Abstract:
A method for estimating the elevation of an wireless terminal inside of a tall structure is described that compensates for differences in temperature and atmospheric pressure between the inside and outside of the structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/472,600, filed on Aug. 29, 2014, which is incorporated herein by reference. 
     This application is related to “Estimating the Lateral Location of a Wireless Terminal Based on Temperature and Atmospheric Pressure,” application Ser. No. 14/472,574, which is incorporated by reference in its entirety. 
     This application claims the benefit under 35 U.S.C. §119(e) to “Smartphone Absolute Altitude Estimation Using Barometer Data,” Provisional Application Ser. No. 62/033,968, which is incorporated by reference in its entirety. 
    
    
     This application is related to U.S. Pat. Nos. 6,518,918, 6,944,465, 7,460,505, 7,383,051, 7,257,414, 7,753,278, 7,433,695, 7,848,762, and 8,306,676, and 8,630,665, each of which are incorporated by reference. 
     This application is related to U.S. Patent Application Publications 2008/0077356, 2008/0077472, and 2008/0077516, each of which are incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to telecommunications in general, and, more particularly, to a technique for estimating the elevation of a wireless terminal based on measurements of temperature and atmospheric pressure. 
     BACKGROUND OF THE INVENTION 
     The salient advantage of wireless telecommunications over wireline telecommunications is 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 signal 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. In order for these 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 estimation technique. Some of these techniques work well even in environments where the measured strengths of the different signals vary significantly, such as where signal obstructions are present, including natural obstructions such as mountains and artificial obstructions such as buildings. 
     SUMMARY OF THE INVENTION 
     It is well known in the prior art that atmospheric pressure, P A , decreases logarithmically with elevation, Z A , according to the formula: 
                     Z   A     =     -     Hln   ⁡     (       P   A       P   0       )                 (     Eq   .           ⁢   1     )               
wherein
         P 0  is the reference atmospheric pressure, and   H is the scale height of the atmosphere, which is the elevation at which the atmospheric pressure has decreased to e −1  times its value at mean sea level (e.g., approximately 7000 meters).       

     It is also well known in the prior art how to estimate the elevation of an object—such as an airplane—using Equation 1. Aircraft altimeters have used this technique for decades, and it is well known to be highly accurate. Furthermore, as U.S. Pat. Nos. 6,518,918 and 8,306,676 illustrate, it is well known in the prior art how to estimate the elevation of a wireless terminal using Equation 1. 
     The inventors of the present invention recognized that estimating the elevation of an object in a tall building using Equation 1 can be inaccurate. For example, if a person in a tall building calls 9-1-1 to report a fire and the fire department attempts to locate the fire based on the location of the person&#39;s wireless terminal, the estimate of the location of the wireless terminal needs to be accurate. If the elevation of the wireless terminal—and the corresponding floor of the building—are estimated using Equation 1, the estimate of the elevation can be wrong by many floors. This is, of course, unacceptable and potentially life threatening. 
     Furthermore, the inventors recognized that the reason these estimates could be inaccurate was because of a phenomenon known as “the stack effect.” In general, the stack effect is common in tall buildings and structures, and it can cause the relationship between atmospheric pressure and elevation inside of those structures to be different than that predicted by Equation 1. 
     For example, when a tall structure is wholly or partially sealed so that air pressure between the inside and the outside of the structure cannot equalize at every elevation, a temperature difference between the inside and the outside of the structure can cause the relationship between elevation and the atmospheric pressure inside of the structure to be different than that predicted by Equation 1. 
     The illustrative embodiment of the present invention ameliorates errors in the estimate of the elevation of an object caused by the stack effect. The is particularly valuable for estimating the elevation of a wireless terminal located in a tall building or structure. The illustrative embodiment comprises: receiving, at a data processing system, a measurement of a location-dependent trait of a radio signal as received by a wireless terminal; receiving, at the data processing system, a measurement of barometric pressure at the wireless terminal; generating an estimate of the lateral location of the wireless terminal based on the measurement of the location-dependent trait of the radio signal; and generating an estimate of the elevation of the wireless terminal based on: (i) the estimate of the lateral location of the wireless terminal, and (ii) the measurement of barometric pressure at the wireless terminal. 
    
    
     
       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 block diagram of the salient components of wireless terminal  101  in accordance with the illustrative embodiment of the present invention. 
         FIG. 3  depicts a block diagram of the salient components of location engine  113  in accordance with the illustrative embodiment. 
         FIG. 4  depicts a flowchart of the salient processes performed in accordance with the illustrative embodiment of the present invention. 
         FIG. 5  depicts a flowchart of the salient processes performed in accordance with task  401 . 
         FIG. 6  depicts an isometric drawing of geographic region  120  in accordance with the illustrative embodiment of the present invention. 
         FIG. 7  a detailed map of the ground level of geographic region  120 . 
         FIG. 8  depicts geographic region  120  divided into a 10 by 10 grid. 
         FIG. 9  depicts a map that depicts where in geographic region  120  it is not improbable for wireless terminal  101  to be located. 
         FIG. 10  depicts a flowchart of the salient processes performed in accordance with task  402 . 
         FIG. 11  depicts a flowchart of the salient processes performed in accordance with task  403 . 
         FIG. 12  depicts a flowchart of the salient processes performed in accordance with task  404 . 
     
    
    
     DEFINITIONS 
     Atmospheric Pressure—For the purposes of this specification, the term “atmospheric pressure” is defined as the force per unit area exerted on a surface by the weight of the air above that surface in the atmosphere of Earth. 
     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. 
     Elevation—For the purposes of this specification, the term “elevation” is defined as the height relative to a reference (e.g., mean sea level, ground level, etc.). 
     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. 
     Height—For the purposes of this specification, the term “height” should be given the ordinary and customary meaning that the term would have to a person of ordinary skill in the art at the time of the invention. 
     Identity of a Radio Signal—For the purposes of this specification, the phrase “identity of a radio signal” is defined as one or more indicia that distinguish one radio signal from another radio signal. 
     Lateral Location—For the purposes of this specification, a “lateral location” is defined as information that is probative of latitude or longitude or latitude and longitude. 
     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. 
     Location-Dependent Trait of a Radio Signal—For the purposes of this specification, the term “location-dependent trait of a radio signal” is defined as a characteristic of a radio signal that varies with:
         (i) the location of the transmitter of the signal, or   (ii) the location of the receiver of the signal, or   (iii) both i and ii.
 
For example and without limitation, the amplitude and phase of a radio signal are generally location-dependent traits of the signal. In contrast, the frequency of a radio signal is generally not a location-dependent trait of the signal.
       

     Location-Trait Database—For the purposes of this specification, a “Location-Trait Database” is defined as a mapping that associates:
         (i) one or more location-dependent traits of one or more radio signals received or transmitted by a wireless terminal, or   (ii) the identity of one or more radio signals received or transmitted by a wireless terminal, or   (iii) both i and ii,
 
at each of a plurality of locations.
       

     Processor—For the purposes of this specification, a “processor” is defined as hardware or hardware and software that performs mathematical and/or logical operations. 
     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 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. 
     DETAILED DESCRIPTION 
       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 , 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 , weather station  114 , and GPS constellation  121 , interrelated as shown. 
     Wireless infrastructure  111 , location-based application server  112 , location engine  113 , weather station  114 , 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. 
     Wireless terminal  101  comprises the hardware and software necessary to perform the processes described below and in the accompanying figures. Furthermore, wireless terminal  101  is mobile and can be at any location within geographic region  120  at any time. 
     Wireless terminal  101  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  provides a different set of services. 
     In accordance with the illustrative embodiment, wireless terminal  101  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  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  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 . And still furthermore, wireless terminal  101  is capable of measuring a difference of a location-dependent trait of two signals it receives, in well-known fashion, and of transmitting such measurements to location engine  113 . 
     In accordance with the illustrative embodiment, wireless terminal  101  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, etc.), in well-known fashion, and of transmitting those parameters to location engine  113 . 
     In accordance with the illustrative embodiment, and as described in detail below, wireless terminal  101  comprises a barometer  205  (shown in  FIG. 2 ) and thermometer  206  (also shown in  FIG. 2 ). Wireless terminal  101  is capable of measuring (periodically, sporadically, and on-demand) the atmospheric pressure and temperature, in well-known fashion, and of transmitting the measurements 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  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 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 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 , and   b. identifying each radio signal transmitted by wireless terminal  101 , 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 , in well-known fashion, and of transmitting the measurements to location engine  113 , and   d. transmitting one or more signals to wireless terminal  101  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  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 , and   b. identifying each radio signal transmitted by wireless terminal  101 , 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 , in well-known fashion, and of transmitting the measurements to location engine  113 , and   d. transmitting one or more signals to wireless terminal  101  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  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 —generated by location engine  113 —in a location-based application, in well-known fashion. Location-based applications are well-known in the art and provide services such as without limitation E-911 routing, navigation, location-based advertising, 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  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. 2  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 . Location engine  113  comprises the location-trait database and GIS databases, which are described in detail below. 
     Weather station  114  comprises hardware and software that continually measures the outdoor temperature and atmospheric pressure, in well-known fashion, and transmits those measurements to location engine  113 . Weather station  114  is at a known location in geographic region and known elevation. Although the illustrative embodiment comprises only one weather station, it will be clear to those skilled in the art how to make and use alternative embodiments of the present invention that comprise any number of weather stations. 
       FIG. 2  depicts a block diagram of the salient components of wireless terminal  101  in accordance with the illustrative embodiment of the present invention. Wireless terminal  101  comprises: radio receiver and transmitter  201 , processor  202 , memory  203 , human interface  204 , barometer  205 , and thermometer  206 , interconnected as shown. 
     Radio receiver and transmitter  201  comprises hardware and software that enables wireless terminal  101  to receive (and analyze) radio signals and to transmit radio signals. In accordance with the illustrative embodiment, wireless telecommunications service is provided to wireless terminal  101  in accordance with the air-interface standard of the 3 rd  Generation Partnership Project (“3GPP”). After reading this disclosure, however, it will be clear to those skilled in the art how to make and use alternative embodiments of the present invention that operate in accordance with one or more other air-interface standards (e.g., Global System Mobile “GSM,” UMTS, CDMA-2000, IS-136 TDMA, IS-95 CDMA, 3G Wideband CDMA, IEEE 802.11 Wi-Fi, 802.16 WiMax, Bluetooth, etc.) in one or more frequency bands. As will be clear to those skilled in the art, a wireless terminal is also known as a “cell phone,” “mobile station,” “car phone,” “PDA,” and the like. It will be clear to those skilled in the art how to make and use radio receiver and transmitter  201 . 
     Processor  202  is hardware under the command of software stored in memory  203  that performs all of the functions described below and in the accompanying figures. It will be clear to those skilled in the art how to make and use processor  202 . 
     Memory  203  is a non-volatile random-access memory that holds all of the programming and data required for the operation of wireless terminal  101 . It will be clear to those skilled in the art how to make and use memory  203 . 
     Human interface  204  is hardware and software that enables a person to interact with wireless terminal  101 . Human interface  204  comprises a display, keypad, microphone, and speaker, and it will be clear to those skilled in the art how to make and use human interface  204 . 
     Barometer  205  is a hardware MEMS sensor that measures the atmospheric pressure at wireless terminal  101 . In accordance with the illustrative embodiment, barometer  205  is the LSP331AP MEMS pressure sensor from ST Microelectronics, but it will be clear those skilled in the art, after reading this disclosure, how to make and use alternative embodiments of the present invention that use a different sensor to measure the atmospheric pressure. 
     Thermometer  206  is a hardware temperature sensor that measures the ambient temperature at wireless terminal  101 . In accordance with the illustrative embodiment, thermometer  206  is the ADT7420 temperature sensor from Analog Devices, but 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 use a different sensor to measure the ambient temperature at wireless terminal  101 . 
     Radio receiver and transmitter  201  are capable of performing the processes described below and in the accompanying figures. For example and without limitation, wireless terminal  101  is capable of:
         a. receiving one or more radio signals transmitted by cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3 , Wi-Fi base stations  103 - 1  and  103 - 2 , and GPS constellation  121 , and   b. identifying each radio signal transmitted by cellular 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, 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 cellular 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, and of transmitting the measurements to location engine  113 , and   d. transmitting one or more signals to cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3 , Wi-Fi base stations  103 - 1  and  103 - 2  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 , and   e. measuring the temperature and atmospheric pressure at wireless terminal  101 , in well-known fashion, and transmitting those measurements to location engine  113 .
 
It will be clear to those skilled in the art how to make and use wireless terminal  101 .
       

     Location engine  113 — FIG. 3  depicts a block diagram of the salient components of location engine  113  in accordance with the illustrative embodiment. Location engine  113  comprises: processor  301 , memory  302 , and receiver and transmitter  303 , which are interconnected as shown. 
     Processor  301  is a general-purpose processor that is capable of executing an operating system, the application software that performs tasks  402  through  407  (described herein and shown in  FIG. 4 ), and of populating, amending, using, and managing a location-trait database, a GIS database, and a stack-effect database, as described in detail below and in the accompanying figures. It will be clear to those skilled in the art how to make and use processor  301 . 
     In general, the location-trait database contains information for the possible locations of wireless terminal and the identity and location-dependent traits of radio signals as if wireless terminal  101  were at each of those locations. It will be clear to those skilled in the art how to make and use the location-trait database. 
     In general, the GIS database contains information for geographic region  120 , including without limitation, the physical characteristics of all of the structures in geographic region  120 . It will be clear to those skilled in the art how to make and use the GIS database. 
     Memory  302  is a non-volatile memory that stores:
         a. the operating system, and   b. the application software, and   c. the location-trait database,   d. the GIS database, and   e. the stack-effect database.
 
It will be clear to those skilled in the art how to make and use memory  302 .
       

     Receiver and transmitter  303  enables location engine  113  to transmit to and receive from wireless terminal  101 , wireless infrastructure  111 , location-based application server  112 , weather station  114 , and Wi-Fi base stations  103 - 1  and  103 - 2 , in well-known fashion. It will be clear to those skilled in the art how to make and use receiver and transmitter  303 . 
     Operation of the Illustrative Embodiment— FIG. 4  depicts a flowchart of the salient processes performed in accordance with the illustrative embodiment of the present invention. 
     At task  401 , the location-trait database, the GIS database, and the stack-effect database are constructed and stored in memory  302  of location engine  113 . Task  401  is described in detail below and in the accompanying figures. 
     At task  402 , location engine  113  collects measurements of temperature and atmospheric pressure from weather station  114  and wireless terminal  101 . Task  402  is described in detail below and in the accompanying figures. 
     At task  403 , location engine  113  collects empirical data on the radio signals received and transmitted by wireless terminal  101 . Task  403  is described in detail below and in the accompanying figures. 
     At task  404 , location engine  113  generates an estimate of the lateral location of wireless terminal  101  based on:
         (i) the empirical data for the radio signals received in task  403 , and   (ii) the location-trait database
 
in well-known fashion. It will be clear to those skilled in the art how to make and use embodiments of the present invention to perform task  404 . 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 are incorporated by reference. In accordance with the illustrative embodiment of the present invention, the estimate of the lateral location of wireless terminal  101  is one grid square in geographic region  120  (as depicted in  FIG. 8 ).
       

     At task  405 , location engine  113  selects the stack-effect compensation model from the stack-effect database that corresponds to the estimate of the lateral location of wireless terminal generated in task  404 . For example, if wireless terminal  101  is estimated to be in grid square (6,3), then the stack-effect compensation model associated with grid square (6,3) is selected. It will be clear to those skilled in the art, after reading this disclosure, how to make and use embodiments of the present invention that perform task  405 . 
     At task  406 , location engine  113  generates an estimate of the elevation of wireless terminal  101  based on:
         (i) the stack-effect compensation model selected in task  405 , and   (ii) the measurements of temperature and atmospheric pressure received in task  402 .
 
Task  406  is described in detail below and in the accompanying figures.
       

     At task  407 , location engine  113  transmits:
         (i) the estimate of the lateral location of wireless terminal  101  generated in task  404 , and   (ii) the estimate of the elevation of wireless terminal  101  generated in task  406 
 
to location-based application server  112  and to wireless terminal  101  for use in a location-based application. It will be clear to those skilled in the art how to make and use embodiments of the present invention that perform task  407 . After task  407  is completed, control passes back to task  402 .
       

     Task  401 : Construct the GIS Database, the Location-Trait Database, and the Stack-Effect Database— FIG. 5  depicts a flowchart of the salient processes performed in accordance with task  401 . 
     At task  501 , the GIS database is constructed and stored in memory  302  of location engine  113 . 
     As part of task  501 , geographic region  120  is delimited and surveyed in three dimensions.  FIG. 6  depicts an isometric drawing of geographic region  120 , which spans approximately four city blocks and comprises, among other things, park  601 , boxy building  602 , empty lot  603 , and cylindrical building  604 . 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 area, any geographic features, and any number, size, height, and shape of structures. 
     In accordance with the illustrative embodiment, geographic region  120  is flat, level, and at an elevation of 1000 meters. It will be clear to those skilled in the art, however, after reading this disclosure, how to make and use alternative embodiments of the present invention in which geographic region is not flat, not level, and/or is at a different elevation. 
     In accordance with the illustrative embodiment, the height of boxy building  602  is 128 meters and the height of cylindrical building  604  is 140 meters. In other words, the elevation of boxy building  602  is 1128 meters and the elevation of cylindrical building  604  is 1140 meters. 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 the structures have any height. 
     In accordance with the illustrative embodiment, geographic region  120  is square and comprises approximately four city blocks of an urban environment. It will be clear to those skilled in the art however, after reading this disclosure, how to make and use alternative embodiments of the present invention in which geographic region  120  has any area of any shape and any population density and development. As part of task  501 , a detailed map of the ground level of geographic region is made in well-known fashion, and as shown in  FIG. 7 . 
     As part of task  501 , grid  800  is overlaid onto geographic region  120  as shown in  FIG. 8 . Grid  800  is an 10 by 10 grid that partitions geographic region  120  into a plurality of possible lateral locations of wireless terminal  101 .  FIG. 8  also depicts the relationship of the footprints of boxy building  602  and cylindrical building  604  with respect to the grid. 
     Although the illustrative embodiment comprises 100 grid squares, it will be clear to those skilled in the art how to make and use alternative embodiments of the present invention that comprise any number of possible lateral locations with any shape. See for example and without limitation, U.S. Pat. No. 7,753,278, which is incorporated by reference. 
     At any instant, the three-dimensional location of wireless terminal  101  can be described by a combination of a lateral location and an elevation. Although it is not improbable for wireless terminal  101  to be at any lateral location in geographic region  120  and it is not improbable for wireless terminal  101  to be at any elevation (up to 140 meters) above geographic region  120 , it is improbable for wireless terminal  101  to be at some combinations of those lateral locations and elevations. For example, it is not improbable for wireless terminal  101  to be at a lateral location in park  601  and to have an elevation of 1001 meters (i.e., be at or near ground level). It is, however, improbable for wireless terminal  101  to be at a lateral location in park  601  and to have an elevation of 1060 meters. 
     Therefore, as part of task  501 , each improbable combination of lateral locations and elevations for wireless terminal  101  in geographic region is determined, indexed by elevation, and stored in the GIS database. Each improbable combination of lateral locations and elevation for wireless terminal  101  can be determined by referencing the three-dimensional survey of geographic region  120 , which is depicted in  FIG. 9 . 
     From the survey (as depicted in  FIGS. 8 and 9 ), it can be easily seen that when wireless terminal  101  is at ground level (e.g., under an elevation of 1003 meters, etc.), it is not improbable for wireless terminal  101  to be at any lateral location in geographic region  120 . 
     In contrast, when wireless terminal  101  is at an elevation above ground level (e.g., above an elevation of 1003 meters, etc.) and below the rooftop of building  602  (1128 meters), it is improbable for wireless terminal  101  to be at any lateral location that is outside of either boxy building  602  or cylindrical building  604 . Therefore, when wireless terminal  101  is in this range of elevations, the plurality of lateral locations of wireless terminal  101  that are not improbable include: (6,2), (6,3), (6,5), (6,6), (6,7), (7,2), (7,3), (7,5), (7,6), and (7,7). 
     When wireless terminal  101  is at an elevation above the rooftop of building  602  (1128 meters) and below the rooftop of building  604  (1140 meters), it is improbable for wireless terminal  101  to be at any lateral location that is outside of cylindrical building  604 . Therefore, when wireless terminal  101  is in this range of elevations, the plurality of lateral locations of wireless terminal  101  that are not improbable include: (6,5), (6,6), (6,7), (7,5), (7,6), and (7,7). 
     This information is stored in the GIS database in memory  302  as shown in Table 1. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Possible Lateral Locations of Wireless Terminal 101 That 
               
               
                 Are Not Improbable Given The Elevation of Wireless Terminal 101 
               
             
          
           
               
                   
                 Possible Lateral Locations 
               
               
                 Elevation of Wireless 
                 of Wireless Terminal 101 
               
               
                 Terminal 101 
                 That Are Not Improbable 
               
               
                   
               
               
                 1000 to 1003 meters 
                 All 100 Grid Squares 
               
               
                 1003 to 1128 meters 
                 (6,2), (6,3), (6,5), (6,6), 
               
               
                   
                 (6,7), (7,2), (7,3), (7,5), 
               
               
                   
                 (7,6), (7,7) 
               
               
                 1028 to 1140 meters 
                 (6,5), (6,6), (6,7), (7,5), 
               
               
                   
                 (7,6), (7,7) 
               
               
                   
               
             
          
         
       
     
     At task  502 , the location-trait database is constructed and stored into memory  302  of location engine  113 . As part of task  503 , the identity—and location-dependent traits for—each radio signal that wireless terminal is expected to be able to receive from cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3 , Wi-Fi base stations  103 - 1  and  103 - 2 , for each possible lateral location of wireless terminal  101 , is determined in well-known fashion. 
     As part of task  502 , the identity of—and location-dependent traits for—each radio signal that each of cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3 , Wi-Fi base stations  103 - 1  and  103 - 2  is expected to be able to receive from wireless terminal  101 , for each possible lateral location of wireless terminal  101 , is determined in well-known fashion. 
     It will be clear to those skilled in the art how to accomplish task  503 , and in accordance with the illustrative embodiment, this is accomplished through a combination of “drive testing” (i.e., empirical data gathering) and radio-frequency propagation modeling. See for example and without limitation, U.S. Patent Application Publications 2008/0077356, 2008/0077472, and 2008/0077516, which are incorporated by reference. 
     At task  503 , the stack-effect database is constructed and stored into memory  302  of location engine  113 . As part of task  503 , a stack-effect compensation model is generated for each grid square in geographic region  120 . 
     In accordance with the illustrative embodiment, the stack-effect compensation model for grid squares (6,2), (6,3), (7,2), and (7,3), which comprise boxy building  602 , is: 
                     P   M     =       CH   602     ⁢       P   W     ⁡     (       1     T   W       -     1     T   T         )                 (       Eq   .           ⁢   2     ⁢   a     )               
wherein:
         P M  is the pressure differential caused by the stack effect,   C is a constant equal to 0.0342 (for SI units) and 0.0188 (for U.S. units),   H 602  is the height of the neutral pressure plane in boxy building  602 ,   P W  is the measurement of atmospheric pressure at weather station  114 ,   T W  is the measurement of temperature at weather station  114 , and   T T  is the measurement of temperature at wireless terminal  101 .       

     In accordance with the illustrative embodiment, the height of the neutral pressure plane for boxy building  602  is estimated to be at one-half of the height of the building or at 64 meters. 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 the height of the neutral pressure plane is determined empirically or estimated by another method. 
     In accordance with the illustrative embodiment, the stack-effect compensation model grid squares (6,5), (6,6), (6,7), (7,5), (7,6), and (7,7), which comprise cylindrical building  604 , is: 
                     P   M     =       CH   604     ⁢       P   W     ⁡     (       1     T   W       -     1     T   T         )                 (       Eq   .           ⁢   2     ⁢   b     )               
wherein: H 604  equals 30 meters.
 
     In accordance with the illustrative embodiment, cylindrical building  604  has an opening to the outside atmosphere every 30 meters, which enables cylindrical building  604  to have multiple neutral pressure planes. 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 the height of the neutral pressure planes is determined to have another value. 
     In accordance with the illustrative embodiment, the stack-effect compensation model for all of the other 90 grid squares (i.e., those that do not comprise a structure) is:
 
 P   M =0  (Eq. 2c)
 
     It will be clear to those skilled in the art that tasks  501 ,  502 , and  503  can be performed concurrently or in any order. 
     Task  402 : Collect Temperature and Atmospheric Measurements— FIG. 10  depicts a flowchart of the salient processes performed in accordance with task  402 . 
     At task  1001 , weather station  114  transmits a measurement of temperature, T W , and a measurement of atmospheric pressure, P W , to location engine  113 . In accordance with the illustrative embodiment, task  1001  is performed every 10 minutes, but it will be clear to those skilled in the art how to make and use alternative embodiments of the present invention that transmit the measurements at other times. 
     At task  1002 , location engine  113  receives the measurement of temperature, T W , and a measurement of atmospheric pressure, P W , transmitted in task  1101 . 
     At task  1003 , wireless terminal  101  transmits a measurement of temperature, T T , and a measurement of atmospheric pressure, P T , to location engine  113 . In accordance with the illustrative embodiment, task  1003  is performed every 5 seconds, but it will be clear to those skilled in the art how to make and use alternative embodiments of the present invention that transmit the measurements at other times. 
     At task  1004 , location engine  113  receives the temperature and atmospheric measurements transmitted in task  1103 . 
     In accordance with the illustrative embodiment, tasks  1001 ,  1002 ,  1003 , and  1004  are performed continuously, concurrently, and asynchronously. 
     Task  403 : Collect Empirical Data on Radio Signals— FIG. 11  depicts a flowchart of the salient processes performed in accordance with task  403 . 
     At task  1101 , each of cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3  and Wi-Fi base stations  103 - 1  and  103 - 2  transmits the identity of each signal it has received from wireless terminal  101  and the measurements of the location-dependent traits of those signals. In accordance with the illustrative embodiment, task  1101  is performed every 20 milliseconds, but it will be clear to those skilled in the art how to make and use alternative embodiments of the present invention that transmit the measurements at other times. 
     At task  1102 , location engine receives the identities and measurements transmitted at task  1101 . 
     At task  1103 , wireless terminal  101  transmits the identity of each signal it receives from cellular base stations  102 - 1 ,  102 - 2 , and  102 - 3  and Wi-Fi base stations  103 - 1  and  103 - 2  and the measurements of the location-dependent traits of those signals. In accordance with the illustrative embodiment, task  1103  is performed every 20 milliseconds, but it will be clear to those skilled in the art how to make and use alternative embodiments of the present invention that transmit the measurements at other times. 
     At task  1104 , location engine receives the identities and measurements transmitted at task  1103 . 
     In accordance with the illustrative embodiment, tasks  1101 ,  1102 ,  1103 , and  1104  are performed continuously, concurrently, and asynchronously. 
     Task  406 : Estimate the Elevation of Wireless Terminal  101 — FIG. 12  depicts a flowchart of the salient processes performed in accordance with task  406 . 
     At task  1201 , location engine  113  generates an estimate of the reference atmospheric pressure for geographic location  120 , P 0 , based on: 
                     P   0     =       P   W       ⅇ     -     (       Z   W     H     )                   (     Eq   .           ⁢   2     )               
wherein:
         P 0  is the reference atmospheric pressure for geographic location  120 ,   P W  is the measurement of atmospheric pressure received from weather station  114  that most closely corresponds in time to the measurement of atmospheric pressure of interest received from wireless terminal  101 , P T ,   Z W  is the elevation of weather station  114  (1000 meters in the illustrative embodiment), and   H is the scale height of the atmosphere, which is the elevation at which the atmospheric pressure has decreased to e −1  times its value at mean sea level (e.g., approximately 7000 meters).       

     At task  1202 , location engine  113  generates an estimate of the pressure differential, P M , based on the stack-effect compensation model selected in task  405  and the measurements of temperature and atmospheric pressure received in task  402 . It will be clear to those skill in the art, after reading this disclosure, how to make and use embodiments of the present invention that perform task  1202 . 
     At task  1023 , location engine  113  generates an estimate of the elevation of wireless terminal  101 , Z T , based on: 
                     Z   T     =     -     Hln   ⁡     (         P   T     -     P   M         P   0       )                 (     Eq   .           ⁢   3     )               
wherein:
         P T  is the relevant measurement of atmospheric pressure received from wireless terminal  101 .       

     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.