Potential Applications of GNSS-R Observations over Agricultural Areas: Results from the GLORI Airborne Campaign  
[PERSON]  
1 CESBIO (CNRS/UPS/IRD/CNES), 18 av. [PERSON], bpi 2801, 31401 Toulouse CEDEX 9, France; 1  
[PERSON]  
1 CESBIO (CNRS/UPS/IRD/CNES), 18 av. [PERSON], bpi 2801, 31401 Toulouse CEDEX 9, France; 1  
[PERSON]  
2 TEITS, IBSTEA, University of Montpellier, 34093 Montpellier, France; [EMAIL_ADDRESS]  
[PERSON]  
1 ISPA, INRA, 33140 Villenave d'Ornon, France; [EMAIL_ADDRESS] (S.D. ); [PERSON] (D.G. ); [PERSON].W.) Footnote 1: [[https://www.mdpi.com/journal/remotesensing](https://www.mdpi.com/journal/remotesensing)]([https://www.mdpi.com/journal/remotesensing](https://www.mdpi.com/journal/remotesensing))  
[PERSON]  
3 ISPA, INRA, 33140 Villenave d'Ornon, France; [EMAIL_ADDRESS] (S.D. ); [PERSON] (D.G. ); [EMAIL_ADDRESS] (J.P.W.) Footnote 2: [[https://www.mdpi.com/journals/pdf/10.1245/](https://www.mdpi.com/journals/pdf/10.1245/)]([https://www.mdpi.com/journals/pdf/10.1245/](https://www.mdpi.com/journals/pdf/10.1245/))  
[PERSON]  
1 CESBIO (CNRS/UPS/IRD/CNES), 18 av. [PERSON], bpi 2801, 31401 Toulouse CEDEX 9, France; 1  
antenna reflected signal-to-noise-ratio (SNR) measurements from commercial geodetic instruments, or dedicated multi-antenna configurations combined with the analysis of correlation waveforms and/or delay-Doppler maps (DDMs). Focusing on the estimation of soil moisture and vegetation parameters, different studies have been conducted over the last ten years [16; 17; 18; 19; 20; 21; 22; 23; 24]. Observations and analyses conducted during the SMEX02 campaign were, to the best of our knowledge, the first large-scale attempt to investigate the airborne potential of GNSS-R for the remote sensing of soil moisture. Using a single-polarization, left-hand circular polarization (LHCP) antenna for nadir-pointing observations, the aircraft flew at approximately 1100 m above ground level (AGL) with a velocity of ~75 m/s over an area where the ground and vegetation parameters were monitored. The authors have computed the GNSS-R signal's sensitivity to lie between 10 and 30 dB per m\({}^{3}\)/m\({}^{3}\) of surface soil moisture. They also observed and modeled the attenuation produced by the vegetation cover, but they did not perform an in-depth investigation of the system's sensitivity to this parameter. In 2009, the LeiMON campaign [16] was performed over a period of more than six months in order to assess the sensitivity of polarimetric GNSS-R observations to soil roughness, soil moisture and vegetation, using a ground-based instrument attached to a mast 25 m above the ground. During the course of the experiment, the soil moisture and roughness conditions varied over a wide range, and the only monitored crop was sunflower. The sensitivity of the cross-polarization (right-left) reflectivity (\(\Gamma_{RL}\)) to soil moisture content was confirmed to be 20 to 30 dB/(m\({}^{3}\)/m\({}^{3}\)) for low to moderate surface roughness conditions, with a root mean square surface height (Hrms) below 3 cm, and its sensitivity to vegetation water content (VWC) was estimated to be ~0.3 dB/(kg/m\({}^{2}\)). The use of observations based on the ratio of \(\Gamma_{RR}\) (co-polarization reflection coefficient) to \(\Gamma_{RL}\) to retrieve soil moisture was shown to limit the effect of roughness conditions. However, as pointed out by the authors, instrumental improvements to avoid saturation at reflectivities below ~17 dB were needed to confirm their results. In an effort to validate these results over a larger area, including various crops and several forest sites, the instrument described in [16] was upgraded and installed on an aircraft for the GRASS campaign organized in Italy [25; 26], during which GNSS-R measurements were recorded from an aircraft at 150 m AGL. These observations confirmed the strong potential of GNSS-R techniques for the estimation of not only soil moisture, but also forest biomass.