Backscatter Coefficient Data Research Articles

Modeling forest signatures at L band: passive, active monostatic and active bistatic systems

The Tor Vergata forest model, based on the radiative transfer theory and a discrete approach, has been completed by introducing routines which generate the input data set as a function of general variables, such as forest biomass and dbh (diameter at breast height) distribution. Comparisons between model simulations and measurements of emissivity and backscattering coefficient, at L band, are shown. Radiometric measurements covered a significant variability of biomass and two seasonal conditions. Backscattering coefficient data have been taken from the literature. A good correspondence between simulated and measured data is generally observed. Effects of both biomass variations and seasonal changes are well represented. The model is used to predict the performances of an L band radiometer in monitoring soil moisture under forests of various biomass values. Finally, since the model gives a full bistatic characterization of the scattering, it is used to predict the trends of the bistatic scattering coefficient as a function of biomass.

Interlaboratory Comparison of Ultrasonic Backscatter Coefficient Measurements From 2 to 9 MHz

As are the attenuation coefficient and sound speed, the backscatter coefficient is a fundamental ultrasonic property that has been used to characterize many tissues. Unfortunately, there is currently far less standardization for the ultrasonic backscatter measurement than for the other two, as evidenced by a previous American Institute of Ultrasound in Medicine (AIUM)-sponsored interlaboratory comparison of ultrasonic backscatter, attenuation, and speed measurements (J Ultrasound Med 1999; 18:615-631). To explore reasons for these disparities, the AIUM Endowment for Education and Research recently supported this second interlaboratory comparison, which extends the upper limit of the frequency range from 7 to 9 MHz. Eleven laboratories were provided with standard test objects designed and manufactured at the University of Wisconsin (Madison, WI). Each laboratory was asked to perform ultrasonic measurements of sound speed, attenuation coefficients, and backscatter coefficients. Each laboratory was blinded to the values of the ultrasonic properties of the test objects at the time the measurements were performed. Eight of the 11 laboratories submitted results. The range of variation of absolute magnitude of backscatter coefficient measurements was about 2 orders of magnitude. If the results of 1 outlier laboratory are excluded, then the range is reduced to about 1 order of magnitude. Agreement regarding frequency dependence of backscatter was better than reported in the previous interlaboratory comparison. For example, when scatterers were small compared with the ultrasonic wavelength, experimental frequency-dependent backscatter coefficient data obtained by the participating laboratories were usually consistent with the expected Rayleigh scattering behavior (proportional to frequency to the fourth power). Greater standardization of backscatter measurement methods is needed. Measurements of frequency dependence of backscatter are more consistent than measurements of absolute magnitude.

Intercomparison of pulsed lidar data with flight level CW lidar data and modeled backscatter from measured aerosol microphysics near Japan and Hawaii

Aerosol backscatter coefficient data were examined from two flights near Japan and Hawaii undertaken during NASA's Global Backscatter Experiment (GLOBE) in May‐June 1990. During each of these two flights the aircraft traversed different altitudes within a region of the atmosphere defined by the same set of latitude and longitude coordinates. This provided an ideal opportunity to allow flight level focused continuous wave (CW) lidar backscatter measured at 9.11‐μm wavelength and modeled aerosol backscatter from two aerosol optical counters to be compared with pulsed lidar aerosol backscatter data at 1.06‐ and 9.25‐μm wavelengths. The best agreement between all sensors was found in the altitude region below 7 km, where backscatter values were moderately high at all three wavelengths. Above this altitude the pulsed lidar backscatter data at 1.06‐ and 9.25‐μm wavelengths were higher than the flight level data obtained from the CW lidar or derived from the optical counters, suggesting sample volume effects were responsible for this. Aerosol microphysics analysis of data near Japan revealed a strong sea‐salt aerosol plume extending upward from the marine boundary layer. On the basis of sample volume differences, it was found that large particles were of different composition compared with the small particles for low backscatter conditions.

Airborne Insects as Potential Sources of Acoustic Reflectivity

Abstract Laboratory measurements of the acoustic backscattering cross sections of a variety of insects at 3.4 kHz are reported. The authors argue from these data, and from what is known (from radar studies) about insect concentrations in the lower atmosphere, that acoustic returns from insects may seriously contaminate sodar data. They suggest that this possibility should be carefully considered when interpreting backscatter coefficient data acquired in acoustic sounding experiments.