## 1 Introduction  
Global navigation satellite system (GNSS) radio occultation was originally developed to measure the planetary atmosphere when exploring the solar system [1, 2]. Then, in the mid-1960s, [PERSON] put forward the GNSS radio occultation theory for remote sensing of the Earth's atmospheric parameters [3, 4], which provides accurate atmospheric parameter profiles worldwide. The theory is useful for numerical weather forecasting, climate monitoring, and atmospheric research. Because this theory requires putting multiple signal transmitting devices and signal receiving devices in space, until the late 1980s the United States Global Positioning System (GPS) tended to be perfect, providing the necessary satellite constellation for GNSS radio occultation [5, 6]. GNOS, a global navigation satellite radio occultation detector carried by China's Fengyun 3 series meteorological satellites, can simultaneously receive GPS navigation signals and BeiDou navigation system (BDS) navigation signals for occultation detection [7]. It is the world's first radio occultation detector compatible with BDS and GPS systems [8]. The BDS occultation observation data in this paper comes from the dual-frequency occultation observations of BDS BII signals and BDS B3I signals carried by the second-generation radio occultation detector (GNOS II) on the FY-3E satellite (FY-3E) in the Fengyun series of missions. The operating frequency of BDS BII signals is 1561.098 MHz. The signal adoptsmultifrequency transmission technology and time-division frequency multiple-access technology, which have strong anti-interference ability and can eliminate errors in navigation and positioning worldwide. The BDS B1I signal has higher accuracy and reliability. The operating frequency of BDS B3I signals is 1268.52 MHz. In this paper, the carrier phase (B\({}_{3}^{*}\)) of a quasi-B3I signal is reconstructed, and the results are compared with the actual BDS B3I signal in dual-frequency measurement. In GNSS atmospheric occultation data processing, the ionospheric error will greatly affect the accuracy of atmospheric occultation retrieval products [9; 10]. Dual-frequency occultation processing means that when the radio-wave signal transmitted by the LEO satellite is received by the high-precision dual-frequency radio occultation receiver carried by the LEO satellite, the radio wave signal will have a certain amount of bending and delay effects through the lower atmosphere and the ionosphere due to atmospheric refraction, scattering, and other reasons, and the bending and delay effects will cause the frequency of the receiver signal to change [11; 12]. In general, the ionosphere-free combination using dual-frequency observations can effectively eliminate the influence of the first-order term of the ionosphere, thus improving the product accuracy [13; 14]. After eliminating the ionospheric delay through the ionospheric combination between the dual-frequency BDS-based carrier-phase observations, the Doppler frequency shift of the BDS signal is calculated using the high-precision velocity, position, and clock-difference information of the receivers of the BDS satellite (reference satellite) and the low-orbit satellite (occultation satellite), so as to invert the curved angular profile of the atmosphere [15; 16; 17; 18]. In some flight missions, single-frequency receivers may also be used for GNSS atmospheric occultation detection. Since sometimes even single-frequency receiver occultation detection cannot eliminate the ionospheric error through combination, the half-sum combination of the single-frequency carrier phase and pseudorange has been developed to eliminate ionospheric error [19; 20; 21]. For example, a GPS radio occultation receiver named Turbooge carried on the Danish Oysted satellite launched in February 1999 only received the single-frequency occultation signal normally [22], so the scientific researchers used the double-difference single-frequency occultation processing method for GNSS atmospheric occultation processing.