# Improving GNSS PPP Convergence: The Case of Atmospheric-Constrained, Multi-GNSS PPP-AR
## 3 Concepts of Functional and Stochastic Atmospheric Constraints
### Existing Atmospheric Products
#### 3.1.1 Existing GIM Products
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Product** & **Agency** & **Type of Product** & **\#Stations** & **\#Satellites** & **GNSS** & **Mapping** \\ \hline c1 pg & AIUB & 1-day predicted & \(\sim\)120 & \(\sim\)56 & \({}^{\star}\)GNSS & NONE \\ c2 pg & AIUB & 2-day predicted & \(\sim\)120 & \(\sim\)56 & \({}^{\star}\)GNSS & NONE \\ carg & AOE & Post-processed (R) & – & – & \({}^{\star}\)MIX & COSZ \\ cag & AOE & Post-processed (F) & – & – & \({}^{\star}\)MIX & COSZ \\ codg & AIUB & Post-processed (F) & \(\sim\)259 & \(\sim\)56 & \({}^{\star}\)GNSS & NONE \\ corg & AIUB & Post-processed (R) & \(\sim\)118 & \(\sim\)55 & \({}^{\star}\)GNSS & NONE \\ e1 pg & ESA/ESOC & 1-day predicted & – & – & GPS & NONE \\ e2 pg & ESA/ESOC & 1-day predicted & – & – & GPS & NONE \\ ehrg & ESA/ESOC & Post-processed (R) & \(\sim\)231 & \(\sim\)54 & GPS & NONE \\ emrg & NRCAN & Post-processed (R) & \(\sim\)351 & \(\sim\)29 & GPS & MOD \\ esag & ESA/ESOC & Post-processed (F) & \(\sim\)300 & \(\sim\)54 & GPS & NONE \\ esrg & ESA/ESOC & Post-processed & \(\sim\)236 & \(\sim\)54 & GPS & NONE \\ igrg & IGS & Post-processed (CR) & \(\sim\)296 & 0 & \({}^{\star}\)MIX & COSZ \\ igsg & IGS & Post-processed (CF) & \(\sim\)328 & \(\sim\)32 & \({}^{\star}\)MIX & COSZ \\ jplg & JPL & Post-processed (F) & \(\sim\)170 & \(\sim\)31 & GPS & NONE \\ jprg & JPL & Post-processed (R) & \(\sim\)170 & \(\sim\)31 & GPS & NONE \\ u2 pg & UPC & Predicted & – & – & GPS & NONE \\ uhrg & UPC & Post-processed (R) & \(\sim\)259 & \(\sim\)31 & \({}^{\star}\)MIX & COSZ \\ upcg & UPC & Post-processed (F) & \(\sim\)272 & \(\sim\)31 & GPS & NONE \\ uprg & UPC & Post-processed (R) & \(\sim\)255 & \(\sim\)31 & GPS & COSZ \\ whug & WHU & Post-processed (F) & \(\sim\)314 & \(\sim\)31 & GPS & MSLM \\ \hline \hline \multicolumn{6}{l}{\({}^{\star}\)MIX/\({}^{\star}\)GNSS = GPS and GLONASS satellites. R = rapid, F = final, CR = combined rapid, CF = combined final.} \\ \end{tabular}
\end{table} 
Table 2: Different existing GIM products available to the PPP user from different ACs [33]. PPP convergence is affected by the kind of product used, as outlined in Table 2. A secondary objective, where ionospheric constraining plays a major role, PPP convergence in relation to the horizontal error was investigated for the first 10 min using the different GIM products. As observed in Figure 1(a),(b), over 24 h, there are millimetre-level variations between the products in the dual- and triple-frequency scenarios, respectively. A similar pattern was observed in the first 10 min of PPP convergence, as depicted in Figure 1(c),(d). However, in the 10-min scenario, centimetre-level differences could be seen, though some of the products still had millimetre range differences. These differences were observed in both the dual- and triple-frequency PPP results. It must be clarified that the purpose of these analyses was not to pitch or promote one product against another but rather highlight the influence that the choice of product could have on PPP initial solution convergence. It is important to point out that the GIM product for a particular day represents a snapshot of the ionospheric activity occurring within that period of time. Hence, the results shown are not conclusive for all other days, as each day can potentially show varying results from those presented for DOY 253 to 259 in 2018. Only 10 post-processed GIM products were investigated given the fact that it was difficult to obtain all the existing products from archiving sources. However, the key intended point is to show that depending on the product used to constrain PPP solutions, especially in the first few minutes, there is the possibility of centimetre- to millimetre-level of differences that could be observed.