# Single-Baseline RTK Positioning Using Dual-Frequency GNSS Receivers Inside Smartphones
###### Abstract  
Global Navigation Satellite System (GNSS) positioning is currently a common practice thanks to the development of mobile devices such as smartphones and tablets. The possibility to obtain raw GNSS measurements, such as pseudoranges and carrier-phase, from these instruments has opened new windows towards precise positioning using smart devices. This work aims to demonstrate the positioning performances in the case of a typical single-base Real-Time Kinematic (RTK) positioning while considering two different kinds of multi-frequency and multi-constellation master stations: a typical geodetic receiver and a smartphone device. The results have shown impressive performances in terms of precision in both cases: with a geodetic receiver as the master station, the reachable precisions are several mm for all 3D components while if a smartphone is used as the master station, the best results can be obtained considering the GPS+Galileo constellations, with a precision of about 2 cm both for 2D and Up components in the case of L1+L5 frequencies, or 3 cm for 2D components and 2 cm for the Up, in the case of an L1 frequency. Moreover, it has been demonstrated that it is not feasible to reach the phase ambiguities fixing: despite this, the precisions are still good and also the obtained 3D accuracies of positioning solutions are less than 1 m. So, it is possible to affirm that these results are very promising in the direction of cooperative positioning using smartphone devices. s 2019 19 4302 10.3390/s19194302 10.3390/s19194302 10.  
a form of assistance composed by fast messages for ephemeris, almanacs, orbits, and corrections. Nowadays, under good multipath conditions, the typical accuracy reachable is about few meters while in adverse conditions it becomes around 10 meters. In general, the accuracy of the latest GNSS navigation messages is approximately 1 meter, making the main sources of errors the front-end antenna and the receiver-dependent noise. Geodetic receivers allow it to reach sub-centimetric positioning accuracy thanks to the quality of the geodetic antenna, its regular gain, the high sensitive oscillator, and the signal processor performances. Moreover, their capability to acquire multi constellation and multi-frequency observations allows it to apply advanced algorithms for positioning. Numerous techniques can be used in real-time and post-processing in order to estimate and compensate for the biases affecting the so called GNSS observables, pseudoranges, and carrier-phase measurements. The three main techniques for increasing the accuracy of the positioning are based on differentiation, modeling, and a combination of these measurements. While pseudoranges are mainly used for low-precision real-time positioning, carrier-phase measurements are used for precise positioning, both in real-time and in post-processing. Until recently, both mass-market receivers and chips installed in smartphones exploited mainly code-phase measurements on the single frequency L1/E1 despite having an internal carrier-phase loop. This was due to their inability to demodulate the carrier-phase signals and due to the need to operate in real-time. Although in the most modern receivers this is no longer a problem, the research on smartphone positioning initially focused on the feasibility of cm-accurate positioning based only on pseudoranges measurements and analyzing the performances of the main parts of the GNSS chipsets like the internal oscillator and the antenna gain. In fact, it is known that the former, namely the internal oscillator, is due to the occurrence of some time correlated gaps in the pseudorange measurements, while the latter leads to poor multipath suppression capability and less sensitivity with respect to a geodetic antenna. Being unable to access the raw observation measurements, the classical approach used to analyze these problems consisted of using a hardware add-on or modification with external tools.