Using elevated radials with grounded towers

This paper describes a technique in which elevated horizontal radials are used in combination with a grounded quarter-wave tower to construct an AM-broadcast transmit antenna. Computer analysis indicates that the radiation efficiency of these antennas can be within one decibel of that of a conventional base-insulated 1/4-wave tower with an extensive buried-radial ground system     

Using elevated radials with ground-mounted towers

Computer-modeling exercises are reported which indicate that it is possible to use four elevated radials together with a conventional ground-mounted base-insulated tower for MF broadcasting purposes, and achieve the same radiated field strength as that emitted by a similar monopole using 120 buried radials. Several different methods of attaching the radials to the tower were investigated, along with variations in radial height, radial length, and tower height. Limited field testing appears to confirm the results of the computer analysis     

AM broadcast antennas with elevated radials: varying the number ofradials and their height above ground

Previous computer-modeling studies indicated that four elevated radials may be used in conjunction with an elevated vertical monopole (tower) to produce radiated field strength which is equivalent to that emitted by a similar monopole using 120 buried radials. Inquiries have been received concerning the effect of increasing the number of elevated radials, and/or changing the height of the radials above ground. Analysis shows that there seems to be an optimum number of elevated radials for each height and soil type. Surprisingly, the continued addition of more and more elevated radials eventually produces a reduction in performance. The computer code used for this work was NEC, a “method of moments” program developed at Lawrence Livermore National Laboratory     

AM broadcast antennas with elevated radial ground systems

Results are presented of computer modeling studies indicating that an elevated vertical monopole antenna with four elevated horizontal radials produces more ground-wave field strength than does a conventional ground-mounted monopole with 120 buried radials. For the analysis, the length of the radials and the height of the monopole were set equal to 0.25 of the free-space wavelength, and the frequency of operation was fixed at 1.0 MHz. Three different sets of ground constants were used, simulating averaging, very good, and very bad soil electrical parameters in accordance with the Numerical Electrical Code (NEC-GS)     

Space/ground systems as cooperating agents

Within NASA and the European Space Agency (ESA), it is agreed that autonomy is an important goal for the design of future spacecraft, and that this requires on-board artificial intelligence. NASA emphasises deep space and planetary rover missions, while ESA considers on-board autonomy as an enabling technology for missions that must cope with imperfect communications. ESA's attention is on the space/ground system. A major issue is the optimal distribution of intelligent functions within the space/ground system. This article describes the multi-agent architecture for space/ground systems (MAASGS). A MAASGS agent may model a complete spacecraft, a spacecraft subsystem or payload, a ground segment, a spacecraft control system, a human operator, or an environment. The MAASGS architecture has evolved through a series of prototypes. This study recommends that the MAASGS architecture should be implemented in the operational Dutch Utilisation Centre.     

Using GPS to measure ground complex permittivity

In personal satellite communications, ground specular reflection may be received along with the direct signal. To characterise this effect at L-band, fade measurements were taken over grass and asphalt surfaces using a GPS receiver. By matching experimental results to a simulation, ground electrical characteristics were derived, showing a novel method for their measurement     

The role of computers in conjunction with analytical instrumentation

The use of a small or medium size computer as a data acquisition and processing system for a mass spectrometer has greatly enhanced the effectiveness of this instrument, particularly when used in conjunction with a gas chromatograph as the sample input device for the spectrometer. The qualitative information obtainable from the complex mixtures usually represented by biological samples is greatly increased, up to the point that an almost automatic identification of all components in such mixtures is achieved. The sensitivity of a mass spectrometer is very high, compared with most other analytical instruments, and it is therefore well suited for the identification of trace components, an important feature for environmental analyses. Similarly unmatched is the high degree of specificity of mass spectrometry and its capability for quantitative measurements at very low levels. These aspects are illustrated on examples involving the identification of drugs in body fluids, contaminants in drinking water, forensic analysis, monitoring of biologically very active trace components (melatonin), detection of absorption through the skin of toxic flame retardants from fibers and the monitoring of the extremely toxic substance tetrachlorodibenzodioxin in a severely contaminated area.     

在8051单片机应用系统中使用DiskOnChip

本文以8051单片机为例探讨了在8位单片机应用系统中，使用M—Systems公司的DiskOnChip作为大容量非易失数据存储器的可行性，给出了在8051单片机应用系统中使用DiskOnChip的软、硬件实现方案。     

Simplified calculation of ground losses in low- and medium-frequency antenna systems

Calculations of ground losses are paramount in obtaining the best performance of a monopole antenna in the low- (LF) and medium-frequency (MF) bands. Ground losses are usually computed numerically, due to difficulties in the mathematical formalism. The novel approach here permits obtaining simple analytical expressions for ground-loss calculations that can be useful for determining the behavior of the ground plane. As a first approximation, the monopole antenna is placed on a perfect electrically conducting (PEC) ground plane in order to obtain the antenna current distribution and the near magnetic field, taking into account the non-zero-radius effect of the monopole. Next, the near magnetic field is used to determine the surface-current density on the ground plane below the antenna. This is divided into two zones: (1) the artificial ground plane, where either a radial-wire ground screen or a metallic layer is used to increase the soil's conductivity; and (2) the natural ground plane or bare soil up to a circular boundary a half wavelength from the antenna's base. The power dissipation is calculated from the artificial and natural ground-plane surface-current densities, and the ground-plane loss resistance is obtained. Also, an effective conductivity is defined as a measure of the ground plane's effectiveness, and the cases of quarter-wave monopoles and short top-loaded antennas are analyzed. Some results are validated by means of numerical computations and moment method simulations     

Design and control considerations for automated ground transportation systems

Fundamental system and control problems associated with proposed automated ground transportation concepts are discussed. The problem formulation provides a systematic approach toward the synthesis and techinical evaluation of a large class and future systems employing automatically controlled single-mode or dual-mode vehicles on intracity guideway networks. Important system engineerig problems--common to some degree to all proposed concepts--are defined and the associated control requirements are discussed. Basic performance criteria, the weighted average travel time and relative safety of travel, are formulated. Tradeoffs between important system parameters are identified and their relations to performance and cost are discussed. Analysis of system operation is based upon idealized models in which perfect information and control are assumed. Upper bounds on performance are thus obtained, and the main constraints are identified. The degradation of system performance resulting from random arrivals and departures is thereafter analyzed quantitatively. Finally, control problems arising from the use of short headways are discussed. The objective of the paper is to help develop a systematic approach to the design of automated transportation systems using digital computers and modern electronic communications. It concentrates on the functional system problems of relatively simple networks, which lend themselves to direct analytical approaches and which constitute a point of departure for more complex networks.     

Three-dimensional cellular systems for air/ground personal communication

A new three-dimensional (3-D) cellular system is proposed for air/ground (A/G) personal communications. The proposed system extends the Global System for Mobile Communication (GSM) to enable its subscribers to send and receive calls while traveling in the air using the same subscriber identity module (SIM)-the same mobile number-that is used for the ground network. Modifications to the GSM system architecture to develop the A/G system architecture are outlined. A multilayer service volume is proposed in which the coverage volume is divided into three coaxial cylindrical layers (cells) with each layer serving a limited part of the airspace. Capacity of the new proposed system based on the standard GSM concept is analyzed and compared to other alternative systems: Code Division Multiple Access (CDMA) and Frequency Division Multiple Access (FDMA).     

Improving JPEG performance in conjunction with cloud editing forremote sensing applications

The authors propose an improved version of JPEG coding for compressing remote sensing images obtained by optical sensors onboard microsatellites. The approach involves expanding cloud features to include their cloud-land transitions, thereby simplifying their coding and subsequent compression. The system is fully automatic and appropriate for onboard implementation. Its improvement in coding stems from the realization that a large number of bits are used for coding the blocks that contain the transition regions between bright clouds, if present in the image, and the dark background. A fully automatic cloud-segmentation algorithm is therefore used to identify the external boundaries of the clouds, then smooth the corresponding blocks prior to coding. Further gains are also achieved by modifying the quantization table used for coding the coefficients of the discrete cosine transform. Compared to standard JPEG, at the same level of reconstruction quality, the new method can achieve compression ratio improvement by 13-161%, depending upon the context and the amount of cloud present in the specific image. The results are demonstrated with the help of several real images obtained by the University of Surrey, U.K., satellites     

In conjunction with Venus [planetary radar astronomy]

Few people know that the value of the astronomical unit-the mean distance from the Earth to the Sun, and the yardstick for gauging distances within the solar system-was determined not by astronomers, but electrical engineers. They did it using military radar and thereby opened up a new field of scientific research, known as planetary radar astronomy. The paper discusses the early history of planetary radar astronomy up until the mid 1960s     

An analysis of the effects of ground reflection in line-of-sight phase systems

The presence of a reflected component in a line-of-sight radio system influences the phase variability resulting from atmospheric changes along and near the path. An analysis is made of a simple case showing the general nature of this effect and the parameters upon which it depends. A method is presented for calculating the phase behavior of the resultant signal, in terms of that expected from a single direct component, and the relevant system parameters.     

Capacity of multiple antenna systems in free space and above perfect ground

Multiple input-multiple output (MIMO) systems are a promising technique for wireless applications that require high data rates. We theoretically study their capacity potential for propagation in free-space and over a ground surface. We investigate how this depends on the array geometry and the electric field polarization. Moreover, we validate our theoretical predictions with measurements above a nearly perfectly flat surface. The experiment shows good agreement with the theory for a large range of distances and deviates from it when secondary effects (roughness etc.) become more significant.     

Transient phase conjunction in a two-level system

An analytical expression for conjugate signals is developed for a case where three rectangular pulses with the same center frequency are injected into a two-level system. The Maxwell's equation is solved with the slowly varying envelope approximation and with the perturbative third-order nonlinear polarization expression of a two-level system. Numerical examples are discussed in such a way that the input pulsewidth is changed relative to the energy relaxation and dephasing times. As the optical pulsewidth becomes short, ripples appear in the signal pulse due to the beat between the optical field and the resonant oscillation of the atomic system     

NASA ground terminal communication equipment: Automated fault isolation expert systems

This paper describes the prototype expert systems that diagnose the Distribution and Switching System I and II (DSS1 and DSS2), Statistical Multiplexers (SM), and Multiplexer and Demultiplexer systems (MDM) at the NASA Ground Terminal (NGT) located at White Sands, New Mexico. A system-level fault isolation expert system monitors the activities of a selected data stream, verifies that the fault exists in the NGT and identifies the faulty equipment. Equipment-level fault isolation expert systems will be invoked to isolate the fault to a Line Replaceable Unit (LRU) level. Input and sometimes output data stream activities for the equipment are available. The system-level fault isolation expert system will compare the equipment input and output status for a data stream and perform loopback tests (if necessary) to isolate the faulty equipment. The equipment-level fault isolation system utilizes the process of elimination and/or the maintenance personnel's fault isolation experience stored in its knowledge base. The DSS1, DSS2, and SM fault isolation systems, using the knowledge of the current equipment configuration and the equipment circuitry, will issue a set of test connections according to the predefined rules. The faulty component or board can be identified by the expert system by analyzing the test results. The MDM fault isolation system correlates the failure symptoms with the faulty component based on maintenance personnel experience. The faulty component can be determined by knowing the failure symptoms.

The NGT fault isolation prototype is implemented in Prolog, C, and VP-Expert, on an IBM AT compatible workstation. The DSS1, DSS2, SM, and MDM equipment simulators are implemented in PASCAL. The equipment simulator receives connection commands and responds with status for the expert system according to the assigned faulty component in the equipment. The DSS1 fault isolation expert system was converted to C language from VP-Expert and integrated into the NGT automation software for offline switch diagnoses.

Potentially, the NGT fault isolation algorithms can be used for the DSS1, SM, and MDM located at Goddard Space Flight Center (GSFC). The prototype could be a training tool for the NGT and NASA Communications (Nascom) Network maintenance personnel.     

Using FPGAs to implement self-timed systems

Asynchronous or self-timed systems that do not rely on a global clock to keep system components synchronized can offer significant advantages over traditional clocked circuits in a variety of applications. In order to ease the complexity of this style of design, however, suitable self-timed circuit primitives must be available to the system designer. This article describes a technique for building self-timed circuits and systems using a library of circuit primitives implemented using Actel field programmable gate arrays (FPGAs). The library modules use a two-phase transition signaling protocol for control signals and a bundled protocol for data signals. A first-in first-out (FIFO) buffer and a simple routing chip are presented as examples of building self-timed circuits using FPGAs.This work was supported in part by NSF award MIP-9111793.