1. Beutler G, Brockmann E, Gurtner W, Hugentobler U, Mervart L, Rothacher M, Verdun A (1994) Extended orbit modeling techniques at the CODE processing center of the international GPS service for geodynamics (IGS): theory and initial results. Manuscr Geod 19:367–386
2. Cai C, Gao Y (2013) Modeling and assessment of combined GPS/GLONASS precise point positioning. GPS Solut 17:233–236
3. Dach R, Schaer S, Hugentobler U (2006) Combined multi-system GNSS analysis for time and frequency transfer. In: Proceedings of the European frequency and time forum, pp 530–537
4. El-Mowafy A, Deo M, Rizos C (2016) On biases in precise point positioning with multi-constellation and multi-frequency GNSS data. Meas Sci Technol 27(3):035102.
https://doi.org/10.1088/0957-0233/27/3/035102
5. El-Mowafy A, Deo M, Kubo N (2017) Maintaining real-time precise point positioning during outages of orbit and clock corrections. GPS Solut 21(3):937–947