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) is a global precise positioning service, since it requires the availability of precise reference satellite orbit and clock products in real-time using a network of GNSS reference stations distributed worldwide.

Combining the precise satellite positions and clocks with a dual-frequency GNSS receiver (to remove the first order effect of the ionosphere), PPP is able to provide position solutions at centimetre to decimetre level, even less than 1 cm-level positioning in static mode. PPP differs from double-difference Real Time Kinematics (RTK) positioning in the sense that it does not require access to observations from one or more close reference stations accurately-surveyed and that PPP provides an absolute positioning instead of the location relative to the reference station as RTK does. PPP just requires precise orbit and clock data, computed by a processing centre with measurements from reference stations from a relatively sparse station network (thousands of km apart would suffice). This makes PPP a very attractive alternative to RTK for those areas where RTK coverage is not available. On the contrary, the PPP technique is still not so much consolidated as RTK and requires a longer convergence time to achieve maximum performances (in the order of tens of minutes). Currently, there exist several consolidated post-processing PPP services. On the contrary, real-time PPP systems are in an incipient development phase.

The PPP algorithm uses as input code and phase observations from a dual-frequency receiver, and precise satellite orbits and clocks, in order to calculate precise receiver coordinates and clock. The observations coming from all the satellites are processed together in a filter that solves for the different unknowns, namely the receiver coordinates, the receiver clock, the zenith tropospheric delay and the phase ambiguities.

The accuracy of the satellite clocks and orbits is one of the most important factors affecting the quality of the PPP. Another relevant factor that affects the 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 both the GPS and GLONASS systems, or, in the future, Galileo.

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 GPS 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 referred 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 or stations.

• 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 GPS receiver.

With respect to challenges, PPP faces several in order to achieve its full potential to applications. Between these challenges:

• A long initialization time, this is a drawback for real-time applications.
• Integer ambiguity resolution, to have a more precise solution.