INNOVATION INSIGHTS with Richard Langley

Richard Langley

SCIENTISTS AND ENGINEERS continue to improve high-accuracy GPS positioning techniques — techniques pioneered a quarter of a century ago. The first GPS satellite, SVN01/PRN04, was launched from Cape Canaveral on February 22, 1978. And between 1978 and 1985, the U.S. Air Force orbited nine more prototype or Block I satellites to test key technologies before deploying the operational constellation.

Surveyors and geodesists were among the earliest users of the Block I satellites. Using the satellite signals, they developed accurate positioning techniques based on the use of carrier-phase observations — about two orders of magnitude more precise than code measurements. To reduce the effect of biases and errors in the measurements, they developed the concepts of between-satellite and between-receiver single differencing of the carrier-phase data as well as double and triple differencing. Raw measurements were recorded by receivers and then post-processed to obtain receiver coordinates. Clever approaches were developed to handle the integer ambiguity of the carrier phases.

With the launch of the Block II satellites beginning in 1989, further improvements in positioning accuracy and efficiency became possible, including real-time carrier-phase-based positioning with a radio link between a reference receiver and a remote receiver. This technique became known as real-time kinematic or RTK, as it permitted the remote receiver to rove and occupy different points in a single positioning exercise. But carrier-phase ambiguity resolution issues coupled with inaccurately modeled satellite orbit and atmospheric effects has limited consistent single-baseline RTK operation between reference and rover receivers to tens of kilometers. On longer baselines, inaccurate modeling can result in significant positioning errors. Network RTK, using simultaneously operating reference stations to better determine error corrections, can extend the area of coverage of RTK but it, too, has limitations.

In this month's column, I am joined by my colleague Don Kim who has developed an innovative approach to long-range RTK. We describe how accurate modeling of atmospheric effects coupled with an ionosphere-free ambiguity resolution process results in successful long-range RTK that can be implemented in either single-baseline or network mode. Has the ultimate RTK approach been developed? Probably not. But we're getting closer.

"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.

Biases and errors such as satellite orbit error and atmospheric signal refraction are the primary limiting factors in successful long-baseline, real-time kinematic (RTK) style processing of GPS measurements — either in real-time or post-processing mode. These error sources are dependent on the distance between a reference and rover receivers. If they are not adequately accounted for, they can result in significant positioning errors in long-baseline applications. This is particularly true for the conventional single-baseline RTK and hence reduces the effective inter-receiver distance of this technique to a few tens of kilometers.

We can apply effective strategies to mitigate these error sources. For example, the ionosphere-free linear combination of the L1 and L2 carrier-phase measurements can completely cancel first-order ionospheric delays. Although this approach is appealing for mitigating the ionospheric errors, we have to be prepared to accept some penalty. As it is difficult to fix integer ambiguities using the ionosphere-free observations for long baselines, float ambiguity solutions (less accurate than fixed ones) are normally used. Due to the amplification of the noise by the linear combination, the solutions are less precise. Errors in broadcast GPS satellite orbits have little effect for baselines up to a few hundred kilometers and, furthermore, can be virtually eliminated using precise ephemerides in post-processing mode. Tropospheric delay is usually estimated based on model atmospheric predictions and/or surface meteorological observations made near the stations at the time of the GPS measurements. As this approach often inappropriately accounts for spatial and temporal variations in water vapor delays, it is a common procedure to estimate a residual zenith delay from the data itself.