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In this paper we present an algorithm that measures the round trip times of IEEE 802.11 MAC packets in order to trilaterate WLAN nodes.We show that if we use packet sequences (e.g., RTS, CTS, DATA and ACK) instead of packet pairs (e.g., DATA and ACK), the TOA measurements can be implemented more easily and a higher observation rate can be achieved. In addition, the receiver and any listening third node can measure the round trip times. We present the open source software ldquoGoodtryrdquo that implements this algorithm and overcomes the low clock resolution and other implementation constraints of off-the-shelf IEEE 802.11 cards. Experimental results show that our software-based trilateration has an accuracy of about four meters.