# The Implementation of Precise Point Positioning (PPP): A Comprehensive Review
## 2 Error Sources in PPP
### Satellite Orbit and Clock Errors  
Knowing the precise satellite's position in space is crucial for the estimation of the distance between the satellite and receiver, which is required to calculate the receiver's position. In SPP, the information in the broadcast ephemeris is employed to determine the satellite position. However, the accuracy offered by broadcast ephemerides is suboptimal for high-precision applications, leading to a demand for more accurate ephemerides. Such precise satellite orbit data can be obtained from commercial providers or the IGS open-access products [26]. The IGS offers a spectrum of products with varying degrees of accuracy and latency. The final IGS products are the most accurate products, which feature an orbital accuracy of approximately 2.5 cm; nevertheless, they are available with a latency of 12-18 days [27].  
Another potential source of error originates from satellite clock instabilities. The clock error model in the broadcast ephemeris also lacks the required accuracy for PPP, creating the need for more precise clocks. This high-accuracy orbital and clock data are conventionally stored in the Standard Product 3 (SP3) format, often with epoch intervals of 15 min or less. The latest version of SP3 at the time of this paper is SP3-d [39]. Both the orbital and clock data need to be interpolated to acquire exact positional and temporal details at the moment of signal transmission. Orbit interpolation is commonly achieved using a Lagrange polynomial of 9 th or 10 th order, yielding centimeter-level accuracy [40; 41]. Linear interpolation, conversely, is generally sufficient for clock errors.  
Although SP3 files do contain clock error data, higher-rate data are preferable due to the stochastic nature of clock errors in contrast to the relatively smooth satellite orbits. The IGS provides such high-rate clock data, typically at intervals of 5 min or 30 s, using the clock Receiver Independent Exchange (RINEX) format [42]. It is feasible to utilize SP3 data for the orbits and high-rate data for the clocks; however, it is necessary to maintain data consistency from the same analysis center.  
Originally, IGS products were generated only for GPS and subsequently for GPS and GLONASS. The extension to support all GNSS constellations was realized through the IGS Multi-GNSS Experiment (MGEX) project [43; 44]. Seven IGS analysis centers now provide orbits and clock products for Multi-GNSS, namely, Centre National d'Etudes Spatiales (CNES), Center for Orbit Determination in Europe (CODE), GeoForschungsZentrum Potsdam (GFZ), Information and Analysis Center (IAC), Shanghai Observatory (SHAO), Wuhan University, and the Japan Aerospace Exploration Agency (JAXA).  
To meet the growing demand for real-time PPP, the IGS officially launched its Real-Time Service (IGS RTS) on 1 April 2013 [45]. The real-time corrections are disseminated to the users via the Internet using protocols such as NTRIP (Networked Transport of Radio Technical Commission for Maritime Services (RTCM) via Internet Protocol) [26]. RTS messages conform to the RTCM State-Space Representation (SSR) format. The SSR parameters are expressed as relative corrections to the broadcast ephemeris-derived values rather than as absolute precise satellite orbits and clocks to minimize the transmission bandwidth [45].