coded modulation (TCM), and rate 4/6 multiplicity 2 8-PSK TCM schemes. An analytical model is used for simulating the fading channel. The simulation results show that under Rayleigh fades, the TCM designed for the fading channel is superior to uncoded QPSK and to the conventional TCM optimized for the additive white Gaussian noise (AWGN) channel. The performance of differentially detected TCM under AWGN and fading is examined based on the preliminary results. Results of the uncoded QPSK with fading channel modeled with the empirical fade data are given.
A spread spectrum system, slow frequency hopping with GMSK (Gaussian minimum shift keying) modulation (SFH-GMSK), is proposed for mobile telephone communications. The system performance is evaluated using computer simulation and is compared with an unspread system. Results show that under multipath fading conditions, when the signal-to-noise ratio (SNR) is greater than 15 dB, slow frequency hopping gives some bit error rate improvement over the unspread system. Theoretical predictions indicate that a system efficiency of 20-65 users per cell can be achieved in the cellular configuration. Joint use of SFH-GMSK and FM is also investigated. It is shown that FM interference can cause serious degradation to the SFH-GMSK performance.
This is a response to a Space Applications Advisory Committee (SAAC) request for information about the status and trends in satellite communications, to be used to support efforts to conceive and recommend long range goals for NASA communications activities. Included in this document are assessments of: (1) the outlook for satellite communications, including current applications, potential future applications, and impact of the changing environment such as optical fiber networks, the Integrated Services Digital Network (ISDN) standard, and the rapidly growing market for Very Small Aperture Terminals (VSAT); (2) the restrictions imposed by our limited spectrum resource; and (3) technology needs indicated by future trends. Potential future systems discussed include: large powerful satellites for providing personal communications; VSAT compatible satellites with onboard switching and having voice capability; large satellites which offer a pervasive T1 network service (primarily for video-phone); and large geostationary communications facilities which support common use by several carriers. Also, discussion is included of NASA particular needs and possible future systems. Based on the mentioned system concepts, specific technology recommendations are provided for the time frames of now - 1993, 1994 - 2000, and 2000 - 2010.
The performance of a proposed NASA 30/20 GHz satellite communications system is studied for multi-h phase coded modulation (MHPM) schemes. The techniques used to model and simulate a satellite communications channel including transmitter, receiver, filters, nonlinearities, and interferers are presented. The performance of various MHPM schemes is compared for several different channel configurations. As a measure of performance, the probability of bit error vs Eb/NO is computed using a Monte Carlo simulation technique. It is found that, regardless of the channel configuration, MHPM schemes can provide power efficiency over serial minimum shift keying modulation.
An assessment is made of the feasibility of providing low data rate service to small earth stations by satellite at Ka-band. Technological as well as economic factors are considered. The results of NASA-sponsored contractual studies are compared and results of internal NASA studies are presented. Several FDMA and TDMA scenarios are critically examined with the objective of establishing the relative utility of such systems to end users. It is shown that FDMA has no advantage over TDMA in a multibeam scenario for 56 Kbs of data by voice, video, or the equivalent. For the same assumptions, significant weight and power advantages are realized in the space segment using TDMA.
Issues and results in a NASA study of the potential concepts and markets for a multibeam 30/20 GHz domestic satellite system in the 1990s are presented. Issues considered include the reduction of signal attenuation due to rain, beam-beam interference isolation in the multibeam system, the method of access/modulation (FDMA, TDMA or hybrid) and the market for reduced reliability and wideband services. A hypothetical demonstration payload configuration which would attempt to resolve these issues is illustrated. The communications payload would employ a system of seven contiguous coverage spots in order to demonstrate a typical cell in a contiguous beam system having extensive frequency re-use, as in a direct-to-user system, and a single spot, typical of a trunking system, to determine signal isolation. The payload could be carried on several existing buses and is illustrated on an MMS bus.
NASA is conducting a series of millimeter wave satellite communication systems and market studies to: (1) determine potential domestic 30/20 GHz satellite concepts and market potential, and (2) establish the requirements for a suitable technology verification payload which, although intended to be modest in capacity, would sufficiently demonstrate key technologies and experimentally address key operational issues. Preliminary results and critical issues of the current contracted effort are described. Also included is a description of a NASA-developed multibeam satellite payload configuration which may be representative of concepts utilized in a technology flight verification program.
Lockheed C-130 Hercules aircraft have during their service life been periodically inspected and growing cracks around rivet holes were recorded. This record has recently been used to determine the statistical distributions of crack initiation times and the distribution of initial crack sizes. When crack initiation times are calculated from such cracks, by backward extrapolation of the growth relation, the resulting distribution of crack initiation times will indicate a preponderance of short times to crack initiation. If however, such distributions are combined with the reliability of the inspection procedure, the statistical distribution of missed initiation times can be estimated. The method used is based on Bayes theorem which permits the calculation of the 'prior' distribution (initiation times before inspection) from a knowledge of the 'posterior' distribution (initiation times obtained from the inspection) and a 'likelihood function' (reliability of the inspection) procedure. The results indicate that during an early inspection a large percentage of initiation times will be missed and that the fraction of located initiation times increases during later inspections.
A preliminary analysis was conducted to establish the requirements of a space-to-earth microwave power transmission system. The need for accurate phase control on the transmitter was established and methods for assessing the impact of power density and thermal constraints on system performance were demonstrated. Potential radio frequency interference was considered. The sensitivity of transmission system scale to variations in power source, transportation and orbital fabrication and assembly costs was also determined.
Statistics such as average power density pattern, variance of the power density pattern and variance of the beam pointing error are related to hardware parameters such as transmitter rms phase error and rms amplitude error. Also a limitation on spectral width of the phase reference for phase control was established. A 1 km diameter transmitter appears feasible provided the total rms insertion phase errors of the phase control modules does not exceed 10 deg, amplitude errors do not exceed 10% rms, and the phase reference spectral width does not exceed approximately 3 kHz. With these conditions the expected radiation pattern is virtually the same as the error free pattern, and the rms beam pointing error would be insignificant (approximately 10 meters).
Ground reflections generate undesirable effects on acoustic measurements such as those conducted outdoors for jet noise research, aircraft certification, and motor vehicle regulation. This paper shows how cepstral techniques developed in speech processing can be adapted to identify the echo delay time and to correct for ground reflection effects. A sample result is presented using an actual narrowband sound pressure level spectrum. The technique can readily be adapted to existing fast Fourier transform type spectrum measurement instrumentation to provide field measurement of echo time delays.
Ground reflections generate undesirable effects on acoustic measurements such as those conducted outdoors for jet noise research, aircraft certification, and motor vehicle regulation. Cepstral techniques developed in speech processing are adapted to identify echo delay time and to correct for ground reflection effects. A sample result is presented using an actual narrowband sound pressure level spectrum. The technique can readily be adapted to existing fast Fourier transform type spectrum measurement instrumentation to provide field measurements/of echo time delays.
A preliminary study was made of a radar imaging satellite for earth applications. A side-looking synthetic-aperture radar was considered and the feasibility of obtaining a wide area coverage to reduce the time required to image a given area was investigated. Two basic approaches were examined; low altitude sun-synchronous orbits using a multibeam/multifrequency radar system and equatorial orbits up to near-synchronous altitude using a single beam system. Surveillance and mapping of ice on the Great Lakes was used as a typical application to focus the study effort.
Experimental evaluation of four transfer functions for single tube boiler and demonstration of functions being independent of exit restrictions
Turbine flow meters dynamic calibration employing sinusoidally perturbed orifice flow
Turbine flowmeters are calibrated dynamically by means of frequency response tests, provided small perturbations are used. The indicated flow is related to the actual flow by a first order lag function. This lag function is completely defined by the breakpoint frequency which is directly proportional to mean flow rate.
Technique for dynamically calibrating turbine flowmeters for small perturbations by means of frequency response tests