Abstract:
A device for activating a signal emitter for use in a firefighter locator system operates in conjunction with the firefighter&#39;s self-contained breathing apparatus (SCBA). The activation device includes a pressure sensor to monitor pressure on an air line that delivers air from the SCBA air tank to the SCBA air regulator. When the firefighter opens the SCBA air tank valve, pressure is sensed in the air line and the emitter is activated to transmit a signal. The signal is received by base stations that then use the signal to locate/track the signal emitter. Preferably, the activating system also includes a user operated reset button(s) and a shutoff circuit to allow the user to stop transmission of the signal by the signal emitter. The shutoff circuit is designed to stop transmission of the signal only after the air line has been depressurized and the reset button(s) is depressed.

Description:
This application is a continuation-in-part of application Ser. No. 09/691,751 filed Oct. 18, 2000 now U.S. Pat. No. 6,965,344, which is currently pending. The contents of application Ser. No. 09/691,751 are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains generally to systems for locating and tracking moving objects such as the position of a person inside a structure. More particularly, the present invention pertains to activation devices for signal emitters that are useful as part of a locator system. The present invention is particularly, but not exclusively, useful as a device for use in a firefighter locator system that automatically activates a firefighter&#39;s signal emitter when the firefighter opens the air tank valve on the firefighter&#39;s self-contained breathing apparatus (SCBA). 
     BACKGROUND OF THE INVENTION 
     There are many circumstances wherein there is a need to establish the accurate positioning and tracking of movable objects or individuals. This is particularly so when the individual or object is moving in a hostile or dangerous environment. One example is when a firefighter enters a structure during a rescue operation. In situations such as this, there is a need to determine the position of the firefighter from outside the structure with accuracies of approximately one meter. Although an object&#39;s position can be determined effectively outdoors using the current global positioning system (GPS), the GPS system is unsuitable, without augmentation, for locating moving objects indoors at accuracies of approximately one meter. 
     To accurately locate and track objects or individuals inside or adjacent to a structure, the tracking signal that is used by the system must have good penetration and little distortion through the walls and other features of the structures. Lack of-adequate signal penetration can result in a loss of signal strength which in turn can cause unacceptable location errors. Also, the signal should have low deflection (refraction and diffraction) to reduce the presence of multipath signals which limit location accuracy. Further, to locate an object&#39;s position accurately indoors, a system must provide sufficient coverage, and be able to acquire the signals quickly. 
     Unfortunately, radiofrequency (RF) systems using high frequency signals are limited in their ability to penetrate the walls and features of a structure. Also, because high frequency signals have wavelengths that are much shorter than the size of typical structural features such as rooms, hallways and staircases, these features can act as waveguides for the high frequency waves, altering the path of the signal. On the other hand, low frequency RF signals offer the potential to penetrate the walls and features of a structure and overcome inaccuracies due to fading and path length perturbations caused by diffraction and reflection. Further, since the wavelength of the low frequency waves are approximately the same or greater than the size of typical structural features, the features do not act as waveguides. Consequently, low frequency RF signals having wavelengths approximating the size of structural features are preferred over high frequency signals for use in and around structures. 
     Traditional positioning technologies use time-of-arrival and the angle-of-arrival methods. In a typical time-of-arrival system, the system measures the time of arrival of a marker modulated onto a signal to determine range. However, in time-of-arrival systems, increased resolution can only be obtained at the expense of increased bandwidth. By way of example, for a desired locating accuracy of one meter, a ranging system based on time of arrival would require a bandwidth on the order of tens of MHz. Unfortunately, this much bandwidth (tens of MHz) is unavailable at the low frequencies required for indoor use. 
     Another traditional positioning technology is the angle-of-arrival system. Typically, the angle of arrival is measured with array antennas or spinning real-aperture antennas. To achieve an unambiguous angle measurement commensurate with a one meter cross-range resolution at a one kilometer distance, each individual antenna (or array) must be on the order of 15 wavelengths across. Consequently, for the low frequency RF signals required for indoor locating, each antenna would be quite large and costly. Further, such large antennas would be unsuitable for a firefighter locator system which requires small, portable equipment that can be setup quickly. 
     Another technique for locating the position of an object includes establishing several known locations to receive a signal emitted from the object. By measuring the phase delay of a cyclostationary feature of the signal at each of the known locations, the position of the object can be determined. For example, U.S. Pat. No. 5,999,131 which issued to Sullivan for an invention entitled “Wireless Geolocation System,” and which is assigned to the same assignee as the present invention, discloses a system for locating mobile phones within a cell which may comprise several square miles. Unlike a mobile phone system which broadcasts over relatively high frequencies and large distances, the present invention is focused on using low frequency RF signals having the ability to penetrate the walls and floors of structures. Further, whereas it is sufficient to locate a mobile phone within a cell to an accuracy of about 50 feet, the present invention is concerned with locating an object positioned inside a structure to an accuracy of one meter. 
     During a typical rescue operation, only a portion of firefighters and other emergency personnel that arrive at the scene may actually enter the structure and require locating/tracking. Although it is preferable to supply each firefighter with a signal emitter in case that firefighter is required to enter the structure, it is also preferable to disable each signal emitter until it is needed (i.e. until the corresponding firefighter is required to enter the structure). This reduction in overall signal generation at the site reduces signal clutter and allows for higher system resolution for the activated signal emitters. It is also preferable to have a signal emitter activation system that does not rely on the firefighter to manually activate the signal emitter. It is also desirable to stop the emitter from signal transmission after a firefighter safely exits the structure. Importantly, the mechanism for emitter shutdown should contain safety features to prevent inadvertent emitter shutdown while the firefighter remains in the structure. 
     Considering the above, it is an object of the present invention to provide a wireless system for locating and tracking the position of a movable signal emitter situated inside a structure with accuracies of approximately one meter. Another object of the present invention is to provide a wireless system for accurately locating the position of a signal emitter that uses penetrating, low frequency RF signals, and requires only a minimal amount of bandwidth. Still another object of the present invention is to provide a wireless system for accurately locating and tracking the position of a plurality of signal emitters situated inside or adjacent to a structure. It is another object of the present invention to provide a system for automatically activating a firefighter&#39;s signal emitter before the firefighter enters a structure. Still another object of the present invention is to provide a system for signal emitter shutdown that contains safety features to prevent inadvertent signal emitter shutdown while the firefighter remains in the structure. Yet another object of the present invention is to provide a wireless locating system that can incorporate a bi-directional data link and is simple to use, and comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, a system and method for locating and tracking the position of a movable signal emitter that is situated inside a structure includes establishing at least three mutually dispersed base station sites outside the structure at known locations. For a multistory structure, the system preferably includes three base station sites located at approximately ground level, and an additional base station site that is elevated. A central processing site is also included in the system, and a wireless link is provided to allow for communication from each of the base station sites to the central processing site. 
     To operate the system, the emitter is turned on to transmit a continuous low frequency (approximately 27 Mhz) RF signal. An omni-directional antenna mounted on the emitter allows for the transmission of the signal in all directions. Each base station site has an antenna for receiving the continuous signal. Preferably, each base station site is self-surveying by using either a global positioning system or other wireless method to accurately establish its position. The positions of the base station sites are then communicated to the central processing site for use in the algorithm which computes the position of the emitter. 
     In one embodiment of the present invention, each base station has access to a reference signal such as a signal that is in phase with the signal generated at the signal emitter. Based on this reference signal, each base station compares the actual signal that is received from the signal emitter to the reference signal in order to measure an actual phase delay at each station. For a given base station, the actual phase delay is indicative of the distance between the signal emitter and the base station. Although indicative of distance, phase-related ambiguities arise in converting the actual phase delay to a distance measurement due to the fact that one actual phase delay could represent more than one possible distance. It is to be appreciated that these possible distances differ by a distance that is related to the signal wavelength. 
     In this embodiment, the measured actual phase delay from each base station site is communicated to the central processing site. At the central processing site, the measured actual phase delay for a given base station can be converted into a set of possible emitter distances from that base station. This process can be repeated for each base station resulting in a set of possible emitter distances from each base station. Next, the processor can determine all possible points where the distance sets overlap using triangulation methods known in the pertinent art. This set of possible points includes the real emitter position and the ambiguities inherent in the phase-only system. 
     Next, the ambiguities can be eliminated by the processor to find the real emitter position. It is to be appreciated that the number of ambiguities will depend on the emitter signal wavelength and the coverage area. Several techniques can be used to reduce or eliminate the ambiguities. For example, increasing the number of base stations will generally reduce the number of ambiguities. Another technique involves determining an initial position for the emitter and tracking the movement of the emitter. This technique allows for some of the ambiguous positions to be eliminated as improbable in light of any known limitations on emitter movement such as speed. Another technique involves using an algorithm known in the pertinent art such as the maximum likelihood method (MLM). Another technique for eliminating ambiguities involves using an emitter that transmits multiple frequencies. Here, each frequency produces a set of possible emitter positions. The set of possible positions produced at one frequency can be compared to the set of possible positions produced at a second frequency and any positions that are not common to both sets can be eliminated as ambiguities. Once the ambiguities have been eliminated, the remaining point is the real position of the signal emitter relative to the base station sites. 
     In another embodiment of the present invention, rather than actual phase delays, relative phase delays from one base station to another can be used to locate the position of a signal emitter. In this embodiment, the location of each base station is known and each base station is synchronized with the other base stations. Synchronization between the emitter and the base stations is not necessary. Each base station measures the phase angle of the emitter signal and records a measurement time. These data are transferred to the central processing site where the processor calculates a set of relative phase delays. Alternatively, the base stations can relay the received signals to the central processing site where the phase angles and measurement times can be determined and used to calculate a set of relative phase delays. For this purpose, the received signal can be time shifted or frequency shifted at the base station and the shifted signal relayed to the central base site thereby reducing signal interference. 
     For a three receiver system, three relative phase delays can be calculated; a first for base stations one and two, a second for base stations two and three and a third for base stations one and three. Each relative phase delay is indicative of a differential range estimate for the two base stations used to establish the relative phase delay. For example, consider an emitter signal having a wavelength, λ. Based on a relative phase delay of one-half λ measured between base station one and base station two, the processor can establish the set of points wherein the distance from base station one is one-half λ greater than the distance from base station two (differential range estimate). Absent any phase-related ambiguities, the emitter must be located at one of these points. Similarly, the processor can establish a differential range estimate for each of the other base station combinations and use the differential range estimates to locate the position of the signal emitter using triangulation algorithms known in the pertinent art. Depending on the signal wavelength and the coverage area, these differential range estimates may contain phase-related ambiguities requiring techniques outlined above such as emitter initialization or using an MLM algorithm to reduce or eliminate the ambiguities. 
     As contemplated by the present invention, in addition to locating a stationary emitter, the path of a moving signal emitter can be tracked. To track a moving signal emitter, the base stations must be synchronized, and the actual or relative phase delays must be measured simultaneously at predetermined measurement times. This allows the central processing site to calculate an emitter position for each measurement time, and thereby track the position of a moving emitter. 
     Also in accordance with the method and system of the present invention, a plurality of movable signal emitters can be located and tracked inside and around a structure. Any multiple access protocol known in the pertinent art such as frequency division multiple access (FDMA), code division multiple access (CDMA) or time division multiple access (TDMA) can be used for this purpose. Each base station can include a filter to separate the signals from the plurality of emitters by frequency, code or time. After separation, the actual or relative phase delays for each emitter can be determined for calculation of the location of each emitter relative to the base station sites. 
     The present invention also includes a system for activating the signal emitter immediately before the firefighter enters the structure. By selectively activating only the signal emitters of those firefighters who are entering the structure, system clutter is reduced. In greater detail, the activating system in accordance with the present invention is used in conjunction with the firefighter&#39;s SCBA unit, and causes the signal emitter to begin transmitting a signal when the pressure valve on the SCBA air tank is opened to release tank air to the SCBA facemask. To achieve this functionality, the activation system includes a pressure sensor to monitor pressure on an air line that delivers air from the SCBA air tank to the SCBA air regulator. When tank pressure in sensed in the air line, the emitter is activated by a pressure switch and a signal is transmitted by the emitter for receipt by the base stations to locate/track the signal emitter. The activating system also includes a user operated reset button(s) that is preferably mounted on the signal emitter. 
     Once activated, the signal emitter continues to transmit a signal until the emitter is instructed to stop transmission. A shutoff circuit is included in the emitter to allow the user to stop transmission of the signal by the signal emitter. As a safety precaution, the shutoff circuit is designed to only stop transmission of the signal if the air line has been depressurized and the reset button(s) is depressed. For these conditions to occur, the user must close the valve on the SCBA air tank, bleed pressure from the SCBA regulator and depress the reset. With this design, the signal emitter continues to transmit a signal after all air from the SCBA air tank is exhausted by the user, as long as the user does not depress the reset button(s). For the present invention, the reset button can be recessed or otherwise protected to avoid inadvertent signal shutoff. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a schematic representation of a system of the present invention showing a firefighter equipped with an emitter situated inside a structure, and showing the base station sites and central processing site used to locate and track the position of the firefighter; 
         FIG. 2  is a functional block diagram showing the interactive components of a representative base station site for the present invention; 
         FIG. 3  is a functional block diagram setting forth the sequential steps performed during operation of the system of the present invention; and 
         FIG. 4  is a schematic representation of a system for emitter activation in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to  FIG. 1 , a firefighter locator system in accordance with the present invention is shown and generally designated  10 . As shown in  FIG. 1 , system  10  can be used to determine the position of a firefighter  12  located inside a multistory structure  14 . In accordance with the present invention, system  10  preferably includes a central processing site  16 , three ground-level base station sites  18  (of which the sites  18   a ,  18   b  and  18   c  are exemplary) and an elevated base station site  20 . The base station sites  18   a - c , and  20  which are shown in  FIG. 1  are arbitrarily located and are only representative of base station sites  18 ,  20  which can be used in system  10  for the present invention. Indeed, it is to be appreciated that the actual positioning of the base station sites  18 ,  20  is generally unimportant so long as they are mutually dispersed and their exact location is known. 
     It is contemplated for the present invention that the base station sites  18 ,  20  will generally be located outside of the structure  14 . It is further contemplated for the present invention that at least three base station sites  18 ,  20  are required to accurately determine the position of a movable object such as a firefighter  12  inside a single-story structure (not shown). For a multi-story structure  14 , four or more base station sites  18 ,  20  are preferred, with at least one elevated base station site  20 .  FIG. 1  also shows that each base station site  18   a - c ,  20  is in direct communication with the central processing site  16  by a respective communications link  22   a - d . For purposes of the present invention, the communication links  22   a ,  22   b ,  22   c  and  22   d  are preferably wireless channels, but can be of any type well known in the pertinent art such as a land line. 
     The basic object of the system  10  is to accurately determine the position of a signal emitter  24  relative to the base station sites  18 ,  20 . Further, this is to be accomplished regardless of whether the signal emitter  24  is stationary or mobile (i.e. being carried by firefighter  12 ). For the present invention, the signal emitter  24  can be any type of communications equipment which emits omni-directional, electromagnetic radiation signals  26  (e.g. radiofrequency (RF) signals). It is contemplated for the present invention that a low frequency RF signal  26 , capable of penetrating the walls and structure of a building is used. Preferably, the signal  26  has a wavelength that is approximately the same or larger size than typical structural features such as hallways, staircases and room dimensions to prohibit these features from acting as a waveguide. For example, a signal  26  with a frequency of approximately 27 Mhz may be used. Also contemplated for the present invention, each signal emitter  24  may have the capability to broadcast both a vertically polarized signal and a horizontally polarized signal. It is to be appreciated that certain horizontally or vertically oriented features of the structure  14  will reflect or diffract signals differently depending on whether the signal is horizontally or vertically polarized. By using both horizontally and vertically polarized signals, position errors due to these oriented features of the structure  14  can be eliminated. 
     The operation of a representative base station site  18 ,  20  can be best understood by cross-referencing  FIGS. 1 and 2 . In  FIG. 2 , it is to be appreciated that a base station  18 ,  20  includes an antenna  28  for receiving signals  26  from the signal emitters  24 . It is contemplated for the present invention that the system  10  is able to track and locate several signal emitters  24  at once. For example, each signal emitter  24  may broadcast a unique frequency. For the present invention, any multiple access protocol known in the pertinent art such as frequency division multiple access (FDMA), code division multiple access (CDMA) or time division multiple access (TDMA) can be used to allow each base station  18 ,  20  to process signals  26  from a plurality of signal emitters  24  contemporaneously. 
     In the preferred embodiment of the present invention, each emitter  24  may include the capability of broadcasting non-position data which can be received by the antenna  28  at each base station site  18 ,  20 . Accordingly, each signal  26  from each emitter  24  may contain both a positioning component and a non-positioning component. Non-position data may include sensor measurements made near the emitter  24  such as oxygen level, carbon monoxide level or temperature. Additionally, firefighter heart rate, air tank level, motion, battery level or similar measurements may be measured by sensors and transmitted to the base station  18 ,  20 . 
     Once the signals  26  are received at the antenna  28  from each signal emitter  24 , the signals  26  and their components must be sorted.  FIG. 2  shows that the base station  18 ,  20  may contain a filter  30  for sorting the received signals  26 . As shown in  FIG. 2 , the signals  26  received at the antenna  28  can be communicated over line  32  to the filter  30  for separation. When a non-position data channel is used, the sorted data can be communicated over line  34  from the filter  30  to a display/recorder  36 . 
     Once separated, the position signals  26  from each signal emitter  24  can be communicated to a phase sensing circuit  44 . As shown in  FIG. 2 , the separated signal  26  from a first emitter  24  can be communicated over line  40  to a phase sensing circuit  44   a , and the separated signal  26  from a second emitter  24  can be communicated over line  42  to a phase sensing circuit  44   b.    
     In one embodiment of the present invention, an actual phase delay (τ A ) for the signal  26  can be determined at the phase sensing circuit  44 . In this embodiment, the signal  26  is compared to a reference signal  50  to determine the actual phase delay (τ A ) of each emitter signal  26 . As shown in  FIG. 2 , the reference signal  50  can be communicated over lines  51   a,b  to each phase sensing circuit  44 . For this embodiment of the present invention, the reference signal  50  is synchronized with the signal emitter  24 . In this embodiment, the actual phase delay (τ A ) determined by each phase sensing circuit  44  is indicative of the distance (range) between the signal emitter  24  and the base station  18 ,  20  that receives the signal  26 . Once determined, the actual phase delay (τ A ) for each signal  26  received at a base station  18 ,  20  can be communicated from each phase sensing circuit  44  over a line  52   a,b  to a transmitter  54   a,b . The transmitter  54   a,b  allows the actual phase delay (τ A ) data to be sent from the base station site  18 ,  20  to the central processing site  16  over the communication link  22 . As described in detail below, the central processing site  16  processes the actual phase delays (τ A ) from each base station site  18 ,  20  to geometrically determine the position of each signal emitter  24  relative to the base station sites  18 ,  20 . 
     In another embodiment, position information can be obtained without synchronizing the reference signal  50  at each base station site  18 ,  20  with the signal emitter  24 . Rather than measuring actual phase delays at each base station  18 ,  20 , a relative phase delay (τ R ) can be determined by comparing the signal  26  received at one base station site  18 ,  20  with the signal  26  received at a second base station site  18 ,  20 . By comparing each base station  18 ,  20  to at least one other base station  18 ,  20 , a set of relative phase delays (τ R ) can be obtained and used to find the location of the signal emitter  24 . In this embodiment, each base station  18 ,  20  has a reference signal  50  that is synchronized with the reference signal  50  at each of the other base stations  18 ,  20 . The phase sensing circuit  44  measures the phase of the signal  26  and the reference signal  50  is used to obtain a measurement time. Once determined, the phase and measurement time for each signal  26  received at a base station  18 ,  20  can be communicated from each phase sensing circuit  44  over a line  52   a,b  to a transmitter  54   a,b . The transmitter  54   a,b  allows the phase and measurement time to be sent from the base station site  18 ,  20  to the central processing site  16  over the communication link  22 . 
     At the central processing site  16 , a relative phase delay (τ R ) can be calculated by comparing the phase and time measurement data received from one base station site  18 ,  20  with the phase and time measurement data received from a second base station site  18 ,  20 . In this embodiment, each relative phase delay (τ R ) determined at the central processing site  16  is indicative of the differential range between the signal emitter  24  and the two base stations  18 ,  20  used to calculate the relative phase delay (τ R ). Stated differently, each relative phase delay (τ R ) indicates that the signal emitter  24  may be further from one base station  18 ,  20  than another base station  18 ,  20 , and indicates the magnitude of this difference. By comparing each base station  18 ,  20  to at least one other base station  18 ,  20 , a set of relative phase delays (τ R ) can be obtained. As described in detail below, this set of relative phase delays (τ R ) can be used to geometrically determine the position of each signal emitter  24  relative to the base station sites  18 ,  20 . 
     In yet another embodiment, each base station  18 ,  20  can relay the received signals  26  to the central processing site  16  for calculation of either actual or relative phase delays (τ). Since the distance between each base station  18 ,  20  and the central processing site  16  is known, the phase delay due to the signal travel between the base station  18 ,  20  and the central processing site  16  can be eliminated using processing techniques known in the pertinent art. At the central processing site  16 , the relayed signals can be compared directly to calculate a set of relative phase delays (τ R ) or the central processing site  16  can include a reference signal in phase with the signal emitter  24  to allow calculation of actual phase delays (τ A ). For this purpose, the received signal  26  can be time shifted or frequency shifted at the base station  18 ,  20  and the shifted signal relayed to the central processing site  16  thereby reducing signal interference. 
     The operation of system  10  of the present invention will, perhaps, be best understood by cross-referencing  FIGS. 2 and 3 . Block  60  indicates that the antenna  28  at each base station site  18 ,  20  receives the low frequency signals  26  from the signal emitters  24  (e.g. emitter “A” and emitter “B”). Next, as indicated by block  62 , the signals  26  from the signal emitters  24  are separated using a filter  30 . As shown in blocks  66   a,b , once separated, the signals  26  can be used to calculate either actual or relative phase delays (τ). As indicated above, if actual phase delays (τ A ) are used, they can be calculated at either the base station  18 ,  20  or the central processing site  16 . If relative phase delays (τ R ) are used, they are calculated at the central processing site  16 . At the central processing site  16 , each calculated phase delay (τ) is converted into a set of possible locations where the signal emitter  24  may be. If actual phase delays (τ A ) are used, each actual phase delay (τ A ) is converted into a set of possible locations that indicate distance from the corresponding base station  18 ,  20 . If relative phase delays (τ R ) are used, each relative phase delay (τ R ) is converted into a set of possible locations that indicate a differential range for the corresponding base stations  18 ,  20  used to determine the relative phase delay (τ R ). 
     In either case, the conversion of phase delays to distance measurements results in phase-related ambiguities that increase the number of possible locations represented by each phase delay. Specifically, each actual phase delay (τ A ) represents a plurality of ranges from the base station  18 ,  20 . It is to be appreciated that these ranges differ by a distance related to the wavelength of the signal  26 . Similarly, each relative phase delay (τ R ) represents a plurality of differential ranges for the base stations  18 ,  20  used to calculate the relative phase delay (τ R ). It is to be appreciated that the distances between these differential ranges are related to the wavelength of the signal  26 . 
     Once each phase delay (τ) is converted into a set of possible locations for the signal emitter  24 , the processor can determine all possible points where the distance sets overlap using triangulation methods known in the pertinent art. This set of possible points includes the real position of the signal emitter  24  and the ambiguities inherent in the phase-only system. 
     Next, as shown in blocks  68   a,b , the ambiguities can be eliminated by the processor to find the real position of the signal emitter  24 . It is to be appreciated that the number of ambiguities will depend on the wavelength of the signal  26  and the size of the area in which the signal emitter  24  may be found. Several techniques can be used to reduce or eliminate the ambiguities. For example, increasing the number of base stations  18 ,  20  will generally reduce the number of ambiguities. Another technique involves determining an initial position for the signal emitter  24  and tracking the movement of the signal emitter  24 . This technique allows for some of the ambiguous positions to be eliminated as improbable in light of any known limitations on signal emitter  24  mobility. Block  70  shows that an a prior database can be used to record the result of each position determination for use in a subsequent position determination. Another technique to reduce or eliminate phase-related ambiguities involves using an algorithm known in the pertinent art such as the maximum likelihood method (MLM). Another technique for eliminating ambiguities involves using a signal emitter  24  that transmits two or more signals  26  contemporaneously, each signal  26  having a different frequency. Since each frequency produces a different set of possible positions for the signal emitter  24 , the set of possible positions produced at one frequency can be compared to the set of possible positions produced at a second frequency and any positions that are not common to both sets can be eliminated as ambiguities. Additionally, a combination of the above techniques can be used to eliminate phase-related ambiguities. Once the ambiguities have been eliminated, the remaining point is the real position of the signal emitter  24  relative to the base station sites  18 ,  20 . 
     Referring now to  FIG. 4 , it can be seen that the present invention also includes a system  72  for activating the signal emitter  24  immediately before the firefighter enters the structure. By selectively activating only the signal emitters  24  of those firefighters who are entering the structure, signal clutter is reduced. As shown, the activating system  72  includes a firefighter&#39;s SCBA  74 , a pressure sensor  75 , a pressure switch  76 , and the signal emitter  24 . The firefighter&#39;s SCBA  74  includes an SCBA air tank  78 , a pressure regulator  80  and a facemask  82 . The firefighter&#39;s SCBA  74  further includes an air line  84  for delivering air from the SCBA air tank  78  to the SCBA pressure regulator  80 , and an air line  86  for delivering air from the SCBA pressure regulator  80  to the facemask  82 . 
     As further shown in  FIG. 4 , the pressure sensor  75  monitors pressure on air line  84 . When tank pressure is sensed in air line  84 , the emitter  24  is activated by the pressure switch  76  (via wire  88 ) and a signal is transmitted by the emitter  24 . Also shown, the SCBA air tank  78  has a pressure valve  90  that can be opened to release tank air to the regulator  80  and facemask  82 . It is to be appreciated that with this combination of structure, the pressure sensor  75  senses tank pressure in air line  84  and the pressure switch  76  activates emitter  24  when valve  90  is opened. 
     The activating system  72  also includes a user operated reset button  92  that is preferably mounted on or is integral with the signal emitter  24 . A shutoff circuit is included in the emitter  24  to allow the user to stop transmission of the signal by the signal emitter  24 . Once activated, the signal emitter  24  continues to transmit a signal until the emitter  24  is instructed to stop transmission. As a safety precaution, the shutoff circuit is designed to only stop transmission of the signal if the air line  84  has been depressurized and the reset button  92  is depressed. For these conditions to occur, the user must close the valve  90  on the SCBA air tank  78 , bleed pressure from the SCBA regulator  80  and depress the reset button  92 . As shown, the reset button  92  can be recessed to avoid inadvertent signal shutoff. Alternatively, a pair of reset buttons  92  (pair not shown) that must be simultaneously depressed to cause shutoff can be implemented in accordance with the present invention to avoid inadvertent signal shutoff. With the shutoff circuit configured in this manner, the signal emitter  24  continues to transmit a signal after all the air from the SCBA air tank  78  is exhausted by the user, as long as the user does not depress the reset button  92 . 
     While the particular firefighter locator with activator as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.