Pulsed CW Signals Research Articles

Linear photonic technique for fixed and time varying RF phase shifts of radar signals

A simple linear photonic technique is proposed to achieve fixed or time varying radio-frequency (RF) phase shifts which can be used in applications such as radar signal manipulation. The technique is based on fixing or tuning the wavelength of an RF modulated optical signal within the reflection band of a fiber Bragg grating (FBG) filter with a step group delay profile. The scheme is verified in a realistic simulation to achieve a Doppler shift in a pulsed CW signal return.

Experimental studies of low-frequency bistatic reverberation in a shallow sea

Experimental data are presented on time and frequency dependences of the reverberation level for bistatic transmission and reception at low acoustic frequencies. The data are obtained from the studies carried out in a coastal shallow-water region and in the central region of the Barents Sea with explosion-generated and pulsed cw signals. By using the simplest computational model, approximate estimates are obtained for the effective coefficient of spatial attenuation and the effective scattering coefficient in the frequency band 40–400 Hz.

Experimental observation and statistics of multipath from terrain with application to overland height finding

Data is presented from an airborne experiment designed to evaluate electromagnetic pulse scattering from a variety of terrain types for the application of overland height finding. The one-way propagation data was collected at ultrahigh frequency (UHF) and very high frequency (VHF) frequencies using a 3-MHz pulsed CW signal with the radars mounted on two aircraft. A statistical analysis of the data shows that the probability of a detectable ground reflection and the strength of that reflection vary as a function of the terrain roughness. We introduce a broad-band synthesis technique in which Fourier theory is used to generate a time-domain response from a CW propagation model. Results from this technique agree well with the experimental observations, but the lack of detailed terrain information does not allow exact reproduction of the fine details.

An estimation method of vertical sound‐speed profiles in shallow water using multipath propagation

To observe changes in shallow water structure, a method of estimating the vertical sound‐speed profile in shallow water is presented. “Shallow” means a water depth of less than about 1500 m in which sound can be propagated a certain distance by repeated reflections at both surface and bottom. In such water, the acoustic characteristics of both the surface and the bottom are important determinants of the sound field. The acoustical characteristics of shallow water in an acoustic transmission experiment that was conducted in Sagami Bay using a 5‐kHz pulsed cw signal at a range of 10 km are examined. According to the results, bottom‐and/or surface‐reflected rays are stable in time and have identifiable ray paths. Using a shallow water model, a simulation to estimate the vertical sound‐speed profile not from the travel times of the rays but from the travel time differences between the bottom‐ and/or surface‐reflected rays was carried out. Travel time differences between individual rays could easily be measured by a clock, and estimated results agree well with given correct models.

FM/AM Noise Test Set for a Pulsed Doppler Radar

A new test method to measure the amplitude noise and phase noise in both CW and pulsed CW signals of a Ku-band pulsed Doppler radar is described. These noises are measured in a simulated environment of radar operation; thus the test results may give direct information to determine radar subclutter visibility. In comparison with the conventional noise test method, this new method not only gives more meaningful results but also can obtain results much faster in testing. Actual test system design is described by block diagrams and theoretical analysis. A method to determine approximate frequency jitter in a transmitter signal is also described.

Spectrum of delay fluctuations in single‐path underwater transmission over a range of 43 nmi

Two single‐path transmission experiments were performed, one at 450 Hz and one at 400 Hz, to measure the spectrum of the fluctuations in acoustic travel‐time difference between two receivers. In the first, a source on the ocean bottom 15 000 ft deep transmitted a pulsed cw signal to two bottomed receivers spaced 4300 ft in line with the source 19 nmi away. Large low‐frequency fluctuations were found in the arrival‐time difference between the two receivers. The energy in the spectrum of these fluctuations peaked around 0.2 to 0.7 cycles/h; it corresponded to a rms fluctuation in relative delay of about 1 ft. The second experiment used the same receivers to examine more rapid fluctuations, with a source suspended at 1200 ft depth 43 nmi away. In this case the results showed an additional peak in the fluctuation spectrum around 0.1 Hz. The energy in these fluctuations corresponded to a rms variation in acoustic path length difference of 0.11 ft. The faster fluctuations are ascribed to reflections from the undulating ocean surface. The slower ones are believed due to slow changes in the sound‐speed profile.

Acoustic propagation in shallow water overlying a consolidated bottom

An experiment designed to measure normal mode amplitude functions and attenuation coefficients was conducted in shallow water on Campeche Bank off the Yucatan Peninsula. Measurements were made at two locations on the bank in water of about 30 m in depth over a bottom consisting of consolidated limestone having a measured and sound velocity of 1900 m/sec. Pulsed cw signals with frequencies of 400, 750, and 1500 Hz were used. Theoretical calculations of the mode amplitude functions using a fluid model of the bottom were found to agree well with the measurements. In order to reconcile the measured mode attenuation coefficients with theory, it was necessary to assume that the shear velocity of the bottom was 1000 m/sec. The latter is lower than the minimum sound velocity in the water column so that the generation of propagating shear waves in the bottom was the dominant attenuation mechanism. Significant differences in the measured mode attenuation coefficients at the two stations were explained by the deepening of the low velocity channel at the bottom of the water column. Subject Classification: [43]30.20, [43]30.50.

Ambiguity Function for Wide- and Narrow-Band Signals with the Effect of the Acclerating Point Target

Conventional ambiguity-function theory is applied when estimates of range and Doppler for moving targets are desired. Unfortunately, previously available literature has not considered the effects of an accelerating target. This paper is extended herein for both linear FM and pulsed CW signals to account for these effects, and the resulting half-power contours are plotted. For the linear FM pulse, the matched contours lengthen with acceleration, if mismatched in both Doppler and acceleration, the ambiguity function assumes the form of an Airy function, and representative contours for these conditions are also given. For the pulsed CW, the ambiguity function assumes a form that includes Fresnel integral; the resulting half-power contours are rotated when accleration is allowed in matched situations. When there is mismatch, the contours are both translated and rotated.

The Reflectivity of Two Bottom Areas as Measured by Total Energy Reflection Coefficients

In a paper presented at the 72nd meeting of the Acoustical Society of America, the author discussed the high variability in the peak amplitude bottom reflection coefficients obtained using pulsed CW signals over an abyssal plain area. These results led to an investigation into the use of a reflected-to-incident energy ratio as a more suitable description of bottom reflectivity. Ratios between the integral-square values of the bottom-reflected and direct-path signal envelopes were computed for 100 msec pulsed CW and 250-msec linear frequency modulated (LFM) signals. The frequencies were 1080 and 3700 Hz for the CW; the LFM bands were centered on these frequencies. Results indicate that the integral-square ratios, which are essentially energy ratios, are as variable as the peak reflection coefficients. The variability, expressed as the interval bounded by the mean plus and minus one sigma, is higher for the 3700-Hz data. For both frequencies and signal types, this variability often exceeds 4 dB over survey areas only a few miles in extent.

Review of the Principal Phenomena of Transient Radiation and Scattering

This paper summarizes two basic processes generating acoustic transients: sound radiation by transiently moving solid boundaries and scattering of transient sound pulses by single scatterers. The velocity history of the radiating surface is related to the farfield pressure history in terms of the source geometry. Transients radiated by an impulsively accelerated solid are described. For “pulsed CW” sources, the transition of radiation loading and directivity patterns is traced from the transient to the steady-state régime. Different aspects of the physical process are emphasized by various analytical formulations (wave-harmonic and creeping-wave solutions; Kirchhoff approximation). This correspondence also holds for the scattering problem. The diffraction of a pressure step wave by a finite object is described. The echo structure of a pulsed CW signal is related to pulse-length and scatterer geometry and, for elastic scatterers, to their transient structural response. [Study supported by the U. S. Office of Naval Research and General Dynamics/Electronics.]

Pertinent Acoustic Characteristics of Cylindrical Wooden Tanks

Measurements of acoustic characteristics of the cylindrical wooden tanks used for sonar calibrations at the NAVAIRDEVCEX have been performed. Characteristics measured include reverberation time for pulsed CW signals, reverberation time for random-noise signals, and the lowest frequency at which these tanks could reasonably be excited. The results of the CW reverberation-time measurements were analyzed to derive attenuation coefficients of the unlined, waterfilled wooden tanks as a function of frequency. Graphs were constructed to permit the determination of useable pulse-repetition rates for desired signal to reverberation levels while using pulsed CW signals. The results of reverberational times for various bandwidths of noise were compared with the CW results. Agreement was found to exist between the CW and noise measurements with regard to the lowest frequency at which the tank could be excited. This frequency corresponded with a quarter-wavelength for the major dimension of the tank being measured. The data obtained are presented graphically and discussed in relation to the capabilities aud the limitations of the tanks for the calibrations of transducers.

Convergence Zone Propagation in the North-East Pacific

Oceanographic factors cause the sound velocity minimum in the Gulf of Alaska and the neighboring portions of the North-East Pacific to occur at a much shallower depth than in more southerly waters. As a result it is fairly easy to make experimental measurements with hydrophones at depths near the vertex. Measurements using pulsed CW signals of sound level against range have been made at several locations with arrays including hydrophones at various depths. These measurements have been compared with predictions based on ray tracing assuming a series of layers each with constant velocity gradient. Use of the Kuwahara sound-velocity relationship predicts too short a range for the convergence. Work is in progress to utilize predictions based on the work of Wilson. The pulse-to-pulse amplitude fluctuation is small, indicating at least short-term stability in the acoustic path without much effect from surface or internal waves.

Fluctuations in Pulsed cw Transmission

Pulsed cw signals were transmitted from an electromagnetic transducer situated in the deep sound channel, and were received by a hydrophone at a similar depth, at ranges of from 1 to 6 kyd. A study of the fluctuations of ∫ P2dt has shown that for the sound channel arrival, the relative fluctuations increase with range. For ranges from 2 to 6 kyd, the relative fluctuation is approximately KR1.1. The surface reflected arrival has a maximum relative fluctuation at 1 kyd, a minimum relative fluctuation at 3.4 kyd, and increasing scatter out to the maximum range. Tentative explanations for these phenomena, justification for the relating of results using ∫ P2dt to those using Bmax, plus correlation with results published by other scientists will be discussed. (This work was supported by the Office of Naval Research.)