The GPS Analysis and Positioning Software (GAPS): A Present and Future Tool for Processing GNSS Data

Precise Point Positioning (PPP) is one of the existing techniques to determine point coordinates using a GNSS receiver. In this technique, observations carried out by a single receiver are used in order to determine the three coordinate components, as well as other parameters, such as the receiver clock error and total neutral atmosphere delay. The technique is said to be "precise" because precise information, such as satellite orbits and clock errors, is used in the data processing (because of this, one should have in mind that the usage of precise orbits, clocks and other precise products is an implicit procedure whenever the term "PPP" is used). More than that, PPP is "precise" also because the resulting parameters are precise (and accurate). The fact that the observation model used for accurate error modelling has to take into consideration the several effects present in GPS signals, and that observations are undifferenced (there are no differences between receivers nor between satellite measurements), makes PPP a powerful data analysis tool which is sensible to a variety of parameters. The PPP application developed at UNB (University of New Brunswick), which is called GAPS (GPS Analysis and Positioning Software), has been designed and built in order to take advantage of precise products, resulting in a data analysis tool for determining parameters other than position, receiver clock error and neutral atmosphere delay. These other estimated parameters include ionospheric delays, code biases, satellite clock errors, and code multipath among others. In all cases, the procedures were developed in order to be suitable for real-time as well as post-processing applications. The ionospheric delay estimation uses a spherical ionospheric shell model, in which the vertical delays are described by means of a zenith delay at the station position and two horizontal gradients. This estimation makes use of carrier-phase measurements only. The use of precise orbits and clocks is a key element for the quality control of the data which goes into the ionospheric estimation filter. The code multipath estimation is based on the assumption that the several effects present in code measurements are dealt with within PPP, but strongly based on carrier-phase measurements. Based on this, these effects can be removed from pseudorange measurements, and the leftover effect is essentially the code multipath plus receiver noise. Another effect which afflicts pseudorange measurements is the code bias. The code biases are important because satellite clock data products are computed using a certain arbitrary convention of observation type, such as P1 code measurements rather than C1 code. One of the analysis tools of GAPS produces values of the satellite code biases, based on a positioning observation model, as opposed to being based on a satellite clock estimation observation model as is usually the case when bias values are provided to users. Regarding satellite clock error estimates, GAPS was enhanced in order to provide estimates of satellite clock offsets. This tool was created aiming at a suitable approach for real-time carrier-phase-based satellite clock estimation. GAPS is available online via a web interface, through the University of New Brunswick website. In addition to signal analysis outputs, GAPS provides state-of-art PPP results, including position, receiver clock errors, and neutral atmosphere delays, in static or kinematic mode. Future plans for enhancing GAPS to process data from modernized GPS and other GNSS constellations will also be discussed as will the possibilities of using GAPS as a real-time tool.