## 1 Introduction  
Global Navigation Satellite System (GNSS) occultation measurement is a meteorological remote sensing technology, which utilizes occultation phenomenon between GNSS navigation constellation and low earth orbit (LEO) satellites in its orbit to measure the earth's ionosphere and atmosphere [1; 2; 3]. By installing GNSS occultation antennas on the LEO satellites, it is possible to obtain the atmospheric refractive index, temperature, pressure from the earth's surface to 40 km and the ionospheric electronic distortion rate from \(\sim\)60 km to the orbital height of the occultation LEO satellite [4; 5]. Atmospheric detection is of great significance for the weather forecast, scientific research and military application [6; 7; 8].  
The occultation signal from GNSS satellites has low and negative elevation angles [9; 10; 11]. Many approaches to data processing in the presence of multipath propagation (methods based on Fourier Integral Operators, like Canonical Transform, Full Spectrum Inversion and Phase Matching) can be found in references [12; 13; 14; 15; 16; 17; 18]. When the occultation signal passes through the lower atmosphere, especially the equatorial low latitude area, serious multipath phenomena appear in radio waves due to the rapid change of atmospheric refractive index gradient [19; 20; 21]. The fluctuation of atmospheric doppler frequency shifts and amplitude signal increases sharply, and the spectrum of occultation signal expands [6; 7; 8]. Due to the attenuation of signal amplitude and the rapid change of doppler frequency, the general phase locked loop (PLL) cannot work normally.  
Until recently, two very important challenges remained outstanding: (1) the ability to record and retrieve rising occultations; and (2) the ability to penetrate the lowest 2 km in the tropical troposphere routinely (for rising or setting occultations) without introducing tracking errors. These challenges have largely been overcome by the use of open-loop (OL) tracking implemented in the GPS receivers onboard the SAC-C satellite, and later Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) [22]. Closed-loop tracking is a method of tracking the carrier frequency and code frequency of satellite launches in real time, through the adjustment of the software's own loop route. The OL tracking technique is different from the traditional closed-loop tracking, in that there is no feedback between the observation signal and the control model. Meanwhile, signal tracking in the OL mode uses the predicted Doppler model and pseudo-range model to track the full signal, and has the ability to track stably [22; 23; 24].  
A mountain-based occultation experiment for three weeks in Wuling Mountain of Hebei Province is planned and implemented, with the following works to be carried out: 1. A hardware and software system supporting Global Position System (GPS)/Beidou satellites constellation (BD, the 2 nd and 3 rd generations) dual-mode occultation being developed for full OL tracking technology's verification; 2. An open-loop data processing method assisted with closed-loop observations, which is proposed to solve the problem to determine the initial sign of the recovered phase; 3. In the mountain-based occultation experiment, the robustness of the hardware and software systems using OL technology to track L1C/GPS and B1/BD signals being verified; and 4. The atmosphere profiles retrieved from observations in open-loop tracking mode being evaluated with the local observations of temperature, humidity and pressure provided by the Beijing Meteorological Bureau.