Propagation Time Difference Research Articles

An Extended Fibre Optic Stress Location Sensor

A new type of stress location sensor is discussed, in which the frequency-modulated continuous wave (FMCW) technique is used to detect the difference in propagation time between two optical paths in an optical fibre interferometer. The interferometer is formed by the polarization modes in a birefringent fibre, and the location of stress experienced by the fibre may be measured to within - 1·2 m. A variation of the interferometer using the LP01 and LP11 mode-sets in a step-index fibre is investigated, and it is found that dispersion within the LP11 mode-set limits the accuracy of the system.

Estimation of the differential pulse propagation times in two-color laser ranging systems

Two-color laser ranging systems can be used to determine the atmospheric delay by measuring the difference in propagation times between two optical pulses transmitted at two different wavelengths. In this paper, the performance of a cross-correlation technique for estimating the differential propagation time is analyzed by considering both speckle and shot noise. For the flat diffuse targets, the differential timing accuracy is highly dependent on the receiver bandwidth and the characteristics of the time-resolved speckle. At low signal levels, when the receiver bandwidth is chosen properly, the performance of the correlation estimator is comparable with that of the maximum-likelihood estimator. At high signal levels, however, speckle places a fundamental limit on the performance of the correlation estimator. For the cube-corner reflector arrays, timing performance is dominated by partially developed speckle. The differential propagation time cannot by resolved to better than the pulse widths of the received signals.

A Fundamental Evaluation of In Vivo Local Velocity Measurement by Crossed Beam Method

The possibility of in vivo local velocity measurements by crossed beam method is described. The local velocity in the homogeneous medium is calculated from the known beam geometry and the difference in propagation time measured by several pairs of transducers. Each propagation time is determined by the center of moment for the echo from the cross region. Experimental results show the estimated error to be about 4%, but corrected values agree well with true values and the effectiveness of this new method is confirmed.

Electron precipitation and VLF emissions associated with cyclotron resonance interactions near the plasmapause

Correlated bursts of bremsstrahlung X rays and VLF emissions were recorded for ∼ 25 min at Siple Station, Antarctica, on January 2, 1971. The bursts occurred quasi-periodically with spectral power predominantly in the period range 4–12 s. A typical VLF burst consisted of 3–5 rising elements of ∼0.1-s duration separated by ∼0.15 s and was confined to the frequency range 1.5–3.8 kHz. Evidence is presented to show that the bursts were triggered by the low-frequency tail of whistlers propagating from the northern hemisphere. The spectrum of the precipitated electrons was characterized by an e folding energy of 45 keV; electrons with energies between ∼70 and 250 keV were the primary contributors to the observed X ray flux. A characteristic feature of the data was a time delay of ∼0.2 s between the reception of a VLF burst and the associated X ray burst. Calculations show that this delay can be related to the difference in wave and particle propagation times from a burst generation region near the equator to the detectors at Siple Station if the waves and electrons leave the interaction region in opposite directions. The interpretation of the observations in terms of an equatorial cyclotron resonance interaction occurring at the outer edge of the plasmapause on the L = 4.2 field line, offered initially by Rosenberg et al. (1971), is given further support by the more extensive analysis presented here of the electron energy-wave frequency relationship in the bursts and the propagation times for the resonant waves and electrons. It is inferred from the X ray data that the equatorial flux of trapped electrons was probably anisotropic and near the stable trapping limit at the time of this event. It is suggested that an important effect of the trigger signal is the increase of the anisotropy of the resonant electrons. Wave growth rates calculated in the random phase approximation for electron pitch angle distributions that might apply in this event can explain certain features of the VLF and precipitation data during and between the bursts.

Time and Doppler Correlation for Passive Acoustic Ranging

In passive acoustic ranging, a correlation function is frequently used to measure the signal propagation time difference between two sensors, or between two paths to a single sensor. The same device can be used to measure the Doppler differences when the source is in motion. The joint use of time and Doppler differences can result in more accurate range estimation than is possible from time or Doppler differences alone. Examples are given of the use of the use of time and Doppler correlation in passive ranging on a submarine.

The relation between group path and polarization rotation

It is shown that for one-hop oblique path propagation through a slowly varying ionosphere with no collisions, the rate of variation of incoming polarization with radio frequency is equal to one-half the group propagation time difference between the two magnetoionic components. This relation suggests that simple experimental techniques to measure the incoming wave polarization may provide the more difficult-to-measure differential group time information necessary for obtaining very accurate electron density profiles from oblique ionograms.

Theoretical analysis of the basic parameters of ultrasonic flowmeters

A comparison of the final results of the two methods shows that the analytical expressions for the interchannel phase difference δϕ and repetition frequency difference δF, and in the case of pulsed circuits also of the pulse propagation time difference δΤ, differ in the two methods by their corrections BΓ and BΓ.

Permalloy film NDRO memory

A thin magnetic film NDRO storage cell has been developed for very high-speed word-organized memories. The storage cell contains two 500-A, 15-mil-square Permalloy film elements with a read and sense line between them. One film element is deposited on a metallic ground plane, so that the read line and its image in the ground plane are coupled to the readout film and decoupled from the storage film. A 3-ns-wide 300-mA read pulse with a 1-ns rise time yields a 4- to 6-mV output signal. Two methods of coincident current writing were investigated. One with the word and digit lines outside the storage cell required 500-mA word current, 80-mA digit current, and a read after write pulse. Writing takes 50 ns and the storage cell will tolerate coincident word and digit disturb currents of 50 and 100 mA, respectively. The other method uses the read and sense lines between the films as word and digit lines. Word currents of 300 mA and digit currents of 50 mA are required to write. The disturb margins are greater, and read after write pulse is not required. Because the eddy currents in the ground plane must decay, writing takes 3 μs. A 512-word, 40-digit test plane has been built and partially populated to determine problems in building a large array. The characteristic impedances and effects of attenuation and delay have been observed. In particular, the output signals are reduced one half because of attenuation and the difference in signal propagation time in the two directions on the sense line.

Determining Ionospheric Parameters over Regions of Denied Access

Utilization of high frequencies for over-the-horizon surveillance necessitated a refined approach to the determination of actual ray paths. Various ray-tracing programs have been developed. The basic problem lies in accurately determining values of the appropriate ionospheric parameters. A possible means is by utilization of oblique sounding devices located at the path terminals to provide 1) relative propagation time difference (Δt) between the modes, 2) maximum frequency propagated by any given mode (MUF), and 3) difference in MUF between the various modes (ΔMUF). The technique has not been fully verified; preliminary results are encouraging.

Discussion of paper by E. J. Stegelmann and C. H. von Kenschitzki, ‘On the interpretation of the sudden commencement of geomagnetic storms’

It was pointed out some years ago [Dessler et al. 1960] that a major part of the rise time of a sudden commencement can be accounted for as the period of time in which the enhanced solar wind sweeps over the outer boundary of the geomagnetic field, plus the difference in propagation time from different points on the boundary. The dimensions of the outer boundary of the geomagnetic field are of the order of 105 km, so that an enhanced wind of 103 km/sec characteristically takes about 102 seconds to engulf the field and enhance the compression of the field. Typical propagation times within the field differ by 102 seconds. The basic idea was simply that the compression of the geomagnetic field at the surface of the earth results from the algebraic sum of all the compressive forces exerted by the wind on the outer boundary of the field [Parker, 1958]. Any changes in the compressive forces on the outer boundary are communicated inward at about the Alfven velocity [Francis et al., 1959]. Enhanced compression of the boundary starts somewhere on the solar side, at which time a slight increase in compression begins to propagate through the geomagnetic field as a hydromagnetic wave. With the passage of time more and more of the boundary is compressed as the enhanced wind sweeps by in space. The rising compression communicates throughout the field with the Alfven speed, and levels off only after the enhanced wind fully encloses the geomagnetic field.

A Comparison of Frequency-Shift Anti-Multipath Signaling Techniques for Digital Communication Systems

An important result of multipath propagation is the lengthening of the received signal due to a single transmitted pulse. The received pulse duration may be increased by the difference in propagation times between the longest and shortest of the active paths. This time difference represents the minimum pulse repetition period which can be used if errors due to inter-symbol interference are to be avoided. This minimum pulse repetition period places a limit on the available data rate per channel. The total data rate can be increased by frequency division multiplexing. Several recent papers have described a technique for synchronous frequency-division multiplexing for use under the multipath conditions described above. This synchronous system is analyzed to show its bandwidth, data rate, and signal-to-noise properties, and these parameters are compared with those of a conventional FSK system. A second refinement of conventional FSK is the use of more than two possible frequencies for each pulse. The bandwidth, data rate, and signal-to-noise properties of multilevel synchronous FSK are presented, and some circumstances under which it may be used to advantage are described.