  This paper presents an adaptive anchor pairs selection algorithm for UWB
(ultra-wideband) TDOA-based (Time Difference of Arrival) indoor positioning
systems. The method assumes dividing the system operation area into zones. The
most favorable anchor pairs are selected by minimizing the positioning errors
in doorways leading to these zones where possible users' locations are limited
to small, narrow areas. The sets are determined separately for going in and out
of the zone to take users' body shadowing into account. The determined anchor
pairs are then used to calculate TDOA values and localize the user moving
around the apartment with an Extended Kalman Filter based algorithm. The method
was tested experimentally in a furnished apartment. The results have shown that
the adaptive selection of the anchor pairs leads to an increase in the user's
localization accuracy. The median trajectory error was about 0.32 m.
