Abstract:
A system for controlling a locomotive, including a plurality of portable communications units, each unit adapted to generate signals conveying commands indicative of functions to be performed by the locomotive. Each unit is also associated with an operational status; the signals generated by each unit have a characteristic dependent on the operational status associated with that unit. Also, the system includes a controller adapted to receive the signals generated by the plurality of portable communications units, to determine the commands conveyed by the received signals and to control the locomotive on the basis of the decoded commands. By making each portable transmitter unit aware of its operational status, battery power can be conserved and airwave congestion reduced by precluding the transmission of commands that are guaranteed to be rejected because of the lack of command authority of an originating unit. Meanwhile, the unit which holds command authority continues to be able to send a complete set of commands.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. provisional application serial No. 60/434,672 filed Dec. 20, 2002. The contents of the above document are incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to remote control systems for locomotives and, more particularly, to remote control systems in which a designated one of several portable transmitter units has the authority to issue locomotive commands at a given time.  
         BACKGROUND  
         [0003]    Economic constraints have led the railway industry to develop portable transmitter units allowing a ground-based operator to remotely control a locomotive in a switching yard. The unit is essentially a battery-powered transmitter communicating by way of a radio link with a slave controller that is either on-board the locomotive or in communication therewith. Typically, the operator carries this unit along with him or her and can perform duties such as coupling and uncoupling cars while remaining in control of the locomotive movement at all times. This allows for placing the point of control at the point of movement thereby potentially enhancing safety, accuracy and efficiency.  
           [0004]    In some instances, a single operator may effectively and safely control a consist that includes a limited number of cars remaining at all times well within the visual range of the operator. However, when the consist is long, two operators may be required, each person being physically close to and monitoring one end of the train. To this end, remote control systems have been designed whereby a locomotive controller is capable of receiving inputs from a designated one of two or more hand-held transmitters. An example of such a remote control system is described in U.S. Pat. No. 5,685,507 to Horst et al., the contents of which are incorporated by reference herein.  
           [0005]    In the two-operator arrangement described in U.S. Pat. No. 5,685,507, each operator is provided with a portable transmitter unit. Circuitry within each unit obtains the current setting of various switches and levers on the unit, resulting in the generation of a digital command word sent periodically via radio frequency (RF) to the locomotive controller. In order to avoid confusion, however, the locomotive controller will accept, at any given point in time, commands from only one of the portable transmitter units, namely the one unit said to hold “command authority”. Commands received from any other portable transmitter unit are rejected by the locomotive controller. The exception is a limited set of emergency and signaling commands that are available to a portable transmitter unit irrespective of whether it holds command authority. Also disclosed in U.S. Pat. No. 5,685,507 is a handoff procedure for transferring the designation of command authority holder from one portable transmitter unit to another.  
           [0006]    The above arrangement therefore solves the problem of allowing only one designated operator to execute, at any given time, the vast majority of commands in relation to the control of a locomotive. However, those skilled in the art will appreciate that this scheme is inefficient from a power and bandwidth standpoint. In particular, the fact that commands received from a portable transmitter unit that does not hold command authority are rejected signifies that such commands have been unnecessarily transmitted to the locomotive controller. That is to say, since the vast majority of its commands will ultimately be rejected, a portable transmitter unit that lacks command authority causes the needless expenditure of battery power and consumption of radio frequency bandwidth.  
           [0007]    Thus, there is a need in the industry to provide a remote control system for a locomotive which alleviates at least in part the deficiencies associated with existing systems.  
         SUMMARY  
         [0008]    The present invention recognizes that battery power can be conserved and airwave congestion reduced by precluding the transmission of commands that will be rejected because of the lack of command authority of an originating portable communications unit. Meanwhile, it is recognized that a portable communications unit which holds command authority must continue to be able to send a complete set of commands. In order to allow this dual transmission capability to take place, each portable communications unit is made aware of its operational status, which is either that of a command authority holder or that of a command authority non-holder. If the operational status is that of a command authority holder, then a full set of commands will be transmitted to the locomotive controller, while if the operational status is that of a command authority non-holder, then only a limited set of commands will be transmitted. This results in a reduction in the bandwidth used by a portable communications unit whose operational status is that of a command authority non-holder, which in turn allows the conservation of battery power.  
           [0009]    According a broad aspect, therefore, the invention provides a system for controlling a locomotive. The system includes a plurality of portable communications units, each unit adapted to generate signals conveying commands indicative of functions to be performed by the locomotive. Each unit is also associated with an operational status; the signals generated by each unit have a characteristic dependent on the operational status associated with that unit. Also, the system includes a controller adapted to receive the signals generated by the plurality of portable communications units, to decode the commands conveyed by the received signals and to control the locomotive on the basis of the decoded commands.  
           [0010]    According to a second broad aspect, the present invention seeks to provide a portable communications unit for use in a remote control system having a controller for controlling a locomotive. The portable communications unit includes a user interface for allowing a user to specify functions to be performed by the locomotive and a command generator adapted to generate commands indicative of the functions specified by the user via the user interface, the command generator being further adapted to determine an operational status assigned to the portable communications unit. The portable communications unit further includes a transmitter connected to the command generator, the transmitter is adapted to send signals conveying the commands generated by the command generator to the controller, wherein the signals generated by the transmitter have a characteristic dependent on the operational status associated with that unit.  
           [0011]    In accordance with a third broad aspect, the present invention provides a controller for controlling a locomotive. The controller includes a transceiver for receiving commands from a plurality of remote communications units and for sending messages to the remote communications units. The controller further includes a processing unit connected to the transceiver, the processing unit being operative for assigning to each remote communications unit an operational status selected from the group consisting of a command authority holder operational status and a command authority non-holder operational status. The processing unit is further adapted to control the locomotive on the basis of the commands received from the remote communications units and on the basis of the operational status of each remote communications unit from which commands are received, the processing unit is further adapted to allow transfer of command authority holder operational status between the remote communications units. Upon a transfer of command authority from a first remote communications unit to a second remote communications unit, the processing unit sends a message to the first and second remote communications units indicative of the change in the operational status of the respective remote communications unit.  
           [0012]    The present invention may be summarized according to a fourth broad aspect as a method implemented by a portable communications unit used for instructing a locomotive controller to control a locomotive. The method includes determining an operational status associated with the portable communications unit; assembling commands indicative of functions to be performed by the locomotive; and generating signals conveying the commands, the signals having a characteristic dependent on the operational status associated with the portable communications unit.  
           [0013]    These and other aspects and features of the present invention will now become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    In the accompanying drawings:  
         [0015]    [0015]FIG. 1 shows in schematic form a remote control system composed of a controller and a plurality of portable communications units;  
         [0016]    [0016]FIG. 2 is a top plan view of one of the portable communications units of FIG. 1;  
         [0017]    [0017]FIG. 3 is a block diagram of the portable communications units of FIG. 1, including a command generator;  
         [0018]    [0018]FIG. 4 is a flowchart illustrating steps in the operation the command generator of FIG. 3;  
         [0019]    [0019]FIG. 5 is a block diagram of the controller of FIG. 1, including a processing unit;  
         [0020]    [0020]FIGS. 6A and 6B show a flowchart describing steps in the operation the processing unit of FIG. 5; and  
         [0021]    [0021]FIG. 7 illustrates a locomotive control word generated by the command generator of FIG. 3.  
     
    
       [0022]    In the drawings, embodiments of the invention are illustrated by way of example. It is to be expressly understood that the description and drawings are only for purposes of illustration and as an aid to understanding, and are not intended to be a definition of the limits of the invention.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    With reference to FIG. 1, there is shown a remote control system in accordance with an embodiment of the invention, including a plurality of portable communications units  10 , each of which generates a digitally encoded radio frequency (RF) signal to convey operator commands to a controller for controlling a set of locomotives  13 ,  15 . Separate controllers may be mounted on-board each of the locomotives  13 ,  15  or, as illustrated, a single controller  46  may be located in a central station which communicates with the various locomotives  13 ,  15 . The controller  46  detects the various RF signals transmitted by the plurality of portable communications units  10 , decodes their contents and causes operation of various actuators on the locomotives  13 ,  15  to carry into effect the commands remotely issued by the operators (not shown).  
         [0024]    It should be understood that not all operators are allowed to control all trains. In general, only a limited number of operators are allowed to control a given train, typically one operator at the front of the train and another at the back. To this end, it is useful to assign to each of the operators an identifier, conveniently denoted X, Y, Z and W in FIG. 1. In a hypothetical scenario, it may be that the operators with identifiers X and Y are allowed to control locomotive  13  and the operators with identifiers Z and W are allowed to control locomotive  15 . As will be described in further detail herein below, the identifiers of those operators allowed to control each train are stored in a memory within the controller  46 .  
         [0025]    [0025]FIG. 2 illustrates the physical layout of a portable communications unit  10  in accordance with an embodiment of the present invention. The unit  10  comprises a housing  12  enclosing electronic circuitry and a battery supplying electric power to operate the circuitry as well as other components on and within the housing  12 . The main user interface is made up of a plurality of manually operable levers and switches  14 - 28 , which project outside the housing to allow adjustment of parameters such as locomotive speed, rate of acceleration, sounding of horn, etc.  
         [0026]    By way of example, various controls identified by reference numerals on the portable communications unit in FIG. 2 are listed in the following table:  
                                       REF.       TYPE OF       NUMERAL   CONTROL   ACTUATOR                   14   Locomotive Speed Control   Multi-Position dial       16   Locomotive Override   Multi-Position dial           Brake Control       18   Reset   Push-Button       26   Direction   Multi-Position Switch           (Forward/Reverse/Neutral)       22   Ring Bell/Horn   Toggle Switch       24   Power on/Lights Dim/Bright   Toggle Switch       20   Train Brake Control   Multi-Position Switch       28   Status Request   Push-Button                  
 
         [0027]    The various manually operable switches, levers and buttons briefly described above are constituted by electric contacts whose state of conduction is altered when the control settings are changed. For instance, the push-buttons  18  and  28  and the toggle switches  22  and  24  have electric contacts that can assume either a closed condition or an opened condition. The multi-position dials  14  and  16 , and the multi-position switches  20  and  26 , have a set of electric contact pairs, only a single pair being closed at each position of the lever or switch. By reading the conduction state of the individual electric contact pairs, the commands issued by the operator can be determined.  
         [0028]    On the front surface of the housing  12  is provided a display panel  34  that visually echoes the control settings of the portable transmitter  10 , as determined from the conduction state of the individual electric contact pairs. The display panel  34  includes an array of individual light sources  36 , such as light emitting diodes (LEDs), corresponding to the various operative conditions of the locomotive that can be selected by the operator. Hence, a simple visual observation of the active LEDs  36  allows the operator to determine the current position of the controls  14 - 28 .  
         [0029]    Among those portable communications units allowed to exert control over each of the locomotives  13 ,  15 , only one unit is authorized, at any given time, to issue the vast majority of commands. This is required in order to prevent the controller  46  from receiving parallel sets of contradictory commands in relation to the same train from different portable communications units. Thus, it can be said that, generally, at most one of the portable communications units  10  has a “command authority holder” operational status at any given time, while the other portable communications units have a “command authority non-holder” operational status. In accordance with the present invention, each portable communications unit will have knowledge of its operational status.  
         [0030]    Due to this arrangement, certain commands will only be accepted by the controller  46  (shown in FIG. 1) if they originate from the portable communications unit that has a “command authority holder” operational status. Thus, for example, a portable communications unit  10  that has a “command authority non-holder” operational status is unable to dim the lights of the locomotive (see reference numeral  24 ). Naturally, the “relinquish control to companion” command is only available to the one portable communications unit  10  which has a “command authority holder” operational status.  
         [0031]    A select subset of commands are available to all portable communications units, regardless of their operational status. For example, of the five types of brake control commands (release, low, medium, full, and emergency) the emergency brake command is available to all the portable communications units  10 , including those that have a “command authority non-holder” operational status. The remaining four brake commands (release, low, medium and full) are only available to the portable communications unit  10  that has a “command authority holder” operational status. Also included in the subset of commands that are available to all the portable communications units is the “ring bell/horn” command, which is self-explanatory. Certain other commands relate to controls specifically directed to those portable communications units that have command authority non-holder operational status. Such commands include the “reset” command, which is indicative of an operator&#39;s desire to gain a “command authority holder” operational status for his or her portable communications unit.  
         [0032]    In accordance with an embodiment of the present invention, the portable communications unit  10  can be equipped with an indicator  37  of the current operational status assigned to the unit  10 . As shown in the illustrated embodiment, the indicator  37  may take the form of an LED which conveys the operational status of the portable communications unit  10  in a visual manner. The binary distinction between “command authority holder” operational status and “command authority non-holder operational status” may be visually conveyed by way of the presence or absence of light, a light intensity, a rate of blinking, color of light, for example.  
         [0033]    In an alternate implementation of the portable communications unit  10 , the sensory indicator  37  indicates the current operational status of the unit  10  by means of an audible sound or a physical movement (e.g., vibration). In still another configuration of the portable communications unit  10 , the sensory indicator  37  may be adapted to emit a signal only when there has been a change in the operational status of the unit  10 . Myriad other ways of providing a sensory indication of the operational status (or of a change in the operational status) of the portable communications unit  10  will become apparent to those of ordinary skill in the art in light of the present specification and need not be described in further detail herein.  
         [0034]    [0034]FIG. 3 provides a functional diagram of the electronic circuitry within the portable communications unit  10 . Specifically, there is provided a command signal generator  38  and a transmitter  40 , and optionally, a receiver  41 .  
         [0035]    The command signal generator  38  scans at short intervals the state of conduction of each pair of contacts. A microprocessor within the command signal generator  38  assembles the results of the scan into a binary sequence, hereinafter referred to as a “locomotive control word”. The transmitter  40  receives the locomotive control word from the command signal generator  38  and generates an RF signal for transmission of the coded sequence by a suitable modulation scheme, such as frequency shift keying (FSK). Other suitable modulation schemes known in the art may be used without detracting from the spirit of the invention.  
         [0036]    In an embodiment of the present invention, the transmitter  40  sends out the modulated locomotive control word in repetition at a fixed rate (e.g., selected in the range from two (2) to five (5) times per second). By providing the transmitter  40  on each portable communications unit  10  with a unique repetition rate, the likelihood of transmission errors is reduced when several units in close proximity broadcast control locomotive control words pertaining to different locomotives. It will be known how to set each transmitter (and the companion receiver) at a unique transmission/reception period so as to maintain secure communication links even when all the transmitters use the same carrier frequency. Of course, other schemes of conveying the locomotive control word in a robust fashion will be apparent to those of ordinary skill in the art. For example, a scheme using variable time intervals between transmission is described in U.S. Pat. No. 6,456,674 issued to Canac Inc, the contents of which are incorporated herein by reference.  
         [0037]    In an alternative implementation, it is within the scope of the present invention for the portable communications units  10 , and the controller  46  to transmit signals using a TDMA type, or spread spectrum type protocol. In such embodiments, each of the communication entities, meaning each portable communications unit  10  (and the controller  46  in the cases where the controller  46  is able to transmit signals to either the portable communications units  10  or to the locomotives  13 ,  15 ) is assigned a time interval within a TDMA frame during which it is able to transmit signals. For example, a portable communications unit  10  is able to transmit a locomotive control word during its respective time interval in the TDMA frame. Such a protocol avoids collisions between transmissions by assigning only one entity per time interval.  
         [0038]    As indicated above, the command signal generator  38  generates a locomotive control word. The locomotive control word may be implemented in a plurality of different ways. In a non-limiting example, as shown in FIG. 7, a locomotive control word  700  includes a function component  710 A,  710 B, an identifier component  720  and an integrity component  730 .  
         [0039]    The function component  710 A,  710 B is indicative of the desired operative state of the locomotive being controlled by the operator of the portable communications unit  10 . The function component  710 A,  710 B is determined on the basis of the state of conduction of each pair of contacts corresponding to the switches, levers and buttons  14 - 32  of the unit  10 .  
         [0040]    In a specific implementation, the function component  710 A,  710 B has a length that may be dependent on the operational status (command authority holder or command authority non-holder) of the portable communications unit  10 . For example, commands relative to speed and braking (except for emergency braking controls) that are input by an operator of a portable communications unit that has “command authority non-holder” status need not be transmitted, as such commands would in any event be rejected by the controller  46 . Hence, a majority of the information gained from the positions of the levers, switches and buttons  14 - 32  can be omitted from the function component when the portable communications unit  10  has “command authority non-holder” operational status.  
         [0041]    Accordingly, the function component  710 A is illustrative of the case where the portable communications unit  10  has “command authority holder” status, while the function component  710 B is illustrative of the case where the portable communications unit  10  has “command authority non-holder” status. As depicted, emergency commands (brake and horn settings) as well as the “reset” control (i.e., a request to assume “command authority holder” operational status) appear as the “initial” bits of the function component  710 B.  
         [0042]    The identifier component  720  uniquely represents the portable communications unit  10  with respect to other portable communications units. The identifier component  720  may be hard-wired in the circuitry of the portable communications unit  10  (e.g., it may be the unit&#39;s serial number) or it may be received from the controller  46  in the case where the portable communications unit  10  includes a receiver  41 . The identifier component  720  will be compared by the controller  46  to a list of identifiers that are allowed to control a particular locomotive. Optionally, the identifier component  720  may also include a component uniquely representing the locomotive which is being controlled.  
         [0043]    The integrity component  730 , which is optional, is used to assist the controller  46  in determining whether it has correctly received the function component  710 A,  710 B and the identifier component  720  of the locomotive control word  700 . In one embodiment, the integrity component  730  may take the form of a cyclic redundancy check (CRC).  
         [0044]    Referring back to FIG. 3, in order to assemble the various components of the locomotive control word  700 , an embodiment of the present invention provides the command signal generator  38  with a microprocessor that runs a program stored on a computer-readable medium. In a different form of construction, the command signal generator  38  may be constituted by an array of hardwired logic gates that generate the locomotive control word upon actuation of the controls.  
         [0045]    Operation of the microprocessor in accordance with its program is now described in greater detail with reference to the flowchart in FIG. 4. At step  410 , the microprocessor records the state of conduction of the electric contacts of the transmitter controls. This may be done on a periodic basis or upon a change being detected in one of the actuators. At step  420 , the microprocessor checks the operational status of the portable communications unit  10 . On the basis of the identity of the closed contacts, and on the basis of the operational status of the portable communications unit  10 , the microprocessor will produce either the full-length function component  710 A or the shortened function component  710 B of the locomotive control word  700 .  
         [0046]    Specifically, if at step  420 , the microprocessor determines that the operational status of the portable communications unit  10  is that of a “command authority holder”, then the microprocessor proceeds to step  430 , where the full-length function component  710 A is assembled. In particular, the function component  710 A will be indicative of the various settings selected by the operator, including speed, brake, power and horn settings, among others. The microprocessor then proceeds to step  450 .  
         [0047]    On the other hand, if at step  420 , it is determined that the operational status of the unit  10  is that of a command authority non-holder, then the microprocessor proceeds to step  440 , where the shortened function component  710 B is created on the basis of a restricted subset of the controls selected by the operator. This subset may be limited to emergency controls, such as an emergency brake control  16 , as well as the control indicative of a desire to gain command authority holder operational status, which could be conveyed by the operator via the reset push-button  18 , or any other input, or sequence of inputs. The microprocessor then proceeds to step  450 .  
         [0048]    At step  450 , the microprocessor appends the identifier component  720  to the function component (either  710 A or  710 B). In order to minimize the design complexity of the controller  46  (to be described later), it may be advantageous to append the identifier component  720  to the “front” of the function component, such that it is released before the function component.  
         [0049]    At step  460 , a data security code enabling the controller  46  to check for transmission errors may be created and appended in the form of the integrity component  730 . As with the identifier component  720 , the integrity component  730  can be appended to an operational-status-invariant location with respect to the remainder of the locomotive control word, such as the “front” of the identifier component  720  or between the identifier component  720  and the function component  710 A,  710 B.  
         [0050]    At step  470 , the completed locomotive control word  700  is supplied to the transmitter  40 , which handles modulation and radio frequency transmission of the locomotive control word to the controller  46 . It will be appreciated that the use of shorter words when the portable communications unit  10  has “command authority non-holder” operational status results in quicker completion of the transmission and hence uses less bandwidth on the wireless link and allows the unit  10  to conserve power. After step  470 , the microprocessor returns to step  410  and waits for the next occasion to record the state of conduction of the electric contacts of the transmitter controls.  
         [0051]    In a non-limiting embodiment, in order to further reduce bandwidth occupied by signals sent from a portable communication unit  10  having a “command authority non-holder” operational status, the transmitter  40  can transmit signals generated by portable communication units  10  having a “command authority non-holder” operational status at a reduced repetition rate in comparison to the signals generated by a portable communication unit  10  having a “command authority holder” operational status. As such, locomotive control words  700  generated by portable communication units  10  having “command authority non-holder” operational status would be transmitted less frequently than a locomotive control word  700  generated by a portable communication unit  10  having a “command authority holder” operational status.  
         [0052]    It should be understood that in such an alternative embodiment, it is not necessary for the portable communication units  10  to generate locomotive control words having shortened function components  710 B. As such, in an alternative embodiment the repetition rate of transmission may be lessened for the portable communication units  10  having a “command authority non-holder” operational status, while the function component  710 B of the locomotive control words  700  generated by the portable communication units  10  having a “command authority non-holder” operational status is of the same length as the locomotive control words  700  generated by the portable communications unit  10  having a “command authority holder” operational status.  
         [0053]    In yet another non-limiting embodiment, in the case where the portable communication units  10  use TDMA, or spread spectrum protocols, it is possible for the portable communications unit  10  having a “command authority holder” operational status to be assigned a time interval in the TDMA frame having a longer length than the time intervals assigned to the portable communications units  10  having a “command authority non-holder” operational status. Alternatively, it is possible for the portable communication unit  10  having the “command authority holder” operational status to be assigned more time intervals in the TDMA frame than the portable communications units  10  having the “command authority non-holder” operational status.  
         [0054]    As described above, it is within the scope of the present invention for the portable communications units  10  to include a sensory indicator  37  for indicating to an operator whether the portable communications unit  10  has a “command authority holder” operational status or a “command authority non-holder” operational status. In order to inform the operator of the current operational status of the portable communications unit  10 , the sensory indicator  37  may be triggered or toggled when appropriate.  
         [0055]    In a first example of implementation, the portable communications units  10  do not include a receiver  41  for receiving signals from the controller  46 , or from other portable communications units  10 . In such an embodiment, the portable communications units  10  include a memory in which is stored a profile associated to the “command authority holder” operational status, and the “command authority non-holder” operational status. Depending on the operational status of the portable communication unit  10 , the memory is set to select either one of the “command authority holder” operational status, or the “command authority non-holder” operational status. In a non-limiting example of implementation, when the operator of a portable communications unit  10  that has a “command authority holder” operational status decides to relinquish that status, the operator activates the “reset” button, or any other input, or sequence of inputs, needed to relinquish the operational status. Upon activation of the relinquish inputs, the memory of the portable communication device is modified such that the memory toggles to select the opposite operational status, which is the “command authority non-holder” operational status. Once this is done, the operator of that portable communication unit  10  communicates either verbally, or through a visual cue, to the operator of another portable communications unit  10  that it should acquire the “command authority holder” operational status. Accordingly, the operator of that other portable communication unit  10  can then activate the “reset” button, or any other input, or sequence of inputs, on his/her portable communication unit  10 , such that its memory toggles to the profile associated to “command authority holder” operational status. In this manner, each of the affected portable communications units will appropriately tailor the format of the locomotive control words they generate meaning the message length, repetition rate or both, in accordance with their respective new operational status.  
         [0056]    It should be understood that by activating the “reset” button, or other input or sequence of inputs, as described above, the portable communications units  10  send signals to the controller  46  indicative of the desire to switch operational status. However, in the above described embodiment, the portable communications units  10  do not receive confirmation from the controller  47  or from the other portable communication units  10  as to the change, or current status, of their respective operational status.  
         [0057]    The controller  46  is responsive to the signals indicative of the desire to switch operational status to update in its own memory the profiles of the portable communications units  10 , such that its memory is up to date as to which portable communications unit  10  has a “authority command holder” operational status. It will be appreciated that in order for a portable communication unit to be assigned the “command authority holder” operational status, the portable communications unit  10  that previously held that status must have sent a signal relinquishing that status.  
         [0058]    In an alternative embodiment, as shown in FIG. 3, the portable communication units  10  include a receiver  41 , and the operational status of the unit  10  (command authority holder or command authority non-holder) is communicated to the receiver  41  by the controller  46 , e.g., via a wireless link. Upon receipt of a signal from the controller at the receiver  41 , the receiver  41  is configured to distribute the operational status to the other components of the portable communications unit  10 , namely the command signal generator  38 , the transmitter  40  and the sensory indicator  37 . Alternatively, when the current operational status of the portable communication unit  10  is checked at step  420 , as described above with respect to FIG. 4, it is within the scope of the present invention to determine whether this current operational status is different from the operational status that was in effect during the previous execution of step  420 . If there has been a change, then the sensory indicator  37  can be triggered (e.g., an audible tone with increasing pitch or volume to indicate assumption of command authority and decreasing pitch or volume to indicate relinquishing of command authority). Alternatively, the sensory indicator  37  can be toggled (e.g., LED on to indicate that the unit  10  has command authority and LED off to indicate that the unit  10  does not have command authority). It should be understood that although the sensory indicator  37  has been described above as being in the form of a light or a sound, any type of display, such as text or pictograph, could also be used to indicate the operational status to an operator, without departing from the spirit of the invention.  
         [0059]    [0059]FIG. 5 provides a functional diagram of the controller  46 , which can be either mounted on board one of the locomotives  13 ,  15  or centrally arranged to communicate with each of the locomotives  13 ,  15  in wireless or wireline fashion. The controller  46  includes a transceiver unit  48 , or a receiver unit in the case where the controller  46  is unable to transmit signals to the portable communication units  10 , a processing unit  50  and an interface  72 . The processing unit  50  includes a CPU  66  with access to a memory  68  in which are stored (1) a list of identifiers specifying those portable communications units allowed to control each locomotive and (2) the identifier of the single portable communications unit which currently has command authority holder operational status. If the controller  46  is mounted on-board a given locomotive, then the interface  72  will typically connect the processing unit to a driver unit  52  and to a plurality of sensors  78  situated on the locomotive. Otherwise, if the controller  46  is mounted remotely from the locomotive, the interface  72  will be connected to another transceiver unit (not shown) which maintains communication with a driver unit  52  and a plurality of sensors  78  located on board the locomotive.  
         [0060]    The transceiver unit  48  senses modulated signals sent out from the portable communications units  10  and decodes the locomotive control words contained therein. Details of channel acquisition and synchronization are not described here as they are assumed to be within the reach of a person ordinarily skilled in the art.  
         [0061]    In general, a received locomotive control word might have been sent out by a portable communication unit that is not allowed to control a particular locomotive. Assuming, however, that the originating portable communications unit is indeed allowed to control the locomotive in question, the operational status (i.e., command authority holder or command authority non-holder) is still not known a priori to the transceiver  48 . Hence, the transceiver should be capable of decoding both types of locomotive control words, namely those originated by a portable communications unit having “command authority holder” operational status as well as those originated by a portable communications unit having “command authority non-holder” operational status.  
         [0062]    In one embodiment of the present invention, the lack of a priori knowledge of the operational status of the originating portable communications unit is inconsequential to design or operation of the transceiver  48 . This is because, in such an embodiment, the only difference between locomotive control words issued by portable communications units having differing operational status appears in the length of the locomotive control word, and possibly, the repetition rate at which the locomotive control word is sent. The mere presence of certain superfluous bits at the tail end of a locomotive control word issued by a portable communications unit that does not have command authority holder operational status will not affect the validity of the initial bits which may contain emergency commands or a command indicative of a desire to assume command authority holder operational status. It is therefore acceptable to allow the transceiver  48  to feed the processing unit  50  with a full-length locomotive control word without knowledge of the operational status of the originating portable communications unit.  
         [0063]    The processing unit  50  receives the locomotive control word from the transceiver  48  as well as input signals received via the interface  72  from various sensors on the locomotive in question. Examples of sensors include direction sensors, speed sensors, pressure sensors, air flow rate sensors, etc. The interface  72  receives the signals produced by the sensors and digitizes them where required so they can be directly processed by the CPU  66 . The CPU  66  generates binary signals for commanding the various controls of the locomotive in question and supplies these binary signals via the interface  72 . Optionally, the processing unit  50  supplies data conveying an indication of each portable transmitters unit&#39;s operational status to the transceiver unit  48 , in the cases where the transceiver  48  transmits signals to the portable communication units  10  regarding the operational status of the portable communication units  10 .  
         [0064]    In order to generate the signals to control a given locomotive and, when necessary, to inform a portable communications unit of its operational status, the CPU  66  runs a program stored on a computer-readable medium. Operation of the CPU  66  in accordance with its program is now described in greater detail with reference to the flowchart in FIGS. 6A &amp; 6B.  
         [0065]    Specifically, upon reception of a locomotive control word  700  at step  610 , the CPU  66  will identify the full-length function component, as well as the identifier component and the integrity component. It will of course be appreciated that the function component actually transmitted may have been a shortened version of the function component, in the event that the originating portable communications unit has command authority non-holder operational status. Although the length of the function component will be an important factor later on (see steps  660 ,  670 ), this is still not relevant at the present stage of processing.  
         [0066]    At step  620 , the CPU  66  compares the identifier component of the locomotive control word to a list of two or more possible identifiers stored in the memory  68 . The list of acceptable identifiers contains the identifiers of all the portable communications units allowed to control the locomotive.  
         [0067]    If, at step  630 , the identifier in the locomotive control word does not correspond to any one of the identifiers in the list, then the CPU  66  rejects the word and takes no action. Otherwise, the system will examine the function component in order to determine what are the requested functions that the locomotive should perform. If, at step  640 , it is determined that the function component is indicative of a command that can be sent by either a “command authority holder” portable communication unit or a “command authority non-holder” portable communication unit. In this non-limiting implementation depicted in FIG. 6A, at step  640  it is determined whether the function component is indicative of a request to apply the emergency brake or to sound the bell or horn, then the system complies with the request. If step  640  answered in the negative, at step  650 , the system will implement the command only if the identifier in the locomotive control word matches a specific identifier in the list that designates the remote transmitter currently holding the command authority (steps  660  and  670 ).  
         [0068]    If, indeed, step  670  is verified, then the locomotive executes the command unless it is determined at step  680  that the command is a request to transfer command authority to another remote controller. The CPU  66  recognizes this request by checking the state of the bit reserved for this function in the locomotive control word. If the state of the bit is 1 (command transfer requested), the CPU  66  will proceed to perform a certain number of safety checks to determine if the command transfer can be made in a safe manner. More particularly, the CPU  66  will determine if the locomotive is stopped and if the brake safety checks (steps  690 ,  692 ) are verified. If the locomotive is moving or the brake safety checks fail, then no action is taken and the command remains with the portable transmitter currently in control.  
         [0069]    If steps  690  and  692  are successfully passed, then the CPU  66  proceeds to monitor the reset bit of all the locomotive control words received that carry an identifier in the list stored in the memory  68  (the reset bit issued by the transmitter currently holding the controls is not considered). The procedure of checking the reset bit, or any other confirmation button or sequence, is used for safety purposes in order to transfer the control of the locomotive only when the target portable communications unit has effectively acknowledged acceptance of the control. To this end, the CPU  66  executes to step  694 , where it determines whether, within a designated amount of time (e.g., 10 seconds), the “reset” bit has been set to the high position. This requires performing an additional series of steps similar to steps  610  through  650 . If the time limit has expired without having received an indication that a portable communications unit wishes to assume command authority holder operational status, the CPU  66  will abort the transfer function and resume normal execution of the program.  
         [0070]    However, if within the designated amount of time since reception of the request to transfer control from the current transmitter the CPU  66  observes a reset bit in the high position, implying that the operator of a remote transmitter in the pool of candidates able to acquire control has depressed the reset button, the program proceeds to step  696 , where the CPU  66  modifies the memory  68  to reflect the identity of the portable communications unit that has assumed command authority. For example, this may be done by shifting in memory  68  the identifier associated with the reset bit at high to the position of the current holder of command authority. From now on, the CPU  66  will accept commands (except the safety-related controls such as applying the emergency brake and sounding the bell/horn) only from the new holder of command authority. In addition, at step  698 , which is an optional step, the CPU  66  signals to the former holder of command authority that its operational status has been changed to that of command authority non-holder and, likewise, signals to the new holder of command authority that its operational status has been changed to that of command authority holder. Both signaling operations are effected via the transceiver unit  48 . Each of the affected portable communications units will appropriately tailor the format of the locomotive control words they generate in accordance with their respective new operational status.  
         [0071]    In the non-limiting embodiments described above, the multiple portable communication units  10  have been described as having a “command authority holder” operational status, or a “command authority non-holder” operational status. It should be understood that instead of having multiple portable communication units  10  with these operational status, it is within the scope of the present invention for one of the portable communication units to have a “student” operational status, and for one or more other portable communication units  10  to have a “trainer” or “teacher” operational status.  
         [0072]    In such an embodiment, the portable communication unit  10  having a “student” operational status would be able to generate locomotive control words having a function component  710 A having commands relative to the desired operative state of the locomotive. Moreover, the portable communication unit  10  having a “student” operational status would be able to control most, if not all, of the functionality of the locomotive, similarly to the portable communications units  10  having an “authorized command holder” operational status.  
         [0073]    However, the portable communication units  10  having the “trainer” operational status would be slightly different from the portable communication units  10  having the “authorized command non-holder” operational status. More specifically, the operator of the portable communication units  10  having the “trainer” operational status would only provide inputs via the switches, levers and buttons when it is desired to override the commands sent by the operator of the portable communication unit  10  having the “student” operational status. As such, the command signal generator  38  of the portable communication units  10  having the “trainer” operational status would scan the contacts of the controls  14 - 28  and would only generate a locomotive control, when there is a change in the controls  14 - 28 , thereby indicating that the operator has entered a command. Upon receipt of the locomotive control word from a portable communication unit  10  having the “trainer” operational status the processing unit  50  of the controller  46  is operative to override any command sent by the portable communication units  10  having the “student” operational status, and implement the commands contained in the function component of the locomotive control word of the portable communication units  10  having the “trainer” operational status.  
         [0074]    In this manner, the remote control system is able to conserve bandwidth by only transmitting a locomotive control word issued by a portable communications unit  10  having the “trainer” operational status when the operator of the portable communications unit  10  having the “trainer” operational status desires to overide a command sent by a portable communication units  10  having the “student” operational status. Alternatively, the portable communication unit  10  having the “trainer” operator status may send periodic contact messages of the type “I am alive”. The controller  46  is adapted to continue to implement commands issued by the portable communication unit having the “student” operational status only as long as it receives the periodic “I am alive” message from the “trainer”. Advantageously, this prevents the controller  46  from implementing commands from the “student” when the “trainer” is unable to override the commands.  
         [0075]    Those skilled in the art will appreciate that various alternative implementations of the present invention are possible. For example, there exist alternatives to the use of a unique repetition rate to allow secure communication between the transmitters in the various portable communications units  10  and the controller  46  while sharing the same frequency band. An example is spread spectrum communication, a specific example of which is code division multiple access (CDMA). In such a scenario, each portable communications unit can be supplied with its own spreading code or its own Walsh code, allowing interference-free communications to occur contemporaneously and within a common frequency range.  
         [0076]    The transmitter and receiver gear of the remote locomotive control system in accordance with a non-limiting example of implementation of the invention has been described above in terms of function of the principal parts of the system and their interaction. The components and interconnections of the electric network necessary to carry into effect the desired functions are not being specified because such details are well within the reach of a person skilled in the art.  
         [0077]    Also, those skilled in the art will appreciate that each of the various processing units described herein above may be implemented as an arithmetic and logic unit (ALU) with access to a code memory which stores program instructions for the operation of the ALU. The program instructions could be stored on a medium which is fixed, tangible and readable directly by the processor, (e.g., removable diskette, CD-ROM, ROM, or fixed disk), or the program instructions could be stored remotely but transmittable to the processor via a modem or other interface device (e.g., a communications adapter) connected to a network over a transmission medium. The transmission medium may be either a tangible medium (e.g., optical or analog communications lines) or a medium implemented using wireless techniques (e.g., microwave, infrared or other transmission schemes).  
         [0078]    Those skilled in the art should also appreciate that the program instructions stored in the code memory can be compiled from a high level program written in a number of programming languages for use with many computer architectures or operating systems. For example, the high level program may be written in assembly language, while other versions may be written in a procedural programming language (e.g., “C”) or an object oriented programming language (e.g., “C++” or “JAVA”). Those skilled in the art should further appreciate that in some embodiments of the invention, the functionality of the processor may be implemented as pre-programmed hardware or firmware elements (e.g., application specific integrated circuits (ASICs), electrically erasable programmable read-only memories (EEPROMs), etc.), or other related components.  
         [0079]    While specific embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope of the invention as defined in the appended claims.