Indoor localization has become increasingly important for sports analysis, automation, and for mass market products like entertainment systems. For such applications, an increasing accuracy of a few centimeters or even millimeters is desired, but it is a huge challenge to develop a positioning system of that high accuracy in an indoor environment with severe multipath characteristics. A high precision real-time indoor positioning system was designed, implemented, and tested, which is capable of achieving a position accuracy in subcentimeter range. The proposed system implements the positioning based on time difference of arrival technology with the usage of <sc>on-off</sc> keying modulated ultrahigh frequency signals in order to deal with multipath interference and achieve high positioning precision. This paper describes the design and evaluation of the demonstrator system. The first measurements are performed within a fully furnished laboratory environment and already prove a positioning precision of a radio frequency source with a root-mean-square error of 8 mm. Moreover, the low latency of less than 2 ms and the high update rate of 100 Hz make the system suitable for real-time applications.