Analytical model of bistatic reflections and radio occultation signals

Abstract

The relationships between the Doppler frequencies, eikonal acceleration, and refractive attenuations of the direct and reflected signals are established for bistatic and radio occultation experiments. These connections allow recalculating the Doppler shifts and the phase delays to the refractive attenuation (reflectivity cross section) and open a new avenue for potentially measuring the total absorption in the atmosphere at low elevation angles. The fundamental characteristics of bistatic remote sensing of the atmosphere and Earth's surface such as the phase delay, reflection coefficient, reflectivity cross section, and Doppler shift of the reflected signals relative to the direct signals are obtained in analytical forms by taking into account the refraction and absorption effects in the atmosphere. Difference in the Doppler frequencies of the reflected and direct signals is proportional to the difference of the modified refractive index at the radio ray perigee and at the Earth's surface. The obtained analytical results are in good agreement with the measurements data obtained during the MIR/GEO (wavelengths 2 and 32 cm), and CHAMP (wavelengths 19 and 24 cm) radio occultation experiments. Detecting the reflected signals in radio occultation data has opened new perspectives for bistatic monitoring of the atmosphere and Earth's surface at low elevation angles. Experimental results of the propagation effects at low elevation angles are of great importance for fundamental theoretical investigation of radio waves propagation.

An overview of GNSS remote sensing

“…Contrary to its importance, WV remains poorly understood and inadequately measured both spatially and temporally, especially in the southern hemisphere, where meteorological data are sparse. GNSS atmospheric remote sensing using LEO satellites such as CHAllenging Minisatellite Payload (CHAMP), Gravity Recovery and Climate Experiment (GRACE) and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) is a robust technique for profiling the atmosphere from LEO satellite orbit heights down to the Earth's surface [3][4][5]. Many studies have demonstrated that the GNSS-RO technique is able to measure the Earth's atmospheric parameters with unprecedented high accuracy, high resolution and global coverage [6][7][8][9][10].…”

“…[63]. Five theoretical 1D delay waveforms are shown corresponding to five different wind speeds(3,4,5,6 and 7 m/s), with a measured delay waveform from GPS satellite PRN#13 superimposed. The data were collected by an airborne receiver at an altitude of about 3 km.…”

Analysis of reflections in GNSS radio occultation measurements using the phase matching amplitude

“…The black lines show the amplitude of the U function when passing segments of the signal to the PM operator using a 10 km sliding window. The relationship between reflected bending angle and impact parameter is wellknown (see, e.g., Pavelyev et al, 2011;Gorbunov, 2016) and can be described as…”

Information content in reflected signals during GPS Radio Occultation observations

“…Signatures of the interaction of the GPS signal with the Earth's surface have been recorded during satellite-based radio occultation (GPSRO) events (Beyerle and Hocke, 2001;Beyerle et al, 2002;Pavelyev et al, 2011). Besides the usual refracted signal (hereafter referred to as "direct signal") at the limb, another signal is frequently detectable with a small frequency offset, corresponding to lower elevation, often among distorting phenomena of multipath propagation, superrefraction (unusually large refractivity gradient) and strong attenuation, and is not always clearly defined.…”