Interference Fluctuations Research Articles

A Broad-Band Noise Technique for Fast-Scanning Radar Observations of Clouds and Clutter Targets

This paper describes a relatively simple technique for suppressing interference fluctuations from precipitation and other cluttertype targets. The technique utilizes noise as a transmitted signal and receives the returns in a simple radiometer-type receiver. For noise of bandwidth B and an averaging time equal to transmitted pulselength T, the root-mean-square (rms) fluctuation in the return from a given volume in range is a fraction, ≃√/√BT of the mean return, down from a fractional value of unity for unaveraged single-frequency returns. Using this technique, a fast-scanning radar has been constructed which scans the hemisphere overhead every 20 s with a 2.2° beam. For this system, B = 30-40 MHz and T = 1 μs, and the error in the reflectivity estimate per transmitted pulse is ≃1-dB rms. Observations of precipitation returns using 300-MHz bandwidth noise are compared to simultaneous single-frequency observations to demonstrate the clutter reducing ability of the noise technique. For 300-MHz bandwidth noise transmissions and an averaging time of 1 μs, interference fluctuations are reduced by 25 dB from the single-frequency case, and the mean reflected signal is determined to within 0.3-dB rms in a single transmitted pulse.

Light-scattering study of the temperature dependence of Escherichia coli motility

Two light-scattering techniques are used to study the temperature dependence of translational and rotational motility in Escherichia coli. The method of number fluctuation spectroscopy is developed theoretically and experimentally to measure the translational swimming speed of a smooth swimming strain of E. coli. Interference fluctuation techniques are used to study the rotational component of the motion. The results demonstrate that the thrust remains proportional the the torque generated by the flagella throughout the range studied and also show that relative changes in translational swimming speed may be inferred from the dynamics of rotational motion.

Scattering in an Inhomogeneous Medium

The standard mathematical procedure formally describes scattering by the superposition of a scattered pressure on the unscattered sound field. At low frequencies, because of the irregular distribution of the inhomogeneities, the phases of the scattered waves are at random and scattering is an interference phenomenon. As the frequency increases, scattering becomes highly collimated in the forward direction and the phase differences decrease to zero. At this point ray theory starts to apply. The scattered pressure, then, essentially describes only a phase change caused by the different sound velocities and the focusing and defocusing by the lens action of the patches. The medium in the neighborhood of the receiver can be shown to contribute only by focusing, the medium farther away only by interference fluctuations. Focusing leads to normally distributed amplitude fluctuations. The distribution of the interference fluctuations, however, passes from normal to Rayleigh with increasing values of range.

Forward Scattering in the Sea

At small ranges and at high frequencies the scattering of sound in the sea is known to be the result of the focusing and defocusing action of the patches of inhomogeneity of the medium. The standard deviation of the transmitted amplitude should increase as the 3/2 power of the distance, and the amplitudes should be normally distributed. As the range increases, scattering passes over into an interference phenomenon because of the accumulating phase differences. The standard deviation of the amplitude should then increase as the square root of the range times the wave number, and the fluctuations should change from a normal distribution to a Rayleigh distribution with increasing range. Recent measurements at sea seem to confirm the more essential points of the theory. Although the behavior with range is generally verified, measured amplitudes tend to depart from theoretical expectations in the focusing and the transition range, where the shape of the patches of inhomogeneity seems to be essential, but approach the theoretical distribution when the interference fluctuations predominate. When the fluctuation of amplitude is greater than expected, the distribution curve usually shows several peaks, although the shape is between normal and Rayleigh when the fluctuation is more typical. Fluctuation measurements made at two frequencies enable the thermal and spacial size of the patches of inhomogeneity to be deduced. An estimate made in this way is in fair agreement with direct microthermal measurements in the sea.