BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to wayside signaling generally and more particularly to wayside signal acknowledgment systems. 
     2. Description of Related Art 
     A wide variety of wayside signal systems are known to the prior art. Traditional wayside signaling systems comprise one or more colored signal lights that are mounted on poles alongside a train track at various locations such as near the beginning of a block of track or near grade crossings, sidings, switches, etc. The signal lights indicate such things as speed restrictions and the status of the next block of track. On some railroads there are over 125 different colored light signal indications that must be recognized and obeyed. An engineer/operator is required to observe the lights and operate the train accordingly. However, because engineers/operators are human, mistakes which can cause serious accidents are sometimes made. Such mistakes include the failure to observe signal lights and misinterpretation as to the meaning of the signal lights. 
     Several known systems address this problem in one form or another. For example, a system described in U.S. Pat. No. 6,112,142 (the contents of which are hereby incorporated by reference herein), which is owned by the assignee of the present invention, provides a signal comparator system and method in which and engineer and a trainman are each provided with a combined display/input device referred to therein as a pendant. In that system, both the engineer and the trainman must agree, by pressing corresponding buttons on the pendant, as to the meaning of the signal as indicated by the lights. If both the engineer and the trainman agree as to the meaning of the signal, that system will automatically enforce any restrictions corresponding to the signal. If the engineer and the trainman do not agree as to the meaning of the signal, or do not obey any restrictions corresponding to the signal (e.g., the signal indicates stop, but the brakes are not activated), the system will take corrective action to enforce the signal and/or stop the train. However, this system, while providing several advantages over other known systems, has some drawbacks. First, it requires the presence of both an engineer and a trainman. Second, it is susceptible to error or intentional defeat by an engineer and a trainman who enter (accidentally or purposely) the wrong signal information. 
     In another known system, referred to as Cab Signal, a display is provided in the cab for the engineer/operator and wayside signals are transmitted to the system and shown on the display. The Cab Signal system forces the engineer/operator to acknowledge signals that are more restrictive than the current signal and, in some systems, forces the engineer/operator to obey the more restrictive signal. However, this system does not force an engineer/operator to acknowledge less restrictive signals. This is disadvantageous because if an engineer/operator misses a less restrictive signal, the engineer/operator may miss an opportunity to operate the train more efficiently by increasing the speed of the train. 
     What is needed is a system and method that overcomes these and other deficiencies in known systems. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention meets the aforementioned need to a great extent by providing a positive signal comparator system comprising a transceiver located on a train for transmitting an interrogation signal to a wayside signal device and receiving a response signal from the wayside signal device, an input device through which the engineer/conductor enters a signal in response to the signal received from the wayside signal device, and a controller including a signal comparator for determining if the signal input by the engineer/operator matches the signal received from the wayside signal device and taking corrective action if the engineer/operator fails to enter the proper signal. In some embodiments, the corrective action comprises activating a warning device and/or activating the train&#39;s brakes. 
     In some embodiments, the invention further comprises a display for displaying a signal received from the wayside signal generator to the engineer/operator. In such embodiments, the wayside signal device may or may not include signal lights or other visual indication of the signal. In other embodiments, no display of the signal is provided and the engineer/operator must rely on a visual indication of the signal from the wayside signal device. 
     In some embodiments, the system includes a positioning system such as a global positioning system that is used to determine the location of the train and a database in which is stored the location of all wayside signals in the system. When the controller determines that the train is near a wayside signal device, the controller automatically activates the transceiver to interrogate the device. In other embodiments, the wayside signal device automatically transmits a wayside signal when the wayside signal device detects that the train is approaching (e.g., with a track occupancy circuit), or continuously transmits a wayside signal on a periodic basis regardless of whether a train is present. 
     In some embodiments, after receiving a signal from a wayside signal device the controller dynamically determines the amount of time necessary to stop the train based on the train&#39;s speed, weight, and other factors and sets a timeout period by which the engineer/operator must enter a matching signal. In other embodiments, the timeout period is predetermined based on a worst-case assumption (e.g., fastest possible speed, greatest weight, steepest downhill grade of track, etc.) of the time required to stop the train. If the engineer/operator fails to enter a matching signal within the timeout period, corrective action is taken. 
     In some embodiments of the invention, a single pendant is provided and the controller requires only a single matching signal to be entered by an engineer/operator. In other embodiments of the invention, a second pendant is provided and the controller requires a second person such as a trainman to match the signal provided by the wayside signaling device. 
     In one aspect of the invention, the engineer/operator is required to match the signal transmitted by the wayside signaling device. This is an improvement over systems in which the engineer/operator is only required to acknowledge the signal (e.g., by pressing a general purpose acknowledgment button regardless of the meaning of the signal) because it ensures that the engineer/operator is alert and is not simply reflexively acknowledging the signal. 
     In another aspect of the invention, all signals, whether or not they are more restrictive than the previous signal, must be matched by the engineer/operator. Having the engineer/operator acknowledge less restrictive signals provides additional indication that the engineer/operator is alert. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant features and advantages thereof will be readily obtained as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of one embodiment of the invention. 
         FIG. 2  is a front view of a pendant of the embodiment of FIG.  1 . 
         FIG. 3  is a flow chart illustrating operation of the system of FIG.  1 . 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will be discussed with reference to preferred embodiments of signal comparator systems. Specific details, such as types of signals, are set forth in order to provide a thorough understanding of the present invention. The preferred embodiments discussed herein should not be understood to limit the invention. Furthermore, for ease of understanding, certain method steps are delineated as separate steps; however, these steps should not be construed as necessarily distinct nor order dependent in their performance. 
     A positive signal comparator system  100  is illustrated in FIG.  1 . The system  100  includes a controller  110 . The controller  110  may be a conventional microprocessor or may be implemented using discrete components. The controller  110  is responsible for implementing the logical operations discussed in detail below. 
     An engineer pendant  120  is connected to the controller  110 . The engineer pendant  120  is illustrated in further detail in FIG.  2 . The pendant  120  includes a series of 12 buttons  231 - 242  labeled as 1 CLR (clear), 2 LTD (limited), 3 APP (approach), 4 MED (medium), 5 DIV (diverging), 6 SLOW, 7 ADV (advance), 8 RES (restricted), 9 STOP/PROC (1 push=stop, 2 pushes=proceed), 10 COND O&#39;RIDE (conditional override), 11 ACK/ENTER (acknowledge/enter—depends upon context); and 12 CANCEL, respectively. Buttons  231 - 240  correspond to various signals defined in the GCOR (General Code of Operational Rules) and various other signaling systems used in the United States. The ACK/ENTER and CANCEL buttons  241  and  242  are used to acknowledge warnings, enter information, and cancel a previous entry, respectively. 
     The buttons  231 - 242  are used by the engineer/operator (and, in embodiments with two pendants, the trainman) to acknowledge a signal from a wayside signaling device. For example, if a “medium approach medium” signal were received from the wayside signal device (which means that the train is allowed to travel at medium speed through turnouts, crossovers sidings and over power operated switches, then proceed, approaching the next signal at a speed not exceeding the medium speed), the engineer/operator would depress the MED button  234 , the APP button  233 , and the MED button  234  in that order to verify that the signal has been correctly received and understood by the engineer/operator. 
     The pendant  120  also includes a window  210 , which is preferably a graphics-capable display (a liquid crystal display is illustrated in  FIG. 2 , but any graphics display could be used). The window includes a current speed field  211 , a maximum speed field  212 , a acceleration field  213  (which indicates the current acceleration in m.p.h. per minute), a distance to next signal field  214 , a milepost field  215 , an elevation profile window  218 , and a track curvature window  220 . The distance to next signal field  214  reads “signal  6438 ” in  FIG. 2 , which signifies that the next signal is 6,438 feet away. In some embodiments, when a signal from the next signal device is received, the word “signal” will be replaced by the meaning of signal (e.g., “clear”) received from that signal device. The window  210  also displays, in fields  222  and  224 , current and upcoming speed restrictions over limited areas of the track. In the example of  FIG. 2 , the speed restrictions are “Form A” speed restrictions, valid from mileposts 198 to 196.2 and 182.2 to 180, with maximum speeds of 35 and 30 miles per hour and no time limits, respectively. The buttons surrounding the window  210  are “soft keys” that have different, programmable functions, which are beyond the scope of the present invention, depending on the content of the display  210  in a manner well known in the art. 
     In embodiments of the invention in which the signal from the wayside signal device  190  is displayed to the engineer/operator, the signal may be displayed in a “pop-up” window in the window  210 . In other embodiments, the signal may only be displayed next to the distance to signal field  214  as discussed above. In other embodiments, no visual indication of the signal device  190  is provided on the pendants  120 ,  130 . Rather, in such embodiments, the engineer/operator relies on a visual indication on the signal device  190  such as colored lights. 
     Referring now back to  FIG. 1 , a transceiver  140  is also connected to the controller  110 . The transceiver  140  is capable of communicating with wayside signal devices  190 . The communication is wireless in preferred embodiments, although those of skill in the art will understand that other forms of communication, such as power line communication, are also possible. 
     Also connected to the controller  110  is a positioning system  150 . The positioning system  150  is a GPS receiver in preferred embodiments. The GPS receiver can be of any type, including a differential GPS, or DGPS, receiver. Other types of positioning systems  150 , such as inertial navigation systems (INSs) and Loran systems, can also be used. Such positioning systems are well known in the art and will not be discussed in further detail herein. [As used herein, the term “positioning system” refers to the portion of a positioning system that is commonly located on a mobile vehicle, which may or may not comprise the entire system. Thus, for example, in connection with a global positioning system, the term “positioning system” as used herein refers to a GPS receiver and does not include the satellites that are used to transmit information to the GPS receiver.] 
     The positioning system  150  continuously supplies the controller  110  with position information for the train to which the system  100  is attached. This position information allows the controller  110  to determine where the train is at any time. The positioning system  150  is preferably sufficiently accurate to unambiguously determine which of two adjacent tracks a train is on. By using train position information obtained from the positioning system  150  as an index into a map database  160  (discussed in further detail below), the controller  110  can determine the train&#39;s position relative to wayside signal devices  190  in the system. As discussed in further detail below, this allows the controller  110  to send an interrogation signal to the wayside signal device  190  at the appropriate time. 
     A map database  160  is also connected to the controller  110 . The map database  160  preferably comprises a non-volatile memory such as a hard disk, flash memory, CD-ROM or other storage device, on which map data and the locations of wayside signal devices is stored. Other types of memory, including volatile memory, may also be used. The map data preferably also includes positions of switches, grade crossings, stations and anything else of which a conductor or engineer is required to or should be cognizant. The map data preferably also includes information concerning the direction and grade of the track. 
     In addition to the positioning system  150 , a tachometer  170  is also connected to the controller  110 . The tachometer  170  measures the axle rotation, from which the speed of the train can be derived if the wheel size is known. In the event that the positioning system  150  becomes unavailable, the system  100  can operate by estimating distance traveled from the rotation of the axle or motor. However, wheel slippage and changes in wheel size over time can effect the accuracy of such a system. The system  100  may be configured to compensate for wheel wear in the manner described in co-pending U.S. patent application Ser. No. 10/157,874, filed May 31, 2002, entitled “Method and System for Compensating for Wheel Wear on a Train,” the contents of which are hereby incorporated by reference herein. 
     Finally, a brake interface  180  connected to the controller  110  allows the controller  110  to activate and control the train brakes when necessary to slow and/or stop the train. Brake interfaces are well known in the art and will not be discussed in further detail herein. 
     A flowchart  300  illustrating operation of the system  100  is shown in FIG.  3 . The process starts with the controller  110  querying the positioning system  150  (or, in some embodiments the tachometer  170  if position information from the positioning system  150  is not available) to determine the position of the train at step  302 . The controller  110  then consults the database  160  to determine the nearest signaling device  190  based on the train&#39;s position at step  304 . Next, the controller  110  determines whether the signaling device  190  is within the range of the transceiver  140  at step  306 . If the nearest device is not within range, steps  302  and  304  are repeated until the next signaling device  190  is within range. When the next device  190  is within range, the controller  110  causes the transceiver  140  to transmit an interrogation message at step  308 . 
     The controller then determines at step  310  a timeout within which a signal must be received from the device  190  and a matching signal must be received from the engineer&#39;s pendant  120 , and, in some embodiments, from the trainman&#39;s pendant  130 . The timeout is chosen such that, at the expiration of the timeout, there will be sufficient distance and time in which to stop the train in the event of a problem (e.g., the device does not respond or the signal entered by engineer and/or trainman does not match the signal received from the device). The timeout is dynamically determined in some embodiments using factors such as the speed and weight of the train, the distance between the train and the upcoming signaling device  190 , the grade of the upcoming section of track, the distribution of weight on the train, and/or the characteristics of the braking system on the train using equations which are well known in the art. In other embodiments, the timeout is a fixed period based upon a worst-case assumption about the distance required 
     If the wayside signaling device  190  responds at step  312 , the received signal is displayed in some embodiments on the engineer&#39;s pendant  120 , and in yet other embodiments on the trainman&#39;s pendant  130  at step  314 . The controller  110  then prompts the engineer (and, in some embodiments, the trainman) to enter a matching signal at step  316 . If the signal entered by the engineer (and, in some embodiments, the trainman) do not match the signal reported by the wayside device  190  via the transceiver  140  at step  318 , and the timeout has not yet been reached at step  320 , steps  316  and  318  will be repeated to provide the operator (and, in some embodiments, the trainman) with an additional opportunity to enter a correct matching signal. If a correct matching signal is received from the engineer&#39;s pendant  120  (and, in some embodiments, the trainman&#39;s pendant  130 ) at step  318 , the controller then monitors the train&#39;s compliance with the signal at step  320 . If the train is in compliance at step  322 , but is not yet past the end of the block corresponding to the signaling device  190  at step  324 , step  322  is repeated until the train is past the end of the block at step  324 , at which point steps  310  et seq. are repeated. 
     If the train is not in compliance at step  322 , the controller activates a warning device at step  330 . The warning device may be a pendant  120  ( 130 ) in preferred embodiments, but also may be a horn, whistle, or other device (not shown in  FIG. 1 ) rather than addition to the pendant  120  in other embodiments. If the train is brought into compliance at step  332 , steps  324  et seq. are repeated. If the train is not brought into compliance at step  332 , corrective action is taken at step  334 . The types of corrective action can include applying a penalty braking algorithm, which will stop the train; notifying a dispatcher (in embodiments that provide for communication between the system  100  and a dispatcher); and slowing the train down to a predetermined safe speed to allow the engineer to visually verify that it is safe to proceed. 
     If the device  190  fails to respond within the timeout period at step  312 , the controller activates a warning device at step  340 . The controller determines whether the train is stopped (or, in other embodiments, has slowed down to a safe speed) at step  342 . If the train has not been stopped (or slowed down) at step  342 , the brakes are activated at step  344 . The process then ends. At this point, some embodiments of the system require authorization from a dispatcher in order to start the train moving again. Other embodiments require the engineer/operator to perform a start up procedure. Yet other embodiments simply require a full stop before further movement is allowed. 
     In some embodiments, the system will become “active” anytime (1) any switch button is used or (2) anytime the speed of the locomotive is greater than 15 mph. These features make the system unobtrusive during railyard switching operations. Also, when speed increases above 15 mph the system will require an initial acknowledgment between the engineer and trainman. This feature provides for positive indication that the system is operational and functioning properly. After this initial acknowledgment the system will require engineer/trainman acknowledgments at set intervals mandatorily such as one (1) hour between pendant activity as long as the train speed is above 15 mph and no signal button has been depressed in the last hour. In the event that speed is reduced to a “stop” and then increased to greater than 15 mph without any intervening button operation, the system will “force” an acknowledgment to further check the system and the crew&#39;s actions. 
     As discussed above, compliance with the signal from the wayside signaling device  190  is monitored at step  322 . An example of non-compliance is if the speed of the train exceeds the “target” speed for a given signal by a prescribed speed over the target speed and the train is not decelerating, at a target deceleration amount (e.g., 1 mph/min). In some embodiments, if an initial determination of non-compliance is made, a response timer will be set and automatic braking will occur upon timeout of the response timer unless (1) the speed of the train is reduced to less than 5 mph above the “target speed”; (2) the train is decelerating at an acceptable rate; or (3) the speed of the train is brought below the “target speed”. 
     In addition to ensuring compliance with wayside signaling devices  190 , the system  100  may also insure compliance with “slow order” or speed restriction information for the territory to be traversed by the train. In such embodiments, “slow order”/speed restriction information is stored in the database  160  and is treated in a manner similar to signals from wayside devices  190  (e.g., when the train approaches the start of a section of track covered by a slow order or speed restriction, the slow order/speed restriction information is displayed to an engineer (and, optionally, a trainman) on the pendant  120  in a “pop up” window, and the controller  110  takes corrective action if a matching signal is not entered by the engineer/trainman and/or if the slow order/speed restriction is not complied with.) 
     Several methods for updating the “slow order”/speed restriction information are available including: 
     A. Operator Update: 
     The train crew must “sign up” before boarding the train. The operator can be given a credit card sized memory device or some similar device having the latest track information at the “sign up” location. After receiving this data, a crewman can board the train and read this latest data into the database  160 . 
     B. Radio Update: 
     At prescribed railroad locations, a low power transmitter can be employed to automatically update the database  160  (which may or may not be accomplished using transceiver  140 ). Additionally, an existing RF infrastructure of the rail system could be employed to update all locomotives with new data. 
     C. Computer Update: 
     During mechanical inspections, a laptop or other memory device could be used to update the database  160 . In such embodiments, the pendant  120 ,  130  preferably displays the date the system was last updated the crew can verify that they have the latest data. 
     In preferred embodiments, each wayside signal device  190  has a unique telemetry identifier. Therefore only the particular signal in advance of the train is interrogated. This information is telemetered to the system  100  and displayed to the crew, which may be only one member. As the train travels closer to the signal, updates of the signal indication are sent to the train to ensure the signal does not change during this period. When the train is within 1500 feet (for example), the crew is forced to acknowledge exactly the signal indication. Should the crew member(s) not correctly acknowledge the signal, the system will automatically stop the train. Additionally, as with the prior system, the speed limit as defined by the signal indication is automatically enforced upon the train. 
     In some embodiments, no interrogation signal is sent by the train to the wayside signal device. In some of these embodiments, the wayside signal device may employ a track occupancy circuit or some other means (e.g., radar) to detect the presence of an approaching train and automatically transmit a message including the wayside signal to such an approaching train. In yet other embodiments, which are particularly useful when the wayside signal device is located near a source of power, the wayside signal device periodically broadcasts a wayside signal without regard to whether or not a train is approaching. In still other embodiments, the train&#39;s position and (optionally) speed information are transmitted to a central dispatch authority and the central dispatch authority instructs the wayside signal device to transmit a signal to the train as it approaches the device. The wayside signal device in these alternative embodiments may or may not include an identification of the device in the message along with the wayside signal. Those of skill in the art will recognize that a system may include a combination of some or all of these types of wayside signal devices (e.g., those that transmit a wayside signal upon receiving an interrogation message, those that transmit a wayside signal in response to detecting an approaching train, those that automatically transmit a wayside without regard to whether or not a train is approaching, and those that transmit a wayside signal under the control of a central dispatch authority). 
     While the invention has been described with respect to certain specific embodiments, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. It is intended therefore, by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.
A positive signal comparator system includes a transceiver located on a train for transmitting an interrogation signal to a wayside signal device and receiving a response signal from the wayside signal device, an input device through which an operator enters a signal in response to the signal received from the wayside signal device, and a controller including a signal comparator for determining if the signal input by the operator matches the signal received from the wayside signal device and taking corrective action if the operator fails to enter the proper signal. In some embodiments, the corrective action comprises activating a warning device and/or activating the train's brakes. In some embodiments, the invention further comprises a display for displaying the signal received from the wayside signal generator to the operator.