ESTIMATING SHADOW-ZONE PARAMETERS OF TROPOSPHERIC REFRACTION FROM THE RADIATION OF REMOTE SOURCES. PART Ⅱ: EXPERIMENT

摘要

Obtaining quasi-real time estimates of troposphere refraction is a task of crucial importance for improving the performance of radio systems. This paper presents experimental results on troposphere refraction over the sea, recovered from radio signals of GPS navigation satellites. The work features measurements in the shadow zone where signal parameters are subject to great variations and happen to be particularly sensitive to changes in refraction. The shadow-zone level of the GPS signals has been increased by the use of a parabolic antenna. The observations of radio occultations of GPS satellites over the sea involved cases of standard and enhanced refraction, and of tropospheric ducting. The measured data have been used to retrieve height gradients of the refractive index profiles n(h) in the near-surface layer of the above-sea troposphere. The appropriate calculations are done within the model developed in Part Ⅰ of the paper, using the data on the occultation angle and slope of the diffraction-controlled portion of the basic propagation loss curve. The magnitudes so obtained are confronted with the 'effective' gradients that are recovered from the signal levels measured along a near-surface trans-horizon propagation path. The correlation factors estimated for elements of the gradient arrays, compiled for the fixed near-surface and the satellite radio links, were about 0.53 to 0.78 (samples taken from neighboring measurements). The figures can increase up to 0.85 or 0.9 upon averaging. The results may prove useful for developing satellite-based systems oj diagnosing the troposphere refraction.