# Characteristics of BeiDou-3 Experimental Satellite Clocks
## 1 Introduction  
The BeiDou-2 constellation presently consists of 14 active satellites and can be grouped into the following three types: five Geostationary Orbit (GEO) satellites, six Inclined Geostationary Orbit (IGSO) satellites, and three Medium Earth Orbit (MEO) satellites [1]. All of them are operated on Rb Atomic Frequency Standard (AFS) [2]. According to the three-step strategy, the BeiDou-3 that was under construction was planned to complete the constellation consisting of 35 satellites, and to provide global service for users around the world [1, 3, 4]. Therefore, by the end of February 2016, China had launched five experimental satellites for validating the BeiDou-3 performance, including two IGSOs (C31 and C32) and three MEOs (C33, C34, and C35). Unfortunately, the signal of C35 cannot be obtained by the ground receivers because of technical reasons [5]. The new generation of experiment satellites have several improvements over the older BeiDou-2 satellite [6]. First, it can broadcast new types of signals, B1C (1575.42 MHz) [7], B2a (1176.45 MHz) [8], and B2b, to improve the compatibility and interoperability with other navigation satellite systems. Second, the BeiDou-3 experiment satellites are equipped with the improved rubidium (Rb) AFS(C31, C33, and C34) and the Passive Hydrogen Maser (PHM) (C32 and C35) produced by China's own technology. Third, Inter-Satellite Links (ISL), one of the most important ingredients for global BeiDou system design, is used to enable autonomous navigation. The detailed information of the BeiDou-3 experiment satellites is shown in Table 1.  
With the above-mentioned characteristics in mind, researchers have been able to further their studies. [PERSON] et al. confirmed that ISL contributes enormously to the space signal accuracy, in particular, when the satellites are not tracked by a regional network [6]. [PERSON] et al. presented an initial assessment of the new-generation signals and of the initial results of single epoch relative positioning [9]. [PERSON] et al. showed the initial results of the precise orbit determination with nine stations [10]. [PERSON] et al. evaluated the multipath effect and the performance of the precise orbit and clock determination [11]. [PERSON], et al. proved that the best orbit determination solution for BeiDou-3e is obtained with five-parameter Extended CODE Orbit Model (ECOM) and a three-day arc [12]. [PERSON] et al. elaborated on a brief introduction of PHM design, and also analyzed the performance of both PHM and the improved Rb AFS on-board clock [13]. Currently, there are two main methods to estimate the precision satellite clock offsets, called Two-Way Satellite Time and Frequency Transfer (TWSTFT), and Orbit Determination and Time Synchronization (ODTS). The BeiDou official clock products in the broadcast ephemeris are estimated by the former method [14; 15], and the performance of the BeiDou-3 experimental clock that was obtained in this way has been fully discussed in the literature [13]. However, this method has some drawbacks. The clock offsets obtained by TWSTFT are calculated independently without orbit information, and as a result, they are not coupled with the orbit solution like the ODTS method is. For this reason, the high-precision clock products obtained by ODTS are the main products, such as the International Global Navigation Satellite System (GNSS) Service (IGS) products, that are used by general users. Therefore, the characteristics of such clock products need to be further studied. This paper focuses on the characteristics of the improved Rb AFS as well as the PHM deployed by the BeiDou-3 experiment satellites. The precise clock products are obtained though the strategy of two-step method of orbit determination. The stabilities of the onboard clocks of both the BeiDou-3 and BeiDou-2 satellites, as well as GPS, GLObal NAvigation Satellite System (GLONASS), and Galileo satellites are evaluated using the Overlapping Allan Deviation (OADEV).