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In this paper we present a novel approach for high precision clock synchronization in wireless systems. A concept similar to the standard FMCW (Frequency-Modulated Continuous Wave) radar principle is used to estimate the offset in time and in frequency between two wireless communication units. The novel approach allows for a synchronization of both offsets significantly below 100 ps and 10 Hz, respectively. This highly accurate synchronization is used in a prototype system to measure the distance between wireless units similar to a secondary radar. The prototype works within the 5.8 GHz ISM-Band and uses a bandwidth of 150 MHz. With the setup presented we can measure the distance between two radio units with a standard deviation of 4 to 5 cm over a range of 200 m. This distance deviation implies a clock and center frequency synchronization of both communication modules significantly below 100 ps and 1 ppb, respectively. The broadband measurement principle is robust towards multi-path interference. It can be extended to other frequency bands and is well-suited for direct integration into communication channels and novel modulation principles such as OFDM (Orthogonal Frequency Division Multiplexing) or SC/FDE (Single Carrier transmission with Frequency Domain Equalization), e.g. for locatable WLAN (Wireless Local Area Network) devices or wireless sensor networks.

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