Abstract:
A method and system that account for one or more propagation-time components in a transmission path between a base station and a wireless terminal in a coverage area being served by the system. One such component is in the base station equipment between the radio that serves a wireless terminal and the antenna element that radiates and/or receives electromagnetic signals that involve the terminal. Another component of the transmission path is the one or more paths over which a radiated signal travels between the base station antenna element and the wireless terminal. By accounting for these propagation components through the use of measurement data provided by possibly a large number of wireless terminals, a disclosed location engine is able to derive adjusted measurements that are more representative of the propagation-time characteristic being measured by the individual terminals. These adjusted measurements can then be used to estimate the location of a wireless terminal more accurately.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application Ser. No. 14/694,151, filed on Apr. 23, 2015, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to telecommunications in general, and, more particularly, to a technique for determining an estimate of the location of a wireless terminal that accounts for one or more propagation-time components of a transmission path between a base station equipment component and the same or a different wireless terminal. 
     BACKGROUND OF THE INVENTION 
     The salient advantage of wireless telecommunications over wireline telecommunications is that the user of the wireless terminal is afforded the opportunity to use his or her terminal anywhere. On the other hand, the salient disadvantage of wireless telecommunications lies in that fact that because the user is mobile, an interested party might not be able to readily ascertain the location of the user. 
     Such interested parties might include both the user of the wireless terminal and a remote party. There are a variety of reasons why the user of a wireless terminal might be interested in knowing his or her location. For example, the user might be interested in telling a remote party where he or she is or, alternatively, the user might seek advice in navigation. 
     In addition, there are a variety of reasons why a remote party might be interested in knowing the location of the user. For example, the recipient of an E 9-1-1 emergency call from a wireless terminal might be interested in knowing the location of the wireless terminal so that emergency services vehicles can be dispatched to that location. 
     There are many techniques in the prior art for estimating the location of a wireless terminal. In accordance with some techniques, the location of a wireless terminal is estimated, at least in part, from measurements that are reported by the wireless terminal. The reported measurements are of signals measured by the wireless terminal that are transmitted by one or more base stations and, in some cases, by Global Positioning System (GPS) satellites. Some techniques rely on signal-strength measurements, while some other techniques rely on time-based measurements, while still some other techniques rely on other types of measurements. In order for these estimation techniques to work, at least some of the transmitted signals have to be strong enough to allow for accurate measurement by the wireless terminal and for reliable processing by the particular technique. 
     In some operating scenarios, conditions exist that might be insufficiently or incorrectly characterized by some entity—for example, by the service provider that controls the infrastructure providing service to the wireless terminal. One such condition is propagation time, in which the base station or wireless terminal measures the round-trip time (RTT), or equivalent, of the signal being measured. Here, the service provider might attempt to correct for the component of the propagation time attributed to the base station equipment, by subtracting off the electrical delay of the equipment from the RTT measurement prior to providing the measurement to an application that uses it to locate the wireless terminal. This type of error, as well as other errors, can impair the performance of at least some location estimation techniques in the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention enables a telecommunications system to account for one or more propagation-time components in a transmission path between a base station and a wireless terminal in a coverage area being served by the system. One such component is in the base station equipment between the radio that serves a wireless terminal and the antenna element that radiates and/or receives electromagnetic signals that involve the terminal. Another component of the transmission path is the one or more paths over which a radiated signal travels between the base station antenna element and the wireless terminal. By accounting for these propagation components through the use of measurement data provided by possibly a large number of wireless terminals, a location engine disclosed herein is able to derive adjusted measurements that are more representative of the propagation-time characteristic being measured by the individual terminals. These adjusted measurements can then be used to estimate the location of a wireless terminal more accurately. 
     In accordance with the illustrative embodiment of the present invention, the location engine, implemented on a server computer or other computing device, receives propagation-time measurements of signals between a base station and a wireless terminal in a coverage area being served by the base station, for one or more base stations and/or wireless terminals. For example and without limitation, a propagation-time measurement can be the round-trip time (RTT) measurement made and reported by wireless terminals in certain third-generation (3G) cellular networks. The location engine estimates the location of the wireless terminal at the location that corresponds to where the propagation-time measurement was made, but without using the measurement in the estimate. In other words, the location is estimated based on evidence that is independent of the propagation-time measurement received. In estimating the location in this way, the location engine establishes a “ground truth” against which the propagation-time measurement can be referenced. The location engine then estimates the spatial displacement between the location of the base station and the estimated location of the wireless terminal. 
     The location engine builds and maintains a data set over time, wherein the data set is made up of comparison values between i) each estimated spatial displacement and ii) the corresponding propagation-time measurement. Each comparison value can be based on the difference between the spatial displacement and propagation-time measurement, provided that one or both of these values are normalized to the other, in terms of units of measure (e.g., time versus distance) and physical condition being represented (e.g., one-way versus round-trip propagation). When the data set is sufficiently large, the location engine generates a statistic of the data set. In at least some embodiments of the present invention, the statistic summarizes i) a measure of location with the data set (e.g., mean, median, predetermined percentile, etc.), ii) a measure of statistical dispersion within the data set (e.g., standard deviation, range, etc.), or iii) a measure of the shape of the distribution of the data set (e.g., skewness, etc.), for example and without limitation. 
     The location engine then estimates the location of one or more wireless terminals based on the generated statistic. For example and without limitation, the location engine can use the statistic in order to account for one or more components in the received propagation-time measurements by adjusting the measurements accordingly; then, the location engine can use the adjusted measurements in performing a location estimation technique that relies on the measurements. The wireless terminals whose locations are estimated by using the propagation-time measurements can be different from the wireless terminals whose locations are estimated using the independent technique described earlier. 
     The location engine can use many data points that are provided by each wireless terminal, provided by potentially many wireless terminals, and related to electromagnetic signal transmissions that involve potentially many base stations, thereby leveraging a crowdsourced effect. By using a sufficient amount of data from a sufficient number of sources, the location engine of the illustrative embodiment can compensate for one or more sources of error, including but not limited to:
         a. the electrical delay introduced by the base station equipment.   b. any service provider error created in attempting to compensate for the foregoing equipment delay.   c. multipath delay, in which signals between a base station and a wireless terminal travel over one or more indirect paths, often making the base station and the wireless terminal seem farther apart than they physically are.   d. quantization effects in a wireless terminal or base station in measuring and reporting RTT or equivalent.       

     An illustrative method comprises: receiving, by a server computer: i) a first propagation-time measurement of a first signal in a transmission between a first wireless terminal and a base station, ii) a second propagation-time measurement of a second signal in a transmission that involves the base station, and iii) evidence of the location of the first wireless terminal; estimating, by the server computer, the location of the first wireless terminal based on the evidence of the location of the first wireless terminal; estimating, by the server computer, a first spatial displacement between the first wireless terminal and the base station, based on the estimated location of the first wireless terminal; generating, by the server computer, a statistic by applying a corresponding, predetermined statistical algorithm to a data set, wherein a first value in the data set is based on i) the first spatial displacement and ii) the first propagation-time measurement, and wherein a second value in the data set is based on the second propagation-time measurement; estimating, by the server computer, the location of a second wireless terminal based on the statistic, resulting in a location estimate; and transmitting, by the server computer, the location estimate to a location-based application. 
     Another illustrative method comprises receiving, by a server computer: i) a first propagation-time measurement of a signal transmitted between a first wireless terminal and a base station, ii) evidence of the location of the first wireless terminal, iii) a second propagation-time measurement of a signal transmitted between a second wireless terminal and the base station, and iv) evidence of the location of the second wireless terminal; estimating, by the server computer: i) the location of the first wireless terminal based on the evidence of the location of the first wireless terminal, and ii) the location of the second wireless terminal based on the evidence of the location of the second wireless terminal; estimating, by the server computer: i) a first spatial displacement between the first wireless terminal and the base station, based on the estimated location of the first wireless terminal, and ii) a second spatial displacement between the second wireless terminal and the base station, based on the estimated location of the second wireless terminal; generating, by the server computer, a statistic by applying a corresponding, predetermined statistical algorithm to a data set, wherein a first value in the data set is based on i) the first spatial displacement and ii) the first propagation-time measurement, and wherein a second value in the data set is based on i) the second spatial displacement and ii) the second propagation-time measurement; estimating, by the server computer, the location of a third wireless terminal based on the statistic, resulting in a location estimate; and transmitting, by the server computer, the location estimate to a location-based application. 
     Yet another illustrative method comprises: receiving, by a server computer: i) a first propagation-time measurement of a signal transmitted between a first wireless terminal and a base station, ii) evidence of the location of the first wireless terminal, iii) a second propagation-time measurement of a signal transmitted between a second wireless terminal and the base station, and iv) evidence of the location of the second wireless terminal; estimating, by the server computer: i) a first spatial displacement between the first wireless terminal and the base station, based on the evidence of the location of the first wireless terminal, and ii) a second spatial displacement between the second wireless terminal and the base station, based on the evidence of the location of the second wireless terminal; generating, by the server computer, a statistic by applying a corresponding, predetermined statistical algorithm to a data set, wherein a first value in the data set is based on i) the first spatial displacement and ii) the first propagation-time measurement, and wherein a second value in the data set is based on i) the second spatial displacement and ii) the second propagation-time measurement, and wherein the statistic is based on a multipath characteristic of a wireless coverage area serviced by the base station; estimating, by the server computer, the location of a third wireless terminal based on the statistic, resulting in a location estimate; and transmitting, by the server computer, the location estimate to a location-based application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a diagram of the salient components of wireless telecommunications system  100  in accordance with the illustrative embodiment of the present invention. 
         FIG. 2  depicts a diagram of the salient components of wireless telecommunications system  100  that provide telecommunications service to at least some of geographic region  220  or that operate within geographic area  220 . 
         FIG. 3  depicts a diagram of the salient components of cellular base station  102 - 1 , in communication with wireless terminal  101 - 1 . 
         FIG. 4  depicts a block diagram of the salient components of location engine  113  in accordance with the illustrative embodiment. 
         FIG. 5  depicts a flowchart of the salient processes performed in accordance with the illustrative embodiment of the present invention. 
         FIG. 6  depicts a flowchart of the salient processes performed in accordance with task  507 . 
         FIG. 7  depicts a probability distribution  700  of data set  701 , developed as a histogram and generated at task  601 . 
         FIG. 8  depicts a flowchart of the salient processes performed in characterizing multipath. 
         FIG. 9  depicts a flowchart of the salient processes performed in estimating the location of a base station. 
     
    
    
     DETAILED DESCRIPTION 
     Based on—For the purposes of this specification, the phrase “based on” is defined as “being dependent on” in contrast to “being independent of”. The value of Y is dependent on the value of X when the value of Y is different for two or more values of X. The value of Y is independent of the value of X when the value of Y is the same for all values of X. Being “based on” includes both functions and relations. 
     Estimate—For the purposes of this specification, the infinitive “to estimate” and its inflected forms (e.g., “estimating”, “estimated”, etc.) should be given the ordinary and customary meaning that the terms would have to a person of ordinary skill in the art at the time of the invention. 
     Generate—For the purposes of this specification, the infinitive “to generate” and its inflected forms (e.g., “generating”, “generation”, etc.) should be given the ordinary and customary meaning that the terms would have to a person of ordinary skill in the art at the time of the invention. 
     Location—For the purposes of this specification, the term “location” is defined as a zero-dimensional point, a finite one-dimensional path segment, a finite two-dimensional surface area, or a finite three-dimensional volume. 
     Processor—For the purposes of this specification, a “processor” is defined as hardware or hardware and software that perform mathematical and/or logical operations. 
     Propagation time—For the purposes of this specification, “propagation time” is defined as the length of time it takes for a signal to move along a transmission path. A measurement related to propagation time can be time-based; timing-based; delay-based; based on a difference in time, timing, or delay; or based on some combination thereof. 
     Spatial displacement—For the purposes of this specification, the term “spatial displacement” is defined as the distance along a straight line between two points in space. 
     Statistic—For the purposes of this specification, the term “statistic” is defined as a single measure of some attribute of a sample, calculated by applying a statistical algorithm to the values of the items of the sample, which are known together as a data set. A “descriptive statistic” can be used to describe the data in a data set. 
     Radio—For the purposes of this specification, a “radio” is defined as hardware or hardware and software that is capable of telecommunications via an unguided (i.e., wireless) radio signal of frequency less than 600 GHz. 
     Receive—For the purposes of this specification, the infinitive “to receive” and its inflected forms (e.g., “receiving”, “received”, etc.) should be given the ordinary and customary meaning that the terms would have to a person of ordinary skill in the art at the time of the invention. 
     Transmit—For the purposes of this specification, the infinitive “to transmit” and its inflected forms (e.g., “transmitting”, “transmitted”, etc.) should be given the ordinary and customary meaning that the terms would have to a person of ordinary skill in the art at the time of the invention. 
     Wireless coverage area—For the purposes of this specification, the term “wireless coverage area” is defined as the geographic area within which a carrier or a set of equipment, or both, provides wireless service. 
     Wireless terminal—For the purposes of this specification, the term “wireless terminal” is defined as a device that is capable of telecommunications without a wire or tangible medium. A wireless terminal can be mobile or immobile. A wireless terminal can transmit or receive, or transmit and receive. As is well known to those skilled in the art, a wireless terminal is also commonly called a cell phone, a pager, a wireless transmit/receive unit (WTRU), a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, and any other type of device capable of operating in a wireless environment are examples of wireless terminals. 
       FIG. 1  depicts a diagram of the salient components of wireless telecommunications system  100  in accordance with the illustrative embodiment of the present invention. Wireless telecommunications system  100  comprises: wireless terminal  101 - 1 , cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3 , Wi-Fi base stations  103 - 1  and  103 - 2 , wireless infrastructure  111 , location-based application server  112 , location engine  113 , and GPS constellation  121 , interrelated as shown. 
     Wireless infrastructure  111 , location-based application server  112 , location engine  113 , and Wi-Fi base stations  103 - 1  and  103 - 2  are all connected to one or more interconnected computer networks (e.g., the Internet, a local-area network, a wide-area network, etc.) and, as such, can exchange data in well-known fashion. 
     Although the illustrative embodiment depicts wireless telecommunications system  100  as comprising only one wireless terminal, it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention that comprise any number of wireless terminals  101 - 1  through  101 -M, wherein M is a positive integer. 
     Wireless terminal  101 - 1  comprises the hardware and software necessary to perform the processes described below and in the accompanying figures. Furthermore, wireless terminal  101 - 1  is mobile and can be at any location within geographic region  120  at any time. 
     Wireless terminal  101 - 1  is capable of providing bi-directional voice, data, and video telecommunications service to a user (not shown), but it will be clear to those skilled in the art, after reading this disclosure, how to make and use embodiments of the present invention in which wireless terminal  101 - 1  provides a different set of services. 
     In accordance with the illustrative embodiment, wireless terminal  101 - 1  is capable of transmitting one or more radio signals—that can be received by one or more of base stations  102 - 1 ,  102 - 2 , and  102 - 3  and Wi-Fi base stations  103 - 1  and  103 - 2 —in accordance with specific parameters (e.g., signal strength, frequency, coding, modulation, timing offset, etc.), in well-known fashion, and of transmitting at least some of those parameters to location engine  113  as well as other information described below. Additionally, wireless terminal  101 - 1  is capable of receiving one or more radio signals from each of base stations  102 - 1 ,  102 - 2 , and  102 - 3 , Wi-Fi base stations  103 - 1  and  103 - 2 , and GPS constellation  121 , in well-known fashion. 
     Wireless terminal  101 - 1  is also capable of identifying each radio signal it receives, in well-known fashion, and of transmitting the identity of each signal it receives to location engine  113 . Wireless terminal  101 - 1  is further capable of measuring one or more location-dependent traits of each radio signal it receives, in well-known fashion, and of transmitting each measurement it generates to location engine  113 . 
     Cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3  communicate with wireless infrastructure  111  via wireline and with wireless terminal  101 - 1  via radio in well-known fashion. As is well known to those skilled in the art, base stations are also commonly referred to by a variety of alternative names such as access points, nodes, network interfaces, etc. Although the illustrative embodiment comprises three cellular base stations, it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention that comprise any number of cellular base stations. 
     In accordance with the illustrative embodiment of the present invention, cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3  are terrestrial, immobile, and base station  102 - 3  is within geographic region  120 . It will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention in which some or all of the base stations are airborne, marine-based, or space-based, regardless of whether or not they are moving relative to the Earth&#39;s surface, and regardless of whether or not they are within geographic region  120 . 
     Cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3  comprise the hardware and software necessary to be 3GPP-compliant and to perform the processes described below and in the accompanying figures. For example and without limitation, each of cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3  are capable of continually:
         a. receiving one or more radio signals transmitted by wireless terminal  101 - 1 , and   b. identifying each radio signal transmitted by wireless terminal  101 - 1 , in well-known fashion, and of transmitting the identity of those signals to location engine  113 , and   c. measuring one or more location-dependent traits of each radio signal transmitted by wireless terminal  101 - 1 , in well-known fashion, and of transmitting the measurements to location engine  113 , and   d. transmitting one or more signals to wireless terminal  101 - 1  in accordance with specific parameters (e.g., signal strength, frequency, coding, modulation, etc.), in well-known fashion, and of transmitting those parameters to location engine  113 .
 
It will be clear to those skilled in the art how to make and use cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3 .
       

     Wi-Fi base stations  103 - 1  and  103 - 2  communicate with wireless terminal  101 - 1  via radio in well-known fashion. Wi-Fi base stations  103 - 1  and  103 - 2  are terrestrial, immobile, and within geographic region  120 . Although the illustrative embodiment comprises two Wi-Fi base stations, it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention that comprise any number of Wi-Fi base stations. 
     Each of Wi-Fi base stations  103 - 1  and  103 - 2  are capable of continually:
         a. receiving one or more radio signals transmitted by wireless terminal  101 - 1 , and   b. identifying each radio signal transmitted by wireless terminal  101 - 1 , in well-known fashion, and of transmitting the identity of those signals to location engine  113 , and   c. measuring one or more location-dependent traits of each radio signal transmitted by wireless terminal  101 - 1 , in well-known fashion, and of transmitting the measurements to location engine  113 , and   d. transmitting one or more signals to wireless terminal  101 - 1  in accordance with specific parameters (e.g., signal strength, frequency, coding, modulation, etc.), in well-known fashion, and of transmitting those parameters to location engine  113 .       

     It will be clear to those skilled in the art how to make and use Wi-Fi base stations  103 - 1  and  103 - 2 . 
     Wireless infrastructure  111  comprises a switch that orchestrates the provisioning of telecommunications service to wireless terminal  101 - 1  and the flow of information to and from location engine  113 , as described below and in the accompanying figures. As is well known to those skilled in the art, wireless switches are also commonly referred to by other names such as mobile switching centers, mobile telephone switching offices, routers, etc. 
     Location-based application server  112  comprises hardware and software that uses the estimate of the location of wireless terminal  101 - 1 —generated by location engine  113 —in one or more location-based applications, in well-known fashion. Location-based applications are well-known in the art and provide services such as, and without limitation, E-911 routing, navigation, location-based advertising, and weather alerts. 
     Location engine  113  is a data processing system that comprises hardware and software that generates one or more estimates of the location of wireless terminal  101 - 1  as described below and in the accompanying figures. It will be clear to those skilled in the art, after reading this disclosure, how to make and use location engine  113 . Furthermore, although location engine  113  is depicted in  FIG. 1  as physically distinct from wireless infrastructure  111 , it will be clear to those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention in which location engine  113  is wholly or partially integrated into wireless infrastructure  111 . 
       FIG. 2  depicts a diagram of the salient components of wireless telecommunications system  100  that provide telecommunications service to at least some of geographic region  220  or that operate within geographic area  220 . In particular, wireless terminals  101 - 1  through  101 -M (wherein M as depicted is equal to 7) operate within area  220 , and at least cellular base station  102 - 1 , wireless infrastructure  111 , location-based application server  112 , and location engine  113  provide service to the wireless terminals and are interrelated as shown. 
     Some are all of wireless terminals  101 - 1  through  101 -M are in communication with base station  102 - 1  at any given moment in time. Additionally, some or all of wireless terminals  101 - 1  through  101 -M can also be in communication with one or more base stations in addition to base station  102 - 1 . 
     As discussed above, wireless terminal  101 - m , wherein m can have a value of 1 through M, is further capable of measuring one or more location-dependent traits of each radio signal it receives, in well-known fashion, and of transmitting each measurement it generates to location engine  113 . At least some of the location-dependent traits are related to propagation time, and, in particular, propagation delay in some cases. Some propagation-time-related measurements that can be provided by terminal  101 - m  are as follows, for example and without limitation:
         a. the round-trip time (RTT) or round-trip delay time (RTD) of all of the signals transmitted and receivable by wireless terminal  101 - m  through one or more of the base stations.   b. the time advance (TA) of all of the signals transmitted and receivable by wireless terminal  101 - m  through one or more of the base stations.   c. the received temporal difference of each pair of multipath components (e.g., one temporal difference for one pair of multipath components, a pair of temporal differences for a triplet of multipath components, etc.) of all of the signals receivable by wireless terminal  101 - m  from one or more transmitters.   d. the received delay spread (e.g., RMS delay spread, excess delay spread, mean excess delay spread, etc.) of all of the signals receivable by wireless terminal  101 - m.      e. the received relative arrival times of two or more multipath components of all of the signals receivable by wireless terminal  101 - m , from one or more transmitters (which can be determined by a rake receiver in well-known fashion).       

     Cellular base station  102 - 1 , as well as other base stations within system  100 , is further capable of measuring one or more location-dependent traits of each radio signal it receives from one or more wireless terminals, in well-known fashion, and of transmitting each measurement it generates to location engine  113 . At least some of the location-dependent traits are related to propagation time, and, in particular, propagation delay in some cases. Some propagation-time-related measurements provided by base station  102 - 1  are the same as those listed above, for example and without limitation, except that the signal propagation directions are reversed. 
     By accumulating the aforementioned measurements that are received from one or more of the wireless terminals or base stations, or both, location engine  113  is capable of performing the tasks described below. 
       FIG. 3  depicts a diagram of the salient components of cellular base station  102 - 1 , in communication with wireless terminal  101 - 1  via a transmission path or paths comprising one or more propagation components. Cellular base station  102 - 1  comprises: one or more antenna elements  301  and base station processing equipment  302 , which comprises one or more radios  303 . Signal path  304  between antenna element  301  and radio  303 , or between element  301  and a different base station equipment component, is characterized by a first propagation delay component that is attributed to the type and length of transmission medium used (e.g., cable, etc.). 
     Additionally, there are one or more signal paths taken by a signal transmitted between antenna element  301  and wireless terminal  101 - 1 . Signal path  305 , which is a direct path, is characterized by a second propagation delay component; signal path  306 , which is an indirect path due to reflection off of building  311 , is characterized by a second propagation delay component; and signal path  307 , which is an indirect path due to reflection off of mountain  312 , is characterized by a third propagation delay component. As those who can appreciate after reading this specification, other signal paths can occur based on reflection from other terrestrial objects and from bodies of water, and on phenomena other than reflection. When radio signals reach a receiving antenna by two or more signal paths, multipath is said to occur. 
     Wireless terminal  101 - 1  and/or base station  102 - 1  are capable of making and providing (e.g., to location engine  113 , etc.) propagation-time measurements, in which the measurements reflect at least some of the propagation delay components described above. 
     Location engine  113 — FIG. 4  depicts a block diagram of the salient components of location engine  113  in accordance with the illustrative embodiment. Location engine  113  comprises: processor  401 , memory  402 , and receiver and transmitter  403 , which are interconnected as shown. In accordance with the illustrative embodiment of the present invention, location engine  113  is a server computer. As those who are skilled in the art will appreciate after reading this specification, however, location engine  113  can be a different type of data-processing system or computing device. 
     Processor  401  is a general-purpose processor that is configured to execute an operating system and the application software that performs the operations described herein, including the operations described in  FIG. 5  and other figures. Processor  401  is also capable of populating, amending, using, and managing propagation-time measurements, data sets based on the measurements, statistics of each data set, and so on. It will be clear to those skilled in the art how to make and use processor  401 . 
     Memory  402  is a non-volatile memory that is configured to store:
         a. operating system  411 , and   b. application software  412 , and   c. database  413  for storing one or more data sets as described below.
 
It will be clear to those skilled in the art how to make and use memory  402 .
       

     Receiver and transmitter  403  is configured to enable location engine  113  to receive from and transmit to wireless terminal  101 - m , wireless infrastructure  111 , location-based application server  112 , and the base stations (i.e., cellular and WiFi), in well-known fashion. It will be clear to those skilled in the art how to make and use receiver and transmitter  403 . 
     Operation of the Illustrative Embodiment— FIG. 5  depicts a flowchart of the salient processes performed in accordance with the illustrative embodiment of the present invention. 
     The processes performed by wireless telecommunications system  100  of the illustrative embodiment are depicted in the drawings (i.e.,  FIG. 5  and subsequent figures) as being performed in a particular order. It will, however, be clear to those skilled in the art, after reading this disclosure, that such operations can be performed in a different order than depicted or can be performed in a non-sequential order (e.g., in parallel, etc.). In some embodiments of the present invention, some or all of the depicted processes might be combined or performed by different devices, either within location engine  113  or other than location engine  113 . In some embodiments of the present invention, some of the depicted processes might be omitted. 
     For purposes of clarity, wireless terminal  101 - 1  and cellular base station  102 - 1  are used as examples of a wireless terminal and base station. However, as those who are skilled in the art will appreciate after reading this specification, the tasks described below are applicable to other wireless terminals and other base stations (e.g., WiFi, etc.) as well. 
     At task  501 , location engine  113  receives one or more propagation-time measurements (e.g., round-trip time, etc.), wherein each measurement is that of a signal in a transmission between wireless terminal  101 - 1  and base station  102 - 1 . Measurements can be received for signals between multiple wireless terminals and a given base station, for signals between a given wireless terminal and multiple base stations, and for signals across multiple paths between each wireless terminal and base station, in any combination thereof. The measurements can be representative of signals from a base station to a wireless terminal, or from a wireless terminal to a base station, or both. In some embodiments of the present invention, a propagation-time measurement can be received in response to location engine  113  transmitting a mobile-terminated location request (MTLR) message, or equivalent. 
     The propagation-time measurements actually received by location engine  113  are based on the propagation-time-related measurements provided by terminal  101 - 1  as described above and in  FIG. 2 . In some embodiments of the present invention, one or more of the propagation-time measurements received by location engine  113  are further based on a predetermined constant. For example and without limitation, a wireless service provider in control of system  100  might choose to adjust (i.e., by a “fudge factor”) one or more of the measurements provided by wireless terminal  101 - 1 , in order to compensate for known signal paths within the equipment itself, such as signal path  304  that is characterized by a first propagation delay component. In this example, the service provider might attempt to correct by subtracting out the delay effects introduced by signal path  304 , in order to obtain a measurement that is more representative of one or more signal paths between antenna element  301  and wireless terminal  101 - 1 , instead of between radio  303  and the wireless terminal. 
     Location engine  113  also receives evidence of the location of one or more wireless terminals, such as terminal  101 - 1 . Evidence of a location is data to which a location estimation algorithm can be applied in order to generate an estimated location (e.g., a geographic location, etc.). For example and without limitation, evidence of the location can comprise a signal-strength measurement, a time-related measurement, or information that, by itself, is not a representation of the geographic location of a wireless terminal, estimated or otherwise, but that is probative of the geographic location. In some alternative embodiments of the present invention, the evidence of a location can comprise a relatively coarse location, whereas the estimated location generated from the evidence can be a relatively fine location. The evidence of the location can be different from and independent of the propagation-time measurements, while concurrently the location to which the evidence applies can be coincident with the location at which and/or time interval during which the propagation-time characteristic was measured and/or reported. 
     Location engine  113  also receives evidence of the location of one or more of the base stations, such as base station  102 - 1 . In some embodiments, location engine  113  receives a geographic location of one or more of the base stations, in which the location or locations have been confirmed to a known degree of accuracy. 
     At task  503 , location engine  113  estimates the geographic location of wireless terminal  101 - 1  based on the received evidence of the location of terminal  101 - 1 , thereby establishing a “ground truth” for the location of the terminal. Engine  113  can estimate the location of other wireless terminals as well, thereby also establishing ground truths for those terminals. There are various techniques that can be used to estimate the location of wireless terminal  101 - 1  based on the received evidence. See for example and without limitation, U.S. Pat. Nos. 6,944,465, 7,460,505, 7,383,051, 7,257,414, 7,753,278, 7,433,695, 7,848,762, and 8,630,665, each of which is incorporated by reference herein. Location engine  113 , in some embodiments of the present invention, can receive an estimate of the geographic location of wireless terminal  101 - 1  in which the estimate has been calculated elsewhere (e.g., by wireless terminal  101 - 1  itself, etc.). In some embodiments of the present invention, the uncertainties of one or more grounds truths are included as a component of the analysis represented by method  500 . 
     At task  505 , location engine  113  estimates a spatial displacement (e.g., shortest distance, etc.) between wireless terminal  101 - 1  and base station  102 - 1  based on the estimated location of terminal  101 - 1 . Engine  113  can estimate the spatial displacements between other combinations of wireless terminals and base stations as well. In some embodiments of the present invention, evidence of the location of base station  102 - 1 , the location itself of base station  102 - 1 , or the location of antenna element  301  is also used in estimating the spatial displacement. 
     At task  507 , location engine  113  generates a statistic of a data set. Task  507  is described in detail below and in  FIG. 6 . 
     At task  509 , location engine  113  estimates the location of a different wireless terminal than terminal  101 - 1 , such as terminal  101 - 2 , based on the statistic generated at task  507 . In some embodiments of the present invention, engine  113  refines the estimate of the location of wireless terminal  101 - 1  based on the statistic generated at task  507 . A location estimate of the wireless terminal is made available as a result of this task. 
     Location engine  113  can determine the location of the wireless terminal in the following manner. Once the statistic is made available at task  507 , engine  113  can use that statistic, or a second statistic based on the first statistic, to further adjust each propagation-time measurement being reported so that the propagation-time measurement can be directly used in a meaningful way to determine location. The adjusted and improved propagation-time measurement can then be directly used as part of one or more well-known techniques for location determination (e.g., OTDOA, Cell ID+RTT, etc.), in order to provide an improved location estimate compared with a location estimate obtained by using the unadjusted propagation-time measurements. 
     In some embodiments of the present invention, the location estimate is based on concurrent or simultaneous propagation-time measurements between a wireless terminal and more than one base station. For example, analysis of the correlated measurements can add to the precision of the estimate. 
     At task  511 , location engine  113  transmits the location estimate that was made available at task  509 , to a location application at application server  112 . In some embodiments of the present invention, engine  113  transmits the location estimate to a device different from server  112  or uses the location estimate for its own purposes. 
     Location engine  113  then repeats one or more of the aforementioned tasks. 
     Task  507 : Generate a Statistic— FIG. 6  depicts a flowchart of the salient processes performed in accordance with task  507 . 
     At task  601 , location engine  113  compares a first value based on the spatial displacement value estimated at task  505 , with a second value based on the propagation-time measurement received at task  501 , resulting in a comparison value. 
     In accordance with the illustrative embodiment, the comparison comprises a calculation of the difference between the first value and the second value, wherein the first and second values have been normalized or converted into comparable units of measure. For example and without limitation, the first value is obtained by converting a spatial distance measurement to a time-related measurement, based on the time it takes for a radio signal to span the one-way spatial distance. As part of this example, the second value is obtained by converting its propagation-time-related value to a time-related value consistent with that of the first value, such as by taking a round-trip-time (RTT) measurement, in chips, and converting it to a one-way-time value in nanoseconds. 
     In regard to wireless propagation components  305  through  307  in  FIG. 3 , in some embodiments of the present invention one or both of the first and second values might be adjusted in order to account for the probability of the propagation-time measurement not being representative of a direct-path radio signal, but of the measurement being influenced by an indirect-path or a multipath radio signal. Alternatively, such an adjustment for indirect-path or multipath can be performed later as described below. 
     As those who are skilled in the art will appreciate after reading this specification, the comparison described above can be performed in a different way than calculating a difference or in different units of measure, or both. 
     At task  603 , location engine  113  stores the result of the comparison between the first and second values, in memory  402 &#39;s database, in order to build a data set  701  as depicted in  FIG. 7  described below. 
     At task  605 , location engine  113  repeats aforementioned tasks  601  and  603  in order to ensure that number of values that constitute data set  701  is sufficient.  FIG. 7  depicts a probability distribution  700  of data set  701 , developed as a histogram of multiple comparison values generated at task  601 , which are being stored into memory at task  603 . Data set  701  can comprise comparison values that are representative only of a single wireless terminal/base station pair, representative of multiple wireless terminals with respect to a single base station, representative of one or more wireless terminals with respect to multiple base stations, and so on. 
     Some characteristics of data set  701  are discussed here. First, the depicted data set extends over to the left side of the y-axis. One situation in which this can occur is when the service provider has overcorrected, in the propagation-time measurement data delivered to location engine  113 , for electrical delays in the equipment (e.g., cabling, antenna amplifiers, etc.) that are present in path  304  of  FIG. 3 . Second, the depicted data set exhibits some positive skewness (i.e., skewness to the right). One situation in which this can occur is when some multipath is present in the coverage area or areas from which the data originates. 
     The comparison values that constitute data set  701  can depend on various factors. For example and without limitation, data set  701  might be developed from comparison values in which some or all of the base stations, in a predetermined group of base stations, are represented in those constituent comparison values, if one of more of the following apply:
         a. similar radio-frequency (RF) propagation conditions (e.g., multipath, etc.) are present in the coverage areas of the base stations.   b. similar base station equipment configurations (e.g., sectorization, etc.) exist.   c. similar propagation-time corrections made by the service provider are in effect.
 
On the other hand, data set  701  might instead be developed from comparison values in which only a single base station, or a limited group of similar base stations, is represented in those constituent comparison values, if one or more of the following apply:
   a. different RF propagation conditions are present.   b. different base station equipment configurations exist.   c. different corrections made by the service provider are in effect.       

     At task  605 , location engine  113  determines when a sufficient number of comparison values have been accumulated as part of data set  701 . It will be clear to those who are skilled in the art after reading this specification, how to determine when a sufficient number has been accumulated. This might depend, for example, one or more sources of error such as the quantization error of the propagation-time (e.g., RTT, etc.) measurements made by the wireless terminals. 
     At task  607 , location engine  113  selects one or more statistical algorithms whose resulting statistical values are to be determined with respect to one or more of the values in data set  701 . In some embodiments of the present invention, a to-be-determined statistic can be a descriptive statistic, in which case the statistic can be summary statistic or can be based on a summary statistic. Summary statistics include, while not being limited to:
         a. a measure of location within data set  701 —arithmetic mean, median, mode, interquartile mean, a predetermined percentile, etc.   b. a measure of statistical dispersion within data set  701 —standard deviation, variance, range, interquartile range, absolute deviation, distance standard deviation, etc.   c. a measure of the shape of the distribution of data set  701 —skewness, distance skewness, etc.       

     As those who are skilled in the art will appreciate after reading this specification, the statistic can be selected based on one or more of: the RF environment (e.g., multipath that is present, etc.), the base station or stations involved (i.e., transmitting and/or receiving signals), the wireless terminal or terminals involved (i.e., transmitting and/or receiving signals), any correction or offset applied by the service provider, or trial-and-error, for example and without limitation. 
     At task  609 , location engine  113  generates a first statistic by applying one or more corresponding, predetermined statistical algorithms to a data set, in well-known fashion. In some embodiments, engine  113  can adjust the generated statistic accordingly or calculate a value of an additional statistic or characteristic of data set  701  based on the first statistic. For example and without limitation, if the skewness of data set  701  indicates the presence of strong multipath (i.e., a distinct, positive skewness is observed), then the characteristic of data set  701  for which a value is calculated and eventually provided to task  509  might be a first characteristic. However, if the skewness of data set  701  indicates the presence of weak or no multipath (i.e., a slight skewness or no skewness is observed), then the characteristic of data set  701  for which a value is calculated and eventually provided to task  509  might be a second characteristic. As multipath might vary significantly from one cell of coverage to another, the mere presence of skewness might dictate that separate data sets be maintained and analyzed for each base station. 
     After task  609 , control of task execution then proceeds to task  509 . 
     As those who are skilled in the art will appreciate after reading this specification, a representation of a data set can be used that is alternative to the probability distribution representation depicted in  FIG. 7 . Moreover, a method of calculating a correction can be used that is alternative to generating a statistic of a data set. 
     In some embodiments of the present invention, location engine  113  can determine one or more multipath characteristics of a wireless coverage area based on one or more of the tasks described here.  FIG. 8  depicts a flowchart of the salient processes performed in characterizing multipath, or one or more RF propagation paths in general. 
     For purposes of clarity, wireless terminal  101 - 1  and cellular base station  102 - 1  are used as examples of a wireless terminal and base station. However, as those who are skilled in the art will appreciate after reading this specification, the tasks described below are applicable to other wireless terminals and other base stations (e.g., WiFi, etc.) as well. 
     At task  801 , location engine  113  receives one or more propagation-time measurements (e.g., round-trip time, etc.), wherein each measurement is that of a signal in a transmission between wireless terminal  101 - 1  and base station  102 - 1 . Measurements can be received for signals between multiple wireless terminals and a given base station, for signals between a given wireless terminal and multiple base stations, and for signals across multiple paths between each wireless terminal and base station, in any combination thereof. The measurements can be representative of signals from a base station to a wireless terminal, or from a wireless terminal to a base station, or both. 
     The propagation-time measurements actually received by location engine  113  are based on the propagation-time-related measurements provided by terminal  101 - 1  as described above and in  FIG. 2 . In some embodiments of the present invention, one or more of the propagation-time measurements received by location engine  113  are further based on a predetermined constant, as discussed above and in task  501 . 
     Location engine  113  also receives evidence of the location of one or more wireless terminals, such as terminal  101 - 1 . For example and without limitation, evidence of the location can comprise a signal-strength measurement, a time-related measurement, or information that, by itself, is not a representation of the geographic location of a wireless terminal, estimated or otherwise, but that is probative of the geographic location. The evidence of the location can be different from and independent of the propagation-time measurements, while at the same time the location to which the evidence applies can correspond to the location at which the propagation-time characteristic was measured and/or reported. 
     Location engine  113  also receives evidence of the location of one or more of the base stations, such as base station  102 - 1 . In some embodiments, location engine  113  receives a geographic location of one or more of the base stations, in which the location or locations have been confirmed to a known degree of accuracy. 
     At task  803 , location engine  113  estimates the geographic location of wireless terminal  101 - 1  based on the received evidence of the location of terminal  101 - 1 , thereby establishing a “ground truth” for the location of the terminal. Engine  113  can estimate the location of other wireless terminals as well, thereby also establishing ground truths for those terminals. There are various techniques that can be used to estimate the location of wireless terminal  101 - 1  based on the received evidence. See for example and without limitation, U.S. Pat. Nos. 6,944,465, 7,460,505, 7,383,051, 7,257,414, 7,753,278, 7,433,695, 7,848,762, and 8,630,665, each of which is incorporated by reference herein. In some embodiments of the present invention, the uncertainties of one or more grounds truths are included as a component of the analysis represented by method  800 . 
     At task  805 , location engine  113  estimates a spatial displacement (e.g., shortest distance, etc.) between wireless terminal  101 - 1  and base station  102 - 1  based on the estimated location of terminal  101 - 1 . Engine  113  can estimate the spatial displacements between other combinations of wireless terminals and base stations as well. In some embodiments of the present invention, evidence of the location of base station  102 - 1 , the location itself of base station  102 - 1 , or the location of antenna element  301  is also used in estimating the spatial displacement. 
     At task  807 , location engine  113  generates a statistic of a data set. Task  807  is similar to task  507  described in detail above and in  FIG. 6 , with an important difference. Instead of using the propagation-time measurements as described in task  601 , location engine  113  in task  807  uses calibrated propagation-time measurements. The calibrated propagation-time measurements are generated by taking each propagation-time measurement as received by the location engine from wireless infrastructure  111  and adjusting the measurement. The measurement is adjusted such that any propagation delay attributed to the equipment at base station  102 - 1  and/or attributed to any correction attempted by the wireless service provider is removed from the received propagation-time measurement. The rationale for doing this is to remove any errors attributed to the base station and wireless infrastructure, thereby making the calibrated propagation-time measurement a true representation of the electromagnetic signal&#39;s propagation delay over the air (i.e., between the wireless terminal and base station antenna element). As those who are skilled in the art will appreciate after reading this specification, the calibrated propagation-time measurements can be derived, at least in part, by accurately measuring the equipment delay for the specific equipment involved and/or by obtaining the service provider&#39;s correction factor, if any. 
     At task  809 , location engine  113  estimates a multipath characteristic based on the statistic generated at task  807 . For example, as discussed above and in  FIG. 7 , depicted data set  701  exhibits some positive skewness (i.e., skewness to the right). One situation in which this can occur is when some multipath is present in the coverage area or areas from which the data originates. Accordingly in this example, skewness might be the statistic generated at task  807 —more specifically, selected at task  607  and generated at task  609 —from which the multipath characteristic can be generated. This skewness statistic can then be compared, for example and without limitation, to various reference statistics that are stored in a database, in order to characterize the multipath that is present. 
     At task  811 , location engine  113  transmits the estimated multipath characteristic that was made available at task  809 , to an application (e.g., an RF engineering application, etc.). In some embodiments of the present invention, engine  113  uses the characteristic for its own purposes. 
     Location engine  113  then repeats one or more of the aforementioned tasks. 
     In some embodiments of the present invention, location engine  113  can determine estimate a more accurate location of one or more base stations based on one or more of the tasks described here.  FIG. 9  depicts a flowchart of the salient processes performed in estimating the location of a base station. 
     For purposes of clarity, wireless terminal  101 - 1  and cellular base station  102 - 1  are used as examples of a wireless terminal and base station. However, as those who are skilled in the art will appreciate after reading this specification, the tasks described below are applicable to other wireless terminals and other base stations (e.g., WiFi, etc.) as well. 
     At task  901 , location engine  113  receives one or more propagation-time measurements (e.g., round-trip time, etc.), wherein each measurement is that of a signal in a transmission between wireless terminal  101 - 1  and base station  102 - 1 . Measurements can be received for signals between multiple wireless terminals and a given base station, for signals between a given wireless terminal and multiple base stations, and for signals across multiple paths between each wireless terminal and base station, in any combination thereof. The measurements can be representative of signals from a base station to a wireless terminal, or from a wireless terminal to a base station, or both. 
     The propagation-time measurements actually received by location engine  113  are based on the propagation-time-related measurements provided by terminal  101 - 1  as described above and in  FIG. 2 . In some embodiments of the present invention, one or more of the propagation-time measurements received by location engine  113  are further based on a predetermined constant, as discussed above and in task  501 . 
     Location engine  113  also receives evidence of the location of one or more wireless terminals, such as terminal  101 - 1 . For example and without limitation, evidence of the location can comprise a signal-strength measurement, a time-related measurement, or information that, by itself, is not a representation of the geographic location of a wireless terminal, estimated or otherwise, but that is probative of the geographic location. The evidence of the location can be different from and independent of the propagation-time measurements, while at the same time the location to which the evidence applies can correspond to the location at which the propagation-time characteristic was measured and/or reported. 
     Location engine  113  also receives evidence of the location of one or more of the base stations, such as base station  102 - 1 . In some embodiments, location engine  113  receives a geographic location of one or more of the base stations, in which the location or locations have been confirmed to a known degree of accuracy. For pedagogical purposes, however, the location of least one base station (e.g., base station  102 - 2 , etc.) is either unknown or is known only to an inadequate degree of accuracy. 
     At task  903 , location engine  113  estimates the geographic location of wireless terminal  101 - 1  based on the received evidence of the location of terminal  101 - 1 , thereby establishing a “ground truth” for the location of the terminal. Engine  113  can estimate the location of other wireless terminals as well, thereby also establishing ground truths for those terminals. There are various techniques that can be used to estimate the location of wireless terminal  101 - 1  based on the received evidence. See for example and without limitation, U.S. Pat. Nos. 6,944,465, 7,460,505, 7,383,051, 7,257,414, 7,753,278, 7,433,695, 7,848,762, and 8,630,665, each of which is incorporated by reference herein. In some embodiments of the present invention, the uncertainties of one or more grounds truths are included as a component of the analysis represented by method  900 . 
     At task  905 , location engine  113  estimates a spatial displacement (e.g., shortest distance, etc.) between wireless terminal  101 - 1  and base station  102 - 1  based on the estimated location of terminal  101 - 1 . Engine  113  can estimate the spatial displacements between other combinations of wireless terminals and base stations as well. In some embodiments of the present invention, evidence of the location of base station  102 - 1 , the location itself of base station  102 - 1 , or the location of antenna element  301  is also used in estimating the spatial displacement. 
     At task  907 , location engine  113  generates a statistic of a data set. Task  907  is similar to task  507  described in detail above and in  FIG. 6 , with an important difference. Instead of using the propagation-time measurements as described in task  601 , location engine  113  in task  907  uses calibrated propagation-time measurements. The calibrated propagation-time measurements are generated by taking each propagation-time measurement as received by the location engine from wireless infrastructure  111  and adjusting the measurement. The measurement is adjusted such that any propagation delay attributed to the equipment at base station  102 - 1  and/or attributed to any correction attempted by the wireless service provider is removed from the received propagation-time measurement. The rationale for doing this is to remove any errors attributed to the base station and wireless infrastructure, thereby making the calibrated propagation-time measurement a true representation of the electromagnetic signal&#39;s propagation delay over the air (i.e., between the wireless terminal and base station antenna element). As those who are skilled in the art will appreciate after reading this specification, the calibrated propagation-time measurements can be derived, at least in part, by accurately measuring the equipment delay for the specific equipment involved and/or by obtaining the service provider&#39;s correction factor, if any. 
     In some embodiments of the present invention, location engine  113  estimates a multipath characteristic based on the statistic generated at task  907  and further adjusts the propagation-time measurement in order to generate the calibrated propagation-time measurement. For example, as discussed above and in  FIG. 7 , depicted data set  701  exhibits some positive skewness (i.e., skewness to the right). One situation in which this can occur is when some multipath is present in the coverage area or areas from which the data originates. Accordingly in this example, skewness might be the statistic from which the multipath characteristic can be estimated and processed as described earlier. 
     At task  909 , location engine  113  estimates the location of a base station whose location is unknown or is known only to an inadequate degree of accuracy (e.g., base station  102 - 2 , etc.) based on the statistic generated at task  907 . In some embodiments of the present invention, engine  113  can instead refine the evidence of the location of base station  102 - 1  discussed earlier, based on the statistic generated at task  907 . 
     Location engine  113  can determine the location of the base station in the following manner. Once the statistic is made available at task  907 , engine  113  can use that statistic to further adjust each calibrated propagation-time measurement so that the measurement can be directly used in a meaningful way to determine the base station&#39;s location. Recognizing that the ground-truth locations of one or more wireless terminals are already available as a result of task  903 , these ground truths, in combination with the adjusted, calibrated propagation-time measurement, can be used as part of one or more well-known techniques for location determination (e.g., OTDOA, Cell ID+RTT, etc.), but with an important difference. In this difference, it is the wireless terminal locations that are known and the base station location that is unknown or inaccurate prior to execution of this task, instead of the other way around. 
     At task  911 , location engine  113  transmits the location estimate that was made available at task  909 , to an application engine. In some embodiments of the present invention, location engine  113  uses the location estimate for its own purposes (e.g., to update its base station location database, etc.). 
     Location engine  113  then repeats one or more of the aforementioned tasks. 
     In some embodiments of the present invention, one might conclude in the first place that the stated location of a particular base station is incorrect, by modeling the spatial-displacement error statistics for multiple sectors (e.g., all sectors, etc.) of the base station. Method  900  can be invoked for the particular base station, for example, based on arriving at the foregoing conclusion. 
     It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.