The Electronic Countermeasures Problem for Extremely Low Frequency (ELF) Communications

An extremely low frequency (ELF) communication system for military purposes needs specialized engineering techniques, in particular signal bandwidth spreading in order to protect against enemy electronic countermeasures (ECM). Qualitative discussion is presented of the processing which the system must have to make jamming difficult, with qualitative discussion of predictive jamming, repeat-back jamming, and high power brute force jamming by adapting electric power systems.     

Extremely Low Frequency (ELF) Communication to Submarines

An ELF (extremely low frequency) communication system has been proposed for communication from land to submerged submarines. We present an analytical study of conceptual feasibility of such a system and discuss some of its properties. We find no fundamental technical reason why such a system is infeasible. The present work yields the general structure of the receiver, and estimates for the important parameters of such a system. Nonlinear processing ahead of the FSK (frequency-shift keying) receiver in the submarine appears essential. Such processing greatly reduces the effects of atmospheric noise due to nearby lightning storms, and hence, makes reliable operation possible at much smaller transmitter power.     

The Radiation of Extremely Low Frequency (ELF) Waves

The excitation of extremely low frequency (ELF) waves by a horizontal electric dipole in the earth-ionosphere waveguide can be obtained by a heuristic approach based upon the observation of a relationship between a horizontal electric dipole and a horizontal magnetic dipole. Then multiple imaging takes into account the effect of the ionosphere. This approximate derivation preserves the physical picture which is obscured by complex mathematics in rigorous analysis.     

Ground Anisotropy and Extremely Low Frequency (ELF) Transmitting Antennas

The surface impedance concept is used to examine the effect of ground anisotropy on the radiated field and power dissipation of a horizontal ELF transmitting array. The principal result, for a simple homogeneous but anisotropic ground, is that even though the ground skews the antenna pattern, any attempt to reorient the array to steer the pattern peak towards the receiver incurs an increase in power dissipation. In fact, the power dissipation is minimized by steering the pattern peak even further away from the receiver.     

Propagation Measurements in the Extremely Low Frequency (ELF) Band

An experimental measurement program has been conducted to determine propagation parameters in the 40-50 and 70-80 Hz ranges. These measurements have utilized a transmitter in Wisconsin radiating a sinusoidal signal at radiated power levels of 0.5 W at 45 Hz and 1 W at 75 Hz. Sites have been located at Nova Scotia, California, Utah, Hawaii, Norway, Greenland, Greece, and Saipan. Using sites on the same great circle path as the transmitter, it has been possible to estimate average attenuation rates and excitation factors with high accuracy subject to the constraints discussed in the paper. Both daytime and nighttime attenuation rates and excitation factors are discussed.     

Extremely Low Frequency (ELF) Atmospheric Noise Level Statistics for Project Sanguine

Statistics of narrow-band extremely low frequency (ELF) noise for use in Sanguine systems analysis were obtained at locations in Malta and Guam. The results of four seasons of ELF atmospheric noise measurements are discussed. In particular the contributions of local thunderstorms to high noise levels, i.e., the 1 percent exceedance levels, are shown. Spectra of the 1 percent and 50 percent exceedance levels of the magnetic-field intensity component, obtained from the sensor aligned for maximum reception from northern Wisconsin are plotted. At both locations, the noise levels were highest during the fall season. The diurnal variation of the median noise levels correlated with worldwide thunderstorm activity, while the diurnal variation of the mean square levels correlated with local thunderstorm activity. When few local thunderstorms occur during a season, the cumulative distributions of the 250 s samples for the magnetic-field intensity can be approximated by a log-normal distribution almost to the 1 percent exceedance level.     

Motion-Induced Noise in Electrode-Pair Extremely Low Frequency (ELF) Receiving Antennas

The pressure fluctuations in the turbulent boundary layer surrounding the towed antenna cable cause it to vibrate in the geomagnetic field and thereby generate noise. This noise mechanism is the dominant one in current long antennas. Its effect can be reduced only by increasing antenna diameter and length. Tension, stiffness, and damping have no effect. Noise voltage spectra derived from pressure fluctuation data show striking agreement with those measured.     

A Signaling Scheme and Experimental Receiver for Extremely Low Frequency (ELF) Communication

The advantages of utilizing the extremely low frequency (ELF) band for communications to submarines are principally the low attenuation rate of ELF signals in seawater and the low attenuation rate in the earth-ionospheric waveguide. ELF transmitters, however, require high input power. Efficient signaling schemes and sophisticated receiver designs help minimize the required transmitter power and hence system cost. One such signaling schemeconvolutional encoding and binary antipodal modulation-and an experimental receiver were successfully field tested and are described. Tests included real-time reception of messages aboard a submerged submarine in the Atlantic Ocean. It is concluded that the receiver, which included adaptive nonlinear noise processing, notch filtering, ocean channel compensation, and sequential decoding makes extremely efficient usage of available signal strength and demonstrates the technical feasibility of communication in the ELF band.     

Nighttime Variations of Extremely Low Frequency (ELF) Signal Strengths in Connecticut

A limited amount of extremely low frequency (ELF) horizontal magnetic field strength measurements have been taken in Connecticut during the past 3 years. The transmission source for these 1.6 Mm range measurements was the U. S. Navy ELF Wisconsin Test Facility. The principal result obtained from these measurements is that there are considerable variations in the ELF nighttime propagation parameters.     

Far-Field Extremely Low Frequency (ELF) Propagation Measurements, 1970-1972

For the past 3 years, we have participated in various extremely low frequency (ELF) propagation tests. During these tests, receiving sites were located in Connecticut, North Carolina, Maine, Utah, Nova Scotia, California, Greenland, the Virgin Islands, Alaska, Norway, Hawaii, Greece, and Saipan. Measurements have also been taken sporadically in Connecticut since June 1970. At each location, the horizontal magnetic field strengths were measured at a band of frequencies centered at 45 Hz and 75 Hz in order to determine the average attenuation rates and relative excitation factors for daytime and nighttime propagation conditions. The U.S. Navy ELF Wisconsin Test Facility was the transmission source. The principal results obtained from these measurements were 1) the daytime attenuation rate is usually higher than the nighttime attenuation rate at both 45 and 75 Hz; 2) the relative excitation factors are quite different for daytime and nighttime propagation conditions; 3) nighttime propagation is more variable than daytime propagation; 4) there is a seasonal variation in the ELF propagation parameters; and 5) auroral-zone effects appear to be present at both frequencies.     

Extremely Low Frequency (ELF) Propagation Measurements Along a 4900-km Path

Extremely low frequency (ELF) electromagnetic wave propagation was investigated by measuring the amplitude of a CW signal transmitted from the Sanguine site in North Carolina to receiving sites located in New York State, Labrador, and Iceland. The attenuation factor α and the reciprocal of the excitation factorh_{i} (sigma_{e}S_{0})^{1/2}were determined. Data for daytime, nighttime, and sunrise transition paths were obtained. The attenuation factors obtained for 78 and 156 Hz are compared with the value of α obtained from measurements of atmospherics. The attenuation values at 78 Hz of 1.29 and 1.01 dB/1000 km, and a value of 0.75 dB/1000 km at 45 Hz, obtained by interpolation of results of this and other experiments, were used in Sanguine systems analysis.     

Extremely Low Frequency (ELF) Ionospheric Radio Propagation Studies Using Natural Sources

The purpose of this paper is to describe methods of studying the propagation of radio waves over the earth's surface in the extremely low frequency (ELF) frequency band (3 Hz-3 kHz) using natural (as distinct from "man-made") sources of radiation. Attention is focused on the characteristics of the radio wave field and on methods of analysis of acquired data in order to obtain information on propagation parameters of interest in Project Sanguine. Wave impedance measurements are identified as being of particular importance in these investigations.     

Surface Impedance and the Efficiency of Horizontal-Dipole Extremely Low Frequency (ELF) Antenna Arrays

The effective conductivity of a potential antenna site is a measure of the field strength that an antenna built on the site would radiate. Only in special cases, however, is this same measure suitable for characterizing the power that the antenna would dissipate. It is shown that in the case of a current-sheet antenna built on a horizontally stratified half-space, a measure that characterizes both the radiation and dissipation properties of the site is the complex surface impedance. This same measure remains substantially valid for an antenna composed of separate cables and for inhomogeneous half-spaces of not too great lateral variation. The most reliable methods of measuring the surface impedance are theE/HandH/Imethods, provided both include the measurement of the phase of the ratio.     

Application of Magnetotelluric Techniques at Extremely Low Frequency (ELF) for Sanguine Transmitting Antenna Site Characterization

Conductivity measurements using the magnetotelluric resistivity method can be used to survey a proposed Sanguine transmitter site for its suitability, and to predict the radiation pattern of the transmitting antenna. The magnetotelluric resistivity method, unlike other conductivity measuring techniques, can account for the anisotropic behavior of the earth, and can obtain measurements rapidly as compared with other techniques.     

Design of a Sanguine Noise Processor Based Upon World-Wide Extremely Low Frequency (ELF) Recordings

This paper describes the design of a candidate noise processor for the Sanguine receiver based on communication theory considerations and detailed experiments using wide-band recordings of extremely low-frequency (ELF) (3-300 Hz) atmospheric noise. This processor consists of the following elements: 1) a compensating (or whitening) filter; 2) nonlinear notch filtering at frequencies of manmade interference; 3) a post-notch filter nonlinearity; and 4) a phase coherent linear matched filter. Due to the impulsive non-Gaussian nature of the noise, nonlinear processing with a bandwidth considerably greater than the 40-80-Hz signal bandwidth is significantly better than a linear receiver (consisting only of a matched filter and appropriate whitening filters). Simulations using noise recordings from a number of widely separated locations in the world have shown improvements of 7 dB to 20 dB at times of high ELF atmospheric noise levels at the receiver input.     

Propagation Theory and Calculations at Lower Extremely Low Frequencies (ELF)

Propagation theory along the lines of Budden's terrestrial waveguide formalism is reviewed. Special emphasis is placed on its application to the lower extremely low frequency (ELF) range for which a number of calculations have been made to demonstrate the relationship between propagation parameters and the physical constants of the ionosphere. Calculations performed with diffuse anisotropic ionospheres yield reasonable agreement with ELF test results.     

潜艇极低频全向接收天线中运动感应噪声研究

通过对极低频拖曳全向接收天线运动感应噪声产生原理的研究,得出了潜艇电极对拖曳天线和磁场天线的运动感应噪声功率谱的表达式,详细指出了天线灵敏度、电缆特性、地磁场等关键因素在影响运动感应噪声方面所起的作用,探讨了减小运动感应噪声的方法,并与实验测得的噪声电压进行比较证实了上述理论。     

用于探测极低频信号的光纤传感器相位生成载波解调方法

提出并实现了一种用于微弱极低频信号探测的干涉式光纤传感器相位生成载波(PGC)解调方法。对PGC解调方法中微分交叉相乘(DCM)及反正切(Arctangent)两种相位抽取算法应用于极低频信号解调进行了理论分析和算法仿真,其结果表明,DCM式PGC算法解调极低频信号时,其结果中存在直流漂移,而Arctangent式PGC算法的解调结果中不存在直流漂移问题。进而基于光纤干涉仪搭建了极低频光纤传感系统,并进行了两种PGC解调算法的对比实验,实验结果表明,Arctangent式PGC算法不存在DCM式PGC算法解调极低频信号时的直流漂移,且能够准确解调极低频信号,与理论分析的结论相符。最后采用Arctangent式PGC算法实现了极低频干涉式光纤传感系统,该系统达到的最低可探测信号频率为0.01 Hz,最小可探测信号为4×10-4rad/Hz,动态范围为110 dB@1 Hz,线性相关系数为99.99%。     

工频磁场感应线圈校准

介绍电磁兼容抗扰度试验中,常用的工频磁场感应线圈及工频磁场发生器的基本原理和相应的校准方法,并对校准结果和校准过程中遇到的问题进行了详细分析.