Abstract:
A system and method for providing precision locations for sensors which make up an array of sensors in a gunshot detection system. In a preferred embodiment sensors employ a commercial GPS which reports a sensor position or a group of pseudoranges to GPS satellites. A server collects differential information from a differential node and, in one preferred embodiment, calculates a precision position for each sensor by adjusting the reported position or pseudoranges with the differential information. In another preferred embodiment differential information is sent from the host to individual sensors which calculate their own precision positions. Differential information may be latitude and longitude corrections, pseudorange corrections, ionospheric delay, GPS satellite clock drift, or other corrective term which will improve the accuracy of a sensor position.

Description:
[0001]     This application hereby claims priority back to U.S. Provisional Application Ser. No. 60/481,915, filed on Jan. 19, 2004, hereby incorporated by reference as if set forth fully herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a system and method for detecting and locating an acoustic event. More particularly, but not by way of limitation, in a system for identifying and locating an acoustic event, the present invention provides a system and method for achieving highly accurate position information for individual sensors.  
         [0003]     Gunfire and sniper detection systems are generally known in the art. Such systems can be broadly grouped into three categories: systems which pinpoint the precise location of the source of gunfire; azimuthal sensors which provide an indication of the radial direction to the source of gunfire; and proximity sensors which merely provide an indication that nearby gunfire was detected. While such systems have been demonstrated to perform well in both law enforcement and military applications, the entire field is presently an emerging technology.  
         [0004]     In many large cities, gun-related violence has become a plague of epidemic proportions. Urban gunfire, whether crime-related or celebratory in nature, results in thousands of deaths per year in the United States alone. Gunfire location systems, such as those installed in the Redwood City, Calif., Glendale, Ariz., Willowbrook, Calif., City of Industry, Calif., and Charleston, S.C. areas, have proven to be effective in reducing law enforcement response time to detected gunfire, apprehending criminals, collecting evidence, and reducing the occurrence of celebratory gunfire. One such system is described in U.S. Pat. No. 5,973,998, issued to Showen, et al., which is incorporated herein by reference.  
         [0005]     Showen, et al. discloses a system wherein sensors are placed at a density of roughly six to ten sensors per square mile. Audio information is sent to a computer at a central location and processed to: detect a gunshot; determine a time of arrival for the gunshot at each sensor; and calculate a location of the shooter from the differences in the times of arrival at three or more sensors. Showen, et al. takes advantage of the long propagation distance of gunfire to place sensors in a relatively sparse array so that only a few of the sensors can detect the gunfire. This permits the processor to ignore impulsive events which only reach one sensor—a concept called “spatial filtering.” This concept of spatial filtering radically reduces the sensor density compared to predecessor systems, which require as many as 80 sensors per square mile.  
         [0006]     Another gunshot location system is described in co-pending U.S. patent application Ser. No. 10/248,511 by Patterson, et al., filed Jan. 24, 2003, which is incorporated herein by reference. Patterson, et al., discloses a system wherein audio information is processed within each sensor to detect a gunshot and determine a time of arrival at the sensor. Time of arrival information, as determined from a synchronized clock, is then transmitted wirelessly by each sensor to a computer at a centralized location where a location of the shooter is calculated in the same manner as in the Showen, et al. system.  
         [0007]     As yet, azimuthal systems have not been as widely accepted as, for example, the Showen, et al. system. Azimuthal sensors typically employ one or more closely-spaced sensors, where each sensor includes several microphones arranged in a small geometric array. A radial direction can be determined by measuring the differences in arrival times at the various microphones at a particular sensor. Presently such systems suffer from somewhat limited accuracy in the determination of the radial angle, which in turn, translates into significant errors in the positional accuracy when a location is found by finding the intersection of two or more radial lines, from corresponding sensors, directed toward the shooter. Since errors in the radial angle result in ever increasing positional error as the distance from the sensor to the source increases, the reported position will be especially suspect toward the outer limits of the sensors&#39; range.  
         [0008]     To provide an absolute location for an event, the location of reporting sensors must be known. In a fixed system, the location of each sensor can be surveyed at the time the sensors are placed. In a system with moving or re-locatable sensors, each sensor typically self-surveys with a global positioning system receiver (“GPS”) or other such system. As will be appreciated by those skilled in the art, several factors can impact the accuracy of a location provided by a GPS receiver which, in turn, impacts the accuracy of a source location provided by the gunshot location system.  
         [0009]     GPS receivers can be broadly divided into two categories, commercial or civilian receivers and military receivers. Commercial GPS receivers use the L1 frequency of the GPS signal to acquire the timing information used to determine position and perhaps the L2 frequency to determine atmospheric delays while military receivers use both the L1 and L2 frequencies to determine the position. Encryption keys to decode the L2 signal are controlled by the U.S. government and generally restricted to military applications. In general, military GPS receivers are more accurate than their commercial counterparts but, for a variety of reasons, tend to be larger, consume more electrical power, and are dramatically more expensive. In times past, selective availability (“SA”) was employed to further degrade the positional accuracy of commercial GPS receivers. However, the U.S. government is now fully committed to eliminating SA except regionally at times of conflict or other such threat.  
         [0010]     A number of schemes have been developed to improve the accuracy of commercial GPS receivers such as: differential GPS (“DGPS”) where a network of fixed ground-based reference stations broadcast the difference between actual pseudoranges and measured pseudoranges; the Wide-Area Augmentation System (“WAAS”) which uses a series of ground-based stations operating in concert with a constellation of geosynchronous satellites to provide WAAS enabled GPS receivers with information such as atmospheric delay, individual satellite clock drift, and the like; Local-Area Augmentation Systems which are WAAS-like in nature but transmit the corrective information from ground-based stations locally, instead of satellites; as well as others. Each system presently suffers from limitations, such as: DGPS requires a second receiver and a nearby ground-based station and DGPS is particularly useful for overcoming the effects of SA but is of less value since SA is generally no longer active and WAAS has broader coverage; the WAAS system is limited to North America, requires a clear view of the southern sky, and is still in deployment such that presently not all areas enjoy reliable WAAS augmentation; and LAAS systems will have very limited coverage, strictly near major airports and require specialized receivers.  
         [0011]     As it relates to gunshot detection systems, the accuracy of commercial GPS receivers is somewhat limiting. Unfortunately, military GPS receivers are generally not available for law enforcement applications and thus, such acoustic sensors are limited by the characteristics of commercial receivers. Even in military applications, with battery-operated gunshot detection sensors, such as soldier-worn sensors, the size, weight, cost, and electrical power consumption of military GPS receivers presently tips the balance towards using a commercial version, despite accuracy concerns.  
         [0012]     It is thus an object of the present invention to provide a system and method for improving the positional accuracy of an array of self-surveying acoustic sensors which incorporate commercial GPS receivers.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention provides a system and method for improving the positional accuracy of acoustic sensors in a gunshot detection system. In a preferred embodiment, the inventive system includes: a plurality of acoustic sensors dispersed throughout a monitored area, each sensor having a commercial GPS receiver; a host processor at a known location, the host processor also having a commercial GPS receiver; and a communication network adapted to deliver information from the sensors to a host processor. Since the location of the host is known, errors in the GPS provided position at the host are used to correct the positional information provided by each sensor to improve the accuracy of the reported position of the source of an event.  
         [0014]     In one preferred embodiment, the host is located at a fixed, surveyed position. Unlike DGPS and WAAS, no special receiver is required since the correction for all reporting sensors is performed at a centralized location.  
         [0015]     In another preferred embodiment, the host further includes a military GPS receiver, or similar highly accurate positioning system, thus making the host portable or re-locatable. Since the position of the host is always known with a high degree of accuracy, differences between the reported positions of the commercial receiver and the military receiver can be determined and applied to the positions of reporting sensors.  
         [0016]     Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  depicts a preferred configuration of the inventive gunshot detection system.  
         [0018]      FIG. 2  provides a perspective view of a preferred embodiment of a sensor as employed in the inventive system.  
         [0019]      FIG. 3  provides a block diagram of a preferred embodiment of the sensor.  
         [0020]      FIG. 4  provides a block diagram for a preferred embodiment of the host node of the inventive system.  
         [0021]      FIG. 5  provides a block diagram for a preferred embodiment of the inventive system having a dual GPS node independent of the host.  
         [0022]      FIG. 6  provides a diagram showing the relationship of the inventive system to the GPS and WAAS constellations. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Before explaining the present invention in detail, it is important to understand that the invention is not limited in its application to the details of the construction illustrated and the steps described herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.  
         [0024]     Referring now to the drawings, wherein like reference numerals indicate the same parts throughout the several views, a representative gunshot detection system  100  is shown in its general environment in  FIG. 1 . In a preferred embodiment, a plurality of sensors  102 - 106  are dispersed over a monitored area. Preferably, each sensor is placed such that it has a relatively unobstructed acoustic view around its immediate area. By way of example and not limitation, suitable sites include: placed atop a building; placed atop utility or light poles; on towers, etc. Typically sensors  102 - 106  communicate through a communication network  108  with a centralized processor  110  wherein information concerning acoustic events is processed to provide details of the event, such as the source location of a gunshot, time of the gunshot, the number of detected gunshots, the type of event, and the like. It should be noted that sensors  102 - 106  may be any combination of wired or wireless sensors, that communications paths  112 - 116  may carry either analog or digital signals, and that network  108  may comprise any combination of sub-networks, such as, by way of example and not limitation: a telephone network; the internet; a private computer network; a wireless network, or even a collection of dedicated wires routed to the sensor array.  
         [0025]     In a military environment it may be preferable that sensors  102 - 106  are man-wearable. In such a system, the host computer  110  and display devices  120  would likely be carried by a squad leader. In such as a system, other ancillary systems may use data collected by the system and reported in real time, or near real time, to higher levels or command centers.  
         [0026]     As will be appreciated by those skilled in the art, information about a detected acoustic event is typically output to a person of interest such as a police dispatcher or directly to individual officers, as through network  118  to display devices  120  or a computer console. When weapon  122  is fired, the muzzle blast reaches sensors  102 - 106  at different times based on the speed of sound and the distance of each sensor from the shooter. Whether the acoustic information is processed at the sensor, or at computer  110 , a time of arrival is determined for each sensor and the differences of the various times of arrival are processed to determine a location of the source of the gunshot. In response to the gunshot, information is provided at device  120 .  
         [0027]     A preferred embodiment of a sensor  102  is shown in  FIG. 2 . Typically sensor  102  includes a housing  200 , a support (not shown) for mounting sensor  102 , and a windscreen  204  for protecting internal elements from the environment, while allowing acoustic waves to pass through to the interior. If sensor  102  is wireless, antenna  206  is provided for radio frequency communication.  
         [0028]     With reference to  FIG. 3 , a preferred embodiment of a wireless sensor  102  includes: a microphone  210  for receiving acoustic information; an amplifier and/or other signal conditioning  212 ; a processor  224 , typically a digital signal processor (“DSP”), as are well known in the art; a satellite positioning receiver, e.g. a GPS  226  and GPS antenna  216 ; and an interface  214  for communication via a communication network. It should be noted that in such a sensor, GPS receiver  226  may play two roles, providing positional information as to the sensor&#39;s location and an exceptionally accurate real time clock. In one preferred embodiment, DSP  224  includes an analog-to-digital converter  218  to digitize the audio signal for processing to detect a gunshot and determine the time of arrival of the gunshot. A sensor suitable for use with the present invention is described in co-pending U.S. patent application Ser. No. 10/248,511 by Patterson, et al., filed Jan. 24, 2003, which is incorporated by reference hereinabove.  
         [0029]     As will appreciated by those skilled in the art, if sensor  102  is man wearable, whether for military or law enforcement applications, size and weight are important considerations. In turn, electrical power consumption is likewise of prime concern since it has a direct impact on the size and weight of batteries required to operate sensor  102 .  
         [0030]     With presently known gunshot detection systems, it is not possible to provide an accurate position of the shooter with a single sensor. As a result, the calculation of a shooter position requires computation based on the outputs of a plurality of sensors. In a preferred embodiment the sensors communicate with a computer or server  110  as shown in  FIG. 4 , also referred to herein as a “host node”. Server  110  preferably comprises: CPU  302 ; an interface  312  for communication via a communication network; commercial GPS receiver  304 ; and GPS antenna  306 . If the host node is portable or re-locatable, preferably host node  110  further includes military GPS  308 , or other precision position locating system. It should be noted that GPS  308  likewise includes an antenna  310  although both GPS  304  and  308  can alternatively share a single antenna.  
         [0031]     With further reference to  FIG. 3 , in practice a gunshot is received by microphone  210  at three or more sensors  102 . At each sensor  102  the received audio is amplified by amplifier  212 , digitized through A/D converter  218 , and processed in processor  224  to determine if the sound is indeed a gunshot and, if so, a time of arrival of the event. A position is then retrieved from GPS  226  and transmitted via interface  214  to the host  110 . At the host  110 , if three or more sensors report the event, the precise positions of the each sensor are determined by comparing the position reported by receiver  304  with the known position of host  110  and applying the correction to the reported positions from each sensor. After determining precision locations for each sensor, the differences in the times of arrivals from the reporting sensors are used to calculate the source location of the gunshot.  
         [0032]     Turning to  FIG. 6 , while the GPS system is well known in the art, a brief description of the system and it relationship to the present invention may be helpful. The GPS constellation, represented by satellites  504 - 510 , presently consists of 24 satellites in orbits which are not geosynchronous. Thus the number of satellites, the particular satellites, and their positions in the sky, relative to a point on earth, is always changing. Above the GPS satellites  504 - 510 , in a geosynchronous orbit, are the WAAS satellites  502 . Each GPS satellite  504 - 510  constantly sends, among other things, timing information on its L1 carrier. Information regarding each satellite and its position in space, sometimes called the almanac, is broadcast to all GPS receivers, i.e.  520 - 524 , so that each receiver can determine the position of each satellite in the sky. Since the receiver, using  522  as an example, knows the position of each GPS satellite  504 - 510  and since it receives time referenced signals from the GPS satellites, using the differences in the time of travel of the signal, receiver  522  can determine the distances  512 - 518  to satellites  504 - 510 , respectively. These distances are known as “pseudoranges.” For receiver  522 , finding its position on earth is simply a matter of finding the intersections of the spheres defined by a satellite at the center and having a radius equal to the corresponding pseudorange. It is generally held that a GPS receiver using only the L1 frequency can normally find its position in two dimensions within 10 meters.  
         [0033]     A number of conditions exist which are outside of the control of receiver  522  which may affect the accuracy of the calculation. One example is ionospheric delay. As radio frequencies enter the atmosphere, there may be some degree of refraction as the wave strike the ionosphere. As can be seen in  FIG. 6 , the incidence angle between a satellite and the atmosphere, relative to receiver  522 , changes with the satellites position in the sky. The resulting refraction slightly distorts the path to receiver  522 , causing the signal to arrive late. Another condition is GPS clock drift. Since pseudo ranges are determined based on time of transmission relative to the speed of light, even slight inaccuracies in the time clocks between satellites will impact the position determination.  
         [0034]     To overcome such errors, ground stations, represented by station  526 , have been established as part of WAAS in the United States and EGNOS in Europe. Each station is at a precise location and, since the precise locations of the satellites are also known from the almanac, errors in the pseudoranges can be determined. Once the errors are determined, the contributing factors are determined, whether from ionospheric delay or clock drift, and the information is up-linked to satellites  502  which return the information to receiver  522 . Receiver  522  can then correct its calculated pseudoranges and calculate an improved position. When WAAS is available to a WAAS enabled receiver, it is generally held that the normal accuracy is about one meter.  
         [0035]     GPS satellites  504 - 510  also transmit information on an L2 frequency. Since L2 and L1 are different, a receiver  512  using both frequencies can determine the differences in the pseudoranges from each signal and determine the degree to which the signals were delayed through the atmosphere.  
         [0036]     In one preferred embodiment sensors  102  report pseudoranges along with a current position to host  110 . At host  110 , the pseudoranges are adjusted to reflect atmospheric delay or clock drift based on the pseudoranges received at GPS receiver  304  and in light of the known position of host  110 . Alternatively, atmospheric delay and clock delay values may be transmitted from host  110  to the sensor array such that pseudorange corrections are applied at each sensor  102  before its position is reported.  
         [0037]     In other preferred schemes, the host position does not need to be known. In one such scheme, as shown in  FIG. 5 , a differential node  400  having a military GPS  402 , and optionally a commercial GPS  404 , is located remote from host  406 . Differential and atmospheric delay information is collected at differential node  404  and periodically transmitted to the host  406  via a network interface  408 . Host  406  either uses the received information to improve the precision of sensors positions or, alternatively, passes the information on to sensors  410  so that sensors  410  can calculate and report precision locations. It should be noted that ionospheric delay could be estimated within GPS  402  without resorting to GPS  404 .  
         [0038]     In another scheme suitable for use where WAAS signals are available, WAAS information is collected by the host, either through its local GPS receiver, from a differential node, or from any sensor which is receiving WAAS information. The WAAS information is then either used by the host to improve the precision of sensor locations or passed on to sensors, which are not receiving WAAS information directly, where the information is used to improve the precession of reported locations.  
         [0039]     It should be noted that the term “differential node” is used to describe any node, whether also a host or server, a specialized sensor, or a dedicated node, which provides differential position, pseudorange correction, ionospheric or atmospheric delay, GPS satellite clock drift, WAAS information, or other information used to improve the accuracy of a position fix provided by a sensor. It should also be noted that while sensors subject to correction from such differential data should be proximate the differential node, since the GPS satellites are thousands of miles above earth, proximate distances between sensors and the differential node may be in excess of one hundred miles. “Proximate” is used to describe distances where the corrective information provides meaningful improvement in the calculated position of a sensor.  
         [0040]     It should also be noted that while preferred embodiments of the inventive systems were described with reference to a GPS positioning system, the invention is not so limited. The inventive system can also be used to improve the accuracy of positions obtained from other positioning system such as, by way of example and not limitation: GNSS, GLONASS, Galileo, MSAS, Look-Down, LORAN, etc. Similarly, the term “WAAS” should be construed broadly to include any satellite based augmentation system, such as EGNOS and the like. Such changes are within the scope and spirit of the present invention.  
         [0041]     It should also be noted that while preferred embodiments of the present invention have been described in connection with gunshot location systems, the techniques for providing precision locations from an array of GPS based sensor can be applied to other types of systems, such as those monitoring environmental conditions, geophysical data collection, and the like.  
         [0042]     Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the scope and spirit of this invention.