## 2 Assessment of Available RT Orbit and Clock Products and RT ZTDs
### Assessment of Real-Time Orbit and Clock Corrections  
We investigate the performance of four real-time products being collected and archived at Geodetic Observatory Pecny (GOP) using the BNC Software [34] since 2013. Three of them (IGS01, IGS02, IGS03) are the official IGS RTS combined products [12], while the last one (CNS91) is provided by individual CNES RT analysis center [35]. Two different strategies and software are used for combining the IGS RTS [33]. While IGS01 is generated on the basis of epoch-wise combination, the Kalman filer technique is exploited for producing IGS02 and IGS03. Two different agencies are responsible for these RT products: European Space Agency (ESA) provides IGS01 and Bundesamt fur Kartographie und Geodasie (BKG) IGS02 and IGS03. All mentioned streams include orbits/clock corrections to GPS satellites, and only IGS03 supports also GLONASS constellation. Navigation data from MGEX [36] were used together with the RT corrections to recover the precise satellite orbit and clocks. First, the availability and completeness of RT corrections were checked and, second, satellite orbit and clocks were compared to IGS final orbit and clocks, both during the period of 2013-2017. Figures 1 and 2 depict monthly completeness of RT corrections for all GPS satellites from the IGS01 and IGS02 combined products. The IGS03 product is not shown here because it is almost identical to IGS02; slightly more outages are related to GLONASS satellites. Figure 3 shows then the same picture for the CNES product. From the comparison, we can classify problems into three groups: (1) temporal unavailability period of some satellites, e.g., G03, G04, (2) source-specific unavailability, e.g., G01 for CNS91, and (3) satellite-specific incompleteness. The first group is usually caused by the loss of observations due to the upgrade of a satellite, such as replacing the old Block IIA satellites with the new Block IIF satellites or a maintenance identified by satellite unhealthy status. The second and third groups of gaps are caused by data unavailability from a global network and the processing strategy including outlier detection in the product generation. The availability of the corrections is significantly lower for some months (June 2015, December 2016) compared to others, which was caused by the Internet connection failures at GOP when receiving the streams. The source-specific loss of data at IGS02 and CNS91 streams are visible in June 2015 and, these are mainly due to the inconsistent navigation message Issue of Date (IOD) available from the MGEX broadcast and those referred by RT corrections. It can be thus recommended to use consistent RT navigation data and precise correction streams optimally guaranteed by the same provider. In general, the availability of RT corrections is well over 90% for most satellites, which agrees with findings in [33]. It indicates that the RT corrections were provided continuously for use in troposphere monitoring, however, problems can be expected in a kinematic positioning which is more sensitive to the product incompleteness. Apart from the availability of the corrections, the precision is critical for the user performance. The orbits are compared in 5-min intervals for three components: radial, along-track and cross-track while the clock comparison is based on the second order difference method. The IGS08 and the IGS14 model is used to correct satellite PCOs prior and after 29 January 2017, respectively, corresponding to the adoption of the IGS2014 reference frame [37]. The clock datum is estimated by calculating a mean over all satellites clocks at each epoch. The datum inconsistencies are then eliminated through  
The orbit difference in radial component shows the smallest RMSE for all products, whereas the along-/cross-track components reached slightly larger values. The IGS01 orbit shows the best agreement with respect to the IGS final orbits.