The ability to measure location using time of flight in 802.11 networks is impeded by the standard one microsecond clock resolution, imprecise synchronization of the 802.11 protocol, and the inaccuracy of available clock oscillators. We demonstrate a technique for off-the-shelf 802.11 hardware that enables accurate determination of location of transmitting 802.11 devices using time difference of arrival (TDOA). The technique refines the PinPoint clock model for 802.11 wireless cards, enabling accurate translation of times from one frame of reference to another for free-running clocks and producing accurate location of transmitting nodes to within 3m. This technique can locate nodes regardless of their participation in the location system and may be applied to other wireless communication protocols where send and receive timestamps are available.