Global navigation satellite system (GNSS) precise point positioning (PPP) using GPS and GLONASS first became achievable around 2007. An increasing number of satellites of GNSS and their constant modernization allow conducting PPP with four constellations, namely GPS, GLONASS, BeiDou, and Galileo. The full orbit constellation (FOC) of BeiDou and Galileo is not completed yet and still under development. It is expected that BeiDou and Galileo FOC will be completed in late 2020. In this paper, repeatability of quad-constellation PPP with different variations namely, GPS, GPS-GLONASS, GPS-GLONASS-GALILEO, and GPS-GLONASS-GALILEO-BeiDou, over Turkey are investigated. Ten consecutive days in 2018 (DOY: 324-333) and six IGS-MGEX stations are chosen within and around Turkey to conduct quad-constellation PPP using daily data. GipsyX scientific software developed by NASA Jet Propulsion Laboratory (JPL) is used to conduct static PPP. Satellite elevation cutoff angle is set to 7° for each processing. The results indicate that adding GLONASS, GALILEO, and BeiDou to GPS not always improve the horizontal and vertical precision of the chosen sites. Some stations show precision degradation, the others show the precision improvement but the improvements are generally marginal. The results are mainly related to the observation time and satellite elevation cutoff angle which effect the improvement in precision compared to GPS significantly.
Anahtar Kelimeler: BeiDou, GPS, GLONASS, GALILEO, PPP
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