Abstract:
A system and method for determining the locations of freight containers in a freight yard is described. A remote unit that includes a GPS receiver is attached to the freight containers. The remote receivers have an independent power supply—a battery. Intermittently, either by a timer or by a motion detector, the remote receivers are operated. During operation, the remote receivers receive signals from the global positioning satellite system and at an allotted time, transmit the GPS data to a base station before shutting down to conserve power. The base station processes the GPS data to determine a position in the freight yard of each freight container. When a particular freight container or contents is desired, a data base in the base station can be consulted and the contents and location of a particular freight container located. The freight yard is typically outside with a view of the GPS constellation, such as a rail yard, airport baggage area, ship yard, truck park, etc. An alternative is described for use where the freight yard is a warehouse and the satellite view is obstructed. The alternative uses psuedolites or repeaters to track freight containers within the warehouse.

Description:
[0001]    The present invention is a continuation-in-part of U.S. patent application Ser. No. 07/804,368 entitled “Golf Distance Measuring System and Method” and a continuation of Ser. No. 08/334,733 entitled “System and Method for Determining Freight Container Locations.” 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to a system and method for tracking inventory and freight using the global positioning satellite system.  
           [0004]    2. Description of the Related Art  
           [0005]    The present invention utilizes the global positioning satellite system (GPS) to determine the location of freight, inventory, packages or the like (“freight”) in a holding area, such as a freight terminal, railyard, airport, warehouse or other storage area. Knowledge of GPS and freight or inventory problems and procedures is useful for an appreciation of the present invention. U.S. patent application Ser. No. 07/804,368 entitled “Golf Distance Measuring System and Method” (incorporated by reference) describes inter alia a system for tracking golf carts and players on a golf course using GPS and is analogous to the present invention which tracks freight.  
           [0006]    THE GLOBAL POSITIONING SATELLITE SYSTEM  
           [0007]    GPS is a spaced based system of satellites which can provide an infinite number of receivers accurate three dimensional position (i.e. horizontal location and altitude), velocity, and time. A general understanding of GPS is useful to appreciate the operation of the present invention. Numerous books and articles are available on GPS operation and theory. See e.g., GPS—A Guide to the Next Utility, Trimble Navigation (incorporated by reference for background).  
           [0008]    The GPS system is an umbrella of satellites circling the earth passively transmitting signals. Each satellite has a very accurate atomic clock which is periodically updated. A GPS receiver with an accurate clock can identify a satellite and determine the transit time of the signal from the satellite to the receiver. Knowing the transit time and knowing that the speed of light is 186,000 miles per second enables a calculation of the distance from the satellite to the receiver. The signal carries with it data which discloses satellite position and time of transmission, and synchronizes the GPS receiver with the satellite clocks.  
           [0009]    As a GPS receiver locates 3 or 4 satellites it determines its distance from each satellite. The intersection of these 3 or 4 spheres enables a precise location of the receiver (and some compensation for timing errors in the receiver&#39;s internal clock). The GPS system should have 21 satellites and 3 spares once the system is fully deployed. The full constellation of 24 ellites was declared operational in 1994.  
           [0010]    There are basically two types of GPS receivers—P (precision) code and C/A (coarse availability) code. P code is for government use only and requires specialized equipment. C/A code receivers are becoming widely available with the continuing deployment of GPS satellites. One difficulty with C/A code receivers is that the government from time to time intentionally degrades the satellite signals—so called “selective availability.” With selective availability turned on horizontal accuracy is on the order of 50-100 meters. With selective availability disabled horizontal accuracy can improve to around 15 meters, often better than 5 meters.  
           [0011]    There are several methods presently available for improving the horizontal accuracy of GPS. One method is called “differential” and generally involves sending a correction signal from a base station located at a known coordinate. For example, the U.S. Coast Guard has placed a number of GPS base stations at known locations around the U.S. coast region. These base stations compare their GPS computed positions with the known coordinates of their location to calculate a differential correction. This differential correction is then broadcast to any GPS receiver in range. This correction may be a position correction, but normally the correction is to the timing signal for each individual satellite so that GPS receivers looking at different satellites may calculate their own correction. This is a “wide area” approach. A “local area” approach is also often used for differential correction where a private gps base station is positioned at a known location and broadcasts a private or local correction.  
           [0012]    Another correction approach which has not yet matured but is promising is a so-called “pseudolite” correction. With a pseudolite a GPS transmitter transmits a timing signal much like a GPS satellite. See, The Use of Pseudo-Satellites For Improving GPS Performance, D. Klein, B. Parkinson, Navigation (1984), reprinted Vol. III GPS Navigation p. 135 (1986); Optimal locations of Pseudolites for Differential GPS, B. Parkinson, K. Fitzgibbon, 30 Navigation J. No. 4, winter 1986-87 (incorporated by reference for background). The pseudolite transmits from a known location on or near the standard GPS carrier frequency (e.g. L 1  or L 2 ) to appear to the GPS receiver like another gps satellite. The difference is the pseudolite does not have normal gps errors (or at least minimal), such as ephemeris, ionospheric, multipath, etc., and more importantly, the pseudolite does not have the intentional degradation, selective availability. Additionally, a differential correction signal can be added to the pseudolite signal if desired. A primary benefit of use of pseudolites is that unlike normal differential correction, pseudolites do not require a separate communications channel. That is, the pseudolites appear as another satellite channel to the receiver. Another benefit is that the timing data from the pseudolite channel is known to be much more precise.  
           [0013]    FREIGHT TRACKING SYSTEMS  
           [0014]    Consider a railyard, airport, or sea terminal. A number of railcars or freight containers are constantly on the move into and out of the terminal. The cargo is generally of high value and often transit time is time critical. Indeed, transit time can be very costly when considering a large number of freight containers delayed by even a day extra. The incidence of mis-directed or misplaced freight or cargo can add significantly to the shipping costs. Keeping track of where a particular freight container is located is a daunting task considering the often dynamic nature of a freight terminal and repositioning of the cargo.  
           [0015]    Tracking inventory in an industrial yard is a similar problem. In manufacturing, it is desirable to track the location and availability of finished goods. Most systems use some form of manual label tracking or bar codes to track the inventory. Unfortunately, manual tracking often requires a person to traverse the inventory and scan labels to identify the presence of the inventory.  
         SUMMARY OF THE INVENTION  
         [0016]    The problems with finding freight in a freight yard are largely solved by the system and method of the present invention. The system tracks individual freight containers by intermittently transmitting the position of a freight container to a base station. The base station is able to post process the GPS data to achieve an accurate location of an individual package—within one meter or better accuracy. The base station preferably has an inventory of the contents of a container. Therefore, when a particular container must be located for reshipment or delivery, the base station need only consult its database to find a particular container&#39;s location and contents. This is particularly important when the containers are moving about a freight yard as containers are relocated.  
           [0017]    Broadly speaking, the system includes a number of remote GPS receivers attachable to freight containers in a freight yard. The remote receivers are configured to intermittently transmit their location data to a base station. A communication network connects the remote receivers to the base station. The base station is configured to receive and display the location of a particular remote receiver attached to a freight container upon request. “Intermittent” means noncontinuous operation in the context of present invention. Continuous operation is usually unnecessary and adds battery bulk to a remote receiver where minimal size and weight is important. In one form, a timer sets the time for a remote receiver to transmit. In another form, a motion detector initiates a remote receiver operation.  
           [0018]    In one form, the remote receivers are simply transmitters for receiving the GPS timing signals, amplifying the signals, and retransmitting the GPS timing signals to the base station. The base station then calculates the location of the remote receiver. In another form, the remote receiver includes a GPS engine which calculates an apparent position based on the GPS timing signals. The base station then applies a differential correction to obtain a more accurate position of the remote receivers.  
           [0019]    The method for determining the locations of freight containers in a freight yard in accordance with the present invention includes attaching a number of GPS receivers to freight containers; intermittently operating the receivers to receive GPS signals, and intermittently transmitting data indicative of container location an identification to a base station. The base station receives and records position data of the receivers, and inferentially, the containers the receivers are attached to. Preferably, the base station receives the position data and refines the data by applying a differential correction to obtain an accurate position of the freight containers. Preferably, an inventory of the freight container contents are maintained at the base station so that the location and contents of the container are known.  
           [0020]    In another form, the present invention contemplates a system for determining freight containers locations in a warehouse building. GPS signals are generally not available inside of structures because of their low power. the system uses two or more pseudolites positioned within the building for transmitting GPS type of signals. a plurality of remote receivers are attachable to freight containers within the building for intermittently receiving GPS type signals from the pseudolites. Each remote receiver intermittently transmits its GPS type data to a base station over a communication network. The base station receives the positioning data from the remote receivers and displays the location of the remote receivers in the building. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a rail yard utilizing the present invention;  
         [0022]    [0022]FIG. 2 is a freight staging area utilizing the present invention;  
         [0023]    [0023]FIG. 3 is a block diagram of a remote unit in accordance with the present invention attached to an aircraft;  
         [0024]    [0024]FIG. 4 is a schematic of the packet radio network used to transmit position;  
         [0025]    [0025]FIG. 5 is a block diagram of the base station in accordance with the present invention;  
         [0026]    [0026]FIG. 6 is a block diagram of another embodiment of a remote unit including a radio link in accordance with the present invention; and  
         [0027]    [0027]FIG. 7 is a block diagram depicting a warehouse freight system according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    The present invention is best illustrated by describing several embodiments which are believed preferable depending on the particular freight container location environment. Typically, a remote unit  10  is attached to a freight container and intermittently operates to determine its position and transmit its position to a base station  12 . Some applications need only infrequent position updates or reporting, but must be useful for a long period of time, e.g. months. Other applications need frequent position reporting over a short, several week period.  
         [0029]    FIRST EMBODIMENT  
         [0030]    Turning to the drawings, the system of the present invention includes a remote unit  10 , base station  12 , and calibration system  40 . A remote unit  10  is attached to a freight container in a freight yard and intermittently reports its position to the base station  12 , at least while the container remains in the freight yard.  
         [0031]    As shown in FIG. 3, the remote unit  10  include a packet radio system  20 , a GPS antenna  21  and receiver  22 , a CPU  24 , storage  25 , battery  26  and a control device  28 . The GPS receiver  22  is preferably the multi-channel receiver such as the SV-6 Model or core module II made by Trimble Navigation of Sunnyvale, Calif. Other commercially available substitutes are acceptable such as made by Magellan or Rockwell/Collins. The antenna  21  is either remote or internal to the receiver  22 , but in any event is mounted on the housing of the remote unit  10  for an upward look angle for optimum GPS signal reception. That is, the remote unit  10  is designed for mounting on the top or sides of a freight container for GPS signal reception.  
         [0032]    In more detail, the remote unit  10  includes a CPU  24 , nonvolatile memory storage  25  and control device  28 . Preferably, the control device  28  is simply an activation switch which supplies power to the remote unit  10  to enable operation. In the preferred remote unit, the CPU  24 , memory storage  25  (e.g. RAM), and GPS engine  22  are integral, and preferably low power. Of course integration or segregation of the components of FIG. 3 is a simple matter of design choice. CPU  24  includes an internal clock, as is conventional, which is used to initiate operation. That is, the internal clock is low powered and at a preset time initiates operation of the remote unit  10 .  
         [0033]    In FIG. 3, the packet radio system  20  is conventional, and includes modem  34 , radio interface  36 , and radio  38  (including an antenna, not shown). The radio system  20  is bi-directional in that it may receive signals and also transmit present position and messages back to the base station  12 . A PAC-COM, Inc. (Orlando, Fla.) packet radio modem  300  baud is believed preferable for the modem  34 . The ability to receive signals may be useful in certain applications where it is desired to locate a particular container and an indicator, e.g. light or tone, can be initiated to aid location.  
         [0034]    In practice the radio system  20  of FIG. 3 may be uni-directional for simply transmitting its apparent location of the remote receiver  10  to the base station  12 . An integrated chip set which combines most of the components of the radio system  20  on a single, low cost, low power chip is believed preferable.  
         [0035]    The remote units  10  communicate to the base station  12  over a packet radio network as shown in FIG. 4. The packet radio system is designed to eliminate protocols and acknowledgments to reduce the communications overhead. That is, each remote unit  10  is assigned a time (or event) to operate and transmit its information.  
         [0036]    [0036]FIG. 5 illustrates the base station  12 , which is desirably placed in a routing office or terminal. The base station  12  includes a calibration section  40  which comprises a GPS receiver  42  and antenna  44 . The calibration section  40  continuously determines apparent position of the antenna  44  and feeds this information to CPU  46 . The CPU is conventional, such as a 486 type personal computer operating a 66 MHz. The control device  47  preferably includes a mouse and a standard keyboard. The antenna  44  is probably placed a short distance from the location of the CPU and monitor  46 , 50 , but may be displaced a large distance depending on the physical constraints of the freight yard.  
         [0037]    A database in storage  48  is connected to the CPU  46  and stores information such as freight yard layout, container inventory, and the present position of the remote units, or at least the last reported position. A monitor  50  is coupled to CPU  46  and is useful not only for initialization, but also is selectable to display the present position of all the remote receiver units  10  in the freight yard. The base station  12  includes a packet radio system similar to FIG. 3 coupled to the CPU  46 , and comprises modem  52 , interface  54 , radio  56  and radio antenna  58 .  
         [0038]    The monitor  50  is capable of displaying the freight yard as shown in FIGS. 1 and 2. The remote units (freight containers)  10  are shown on the physical layout of the freight yard. A special symbol may be used (e.g. flashing container) for a container that is to be located. In the present application the term “freight yard” is used to denote any area for marshaling or holding the freight containers. FIG. 1 depicts the freight yard as a rail terminal switching yard while FIG. 2 illustrates a freight yard where shipboard containers are marshaled in a sea port. Other freight yards, such as a trucking trailer marshaling area or an airport container holding area are of course equally applicable.  
         [0039]    Different signal processing techniques may be employed at the base station  12  as desired, such as filtering and compressing. The base station  12  collects each position from the remote units  10  and processes the apparent position to determine a more accurate location of the remote unit. The base station  12  can employ the amount of processing desired to improve the accuracy estimation of the location of the remote unit—commensurate with the time available, the processing load, accuracy desired, etc.  
         [0040]    SECOND EMBODIMENT  
         [0041]    The embodiment illustrated in FIG. 6 is useful to illustrate several alternatives that may be incorporated into the first embodiment illustrated in FIG. 3. The GPS “engine” is eliminated in the remote units  10 . Rather, each remote unit  10  comprises a GPS repeater, such as a Tidget GPS sensor made by Navsys Corp. of Edinburgh, Scotland. The repeater  50  operates to receive the GPS raw data timing signals from the GPS satellites, digitize and compress the timing signals. Preferably, the repeater  50  can be set to look at a certain number of satellites, e.g. 5 satellites. The satellite timing signals are not processed. Instead, the signals are amplified and periodically relayed to the base station  12  via the radio interface  20 .  
         [0042]    In FIG. 6, the remote unit  10  includes a separate timer  52  and motion detector  54  for initiating operation. That is, the timer  52  can be set to initiate operation of the remote unit  10  at preset times. Additionally, or alternatively, the motion detector  54  can initiate operation when motion is detected—i.e. movement of the freight container to which the remote unit  10  is attached. Although a low-cost mercury switch is used as the motion detector  54 , many other types of motion detectors may be used. The battery  56  is sized depending on the load imposed. A rechargeable 6 volt D-cell nickel cadmium rechargeable battery works for most applications.  
         [0043]    The repeater system of FIG. 6 uses GPS time to allocate a transmit window to each remote unit, thus avoiding the handshake protocol communications overhead associated with conventional communications schemes. Each repeater  50  has a unique identification which is transmitted along with position data. Each repeater  50  is allocated, for example, a 5 second transmit time window to transmit its data. Because the base station  12  and all of the repeaters  50  have accurate GPS time data, such a time window allocation is possible. The timer  52  initiates operation of the remote unit, and during operation, the timer is reset to GPS time to ensure accurate time in the timer  52 . A repeater  50  receives timing signals from 4 satellites and stores the signals in a temporary memory buffer (compressing if desired) for transmission in its allocated time window. These raw data timing signals include an identification of the satellite.  
         [0044]    Different signal processing techniques may be employed if desired to obtain an accurate position estimate of these raw data timing signals, such as filtering and compressing. The base station collects each timing signal from the repeaters  50  and processes the timing signals to determine a location of the repeater. The base station  12  can employ the amount of processing desired to the timing signals to improve the accuracy estimation of the repeater—commensurate with the time available, the processing load, accuracy desired, etc.  
         [0045]    The base station  12  receives the timing signals from a certain repeater  50  in the repeater&#39;s allocated timing window. The base station has already coprocessed a timing correction (from calibration section  40 ) for each satellite timing signal, and therefore can apply the correction upon receipt of the repeater timing signal. The repeaters  50  are receiving the timing signals from predominantly the same satellites, so the base station needs to only keep a current correction for the limited number of satellites in view. Using the corrected timing signals, the base station can accurately process the repeater timing signals to derive a location of the repeater in the freight yard.  
         [0046]    This embodiment contemplates the use of time windows to avoid the communication overhead associated with hand shake protocols. With this method, it is believed that repeaters on over one thousand freight containers may transmit their timing signals on a single frequency on a daily basis without interference.  
         [0047]    THIRD EMBODIMENT  
         [0048]    [0048]FIG. 7 illustrates a number of freight containers  70  inside of a warehouse building  72 . Remote units  10  in accordance with the first or second embodiments, FIGS. 3 and 6, are attached to the freight containers  70 . Because the freight containers are inside of a building  72 , reception of GPS signals from the satellite constellation is not normally possible. Therefore, pseudolites  74 , 75  are employed within the building  72  and operate like conventional GPS satellites. Each pseudolite includes an antenna  76  to receive GPS time from the GPS satellites. This eliminates the necessity for an atomic clock in the pseudolites  74  (with a concomitant reduction in cost).  
         [0049]    While two pseudolites  74 , 75  are sufficient to give accurate 3D position to the remote receivers  10 , the system of FIG. 7 preferably uses three pseudolites. The elevation of the warehouse floor is known and freight containers may be positioned on the floor which simplifies position calculations. However, because of the closeness of the pseudolites  74 , 75  to the remote receivers  10 , three pseudolites add precision to the location determination. The pseudolites  74  have few of the errors associated with the GPS satellites, e.g. selective availability, ephemeris, ionospheric, multipath, geometry, etc.  
         [0050]    FOURTH EMBODIMENT  
         [0051]    As an alternative to conventional pseudolites, the pseudolites  74 , 75  in FIG. 7 may be replaced with relays  74 , 75  to rebroadcast GPS timing signals from the GPS constellation. Consider a GPS satellite S 1  and a specific freight container  71  in the warehouse  72 . The total time for the GPS signal to reach container  71  is the time t 1  from the satellite S 1  to relay  74  plus the time t 2  from relay  74  to the container  71  plus the rebroadcast delay d. The rebroadcast delay can be made very accurate by updating a clock in the relay  74  with the accurate GPS time from the satellite. 
           Ttotal =( t   1 + t   2 + d ) 
         [0052]    The method for determining the position of the container  71  from the relay  74  can take several forms. In one form, the total time is used from satellite S 1  to container  71  minus the delay d to determine a distance from the satellite S 1  to container  71 . This ignores the angular relationship between container  71  and relay  74 . With the distance from container  71  to satellite S 1  deduced, the distance between container  71  and another satellite S 2  can be deduced in similar fashion and so on. It is not necessary to determine distances to multiple satellites, but in many case the distance from container  71  to relay  74  can be made more accurate. A similar method using relay  75  can be used to determine the signal transit time between container  71  and relay  75 , and hence the distance. Using conventional GPS algorithms, a suspected position of container  71  can be determined using  2  or more relays to determine the position of container  71 .  
         [0053]    In another form, the time t 2  from relay  74  to container  71  is determined. The delay d is known and time t 1  from the satellite S 1  to relay  74  can be determined. That is, the almanac gives the position of the satellite S 1  and the precise position of relay  74  can be determined ahead of time which means time t 1  can be accurately computed. A measured time at the container  71  is Time total and the delay and t 1  times are subtracted to give t 2 , the signal transit time from relay  74  to container  71 . Knowing time t 2  determines a distance between relay  74  and container  71 . To determine the position of the container  71  in the warehouse  72  another distance is determined. The same procedure can be used to determine a distance between relay  75  and the container  71 . That is, knowing the precise location of relay  75  and the rebroadcast delay d enable determination of the time and distance between the relay  75  and the container  71 .  
         [0054]    It is important that the rebroadcast delays d associated with each relay  74 , 75  be either known or constant. Additionally, it is usually important for the remote receiver  10  on container  71  to be able to identify which relay  74 , 75  is being used to rebroadcast the GPS satellite signals. One method has relays  74 , 75  append an identification message onto the rebroadcast GPS signal. However, it is believed to be preferable to time delimit the rebroadcasts. That is, relay  74  is allocated a time window, e.g. every even second to rebroadcast and relay  75  is allocated another time window to rebroadcast, e.g. every odd second.  
         [0055]    It should be understood that the remote unit  10  on the container  71  preferably does not perform these calculations. That is, the remote unit  10  is configured as in FIGS.  3  or  6  (first or second embodiments) and the data is transmitted to the base station  12  for determination of the position of the freight container  71 . The base station receives the transmitted data with a time stamp and can determine that the data was rebroadcast through a particular relay by comparing the time stamp with the time windows allotted to the relays.  
         [0056]    It should also be understood that this embodiment is illustrated for overcoming the obstruction of a building to receiving GPS signals inside. However, the same technique can be used to eliminate other obstructions to GPS signals. For example, in FIG. 2 the freight containers  90  may be stacked vertically and horizontally spaced close so that GPS signals to the remote receiver units  10  are partially or totally blocked. In this case, one or more relays can be positioned to augment or supplant the normal transmission path of the GPS signals to the remote units  10 . For example, a relay may be positioned at the end of each accessway  92  in FIG. 2.  
         [0057]    OPERATION  
         [0058]    In FIG. 1, a number of railroad cars (i.e. freight containers) are marshalled in a rail yard (i.e. freight yard). As trains are assembled and disassembled, the railroad cars are constantly moved about the rail yard. to assemble the next train, it is important to know where a particular rail car with a particular inventory is located for inclusion in the next train. There is also an optimal movement of rail cars that will minimize the time and effort to assemble the next train. For example, if it is desired to configure the next train with cars  80 - 88 , knowing the contents and locations of the rail cars  80 - 88  can minimize the effort (and cost) in assembling the next train.  
         [0059]    For illustrative purposes, assume the remote receivers  10  of FIG. 3 are attached to the cars  80 - 88 . Every 8 hours, the CPU  24  initiates operation of the remote unit  10 . The GPS  22  powers up and begins determining its apparent location. After a nominal power up cycle, e.g. 3 minutes, the remote unit  10  transmits its apparent location and identification through radio system  20  to base station  12 . The base station  12  of FIGS. 1,4, and  5  receives the apparent position from a remote receiver and applies a calibration or differential correction from calibration section  40  to the apparent position to realize an accurate position of the rail cars within a meter or two. It should be appreciated that the apparent position may be sufficiently accurate for the rail car application of FIG. 1.  
         [0060]    Knowing the location and identification of a remote unit  10  on a rail car  80 - 88 , the base station consults the database in storage  48  to determine rail car contents. When a particular car or contents is desired, the database tells the user the location of the rail car in the rail yard.  
         [0061]    Turning to FIGS. 2 and 6, another illustration of the operation of the present invention is illustrated. In FIG. 2, the freight yard has a number of freight containers  90  positioned along accessways  92 . A number of the freight containers  90  have remote receivers  10  configured as depicted in FIG. 6 attached. The remote unit  10  is attached to a freight container  90  when it enters the freight yard. The remote unit  10  is configured to operate once a week unless it is moved. That is, the timer  52  is programmed to initiate operation of the remote unit  10  once every 7 days. Alternatively, motion detector  54  will initiate operation of the remote unit  10  whenever it senses movement.  
         [0062]    When operation is initiated, the repeater  50  simply begins to amplify an transmit the GPS timing signals it receives. Therefore, the repeater  50  transmits an identification and a number of GPS timing signals. Each timing signal includes a satellite identification. The base station  12  receives the GPS timing signals (FIG. 5) and determines a location for each remote receiver. A differential correction from the calibration section  40  is applied to achieve a more accurate location if desired.  
         [0063]    It should be understood that a variety of combinations of the above embodiments can be easily made. For example, a remote unit  10  may be configured to only operate when it is moved—therefore it includes only the motion detector  54 . Movement initiates operation of the repeater  50  and radio system  20  for a short time period or alternatively, when movement stops.