# Round-Trip Time Ranging to Wi-Fi Access Points Beats GNSS Localization
## 14 Conclusions  
The Bayesian grid method of localization using FTM RTT has been extended for use outdoors. An \"inverse\" Bayesian grid method has been used to determine the locations of the APs. A simplified observation model for outdoor use has been presented. A strategy  
Figure 8: The histogram of error in RTT measurement has a bell shape somewhat like a Gaussian, although also sporting long tails. (the error here is in the RTT measurement, given the AP positions). Superimposed in red is a Gaussian with standard deviation 3 m.  
has been introduced for selecting the set of APs to range to in order to avoid dead ends where no AP being ranged to respond. Experimental results are presented and analyzed for a large complex (210 \(\times\) 120 m) containing multiple buildings with over a hundred APs. The methods were demonstrated in a situation where there are no line-of-sight (LOS) connections, where the APs are inaccessible, and where the APs are concentrated in a few clusters. The RTT-based localization results are compared with the smartphone's code-based GNSS.  
Of course, many factors influence the performance of GNSS and FTM RTT. For real-time use, smartphones provide code-based GNSS. Much greater accuracy can be achieved with geodetic grade GNSS devices with survey-grade circularly polarized antennas if locations are _not_ required in real-time and data can be collected over a long time. Such setups typically require equipment that costs several thousand dollars, plus a second device in a known fixed location, a subscription to a service providing correction data, as well as post-processing (i.e., positions are not available in real-time).  
Some Wi-Fi APs do not support the FTM RTT protocol and so can be used only with \"one-sided\" RTT, which has lower accuracy [27]. Going in the other direction, the IEEE 802.11 az standard will enable ranging with even higher accuracy. The results in this paper apply to current smartphone implementations of code-based GNSS positioning and current \"two-sided\" IEEE 802.11 mc FTM RTT distance measurements with respect to Wi-Fi access points.  
The adoption of IEEE 802.11 mc may not have been as rapid as one might have hoped. Meantime much progress has been made in finalizing IEEE 802.11 az, the next generation positioning (NGP) protocol [59, 60]. IEEE 802.11 az promises to further increase accuracy, likely to sub-meter level, by exploiting MIMO path diversity and channel sounding repetition. In addition 802.11 az will provide reduced measurement time and added privacy and security. This will further improve localization using Wi-Fi APs, both indoors and outdoors.  
This research received no external funding.  
Not applicable.  
The PostScript code on which Figures 1 and 4-8 are based contains all the raw data and is available upon request from the corresponding author.  
I would like to thank [PERSON] of Google for helpful feedback on a draft of this paper and discussions relating to FTM RTT and the IEEE 802.11 standards. I also wish to express my gratitude for the helpful suggestions of the reviewers.  
The author declares no conflicts of interest.