Wi-Fi butterfly effect in indoor localization: The impact of imprecise ground truth and small-scale fading

The increasing accuracy of indoor positioning systems makes their evaluation an increasingly challenging task. A number of factors are already known to affect performance of fingerprint-based systems: hardware diversity, device orientation, environment dynamics. This paper presents a new butterfly-like effect in localization experiments. The effect is caused by minor ground truth (GT) errors — that is, small deviations between calibration and test positions. While such deviations are widely considered as purely additive and thus negligible, we demonstrate that even centimeter-scale GT errors are amplified by small-scale radio fading and lead to severe multi-meter Wi-Fi positioning errors. The results show that fingerprint-based localization accuracy quickly deteriorates as GT errors increase towards 0.4 wavelength (5 cm for 2.4 GHz). Beyond that threshold, system's accuracy saturates to about one-third of its original level achievable with precise GT. This effect challenges the impact of the already known accuracy-limiting factors (such as cross-user tests, receiver diversity, device orientation and temporal variations), as they can be partially explained by minor GT errors. Moreover, for smartphone-in-a-hand experiments, this effect directly associates the evaluation outcomes with experimenters' diligence.