## 6 Conclusions  
In this contribution, we studied the single-baseline real-time kinematic (RTK) performance while having the smartphones upright and in a lying down position, in a zero-baseline and short-baseline setup, respectively. We made use of radio frequency (RF) shielding boxes and re-radiating antennas to track GNSS signals from external low-cost antennas, consisting of L1, L5 GPS, E1, E5a Galileo, L1, L5 QZSS and B1 BDS code and carrier-phase observations, while using two Google Pixel 4 and two Samsung Galaxy S20 smartphones. We assessed their instantaneous (single-epoch) RTK performance and the double-differenced (DD) carrier-phase and code residuals, while also using the internal smartphone antennas. We found that when having the smartphones lying down, the RTK positioning performance, for S20 in particular, will deteriorate. This was also true for the GP4 smartphones when using internal antennas in a short-baseline experiment. Our zero-baseline instantaneous RTK evaluation consisted of a formal and empirical analysis of the smartphones, in an upright and lying down position, while using 12 h of data with a one second measurement interval. When the smartphones were lying down, we found that the S20 smartphones seem particularly affected by the setup configuration with many incorrectly fixed solutions at the meter-level precision. The integer least-squares (ILS) success-rate (SR), i.e., the probability of correct integer estimation, while using full ambiguity resolution, improved significantly when having the S20 smartphones in an upright position (cf. Figure 5). The GP4 smartphones showed more or less no ILS SR improvement between the two zero-baseline setup configurations. However, the formal and empirical positioning confidence ellipses/intervals and probability density functions (PDFs), respectively, showed a contrast difference from each other when all the smartphone models were lying down. This implies that the GP4 antennas were also affected by interferences from the smartphone configuration. The vital influence of the setup configuration was also reflected in the observation data quality, in which linear trends were found to be present in the S20 DD phase residuals while the phones were lying down. The mean root mean square (RMS) values of these DD phase residuals were higher when the smartphones were in a lying down position rather than upright. This was also particularly pronounced in the S20 smartphones for the DD code residuals. It is worth noting that in these zero-baseline experiments, the multipath from the external antennas is expected to be significantly reduced and any remaining small multipath effects would mainly be due to the non-simultaneity of sampling between the receivers. Nevertheless, we found that the RMSs of the GPS and QZSS DD phase residuals resembles those of Trimble NetR9 and ulbox ZED-F9P receivers in the zero-baseline setup, whereas the corresponding RMSs of the L1 DD code residuals were several times larger for both smartphone models (cf. Table 3). Finally, we found that both smartphone models obtained correctly fixed positioning error STDs that also resemble that of the Trimble NetR9 and ulbox ZED-F9P receivers when in an upright position. In the short-baseline instantaneous RTK results, when multipath from the external/internal antennas is expected to be present, we found consistent worsening in the results when the smartphones were lying down (similar to that of the zero-baseline analysis). Most importantly we found encouraging instantaneous, short-baseline, cm level RTK positioning results for the GP4 and S20 smartphones when observing signals from external antennas, with ILS SR close to 100% (99.9%) for GP4 and 99.2% for S20, respectively, while using 8 h of data. When use was made of internal antennas and having the smartphones in an upright position, we achieved a corresponding ILS SR of 98.7% and 64.4% for GP4 and S20, respectively, based on 3.5 h of data.