# Analysis of Key Issues on GNSS-R Soil Moisture Retrieval Based on Different Antenna Patterns
## 1 Introduction  
An important branch of the global navigation satellite system (GNSS) is GNSS reflectometry (GNSS-R), which has emerged in recent years. It utilizes the code-modulated L-band microwave signal that has long-term stability and high spatiotemporal resolution provided by navigation satellites, combining the advantages of both active radar and passive radiometers [1]. GNSS-R has demonstrated great potential in applications such as sea surface height, sea surface wind field, sea ice detection, vegetation status, soil moisture, and snow depth [2; 3; 4; 5; 6]. Currently, various ground-based, air-based and even satellite-based observation experiments are widely conducted worldwide to actively promote its relevant applications. For example, the TDS-1 satellite launched by the UK in 2014 was equipped with SGR-ReSI sensors to facilitate GNSS-R measurements and is currently used in studies for the retrievalof soil moisture [7; 8]. The National Aeronautics and Space Administration (NASA) launched the CYGNSS Satellite Constellation in October 2016 [9], and the European Space Agency (ESA) deployed the GEROS-ISS [10] and the PARIS IoD Program [11]. Although the Chinese GNSS-R study focuses on ground-based and air-based methods, China has included a satellite-based GNSS-R observation program in its \"Thirteenth Five-Year Plan\" and \"Fourteenth Five-Year Plan\". In addition, the radar transmitter originally installed in the Soil Moisture Active Passive (SMAP) malfunctioned in July 2015. The receiver is currently switched to receive GPS L2C frequency, and it also becomes a satellite-borne GNSS-R observation [12; 13]. In the future, more abundant GNSS-R data will be used to measure parameters of the Earth's surface. Soil moisture is an important parameter in the fields of agriculture, hydrology, meteorology and ecology. In recent years, scientists have conducted extensive investigations in the field of soil moisture remote sensing via land- and air-based GNSS-R. [PERSON] et al. calculated the reflection coefficient by calculating the maximum value of the reflected power and the delayed power waveform of the direct signal, and then obtained the dielectric constant through the bistatic radar equation, and finally estimated the soil moisture through an empirical model. This is also known as the GNSS-ICF method [14]. In 2002, NASA added GPS bistatic radar measurements in the Soil Moisture Experiment 2002 (SMEX02) for the first time and confirmed that the intensity of the reflected signal has a spatiotemporal correlation with soil moisture [15]. ESA conducted ground- and air-based GNSS-R tests in Italy in March to September of 2009 and in July and August of 2011, respectively, to acquire the direct signal of right-handed circular polarization (RHCP) and the reflected signal of RHCP/left-handed circular polarization (LHCP); the results showed that the ratio of the direct signal of RHCP to the reflected signal of LHCP can be used as a reliable measurement for soil moisture [16; 17]. Multiple ground- and air-based GNSS-R measurements on soil moisture have been performed by the Polytechnic University of Catalonia, Spain, from 2013 to 2015; in these studies, multipolar measurements of direct and reflected GNSS signals were conducted, and the effect of the instrument parameters on the calculation of the reflection coefficient was examined [18; 19; 20]. [PERSON] et al. used the GPS L2C band reflection signal data provided by the SMAP satellite to identify the surface freezing and fusion status [21]. In terms of the interference of direct and reflected GNSS signals, [PERSON] developed the technique of GPS-multipath reflectometry (GPS-MR) for soil moisture measurement using the GPS signal-to-noise ratio (SNR) data from the crustal deformation monitoring network [22]. [PERSON] et al. developed the Interference Pattern Technique (IPT) for measuring soil moisture using a special receiver and a vertically polarized antenna.