# Relative Positioning in Remote Areas Using a GNSS Dual Frequency Smartphone
## 2 Materials and Methods  
### Campaigns and Equipment Setup  
The experimental works, divided into six tests performed in February, March, and November 2021, took place on the facilities of the Astronomical Observatory of the University of Porto (AOUP), using as control points the pillars from a North-South oriented calibration base (Figure 1) with known precise coordinates in ETRS89. The devices used in the experiments were centered at the top of the pillars using a leveling base. The smartphone was positioned so that the GNSS antenna, located in the upper left corner of the screen, was centered over the point. The smartphone available for the experiments was the Xiaomi Mi8 which, at the time of its acquisition, in 2018, was one of the few that allowed access to raw dual frequency data.  
For assessing the quality of the smartphone's measurements two high-grade geodetic receivers were also used in the tests: a Trimble Alloy equipped with a Zephyr Geodetic  
Figure 1: Calibration base at Astronomical Observatory of the University of Porto.  
II antenna and a Septentrio PolaRx5 equipped with PolaNt-MF antenna. The differences between these high-grade devices and low-grade smartphones are several, starting with the mobile device only allowing dual frequency measurements, while geodetic receivers allow multi frequency measurements. The smartphones use a linear polarization antenna type, the PIFA (Planar Inverted F Antenna), with a not controlled radiation pattern, which has a narrow bandwidth, low sensitivity, low multipath suppression, and minimal interference rejection. These antennas are passive and without a signal amplifier due to the critical power consumption in a smartphone. In response to this difficulty, the manufacturers implemented the duty-cycle mechanism to save power, but with impact in the GNSS measurements, making it impossible to acquire continuous carrier-phase measurements. Therefore, in the experiments performed, the duty-cycle off option, available on the Xiaomi, was used to switch off that mechanism.  
The duty cycle does not exist in the geodetic receivers, and they use mostly active antennas with a signal amplifier. The polarization is RHCP (Right Hand Circular Polarization), and the radiation pattern coverage is controlled. The bandwidth in these antennas is wide and they have good sensitivity, as well as a high multipath suppression and good interference rejection. The phase center in the geodetic antennas is calibrated and known in the Antenna Exchange Format (ANTEX), playing a very important role in achieving millimeter accuracy, while for the smartphone antennas no calibration parameters are present in the ANTEX files. The receivers also have a more efficient front-end than the smartphone; this mechanism filters the signals from the antenna reducing the noise and rejecting signals out of the GNSS bands, and also amplifies the signals (ESA (2018) [13]).  
For reference stations, four Continuously Operating Reference Stations (CORS) from the EUREF Permanent GNSS Network [14] were chosen (see Table 1 and Figure 2) in order to have different baseline lengths. Distances vary between 0.2 and 645 km, respectively, for stations Gaia and CEUTA. The GNSS observations from these reference stations were obtained at 1 Hz using the BKG Ntrip Client (BNC) [15] software.