Precise Point Positioning (PPP)
Precise Point Positioning (PPP) relies on the availability of precise GNSS satellite orbits and clocks. This is fundamentally different from positioning with respect to a local base station or network with RTK. PPP enables precise positioning of a single receiver and does not depend on the availability of tracking data from reference stations around the user location. PPP positioning precision is determined by the uncertainty of the orbit and clock products used in the solution, not proximity to a base station. As PPP establishes a direct connection between the user and GNSS satellite coordinates, it can be used globally for a broad range of applications in remote areas and offshore, where reference stations are distant and offer limited inter-visibility for differential processing. Most PPP augmentation services (e.g. IGS) rely on global networks of sparsely distributed GNSS tracking stations to compute precise satellite orbit and clock products. These products are then made available in real-time or post-mission, depending on user requirement and service specifications.
At the user-end, a dual-frequency receiver is most commonly used to remove the first-order ionospheric delay. In contrast to differential methods where errors cancel by differencing observations made simultaneously at nearby locations, PPP requires that all measurement errors introduced by the satellite, station or atmosphere be accounted using empirical or stochastic models. To reach a specific precision threshold, PPP solutions need a longer convergence time compared to differential methods. While site occupations of a minute or less may be sufficient to obtain RTK positions with centimetre accuracy, 15 minutes to an hour may be required from PPP. Recently, some methods have been developed to accelerate integer ambiguity resolution in PPP by applying local ionospheric constraints.
While PPP solutions can provide coordinates with centimetre accuracy in a global reference, user occupation time must be sufficient to ensure full ambiguity convergence. For the most demanding users who require centimetre level day-to-day repeatability, sessions of up to 24-hours may be necessary to average out errors with systematic daily signatures (i.e. earth-tides and ocean loading).
For post-mission PPP processing, satellite orbit and clock corrections are made available online in standard file formats from IGS global data centers. As the ability to track and process all satellites in view improves PPP precision and convergence, some IGS orbit products now compute corrections for multiple constellations of navigation satellites in support of “multi-GNSS PPP” solutions. In Canada, the Canadian Geodetic Survey (CGS) has implemented an online software as service (SaS) for post-mission processing known as CSRS-PPP. CSRS-PPP is available 24/7 and processes GPS and/or GLONASS RINEX files on-demand. User solutions are estimated within a couple hours of data collection once GNSS precise orbit products have been validated. Results are usually posted and accessible to end-users a few minutes after initiating their GNSS data upload. Typically, positions computed with dual-frequency GNSS observations collected in static mode for periods of 1-2 hours are accurate to better than 5 centimetres. The averaging of systematic errors with longer time periods provides centimetre-level accuracy for 24-hour datasets. Kinematic solutions can offer decimetre-level precision but are more vulnerable to changes in satellite geometry and data outages that are more prevalent for mobile applications. Special attention should be given to the precision estimates associated to the coordinates for each epoch of a kinematic survey.
For Real Time PPP (RT-PPP), streams of precise satellite orbits and clocks are also made publicly available by IGS contributing agencies. Commercial providers also distribute their own proprietary correction streams using different global tracking networks. For real-time applications, wireless access to corrections is usually offered under a subscription-based model to recover costs related to tracking network operations and correction distribution. With positioning errors increasing by about 1 centimetre for each second of latency introduced while relaying correction messages to end-users, efficiencies in network computing and messaging are critical to gain competitive advantage. In Canada, real-time PPP augmentation services are available mainly from commercial providers (see Table 1).
Table 1: Main PPP service providers in Canada
|IGS||Internet||Global||GPS, GLONASS, Galileo, BeiDou and QZSS Precise orbits / clocks||Earth science researches, Agriculture, GIS/Mapping, Exploration, land survey, etc.||http://www.igs.org/rts/access|
|Trimble RTX/Omnistar||SatCom||Global||GPS, GLONASS, BeiDou and QZSS Precise orbits / clocks||Agriculture, GIS/Mapping, Exploration, Airborne Survey||http://www.trimble.com/agriculture/CorrectionServices/centerPointRTX-cell.aspx|
|John Deere NavCom||SatCom||Global||GPS, GLONASS, Precise orbits / clocks||Land survey, agriculture, machine guidance||https://www.navcomtech.com/navcom_en_US/support/faqs/starfire/starfire.page|
|Hexagon Veripos||SatCom||Global||GPS, GLONASS, BeiDou and QZSS Precise orbits / clocks||Primarily marine off-shore services & land navigation||http://www.veripos.com/|
|Fugro Seastar||SatCom||Global||GPS, GLONASS, Galileo, BeiDou Precise orbits / clocks||Marine (off-shore) survey||http://www.fugroseastar.com/|