Radiometry offers the possibility to measure slant-path attenuation through the radiometer formula. But lack of a clear method to obtain an accurate estimate of the effective medium temperature (Tmr) and limited experimental characterization has contributed to uncertainty it is use. This contribution examined the use of the radiometer formula with measurements of brightness temperature, attenuation, and therefore Tmr, for many attenuating rain events made with the Sun-switching method. Monte-Carlo simulations were used to calculate the radiometer response for scattering atmospheres including finite rain cells. Data and analysis showed that use of the clear air Tmr provided a satisfactory estimate of attenuation statistics to over 10 dB.

On the use of the radiometer formula for atmospheric attenuation measurements at GHz frequencies

Ground-based radiometric techniques were applied to measure the slant-path attenuation cumulative distribution function to over 20 dB of attenuation and to less than 1% exceedance probability at the V and W band frequencies of 72.5 and 82.5 GHz. These are the first such measurements in these frequency bands. Brightness temperature measurements were collected at an elevation angle of 36° in Rome, NY, using a four-channel radiometer that included 23.8 and 31.4 GHz receivers. A model-based approach was used to invert brightness temperature to attenuation. An atmospheric model relevant to the geographic location and statistically representative of the attenuating conditions was developed for this purpose. The main assumption of the atmospheric model was that the sources of attenuation for exceedance probabilities of concern were dominated by stratiform rain. Monte Carlo solutions to the radiative transfer equation for the precipitating atmosphere were used to generate the attenuation retrieval algorithm. Sensitivity analysis showed that the attenuation retrieval algorithm was robust to uncertainties in the model parameters. Slant-path attenuation was also measured with the radiometer using the Sun as a source of radiation. Over 30 dB of attenuation dynamic range was possible with this technique. Sun-beacon measurements were used to test model predictions.

/pdf/ground-based-radiometric-measurements-of-slant-path-3you4wbmbm.pdf

Ground-based radiometric measurements of slant-path attenuation in the V/W bands

Slant path brightness temperatures were measured at 72.5 and 82.5 GHz with a multi-channel radiometer. One year of measurements collected at 36° elevation angle in Rome, NY were examined in terms of the attenuation statistics. The radiometric response was modeled with Monte-Carlo solution of the radiative transfer equation. The measured statistics were examined in relation to attenuation prediction models.

http://ieeexplore.ieee.org/iel7/7209133/7228134/07228810.pdf

Slant path attenuation at 72.5 and 82.5 GHz

Atmospheric emission measurements have routinely been used to determine the total attenuation on an earth-space path at the centimeter and millimeter wavelengths. In the presence of rain, scattering effects have to be taken into account at frequencies >20 GHz when interpreting emission data to derive the total attenuation. If one uses the radiometric relation to derive the attenuation assuming an absorptive medium, one finds that the predicted attenuation underestimates the true attenuation. The reason for this is that the rain scatters more emission energy out of the direct ray path than into it. Calculations of the atmospheric emission temperature and attenuation in light to moderate rain at different frequencies and rain rates are performed, taking into account molecular absorption, and varying cloud cover and ground temperature. The results show that for typically encountered meteorological conditions, the radiometric formula can be used to derive the total attenuation ( < 10 dB) from emission measurements with accuracies better than about 1 dB provided that the effective medium temperature in the radiometric formula is a function of frequency and ground temperature.

EHF attenuation derived from emission temperatures in light rain

: Atmospheric emission measurements have routinely been used to determine the total attenuation on an earth-space path at the centimeter and millimeter wavelengths. In the presence of rain, scattering effects have to be taken into account at frequencies 20 GHz when one interprets emission data to derive the total attenuation. If one uses the radiometric relation to derive the attenuation, assuming an absorptive medium, one finds that the predicted attenuation underestimates the true attenuation. The reason for this the rain scatters more emission energy out of the direct ray path than into it. Calculations of the atmospheric emission temperature and attenuation in light-to-moderate rain at different frequencies and rain rates are performed, taking into account molecular absorption, and varying cloud cover and ground temperature. The results show that for typically encountered meteorological conditions, the radiometric formula can be used to derive the total attenuation ( 10 dB) from emission measurements with accuracies of the order less than 1 dB, provided that the empirical constant in the radiometric formula, dependent on the ambient temperature, is a function of frequency and ground temperature.

D. S. Chang

Papers

EHF attenuation derived from emission temperatures in light rain

EHF attenuation derived from emission temperatures in light rain