## 1 Introduction  
The global navigation satellite system (GNSS), including the US GPS, Europe Union GALILEO, Russia GLONASS, and China BeiDou system has achieved great success with an unprecedented impact on all positioning-related areas. It can not only provide spatial information for global users with navigation, positioning information, speed measurement, timing, but also have the opportunity of L-band microwave signals with high time-resolution. As further development of GNSS, the target's reflected signal can be received and utilized [1; 2; 3]. Then the way of utilizing the GNSS reflected signals were employed to detect the targets. This is a new concept of remote sensing called GNSS-reflectometry (GNSS-R), featured with no special radar transmitter. Besides, it is a low-cost option with wide global coverage, a large amount of data acquisition, and can also be a powerful complement to other traditional remote sensing methods. GNSS-R can be regarded as a bi-static radar concept system. In the past 20 years, theoretical [4] and experimental [5] studies using GNSS-R have demonstrated the potential of GNSS-R in remote sensing measurements. There are mainly two types of GNSS-R applications: Altimetry and scatterometry. This GNSS-R technique was firstly proposed for ocean altimetry [5], which is one of the main applications. The altimetry makes use of propagation delay of the reflected signals (from waveform or carrier phase) to measure the surface elevation [6; 7]. Another main GNSS-R application is scatterometry that was proposed by [PERSON] and [PERSON] [4], which used the power/shape information of the waveform (or DDM) to characterize the surface roughness or reflectivity for wind speed retrieval [8; 9; 10; 11], soil moisture measurement [12; 13], or sea ice detection [14; 10]. In addition, with the continuous development of GNSS-R remote sensing technology, it has been widely used in many fields such as measuring the snow depth [15], tsunami [16], vegetation biomass [17], flooding inundation [18], and inland water [19; 20]. The experimental platform has also evolved from ground-based experiments [21] to aircraft [12], balloons [22], and the latest low-orbit satellite [23] platform for measuring hurricanes. In 2002, NASA took the lead in launching a series of soil moisture remote sensing flight experiments (SMEX02-03) using GPS reflection signals. The entire system effectively measured the signal power that varies with the soil moisture content [12]. Based on the bistatic radar configuration, two antennas were used respectively to receive the direct signal from the satellite and signals reflected from the ground. A right-hand circularly polarized antenna (RHCP) was oriented toward the sky, and a left-hand circularly polarized (LHCP) antenna (single-polarized) or added a right-hand circularly polarized antenna (constituting dual-polarization) perpendicular to the ground [21]. The dielectric constant was solved by using the soil reflectivity and the bistatic radar equation. Then, the soil water content can be obtained by various permittivity inversion models (permittivity-soil moisture). As an extension of the earlier work, a calibration process was added to the subsequent soil moisture remote sensing experiment, and a new reflectometer was used to record the data from the satellite with a high elevation angle (greater than 65\({}^{\circ}\)) in the visible range. The results showed that the received calibrated soil reflectivity could be detected and used to estimate the expected relationship between the dielectric constant and soil moisture [24]. After that, researchers proposed another interference pattern technique (IPT) to retrieve the soil moisture content [25]. A left-hand circularly polarized antenna or a vertically polarized antenna, which oriented towards the horizontal, was used to receive the interference signals from dual paths of direct and reflected.