Status and Trends for High Precision GPS Kinematic Positioning

Precise GPS kinematic positioning in the post-processed or in the real-time mode is now increasingly used for many surveying and navigation applications on land, at sea and in the air. The distance from the mobile receiver to the nearest reference receiver may range from a few kilometres to hundreds of kilometres. As the receiver separation increases, the problems of accounting for distance-dependent biases grows and, as a consequence, reliable ambiguity resolution becomes an even greater challenge. In this paper, the status and challenges for high precision GPS kinematic positioning for the short-range, medium-range and long-range cases, in both the post-processing and real-time modes, will be presented. The standard mode of carrier phase-based positioning is the result of progressive R&D innovations. Over the last half decade or so several significant developments have resulted in this high accuracy performance also being available in "real-time" -that is, in the field, immediately following the making of measurements, and after the data from the reference receiver has been transmitted to the (second) field receiver for processing. Real-time precise positioning is even possible when the GPS receiver is in motion. These systems are commonly referred to as RTK systems ("real-time-kinematic"), and make feasible the use of GPS-RTK for many time-critical applications such as machine control, GPS-guided earthworks/excavations, automated haul truck operations, and other autonomous robotic navigation applications. If the GPS signals were tracked and loss-of-lock never occurred, the integer ambiguities resolved at the beginning of a survey could be kept for the whole GPS kinematic positioning span. However, the GPS satellite signals are occasionally shaded (for example, due to buildings in "urban canyon" environments), or momentarily blocked (for example, when the receiver passes under a bridge or through a tunnel), and in most cases the integer ambiguity values are "lost" and must be redetermined. This process can take from a few tens of seconds up to several minutes with present commercial GPS systems for short-range applications. During this "re-initialisation" period the GPS carrier-range data cannot be obtained, and hence there is "dead" time until sufficient data has been collected to resolve the ambiguities. If interruptions to the GPS signals occur repeatedly, ambiguity re-initialisation is, at the very least, an irritation, and at worse a significant weakness of commercial GPS-RTK positioning systems. The goal of all GPS manufacturers is to develop the ideal real-time precise GPS positioning system, able to deliver positioning results, on demand, in as easy a manner as is presently the case using pseudorange-based differential GPS (DGPS) techniques, which typically deliver positioning accuracies of 1-10 metres. For example, the DGPS technique is robust, implemented in real-time via the transmission of correction data, and there is negligible delay in obtaining results. Over the last few years several important developments have occurred that appear to have overcome some of the main constraints: (a) Under certain conditions decimetre level positioning accuracy has been possible even when the baseline lengths have been up to hundreds of kilometres in length. (b) Reliable OTF-AR in the shortest period of time possible, even with just one measurement epoch, has been demonstrated. (c) Given very short periods of time-to-AR the notion of cycle slips, or having to re-initialise the ambiguities, has no meaning because so-called "instantaneous" OTF (IOTF) is the normal mode of kinematic positioning for all epochs. (d) Improved multipath mitigation wthin the GPS receivers themselves. (e) For certain applications single-frequency GPS instrumentation can be used. (f) The release of several commercial integrated GPS-GLONASS receivers. However, the two most significant algorithm improvements have been in: (a) overcoming the baseline length constraint, and (b) shortening the "time-to-AR" to just one epoch of data. This paper discusses these advances and comments on the impact of these on typical GPS positioning applications.