Impact of the Elevation Angle on Cygnss GNSS-R Reflectivity over Different Scattering Media over Land and Ocean

Earth's surface bistatic reflectivity <tex>$\Gamma_{\mathrm{LHCP},\mathrm{CyGNSS}}$</tex> profiles are obtained using Global Navigation Satellite Systems Reflectometry (GNSS-R) products from NASA's Cyclone Global Navigation Satellite Systems (CyGNSS). The focus of this study is to evaluate the influence of the GNSS satellites' elevation angle <tex>$\theta_{\mathrm{e}}$</tex> on <tex>$\Gamma_{\mathrm{LHCP},\mathrm{CyGNSS}}$</tex>. Specific target areas with differentiated scattering media are selected to characterize these profiles as a function of the land-cover type and the wind-speed regimes for ocean-surfaces. In the former case, the main interest is to further understand the function <tex>$\Gamma_{\mathrm{LHCP},\mathrm{CyGNSS}}=f(\theta_{\mathrm{e}},\ldots)$</tex> in preparation of the potential application of the so-called Tau-Omega model to GNSS-R. In the latter case, the objective is to analyze the impact of the coherent scattering component in GNSS-R observables. Preliminary results show <tex>$\Gamma_{\mathrm{LHCP},\mathrm{CyGNSS}}$</tex> increases ~ 3.7 dB over land and ~ 5.3 dB ocean, as <tex>$\theta_{\mathrm{e}}$</tex> moves from~ [70], [90]° to ~ [20], [40]°.


INTRODUCTION
Improving the spatio-temporal sampling properties of GNSS-R [1][2][3] depends on the number of in-orbit receivers, the use of beamforming strategies, and the directivity of the down-looking antennas. GNSS-R sampling properties improve the performance for mesoscale studies, Soil Moisture Content (SMC) determination, and Vegetation Water Content (VWC) monitoring, as compared to conventional Nadir-looking missions. The interferometric GNSS-R (iGNSS-R) proposes the use of several high-gain beams to increase the altimetric precision (Root Mean Square Error RMSE), while providing good sampling properties. On the other hand, the use of the conventional GNSS-R (cGNSS-R) for scatterometry purposes requires only relatively low-gain antennas. The recently launched CyGNSS 8-microsatellite constellation provides an unprecedented spatio-temporal sampling of the Earth's surface. The selected signal correlation technique is cGNSS-R, the Left Hand Circular Polarization (LHCP) downlooking antennas gains is ~ 14.7 dB (antenna boresight), and the orbital inclination is ~ 35º [4]. The mission key-requirement is to provide wind speed estimation over tropical cyclones.
Here, CyGNSS Level 1 Science Data Record is used trying to extend the mission scientific applications [4,5] also to land surfaces. In particular, the objective of this work is to evaluate the impact of the GNSS satellites' elevation angle e  on the reflected signals' power levels for different surface types. GNSS-R land applications require further research due to the complex properties of this scattering media as compared to ocean. In the past, several research groups provided useful information about the scattering properties and the possibility e.g. to apply GNSS-R for SMC determination. More recently, new important conclusions were derived: a) over nearly bare-soil target areas, the measured sensitivity to SMC using data from UK TDS-1 was ∼ 38 dB/(m 3 /m 3 ) [6]; b) over the complete Earth's surface, 1-year averaged Polarimetric Ratio (PR) from a GNSS-R experiment onboard SMAP and SMC based on the multi-temporal dual-channel algorithm [7] showed a Pearson r ~ -0.6 [8]; c) e  should be considered to apply the so-called Tau-Omega model in the GNSS-R case [6], d) the vegetation introduces short-term (volume scattering) and long-term (interference from "big" scatterers and canopy inhomogeneity) fluctuations on the GNSS signals [9].

GNSS-R BISTATIC REFLECTIVITY AS A FUNCTION OF THE ELEVATION ANGLE
The signal power received by a GNSS reflectometer [1,4,8] at a certain polarization p or q , can be derived under the bistatic radar equation as follows [  Ratio values filtered for values higher than 3 dB. Over rainforests the signal is highly attenuated so that it can be understood as noise power floor (see areas in "white" in Fig. 2b), however it is found the potential capability to observe the hydrosphere for low e  .
GNSS satellites signals are emitted at Right Hand Circular Polarization (RHCP), although with a certain degree of ellipticity. After scattering over a surface, they become LHCP for e  higher than the Brewster angle, while they are mainly RHCP for lower e  . Additionally, the scattering over the biomass introduces a significant degree of depolarization in the signals. The two components RHCP (co-polar) and LHCP (cross-polar) should be considered simultaneously in this case.
The scattering over land and ocean surfaces is composed of both  is the Nadir optical depth of the vegetation layer [6]. This attenuation is due to signal propagation through vegetation, and thus it increases for lower e  . On the other hand, the vertical stalks scatter more vertically polarized waves, contributing to depolarize the GNSS signals [8].

IMPACT OF THE ELEVATION ANGLE ON SPACEBORNE GNSS-R EXPERIMENTAL DATA
In this work, the effect of vegetation is assessed over two different target areas: a) Cropland [ Figs is dominant for moderate-to-strong wind speed conditions [10]. In this situation, the footprint is limited by the first iso-delay