# Single-Baseline RTK Positioning Using Dual-Frequency GNSS Receivers Inside Smartphones
###### Abstract
In reference [11], a decimeter level of accuracy in terms of positioning performances has been achieved following the post-processing approach, made by double differencing raw smartphone observations with those coming from a GNSS Continuously Operating Reference Stations (CORS) network. Also the present authors in reference [12] performed a Network Real Time Kinematic (NRTK) positioning to demonstrate the possibility of obtaining a centimeter-level accuracy through the use of differential corrections provided by CORS networks, even if it was not possible to fix the phase ambiguities in a correct way. Despite this challenge, sub-meter accuracy was obtained using that type of technique. Another important milestone in smartphone positioning and navigation was in September 2017, when a chip manufacturer company, Broadcom Limited, announced the release of the first dual-frequency GNSS chipset, the BCM47/755. Up to 2017, most GNSS chipsets installed inside smartphones were single-frequency receivers able to only provide measurements related to one frequency (the L1 band). In these cases, it is not possible to apply the double or triple differences approach [13; 14], or to combine different observations [15]. The ability to use dual-frequencies signals, however, enables a higher level of positioning performances, mainly increasing the signal robustness to environmental impairments and the positioning accuracy. The release of the first dual-frequency chipset, the first smartphone equipped with dual-frequency GNSS appeared in early June 2018, which was the Xiaomi Mi 8. The availability of signals from two frequencies allows us to correct most of the error sources introduced by ionospheric propagation. At the same time, it opens the way to solving the carrier phase integer ambiguity, enabling Real-Time Kinematic (RTK) and Precise Point Positioning (PPP) algorithms directly on smartphones, as long as the phase measurements are stable; furthermore, the design of the signals in the E5/L5 frequency makes it easier to distinguish real signals from the ones reflected by buildings, reducing the multipath effect [16]. Finally, frequency diversity is among the most regularly quoted solutions to increase robustness to interference and jamming. In addition to such benefits, the enhanced position precision also creates interesting opportunities for novel applications, such as augmented reality, autonomous vehicle navigation, and mapping. The first research on this GNSS chipset has been made by reference [17] which evaluates the performances of the Xiaomi Mi 8 comparing the L5/E5a and L1/E1 frequencies. Applying double differences on GPS L5 and L1 code observations in a short baseline configuration, the precision of the measurements is 1.3 meters and 2.1 meters, respectively. Carrier phase-based static differential positioning using GPS and Galileo on a very short baseline has provided cm-level precision in the horizontal component and decimeter-level in the vertical component. In reference [18], the dual-frequency GNSS chipset mounted on the Xiaomi Mi 8 has been considered for multipath characterization, comparing the obtained results with those obtainable with a geodetic receiver. The comparison between GPS and Galileo measurements showed a lower multipath error in the last one,although these results are still very far from the values of the geodetic receiver. Again, the antenna represents the main limitation to achieving cm-level accuracy in positioning. Starting from the previous work made by the authors of reference [12] on single frequency smartphone receiver, in which the RTK positioning has been performed using the differential corrections provided by the CORS network; the main aim of this work is to perform a relative positioning using two different Xiaomi Mi8 smartphones with a double-frequency GNSS chipset embedded.