Innovation: Precise Point Positioning

April 1, 2009 By: Sunil B. Bisnath, Yang Gao GPS World

A Powerful Technique with a Promising Future

INNOVATION INSIGHTS with Richard Langley

MORE THAN 10 YEARS AGO in an Innovation column, I wrote, "Although RTK is the latest word, or should we say acronym, in GPS positioning, it will not be the last. Scientists and engineers will continue to invent faster, more accurate, more convenient, and more reliable ways to use GPS in navigation, surveying, and a host of other areas, some of which we haven't even dreamt of yet."

In the intervening decade, RTK — or real-time kinematic — positioning has become an industry standard procedure in surveying, machine control, and other high-precision applications. RTK makes use of carrier-phase and pseudorange measurements recorded at a (usually) fixed reference location with known coordinates and transmitted in real time to a user's rover receiver using a radio link of some kind. The rover processes the double differences of observations between satellites and receivers to determine its coordinates with better than 10-centimeter accuracy. It can do this successfully if it can resolve the integer ambiguities in the carrier-phase measurements. Ambiguities are the bane of carrier-phase positioning. They must be resolved to turn carrier-phase measurements into unbiased range measurements.

In RTK positioning, the ability to resolve ambiguities is determined by many factors, such as the distance between the reference station and the rover and atmospheric effects. RTK is a much more efficient technique than the earlier developed (but sometimes still used) post-processing surveying techniques. However, it does require an investment in reference station infrastructure or the purchase of commercial RTK services.

Is there a viable alternative to RTK? In this month's column, we take a look at the technique of precise point positioning (PPP). Like RTK, PPP makes use of ambiguous carrier-phase measurements but only from the user's receiver. Rather than measurements from a reference receiver, it needs ultra-precise (and accurate) satellite orbit and clock information such as that provided by the International GNSS Service. Currently, there are issues with how long solutions take to converge and the difficulty in resolving the ambiguities, for example, but research is targeting these and other practical issues. How close is PPP to prime time? Read on.

"Innovation" is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments and topic ideas. To contact him, see the "Contributing Editors" section.

The main goal of this article is to describe the current performance of what has become known as the precise point positioning (PPP) technique, and to discuss the future potential of the technique, along with its technical limitations. We begin with a review of the current state of PPP, covering performance and usage. We then discuss current technical limitations of the approach, including solution convergence period, accuracy, and integrity of solutions. The next section considers potential improvements upon the current approach, in terms of integer ambiguity resolution; integration with other data, for example, from real-time kinematic (RTK) solutions or an inertial navigation system (INS); and the use of other external modeling data, such as atmospheric refraction models. Equally important are PPP-infrastructure challenges, including the availability of precise satellite orbits and clock offsets, precise orbit and clock prediction, real-time dissemination of predicted orbits and clocks, and reference frame realizations. Given the upcoming great changes due to GPS modernization and the development of other global navigation satellite systems (GNSS), we would be remiss not to speculate on the potential significant positive impacts of these added signals on future PPP performance. Finally, we end with a rather provocative discussion of the potential of PPP to perform in a similar manner as the RTK technique.

Current Status

This section is designed to summarize current PPP performance using a number of metrics, and to set the technique's impact within the context of the wider field of positioning and navigation. What we mean by PPP is the state-space solution to the processing of pseudorange and carrier-phase measurements from a single GNSS receiver, utilizing satellite constellation precise orbits and clock offsets determined by separate means. Typically, a dual-frequency GNSS receiver is used with dual-frequency code and phase measurements linearly combined to remove the first-order effect of ionospheric refraction. The real-valued carrier-phase ambiguity terms are estimated from the measurement model. The tropospheric refraction is also estimated, along with the receiver position and ambiguity parameters from the measurements. PPP using a single-frequency GNSS receiver has also been investigated with great promise for certain applications. However, we will not discuss these further in this article; see Further Reading for publications reporting developments in single-frequency PPP and other advances in the technique. To achieve the best position accuracy possible from PPP, effects such as carrier-phase wind-up, transmitter-antenna phase offset, solid Earth tides, and the contribution of ocean tide loading must be corrected using models. Residual terms such as receiver noise and multipath are generally ignored or minimally handled using stochastic procedures.

A unique aspect of PPP is that it is an area of research being actively pursued by academia, government, and industry, in concert and individually. As is typical, early development occurred in research settings for scientific goals. Governments, as service providers, have in some cases engaged in providing PPP services to the public, given the socioeconomic benefits. Industry has embraced and advanced the technology to better serve its clients. The results are: 1) rapid development and use of PPP in a variety of application areas, and 2) significant overlap between the three sectors in terms of research and development, and service models. The latter point will be discussed further in the infrastructure section.