Precise Point Positioning: Is the era of differential GNSS positioning drawing to an end?

For three decades the differential GPS (DGPS), and subsequently DGNSS, technique has been the dominant operational mode for precise positioning for the geoscience, geospatial and navigation communities. All DGNSS techniques perform positioning relative to one or more reference GNSS receivers located at points of known coordinates. Depending upon the type of GNSS measurement, user equipment, reference receiver infrastructure, data processing algorithm, ancillary products or services, and operational requirements (e.g. real-time or postmission, moving or static user equipment, good or poor satellite and reference receiver geometry, etc.), different levels of performance are obtained. For example, the basic DGNSS technique using single-frequency pseudo-range measurements can deliver few-metre to submetre positioning accuracy, in either local-area DGNSS or wide-area DGNSS implementations. The processing of carrier phase data enables sub-decimetre-level accuracy, with the highest operational accuracy being possible (at the few-centimetre level) using the most sophisticated receiver equipment and algorithms, even in real-time and with the user equipment in motion (the so-called RTK mode). RTK techniques are now indispensible for precise navigation, machine automation, surveying and mapping. Over the last decade many government agencies and private companies have established permanent reference receiver networks (or CORSs – continuously operating reference stations) to support RTK users. This positioning infrastructure is expensive to establish and maintain at the appropriate density (e.g. number and distribution of CORS), quality (e.g. monumentation), functionality (e.g. multi-GNSS receivers), integrity (e.g. signal quality monitoring and datum) and robustness (e.g. terrestrial or satellite communications link between CORS and users). Over the same time period the technique of Precise Point Positioning (PPP) has been touted as an alternative for the “infrastructure-hungry” carrier phase-based DGNSS techniques. Although PPP suffers from several disadvantages – slow convergence times, no user equipment supports real-time algorithms, no real-time satellite orbit and clock data streams, uncertain coordinate datum – recent PPP developments could address most of these disadvantages. This paper describes the current knowledge of PPP, progress in the development of software, algorithms and data standards, and the provision of real-time services to support PPP, with a particular focus on the IGS (and commercial) products and the new role of CORS to provide augmentation services to PPP users. The paper also speculates on the evolving role(s) of CORS networks in the context of future PPP, and whether DGNSS techniques will be largely replaced by PPP techniques.