Abstract:
A compact, autonomous motion detecting and alerting system alerts to the movement of objects of interest. Mounted on an environmentally sealed PC board are a transceiver such as a CW radar front-end, connectors, signal processors and a communications device. The system provides early warning of movement of an ice sheet or rubble field via the communication device that may be a cellular telephone. This system is mounted proximate the target surface under observation, oriented at pre-specified offset angles both laterally and in elevation. The target is illuminated and energy reflected therefrom is mixed with a portion of the transmitted signal to produce a difference frequency signal that is processed to establish existence of motion within a pre-specified velocity range. Upon verification of motion, notification is sent to a responsible authority. An autonomous or semi-autonomous power source and integral power management function may be incorporated on the same PC board.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without the payment of any royalties thereon. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to autonomous detection and alerting to motion, such as ice flow in a river. More particularly, it provides a sealed, inexpensive, fully integrated, remote, early-warning system for continuous detection and early warning of river ice flow hazards. It may be solar powered with battery backup, with an option for at least one battery to be solar rechargeable. 
     BACKGROUND 
     Alerting to a waterway&#39;s ice motion during winter frazil ice flow and spring ice breakup is important. The ability to detect river ice sheet and rubble motion can provide early warning to communities and facilities down-river, helping prevent the loss of life, flooding, and infrastructure damage often associated with massive ice rubble flows. Unfortunately, this information is often not available at all, and, when available, it may be hazardous, difficult, time consuming, and expensive to obtain. 
     Currently, there are three methods employed to detect movement of an ice sheet. The first, visual observation, requires the constant attention of a human observer. This method is particularly inefficient, as it can be hindered by darkness or by low-visibility weather conditions. 
     Video monitoring of the ice sheet is similarly limited by the short periods of daylight in northern latitudes in the winter, weather conditions and the need for the presence of both equipment and operator at the time the ice sheet moves. Video monitoring has the further disadvantage of requiring tedious, frame-by-frame motion analysis and the inclusion of known benchmarks in the video frame from which to measure or detect motion. 
     A third method uses a frangible wire embedded in the river ice to detect movement. When the ice moves, the wire breaks, opening a circuit and triggering an alarm. However, deploying the wire across the ice sheet or rubble field may be extremely hazardous. Furthermore, this primitive method provides only a one-time indication of movement, thereby necessitating repeated hazardous deployments in order to reset the alarm. 
     A number of patents utilizing the Doppler effect at various frequencies for motion detection and other applications have been issued. Each is distinguishable from the instant invention. 
     U.S. Pat. No. 5,585,799, Microwave Doppler Radar System for Detection and Kinematic Measurements of River Ice, to Yankielun and Ferrick, Dec. 17, 1996, incorporated herein by reference, teaches a Doppler ice motion detection system for measuring the velocity of an ice sheet. However, the device disclosed in the &#39;799 patent is not capable of autonomous operation, and is thus not well suited to long-term monitoring of an ice sheet. Further, it employs expensive components, making it relatively less suitable for widespread use in simple detection of the dynamics of ice sheet motion. Rather, it is best used when seeking highly accurate scientific data during a relatively short-term movement of an ice sheet. 
     U.S. Pat. No. 6,333,691 B1, Motion Detector, to Janus, Dec. 25, 2001, employs the Doppler phenomena to detect motion by comparing phase or amplitude changes as determined from separate transmit and receive antennas. It also permits operation of the system through otherwise interfering materials via judicious selection of the frequency of operation. 
     U.S. Pat. No. 6,324,912 B1, Flaw Detection System Using Acoustic Doppler Effect, to Wooh, Dec. 4, 2001, employs an acoustic transducer and makes use of the Doppler effect at acoustic frequencies to detect flaws in material under inspection. A particular application is detecting flaws in railroad tracks in real time during operation of a rail riding vehicle. 
     U.S. Pat. No. 5,587,713, Radar Transmitter/Receivers, to Pfizenmaier, et al., Dec. 24, 1996, provides an RF transmit-receive capacity on a single PC board using a ratrace device for inputting RF energy to a mixer from the transmitter and the transmit-receive antenna. 
     U.S. Pat. No. 5,966,090, Differential Pulse Radar Motion Sensor, to McEwan, Oct. 12, 1999, details a pulsed Doppler system for overcoming inherent disadvantages of CW Doppler systems. It yields a constant response versus distance to motion within at least one gated range. It may also provide target direction using a quadrature receive channel. 
     U.S. Pat. No. 6,380,882 B1, Motion Detector Based on the Doppler Principle, to Hegnauer, Apr. 30, 2002, uses two separate RF frequencies to detect motion within a room, comparing the phase difference between these two to detect motion. The design also incorporates suppression circuitry to deal with multi-path interference. 
     U.S. Pat. No. 5,977,874, Relating to Motion Detection Units, to Konstandelos, Nov. 2, 1999, provides an RF transceiver on 2 separate PC boards, one for the electronics and the other for the transmit-receive antenna. The two boards are separated by a common ground plane in a preferred embodiment. The system employs the Doppler phenomenon to detect motion and is of a smaller size than previous designs. 
     U.S. Pat. No. 5,262,783, Motion Detector Unit, to Philpott et al., Nov. 16, 1993, is similar to the &#39;874 patent in that it consists of two PC boards, an antenna board and the electronics board. However, the boards have no electrical connection therebetween, relying on two slots resonant at the oscillator&#39;s fundamental frequency to couple the antenna to the transceiver. Feed striplines on the boards lie orthogonal to the slots, suppressing the oscillator&#39;s 2 nd  harmonic frequency. 
     U.S. Pat. No. 5,497,163, Doppler Radar Module Using Micro-Stripline Technology, to Lohninger et al., Mar. 5, 1996, provides a transceiver and mixer-antenna configuration for utilizing the Doppler effect. It is mounted on a compact multi-layer motherboard, enclosed in a housing of electrically conductive material. 
     U.S. Pat. No. 5,684,458, Microwave Sensor with Adjustable Sampling Frequency Based on Environmental Conditions, to Calvarese, Nov. 4, 1997, provides an adjustable motion detector using the Doppler effect. For adverse environmental conditions, such as thermal noise, that exceed a pre-specified threshold, a processor incorporated in the system changes the sampling frequency. 
     Thus, needed is an autonomous, reliable and inexpensive detection and warning device that also eliminates exposure of personnel to environmental hazards when installing or maintaining it. Applications in nature include warning of flash floods, mudslides, tidal waves, tsunamis, land slides (falling rocks), and snow avalanches. In industry, applications include landslides in open-pit mining operations, instabilities in below ground mines, and security operations including perimeter or border monitoring and intrusion sensing. 
     SUMMARY 
     A preferred embodiment of the present invention is a compact and relatively inexpensive motion detection and alerting system implemented in a single, environmentally secure and benign package. Further it is capable of autonomous continuous operation, if necessary. It operates autonomously detecting motion within a pre-specified velocity range. In general its components include a transceiver sub-assembly and a signal processing sub-assembly. 
     The transceiver sub-assembly propagates electromagnetic energy as a transmitted signal in a pre-specified pattern and direction to illuminate a non-smooth surface. It receives at least some energy reflected therefrom as a reflected signal and provides a reference signal. 
     The signal processing sub-assembly communicates with the transceiver sub-assembly, determining the difference in frequency between a reference sample of the transmitted signal and the reflected signal. Further processing establishes the existence of a pre-specified range of Doppler frequencies that may be correlated to motion of the non-smooth surface, e.g., an ice sheet. Using a decision algorithm to identify non-transient velocities within a pre-specified velocity range enables alerting via a cellular telephone. 
     Components of the transceiver sub-assembly include a transceiver, a T-connector between the transceiver and an antenna, an impedance matching device between the T-connector and the antenna, and a signal mixer connected to the “base” of the T-connector. The signal mixer receives signal inputs from the transceiver and the antenna via the T-connector and outputs signal products, to include a signal at a Doppler frequency as appropriate. 
     In a preferred embodiment, the system is mounted on a single printed circuit (PC) board. The PC board includes a CW microwave source having an average output power between 10 and 20 dBm, the T-connector, an impedance matching transformer, a strip line beam antenna and a single-end signal mixer. 
     The antenna may be any of: a stripline antenna, a stripline beam antenna, a Yagi stripline beam antenna, a log periodic array (LPA) stripline beam antenna, a wire element beam antenna, a tubular element beam antenna, a microwave horn antenna, a microwave dish antenna, and any combination thereof. However, for the system to be fitted to a preferred embodiment, i.e., a single PC board, only the stripline antennas may be used. Preferably, the antenna offers forward gain of approximately at least 10 dB and antenna main lobe dimensions of approximately 60° or less. 
     Preferably, the processing sub-assembly includes analog circuitry connected to the transceiver sub-assembly through a mixer. This analog circuitry conditions output signals from the mixer for digital processing. Digital circuitry receives conditioned output signals from the analog circuitry, converts it to digital format, and employs a decision algorithm for determining movement of the non-smooth surface. A communications device receives output from the digital circuitry that indicates target movement and provides the necessary alert. 
     In a preferred embodiment, the analog circuitry includes a first amplifier for amplifying the signal products from the mixer, a bandpass filter that passes only those Doppler frequencies that correlate to the pre-specified velocity range, a second amplifier for amplifying the output of the bandpass filter, and a Schmitt trigger for final conditioning of the output from the second amplifier. 
     The digital circuitry includes an analog-to-digital (A/D) converter for converting the output of the Schmitt trigger, a digital signal processor (DSP), and a communications device including an autodialer, a cellular phone and a cellular phone antenna. The DSP implements a decision algorithm and an optional power management function. Preferably, all system components noted above are enclosed within a weatherproof enclosure. 
     Preferably, the system includes it own power source installed on the single PC board. The power source may include a solar panel augmenting a rechargeable battery. Further, a preferred embodiment may include its own mounting bracket. In addition to the system itself, a unique method of implementing a motion detection capability is provided. 
     Provided is a method for detecting and alerting to the movement of an irregular or non-smooth target surface. It comprises mounting a motion detection and alerting system of the present invention at a pre-selected look angle in azimuth and elevation; providing power to the system; illuminating at least part of a target surface with a signal containing electromagnetic energy; receiving energy reflected from the target surface as a result; processing the reflected energy together with a reference signal provided from the transceiver to produce a difference frequency signal representing the difference in frequency between the reference signal and the reflected energy; establishing a value of the difference frequency signal, such that a non-zero value indicates motion that may be of a pre-specified type; processing any non-zero value difference signals by implementing a decision algorithm to minimize false alarms; and using the output of the decision algorithm, providing notification of any occurrence of the pre-specified type of motion. The notification, at least in part, may comprise placing a telephone call automatically. Establishing the value of the difference frequency signal may further comprise introducing a pre-specified delay period prior to sending the notification. 
     Further, the method may include selectively powering the components based on system demand, i.e., power management. It may also include mounting the system such that the antenna is oriented in respect to the target surface at an offset angle less than approximately 60° both laterally and in elevation and more preferably at an offset angle between approximately 0° and 30° both laterally and in elevation. Should one desire to use the system for capturing precise measures of movement velocity, one may employ an inclinometer to achieve an accuracy of at least ±1.0° in orienting said system. 
     In a preferred embodiment, an autonomous ice sheet and rubble motion detecting and alerting system is provided in which components of the system as described above are mounted on a single printed circuit (PC) board. The existence of a pre-specified range of Doppler frequencies is correlated to an expected velocity of motion representative of the ice sheet and rubble. 
     Any embodiment of the system may be powered by any of a number of sources including a source remote from its location and a backup source. 
     In one application, a preferred embodiment of the present invention is mounted at a location proximate a target surface to be observed such as an ice sheet or rubble (debris) field. The target may be remote from the responsible authority who may be mobile, thus a wireless communications device is provided in a preferred embodiment. A reference signal, ƒ source , is transmitted from the antenna towards the target surface. Resultant reflected radiation, i.e., the “backscattered” portion, is mixed with a portion of the transmitted signal sampled for that purpose. This mixing produces a difference, or Doppler, frequency ƒ DOP , which is then processed to establish movement of the target surface. 
     Digital circuitry implementing a decision algorithm establishes movement of the target surface, including movement on the target surface. Using the decision algorithm, the difference frequency is established to be non-zero for a sufficient amount of time to rule out transient motion, e.g., a hiker crossing the transceiver&#39;s field of vision. Upon such determination, the system sends a notification, preferably over a wireless communications device, to a responsible authority at a remote location. 
     Further advantages of the present invention will be apparent from the description below with reference to the accompanying drawings, in which like numbers indicate like elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts signal scatter in two dimensions from a smooth surface, the signal being oriented from the vertical at an angle, β. 
     FIG. 2 depicts signal scatter in two dimensions from a rough surface with the expected direction of target surface movement indicated by an arrow. 
     FIG. 3 depicts the relative lateral positioning of a preferred embodiment of the present invention with respect to a target surface under observation, such as a waterway. 
     FIG. 4 is a graph of target velocity versus Doppler frequency for various angles of a transceiver&#39;s offset in elevation from the target surface. 
     FIG. 5 is a schematic diagram of a preferred embodiment of the present invention. 
     FIG. 6 is a schematic diagram of a preferred embodiment of the present invention as laid out on a printed circuit board. 
     FIG. 7A is a side view of a preferred embodiment of the present invention as packaged. 
     FIG. 7B is a top view of a preferred embodiment of the present invention as packaged. 
     FIG. 8 is a block diagram illustrating the method of detecting and alerting to movement of a slow-moving surface in using a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Refer to FIG. 1. A signal  14  is transmitted towards a smooth surface  12 . For example, in one application a continuous wave (CW) radar front-end (transceiver)  16  employing a signal  14  having a single frequency radio frequency (RF) carrier is transmitted from an antenna  10  at an oblique depression angle, β. If the radar-illuminated area (target surface) were perfectly smooth, as shown in FIG. 1, energy in the transmitted signal  14  incident to the smooth surface  12  results in reflections  18  away from the radar&#39;s transmit/receive antenna  10 . 
     Refer to FIG.  2 . Due to the inherent roughness of many surfaces such as sheet ice and rubble fields as depicted at the surface  23 , there is sufficient reflection (backscatter)  25  towards the radar antenna  10  to facilitate Doppler velocity measurements. If this target surface  23  is moving, the reflected radar signal  25  exhibits an apparent shift from the carrier frequency of the transmitted signal  14 . The backscattered radar signal  25  has a higher frequency than that of the transmitted signal  14  if the target surface  23  is moving towards the radar antenna  10 , and a lower frequency than that of the transmitted signal  14  if the target surface  23  is moving away from the radar antenna  10 . The difference between the frequency of the transmitted signal  14  and the apparent frequency of the backscattered signal  25  is the Doppler, or difference, frequency, ƒ DOP , and is proportional to the velocity, ν, of the target surface  23 . 
     Refer to FIG.  3 . Shown is a top view of a waterway with sides  31 . The transceiver  16  and antenna  10  are oriented with respect to the direction of flow (along the z-axis)  21  in the waterway such that the transmitted signal  14  is at an angle, α, with respect to the direction of flow (z-axis)  21  and the x-axis at that point in the waterway. 
     The magnitude of the Doppler frequency shift, ƒ DOP , for a target surface  23  moving at a given velocity, ν ( m/s), is                f   DOP     =       2      v                   cos        (   β   )            cos        (   α   )         λ             (   1   )                                
     where β represents the vertical depression look-angle, α is the horizontal (“off-stream”) look angle, and λ is the wavelength (m) of the transmitted signal  14 . λ is given by the equation λ=c/ƒ source , where c is the velocity of light in a vacuum (3×10 8  m/s) and ƒ source  is the carrier frequency (Hz) of the transmitted signal  14 . The geometry relevant to the above calculations is illustrated in FIGS. 1 (y-z plane), and  2  (x-z plane), where the arrow  21  represents the direction of flow in a waterway, i.e., along the z-axis. 
     Refer to FIG.  4 . The velocity, ν, of the target surface  23  is plotted versus the vertical depression (offset) angle, β, for the offset angles of 0°, 30°, 45°, and 60°. As can be seen, the greater the offset angle, β, the more sensitive the system is to small changes in Doppler frequency, ƒ DOP . Thus if one were interested in not only the velocity but a rate of instantaneous change, i.e., acceleration, then a large offset angle may be used, but otherwise a small offset angle (in both lateral (α) and vertical (β) directions), yields the greatest discrimination and is used in a preferred embodiment of the present invention. 
     In an application using a CW radar front-end as the transceiver  16 , a sample of the transmitted signal  14  at its carrier frequency, ƒ source , “sampled” or “bled off” at a level sufficient to assure its accuracy as a current (real time) reference, is used as the reference signal. It is mixed with the received backscatter signal  25 . The mixing process results in four frequency products: that of the reference signal, that of the backscattered signal  25 , and a sum and a difference, respectively, of the reference and backscattered frequencies. The difference frequency resulting from the mixing process is the Doppler frequency, ƒ DOP . If the target surface  23  were stationary, then ƒ DOP =0. Thus, the presence of a non-zero Doppler frequency signal indicates possible movement of the target surface  23 , thereby providing a means for motion detection and alerting. Of course, movement of objects on the target surface  23  would also be indicated. As discussed below, a means for distinguishing movement of the target surface  23  from movement on the target surface  23  is provided for a preferred embodiment of the present invention. 
     Referring now to FIG. 5, there is shown a schematic diagram of a preferred embodiment  50  of the present invention, such as may be used as an ice motion detection and alerting system. The motion detection and alerting system  50  includes an antenna  10 , a T-connector  55 , an impedance matching transformer  57 , a radar front-end  16 , preferably a low-power CW microwave source having an average output of 10-20 dBm, analog circuitry  56 , a digital circuit  58 , a wireless communications device  51 , and a mixer  53 . A power source (not separately shown) may also be provided. Optionally, for those remotely located systems, an integrated power source  59  shown in FIG. 5 as a battery  59 A supplemented via a solar panel  59 B, may be used. The antenna  10 , CW radar front-end  16 , and mixer  53  are interconnected, for example, by a T-junction  55 , or a microwave circulator (not separately shown), with the antenna  10  being connected through the impedance matching transformer  57 . Though less efficient, a T-junction  55  is preferred over a microwave circulator because of its lower cost. 
     The antenna  10  is preferably a stripline beam antenna, such as a Yagi or log periodic array (LPA). A stripline antenna (that is, an antenna etched directly into a printed circuit board) is economical and directly interfaces with system electronics mounted on the same circuit board  64 . A Yagi or LPA antenna offers both forward gain of at least 10 dB and antenna main lobe dimensions of 60° or less. In alternative embodiments, wire element beam, tubular element beam, microwave horn, and microwave dish antennas (all not separately shown) may be used. The mixer  53  may be a single-end mixer or a detector diode (not separately shown) operating as a mixer. It outputs the signal products described above, i.e., a reference signal, a backscattered signal  25 , a difference frequency signal at ƒ DOP , and a summed frequency signal. 
     Analog  56  circuitry conditions the difference frequency signal output from the mixer  53  for use by the digital circuit  58 . Using a decision algorithm in the digital circuit, movement of the target surface  23  may be established, e.g., for an ice motion detection and alerting system, movement of the river ice sheet or rubble. Analog circuitry  56  includes a bandpass (BP) filter  56 A, at least one amplifier  56 B, and a threshold circuit  56 C, such as a Schmitt trigger. Preferably, the analog circuitry  56  includes two amplifiers  56 B, one each positioned on the input and output sides of the BP filter  56 A. For an ice motion detection and alerting system, the cutoff frequencies of the BP filter  56 A may be selected to reject frequency products outside the range of expected velocity of the ice sheet or rubble, thereby providing a degree of false alarm protection from events that are too fast to be moving ice, such as snowmobiles traversing the ice cover. See for example, FIG. 4 indicating a difference frequency of approximately 33 Hz for a velocity of 10 m/s at a 0° offset. Should one expect ice to move at less than about 10 m/s then the cutoff frequency could be established, via a decision algorithm, to yield only those difference frequencies, ƒ DOP , of less than 33 Hz for those installations of the system that employ a 0° offset: 
     The digital circuit  58  is programmed with a decision algorithm for determining movement of the target surface  23  only, based upon both the value and the duration of the difference frequency signal output by the analog circuitry  56 . That is, notification may be delayed past any initial detection of motion by a predetermined period of time. This serves to prevent false alarms due to motion of transient objects (not separately shown) on the target surface  23  that are within the field of view of the motion detection and alerting system  50 , e.g., for an ice motion detection and alerting system, those items other than the river ice sheet or rubble. When the digital circuit  58  establishes movement of the correct type, it sends appropriate notification to a responsible authority, preferably through a wireless communications device  51 . 
     The use of a wireless communications device  51  allows the responsible authority to be at any location remote from the motion detector system  50 . In a preferred embodiment of the invention, the wireless communications device  51  is a telephone autodialer  51 A and a cellular telephone (not separately shown) or antenna  51 B. However, other wireless communication devices  51 , such as a personal digital assistant (PDA), RF telemetry transceiver, RF transceiver, and acoustical or optical communications devices, are also within the scope of the present invention. 
     In preferred embodiments, the digital circuit  58  also includes power or energy management functionality (not separately shown except in FIG. 8 as a function  85 F). This permits certain sub-systems within a motion detection and alerting system  50  to be taken off line or to be operated cyclically to save energy. For example, if environmental conditions are taken into consideration, perhaps a target surface  23  need not be monitored continuously, and thus illumination may be switched on and off, e.g., maintaining marginal coverage during times when ice breakup is not expected and optimal coverage at other times, but not necessarily continual coverage unless pre-specified movement has been detected. This power management capability may be “programmed in” using a simple algorithm suitable for modification to address specific applications. Since the motion detection and alerting system  50  may be powered by a battery  59 A augmented by a solar panel  59 B, power management functionality improves the power budget by optimizing average demand, thereby reducing the size of the battery  59 A and any connected solar panel  59 B. Lithium cells or lead-acid gel cells may be used. Further, the batteries need not be installed on the circuit board but may be in another weather-tight box separated from the Doppler radar and interconnected by a power cable (not shown separately). 
     Refer to FIG.  6 . As alluded to above, the use of a stripline antenna  10  allows the motion detection and alerting system  50  to be integrated onto a single circuit board  64 . In a preferred embodiment, a stripline beam antenna  10 ,  10 ′ is etched onto the top (antenna  10 ) and bottom (antenna  10 ′) surfaces of the circuit board  64 . A CW radar front-end  16  and a mixer  53  are included within a CW radar sub-assembly  66  that may include the antenna  10 ,  10 ′, a mixer  23 , a T-connector  55  and an impedance matching transformer  57 . In a preferred embodiment, analog circuitry  56 , a digital circuit  58 , and a wireless communications device  51  are included within a processing sub-assembly  62 . 
     Refer to FIGS. 7A and 7B. The packaged motion detection and alerting system  50  is shown in side and top views, respectively. In order to securely mount the motion detection and alerting system  50  in the proximity of the target surface  23  of interest, such as a river ice sheet or rubble field, a mounting bracket  76  is provided. The mounting bracket  76  may be either fixed or adjustable. Further, to protect the motion detection and alerting system  50  from the environment, a weatherproof enclosure  71  may be provided around the various electronic components. 
     The operation of the motion detection and alerting system  50  is described with reference to FIG.  8 . In step  80 , the motion detection and alerting system  50  is mounted and powered up from a fixed point overlooking the target surface  23 , such as a river. Since the system  50  is intended for detecting and alerting to motion, e.g., in a preferred embodiment movement of ice or rubble, and not necessarily for velocity or acceleration measurement, the selection of α and β may not be critical. However, for simple motion detection and alerting, both α and β should be kept as small as possible to obtain the highest difference frequency output relative to the velocity of ice or rubble as calculated using Eqn. (1). See also FIG.  4 . In the event measurement of actual velocity of ice movement is desired, the system  50  may be accurately positioned to within one degree in the vertical plane using an inclinometer (not separately shown). One skilled in the art may add functionality within the digital circuit  58  to allow measurement of velocity. To achieve the horizontal relationship, the system  50  may be aimed visually upstream at a pre-specified angle, α, relative to the direction of river flow at that point. 
     In step  81 , the transmitted signal  14  is sent at the frequency, ƒ source . The backscattered signal  25  is received in step  82  and mixed with the reference signal in step  83 . As described above, this mixing  83  produces several signal products, including the difference frequency signal at ƒ DOP . 
     These signal products are then passed to the analog circuitry  56  where they are processed in step  84 . Analog processing conditions the signal products for digital processing. First, three of the signal products, the reference signal, the backscatter signal  25 , and the difference frequency signal are amplified in step  84 A. These signal products are then bandpass-filtered in step  84 B. The cutoff frequencies are selected based on the expected velocity range of motion desired to be detected. Here, the reference and backscatter  25  signals, as well as their sum, being at microwave frequencies, are filtered out, leaving only the difference frequency signal, ƒ DOP . The difference frequency signal is amplified in step  84 C to an appropriate level to be input into a threshold circuit  56 C, such as a Schmitt trigger. In step  84 D, the threshold circuit converts the analog output into a level appropriate for digital processing. 
     Analog to digital conversion, subsequent digital signal processing and the notification decision process occur in step  85 . First, the digitized difference frequency signal is analyzed in step  85 A, using a decision algorithm to determine if there is motion, i.e., if ƒ DOP &gt;0. If ƒ DOP =0, no notification is sent as indicated at step  85 B. A non-zero ƒ DOP  indicates potential motion of interest. Since the system  50  is sensitive to motion of objects that fall within its field of view, transient passage by animals, skiers, hikers, and motor vehicles also causes a non-zero difference frequency signal. To reduce the occurrence of false alarms, a delay of between five and sixty seconds, preferably approximately fifteen seconds is inserted, as indicated in step  85 C. If, after the delay  85 C, the system  50  detects a non-zero difference frequency signal as indicated at step  85 D, notification is sent as indicated in step  85 E. This “sample and delay” routine may be repeated to account for multiple occurrences of transient passage within a short period, e.g., to enable verification where more than one animal, person or vehicle crosses the target surface  23  within a short period of time. Of course, if a decision algorithm has been implemented at the BP filtering step  84 B so that Doppler frequencies above a pre-specified frequency are cut off, this delay step  85 C may need to operate as a check against only slow moving vehicles, personnel or animals. In a preferred embodiment, notification at step  85 E is sent via cellular telephone, either by simply ringing the receiving telephone or by providing a voice-synthesized warning message upon answer. Optional power management occurs continuously as indicated in step  85 F. 
     In a preferred embodiment, an optimum frequency range is 1.0 GHz-10 GHz, depending on the required resolution and specification application, as dictated by Eqn. (1). Power requirements are minimal, e.g., for ice and rubble field movement, 10-20 mW as provided by modular COTS RF oscillators is preferred, although systems with greater power levels may be implemented, depending upon the application. The sampling rate is in accordance with the Nyquist criterion for the highest audio Doppler output frequency expected to be usable by the system. This is most likely from approximately 100 Hz-10 KHz, again depending on the application and concomitant system parameters, e.g., the expected speed of phenomena being monitored as dictated by Eqn. (1). 
     While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims. For example, although the system is described in specific examples for detecting movement of an ice or rubble surface, it will operate on any non-smooth surface so that it may also be useful for detecting movement prior to and during events such as earthquakes, mudslides, flash floods, and for monitoring certain industrial processes. Further, the source may be operated with other types of electromagnetic energy such as acoustical, ultrasonic, and optical, including visible, IR, and UV. Additionally, these motion detectors may be arranged in arrays to cover a resultant large field of view such as may be expected when monitoring a glacier. Thus, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting, and the invention should be defined only in accordance with the following claims and their equivalents.