Combination of multiple baselines for location estimation

#The present invention relates generally to methods and apparatus for locating wireless devices, also called mobile stations (MS), such as those used in analog or digital cellular systems, personal communications systems (PCS), enhanced specialized mobile radios (ESMRs), and other types of wireless communications systems. More particularly, but not exclusively, the present invention relates to a method for increasing the accuracy of location estimates produced by a network-based wireless location system

#Early work relating to network-based Wireless Location Systems (WLS) is described in U.S. Pat. No. 4,728,959; “Direction Finding Localization System” (issued Mar. 1, 1998) which discloses a system for locating cellular telephones using angle of arrival (AOA) techniques and U.S. Pat. No. 5,327,144, (Issued Jul. 5, 1994) “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the '144 patent are disclosed in U.S. Pat. No. 5,608,410, (Issued Mar. 4, 1997), “System for Locating a Source of Bursty Transmissions”. Location estimation techniques for wide-band wireless communications systems were further developed in U.S. Pat. No. 6,047,192 (Issued Apr. 4, 200), “Robust, Efficient Localization System”.

#All of these patents are assigned to TruePosition, Inc., the assignee of the present invention. TruePosition has continued to develop significant enhancements to the original inventive concepts. First commercially deployed in 1998, overlay network-based wireless location systems have been widely deployed in support of location-based services including emergency services location. The use of multiple location calculations for a single location estimate was previously explored in U.S. Pat. No. 6,097,336; “Method for improving the accuracy of a wireless location system”, by Stilp, Issued Aug. 1, 2000; U.S. Pat. No. 6,334,059; “Modified transmission method for improving accuracy for e-911 calls” by Stilp et al. Issued Dec. 25, 2001; U.S. Pat. No. 6,603,428; “Multiple pass location processing”; by Stilp, issued Aug. 5, 2003; and U.S. Pat. No. 6,873,290; “Multiple pass location processor”, by Anderson et al., issued Mar. 29, 2005.

#The foregoing summary as well as the following detailed description are better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings exemplary constructions of the invention; however, the invention is not limited to the specific methods and instrumentalities disclosed. In the drawings:

#FIG. 1 depicts an exemplary multiple measurement collection using a network-based wireless location system integrated with the wireless communications system;

#FIGS. 2a, 2b, and 2c illustrate the multiple measurement collection in a multipath radio frequency environment;

#We will now describe illustrative embodiments of the present invention. First, we provide a detailed overview of the problem and then a more detailed description of our solutions.

#In radio communication networks, transmission diversity is used to mitigate the effects of multi-path and reflection signals that arise in Non-Line-of-Sight (NLOS) environment. In a network-based wireless location system with multiple, geographically distributed receivers, transmission diversity is used to determine the time of arrival (TOA) or time-difference-of-arrival (TDOA). Using the available uplink radio signals only, the wireless location system takes a series of signal measurements and does not rely on re-broadcasting of radio signals or multiple simultaneous broadcast transmission antennas as is typical in transmission diversity schemes for radio communications. To achieve transmission diversity for use in a wireless location system, a series of positioning measurements may be executed in order to find a single device's geographical position. When multiple UTDOA measurements exist, they may be combined to eliminate redundancy and improve accuracy. Use of the velocity (obtained from the Frequency-Difference-of-Arrival (FDOA) calculations, historical data or predictions based on the local area) allows priority scheduling and use of multiple signal timing measures from the same transmitter without undue accuracy degradation.

#Location Request to Location Report latency is a critical quality of service parameter for location-based services such as emergency services calls (e.g. (9-1-1, 9-9-9, 1-1-2). A time requirement on the initial position estimate prevents the use of longer signal integration lengths in correlation processing which would otherwise improve location accuracy in a more ideal way by obtaining more accurate individual uplink-Time-Difference-of-Arrival (UTDOA) measurements. U.S. Pat. No. 7,924,224 “Variable coherence integration for the location of weak signals”, Lefever et al, assigned to TruePosition Inc. teaches one such method for increasing accuracy without delaying the location report.