Precise Point Positioning

Fundamentals
Title	Precise Point Positioning
Edited by	GMV
Level	Basic
Year of Publication	2011

Precise point positioning (PPP) stands out as an optimal approach for providing standalone static and kinematic geodetic point positioning solutions using all the available GNSS constellations. Combining precise satellite orbits and clocks with un-differenced, dual-frequency, pseudo-range and carrier-phase observables, PPP is able to provide position solutions at centimeter-level precision. PPP offers an attractive alternative to Differential Global Navigation Satellite System (DGNSS), with the advantage that it does not require simultaneous observations from multiple stations, i.e., it only needs a single geodetic receiver. In practice, PPP makes use of a network of reference stations in order to compute precise estimates of GNSS satellites orbits and clock errors. Nevertheless, it requires fewer reference stations globally distributed as compared with classic differential approaches (e.g. Real Time Kinematics, RTK), and one set of precise orbit and clock data (computed by a processing center) is valid for all users everywhere. Furthermore, as the precise orbits and clocks are calculated from a global network of reference stations, the same set of satellites is simultaneously observed by multiple stations, which enables PPP to provide position solutions rather robust to individual reference station failures.

PPP Introduction

Precise Point Positioning (PPP) provides a global precise positioning service by leveraging precise reference satellite orbit and clock products in real-time using widespread networks of GNSS reference stations distributed worldwide. Although being typically a global positioning service, PPP service might be regional too.

PPP is able to provide position solutions at centimeter to decimeter level by combining precise satellite positions and clocks with un-differenced, dual-frequency (to remove the first order effect of the ionosphere), pseudorange and carrier-phase GNSS observables. In static mode, PPP can provide even sub-centimeter positioning precision. PPP differs from traditional Double-Difference (DD) relative baseline positioning (e.g., Real Time Kinematics, RTK) in the sense that it does not require access to simultaneous observations from one or more close reference stations accurately-surveyed. As a result, PPP provides absolute positioning information, contrarily to RTK, which instead provides relative positioning information with respect to a reference station. PPP just requires precise orbit and clock data, which are computed by a processing center with measurements coming from reference stations belonging to a relatively sparse network (i.e., thousands of km apart would suffice). This makes PPP a very attractive alternative to RTK for those areas where RTK coverage is limited or not available.

One of the main drawbacks of PPP techniques is that they require a fairly long convergence time to achieve the utmost performance. Standard PPP techniques generally take many tens of minutes to initially converge. However, many novel PPP techniques recently proposed are capable to significantly reduce this initial convergence time (e.g., down to approx. 10-15 minutes) or, in case external precise ionospheric information is available, even to eliminate it.

PPP Algorithm

The PPP algorithm is a point positioning technique which makes use of i) precise satellite orbits and clocks instead of the corrections broadcast by the satellites; ii) very accurate additional error models; iii) sequential filtering of dual-frequency pseudorange and carrier-phase observables. By this processing, PPP is able to compute precise receiver coordinates, together with the receiver clock, Zenith Tropospheric path Delay (ZTD) and the initial phase ambiguities to all satellites (as carrier-phase measurements are used as well).

The accuracy of the satellite clocks and orbits is one of the most important factors affecting the quality of the PPP algorithm. Another relevant factor that affects PPP performances is the amount and quality of the observations. Like any GNSS technique, PPP is affected by satellite line-of-sight obstructions. Even the most precise orbit and clock data is useless if the user cannot track particular satellites. When satellite visibility is partially obstructed, a best possible service can be ensured by using the full range of satellites from all the available constellations (GPS, Galileo, GLONASS, QZSS, etc.).

Benefits and Prospects

As it has been mentioned before, PPP technique offers significant benefits compared to differential precise positioning techniques:

• PPP involves only a single GNSS receiver and, therefore, no reference stations are needed in the vicinity of the user.

• PPP can be regarded as a global position approach because its position solutions refer to a global reference frame. As a result, PPP provides much greater positioning consistency than the differential approach in which position solutions are relative to the local base station(s).

• PPP reduces labor and equipment cost and simplifies operational logistics to field work since it eliminates the dependency on base station(s).

• PPP can support other applications beyond positioning. For example, as PPP technique estimates receiver clock and tropospheric effect parameters in addition to position coordinate parameter, it provides another way for precise time transfer and troposphere estimation using a single GNSS receiver.

With respect to challenges, PPP technique poses several practical issues in order to achieve its full potential to applications. These include the long initialization time and the integer ambiguity resolution. However, as mentioned, over the last decade, lots of novel solutions and refinements have been proposed for PPP algorithms to cope with the issues above, thus going in the direction of making PPP practical in all service areas.

PPP Implementations

Nowadays, there exist several consolidated implementations of PPP services, consisting in both post-processed and Real-Time (RT) solutions. Post-processed PPP solutions, which have been on the market for many years already, usually achieve better results (i.e., mm-level precision) than RT solutions.

Post-processed PPP solutions can come either in the form of web-based services publicly or privately accessible by the users, or in the form of routines included by GNSS manufacturers within their post-processing software.

Real-Time (RT) PPP implementations, which have appeared on the market only in recent years, are generally more costly than the post-processed ones. This is mainly due to the fact that they require precise orbit and clock corrections to be sent real-time to the location of the GNSS receivers, where a position estimate is computed through the PPP algorithm. The real-time communication with the reference stations is commonly established over the Internet via specifically designed communication protocols, such as the Networked Transport of Radio Technical Commission for Maritime Services via Internet Protocol (NTRIP).

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