Abstract:
A position location system is described for determining position of mobile Tags attached to assets or people. Zone Tags are installed in the venue to assist in position location estimation of mobile Tags. Time is divided into intervals, each interval comprising at least one range measurement sub-interval, and an idle sub-interval where zone Tags and mobile Tags go to sleep to save power. A zone Tag or mobile Tag is designated as the anchor network element. The anchor network element contends and seizes the communications channel at the beginning of each interval to start the range measurement phase. Mobile Tag position location estimates are made periodically. Between the periodic range measurement based position estimates, mobile Tag sensor readings are used to update the mobile Tag&#39;s position location. The period between range measurement based position updates is adjusted based on the statistics of the differences between the range measurements based position estimate and the sensor based position updated.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to PCT Patent Application No. PCT/US15/017423 entitled “Systems and Methods of Location and Tracking” filed Feb. 25, 2015, which application claims priority to U.S. Non-provisional patent application Ser. No. 14/284,384, entitled: “Systems and Methods of Location and Tracking” filed May 21, 2014 and to U.S. Provisional Application Ser. No. 61/944,115, filed Feb. 25, 2014 and entitled: “Systems and Methods of Location and Tracking”, the contents of each which are incorporated by reference in their entirety. 
     
    
     FIELD 
       [0002]    The invention generally relates to location and tracking systems and methods and more specifically to system and methods for tracking the physical position of assets. 
       COPYRIGHT 
       [0003]    A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright rights whatsoever. 
       BACKGROUND 
       [0004]    A number of systems have been developed and deployed to locate and track the position of people and assets collectively (“assets”) in local area environments such as office buildings, hospitals, shopping malls, and hotels, industrial environments such as refineries, oil rigs, and manufacturing facilities. Many of these systems, however, provide position accuracy of within a few meters using an existing Wi-Fi or Bluetooth network. 
         [0005]    Position location accuracy of a few meters may suffice in some applications such as locating a tool in an industrial environment such as in a refinery, in an aircraft hangar etc. In some applications, however, accuracy of less than one meter is required. Many of the applications requiring high position location precision involve establishing close association of two assets (e.g. two persons, a person and some equipment or two pieces of equipment. In many cases, it is not necessary that the precise location of a person or object (i.e. asset) to be known, but rather the association of the two is needed which requires determining that an asset is in the proximity of another asset. For instance, hospitals would like to record if a doctor or a nurse has visited a certain patient during the day. In this application, what is important is to record that the doctor/nurse was in proximity of the patient at certain time. The position location accuracy requirement for this case is often referred to as “room level” accuracy meaning that the doctor/nurse must be located in the right room. However, as mentioned above if the objective is to ensure that the doctor/nurse visited the patient during the day, then it is really detecting the close proximity of the doctor/nurse and the particular patient that is needed and not necessarily accurately locating the doctor/nurse. In rooms with multiple beds, the doctor/nurse must be associated with the right patient in that room. Another example of room level accuracy is in hotels where it is desirable to locate a maid in the right room in case there is an emergency, or to be able to verify that someone has cleaned a specific room. 
         [0006]    One manufacturing use case is to adjust the torque of a wrench depending on the specific manufacturing part the tool is trying to fasten. This again is a case of detecting proximity of the tool and a manufacturing part. Another example of the need to detect proximity is to ensure that certain medical equipment such as an IV pump is in fact attached to a certain patient. Another example is to register that doctors/nurses have washed their hands prior to certain functions such as visiting a patient. This application again requires detection of close proximity of the doctor/nurse and a sink/disinfectant station. 
         [0007]    Another application for accurate position location in retail environment is to direct the customer to the right location in the store for a given item. Here one would want a very accurate positioning scheme to navigate the customer through the store so that the customer gets very close to the item he/she is searching. 
       SUMMARY 
       [0008]    In one embodiment principles of the present disclosure provide a position location determination system having mobile tags and zone tags comprising a first network comprising at least one Access Point (AP) network element, each AP creating a coverage area referred to as a Basic Service Set (BSS), configuring other network elements in its BSS, and providing access to the internet to network elements in its BSS, a second network of zone tag network elements underlying the first network capable of making range measurements with mobile tags, time being divided into range-measurement-period intervals, each interval comprising a measurement period when tags make range measurements, followed by a sleep period where tags are sleep to save power, network elements waking up at the beginning of range-measurement-period, and an anchor network element contends for the channel and seizes the channel, mobile and zone tag tags making range measurements according to a schedule and the range measurements are used by triangulation algorithms to estimate the mobile tag position location. 
         [0009]    In one embodiment principles of the present disclosure provide A process for determining relative location of assets within a transmission zone of a wireless network comprising the steps of providing at least one zone tag for placement within said transmission zone at a known position, said zone tag having a wireless transceiver configured to communication over said wireless network, and receiving said position of said zone tag within said transmission zone; providing at least one mobile tag, for securing to an asset within said transmission zone, said mobile tag having a wireless transceiver configured to communication over said wireless network; associating each said mobile tag with at least one of said zone tag, said zone tag within transmission range of said mobile tag; said zone tags periodically receiving mobile tag communication; determining relative position of said mobile tag as a function of zone tag to mobile tag communication time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Following drawings in conjunction with detailed description help clarify the features and advantages of the present disclosure. In the figures, similar components are identified using the same reference label. Multiple instances of the same component in a figure are distinguished by inserting a dash after the reference label and adding a second reference label. 
           [0011]      FIG. 1  is an exemplary diagram of elements of a position location system. 
           [0012]      FIG. 2  is an exemplary temporal time line of round trip delay measurements between mobile and zone tags according to an embodiment of the current invention. 
           [0013]      FIG. 3  is an exemplary temporal time line of time of arrival measurements between mobile and zone tags according to an alternate embodiment of the current invention. 
           [0014]      FIG. 4  is an exemplary temporal time line of time of arrival measurements between mobile and zone tags according to an alternate embodiment of the current invention. 
           [0015]      FIG. 5  is an exemplary flow chart of round trip delay measurements between mobile and zone tags according to an embodiment of the current invention. 
           [0016]      FIG. 6  is an exemplary flow chart of time of arrival measurements between mobile and zone tags according to an embodiment of the current invention. 
           [0017]      FIG. 7  is an exemplary flow chart of time of arrival measurements between mobile and zone tags according to one embodiment. 
           [0018]      FIG. 8  is an exemplary flow chart of an embodiment to determine the required rate of position location updates of the mobile tags based on range measurements. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The Figures and text below, and the various embodiments used to describe the principles of the present invention are by way of illustration only and are not to be construed in any way to limit the scope of the invention. A Person Having Ordinary Skill in the Art (PHOSITA) will readily recognize that the principles of the present invention may be implemented in any type of suitably arranged device or system. Specifically, while the present invention is described with respect to use in Wi-Fi networks and Access Points therein, a PHOSITA will readily recognize other types of networks and other applications without departing from the scope of the present invention. 
         [0020]    Before the present invention is described in further detail, it is to be understood that the invention is not limited to the particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims. 
         [0021]    Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention. 
         [0022]    Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by a PHOSITA to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, a limited number of the exemplary methods and materials are described herein. 
         [0023]    It must be noted that as used herein and in the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
         [0024]    All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided may be different from the actual publication dates, which may need to be independently confirmed. 
         [0025]    The indoor position location system complements an existing IEEE 802.11 wireless local area network (WLAN) network to provide high precision indoor position location information for mobile tags secured to generally immobile assets, such as equipment or mobile assets such as cellular telephones. 
         [0026]      FIG. 1  shows a typical IEEE 802.11 network deployment. Access Points (APs) are labeled as APi, where i is an integer identifying different APs. There are multiple IEEE 802.11 APs designated as AP 1 , AP 2 , etc. Each of the APs form a Basic Service Set (BSS), which is identified by a unique BSSID, or a local monitoring zone. The BSSs are typically interconnected to form an Extended Service Set (ESS) or extended monitoring zone, and share the same security credentials thereby facilitating easy movement of WiFi devices between different BSSs.  FIG. 1  shows a sample IEEE 802.11 network with 2 BSSs, each of the BSSs has one AP. The BSSs can optionally be linked to form an ESS. Note that the different BSSs, such as for instance the adjacent BSSs, may be operating on different frequency channels to minimize interference between BSSs. 
         [0027]    APs are the wireless network WiFi elements that may already have been installed for internet access in a given building or campus that usually connect to a wired network. In addition to APs, a number of auxiliary APs referred to as zone tags, labeled as zTi, where i is an integer identifying different zone tags, are also installed in the venue. The zone tags are disposed at known positions within the local monitoring zone, which is defined the transmission distance of the APs. The reason for use of the term zone tags is that these are IEEE 802.11 capable devices, capable of being network nodes or passively analyzing the network traffic, which are installed on walls or ceilings and cover a certain “zone” in the venue. Zone tags have similar capability as the WiFi APs do, and provide range measurements with mobile tags to determine the mobile tags&#39; position location. In other words, zone tags complement the APs in the legacy WiFi network in a given venue. Zone tags may provide the same functionality as the IEEE 802.11 APs or IEEE 802.11 Stations (STAs), but whose primary function is radio frequency communication with the mobile tags in order to provide range measurement data. In fact, in a venue where there is no legacy WiFi network, then some of the zone tags will be configured to act as APs. 
         [0028]    Referring to  FIG. 1 , in addition to APs, the system includes a number of auxiliary zone tags zT 1 , zT 2  etc. As mentioned above, mobile tags are WiFi enabled devices secured to assets, and are capable of communicating with the APs and zone tags via radio frequency. Mobile tags are labeled by Tj, where j is an integer and denotes occurrences of mobile tags. The position location server  140  schedules Round Trip Delay (RTD) or Time Of Arrival (TOA) measurements between mobile tags and multiple zone tags, and carries out triangulation using the RTD or TOA measured values to determine the position of the mobile tag, and sends the computed position location to other parts of the network (not shown in  FIG. 1 ). 
         [0029]    In certain configurations, in order to save power, zone tags and mobile tags go into sleep mode, and wake up at certain time instants specified by the position location server to carry out RTD or TOA measurements with the mobile tags. Note that the task of the position location server may be located in other network elements, or even be distributed among network elements. It is assumed that the frame timing of the zone tags and the mobile tags are aligned so that the mobile and zone tags may wake up at approximately the same frame boundaries scheduled by the position location server. The position location server assigns mobile tags to carry out RTD/TOA measurements with a subset of zone tags whose signals the mobile tag can detect. As mentioned above, mobile and zone tags are in sleep mode when they are not making range measurements, and wake up at certain time instants scheduled by the position location server. When a zone tag wakes up, it must first contend with other devices on the network to seize the communications channel. Once a zone tag has seized the channel, it needs to hold on to the channel until it has made range measurements with all mobiles in its coverage, and then pass the channel to other zone tags in the given BSS. 
         [0030]    In the IEEE 802.11 protocol, devices measure the signal strength on the communications channel, and if the measured signal strength is above a threshold, the devices decide that the channel is occupied and go into channel monitoring mode. When in channel monitoring mode, devices only begin transmitting if the channel has become idle for a time interval of DIFS (DCF Inter-Frame Space) plus a random backoff time interval. Note that the channel is determined to be idle if the measured signal strength on the channel is below a threshold. The random backoff time interval is chosen randomly by each device that is contending for the channel, and is designed to avoid collision among the multiple devices that may be simultaneously contending for the channel. In IEEE 802.11, there is also a Short Inter-Frame Space (SIFS), which is defined as a shorter time period than DIFS. SIFS is the time interval between receiving a Data frame and sending back an Ack frame. When a pair of IEEE 802.11 devices/APs are exchanging data, they wait a time interval of SIFS between receiving a packet and transmitting/responding with a packet such as an acknowledgment. Since SIFS is shorter than DIFS, and contending devices wait at least a DIFS interval before attempting to transmit, the device/AP pair that currently occupy the channel prevent other devices in the network from attempting to seize the channel, by using SIFS time interval between their transmissions. Therefore, once the zone tags seize the channel to make range measurements with mobile tags, the zone tags may hold on to the channel by spacing their transmissions by a SIFS or shorter time interval so that other devices do not attempt to access the channel. In certain configurations, the range-measurement-period is defined as the period during which the zone tags in a given BSS first contend and seize the communications channel, sequentially make range measurements with the scheduled mobile tags in that BSS and certain mobile tags in adjacent BSSs, and then relinquish the channel and go in sleep period. Therefore, the range-measurement-period consists of three intervals of: contending for the channel; carrying out range measurements; and going to sleep. 
         [0031]    Some of the embodiments that are described in this disclosure, use the technique of only waiting a SIFS or a shorter time interval between transmissions in order to avoid ceding the channel to other devices; until zone tags have made all scheduled range measurements with the mobile tags for the given range-measurement-period. Moreover, when zone tags and mobile tags wake up to begin the range-measurement-period, zone tags may seize the channel more quickly by only waiting a time interval with duration between SIFS and DIFS, or even a time interval less than SIFS, once the channel becomes idle. Note that since other devices will wait DIFS plus a random time interval before attempting to transmit, then a zone tag that only waits a time interval between SIFS and DIFS will preempt other contending devices from seizing the channel, and will get access to the channel first. 
         [0032]    An embodiment whereby zone tags seize the channel and make RTD measurements with the mobile tags is described next using  FIG. 1  and  FIG. 2  as illustrations. In the example of  FIG. 1 , there are three mobile tags T 1 , T 2  and T 3  in BSS 1  with which zT 1  needs to make RTD measurements. Each zone tag has a mobile-tag-measurement-list that includes all mobile tags with which a given zone tag makes range measurements. The three tags T 1 , T 2  and T 3  are in the mobile-tag-measurement-list of zone tags zT 1 , zT 2  and zT 3 .  FIG. 2  illustrates an exemplary temporal diagram, showing the time sequence when the zone tag zT 1  seizes the channel, and makes RTD measurements with mobile tags T 1 , T 2  and T 3  in its mobile-tag-measurement-list. Zone tag zT 1  seizes the channel, and sends Data frame  1  to mobile tag T 1  at time TOD 1 . Mobile tag T 1  sends an Acknowledgement (Ack) frame back to zT 1  a SIFS (Short Inter-Frame Space) time after receiving Data frame  1 . Once zT 1  has received Ack frame  1  from T 1 , it can estimate the RTD from zT 1  to T 1 . zT 1  after a time interval of length SIFS or shorter upon receiving an Ack frame from T 1 , sends a Data frame  2  to T 2 . T 2  in turn responds with Ack frame SIFS time interval after receiving Data frame  2 . zT 1  repeats the same Data/Ack frame sequence to measure RTD with T 3 . Note that as described previously in this disclosure, zone tag zT 1  sends Data and Ack packets spaced at SIFS or shorter time interval in order to prevent other devices in the BSS from contending for the channel. Once zT 1  has completed RTD measurements with all mobile tags in BSS 1 , then zT 1  may send a Data frame to zT 2 , using SIFS as inter-frame time spacing, to allow zT 2  to seize the channel before other devices in the network may contend for the channel. Similarly, zone tag zT 2  carries out RTD/TOA range measurements with mobile tags T 1 , T 2  and T 3  in BSS 1 , and transfers control of the channel to zone tag zT 3  by sending a Data frame to zT 2  using SIFS as inter-frame time interval. In the above description, SIFS was used as inter-frame time interval by zone tags to prevent other devices from contending for the channel. However, as was described previously any inter-frame time interval that is smaller than DIFS would also be effective. Therefore, SIFS is just one example of inter-frame time interval that prevents other devices from contending for the channel. 
         [0033]    A zone-tag-list, also referred to as a zone tag directory, is the list of zone tags in a BSS that need to make range measurements with mobile tags. A mobile-tag-list, also referred to as a mobile tag directory, is the list of mobile tags that need to make range measurements with zone tags of a BSS. That is to say mobile tags within a BSS are associated with one or more zone tags.  FIG. 5  is an exemplary flow chart of an embodiment used to make RTD measurement between zone tags and mobile tags. The process iterates through each of the zone tags in the zone tag list. The zone tag in the zone-tag-list in the BSS is chosen as the “current” zone tag. In step  502 : the current zone tag contends for the channel, and seizes the channel at the beginning of the range-measurement-period; and the mobile tag at the top of the mobile-tag-list is currently designated as the current mobile tag. The current zone tag is the one that starts RTD measurements with mobile tags. In step  504 , the current zone tag sends a data frame to the current mobile tag. In step  506 , the current mobile tag responds to the Data frame with an Acknowledgement frame. In step  508 , the current zone tag estimates RTD to the current mobile tag using the TOD (Time Of Departure) of Data frame from current zone tag and the TOA of Acknowledgement frame at the current zone tag. In step  510 , the current zone tag verifies if all associated mobile tags on the mobile-tag-list have made RTD measurements with current zone tag; if not the next mobile tag on the mobile-tag-list is chosen as the current mobile tag in step  512  and the process goes back to step  504 . If all mobile tags have made RTD measurements with the current zone tag, then in step  514  the current zone tag determines if it is the last zone tag on the zone tag-list; if not, then in step  516  the next zone tag on the zone-tag-list is set as the current zone tag and the process moves to step  504 . If the current zone tag is the last zone tag on the zone-tag-list, then the process moves to step  518  where zone tags send their RTD measurements to the position location server. 
         [0034]    As shown in  FIG. 1 , zone tag zT 4  that is associated with BSS 2  is also close enough to mobile tag T 1  of BSS 1  to be able to make RTD measurement with T 1 . As shown with dashed arrow lines in  FIG. 1 , zT 4  is assigned by the position location server to make range measurements with mobile tag T 1  in BSS 1  and mobile tags T 4 , T 5  and T 6  in BSS 2 . In other words the mobile-tag-measurement-list of zT 4  contains three tags in BSS 2  and one tag in BSS 1 . Therefore, mobile tags that are in close proximity of zone tags of multiple BSSs may be in the mobile-tag-measurement-list of zone tags that are associated with different BSSs; these mobile tags will need to wake up during the range-measurement-interval of each of the corresponding BSSs. 
         [0035]    Now disclosing an alternate embodiment,  FIG. 3  is a temporal illustration of packet exchanges between mobile tags and zone tags whereby at least two distal zone tags measure TOA of packets transmitted by each mobile tags. In this embodiment, one zone tag in the BSS, zT 1  in the example of  FIG. 3 , is chosen as the anchor zone tag to control the sequence of TOA measurements as described next. Zone tag zT 1  contends for the channel at the beginning of the range-measurement-period, and after seizing the channel, initiates TOA measurement with mobile tag T 1 . At time TOD 1 , zT 1  sends a Data frame  1  to mobile tag T 1 , where TOD stands for Time of Departure in this disclosure. Mobile tag T 1 , following reception of Data frame  1  and after a time interval of SIFS, sends Ack frame  1 , encoding a packet frame identifier therein in exemplary configuration. Zone tags zT 1 , zT 2 , zT 3  and zT 4  receive Ack frame  1  at times TOA 1 , TOA 2 , TOA 3  and TOA 4 , where TOA stands for Time OF Arrival. Zone tag zT 1  after a SIFS time duration following reception of Ack frame  1 , at time TOD 2  sends a Data frame  2  destined for mobile tag T 2 . Mobile tag T 2  sends an Ack frame  2  in response to Data frame  2 . All zone tags measure the TOA of Ack frame  2  (TOA 5 , TOA 6 , TOA 7 , and TOA 8 ). This process is repeated for mobile tag  3 . All zone tags send the measured TOA values from all mobile tags to the position location server for triangulation to determine the position of the mobile tags. In exemplary configuration, the position location server employs the packet identifier and time window for grouping frames for TOA calculation. Note that, instead of zone tag zT 1 , the task of sending Data frame to the mobile tags may be alternatively taken up by the AP of the BSS. 
         [0036]      FIG. 6  is an exemplary flow chart of an embodiment used by zone tags to make TOA measurements on the received Acknowledgement frames from the mobile tags. In step  602 : a zone tag is chosen as the anchor zone tag; anchor zone tag contends for and seizes the channel at the beginning of the range-measurement-period; anchor zone tag chooses the mobile tag at the top of mobile-tag-list as the “current” mobile tag. In step  604 , the anchor zone tag sends a Data frame destined for the current mobile tag. In step  606 , the current mobile tag responds by sending an Ack frame to the anchor zone tag. In step  608 , all zone tags that can detect the Ack frame make TOA measurements of the Ack frame. In step  610 , the anchor zone tag verifies if it has sent Data frames to all mobile tags in the mobile-tag-list: if not, then in step  612  the next mobile tag on the mobile-tag-list is chosen as the current mobile tag and the process moves to step  604 ; if yes, the process moves to step  614  where zone tags send the TOA measurements to the position location server. 
         [0037]    In an alternate embodiment,  FIG. 4  is a temporal illustration of another embodiment whereby mobile tags transmit Data frames to allow the zone tags to make TOA measurements with mobile tags. In this embodiment, the TOA is measured by zone tags using the received Data frame from mobile tags, encoding a packet frame identifier such as the mobile tab identification number therein in exemplary configuration, without a need for the receiving station to send back an Ack frame. This approach shortens the TOA measurement period. In order to request that the receiver not sent an Ack frame, the mobile tags may send an IEEE 802.11 QoS (Quality of Service) Data frame with the Ack Policy subfield set to No Ack, which will result in the recipient of the QoS Data frame not sending an Ack frame. In exemplary configuration, the position location server employs the packet identifier and time window for grouping frames for TOA calculation. 
         [0038]    As illustrated by  FIG. 4 , mobile tag T 1  is assumed to have seized the channel at the beginning of the range-measurement-period, and starts the TOA measurement by sending Data frame  1 . As mentioned above, the Data frame will request that no Ack be sent by the recipient station. Zone tags zT 1 , zT 2 , zT 3  and zT 4  that receive Data frame  1 , measure the TOA of Data frame  1 , TOA 1 , TROA 2 , TOA 3  and TOA 4 . Mobile tag T 2 , i.e. the next mobile tag on the mobile-tag-list, also receives Data frame  1  and, after a time duration of SIFS, sends Data frame  2  to allow zone tags to make TOA measurements on Data frame  2  to be used to estimate the position coordinates of mobile tag  2 . This process continues until all mobile tags have send a Data frame, whose TOA is measured at each zone tag. All zone tags send the measured TOA values from each received Data frame to the position location for triangulation to estimate the position of each mobile tag. In other words, mobile tags take turn in sending Data frames during the range-measurement-period. Note that as shown in  FIG. 4 , the time space between the Data frames sent by mobile tags is chosen to be SIFS or shorter so that other devices in the network do not attempt to access the channel. 
         [0039]      FIG. 7  is an exemplary flow chart of an embodiment, whereby mobile tags may initiate and control the TOA measurements with the zone tags. In step  702 : the mobile tag at the top of the mobile-tag-list is chosen as the “current” mobile tag; the current mobile tag contends for and seizes the channel. In step  704 , the current mobile tag sends Data frame. In step  706 , all zone tags that can detect the Data frame sent by the current mobile tag make TOA measurement on the received Data frame. In step  708 , the current mobile tag verifies if it is the last mobile tag on the mobile-tag-list: if no, in step  710  the next mobile tag on the mobile-tag-list is chosen as the next current mobile tag and the process moves to step  704 ; if yes, the process moves to step  712  where the zone tags send their Toa measurements to the position location server. 
         [0040]    Mobile tags may also be equipped with sensors such as accelerometer, gyroscope, compass, etc to update the position coordinates of the tag as the asset or the people carrying the mobile tag moves in a venue. The position of the mobile tag needs to be first determined using a scheme such as the triangulation methods based on RTD or TOA measurements between mobile tags and multiple zone tags. Once the mobile tag position is initially determined, then the sensors may update the tag&#39;s position. After certain time period, however, the errors in position updates using sensor readings will accumulate and the sensor updated mobile tag position will become inaccurate. Therefore, the position coordinates of the mobile tag need also be periodically refreshed using the triangulation methods based on RTD/TOA measurements. Various embodiments, employ different processes for refreshing range measurement data. In one embodiment, the range measurement based position location update period, referred to as range-measurement-based-positioning-period, is determined adaptively as described next. An initial measurement-based-positioning-period is chosen. The difference between the range measurement based position location estimates and the position location estimates from the sensor updates are computed at the end of each range-measurement-based-positioning-period. If the average, or some other statistical metric, of the difference between the range measurement based position estimate and the sensor based estimates is above a threshold the range-measurement-based-positioning-period interval would be decreased. On the other hand, if average, or some other statistical metric, of the difference between the range measurement based position estimate and the sensor based estimates is below a threshold the range-measurement-based-positioning-period interval would be increased. 
         [0041]      FIG. 8  is a flow chart of an embodiment used to determine the required position location updates of the mobile tags based on range measurements for a system that uses sensor measurements to update position location of mobile between range measurement position estimates. In step  802 , the position location server makes an initial position location estimate of the mobile tag based on range measurements between mobile and zone tag tags. In step  804 , the mobile tag position location is updated at discrete time epochs using sensor readings at the mobile tag, and a time counter is incremented. In step  806 , it is verified if the time counter is higher than the range-measurement-based-positioning-period: if no, the process moves back to step  804 ; if yes, the process moves to step  808 . In step  808 , a new position location of the mobile tag based on range measurements is made, and the time counter is initialized to 0. In step  810 , difference of the sensor based and range measurement based position location estimates is computed. Step  812  computes statistics of the difference between the two types of position location estimates. Step  814 , updates the range-measurement-based-positioning-period duration based on the statistics of the difference between the two types of position location estimates.