MULTI-GNSS PPP: An Alternative Positioning Technique for Establishing Ground Control Points
Authors: Bahadur Berkay , Nohutcu Metin 
Published: March 2020

Traditionally, ground control points (GCPs) are utilized to determine absolute image orientations indirectly in aerial triangulation. For a long time, differential and relative GNSS (Global Navigation Satellite System) positioning techniques have been extensively used to establish GCPs. In our country, the establishment and measurement of GCPs are instructed in the related regulation based on differential GNSS techniques. One of the two methods described in the related regulation is based on establishing, at least, C3 level networks with maximum base length of 10 km and with minimum 35-minute observation time. In an alternative method, without base length restriction, GCP coordinates can be determined being connected to at least 3 TUSAGA-Active stations and with minimum 120-minute static observation. The expected precision for the coordinates of GCPs are described to be better than 5 cm in horizontal and 6 cm in vertical within the regulation. Although differential techniques can provide highly accurate positioning solutions, they are required at least two receivers to mitigate GNSS error sources. Additionally, positioning accuracy obtained from these techniques are strictly dependent on the distance from reference stations. It is clear that all these raise the operational cost and system complexity of differential GNSS techniques. In recent years, Precise Point Positioning (PPP), which enables centimeter- or millimeter-level positioning accuracy with only one receiver on a global scale, has emerged as an alternative positioning method. Over the last decade, PPP has attracted considerable attention within the GNSS community due to its exceptional benefits such as operational simplicity, cost-effectiveness, elimination of base station requirement. However, the main drawback of PPP is relatively long observation period required to achieve a specific positioning accuracy, for example, nearly 50 min to reach 10 cm or better horizontal accuracy with 30 seconds sampling rate. On the other hand, the completion of GLONASS constellation and the emergence of new satellite systems, such as Galileo and BeiDou, offers considerable opportunities to improve the PPP performance. The combinations of different GNSS constellations, namely multi-GNSS, strength the number and geometry of visible satellites, and therefore, reduces the convergence time significantly. Additionally, the new generation GNSS receivers make possible to collect more observations (even up to 100Hz), which provides abundant data for PPP processing. Taking all these into account, the principal objective of this study is to investigate the usability of PPP in establishing GCPs for aerial triangulation. For this purpose, an experimental test was conducted to evaluate the positioning performance of multi-GNSS PPP with high-frequency GNSS receivers (1 Hz). The results indicate that 5 cm or better horizontal and vertical positioning accuracy can be achieved by multi-GNSS PPP process within approximately 30 minutes using high-frequency GNSS receivers. Considering these results and its operational simplicity, it can be said that PPP is a robust alternative for the establishment of GCPs.