Patent Publication Number: US-2004056182-A1

Title: Railway obstacle detection system and method

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention is directed to railway detection systems, in general, and more particularly, to a scanning laser beam railway obstacle detection system and method.  
       [0002] Early on in the history of the railroad steam engine was the invention of the so-called cow catcher, a device on the front of the train engine car to divert livestock and other obstacles off the track. Today, this device in one form or another is used to clear the track just forward of the wheels to prevent obstacles from entering under the train and causing a derailment. This one safety device was followed by a number of inventions to improve safety, such as the crossing gate, warning lights, fences, and numerous other devices. Despite these safety enhancements, large trains today suffer from a number of transportation accidents often ending in fatalities and harmful environmental effects.  
       [0003] Unfortunately, due to the momentum of the moving train, the size of the vehicle that is struck, and in some cases the configuration of the tracks at the point of collision, significant bodily injury can occur to passengers, including fatalities due to blunt force trauma as well as life threatening lacerations and vehicle entrapment. Moreover, railway systems particularly in the US involve the transportation of industrial chemicals resulting in serious post crash environmental and fire safety hazards that represent significant issues for first responders and citizens near the accident site. These two factors combined can result in significant financial liabilities.  
       [0004] However, the majority of these railway accidents are due to vehicular traffic going around crossing gates, unmarked crossings, crossing gates that fail to alert, or poorly designed crossing configurations. Additionally, railway systems can also be subject to failure or sabotage, resulting in derailment or trains operating on the same track. Yet other obstacles such as intentionally placed devices to derail trains, unsuspecting pedestrians, or livestock can also be in the way of on-coming trains. Therefore, it is of paramount importance to find a way of avoiding train accidents or at least mitigating the effects thereof.  
       [0005] A system having the ability not only to detect railway obstacles forward of the train along the line of sight, but also to detect the integrity of the railway track by examining rail spacing is clearly desirable. Additionally, since the railway track is metallic and of a high reflective nature versus the rail ties and rail gravel bed, a system that can detect, measure, and visualize such track kilometers in advance of the approaching train is also greatly desired. With such systems, visual images of range to obstacles on or close to the track, as well as deviations in track gage and integrity can be determined in sufficient time to take evasive action including speed reductions and stopping the train.  
       [0006] With such systems, it would be possible to detect railway obstructions and potential high rate of closure vehicles and trains that may result in imminent collision. This information can be coupled with GPS positioning and database mapping to identify upstream track features and configurations such as switches, turns, and the likelihood of trains switching to other tracks to avoid a possible collision. This enables safer railway operation, higher rates of speed, and integration with safety control systems to augment man-in-the-loop control.  
       SUMMARY OF THE INVENTION  
       [0007] In accordance with one aspect of the present invention, a scanning laser beam railway obstacle detection system disposable on-board a railway vehicle transportable over railway tracks comprises: a laser source for generating a laser beam; a laser scanning module optically coupled to the laser source for scanning the laser beam over the tracks ahead of the vehicle with a predetermined pattern; a light detector for receiving light echoes from the scanned laser beam and for converting the light echoes into electrical signals representative thereof; and a processor for processing the electrical signals from the light detector to detect an obstacle ahead of the vehicle.  
       [0008] In accordance with another aspect of the present invention, a method of detecting a threat to a railway vehicle transportable over railway tracks comprising the steps of: generating a laser beam; scanning the laser beam over the tracks ahead of the vehicle with a predetermined pattern; receiving light echoes from the scanned laser beam and converting the light echoes into electrical signals representative thereof; determining positions of the light echo signals along the scan pattern; and processing the light echo signals and corresponding positions to produce an image of a scene ahead of the vehicle for use in detecting a threat to the vehicle. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIGS. 1A and 1B are plan and elevation view illustrations, respectively, of a railway system suitable for embodying the present invention.  
     [0010]FIG. 2 is a block diagram schematic of a laser control and processing unit  16  suitable for use in the embodiment of FIGS. 1A and 1B.  
     [0011]FIG. 3 is an exemplary illustration of a scan head suitable for embodying a laser beam scanning module for use in the embodiment of FIGS. 1A and 1B.  
     [0012]FIG. 4 is a sketch exemplifying suitable optical elements for use inside the scan head embodiment of FIG. 3.  
     [0013]FIG. 5 is a block diagram schematic of an embodiment for the signal processing of laser beam echoes suitable for use in the system of FIGS. 1A and 1B.  
     [0014]FIGS. 6A and 6B are exemplary illustrations of varying terrain conditions along the railway tracks of the railway system of FIGS. 1A and 1B.  
     [0015]FIG. 7 is an exemplary illustration of the railway vehicle moving in a direction toward a curve in the railway tracks.  
     [0016] FIGS.  8 - 10  are program flowcharts suitable for use in programming a processor in accordance with an embodiment of the present invention.  
     [0017]FIG. 8A is an exemplary scan scene or field of view (FOV) produced by the embodiment of FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0018]FIGS. 1A and 1B illustrate by plan and elevation views a railway system suitable for embodying the present invention. A railway vehicle, like an engine car  10 , for example, may be pulling (or pushing) one or more other cars  12  of a train over railway tracks  14 . In the present embodiment, a railway obstacle detection system using the principles of scanning laser obstacle detection is disposed on-board the train, preferably at the front or engine car  10  and comprises a laser control and processing unit  16  linked optically and electrically over lines  18  to a laser scanning module  20 , preferably located at the front of the engine car  10 . The unit  16  produces a pulsed laser beam over lines  18  to the scanning module  20  which scans the beam over the tracks  14  forward of the train both in azimuth as illustrated by the dashed lines  22  and  24  in FIG. 1A, and in elevation as illustrated by the dashed lines  26  and  28 , for example. While a train is used for the present embodiment, it is understood that the present invention may be embodied in other railway vehicles without deviating from the broad principles of the present invention.  
     [0019] In the present embodiment, the scanning module  20  is operative to scan the beam approximately ±5-10° from the center of the tracks  14  and to oscillate the beam approximately ±10° in elevation from a line of sight axis as it is being rotated in azimuth, thus creating a predetermined pattern which for the present embodiment may be sinusoidal. The unit  16  produces laser pulses at a wavelength of 1.5 microns, and at a pulse repetition rate of around 40,000 pulses per second (PPS) with an inter-pulse period of approximately 25 microseconds. Under these conditions, a scene or field of view (FOV) from two meters (2 m) to two kilometers (2 Km) ahead of the train may be created from a processing of laser pulse echoes in the unit  16  as will become better understood from the description found herein below. For the present embodiment, the scene information forward of the train may be updated at a rate of 2 hertz (Hz). It is understood that the settings and conditions used for the present embodiment are merely provided by way of example and other settings and conditions may be used just as well without deviating from the broad principles of the present invention.  
     [0020] The railway obstacle detection system has the ability to not only detect railway obstacles up to 2 km or more under straight line conditions, but also has the ability to detect the integrity of the railway track by examining rail spacing. The scanning module can be tasked to follow the rail and look into turns. Additionally, since the rail-way track is metallic and of a high reflective nature versus the rail ties and rail gravel bed, track can be easily detected, measured, and visualized kilometers in advance of the approaching train. As such, visual images of range to target as well as deviations in track gage and integrity can be sampled in sufficient time to take evasive action including speed reductions and stopping the train automatically.  
     [0021] With this device it is possible to detect railway obstructions and potential high rate of closure vehicles and trains that may result in imminent collision. As will be described in greater detail below, scene information forward of the train obtained by laser scanning can be coupled with GPS positioning and terrain and railway mapping to identify upstream track features and configurations such as switches, turns, and the likelihood of trains switching to other tracks to avoid a possible collision. As such, adapting the fine detail detection, field of view scanning, ranging capability, and merging GPS mapping information permits a high level of automated railway control. This enables safer railway operation, higher rates of speed, and integration with safety control systems to augment man-in-the-loop control and enhance railway/vehicular/pedestrian safety.  
     [0022]FIG. 2 is a block diagram schematic of a laser control and processing unit  16  suitable for use in the embodiment of FIGS. 1A and 1B. Referring to FIG. 2, a laser source  30  generates laser beam pulses at the pulse repetition rate as controlled by a processing unit  32  via firing signal over line  34 , for example. The laser pulses are directed over an optical path  36  to an optical element  38  which may be a polarizing beam splitter, for example. The element  38  passes the laser pulses to an optical path  40 . A portion of each generated laser pulse may be reflected to a light detector  48  for indicating a start time thereof. A folding mirror  42  may be disposed in path  40  to direct the laser pulses to the scanning module  20 , preferably over a fiber optic cable  44 . In the present embodiment, return or echo pulses from obstacles in the path of the pulsed laser beam are received by the scanning module  20  and directed back over the fiber optic cable  44 , fold mirror  42  and along path  40  to the optical element  38 . However, instead of being passed by element  38 , the return echoes are reflected along an optical path  46  to the light detector  48  which may be an avalanche photo-diode or PIN diode, for example. The detector  48  converts the start pulses and light echoes into electrical pulses which are passed on to the processing unit  32  over signal line  50  for further processing as will become more evident from the description to follow. It is understood that the foregoing described embodiment is provided by way of example and that other optical elements and arrangements may be used to generate the laser pulses without deviating from the principles of the present invention.  
     [0023] The scanning module  20  may be embodied in a scan head  60  located preferably at the front of the engine car  10  remotely from the optical elements of the unit  16  described above. An exemplary illustration of a suitable scan head  60  is shown in FIG. 3. In this embodiment, the optical elements of the railway obstacle detection system may be disposed within the front or engine car and well supported and protected from the outside environment. The fiber optic cable bundle  44  may be used for the optical path or paths coupling the scan head  60  to the unit  16  as was previously described. The fiber optic cabling  44  may take a circuitous route within the vehicle to reach the scan head  60  which may be mounted to an external surface of the engine car  10  to permit the beam scan patterns to be projected out from the front of the train as described herein above. More than one scan head may be used in the present embodiment without deviating from the principles of the present invention.  
     [0024] Referring to the illustration of FIG. 3, the scan head  60  may control movement of the pulsed laser beam scan patterns along three axes  62 ,  64  and  66 . A top  68  of the scan head  60  may be mounted to a front surface of the engine car  10 , for example, such as shown in the illustrations of FIGS. 1A and 1B. A window area  70  of the scan head  60  through which the beam scans are emitted would be pointed in the direction of movement of the engine car  10  along the tracks  14 . The fiber optic cable bundle  44  may be passed through a hole in the body of the engine car  10  and into the scan head  60  through an opening  72  at the top  68  thereof. The optical elements within the scan head  60  which will be described in greater detail herein below cause the pulsed laser beam conducted over the path  44  to be scanned vertically up and down about the axis  66 . A conventional motor assembly (not shown) within the scan head  60  controls movement of a lower portion  74  thereof in azimuth about the axis  62  with respect to the railway tracks of the train. This movement occurs along a seam  76  between the top and bottom portions,  68  and  74 , respectively, and effectively moves the axis  66  along with the lower portion  74  which projects the beam scan pattern through a sinusoidal pattern, for example.  
     [0025] If elevation control of the line of sight axis  66  of laser beam is also desired, another conventional motor (not shown) may be disposed within the scan head  60  to control movement of a portion  78  of the scan head  60  about the axis  64 , for example. This movement causes the axis  66  and scan pattern to move in elevation with the portion  78  which includes the window area  70  and falls within the portion  74 . In the present embodiment, the window area  70  of the portion  78  may be controlled to move inside the portion  74  to protect it from the environment when not in use. The corrugated skin or surface in the area  80  at the top portion  68  acts as a heat sink to improve the transfer of heat away from the scan head  60  during operation thereof.  
     [0026] A sketch exemplifying suitable optical elements inside the scan head  60  is shown in FIG. 4. Referring to FIG. 4, the fiber optic cabling  44  providing the optical path for the pulsed laser beam and return echoes is aligned with the axis of the input aperture of a beam expander  82 , if used in the present embodiment. The beam exiting the expander  82  may be directed over an optical path  84  to an optical resonator element  86 . In the present embodiment, the optical resonator element  86  comprises a vibrating optical mirror element  88  centered and vibrated about the axis  66  by a resonator motor  90  mechanically linked thereto. For a more detailed description of an optical resonator element suitable for use in the present embodiment, reference is made to the pending U.S. patent application Ser. No. 10/056,199, filed Jan. 24, 2002, entitled “Silicon Wafer Based Rotatable Mirror” and assigned to the same assignee as the instant application, which pending application being incorporated by reference herein.  
     [0027] Movement of the mirror element  88  causes the laser beam from the path  84  to be directed vertically up and down across the axis  66  through a maximum predetermined angle  92  which may be on the order of ±10°, for example. The patterned laser beam  100  exits the scan head  60  through the window  70  and is projected along the axis  66 . Accordingly, as the axis  66  is moved azimuthally through the path  94 , the up and down beam pattern evolves into a sinusoidal pattern  96  as shown in FIG. 4. If desired the axis  66  may also be controlled to move in elevation through a path  98  as described above. It is understood that other optical elements may be used for scanning the laser beam through other patterns, like a rotating optical wedge element or non-mechanical transparent liquid crystal scanner or a microlens array scanner, for example, without deviating from the broad principles of the present invention.  
     [0028] If a rotating optical wedge element is used, for example, the beam conducted over path  84  is aligned with a rotational axis of the element and passed from an input side to an output side thereof. The light beam is refracted in its path through the wedge element and exits perpendicular to an inclined output surface thereof. This refraction of the light beam causes it to exit the scan head  60  as beam  100  through the window area  70  at the angle  92  to the axis  66 . Accordingly, as the wedge optical element is rotated 360° about the axis  66 , the beam  100  may be projected conically from the scan head  60  to form a helical like scan pattern. The window area  190  may comprise a clear, flat, zero power optical element made of a material like BK7, for example, so as not to interfere substantially with the scan pattern of the exiting beam  100 . In this example, the wedge optical element and window  70  are structurally coupled to move together along the azimuth path  94  and elevation path  98  to cause the optical axis  66  to move along therewith. In this manner, the scan pattern  100  is forced to move in azimuth and elevation with the portions  74  and  78  of the scan head  60 .  
     [0029] Backscattered light or return echoes will follow the same optical paths as their respectively emitted beam pulses and be returned to the optical elements of unit  16  via the fiber optic bundle  44 . While the present embodiment uses a common optical path for both the emitted and return light, it is understood that separate optical paths for emission and return light may be implemented, i.e. a bistatic optical technique, without deviating from the broad principles of the present invention. In such a bistatic system example, one or more fiber optic paths of bundle  44  may be designated for emission light and certain fiber optic paths thereof may be designated for return light. Accordingly, the fiber optic paths of the return light may be coupled directly to the light detector  48  without the use of a beam separator  38 , for example.  
     [0030] A suitable embodiment for the signal processing portion  32  of unit  16  is shown by the block diagram schematic of FIG. 5. In connection with this embodiment, the azimuth scan module  110  and optical resonator module  112  of the scan head  60  may each include a position sensing unit  114  and  116 , respectively. The position sensing unit  112  may sense the azimuth position of the axis  66  and generate an azimuth position signal (AZ) representative thereof which may be coupled to a signal processor  120  of the unit  16  via an appropriate interface. Likewise, the position sensing unit  114  may sense the position of the beam along the projected pattern and generate a beam position signal (RES) representative thereof which may be also coupled to the signal processor  120  via an appropriate interface. In addition, if an elevation control is used in the embodiment which is optional, the elevation scan module  117  of the head  60  may comprise a similar position sensing unit  118  for sensing the elevation position of the axis  66  and generate an elevation position signal (EL) representative thereof which may be also coupled to the processor  120  through an appropriate interface. Accordingly, the processor  120  may record the position of the laser beam at any given time utilizing the signals AZ, EL (if desired), and RES coupled thereto.  
     [0031] Referring to FIG. 5, in the present embodiment, the signal processor  120  receives the electrical signals, representative of the start pulses and return echoes, over line  50  from the light detector  48  and processes such signals to determine the time of arrival (TOA) of the return pulses. Since in the present embodiment, the processor  120  controls the firing of laser pulses, it can ascertain the start of the time of flight of a return signal or alternatively, it can use the start pulses. With this information and the TOA, the processor  120  may compute the time of flight of each of the return signals which is representative of the distance or range from the front of the train to the object causing the reflection. As will become more evident from the more detailed description found herein below, the processor  120  utilizes the range and beam position data of each of the return signals of a complete scan to formulate an electronic scene forward of the train in a scene memory thereof. This scan scene which may represent a full azimuth scan is then used by the processor  120  to determine if any obstacles or threats are in the path of the moving train. The processor  120  is coupled to three lights  122 ,  124  and  126  which may be colored green, yellow and red, respectively. Based on the results of the threat determination, the processor  120  may light one of these lights to indicate a safe condition or warn the train operator of a perceived impeding collision in sufficient time to avert or minimize the collision. While only three lights are used in the present embodiment, it is understood that more lights or different colored lights may also be used just as well. In addition, a visual display representative of the scene scan forward of the train may be displayed to the train operator through a display monitor  128 , if considered desirable. The display monitor  128  may be coupled to the processor  120  through an appropriate display interface.  
     [0032] Since the train does not always travel over flat terrain or in a straight line, another aspect of the present invention compensates for these variations in terrain features or track configurations by dynamically varying a distance vector D (see FIG. 1A) in range for the forward looking laser scan. For flat terrain and straight track conditions, the range vector D may be set at a maximum distance, and a maximum time of flight T max  within the interpulse period of the laser pulse repetition rate may be determined from the vector D and train speed. Accordingly, any return signals received within the interpulse periods outside T max  will not be processed. It is understood that the vector D may be adjusted based of the terrain conditions and/or curves in the track forward of the train.  
     [0033] Examples of varying terrain conditions along the railway tracks are shown in the illustrations of FIGS. 6A and 6B. In the illustration of FIG. 6A, the train is traveling up a graded portion of track  130  which will eventually flatten at a juncture  132 . Under these conditions, the range vector D will remain parallel to the tracks through the graded portion  130  up to the juncture  132  whereat the tracks become substantially flat. Thus, if range vector D is left unadjusted, return signals to the laser scanner  20  may result from obstacles along the large dashed line  134  which are not along the tracks, but rather in the space above the level of the train. Such return signals may result in a false perception of an impeding collision. In the illustration of FIG. 6B, the train is traveling down a graded portion of track  136  which will eventually flatten at a juncture  138 . Under these conditions, the range vector D will remain parallel to the tracks through the graded portion  136  down to the juncture  138  whereat the tracks become substantially flat. Thus, if the range vector D is left unadjusted, return signals to the laser scanner  20  may result from obstacles along the large dashed line  140  which may result in a false perception of an impeding collision of the train with the ground ahead of the train.  
     [0034] In either example, if the range vector D is limited in dimension to only the distance to the terrain change juncture  132  or  138  in each case, no return signals will be processed from obstacles beyond this limited range. Therefore, return signals from obstacles beyond the adjusted range will be ignored reducing the chances of a false perception of train collision. Note that as the train becomes closer and closer to the terrain change junctures, the range vector will be dynamically adjusted smaller and smaller until the train reaches the respective juncture. At the juncture, the range vector will be restored to its maximum dimension along the small dashed line  142  in each case. This will become better understood from the more detailed description heretofollow.  
     [0035] In the illustration of FIG. 7, the train is shown moving in a direction toward a curve in the railway tracks  14  at  144 . Thus, if the range vector D is left unadjusted, return signals will result from obstacles along the large dashed line  146  beyond the curve  146  which are no longer along the tracks  14 . Once again, if these return signals are processed by the processor  120 , a false perception of an impending collision may result. However, if the range vector D is limited to a dimension along the track  14  up to the curve  144 , then no return signals resulting from obstacles beyond this distance will be processed and the chances of a false detection of collision will be substantially reduced. Note also that as the train approaches the curve  144 , the range vector D will be reduced in dimension until the train comes out of the curve  144 , at which time, the range vector D may be restored to its maximum dimension along the small dashed line  148 . This will also become better understood from the more detailed description heretofollow.  
     [0036] In connection with adjusting the range vector D, a global positioning satellite (GPS) receiver unit  150  is coupled through an appropriate interface with the signal processor  120  as shown in FIG. 5 to provide the processor  120  with a current position of the train. In addition, a digital terrain elevation map database (DTED)  152  in some form of digital memory is coupled to the processor  120  through an appropriate interface. The DTED may be in the form of a CD-ROM, flash memory card, DVD or the like, for example, and contain map data of the terrain along the railway track route on which the train is moving. In whatever memory form, the DTED map data may include configurations of the track along the route and railway features along the track, such as warning and condition lamps, switches, curves, adjacent tracks, train traffic on the same or adjacent track and the likelihood of trains switching to other tracks to avoid a possible collision, for example. The DTED  152  may be updated by the processor  120  in real time with expected train traffic data from information received from other sources over signal lines  154 . Data of terrain, track configuration and features, and expected train traffic ahead of the train may be accessed from the DTED  152  based on the current position of the train obtained from the GPS receiver  150 . From this data, the processor  120  may compute the dimension of the range vector D ahead of the train which will become evident from the more detailed description below.  
     [0037] In the present embodiment, the processor  120  may be a microprocessor or microcontroller of the type manufactured by Texas Instruments, bearing model no. TMS320c6711, for example. The processor  120  may be programmed with various algorithms to perform the tasks described herein above. FIGS. 8, 9 and  10  are flowcharts which exemplify these algorithms. Referring to FIG. 8, for example, in block  160 , the laser is caused to fire or generate a laser pulse by a signal generated over line  34 . Concurrently with the detection of the corresponding start pulse or after a predetermined delay, an internal processor timer is started to count up from preferably a zero count at a predetermined rate based on a desired resolution for determining time of flight of a return signal received within the interpulse period of the corresponding laser pulse. In block  162 , the processor processes the return signals received from the light detector within the corresponding interpulse period and in block  164 , detects a return pulse which stops the internal timer at a count (TOA) which represents the time of flight or range (distance) of the detected pulse.  
     [0038] Before further processing of the pulse, the processor determines in decisional block  166  if the detected return pulse arrived within the time T which represents the dynamically adjusted range vector D as described herein above. The range vector D may be dynamically set by the algorithm exemplified by the flow chart of FIG. 9 which may be called for execution from time to time or periodically by an executive program of the processor. Referring to FIG. 9, in block  170 , a signal from the GPS receiver  150  is received and processed to determine the train position. The speed of the train may be also determined from the GPS signal or from another source which may be a speed sensor located on the train, for example. Then, in block  172  data is extracted or accessed from the DTED  152  forward of the determined train position. Such data would include track configuration, track features, terrain elevation of the ground forward of the train and the most recent update of train traffic. From this accessed data, it is determined in blocks  174  and  176  if there is a bend or curve in the track and/or a terrain elevation change ahead of the train. If no bend or terrain elevation changes exists or in other words, the train is moving along straight and flat track, then the range vector D is set to its maximum dimension D max  and time T is set to the computed time T max  based on D max  and the train speed in block  178 .  
     [0039] If a bend or terrain elevation change is detected, then decisional block  176  diverts program flow to block  180  wherein a distance from the train position to the bend or elevation change is determined using the date accessed from the DTED  152 . From this distance, the time T is calculated based on the train speed. If the calculated time T is greater than T max  as determined by decisional block  182 , then time T is set to T max  in block  178 . Otherwise, time T is retained as calculated. In either case, it is next determined in decisional block  184  if there is any expected train traffic within the distance or time T. If so, a train traffic (TT) flag is set in block  186 . The program is then returned to the executive of the processor.  
     [0040] Returning to the program of FIG. 8, if the detected pulse arrived within the time T of the interpulse period of the corresponding pulse, then program execution continues at block  190 . Else, the detected pulse is not processed and program execution is diverted to block  192  which causes a delay in program execution until the end of the interpulse period and then re-executes block  160 . In block  190 , the AZ and RES signals are recorded along with the time of flight for the corresponding laser pulse and stored in the scene memory based on the recorded position and range of the corresponding pulse in block  194 . In decisional block  196 , it is determined if the scan scene is complete. In other words, has the pulsed laser beam scanned through the intended azimuth angle across the track ahead of the train to gather enough data to complete an image of a scan scene in memory. An illustration of such an image scene in memory is shown in FIG. 8A. Referring to FIG. 8A, the exemplary scan scene or field of view (FOV) includes the tracks  14 , the range vector D and a vehicle V residing across the tracks ahead of the train. If the scene is complete, a feature extraction algorithm is called for execution in block  198 ; otherwise, program execution is diverted to blocks  192  and  160  to re-execute the algorithm which will be executed in accordance with the pulse repetition rate of the laser source which may be on the order of 40K pps, for example.  
     [0041] A feature extraction algorithm is exemplified by the flowchart of FIG. 10. Referring to FIG. 10, in block  200 , the data of the scene memory is analyzed to determine whether or not an imminent perceived threat is present which is likely to cause a train collision. The analysis may include determining detected return pulses which are all within a common range band or bin and determining a grouping or pattern of such pulses based on position proximity to one another. Using well-known pattern recognition techniques, the shape of the grouping from the scene may be compared with known shapes to determine if the obstacle is a threat to collision, like the vehicle V is FIG. 8A, for example, or a known railway feature. Track integrity ahead of the train may be also determined in this step based on a deviation of the determined pattern from a known track pattern, for example. In decisional block  202 , the program determines if an object was extracted from the scene, and if so, is it on or near the tracks and considered a threat to collision or is the condition of the tracks a threat to collision. If no object or no threat is determined in block  202 , program execution is diverted to block  204  wherein the green light  122  is turned on. Otherwise, in block  206 , the program determines the distance to the object or threat condition, which may be the closest range of the return pulses making up the grouping or pattern of the object or threat as a worst case scenario, for example. From the determined distance, an anticipated time to collision, Tc, may be calculated based on the speed of the train.  
     [0042] Thereafter, in block  208 , it is determined whether or not the train traffic (TT) flag is set. If so, as a precaution, it may be next determined, in block  210 , if the train traffic is expected at or around the anticipated time of collision Tc. The program may accomplish this determination by accessing the DTED  152  which has the most recent update of train traffic stored therein. Knowing the position of the train from the GPS receiver  150 , the program may calculate the distance from the train to the expected train traffic and therefrom compute the time to a passing between trains based on the closing speed of the two trains. Of course, it is presumed that any expected train traffic in proximity to the train would not be on the same tracks, but rather on adjacent tracks. Therefore, if the program determines that the train traffic is expected to pass at or around Tc, then, a safe condition is considered to exist and the green light is lit in block  212 .  
     [0043] As a further precaution before setting the green light, the program may also determine whether the shape of the extracted object resembles a cross-section of the front of a train using pattern recognition techniques as described above, and if so, whether the extracted object is on the same tracks or tracks adjacent thereto. This may all be determined from the grouping of pulse returns extracted from the scene as the object in question and from the position of the return pulses thereof. For example, if a majority of the return pulses of the object grouping have azimuth positions within the same tracks, then it is presumed that the object is on the tracks and an unsafe condition exists.  
     [0044] If the TT flag is not set or if train traffic is not expected at or around time Tc or if an unsafe condition is otherwise considered to exist, then a collision threat is perceived and block  214  is executed to calculate a time, Ts, to slow the train or reduce speed to less than a predetermined speed, which may be five miles per hour (5 mph), for example, based on its current speed. Thereafter, in decisional block  216 , the program determines if Ts is less than or equal to Tc, i.e. the train is capable of reducing throttle and slowing to less than 5 mph before collision with the object threat. Of course, if the object threat is another train on the same tracks, it is presumed that the other train will have the same or similar railway object detection system and will take the same action. If the decision of block  216  is affirmative, then the yellow light  124  is lit in block  218 . Otherwise, the red light  126  is lit by block  220  which indicates to the train operator that emergency action is needed to avoid or mitigate a perceived collision. It is understood that when a light is lit by any of the blocks  204 ,  212 ,  218  or  220 , it supercedes another light which may be lit. Preferably, only one of the lights  122 ,  124  or  126  may be lit at any given time.  
     [0045] After a light is lit in the feature extraction algorithm, program execution returns to the processor&#39;s executive program until it is called again by the program described in connection with the embodiment of FIG. 8. In the present embodiment, the program exemplified by FIG. 8 may be executed in the processor  120  periodically at forty thousand times a second (40 K/sec) and will produce an image of a scan scene approximately every half second (0.5 sec) or at a rate of two scan scene images a second. Optionally, after a scan scene is complete as identified by decision step  196 , the processor may cause the stored scene image to be displayed on the display monitor  128  based on the indices of the stored laser echo signals. The update program described in connection with the embodiment of FIG. 9 may be executed often enough to maintain current measurements of train position and speed data, track features and configurations and train traffic ahead of the train, and to update dynamically the range or distance vector and corresponding time T. It is understood that the program flowchart embodiments described in connection with FIGS.  8 - 10  are provided by way of example to illustrate tasks being carried out within the processor  120  and that other programs may be used just as well without deviating from the broad principles of the present invention.  
     [0046] While the present invention has been described above in connection with one or more embodiments, it is understood that such embodiments were presented merely by way of example with no intention of limiting the invention in any way, shape or form. Rather, the present invention should be construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.