# US 7,609,207 B2 · System and method for performing time difference of arrival location without requiring a common time base or clock calibration > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 7,609,207 B2 - Document ID: US7609207B2 - Status: Granted - Kind: B2 - Application number: 12/345,706 - Filed: 2008-12-30 - Published: 2009-04-30 - Granted: 2009-10-27 - Earliest claimed: 2006-03-03 - Assignees: Cisco Technology, Inc. - Inventors: Daniel Lyons - Web page: https://patentgrep.com/patent/US7609207B2 ## CPC - G01S 5/06: Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements (G01S5/12 takes precedence) - G01S 5/021: {Calibration, monitoring or correction (G01S5/0252 takes precedence)} - G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES ## Abstract A method for performing Time Difference Of Arrival (TDOA) that eliminates the need for a common time base or clock calibration and a system for implementing the method. The method relies on a packet transmitted from a reference wireless device (first wireless device) with a known propagation delay between the first wireless device and a second device, which serves as a common reference point for all TDOA estimates. When a packet is received from a wireless device by the first wireless device and the second wireless device, the time difference of arrival is computed based on when the signal was received by the first device and the second device, using the known propagation delay to compensate for differences in clocks and frequencies between the first wireless device and the second wireless device. ## Claims ### Claim 1 A method, comprising: receiving data from a first wireless device representative of a time the first wireless device transmitted a first signal and a time the first wireless device received a second signal from a third wireless device based on a first clock associated with the first wireless device; receiving data from a second wireless device representative of a time the second wireless device received the first signal and the time the second wireless device received the second signal based on a second clock associated with the second wireless device; calculating a first time difference, the first time difference being the difference between the time the first wireless device received the second signal and the time the first wireless device transmitted the first signal; calculating a second time difference, the second time difference being the difference between the time second wireless device received the second signal and the time the second wireless device received the first signal; calculating a relative frequency difference between the first clock and the second clock; adjusting the second time difference by multiplying the second time difference by the relative frequency difference; and determining a time difference of arrival for the second signal based on a difference between the first time difference and the adjusted second time difference. ### Claim 2 (depends on 1) The method set forth in claim 1, wherein the first wireless device is an access point and the second wireless device is an access point. ### Claim 3 (depends on 2) The method set forth in claim 2, further comprising determining a propagation delay between the first access point and the access point. ### Claim 4 (depends on 3) The method set forth in claim 3, wherein the propagation delay is a measured delay. ### Claim 5 (depends on 3) The method set forth in claim 3, wherein the propagation delay is determined dynamically. ### Claim 6 (depends on 5) The method set forth in claim 5, wherein determining the propagation delay further comprises exchanging signals between the first wireless device and the second wireless device. ### Claim 7 (depends on 2) The method set forth in claim 2, wherein the second signal is transmitted before the first signal. ### Claim 8 (depends on 1) The method set forth in claim 1, further comprising determining a propagation delay between the first wireless device and the second wireless device. ### Claim 9 (depends on 8) The method set forth in claim 8, wherein the propagation delay is a measured delay. ### Claim 10 (depends on 9) The method set forth in claim 9, wherein the propagation delay is determined dynamically. ### Claim 11 (depends on 10) The method set forth in claim 10, wherein determining the propagation delay further comprises exchanging signals between the first wireless device and the second wireless device. ### Claim 12 (depends on 1) The method set forth in claim 1, wherein the second signal is transmitted before the first signal. ### Claim 13 An apparatus, comprising: means for receiving data from a first wireless device representative of a time the first wireless device transmitted a first signal and a time the first wireless device received a second signal from a third wireless device based on a first clock associated with the first wireless device; means for receiving data from a second wireless device representative of a time the second wireless device received the first signal and the time the second wireless device received the second signal based on a second clock associated with the second wireless device; means for calculating a first time difference, the first time difference being the difference between the time the first wireless device received the second signal and the time the first wireless device transmitted the first signal; means for calculating a second time difference, the second time difference being the difference between the time second wireless device received the second signal and the time the second wireless device received the first signal; means for calculating a relative frequency difference between the first clock and the second clock; means for adjusting the second time difference by multiplying the second time difference by the relative frequency difference; and means for determining a time difference of arrival for the second signal based on a difference between the first time difference and the adjusted second time difference. ### Claim 14 (depends on 13) The apparatus of claim 13, wherein the first wireless device is an access point and the second wireless device is an access point. ### Claim 15 (depends on 13) The apparatus of claim 13, wherein the first signal is transmitted before the second signal. ### Claim 16 (depends on 13) The apparatus of claim 13, further comprising means for determining a propagation delay between the first wireless device and the second wireless device. ### Claim 17 (depends on 16) The apparatus of claim 16, wherein the means for determining a propagation delay further comprises means for measuring the propagation delay. ### Claim 18 (depends on 16) The apparatus of claim 16, wherein the means for determining a propagation delay further comprises mean for determining the propagation delay dynamically. ### Claim 19 A method, comprising: receiving a first signal from a first wireless device by a second wireless device at a first time according to a clock at the second wireless device; calculating a computed transmit time for the first signal, calculating the computed transmit time comprises subtracting a known propagation delay between the first wireless device and the second wireless device; obtaining a second transmit time for the first signal, the second transmit time corresponding to a transmit time according to a clock in the first wireless device; determining a time adjustment factor to calibrate a clock difference between the first wireless device and the second wireless device, the determining a time adjustment factor comprises subtracting the computed transmit time from the second transmit time; determining a first received time for a second signal, the first received time for the second signal is a time that the second signal sent by a third wireless device was received by the first wireless device; determining a second received time for the second signal, the second received time for the second signal is a time that the second signal is received by the second wireless device; calculating an adjusted second received time for the second signal, the adjusted second received time for the second signal is determined by subtracting the time adjustment factor from the second time; and determining a time difference of arrival for the second signal, the time difference of arrival is the difference between the first received time for the second signal and the adjusted second received time for the second signal.