# Preliminary Validation of Surface Reflections from Fengyun-3C Radio Occultation Data
## 1 Introduction  
Global Navigation Satellite Systems Radio Occultation (GNSS-RO) is a satellite remote sensing technique that uses GNSS measurements for sounding Earth's atmosphere [1, 2, 3, 4]. GPS, GLONASS, Galileo and BeiDou are fully used for precise positioning services [5, 6], and are widely used in GNSS-RO missions. As GNSS signal passes through the atmosphere, the signals received by the receiver on the Low Earth Orbit (LEO) are bent and delayed, which accurately produces the high-quality atmospheric profiles containing humidity, pressure and temperature information [7, 8, 9, 10]. In recent years, a number of studies have focused on the GNSS RO reflected signals. Usually, signals of GNSS RO events may propagate to Earth's surface and then be received by LEO satellites as reflected signals. Around half of RO events, frequently over the ocean, snow and ice surfaces, could present the reflection signatures [11]. The reflected signals enable the capture of variations in the troposphere profiles. At the bottom of the troposphere, the Signal to Noise Ratios (SNR) of the occultation signal drops to a noise level, and the occultation reflected signals may provide valuable meteorological information of the lower atmosphere [12, 13]. Furthermore, the bending angles of profiles that present reflected signals have better departure statistics in the troposphere compared to European Centre for Medium-Range Weather Forecasts (ECMWF) analyses. The reflections with good instrumental and atmospheric conditions were proven good enough to produce highquality data without disturbing the structure of atmospheric retrievals [14]. The reflected signal component during occultation events can be used to reduce refractivity bias under the ducting layer [15]. Moreover, recent studies have found that reflections in the grazing angle of observations can provide a few cm of precision at sea surface [16] and sea-ice [17] altimetry, with the latter using GNSS RO payloads. Currently, the usual implemental approach for reflection signals identification is radio holographic analysis. This method has been successfully applied to separate the direct and reflected signals according to the spectrum of the GNSS RO events, such as reflection in GPS/Meteorology (GPS/MET) and Challenging Ministatellite Payload (CHAMP) RO data, allowing the estimation for bending profiles with a strong error and removing the contribution of reflection in temperature retrievals in the lower troposphere [18; 19; 20]. Taking advantage of the extensive number of RO data, [PERSON] et al. adapted a supervised learning algorithm, named super vector machines (SVM) [21], to train thousands of radio holographic images and automatically classify millions of occultations (ROM SAF reflection flag database) [22]. Alternatively, the techniques developed for retrieving RO in multipath areas, such as canonical transform (CT) methods [23; 24] and phase matching (PM) [25], can also retrieve the reflected signals of the bending angle profiles. A modified CT technique was reported to retrieve the reflected bending angles, and even achieved good agreement with the ROM SAF database [26]. The PM method can highly enhance the signatures of surface reflections by means of processing only an appropriately selected segment of the received signal [27]. Although lots of studies have introduced and investigated the reflection signals on Earth's surface by means of GNSS RO missions, little research reports the reflections of the Chinese Fengyun 3C (FY-3C) measurements. The FY-3C RO satellite was launched by the Chinese Meteorology Agency (CMA) in 2013, which is the third unit of the FY-3 series, and its main mission is operational meteorology [28; 29; 30]. The occultation receiver of FY-3C is Global Navigation satellite system Occultation Sounder (GNOS), which is designed to receive both GPS and BeiDou (BDS) signals [31]. This design employs the open-loop mode, which guarantees an accurate tracking of signals at the lower impact height, where multipath and reflection often occur [32].