# A Testbed for GNSS-Based Positioning and Navigation Technologies in Smart Cities:
## 6 Test Campaigns and Results
### Wi-FiTM RTT Ranging  
#### 6.2.1 Material and Experimental Setup  
In the context of data processing of Wi-FiTM RTT measurements, the quality of the measurements has been assessed, using the setup shown in Figure 10. In particular, two criteriums were used to characterise the Wi-FiTM RTT measurements:  
* Variability of the measurements under static conditions (i.e., observable noise);
* Potential biases, that could be related to the WAP and/or device.  
Figure 9: Cumulative Density Function (CDF) of the 2D horizontal position errors obtained by the SNAP sensors using GNSS and cellular signals either in standalone mode or hybridised in a representative working scenario.  
To conduct this quality assessment, a Google(tm) Pixel 2 smartphone was placed at an exactly well known (measured) distance relative to 3 Google(tm) Wi-Fi(tm) routers. With this setup, several data-takes were taken and analysed. Figure 11 shows the RTT ranges minus the actual range (measured with the measuring tape). In an ideal case, this difference should yield a constant 0 with a certain noise. The plot evidences a certain bias that is router dependent (see the bias values for each router MAC address in the figure) but might as well depend on the smartphone. These biases will directly impact the performance of a potential navigation solution based on RTT.  
In addition to the biases, the noise of these ranges might be derived from this test. Note that the tests have been performed under a benign environment, where no nearby objects or bodies were  
Figure 11: Bias estimation of the RTT measurements for the 3 Wi-Fi(tm) routers and Google(tm) Pixel 2.  
Figure 10: Setup for the Wi-Fi(tm) RTT (Round Travel Time) experimentation in the HANSEL project. Yellow dots corresponds to the position of the WiFi Access Points, the white circle is the location of the smartphone, yellow and red circles are reference lines at 3 and 5 m, and blue points are the positioning solution using WiFi only.  
present and thus the multipath patterns might be minimised. Under these circumstances, the standard deviation of the RTT time series for the 3 routers amount to ca. 20 cm.  
Knowing the standard deviation (i.e., noise) of the measurements will be of relevance when setting up the parameters and configuration of the navigation filter it is based on e.g., Kalman filter. Under real conditions, the weighting of the measurements within the Kalman filter can be performed using either the RTT error measurement given by the Android API, the RSS (prioritising those measurements with higher received power), or a combination of both. However, under ideal cases a theoretical accuracy using RTT of 20 cm (driven by the RTT noise) could be in principle achieved. This is inline with previous works (see for instance [33]).  
In addition to the Wi-FiTM RTT measurement quality, the WALS service and Hansel app have been used to test and demonstrate the capability of the algorithms developed in the _testbed_ to perform purely indoor positioning using only Wi-FiTM RTT measurements (no GNSS data) and the known locations of the WAPs.  
The data collection was done on the 25 February 2020 at the OpenLab of the UAB campus, where several GoogleTM Wi-FiTM routers were deployed. The data collection was performed using the GoogleTM Pixel 2 smartphone of the project for roughly 20 min, placed on a table in the main room of the OpenLabs.