Patent Publication Number: US-2023161074-A1

Title: Systems and methods for determining, broadcasting and using reference atmospheric data in a network of transmitters

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/930,315, filed on May 12, 2020, which claims priority to U.S. Provisional Patent Application No. 62/874,811, filed on Jul. 16, 2019, all of which is hereby incorporated by reference in its entirety and for all purposes. 
    
    
     BACKGROUND 
     Determining the exact location of a mobile device (e.g., a smart phone operated by a user) in an environment can be quite challenging, especially when the mobile device is located in an urban environment or is located within a building. Imprecise estimates of the mobile device&#39;s altitude, for example, may have life or death consequences for the user of the mobile device since the imprecise altitude estimate can delay emergency personnel response times as they search for the user on multiple floors of a building. In less dire situations, imprecise altitude estimates can lead a user to the wrong area in an environment. 
     Different approaches exist for estimating an altitude of a mobile device. In a barometric-based positioning system, altitude can be computed using a measurement of pressure from a calibrated pressure sensor of a mobile device along with ambient pressure measurement(s) from a network of calibrated weather stations and a measurement of ambient temperature from the network or other source. An estimate of an altitude of a mobile device (h mobile ) can be computed by the mobile device, a server, or another machine that receives needed information as follows: 
     
       
         
           
             
               
                 
                   
                     
                       h 
                       mobile 
                     
                     = 
                     
                       
                         h 
                         sensor 
                       
                       - 
                       
                         
                           
                             RT 
                             remote 
                           
                           gM 
                         
                         ⁢ 
                         
                           ln 
                           ⁡ 
                           ( 
                           
                             
                               P 
                               sensor 
                             
                             
                               P 
                               mobile 
                             
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     where P mobile  is the estimate of pressure at the location of the mobile device by a pressure sensor of the mobile device, P sensor  is an estimate of pressure at the location of a weather station that is accurate to within a tolerated amount of pressure from true pressure (e.g., less than 5 Pa), T remote  is an estimate of temperature (e.g., in Kelvin) at the location of the weather station or a different location of a remote temperature sensor, h sensor  is an estimated altitude of the weather station that is estimated to within a desired amount of altitude error (e.g., less than 1.0 meters), g corresponds to the acceleration due to gravity (e.g., −9.8 m/s 2 ), R is a gas constant, and M is molar mass of air (e.g., dry air or other). The minus sign (−) may be substituted with a plus sign (+) in alternative embodiments of Equation 1, as would be understood by one of ordinary skill in the art (e.g., g=9.8 m/s 2 ). The estimate of pressure at the location of the weather station can be converted to an estimated reference-level pressure that corresponds to the weather station in that it specifies an estimate of pressure at the latitude and longitude of the weather station, but at a reference-level altitude that likely differs from the altitude of the weather station. The reference-level pressure can be determined as follows: 
     
       
         
           
             
               
                 
                   
                     
                       P 
                       ref 
                     
                     = 
                     
                       
                         P 
                         sensor 
                       
                       × 
                       
                         exp 
                         ⁡ 
                         ( 
                         
                           - 
                           
                             
                               gM 
                               ⁡ 
                               ( 
                               
                                 
                                   h 
                                   ref 
                                 
                                 - 
                                 
                                   h 
                                   sensor 
                                 
                               
                               ) 
                             
                             
                               RT 
                               remote 
                             
                           
                         
                         ) 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                         
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     where P sensor  is the estimate of pressure at the location of the weather station, P ref  is the reference-level pressure estimate, and h ref  is the reference-level altitude. The altitude of the mobile device h mobile  can be computed using Equation 1, where h ref  is substituted for h sensor  and P ref  is substituted for P sensor . The reference-level altitude h ref  may be any altitude and is often set at mean sea-level (MSL). When two or more reference-level pressure estimates are available, the reference-level pressure estimates are combined into a single reference-level pressure estimate value (e.g., using an average, weighted average, or other suitable combination of the reference pressures), and the single reference-level pressure estimate value is used for the reference-level pressure estimate P ref . 
     Reference pressures from one or more weather stations can be combined into a reference pressure value that a cellular transmitter transmits (e.g., broadcasts) to mobile devices for use in computing their estimated altitudes or for other uses (e.g., calibrating pressure sensors of the mobile devices). By way of example,  FIG.  1    illustrates a cellular network that includes a set of transmitters  110  configured to transmit signals  113  using known transmission technologies. The signals  113  can be transmitted at different times for acquisition by a mobile device  120  as the mobile device  120  moves through the cellular network. The mobile device  120  may take different forms, including a mobile phone or other wireless communication device, a portable computer, a navigation device, a tracking device, a receiver, or another suitable device that can receive the signals  113 . A network backend  130  with a control unit—e.g., a positioning server such as eSMLC/LMF (Evolved-Serving Mobile Location Center/Location Management Function)—is connected to the transmitters  110 . The backend  130  includes processors (e.g., servers) for performing different types of processing—e.g., collecting reference pressures from weather stations, generating reference pressure values based on collected reference pressures, and other types of processing. Examples of possible components in the transmitters  110 , the mobile device  120 , and the backend  130  are shown in  FIG.  12    and discussed in the ‘Other Aspects’ section near the end of this disclosure. 
     The signals  113  transmitted from the transmitters  110  contain different types of information. One type of information is often referred to as “assistance data”. Assistance data can be used by the mobile device  120  (or backend  130 ) to compute an estimated position of the mobile device  120  in terms of latitude, longitude and/or altitude. As previously described, one approach for computing an estimated altitude uses assistance data that includes reference pressures that are based on measurements of pressure from one or more weather stations, and in some cases uses reference temperatures that are based on measurements of temperature from one or more weather stations. As shown in  FIG.  1   , different weather stations  115  are positioned at different locations throughout the cellular network. Each of the weather stations  115  can determine a reference pressure for a reference altitude (e.g., sea-level altitude), and may measure temperatures. Reference pressure values to be transmitted by the transmitters  110  can be determined based on reference pressures from the weather stations  115 . Similarly, a reference temperature can be transmitted by the transmitters  110 , where that reference temperature can be determined based on temperatures measured by the weather stations  115 . Ideally, each transmitter transmits a reference pressure value that was generated using recently-generated and highly-accurate reference pressures from weather stations that are near that transmitter. In some embodiments, each transmitter transmits a reference temperature value that was generated using recently-measured and highly-accurate temperatures measured by weather stations that are near that transmitter (e.g., using an average of measured temperatures). Ideal circumstances cannot be expected for all cellular networks. Instead, factors that affect the reliability of available reference pressures and/or temperatures must be addressed, including (i) the proximity of a transmitter to weather stations that provide reference pressures and/or temperatures, (ii) the age of reference pressures and/or temperatures—e.g., the time since measurements of pressure used to determine the reference pressures were made or since the temperatures were measured, and (iii) the quality of the reference pressures and/or temperatures—e.g., how recently the weather station was calibrated, the resolution of data from the weather station, exposure of the weather station to over-heating, or some other known conditions that may affect the reliability of the reference pressure and/or temperature. The relative locations of transmitters and weather stations will often vary in a network of transmitters. For example: a transmitter and a weather station can be co-located (e.g., weather station  115   a  is located within the footprint of transmitter  110   a ); two or more weather stations can be near a transmitter (e.g., weather stations  115   b  and 115 c  are within a threshold distance of transmitter  110   b ); or no weather station is near a transmitter (e.g., transmitter  110   c ). The age and quality of reference pressures and/or temperatures can also vary among weather stations. Given the potential differences in relative locations, ages, and/or qualities associated with reference pressures and/or temperatures from weather stations in different cellular networks, different methods for collecting reference pressures and/or temperatures from weather stations are needed to optimize use of available reference pressures for computing a reference pressure value and/or use of available temperature measurements for computing a reference temperature value under different circumstances impacting different transmitters. 
     Even after reference pressure values and/or reference temperature values are computed, those values must be transmitted to mobile devices (e.g., UEs) using limited and highly valuable bandwidth. In some wireless networks, the reference values are transmitted to mobile devices via point-to-point protocols, such as LPP (LTE Positioning Protocol) [TS 36.355] or SUPL (Secure User Plane Location) Protocol [OMA SUPL] over the wireless network. However, in use cases where there is a high concentration of mobile devices, such as a high-rise building or stadium, it is more efficient to broadcast the reference values via point-to-multi-point. In order to support accurate computations of altitude at any time, transmitter networks must allocate valuable bandwidth for broadcasting updated reference pressure values and/or reference temperatures on a regular basis. However, regular broadcasts of reference pressure values and/or reference temperatures may unnecessarily occupy bandwidth during periods of time when changes in pressure and/or temperature within the cellular network are so limited that older reference pressure values still reflect actual pressure conditions and/or older temperature measurements still reflect actual temperature conditions in the cellular network. On the other hand, regular broadcasts of reference pressure values may not keep up with changes in pressure within the cellular system such that reference pressure values stored on a mobile device become stale before updated reference pressure values are received. Thus, methods for adaptively modifying when to broadcast reference values can decrease consumption of valuable network resources and also improve accuracy of estimated altitudes over time, which is highly desirable. Alternatively, methods for adaptively modifying when mobile devices search the broadcast channel for reference values when previous reference values are stale can increase power savings, which is highly desirable. 
     Although adjustments to a broadcast schedule offer advantages in some situations, those adjustments may not provide sufficiently-accurate reference pressure values in other situations, such as when weather stations do not produce new reference pressures at a rate that is equal to or greater than the desired adjusted rate of broadcasts, or when a mobile device enters an area in which no broadcasts of reference pressure values are available. Hence, methods for using information about weather changes that are expected within a transmitter network to adjust reference pressure values offers advantages during situations when adjustments to broadcast scheduling would not necessarily provide sufficiently accurate reference pressure values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    depicts an operational environment in which systems and methods for determining, broadcasting and using reference pressure data in a network of transmitters may operate. 
         FIG.  2 A  through  FIG.  2 F  illustrate different cellular elements for use in determining reference pressure data in a network of transmitters. 
         FIG.  3    depicts a process for determining reference pressure data that is transmitted to mobile devices. 
         FIG.  4    depicts a process for selecting one or more reference pressures from one or more weather stations within a transmitter network. 
         FIG.  5    depicts a process for determining a reference pressure value of a reference altitude to be transmitted. 
         FIG.  6    depicts a process for determining if a reference pressure value is valid. 
         FIG.  7 A  depicts a process for generating a new reference pressure value by adjusting a reference pressure value. 
         FIG.  7 B  depicts a process for generating a new reference pressure value by adjusting reference pressures used to compute a previous reference pressure value. 
         FIG.  8    depicts a process for determining reference pressure data that is used to compute altitude estimates of mobile devices or calibrate pressure sensors of mobile devices. 
         FIG.  9 A  and  FIG.  9 B  depict processes for determining if a received reference pressure value is valid. 
         FIG.  10 A  through  FIG.  10 D  depict processes for determining a representative reference pressure value by combining reference pressure values. 
         FIG.  11 A  through  FIG.  11 C  depict processes for determining a new reference pressure value. 
         FIG.  12    illustrates components of a transmitter, a mobile device, and a network backend. 
     
    
    
     DETAILED DESCRIPTION 
     One approach for estimating of an altitude of a mobile device using reference pressures from weather stations is described in the Background section of this disclosure. Typically, weather stations that are dispersed throughout a cellular network provide reference pressures for use in determining a reference pressure value that is transmitted from a cellular transmitter to a receiver that is within range of that transmitter. The reliability of the resultant reference pressure value is impacted by different aspects of a cellular network, including proximity of weather station to transmitter, age of reference pressure, and quality of reference pressure value, as well as changes in weather conditions over time. 
     Unfortunately, many cellular networks do not include sufficient numbers of weather stations and/or sufficient distribution of weather stations in the vicinity of each transmitter in those networks. Given the varied numbers and distributions of weather stations around different transmitters in a cellular network, different approaches are needed for evaluating candidate reference pressures that can be used to generate a reference pressure value to be transmitted by a transmitter. As discussed further below, different types of cellular elements within a cellular network can be used to select reference pressures from weather stations based on relationships between the weather stations and the cellular element. Examples of cellular elements, which are illustrated in  FIG.  2 A  through  FIG.  2 F , include a transmitter, a coverage area of the transmitter, a sub-cell occupied by the transmitter (e.g., micro, pico or other sub-cell), a cell sector used by the transmitter, a coverage area of a transmission beam, an area along a direction of a signal transmitted by the transmitter, or sets of the foregoing cellular elements.
     (i) For deployments that include multiple weather stations within each cellular element (e.g., large cellular elements like coverage areas of different transmitters), a control unit in the network such as a positioning server (e.g., eSMLC/LMF) can compute a reference pressure value for a transmitter based on a mathematical combination of selected reference pressures from weather stations within a cellular element associated with that transmitter. Different mathematical combinations of selected reference pressures are possible, including weighted averages where weights are based on proximity of weather stations to the cellular element (or a portion of the cellular element), age of the reference pressures, qualities of the reference pressures, and/or other characteristics of the reference pressures or weather stations. In some implementations, a control unit may acquire or construct an isobar plot for an area that includes the cellular element, and use the plot to determine a reference pressure value that is best-suited for the mobile device (e.g., that is located at an estimated position of the mobile device).   (ii) For deployments where there is a many-to-one mapping between cellular elements and weather stations (or where a particular cellular element associated with a transmitter does not include any weather station), a control unit may select reference pressures based on proximity of weather stations to a cellular element even if the weather stations are not in a threshold vicinity of the cellular element, and/or may select reference pressures based on the age of reference pressures, based on quality of the reference pressures, and/or based on some other condition.   (iii) For deployments where there is one-to-one mapping between cellular elements and weather stations, a control unit may use a reference pressure from the weather station within a cellular element as the reference pressure value to be transmitted by the transmitter associated with that cellular element.   (iv) For deployments with a mix of (i), (ii) and (iii) above, a control unit may use different ways of selecting reference pressures depending on the number of weather stations in each cellular element associated with each transmitter, such that different approaches for selecting reference pressures can be used for different transmitters—e.g., using (i) for a first transmitter, using (ii) for a second transmitter, and (iii) for a third transmitter. Alternatively, different types of cellular elements can be used to select a threshold amount of reference pressures for each transmitter—e.g., using a small cellular element (e.g., cell sector) for a first transmitter when that small cellular element includes weather stations that provide at least the threshold amount of reference pressures, and using a large cellular element (e.g., cell) for a second transmitter because a small cellular element for the second transmitter does not include weather stations that provide at least the threshold amount of reference pressures, but the large cellular element includes weather stations that provide at least the threshold amount of reference pressures.
 
In any deployment, different types of cellular elements can be used to select reference pressures for different transmitters depending on circumstances impacting each transmitter.
   

     Changes in weather conditions over time can also affect the reliability of reference pressure values. Different cellular networks are deployed in different regions that experience different types of pressure changes over time. These changes in pressure affect the validity of reference pressure values. As discussed further below, a control unit of a cellular network can monitor weather within the cellular network—e.g., by accessing weather reports that detail actual and expected changes in pressure over time and distance in different areas of the network—and to determine expected changes in pressure within cellular elements. Expected changes in pressure can be used to determine expiration times for reference pressure values associated with particular cellular elements, which can be used to increase or decrease the rate at which new reference pressure values are determined and transmitted to mobile devices from transmitters associated with those cellular elements. Modifications to the rate at which new reference pressure values are determined and transmitted to mobile devices allow for more-efficient bandwidth usage while ensuring more accurate reference pressure values (and more-accurate altitude estimates) over time compared to systems that do not adjust broadcast schedules in the same way. 
     Alternatively, changes in pressure over time can be determined and used to adjust previously computed reference pressure values without any need for computing a new reference pressure value using new reference pressures, which reduces the time needed to compute reliable reference pressure values, and provides reliable reference pressure values during times when a new reference pressure value based on new reference pressures cannot be determined or received. Changes in pressure across distances can be monitored and used to limit the validity of reference pressure values to particular areas within the cellular network, such that a moving mobile device can determine if previously received reference pressure values can be used for the mobile device&#39;s current location. When a mobile device has moved since last receiving a reference pressure value, changes in pressure across the distance traveled by the mobile device can be determined and used to adjust that reference pressure value without any need for obtaining a new reference pressure value, which reduces bandwidth use, reduces times needed to compute reliable reference pressure values, and provides reliable reference pressure values during times when a new reference pressure value based on new reference pressures cannot be determined or received. Changes across distance may be represented in different ways, including computed gradients. 
     Different embodiments that incorporate the above aspects are discussed below. 
     Determining Reference Pressure Data That is Transmitted to Mobile Devices 
       FIG.  3    depicts a process for determining reference pressure data that is transmitted to mobile devices. 
     Initially, reference pressures from weather stations within a transmitter network are selected for use in determining a reference pressure value of a reference altitude to be transmitted by a transmitter (step  310 ). One embodiment of step  310  is depicted in  FIG.  4   , which is described later. 
     The selected reference pressures are used to determine a reference pressure value to be transmitted by a transmitter (step  320 ). One embodiment of step  320  is depicted in  FIG.  5   , which is described later. 
     The transmitter transmits the determined reference pressure value (step  330 ), which is received by any mobile device that is inside a coverage area of the transmitter. By way of example, the transmitter may be a cellular transmitter that transmits scheduled broadcast signals at regular intervals. Alternatively, in other implementations, the transmitter transmits the reference pressure value to a particular mobile device after receiving a request for the reference pressure value from that mobile device. 
     At a time after initially transmitting the previous reference pressure value, a determination is made if transmission of a new reference pressure value is scheduled for that time (step  340 ). If transmission of a new reference pressure value is not scheduled for that time, step  340  is repeated. 
     In one embodiment (option 1), if transmission of a new reference pressure value is scheduled for that time, the process returns to step  310 . In another embodiment (option 2), if transmission of a new reference pressure value is scheduled for that time, a determination is made as to whether a previously-determined (e.g., most-recently determined) reference pressure value is valid (step  350 ). One embodiment of step  350  is depicted in  FIG.  6   , which is described later. 
     If the reference pressure value is still valid, the process returns to step  330 , and the reference pressure value is transmitted to the mobile device. 
     If the reference pressure value is not valid, a new reference pressure value is generated (step  360 ). 
     In a first embodiment of step  360 , the new reference pressure value is generated by selecting newer reference pressures from weather stations and determining the new reference pressure value using the same process as used for step  320 , but with the newer reference pressures instead of the older reference pressures from step  310 . Optionally, any schedule for determining and transmitting reference pressure values over time can be adjusted based on monitored pressure conditions. For example, (i) a rate of pressure change in the network or in an area that includes the transmitter and/or weather stations that provided selected reference pressures can be determined (e.g., from regional or more defined weather reports), and (ii) the rate of pressure change can be used to determine a maximum time between reference pressure value determinations and transmissions by dividing a maximum amount of tolerated change in pressure (e.g., 10 Pa) by the rate of pressure change, and using the resulting time as the maximum time between reference pressure value determinations and transmissions. 
     In a second embodiment of step  360 , the new reference pressure value is generated by adjusting the older reference pressure value from step  320  based on measured or expected changes in pressure since the time when the reference pressure value was determined during step  320 . Example implementations of the second embodiment of step  360  are depicted in  FIG.  7 A  and  FIG.  7 B , which are described later. 
     Lastly, the transmitter transmits the new reference pressure value (step  370 )—e.g., to any mobile device that resides within a coverage area of the transmitters, or to a particular mobile device for embodiments that permit requests for new reference pressure values from such a mobile device. 
     In an alternative embodiment (not shown), steps  310  through  340  are performed, except that a determination during step  340  that transmission of a new reference pressure value is not scheduled for that time results in determining if the reference pressure value is valid (step  350 ), after which the process (i) returns to step  340  if the reference pressure value is valid, and (ii) advances to  360  if the reference pressure value is not valid. After step  360 , the process returns to step  340 . 
     In an alternative embodiment (not shown), step  340  is omitted, and step  350  is performed after step  330 . 
     By way of example, the network backend  130  and/or a processor of the transmitter can be used to perform steps  310 ,  320 ,  340 ,  350  and  360 . 
     Selecting One or More Reference Pressures From One or More Weather Stations Within a Transmitter Network (Step  310 ) 
       FIG.  4    depicts a process for selecting one or more reference pressures from one or more weather stations within a transmitter network for use in determining a reference pressure value during an embodiment of step  310 . 
     A cellular element associated with the transmitter is optionally determined (step  411 ). By way of example, different cellular elements are illustrated in  FIG.  2 A  through  FIG.  2 F , including the transmitter itself, a coverage area of the transmitter, a sub-cell occupied by the transmitter, a cell sector used by the transmitter, a coverage area of a transmission beam, an area along a direction of a signal transmitted by the transmitter, or other cellular elements. Sets of these cellular elements could also be used as a “cellular element” that is determined in some embodiments. Any process known in the art for defining the boundaries and characteristics of each cellular element can be used. 
     Reference pressures from weather stations that meet one or more conditions are selected (step  412 ). 
     In a first embodiment of step  412 , n reference pressures from n respective weather stations that meet a proximity-based condition are selected. In one implementation, reference pressures from each weather station within the cellular element, within a threshold distance (for example: 100 yards; 1 mile; or 10 miles) from part of the cellular element (e.g., a coordinate of the cellular element), or within an area that includes the cellular element can be selected. In another implementation, reference pressures from the nearest n weather stations to part of the cellular element are selected. 
     In a second embodiment of step  412 , n reference pressures from n respective weather stations that meet an age-based condition are selected (for example: reference pressure measure is less than 10 minutes old, or less than 1 hour old). In one implementation, each reference pressure with an age that is below a threshold age value is selected. In another implementation, the n most-recent reference pressures are selected. In yet another implementation, each reference pressure that is valid—e.g., has not expired—is selected. 
     In a third embodiment of step  412 , n reference pressures from n respective weather stations that meet a quality-based condition are selected. In one implementation, each reference pressure with a quality that is above a threshold quality value is selected. In another implementation, the n reference pressures with the highest quality are selected. A quality metric can be determined for each reference pressure, where higher values of the metric represent higher quality. By way of example, if the metric is based on time since a weather station was calibrated, a higher metric value would be given for more recent calibration. By way of another example, if the metric represents if a weather station experiences over-heating (e.g., from sun exposure or HVAC exhaust), a high metric value is given to reference pressures that originate from weather stations that do not experience over-heating while a low metric value is given to reference pressures that originate from weather stations that experience over-heating. By way of example, a quality metric may be a tolerance in the pressure measurement where a lower tolerance indicates a higher quality. 
     In a fourth embodiment of step  412 , n reference pressures from n respective weather stations that meet two or more conditions selected from proximity-based, age-based, or quality-based conditions are selected. Different orders of conditions are possible—e.g., apply a proximity-based condition to identify a set of candidate reference pressures, and then apply an age-based or quality-based condition to identify the selected reference pressures, or vice versa. 
     In a fifth embodiment, a series of conditions are tested until reference pressures are selected or until there are no more conditions to test. In one implementation, a first condition is tested (e.g., proximity), and if no reference pressures are selected based on the first condition then a second condition is tested (e.g., age), and so on for other conditions. In another implementation, a set of conditions are tested (e.g., proximity and age), and if no reference pressures are selected based on the set of conditions then a portion of the set of the conditions is tested (e.g., proximity only), and if no reference pressures are selected based on that portion of the set of conditions then another portion of the set of the conditions is tested (e.g., age only), and so on for other portions of the set of conditions. 
     Optionally, if no reference pressures from weather stations meet the one or more conditions, a new cellular element may be determined (step  413   a ), and the process returns to step  412  for the new cellular element. 
     In one embodiment of the process depicted in  FIG.  4   , the new cellular element is another type of cellular element associated with the transmitter—e.g., the first type of cellular element is a cellular element associated with a smaller area such as a transmission beam directed from a transmitter towards the mobile device or a cell sector, and the second type of cellular element is a type of cellular element associated with a larger area such as a coverage area of the transmitter. Initially using a cellular element associated with a smaller area to select reference pressures increases the accuracy of a reference pressure value relative to a mobile device since the smaller cellular element is more likely to select reference pressures that are experiencing similar pressure conditions as the mobile device. 
     In another embodiment of the process depicted in  FIG.  4   , the new cellular element is the same type of cellular element, but is associated with a different transmitter (e.g., a different transmitter adjacent to the transmitter). The new cellular element is a neighboring (e.g., adjacent) cellular element of the same type that is associated with the same transmitter or a different transmitter—e.g., if the first cellular element is a cell sector of a cell in which a transmitter resides, then the new cellular element may be an adjacent cell sector in that same cell; e.g., if the first cellular element is a cell in which a transmitter resides, then the new cellular element may be an adjacent cell in which a neighboring transmitter resides. 
     Optionally, if no reference pressures from weather stations meet the one or more conditions, a message indicating that no reference pressure value is available can be generated and transmitted (step  413   b ). 
     Different embodiments of  FIG.  4    include different sets of the steps. A first embodiment includes steps  411 ,  412  and  413 . A second embodiment only includes step  412  (i.e., there is consideration of conditions, but there is no consideration of cellular elements). 
     The processes of  FIG.  3    and  FIG.  4    can be performed for different transmitters, and the results may be different for different transmitters depending on the distribution of weather stations relative to the different transmitters—e.g., no performance of step  413   a  may be required to produce selected reference pressures for a first transmitter, but performance of step  413   a  may be required to produce selected reference pressures for a second transmitter. 
     Determining a Reference Pressure Value at a Reference Altitude to be Transmitted (Step  320 ) 
       FIG.  5    depicts a process for determining a reference pressure value at a reference altitude to be transmitted during an embodiment of step  320 , which comprises computing the reference pressure value using an average of the selected reference pressures (step  521 ). 
     In one implementation of step  521 , the average is a weighted average that uses weights based on an inverse of respective distances between weather stations of the selected reference pressures and part of the cellular element (e.g., a coordinate or other feature of the cellular element), such that a reference pressure from a weather station that is closer to the cellular element is given greater weight than a reference pressure from another weather station that is further away from the cellular element. Since reference pressure can change over distance, closer reference pressures more-accurately reflect reference pressure at the two-dimensional location of the transmitter than reference pressures that are further away. 
     In another implementation of step  521 , the average is a weighted average that uses weights based on an inverse of respective ages of the selected reference pressures, such that a reference pressure that is more-recently determined is given greater weight than a reference pressure that was determined further in the past. Since pressure can change over time, recently-determined reference pressures more-accurately reflect reference pressure at the current time than older reference pressures. 
     In yet another implementation of step  521 , the average is a weighted average that uses weights based on respective qualities of the selected reference pressures, such that a reference pressure that originates from a more-reliable source is given greater weight than a reference pressure that originates from a less-reliable source. Since weather stations can experience different issues (e.g., rate of drift, over-heating, etc.), reference pressures from more-reliable weather stations (e.g., weather stations that have been recently calibrated, that have less or no overheating, etc.) to more-accurately reflect actual reference pressure than reference pressures from less-reliable weather stations (e.g., weather stations that have not been recently calibrated, that are affected by overheating, etc.). 
     In yet another implementation of step  521 , the average is a weighted average that uses weights based on two or more of respective distances, respective ages, or respective qualities. 
     Determining if a Reference Pressure Value is Valid (Step  350 ) 
       FIG.  6    depicts a process for determining if a reference pressure value is valid during an embodiment of step  350 . 
     Initially, a time at which pressure in the network is expected to change by a threshold amount of pressure since a past instance of time is determined (step  651 ). In different embodiments, the past instance of time can be when the reference pressure value was determined, when a selected reference pressure was determined, or another time. In one embodiment, the time is determined by dividing the threshold amount of pressure by an expected rate of pressure change to yield an amount of time, and then adding the amount of time to the past instance of time to determine the time at which pressure in the network is expected to change by the threshold amount of pressure. The threshold amount of pressure can vary in different embodiments. In one embodiment, the threshold amount of pressure is set to an amount of pressure corresponding to a maximum amount of tolerated error for estimated altitudes (e.g., 10 Pa). In some embodiments, the determination during step  651  is of a time at which pressure is expected to change by the threshold amount of pressure within a particular area of the network that includes the transmitter and/or a weather station that provided a reference pressure used to compute the reference pressure value. Restricting step  651  to pressure changes within particular areas makes the determination more relevant to pressure conditions that are actually expected to affect the transmitter and/or weather stations. In some embodiments, the area may be determined by cellular element as discussed above. 
     An expiration time for the reference pressure value, before which the reference pressure value is valid and after which the reference pressure value is invalid, is determined based on the determined time (step  652 ). In one embodiment of step  652 , the expiration time is the determined time. In another embodiment of step  652 , the expiration time is a time that precedes the determined time by a threshold amount of time (e.g., t units of time). 
     An area in which the reference pressure value is valid can be determined with or without an expiration time. The area may be a particular localized pressure zone among other localized pressure zones, such that the reference pressure value is only valid in the particular localized pressure zone. Alternatively, the area may be an area in the network that is not expecting changes in pressure that are expected for other areas in the network, such that the reference pressure value is not valid in those other areas. 
     Generating a New Reference Pressure Value (Step  360 ) 
       FIG.  7 A  and  FIG.  7 B  depict processes for generating a new reference pressure value during different embodiments of step  360 . 
       FIG.  7 A  depicts a process for generating a new reference pressure value by adjusting a reference pressure value, which comprises adjusting the reference pressure value by an amount of pressure change that occurred since the reference pressure value was determined (step  761   a ). The amount of pressure change that occurred can be determined in different ways, for example, including measurements of the pressure change from weather stations in the network or from a weather report. 
       FIG.  7 B  depicts a process for generating a new reference pressure value by adjusting reference pressures that were used to compute the previous reference pressure value. For each reference pressure of the selected reference pressures that originated from an area in which pressure changed in excess of a threshold amount of pressure since the reference pressure was determined, that reference pressure is adjusted by an amount of pressure change that occurred since that reference pressure was determined (step  761   b ), and the new reference pressure value is computed using an average of the adjusted reference pressures (step  762   b ). In one implementation of step  762   b,  the average is a weighted average that uses weights based on an inverse of respective distances between weather stations of the adjusted reference pressures and part of the cellular element. In another implementation of step  762   b,  the average is a weighted average that uses weights based on an inverse of respective ages of the reference pressures that were adjusted. In yet another implementation of step  762   b,  the average is a weighted average that uses weights based on respective qualities of the reference pressures that were adjusted. Alternatively, the average may be a weighted average that uses weights based on two or more of respective distances, respective ages, or respective qualities 
     As indicated above with respect to step  330  of  FIG.  3   , a mobile device that is within a coverage area of the transmitter can receive a reference pressure value transmitted by the transmitter. That mobile device can use the received reference pressure value to determine reference pressure data for use in estimating an altitude of the mobile device or for other uses (e.g., calibrating a pressure sensor of the mobile device). As discussed below with respect to  FIG.  8   , different approaches can be used to determine such reference pressure data. 
     Determining Reference Pressure Data That is Used to Compute Altitude Estimates of Mobile Devices or Calibrate Pressure Sensors of Mobile Devices 
       FIG.  8    depicts a process for determining reference pressure data that can be used to compute altitude estimates of mobile devices. Alternatively, the reference pressure data can be used to calibrate pressure sensors of mobile devices, or other uses that are known in the art. 
     Initially, a mobile device receives a reference pressure value for a reference altitude that was transmitted by a transmitter (step  810 ). 
     At any time after receiving the reference pressure value, and before receiving any new reference pressure value from the transmitter, a determination is made as to whether the reference pressure value is valid (step  820 ). Embodiments of step  820  are depicted in  FIG.  9 A  and  FIG.  9 B , which are described later. 
     If the reference pressure value is determined to be valid during step  820 , a first representative reference pressure value is determined by combining the reference pressure value received from the transmitter with any other valid reference pressure values received from other transmitters (step  830 ). Embodiments of step  830  are depicted in  FIG.  10 A ,  FIG.  10 B ,  FIG.  10 C , and  FIG.  10 D , which are described later. 
     After the first representative reference pressure value is determined, it can be used with the reference altitude and a pressure value measured by a pressure sensor of the mobile device to estimate an altitude of the mobile device (step  840 ). 
     If the reference pressure value is determined to be not valid during step  820 , a new reference pressure value is determined by adjusting the reference pressure value received from the transmitter based on changes in pressure in the network that have occurred since the reference pressure value was originally determined or since the reference pressure value was received (step  850 ). Embodiments of step  850  are depicted in  FIG.  11 A ,  FIG.  11 B , and  FIG.  11 C , which are described later. 
     After the new reference pressure value is determined, a second representative reference pressure value is determined by combining the new reference pressure value with one or more other new reference pressure values and or valid older reference pressure values corresponding to other transmitters (step  860 )—e.g., using the same process as used for step  830 . 
     After the second representative reference pressure value is determined, it can be used with the reference altitude and the pressure value measured by the pressure sensor of the mobile device to estimate the altitude of the mobile device (step  870 —e.g., using the same process as used for step  840 . 
     By way of example, a processor of the mobile device can be used to perform steps  820 ,  830 ,  840 ,  850 ,  860  and  870 . Alternatively, information needed to perform some or all of these steps can be provided from the mobile device to the network backend  130  of  FIG.  1    using known communication pathways, and the backend  130  performs those steps. 
     Determining if a Received Reference Pressure Value is Valid (Step  820 ) 
       FIG.  9 A  and  FIG.  9 B  depict processes for determining if a received reference pressure value is valid during embodiments of step  820 . 
     During  FIG.  9 A , an expiration time of the reference pressure value is received by the mobile device from a transmitter (step  921   a ), and a determination is made if the reference pressure value is valid (step  922   a ). In one implementation, the reference pressure is valid when the time at which step  922   a  is performed is before the expiration time, and the reference pressure value is not valid if the time at which step  922   a  is performed is after the expiration time. 
     During  FIG.  9 B , a time at which pressure in the network is expected to change by a threshold amount of pressure since a past instance of time is determined (step  921   b ). In different embodiments, the past instance of time can be when the reference pressure value was determined, when a selected reference pressure was determined, or another time. In one embodiment, the time is determined by dividing the threshold amount of pressure by an expected rate of pressure change to yield an amount of time, and then adding the amount of time to the past instance of time to determine the time at which pressure in the network is expected to change by the threshold amount of pressure. The threshold amount of pressure can vary in different embodiments. In one embodiment, threshold amount of pressure is set to an amount of pressure corresponding to a maximum amount of tolerated error for estimated altitudes (e.g., 10 Pa). In some embodiments, the determination during step  921   b  is of a time at which pressure is expected to change by the threshold amount of pressure within a particular area of the network that includes the transmitter and/or a weather station that provided a reference pressure used to compute the reference pressure value. Restricting step  921   b  to pressure changes within particular areas makes the determination more relevant to pressure conditions that are actually expected to affect the transmitter and/or weather stations. 
     An expiration time for the reference pressure value, before which the reference pressure value is valid and after which the reference pressure value is invalid, is determined based on the determined time (step  922   b ). In one embodiment of step  922   b,  the expiration time is the determined time. In another embodiment of step  652 , the expiration time is a time that precedes the determined time by a threshold amount of time. 
     A determination is made if the reference pressure value is valid (step  923   b ). In one implementation, the reference pressure is valid when the time at which step  923   b  is performed is before the expiration time, and the reference pressure value is not valid if the time at which step  923   b  is performed is after the expiration time. 
     In some embodiments, an area of validity for a reference pressure value can also or alternatively be determined and then provided to a mobile device. In these embodiments, an estimated position of the mobile device can be computed (e.g., latitude and longitude). A determination is made if the estimated position is within the area of validity, or within a threshold distance of the area of validity. If so, the reference pressure value is treated as valid. If not, the reference pressure value is treated as not valid. 
     Using Validity of Reference Pressure Value to Control Whether a Mobile Device Searches a Broadcast Channel 
     In some embodiments, step  820  of  FIG.  8   , including the sub-steps of  FIG.  9 A  or  FIG.  9 B  can be performed to determine the validity of a reference pressure value that was previously received from a transmitter. If the value is valid, a decision is made at the mobile device to not have the mobile device search for a broadcast of a new reference pressure value. Not searching can continue until the previously received reference pressure value is no longer valid, after which the mobile device searches for a new reference pressure value. Alternatively, the search for a new reference pressure value may start at a predetermined amount of time prior to the end of validity under circumstances when the end of validity is known. Advantages of using the validity of a reference pressure value to control whether a mobile device searches a broadcast channel for a new reference pressure value include power savings and increases in processing capacity while searching is not being performed. 
     Determining a Representative Reference Pressure Value by Combining Reference Pressure Values (Step  830 ) 
       FIG.  10 A  through  FIG.  10 D  depict processes for determining a representative reference pressure value by combining reference pressure values during embodiments of step  830 . 
     During  FIG.  10 A , the n most-recently determined reference pressure values are selected from m received reference pressure values, wherein m is greater than n (step  1031   a ), and the representative reference pressure value is computed by combining the n selected reference pressures (step  1032   a ). In one implementation, the combination is a weighted average based on an inverse of the ages. 
     During  FIG.  10 B , the n reference pressure values having the longest validity times are selected from m received reference pressure values, wherein m is greater than n (step  1031   b ), and the representative reference pressure value is computed by combining the n selected reference pressures (step  1032   b ). In one implementation, the combination is a weighted average based on an inverse of the validity times. 
     During  FIG.  10 C , the n reference pressure values from n respective transmitters that are closest to an estimated location of the mobile device are selected from m received reference pressure values, wherein m is greater than n (step  1031   c ), and the representative reference pressure value is computed by combining the n selected reference pressures (step  1032   c ). In one implementation, the combination is a weighted average based on an inverse of distances between the n transmitters and the estimated position of the mobile device. 
     During  FIG.  10 D , the representative reference pressure value is computed using a weighted average of the received reference pressure values (step  1031   d ). In different implementations, the weights can be based on any of inverses of ages, inverses of lengths of validity times, and/or inverses of distances (e.g., between the n transmitters and the estimated position of the mobile device). 
     Determining a New Reference Pressure Value (Step  850 ) 
       FIG.  11 A  through  FIG.  11 C  depict processes for determining a new reference pressure value during embodiments of step  850 . 
     During  FIG.  11 A , the reference pressure value is adjusted by an amount of pressure change that occurred since the reference pressure value was determined (step  1151   a ). 
     During  FIG.  11 B , an estimated position of the mobile device is determined (step  1151   b ), an expected amount of change in pressure between the estimated position of the mobile device and a previous estimated position at which the reference pressure value was received is determined (step  1152   b ), and the reference pressure value is adjusted by the expected amount of change in pressure (step  1153   b ). 
     During  FIG.  11 C , an estimated position of the mobile device is determined (step  1151   c ), an expected amount of change in pressure between the estimated position of the mobile device and the transmitter is determined (step  1152   c ), and the reference pressure value is adjusted by the expected amount of change in pressure (step  1153   c ). 
     Additional Operations for Networks That Use Requests for Assistance Information From Mobile Devices 
     Different systems can benefit from the approaches described above, including (i) systems that broadcast assistance information (e.g., reference pressure values) on a schedule, and (ii) systems that transmit reference pressure values in response to receiving requests from mobile devices for a reference pressure value. In the latter systems, additional functionality may be used. For example, mobile devices can monitor times when particular reference pressure values expire. If areas of validity are used, the mobile devices can also monitor areas in which particular reference pressure values are not valid. If a reference pressure value from a transmitter has expired, or is not valid in an area in which the mobile device is believed to reside (e.g., based on comparison of an estimated position of the mobile device and an area of validity for the reference pressure value), the mobile device can ignore that reference pressure value, adjust it, request an adjustment to the reference pressure value (or an adjusted reference pressure value) from the transmitter or the network backend, or request a new reference pressure value from the transmitter if still in range of the transmitter. 
     Application of Methods to Other Data Provided by Weather Stations 
     Discussion above has been generally focused on reference pressures and reference pressure values. However, the approaches can be modified as would be understood by one of ordinary skill in the art to select temperature measurements of weather stations before using the selected temperature measurements to determine a reference temperature value for use in the altitude computation that is based on selected temperature measurement(s) from weather station(s). For example, each of the methods could be modified to replace “pressure” with “temperature”. For embodiments where reference temperatures are not used, then measured temperatures that were measured at different altitudes can replace reference pressures of the reference altitude. Selection of temperature or reference temperatures could be based on proximity, age, quality, or another condition. Similarly, the approaches can be modified as would be understood by one of ordinary skill in the art to select humidity (or reference humidity), other atmospheric parameters, or any data that is available from weather stations. Use of measured temperatures to determine a reference temperature value can involve averaging the measured temperatures (e.g., based on weights) to determine the reference temperature value, or adjusting the measured temperatures to reference temperatures at a reference altitude (e.g., using techniques that are known in the art) before averaging the resultant reference temperatures to determine the reference temperature value. Selection of measured temperatures from weather stations can also be limited to weather stations with altitudes that are within a range of acceptable altitudes (e.g., altitudes within X units of distance from the reference altitude or another altitude, such as a ground-level altitude in the vicinity of the mobile device). 
     The term “reference data” refers to any of: reference pressure(s) from weather station(s); measured temperature(s) or reference temperature(s) from weather station(s); or other atmospheric data from weather station(s). The term “reference value” refers to any of: reference pressure value(s) determined from reference pressure(s); reference temperature value(s) determined from measured temperature(s) or reference temperature(s); or other reference atmospheric data values determined from atmospheric data collected from weather station(s). 
     Other Aspects 
     Discussion above has been generally focused on reference pressures and reference pressure values. However, the approaches can be modified as would be understood by one of ordinary skill in the art to select temperatures for weather stations before using the selected temperatures to determine a reference temperature value for use in the altitude computation that is based on reference temperature(s) from weather station(s). Selection could be based on proximity relative to a cellular element, age, quality or other condition. Similarly, the approaches can be modified as would be understood by one of ordinary skill in the art to select humidity or other atmospheric parameters, or to select any data that is available from weather stations. 
     Different protocols known in the art for transmitting reference pressure values, reference temperature values, locations of transmitters, etc., are contemplated, including use of system information blocks (SIBs). 
     Any method (also referred to as a “process” or an “approach”) described or otherwise enabled by disclosure herein may be implemented by hardware components (e.g., machines), software modules (e.g., stored in machine-readable media), or a combination thereof. In particular, any method described or otherwise enabled by disclosure herein may be implemented by any concrete and tangible system described herein. By way of example, machines may include one or more computing device(s), processor(s), controller(s), integrated circuit(s), chip(s), system(s) on a chip, server(s), programmable logic device(s), field programmable gate array(s), electronic device(s), special purpose circuitry, and/or other suitable device(s) described herein or otherwise known in the art. One or more non-transitory machine-readable media embodying program instructions that, when executed by one or more machines, cause the one or more machines to perform or implement operations comprising the steps of any of the methods described herein are contemplated herein. As used herein, machine-readable media includes all forms of machine-readable media, including but not limited to one or more non-volatile or volatile storage media, removable or non-removable media, integrated circuit media, magnetic storage media, optical storage media, or any other storage media, including RAM, ROM, and EEPROM, that may be patented under the laws of the jurisdiction in which this application is filed, but does not include machine-readable media that cannot be patented under the laws of the jurisdiction in which this application is filed. Methods disclosed herein provide sets of rules that are performed. Systems that include one or more machines and one or more non-transitory machine-readable media for implementing any method described herein are also contemplated herein. One or more machines that perform or implement, or are configured, operable or adapted to perform or implement operations comprising the steps of any methods described herein are also contemplated herein. Each method described herein that is not prior art represents a specific set of rules in a process flow that provides significant advantages in the field of determining, broadcasting and using reference pressure data in a network of transmitters. Method steps described herein may be order independent and can be performed in parallel or in an order different from that described if possible to do so. Different method steps described herein can be combined to form any number of methods, as would be understood by one of ordinary skill in the art. Any method step or feature disclosed herein may be omitted from a claim for any reason. Certain well-known structures and devices are not shown in figures to avoid obscuring the concepts of the present disclosure. When two things are “coupled to” each other, those two things may be directly connected together, or separated by one or more intervening things. Where no lines or intervening things connect two particular things, coupling of those things is contemplated in at least one embodiment unless otherwise stated. Where an output of one thing and an input of another thing are coupled to each other, information sent from the output is received in its outputted form or a modified version thereof by the input even if the information passes through one or more intermediate things. Any known communication pathways and protocols may be used to transmit information (e.g., data, commands, signals, bits, symbols, chips, and the like) disclosed herein unless otherwise stated. The words comprise, comprising, include, including and the like are to be construed in an inclusive sense (i.e., not limited to) as opposed to an exclusive sense (i.e., consisting only of). Words using the singular or plural number also include the plural or singular number, respectively, unless otherwise stated. The word “or” and the word “and” as used in the Detailed Description cover any of the items and all of the items in a list unless otherwise stated. The words some, any and at least one refer to one or more. The terms may or can are used herein to indicate an example, not a requirement—e.g., a thing that may or can perform an operation, or may or can have a characteristic, need not perform that operation or have that characteristic in each embodiment, but that thing performs that operation or has that characteristic in at least one embodiment. Unless an alternative approach is described, access to data from a source of data may be achieved using known techniques (e.g., requesting component requests the data from the source via a query or other known approach, the source searches for and locates the data, and the source collects and transmits the data to the requesting component, or other known techniques). 
       FIG.  12    illustrates components of a transmitter, a mobile device, and a server. Examples of communication pathways are shown by arrows between components. 
     By way of example in  FIG.  12   , each of the transmitters may include: a mobile device interface  11  for exchanging information with a mobile device (e.g., antenna(s) and RF front end components known in the art or otherwise disclosed herein); one or more processor(s)  12 ; memory/data source  13  for providing storage and retrieval of information and/or program instructions; atmospheric sensor(s)  14  for measuring environmental conditions (e.g., pressure, temperature, humidity other) at or near the transmitter; a server interface  15  for exchanging information with a server (e.g., an antenna, a network interface, or other); and any other components known to one of ordinary skill in the art. The memory/data source  13  may include memory storing software modules with executable instructions, and the processor(s)  12  may perform different actions by executing the instructions from the modules, including: (i) performance of part or all of the methods as described herein or otherwise understood by one of skill in the art as being performable at the transmitter; (ii) generation of positioning signals for transmission using a selected time, frequency, code, and/or phase; (iii) processing of signaling received from the mobile device or other source; or (iv) other processing as required by operations described in this disclosure. Signals generated and transmitted by a transmitter may carry different information that, once determined by a mobile device or a server, may identify the following: the transmitter; the transmitter&#39;s position; environmental conditions at or near the transmitter; and/or other information known in the art. The atmospheric sensor(s)  14  may be integral with the transmitter, or separate from the transmitter and either co-located with the transmitter or located in the vicinity of the transmitter (e.g., within a threshold amount of distance). 
     By way of example  FIG.  12   , the mobile device may include: a transmitter interface  21  for exchanging information with a transmitter (e.g., an antenna and RF front end components known in the art or otherwise disclosed herein); one or more processor(s)  22 ; memory/data source  23  for providing storage and retrieval of information and/or program instructions; atmospheric sensor(s)  24  for measuring environmental conditions (e.g., pressure, temperature, other) at the mobile device; other sensor(s)  25  for measuring other conditions (e.g., inertial sensors for measuring movement and orientation); a user interface  26  (e.g., display, keyboard, microphone, speaker, other) for permitting a user to provide inputs and receive outputs; another interface  27  for exchanging information with the server or other devices external to the mobile device (e.g., an antenna, a network interface, or other); and any other components known to one of ordinary skill in the art. A GNSS interface and processing unit (not shown) are contemplated, which may be integrated with other components (e.g., the interface  21  and the processors  22 ) or a standalone antenna, RF front end, and processors dedicated to receiving and processing GNSS signaling. The memory/data source  23  may include memory storing software modules with executable instructions, and the processor(s)  22  may perform different actions by executing the instructions from the modules, including: (i) performance of part or all of the methods as described herein or otherwise understood by one of ordinary skill in the art as being performable at the mobile device; (ii) estimation of an altitude of the mobile device based on measurements of pressure form the mobile device and transmitter(s), temperature measurement(s) from the transmitter(s) or another source, and any other information needed for the computation); (iii) processing of received signals to determine position information (e.g., times of arrival or travel time of the signals, pseudoranges between the mobile device and transmitters, transmitter atmospheric conditions, transmitter and/or locations or other transmitter information); (iv) use of position information to compute an estimated position of the mobile device; (v) determination of movement based on measurements from inertial sensors of the mobile device; (vi) GNSS signal processing; or (vii) other processing as required by operations described in this disclosure. 
     By way of example  FIG.  12   , the server may include: a mobile device interface  31  for exchanging information with a mobile device (e.g., an antenna, a network interface, or other); one or more processor(s)  32 ; memory/data source  33  for providing storage and retrieval of information and/or program instructions; a transmitter interface  34  for exchanging information with a transmitter (e.g., an antenna, a network interface, or other); and any other components known to one of ordinary skill in the art. The memory/data source  33  may include memory storing software modules with executable instructions, and the processor(s)  32  may perform different actions by executing instructions from the modules, including: (i) performance of part or all of the methods as described herein or otherwise understood by one of ordinary skill in the art as being performable at the server; (ii) estimation of an altitude of the mobile device; (iii) computation of an estimated position of the mobile device; or (iv) other processing as required by operations described in this disclosure. Steps performed by servers as described herein may also be performed on other machines that are remote from a mobile device, including computers of enterprises or any other suitable machine. 
     Certain aspects disclosed herein relate to estimating the positions of mobile devices—e.g., where the position is represented in terms of: latitude, longitude, and/or altitude coordinates; x, y, and/or z coordinates; angular coordinates; or other representations. Various techniques to estimate the position of a mobile device can be used, including trilateration, which is the process of using geometry to estimate the position of a mobile device using distances traveled by different “positioning” (or “ranging”) signals that are received by the mobile device from different beacons (e.g., terrestrial transmitters and/or satellites). If position information like the transmission time and reception time of a positioning signal from a beacon are known, then the difference between those times multiplied by speed of light would provide an estimate of the distance traveled by that positioning signal from that beacon to the mobile device. Different estimated distances corresponding to different positioning signals from different beacons can be used along with position information like the locations of those beacons to estimate the position of the mobile device. Positioning systems and methods that estimate a position of a mobile device (in terms of latitude, longitude and/or altitude) based on positioning signals from beacons (e.g., transmitters, and/or satellites) and/or atmospheric measurements are described in co-assigned U.S. Pat. No. 8,130,141, issued Mar. 6, 2012, and U.S. Pat. Pub. No. 2012/0182180, published Jul. 19, 2012. It is noted that the term “positioning system” may refer to satellite systems (e.g., Global Navigation Satellite Systems (GNSS) like GPS, GLONASS, Galileo, and Compass/Beidou), terrestrial transmitter systems, and hybrid satellite/terrestrial systems. 
     This application relates to the following related application(s): U.S. Pat. Appl. No. 62/874,811, filed 2019 Jul. 16, entitled SYSTEMS AND METHODS FOR DETERMINING, BROADCASTING AND USING REFERENCE ATMOSPHERIC DATA IN A NETWORK OF TRANSMITTERS. The content of each of the related application(s) is hereby incorporated by reference herein in its entirety.