Abstract:
A method and system for detecting a signal source at a specified frequency in the presence of background noise includes a processor; a first sensor mounted at a first location operatively connected to the processor; a second sensor mounted at a second location operatively connected to the processor; the processor operating to compute the amplitudes of the first and second Fourier transforms of the outputs of the first and second sensors, respectively, the difference in the amplitudes of the first and second Fourier transforms being determinative of the existence of a signal being generated at the predetermined frequency.

Description:
GOVERNMENT INTEREST 
     The embodiments herein may be manufactured, used, and/or licensed by or for the United States Government without the payment of royalties thereon. 
    
    
     BACKGROUND 
     Description of the Related Art 
     There are many potential military and commercial applications for an improved magnetic sensing system that can more effectively and rapidly detect the presence of an electromagnetic field, magnetic object, or a target. For example, all types of land vehicles, ships, and aircraft have structural and power systems capable of generating substantial magnetic signatures. Even small, inert objects may exhibit sufficient magnetization to be observed from a distance. 
     Magnetoresistive sensor technology has the capability of producing low cost magnetic sensors. Magnetic sensors or transducers are generally passive sensors with desirable attributes for several types of applications that include insensitivity to weather conditions, the requirement of only a small bandwidth, and the unique ability to “see through” walls and foliage without attenuation. Furthermore, in military applications it is generally difficult to make a weapon or vehicle that does not include ferrous material that can be detected by magnetic sensors. Though the permanent magnetic moment of the ferrous material can be minimized by “deperming,” which is a process of reduction of permanent magnetism, the distortion of the earth&#39;s magnetic field due to the magnetic permeability is typically difficult to hide. Data from magnetic sensors can be combined with the data from other sensor modalities such as acoustic and seismic sensors to characterize or identify and track targets. Specifically, in military applications magnetic sensors can be used for perimeter defense, at check points, as part of a suite of sensors in unattended ground sensor networks, and on unmanned ground vehicles (UGVs) and unmanned air vehicles (UAVs). Moreover, magnetic sensors or transducers can also be employed to monitor rooms and passageways that have been cleared by military personnel. 
     The magnetic signals from military targets come from the internal motion of ferromagnetic parts, electrical currents, and the motion of targets containing ferromagnetic material relative to the magnetic sensor. The latter can arise either from the motion of the target or the sensor. All of these magnetic signals often occur at low frequencies, typically less than 100 Hz. Because of background generation of magnetic fields, it is generally difficult to detect the magnetic signals that occur at low frequencies. 
     Conventional magnetic sensing systems, however, are ill-suited for detecting specific frequencies (e.g., magnetic signatures) in environments with excessive background magnetic noise. In addition, conventional systems are generally unable to detect a specific frequency when the magnetic noise in the environment includes the frequency. With a limited capacity to provide highly selective frequency detection, mobile magnetic sensing systems (e.g., mounted to a motorized vehicle) typically become inoperable and impractical due to excessive magnetic noise associated with the surrounding environment. 
     As used herein, the term type of geophysical instrument used for magnetic surveys in which a pair of magnetometers are normally mounted one above the other on a single support staff. Various kinds are available, but the most commonly used in archaeology is the fluxgate gradiometer with the direction-responsive sensors between 0.5 m and 2.0 m apart. This measures the gradient in a magnetic field and will detect shallowly buried features and structures. The use of dual sensors overcomes many of the problems associated with single-sensor instruments, for example variations in the strength of the Earth&#39;s magnetic field and deep-seated geological anomalies. By systematically scanning an area on a grid system and logging the readings at close intervals it is possible to build up detailed plots showing the shape and form of the archaeological anomalies. These can be used to propose the nature and extent of buried features. 
     SUMMARY 
     In view of the foregoing, a preferred embodiment provides a system for detecting a signal source (which may be at a specified frequency) whether or not background noise is present. The preferred embodiment comprises an assembly  30  having at least two spaced apart sensors/receivers  10 ,  20  for receiving signals from a potential target area, a gradiometer comprising a first magnetometer coupled to a first receiver and producing first signal information; and a second magnetometer coupled to a second receiver and producing second signal information; and a processor for processing information from the first and second sensors/receivers  10 ,  20 . As used herein, the terminology processor means digital signal processor, computer, personal computer, laptop, CPU, multiprocessor, microprocessor, multiple processors, multiprocessors, general purpose computer, or the like. As used herein, the terminology assembly (such as assembly  30 ) means an operative association of components and is not intended to imply a necessary physical association or connection, although the components may be physically connected. 
     The preferred embodiment assembly  30  may be vehicle mounted and the first and second sensors/receivers  10 ,  20  may be positioned on opposites sides of the vehicle. Optionally, the sensors may be remotely positioned and transmit wirelessly or by a connector to a processor  38  which is remotely located. 
     The processor may comprise, or have associated therewith, an analog to digital converter (A/D converter) whereby the signal information from the respective sensors  10 ,  20  is converted to a digitized output in a narrow band around a frequency f o . The A/D converter  32  produces a digital output from analog input. 
     In addition, the assembly  30  may comprise a Fourier transform unit  34  which computes a first Fourier transform for the signal SIG 1  from sensor  10  in a narrow band around frequency f 0  and a second Fourier transform for the signal SIG 2  from sensor  20  in a narrow band around frequency f 0 . Moreover, the assembly  30  may comprise a ratio unit coupled to the Fourier transform unit for outputting a ratio of the amplitudes of the first and second signal Fourier transforms at the specified frequency. 
     Furthermore, the assembly  30  comprises a processor  38 . In a preferred embodiment, the processor  38  computes the amplitudes S 1  &amp; S 2  which are the Fourier transforms of the signals of the sensors  10  and  20  at the frequency f 0 . The difference in amplitudes is S 1 −cS 2 , where S 1  is input derived from the first sensor  10 , S 2  is input derived from the second sensor  20 , and c is the input from the ratio unit  36 . Optionally, the processor  38  may convert the signal from analog to digital, compute the ratio and/or the Fourier transforms without departing from the scope of the present invention. 
     For calibration purposes, the test signal generator  40  may comprise a signal generator and coil system, wherein the signal generator and coil system emits a signal at the specified frequency and is positioned symmetrically relative to the first sensor  10  (or magnetometer) and the second sensor  20  (or magnetometer). The test signal generator may be a conventional appliance, broadcast source, signal generator or the like. Upon reception of the test signal, the processor  38  computes a sensitivity of said first magnetometer and said second magnetometer and computes c. 
     A preferred embodiment for detecting an object emitting a specific frequency in an environment with magnet noise also being emitted at the specific frequency may comprise first and second gradiometers producing first and second signal information respectively, and a processor for processing the first and second signal information and computing first and second Fourier transforms of the first and second information signals, respectively. In such an apparatus, the processor may determine a first amplitude from the first Fourier transform of the first information signal and a second amplitude from the second Fourier transform of the second information signal at the specified frequency. Moreover, the processor may calculate a ratio of the first amplitude and the second amplitude at the specified frequency. Furthermore, the processor may compute S 1 −cS 2 , where S 1  is the first signal information, S 2  is the second signal information, and c is the ratio at the specified frequency and is a fixed constant. 
     Another embodiment herein provides a method of detecting a specific frequency in an environment with magnet noise emitting the specific frequency, the method comprising capturing first signal information; capturing second signal information; computing a first amplitude comprising calculating a first Fourier transform of the first signal information; computing a second amplitude comprising calculating a second Fourier transform of the second signal information; computing a ratio of the first amplitude to the second amplitude; calculating S 1 −cS 2 , where S 1  is the first signal information, S 2  is the second signal information, and c is the inverse ratio and is a fixed constant; and outputting the result. 
     In such a method, the processor  38  may detect a known or unknown object at the specific frequency when the calculation S 1 −cS 2  produces a non-zero value, and wherein an object is not detected when S 1 −cS 2  produces a zero value. In addition, the signal information may be captured using first and second magnetometers. Furthermore, when the computation S 1 −cS 2  is greater than zero, this may indicate that the source  60  is emitting the fixed frequency closer to the first magnetometer  10  compared to the second magnetometer  20 , and wherein when the computation S 1 −cS 2  is less than zero, such a method may indicate the source  60  is emitting the fixed frequency closer to the second magnetometer  20  compared to the first magnetometer, and wherein when both S 1  and S 2  increase, and the computation S 1 −cS 2  is near zero, such a method may indicate the source is emitting the fixed frequency that is equidistant the first magnetometer and the second magnetometer. Moreover, the first magnetometer  10  and the second magnetometer may be components of a gradiometer. Additionally, the capturing of the first and second signal information may be done over a period of time. 
     These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
         FIG. 1A  illustrates a schematic diagram of a signal detection apparatus according to an embodiment herein comprising a signal generator  40 ; 
         FIG. 1B  illustrates a schematic diagram of a signal detection apparatus according to an embodiment herein illustrating an unknown signal generator  60 ; 
         FIGS. 2(A) through 2(C)  illustrate a schematic diagram of a signal detection apparatus coupled to a vehicle according to an embodiment herein; 
         FIG. 3  is a flow diagram illustrating a preferred method according to an embodiment herein; and 
         FIG. 4  illustrates a schematic diagram of a computer architecture used in accordance with the embodiments herein. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The embodiments of the invention and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments of the invention. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments of the invention may be practiced and to further enable those of skilled in the art to practice the embodiments of the invention. Accordingly, the examples should not be construed as limiting the scope of the embodiments of the invention. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the full scope of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It will be understood that when an element such as an object, layer, region or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. For example, when referring first and second photons in a photon pair, these terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. 
     Furthermore, relative terms, such as “lower” or “bottom” and “upper” or “top,” may be used herein to describe one element&#39;s relationship to other elements as illustrated in the Figures. It will be understood that relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in the Figures is turned over, elements described as being on the “lower” side of other elements would then be oriented on “upper” sides of the other elements. The exemplary term “lower”, can therefore, encompass both an orientation of “lower” and “upper,” depending of the particular orientation of the figure. Similarly, if the device in one of the figures is turned over, elements described as “below” or “beneath” other elements would then be oriented “above” the other elements. The exemplary terms “below” or “beneath” can, therefore, encompass both an orientation of above and below. Furthermore, the term “outer” may be used to refer to a component that is farthest away. 
     Embodiments of the present invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments of the present invention. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the present invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, a region or object illustrated as a rectangular will, typically, have tapered, rounded or curved features. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the precise shape of a region of a device and are not intended to limit the scope of the present invention. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     The embodiments herein provide an improved magnetic sensing system capable of detecting specific frequencies in environments with excessive magnetic noise. Referring now to the drawings, and more particularly to  FIGS. 1 through 4 , where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments. 
       FIG. 1A  illustrates a schematic diagram of a signal detection apparatus  1  according to an embodiment herein comprising a test/calibration signal generator  40 .  FIG. 1A  illustrates a schematic diagram of a signal detection apparatus  1  after calibration in an environment where there may be an unknown signal generator  60 . As shown in  FIGS. 1A and 1B , signal detection apparatus  1  includes a first sensor  10  and a second sensor  20 . Also shown in  FIGS. 1A and 1B  is an assembly  30 , which may include a signal converter  32 , a Fourier transform unit  34 , a ratio unit  36 , and a processor  38 . Although signal conversion unit  32 , Fourier transform unit  34 , and ratio unit  36 , and processor  38  are shown in  FIG. 1  as separate units housed within an assembly  30 , those skilled in the art understand the components may be combined or arranged in alternative configurations within the scope of the present invention. 
     In  FIG. 1A , a signal generator and/or coil  40  are situated an equal distance from first sensor  10  and second sensor  20 . The signal generator and coil  40  are used to generate a magnetic field at a specified frequency f 0 . Because of possible varying sensitivity, the input to sensor  10  is termed SIG 1  and the input to sensor  20  is designated SIG 2 . The output from first sensor  10  and output from second sensor  20  are fed into signal processor  30  as separate input channels. In addition, the separate input channels of signal processor  30  (e.g., output from first sensor  10  and second sensor  20 ) are fed into signal converter  32 . While not shown, signal converter  32  may include at least one analog-to-digital converter and may be configured as a two-channel analog-to-digital converter or two separate analog-to-digital converters (e.g., one analog-to-digital converter for each input source). The output of signal converter  32  may include two separate channels of digitized output (e.g.,  32   a  and  32   b ), where in the digitized output is a discrete series of numeric values based on a continuous input (e.g., output from first sensor  10  or second sensor  20 ). 
     As further shown in  FIG. 1A  the two output channels of signal converter  32  (e.g.,  32   a  and  32   b ) are subsequently fed into Fourier transform unit  34 . Fourier transform unit  34  performs a Fourier transform operation on each output channel of signal converter  32  (e.g.,  32   a  and  32   b ) to produce an amplitude of that channel at the specific frequency f o  (e.g., α 1  and α 2 ). Alternatively, Fourier transform unit  34  may perform an approximation of a Fourier transform, such as a fast Fourier transform. The amplitude values produced by Fourier transform unit  34  (e.g., α 1  and α 2 ) are then fed into ratio unit  36 . Ratio unit  36  produces the ratio of α 1  to α 2  (or α 1 /α 2 ) as output at the specified frequency f o . This ratio is denoted as c and provides the calibration for the sensors  10 ,  20 . In other words, the ratio c accounts for the difference in sensitivity between the sensors  10 ,  20 . The value c is stored for subsequent calculations such as in an environment depicted in  FIG. 1B . 
       FIGS. 2(A) through 2(C) , with reference to  FIG. 1 , illustrate a schematic diagram of a signal detection apparatus  1 , coupled to a vehicle  40 , according to an embodiment herein. In addition, another embodiment (not shown) of signal detection apparatus  1  is used in a stationary mode—i.e., signal detection apparatus  1  is not mobile.  FIG. 2(A)  illustrates signal detection apparatus  1 , including sensor  10  and sensor  20 , and detection target  60 , such that signal detection apparatus  1  is positioned away from detection target  60 . In this configuration, as described in further detail below, sensor  10  and sensor  20  record signals SIG 1  and SIG 2 , respectively and perform a Fourier transforms of signals (F SIG1 (f)) &amp; F SIG2 (f)) from detection target  60 , where such signals include a signal at frequency f o . Signal SIG 1  is sent to a A/D converter and then a Fourier transform is performed on the signal and the amplitude α 1fo  at the frequency f o  is determined. Similarly for the second sensor or receiver, the signal SIG 2  received by sensor  2  is sent to a A/D converter and then a Fourier transform is performed on the signal and the resulting amplitude at f o , is defined as α 2fo . The calibration procedure performed by the preferred embodiment system  1  defines c as α 1fo /α 2fo . The quantity c becomes a fixed constant after this calibration. 
     After the initial calibration that is used to determine c is completed, one may define the amplitudes S 1  and S 2  as corresponding to the amplitudes of the Fourier Transform of sensor  1  and sensor  2  at the frequency f o , respectively. The difference S in the amplitudes of the Fourier transforms of the signals SIG 1 , SIG 2  is defined by the equation S=S 1 −cS 2  (Equation 1). This amplitudinal difference S (at the frequency f o ) will be zero everywhere except when one is near a source (e.g., target  60 ) emitting a signal that contains f o . The difference in amplitudes S of the Fourier transforms will be either plus (when sensor  10  is closer to the source  60  than sensor  20 ), minus (when sensor  20  is closer to the source  60  than sensor  10 ), or zero (when the sensors  10 ,  20  are equidistance from the source  60  or when they are both a substantial distance away). 
     As is known in the art, the Fourier transform defines a relationship between a signal in the time domain and its representation in the frequency domain. S is the amplitude of the Fourier transform at the frequency f o  in question. 
     When in motion (vehicle or target or both), for example, while coupled to a motor vehicle, the signals (SIG 1 , SIG 2 ) from sensors  10  and  20  are measured as a function of time. As described in further detail below, while in motion, signal detection apparatus  1  again computes S=S 1 −cS 2 ; the difference amplitudes after taking the Fourier transform. The amplitudinal difference S is non-zero as signal detection apparatus (vehicle  1 ) approaches a detection target  60  or visa, versa (as the target  60  approaches the apparatus  1 ) whenever the sensors  10 ,  20  are at different distances from the detection target  60 . As stated previously, the constant c eliminates any difference in sensitivities of the sensors  10 ,  20 . This interaction of sensors  10 ,  20  and detection target  60  while signal detection apparatus  1  is in motion (or while the target  60  is in motion or both) is described in further detail below with reference to  FIGS. 2(B) and 2(C) . 
     As shown in  FIG. 2(B) , signal detection apparatus  1  may be mounted on a vehicle  40 . Vehicle  40  is shown in  FIG. 2(B)  on road  50 , at position  43 , heading towards detection target  60 . Vehicle  40  is also shown in  FIG. 2(C)  on road  50 , at position  46 , near detection target  60 . In  FIGS. 2(B) and 2(C) , vehicle  40  is moving in a linear direction (e.g., traveling forward on a road  50 ) towards detection target  60 . First sensor  10  and second sensor  20  are shown in  FIGS. 2(B) and 2(C)  as being coupled to opposing sides of vehicle  40 . For example, in  FIGS. 2(B) and 2(C) , first sensor  10  is coupled to a left side of vehicle  40  and second sensor  20  is coupled to a right side of vehicle  40 , however the embodiments herein are not restricted to any particular placement of the sensors  10 ,  20  in relation to the vehicle  40 . In addition, while signal detection apparatus  1  mounted on vehicle  40  in  FIGS. 2(B) and 2(C) , signal detection apparatus  1  is not limited to such a coupling and may, in general, be coupled to any mobile device or apparatus. Examples include, but are not limited to, all forms of terrestrial vehicles (either military or civilian), portable devices and handheld devices. 
     While not shown in  FIGS. 2(A) through 2(C) , first sensor  10  may produce a first signal and second sensor  20  may produce a second signal. First sensor  10  and second sensor  20  may produce signals SIG 1  and SIG 2  that are processed independently. 
     The combination of first sensor  10  and second sensor  20  together may operate as a gradiometer for measuring the difference between two signals (e.g., first signal and second signal), with an aim to facilitate rejection of common mode noise signals and improved reduction in errors due to sensor calibration at frequency f o . The terminology “SIG 1  and SIG 2  has been used generically above; and both first sensor  10  and second sensor  20  may comprise magnetometers, in which case the signal that is being detected would be a magnetic field. The signal processing technique of the preferred embodiment may utilize sensors  10 , comprising electric field sensors or, in addition, seismometers, or some other kind of electromagnetic field or acoustic sensor. 
     The output  38   a  of processor  38  (shown in  FIG. 1 ) indicates whether signal detection apparatus  1  is near detection target  60 . For example, the output  38   a  of processor  38  (shown in  FIG. 1 ) may be zero when the signal detection apparatus  1  is unable to detect detection target  60  and the output  38   a  new signal generator  38  (shown in  FIG. 1 ) may be non-zero when the apparatus  1  is able to detect detection target  60 . The output  38   a  also detects whether sensor  10  or sensor  20  is closer to the target  60 . If sensors  10  and  20  are equidistant from the target  60 , then the output  38   a  would equal 0. Both signals S 1  and S 2  are larger when apparatus  1  is at position  46  compared to when apparatus  1  is at position  43 . 
     As an example, signal detection apparatus  1  may be searching for detection target  60 , which is known to emit a frequency f 0  at 60 Hz. In addition, one could scan the frequencies for detecting signals at other frequencies; other than 60 Hz. For example, if looking for an underground facility where an appliance (such as a fan) is present. Assuming most electrical products operate in the surveillance area operate at 60 Hz, by moving the detection apparatus around, variations above the background noise may be detected. Signal detection apparatus  1  uses first sensor  10  and second sensor  20  to capture signal information on either side of vehicle  40 . In addition, the detection apparatus could be handheld and carried. In addition, only the sensors may be carried with provision being made for radioing signals receive by sensors  10 ,  20  to a base unit at a remote location. The signal information captured by each sensor (e.g., S 1  is captured from first sensor  10  and S 2  is captured from second sensor  20 ) is processed by signal processor  30 ,  38  ( FIG. 1 ). Signal processor  38  takes the Fourier transform (e.g., by using Fourier transform unit  34 ) of the signal information captured by each sensor to produce amplitude values α 1  and α 2 . Signal processor  38  then signal information by computing the amplitudinal difference S=S 1 −cS 2  at frequency f 0 , where c was calibrated and stored earlier as c using signal generator and coil  40 , as described above. As a consequence of the above calculations performed by signal processor  30 , signal detection apparatus  1  detects detection target  60 , emitting a frequency f 0 , when S≠0. If S&gt;0, target  60  is closer to sensor  10 , whereas if S c &lt;0, target  60  is closer to sensor  20 . 
       FIG. 3 , with reference to  FIGS. 1A ,  1 B,  2 A, &amp;  2 (B), illustrates a flow diagram according to an embodiment herein. Generally, the process involves finding c using the signal generator and coil  40  and then using the sensors  10 ,  20  and c to determine S c . Step  70 , of the method shown in  FIG. 3 , describes outputting first signal information using a signal generator and coil (e.g., signal generator and coil  40 ). Step  72  describes receiving the signal information (e.g., via sensors  10 ,  20 ). Step  74  describes computing a first amplitude comprising calculating a first Fourier transform of the first signal information (e.g., via Fourier transform unit  34 ) at frequency f 0 . Step  76  describes computing a second amplitude comprising calculating a second Fourier transform of the second signal information (e.g., via Fourier transform unit  34 ). Step  78  describes computing c as a ratio comprising calculating a ratio of the first amplitude to the second amplitude and negating a result of the calculation (e.g., via ratio unit  36 ). Step  80  describes using the computation at frequency f 0  of S 1 −cS 2 , where S 1  is the first signal information, S 2  is the second signal information and c is the inverse ratio (e.g., via new signal generator  38 ). In step  82 , the method shown in  FIG. 3 , describes outputting the new signal (e.g., to create a spectrogram). 
     The techniques provided by the embodiments herein may be implemented on an integrated circuit chip (not shown). The chip design is created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed. 
     The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     The embodiments herein include both hardware and software elements. The embodiments that are implemented in software include but are not limited to, firmware, resident software, microcode, etc. Furthermore, the embodiments herein can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer-readable medium can be any apparatus that can comprise, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. 
     The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD. 
       FIG. 4  illustrates a data processing system suitable for storing and/or executing program code will include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution. 
     Input/output (I/O) devices (including but not limited to keyboards, displays, pointing devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters. 
       FIG. 4  is a schematic drawing illustrates a hardware configuration of an information handling/computer system  100  for use with the embodiments herein. The system comprises at least one processor or central processing unit (CPU)  110 . The CPUs  110  are interconnected via system bus  112  to various devices such as a random access memory (RAM)  114 , read-only memory (ROM)  116 , and an input/output (I/O) adapter  118 . The I/O adapter  118  can connect to peripheral devices, such as disk units  111  and tape drives  113 , or other program storage devices that are readable by the system. The system can read the inventive instructions on the program storage devices and follow these instructions to execute the methodology of the embodiments herein. The system further includes a user interface adapter  119  that connects a keyboard  115 , mouse  117 , speaker  124 , microphone  122 , and/or other user interface devices such as a touch screen device (not shown) to the bus  112  to gather user input. Additionally, a communication adapter  120  connects the bus  112  to a data processing network  125 , and a display adapter  121  connects the bus  112  to a display device  123  which may be embodied as an output device such as a monitor, printer, or transmitter, for example. 
     The Fourier transform of the signals from sensors  10  and  20  may be calculated. In general, Fourier transform X(f) is composed of a real and imaginary function. The real function is x r (f) and the imaginary function is x i (f). Thus, x(f) is given by
 
 X ( f )= x   r ( f )+ ix   i ( f )
 
To determine the amplitude, these functions are evaluated at the frequency f 0 , the functions are squared and summed together. By taking the square root of the sum, the amplitude is derived, which is measure of the signal strength at the frequency f 0 .
 
     The foregoing description of the specific embodiments are intended to reveal the general nature of the embodiments herein. While others can, by applying current knowledge, readily modify and/or adapt for various applications; it is not intended that such specific embodiments be interpreted as departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.