Patent Application
20140070942
1. Field of the Invention
The instant invention relates generally to systems, apparatus, and methods for automatic detection and location of acoustic sources in the presence of high levels of background noise. In particular, the invention relates to a process of detecting and rapidly locating the Personal Alert Safety System (“PASS”) carried by firefighters and other first responders when the PASS is in Alarm Mode.
2. Description of the Related Art
Firefighters and other first responders throughout the US and in many parts of the world carry a Personal Alert Safety System (PASS), a device that produces a loud alarm tone if the user is in peril. The alarm tone is intended to perform two primary functions: (1) notify others that the user is in need of immediate assistance, and (2) assist the rescue operation by providing an acoustic signal that can be located by the rescue team. The PASS device automatically switches from Sensing Mode to Alarm Mode if the user is motionless for thirty seconds. Alternatively, the user can manually trigger Alarm Mode by pressing a push-button.
PASS devices are certified to standards generated by the National Fire Protection Association. For the 2007 edition of NFPA 1982 Standard on Personal Alert Safety Systems (PASS), design requirements for the PASS alarm signal include:
The NFPA Electronic Safety Equipment Technical Committee is responsible for the NFPA 1982 document, which is reviewed and updated approximately every five years. PASS devices certified to the 2007 Edition of NFPA 1982 generally have different alarm tones, depending on the particular manufacturer of the PASS device. The 2013 Edition of NFPA 1982 will specify and standardize the alarm tone so that all PASS devices will sound the same.
In addition to the usual visibility, contamination, moisture, and temperature issues surrounding a fire scene, the detection of an acoustic signal must deal with the presence of multiple echoes from the structure; in wave propagation terminology, this is known as a high multipath environment. The problem is particularly difficult in smaller structures with highly reflecting surfaces, such as stairwells with concrete walls or shower stalls with tile walls.
The Pathfinder System developed by Summit Safety solves the multipath problem by use of a continuous-wave (CW) ultrasonic transmitter (Beacon) and a directional receiver (Tracker), which detects waves propagating only from a narrow angle. The system is more fully disclosed in U.S. Pat. No. 6,504,794, entitled “Tracking, safety and navigation system for firefighters” and which issued Jan. 7, 2003, and U.S. Pat. No. 6,826,117, with the same title and which issued Nov. 30, 2004. The user must manually scan the area with the Tracker to determine the direction of the strongest signal, which implies the direction of the shortest path to the Beacon. In order to achieve a narrow receiving beam angle, a receiving sensor must have a minimum width of 5-10 wavelengths. For the Pathfinder Tracker, this requirement necessitates the use of ultrasound to ensure portability. The same approach could be used to detect a PASS device, but the size of the sensor would be prohibitive. For example, the wavelength at 1 KHz is approximately 1.13 feet and at 4 KHz is approximately 3.4 inches; a five-wavelength requirement would mean the sensor width would be a minimum of 17 inches (at 4 KHz) and maximum of 5.6 feet (at 1 KHz). In addition, since the 2007 edition of NFPA 1982 allows sequential alarm tones, a manual scanning operation would need to be very slow to ensure that the loudest section of the PASS tone was present at all scan angles.
U.S. Pat. No. 7,639,147 B2 by Berezowski et al. entitled “System and Method of Acoustic Detection and Location of Audible Alarm Devices” which issued 29 Dec. 2009 describes a system of audio sensing modules that comprise a pre-installed infrastructure inside a building. Each of the audio sensing modules incorporates a single sensor (microphone) to collect a time-based record for signal processing. The maximum SPL (sound pressure level) and the minimum SPL for the recording form the basis for PASS alarm detection: if the minimum SPL is not less than a predetermined threshold level or if the difference between the maximum and minimum SPL is below a predetermined threshold level, the module is unable to reliably detect a PASS device. If the SPL levels pass these two threshold tests, the module then determines if an alarmed PASS device is present by analyzing the frequency content of the signal; if the frequencies match the frequency characteristics of the expected PASS device, the module then identifies the repetition pattern of the frequencies. Only after passing the two threshold tests, the frequency content test and the frequency repetition test does the module report the detection of an alarmed PASS device. According to the patent, the process of “locating” a PASS device is accomplished by having multiple sensing modules distributed throughout the building; while not stated explicitly, detection by a particular module implies that the PASS device has been “located” (i.e., its location is within detection range of the particular module). Unfortunately the accuracy of the “location” would be crude at best: the distressed firefighter could still be at a considerable distance from the module. Furthermore, any rescuers would need a map of the building with the locations of the pre-installed modules identified. In addition, the modules would require either a wired or a wireless RF telemetry link in order to notify personnel outside the building that an alarmed PASS device had been detected.
Non-acoustic technologies have also been proposed for locating firefighters in distress. For example, radio frequency systems have been developed to locate firefighters. Such systems have limited capabilities inside a building due to difficulties in wave propagation resulting from the metal and dielectric materials used in the building construction.
The instant invention, as illustrated herein, is clearly not anticipated, rendered obvious or even present in any of the prior art mechanisms, either alone or in any combination thereof.
According to one aspect of the invention, the PASS-Tracker is a hand-held device that improves the ability of a rescuer to quickly locate a distressed firefighter by two processes: (1) detecting and recognizing the acoustic alarm sound from a PASS device in Alarm Mode, and (2) providing an indication to rescue personnel of the shortest path to the victim. The invention does not require a pre-installed infrastructure in a particular building; rather the device can be used in an ad hoc fashion at any fire scene. The PASS-Tracker utilizes a plurality of small microphones to detect the acoustic signal from the PASS device. Internal electronics in the PASS-Tracker measure the time-of-arrival (TOA) of the leading edge of the acoustic wave at each microphone and calculate and display the angle-of-arrival (AOA) of the wave. Additional inertial sensors inside the PASS-Tracker compensate for motion of the device in order to keep the display indicator pointing in the direction of the path to the distressed firefighter. Knowledge of the specific format of the PASS alarm tone—in particular, its swept-frequency nature—allows the PASS-Tracker to use pulse-compression and cross-correlation techniques to detect the alarm sound even in the presence of significant fireground noise.
There has thus been outlined, rather broadly, the more important features of the PASS-Tracker in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are additional features of the invention that will be described hereinafter and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways, including applications involving not only firefighters. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.
These together with other objects of the invention, along with the various features of novelty, which characterize the invention, are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be made to the accompanying drawings and descriptive matter in which there are illustrated preferred embodiments of the invention.
Table 1 provides time-of-arrival data for various spacing between the microphones of
Table 2 provides estimates of PASS resonances for commercial PASS devices based on the spectra of
Table 3 provides typical sound pressure levels for common sounds.
Table 4 shows the difference between AOA (Angle of Arrival) estimates using the methods of
The detailed description set forth below in connection with the appended drawings is intended as a description of presently preferred embodiments of the invention and does not represent the only forms in which the present invention may be constructed and/or utilized. The description sets forth the functions and the sequence of steps for constructing and operating the invention in connection with the illustrated embodiments.
A preferred embodiment of the instant invention would employ three omni-directional sensors as illustrated in
In order for the system of
The performance of a TDOA approach in terms of angular resolution depends on the system's ability to accurately detect the arrival time of the wave for each sensor.
The measurement accuracy for time of arrival (TOA) for a sensor depends heavily on the specific waveform of the alarm sound generated by the particular PASS device and on the signal-processing method used to measure TOA. For example, if the PASS alarm sound is a constant-frequency tone that is gated on and off as illustrated in
An improved embodiment uses a process known as cross-correlation to detect the acoustic signal; the signal processor uses a reference “image” of the ideal signal as a matched filter and essentially “slides” the image past the received signal, multiplying the two at all points in time and adding the products to form a single value for the particular shift position. When the image and the signal are lined up, the cross-correlator has a maximum output. If the two signals are identical, the process is known as “autocorrelation”; if the two signals are different, the process is called “cross-correlation.” Detection of an alarm signal is done with cross-correlation, because one input to the correlator is the received acoustic signal, and the second input is the ideal reference. The auto-correlation process is useful to illustrate the general performance of a cross-correlator when (a) the received acoustic signal is a perfect match to the reference and (b) no additional noise is present in the received signal.
In reality, PASS alarm sounds typically last much longer than the 5 ms illustrated in
As noted above, the typical fireground detection environment has a high level of multipath.
The previous discussion has demonstrated that for tone bursts of a single frequency, a cross-correlation detector works only for very short bursts. However, if the burst is modified to allow more than one frequency, the situation changes dramatically. Radar, sonar, and echography system designers have long known of a technique to improve range resolution known as pulse compression. Several variations are based on a “chirp” waveform, including linear frequency modulation (LFM), nonlinear frequency modulation (NLFM), linear period modulation (LPM), and nonlinear period modulation (NLPM). The term “chirp” was coined because the sound of an LFM burst emulates the sound of a bat or bird, demonstrating that Nature has found the technique advantageous. In addition to chirps, phase coding modulation can also be used for pulse compression. For example, all of the satellites of the Global Positioning System (GPS) transmit on the same frequency with pseudorandom phase modulation unique to each satellite; the signals can be separated by receivers using cross-correlation. This type of communication is also known as code division multiple access (CDMA).
LFM Chirp with Resonant Transmitter
The discussion above has assumed that the transmit bursts can be generated with constant amplitude. However, most PASS devices generate the audible signal with a piezoelectric transducer element, which has a resonant behavior. Typical frequency spectra for five different manufacturers of PASS devices are shown in
The plots of
The resonance of the PASS device transducer element causes two primary effects on the transmit acoustic burst. First, the amplitude will peak at the center frequency of the resonance. Second, the phase of the output signal will change, being +90° for frequencies below resonance and transitioning to −90° for frequencies above resonance.
The discussion above has shown that the cross-correlation detector would be able to accurately determine the TOA of the first received burst if a linear FM chirp is used as the source signal in a noise-free environment. However, the fireground typically has additional sources of sounds that would be considered noise to the correlation detector.
In addition to the fireground sounds, any electronic system, such as the microphone sensors and preamplifiers, will have internal noise which is typically “white,” meaning it has constant amplitude for all frequencies, as illustrated in
The combination of (1) a minimum requirement of −30 dB SNR for reliable TOA measurements, and (2) the NFPA requirement to have a minimum sound pressure level of 95 dBA at 3 meters (9.9 feet), provides a method to estimate the maximum detection distance for a PASS device using a cross-correlation detector as a function of the ambient noise sound pressure level (SPL).
This high sensitivity to chirp rate for LFM implies that a system could be designed to accommodate a number of different chirps. For example, different acoustic transmitters could be coded with different chirp rates and be individually sensed by the correlation detector.
Chirps could also be distinguished by the direction of the chirp: up-chirps would not correlate well with down-chirps. Thus the technique of using different chirp rates could be extended to two groups of different chirp rates.
The high sensitivity to chirp rate for LFM also implies that the system would be negatively impacted by Doppler shift, which would occur if the source and/or the detector were moving. Thus the use of LFM would need to be restricted to low-Doppler situations. While this restriction may be marginally acceptable to firefighting, an alternative type of chirp (discussed in the next section) would be preferred.
Pulse Compression with LPM: Linear Period Modulation
Generation of a high-quality, linear frequency modulation chirp would require a circuit with the ability to accurately control frequency in a linear manner. Unfortunately, many of the techniques such as direct digital synthesis (DDS) are complicated and require high time resolution. In contrast, it is quite easy to accurately control the period of a signal using low-cost microprocessors such as the PIC12F629 manufactured by Microchip Technology, Inc., of Chandler, Ariz. For example, generating a square-wave drive with successive cycles of 1000 μs, 998 μs, 996 μs, etc., can be done with a very simple program; high accuracy would be achieved by using a crystal oscillator for the microprocessor.
Linear period modulation (LPM) forms of chirps have a number of advantages over LFM, including insensitivity to Doppler shift. In fact, certain bats use LPM chirps to their advantage in detecting and tracking insects.
The spectrum of LPM (
It can be noted that the correlation sidelobes for LPM (
The cross-correlation detector's performance in the presence of additive white noise is approximately the same for both LFM and LPM. A SNR on the order of −30 dB would be required for reliable detection of time of arrival.
The above discussion has focused on the use of the center peak of the correlation detector waveform for accurate determination of the time-of-arrival (TOA) of the primary wave from the PASS device and to distinguish it from subsequent echoes. However, in order to determine the angle-of-arrival (AOA) of the wave using multiple sensors, the time-difference-of-arrival (TDOA) between the sensors is required. As a result, any error in the TOA estimate would be tolerated if the same error occurred for all of the sensors; i.e., the errors for the individual sensors would cancel when the difference is calculated.
Other characteristics of the cross-correlation detector output could also be used instead of the center peak. For example, instead of using the time for the actual peak of the center pulse, an embodiment could instead use the phase of the correlation-detector output. For example, the zero-crossing of the signal either before or after the peak could be used as a measure of the phase of the cross-correlation function as long as the technique was used for all of the sensor signals. Similarly, the phase could be determined based on the zero crossings of the waveform several cycles away from the peak as long as the technique was used for all of the sensor signals.
The technique of using the phase (or zero-crossings) rather than the center peak for TDOA estimates is limited primarily by two issues. The first relates to the extent of multipath in the environment. If echoes are generated by waves that emanate from directions that are significantly different from the primary wave, (e.g., from the opposite direction), the zero-crossings for the TDOA measurements should be taken from times prior to the center peak to ensure that the subsequent echoes do not affect the measurement. This suggests that the total duration of the autocorrelation signal should not be so long as to confuse the approximate time of the peak from the primary wave.
The second issue that limits the zero-crossing technique concerns how to make sure that all sensors use the same cycle for measuring the time of the zero-crossing. It can be shown that if the spacing between the sensors (i.e., “D” in
For a three-sensor system using time-difference-of-arrival (TDOA) to determine angle-of-arrival (AOA) as illustrated in
Several techniques can be used to determine the AOA from the TDOA values. For example, if (T3-T1) is 0,
However, since the TDOA values will be affected by noise, a third approach—one that produces more-accurate results—is to use both TDOA values to estimate the AOA that minimizes the mean square error. The embodiment of this third approach will pick which of the four possible solutions produces the smallest error. The technique is illustrated in
One of the advantages of LPM over LFM relates to the circuitry required to generate the chirp signal, particularly at the accuracy level required for reliable detection by a cross-correlation detector. Crystal-controlled digital techniques are preferred to assure sufficient accuracy in the signal.
For the actual generation of the PASS alarm sound, a switched power transistor and a transformer are typically used to drive the piezoelectric transducer. The drive signal to the transistor is typically a square wave at the fundamental frequency of the acoustic output signal; the transformer and piezo resonance combine to produce an acoustic output with relatively low harmonics. Since the desired drive signal to the transistor is simply a square wave, one of the simpler digital approaches is to determine the appropriate time to turn the transistor on and off using a counter driven by a higher-frequency crystal oscillator. The digital circuit determines the switching time based on the half-period of the signal. It is a relatively simple matter to change the count threshold at which to switch the transistor on and off. Simple microcontrollers, such as the Microchip PIC12F629 manufactured by Microchip Technology, Inc., of Chandler, Ariz., can easily generate such chirps; this device costs approximately $1 in 1000 quantity. Thus period modulation is a relatively simple process. Linear period modulation would be accomplished by changing the count threshold at regular time intervals.
A further embodiment uses a variation of linear period modulation and changes the count threshold for the period generator not at regular time intervals but rather at the completion of each cycle of the waveform. This approach, which is coined “discrete” linear period modulation, or DLPM, is even easier to implement in a simple microcontroller.
In contrast to period modulation, frequency modulation with crystal-controlled accuracy is significantly more difficult to generate. One approach is to use a Direct-Digital-Synthesizer (DDS) chip which generates a digital output signal using a look-up table; the digital output is then converted to an analog sine wave using a D/A converter. For example, the Analog Devices AD5930, which operates at 50 MHz, draws 8 mA, and costs about $5 in 1000 quantity, could be used to implement frequency modulation. The DDS chip must be further controlled by another processor which generates a digital control signal corresponding to the desired frequency. Compared to the simplicity of the period-modulation approaches discussed above, generating a smooth chirp for LFM involves a rather complex and potentially costly process. A second approach to generate a frequency-modulation chirp would be to use a math processor to calculate the instantaneous period and to use its output to control a period-modulation generator. For fixed chirps, the calculations could be done in advance, with period values stored in a look-up table for the period-modulation generator. This process would become cumbersome with long chirps due to the number of values that would need to be stored. For example, a 2 KHz to 4 KHz DLPM chirp would require 626 different values, one for each cycle of the chirp waveform. Thus frequency modulation could be employed for the chirp, but would not be the preferred implementation, particularly for those situations with the potential for Doppler shift due to relative motion between the acoustic source and the acoustic receiver.
The discussions above have focused on the use of a cross-correlation detector with a single chirp signal. In reality, PASS devices generate an alarm that typically repeats every 3-5 seconds. In addition, the alarm sound may consist of multiple chirps. For example, the 2013 edition of NFPA 1982 will specify an alarm signal consisting of the following sequence:
If the timing of the repetition rate is accurately controlled in addition to the chirp signal itself, then additional improvements to the system can be realized. For example, the correlation detection could be applied only to the single down-chirp which starts the above sequence with the remaining up-chirps being ignored. Alternatively, additional chirps, or even the entire sequence could be used by the correlation detector for improved performance in high-noise conditions at the expense of somewhat higher sidelobes; this combination would be preferred more for detecting the alarm signal compared to locating the source. If the background noise level is unusually high, the correlation detector may either indicate the detection of a non-existent chirp or may miss the detection of an existent chirp; in either situation, the noise would be causing a false indication of an alarmed PASS device. However, in a further improved embodiment, the system can monitor the past history of the correlation detector with signal-processing elements such as phase-lock loops and/or Kalman filters; the likelihood of both false positives and false negatives could be significantly reduced and the effective SNR further improved.
Several different types of PASS-Trackers could be developed for determining and displaying the angle of arrival of the incident acoustic wave utilizing cross-correlation detectors to determine the time of arrival at multiple microphones. Perhaps the simplest embodiment would use only two sensors/microphones and two cross-correlation detectors.
In another embodiment of the concept, three microphones would be used as originally suggested in
The Tracker embodiment with three sensors could also be used in an alternate configuration with the plane of the sensors oriented in a vertical direction in the x-z plane, as suggested by
Another embodiment with four sensors—each located at the four corners of a tetrahedron—would be capable of determining the AOA of the acoustic wave in all three dimensions. Four sensors would be the minimum number required to uniquely determine the angle of arrival for the wave for any 3-D location. Additional sensors could also be added to this embodiment as well as any of the previous embodiments simply to provide more data for increased measurement accuracy and improved SNR.
Accelerometers, rate gyroscopes, and/or earth-magnetic-field sensors could be added to the embodiments described above to enhance the operation by sensing the orientation and rotation of the PASS-Tracker. Since the acoustic chirps from the PASS device would be intermittent—typically occurring once every 3-5 seconds—the addition of the orientation/rotation sensors would allow the display of the direction of the source to compensate for the motion of the Tracker. Thus the Tracker could be pointed in a new direction but the Tracker display of the PASS device would continue to point in the original direction. Such enhancement would allow the user to “line up” the Tracker directly toward the PASS device even though the original measurement was made with the Tracker pointing in a different direction. Such a mode of operation would be particularly useful if the PASS-Tracker were combined with a thermal imaging camera: the displayed image of the environment could be lined up with the direction to the PASS device to assist a rescue team in finding the distressed firefighter; i.e., the rescue team could “see” in which direction they should move in order to rescue the distressed firefighter. For subsequent acoustic chirps from the PASS device, the PASS-Tracker would continue to indicate the desired direction.
Radio-frequency (RF) communications could be added to all of the above embodiments to provide information to estimate the acoustic distance from the PASS-Tracker to the PASS device. If the PASS device transmits a timing pulse via an RF link simultaneously with the transmission of the acoustic chirp, the PASS-Tracker can measure the time difference between the arrival of the RF pulse and the arrival of the acoustic chirp and can estimate the distance along the acoustic path between the PASS device and the PASS-Tracker based on the difference in propagation velocities between the two signals. This technique would be an electronic equivalent of the technique of estimating the distance to a lightning flash based on five seconds per mile using the time delay from the flash to the sound of thunder. Since propagation of an RF pulse is virtually instantaneous compared to the much slower acoustic propagation, small variations in RF propagation due to a building's construction material would have negligible effect on the estimate of acoustic distance. A PASS-Tracker equipped with the RF link could display the distance in either analog or digital form—e.g., by an LED bar graph or by a numeric readout—to indicate an approximate distance along the acoustic path that the rescuer must travel to reach the PASS device of the distressed firefighter. Since some commercial PASS devices include RF telemetry, the addition of an RF timing pulse would be a relatively straightforward design effort. Adding an RF receiver in the PASS-Tracker would still be required in order to implement this distance-measurement capability.
Another embodiment of the techniques described above would focus only on the detection of the PASS alarm chirp without consideration of the time of arrival of the acoustic wave. Such an embodiment could be implemented using only a single microphone and a single cross-correlation detector. This embodiment could be deployed at a fire scene by positioning it at a building exit to assist firefighters in recognizing that a firefighter's PASS device was in alarm mode. By adding a separate RF telemetry link, the device could notify Incident Command or a rapid intervention team (RIT) that a PASS device was being detected at a particular exit of the building. Multiple units at different exits could assist Incident Command or the RIT in determining the best location to initiate a rescue. If the PASS device were to additionally implement the RF link with a timing pulse, as described previously, this embodiment of the PASS-Tracker could estimate the distance to the PASS device. By deploying multiple units with the distance-measuring capability at different exits, the exit with the shortest distance to the downed firefighter could be determined to augment the rescue process. These single-channel PASS-Trackers could also be deployed at locations other than exits—e.g., in stairwells, at rope bags, at spare air cylinders, on specific command firefighters, etc.—to further recognize that another firefighter's PASS device was in alarm mode and to assist in rescue.
Another embodiment of the techniques described above could use two microphones and one cross-correlation detectors for deployment in stairwells. If the two microphones were positioned vertically, i.e. with one microphone above the second, the PASS-Tracker could determine whether the rescue team should go up or down the stairs to reach the victim. Such an orientation could be assured simply by hanging the device in the stairwell. The device could indicate the direction by up or down LEDs or arrows. By adding an RF telemetry link, the device could notify Incident Command that it was detecting an alarmed PASS device and whether the unit was above or below the PASS-Tracker. By adding the RF timing pulse capability described above, the PASS-Tracker could also estimate and indicate the approximate distance to the PASS device.
The PASS-Tracker could also be integrated into the firefighter Self Contained Breathing Apparatus (SCBA), particularly the mask worn by the firefighter. A simple heads-up display could indicate to the rescue firefighter the direction of the distressed firefighter, allowing hands-free operation.
Different embodiments of the invention could be integrated with other types of rescue equipment used by the fire service. In one embodiment, the PASS-Tracker would be combined with the ultrasonic Pathfinder system more fully described by U.S. Pat. No. 6,504,794, entitled “Tracking, safety and navigation system for firefighters” and which issued Jan. 7, 2003, and U.S. Pat. No. 6,826,117, with the same title which issued Nov. 30, 2004. The combination of the two systems would further enhance the rescue effort by realizing the advantages of each. For example, the limited range of the Pathfinder system (120-150 feet) would be aided by the much longer range of the PASS-Tracker as suggested by
In a further embodiment, the PASS-Tracker could be integrated into thermal imaging cameras (TICs). The TIC has the capability to “see” through the dense smoke at a fire. If the direction of the distressed firefighter is indicated on the TIC display, the user would be directed along the path to the firefighter. The combination of the two technologies could reduce rescue time dramatically by ensuring that the rescuer does not waste precious time searching in the wrong location or heading in the wrong direction.
In a further embodiment, the PASS-Tracker could be integrated into other location technologies, such as RF and inertial. For example, US Patent Application 20110029241 entitled “Personal Navigation System and Associated Methods” by Miller, et al.; US Patent Application 20100007485 entitled “Devices, Systems and Method of Determining the Location of Mobile Personnel” by Kodrin, et al., and US Patent Application 20070229356 entitled “Devices, systems and method of determining the location of mobile personnel” by Kodrin et al., describe inertial-based and RF-based locator systems designed to track the position of firefighters at a fire scene. As another example, the Geospatial Location Accountability and Navigation System for Emergency Responders (GLANSER) program initiated by the Department of Homeland Security Science and Technology Directorate has funded several grants to develop a system to track first responders at a fire scene. While the purpose of these systems and technologies is to keep track of the current location of individual firefighters, they are incapable of determining the shortest path to the victim: without an accurate map of the building, they can only provide information on the victim's coordinates and are thus incapable of determining what path the rescuer should take to reach the victim. The combination of the two technologies—PASS-Tracker and the RF/inertial system—would facilitate the rescue effort and reduce rescue time.
A further embodiment of the invention would use cross-correlation techniques to detect the time-of-arrival (TOA) of Alarm signals from legacy PASS devices (those certified to the 2007 and earlier editions of NFPA 1982). TOA measurements with three sensors/microphones would then be used to determine the angle-of-arrival (AOA) of the first PASS signal (to mitigate the effects of multipath). Initial tests with PASS devices from six different manufacturers suggest that at least three of the PASS alarms could be detected with “reasonable” performance. Measurements indicate that the performance would be inferior to use with the PASS devices certified to the 2013 Edition of NFPA 1982—typically requiring 10 to 20 dB higher SNR for the same level of performance. Nevertheless such an embodiment may still prove valuable in some rescue situations, particularly if the noise level is not excessive. To implement this embodiment, the reference waveform for the correlation detector would need to match the alarm sound for the PASS device of the particular manufacturer. This embodiment would allow fire departments that had not yet upgraded their PASS devices to the 2012 Edition of NFPA 1982 to benefit from the invention.
Two well-known techniques can be used to implement the cross-correlation detector. The first is a time-based technique whereby the sampled waveform of the received signal is multiplied by the reference signal as illustrated in
The shift-multiply-add process illustrated in
An alternative technique to implement the cross-correlation detector involves the use of Fourier transforms, as illustrated in
This application takes priority from and claims the benefit of U.S. Provisional Patent Application Ser. No. 61/465,700 filed on Mar. 23, 2011, the contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61465700 | Mar 2011 | US |
