Patent Publication Number: US-11022674-B2

Title: Vehicle surroundings monitoring apparatus

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a vehicle surroundings monitoring apparatus which detects an object around a vehicle (own vehicle) by use of radars. 
     Description of the Related Art 
     A vehicle surroundings monitoring apparatus which detects an object around an own vehicle by use of a plurality of radars is known (see, for example, Japanese Patent Application Laid-Open (kokai) No. H07-248382). Each of the radars transmits a radio wave and receives a reflected wave generated by an object reflecting the transmitted wave, to thereby detect a relative position, a relative speed, and the like of the object with respect to the own vehicle. The vehicle surroundings monitoring apparatus is installed on a drive support system which performs drive assist controls so as to assist drive operation of a driver and provides information (the relative position, the relative speed, and the like) about an object detected by the radars to the drive support system. The drive support system performs a variety of drive assist controls on the basis of the information about the object. 
     As an example of the drive assist controls, a blind spot monitoring (BSM) control is known. The BSM control is a control for generating a warning to a driver of the own vehicle in a case where an object is present in a blind area in which the driver cannot see an object through side mirrors of the own vehicle in many cases. In a case where a drive support system performs the BSM control, the drive support system is equipped with a vehicle surroundings monitoring apparatus which includes radars disposed at the rear right corner part and the rear left corner part of the body of the own vehicle. 
     As another example of the drive assist controls, a forward collision warning control is known. The forward collision warning control is a control for generating a warning to the driver in a case where an other vehicle approaches the own vehicle in a manner that a predicted (extrapolated) travel route of the own vehicle extending forward from the own vehicle intersects crosswise with a predicted (extrapolated) travel route of that other vehicle. In a case where a drive support system performs the forward collision warning control, the drive support system is equipped with a vehicle surroundings monitoring apparatus which includes radars disposed at the front right corner part and the front left corner part of the vehicle body. 
     In view of the above, a drive support system, such as a drive support system which performs both the BSM control and the forward collision warning control, which need to detect both an object present in a forward region (specifically, both in a forward right region and in a forward left region) and an object present in a backward region (specifically, both in a backward right region and in a backward left region) is preferably equipped with four radars disposed at each of the four corners of the vehicle body. 
     In a case where a vehicle surroundings monitoring apparatus is equipped with the four radars disposed at each of the four corners and detects an object around the own vehicle, because a detection angle range of each of the four radars is relatively wide in the horizontal direction which is parallel to the ground surface, as shown in  FIG. 9A  and  FIG. 9B , detection areas A (radio wave transmission areas) of two radars adjacent to each other among the four radars R overlap with each other. In the overlapping areas X where two detection areas A overlap with each other, the radio waves transmitted by the radars interfere with each other, and thus, the accuracy for detecting an object may degrade. It should be noted that it is effective to widen the detection angle ranges of each of the four radars in the horizontal direction in order to reduce blind area of those radars. 
     Incidentally, each of the radars transmits the radio wave periodically. The interference of the radio waves occurs in a case where frequencies of the radio waves transmitted from the two radars adjacent to each other are identical to each other and transmission periods of the radio waves overlap with each other. For example, if the transmission periods which are mutually exclusive to each other are assigned for each of the four radars, the occurrence of the interference of the radio waves can be prevented. However, in this case, the time duration for completing the transmission of the radio waves from all of the four radars becomes relatively long, thereby increasing the possibility that the time required for detecting an object also becomes relatively long. As a result, a considerable delay may occur in performing drive assist controls such as in predicting a probability of a collision. 
     SUMMARY 
     In view of the foregoing, the present invention is made in order to solve the problems described above. That is, one of objects of the present invention is, in a vehicle surroundings monitoring apparatus having at least four radars, each disposed at each of the four corners of a vehicle body, in order to detect an object present around the own vehicle, to prevent the occurrence of the interference of the radio waves transmitted by the radars while reducing the possibility of occurrence of the delay in monitoring (detecting) the object. 
     A vehicle surroundings monitoring apparatus ( 10 ) which achieves the above-described object comprises four radars ( 12 ) and a transmission timing control means ( 13 ). 
     The four radars includes a forward right radar ( 12 FR), a forward left radar ( 12 FL), a backward right radar ( 12 RR), and a backward left radar ( 12 RL), each of said radars detecting an object in a respective detection area around a vehicle through transmitting a radio wave and receiving a reflection wave generated as a result of reflection by said object of the transmitted radio wave, said forward right radar being disposed at the front right corner part of a body of a vehicle, said forward left radar being disposed at the front left corner part of said body, said backward right radar being disposed at the rear right corner part of said body, said backward left radar disposed at the rear left corner part of said body. 
     The transmission timing control means for controlling “first diagonal radars” including said forward right radar and said backward left radar to perform an operation of alternating, at a predetermined interval (the sum of the transmission time Ton and the delay time Tdelay) (steps S 11  to S 14  and steps S 41  to S 43  of  FIG. 5 ), a first transmission period for which each of the first diagonal radars transmits said radio wave in a synchronized manner and a first transmission stop period for which each of the first diagonal radars does not transmit said radio wave, and for controlling “second diagonal radars” including said forward left radar and said backward right radar to perform an operation of alternating, at said predetermined interval (steps S 21  to S 23  and steps S 31  to S 34  of  FIG. 5 ), a second transmission period for which each of the second diagonal radars transmits said radio wave in a synchronized manner and a second transmission stop period for which each of the second diagonal radars does not transmit said radio wave, in such a manner that said first transmission period and said second transmission period do not overlap each other. 
     The vehicle surroundings monitoring apparatus according to the present invention is equipped with the radars disposed at (fixed to) the front right corner part, the front left corner part, the rear right corner part, and the rear left corner part, of the own vehicle. These radars transmit the radio waves and receive the reflection waves of the transmitted radio waves in order to detect an object (i.e., an obstacle, or a 3-D object) which is present around the own vehicle on the basis of the reflection waves. In order to detect an object which is present around the own vehicle by use of the radars disposed at the four corners, the detection angle ranges of each of the four radars in the horizontal direction (plane) should be relatively wide. Accordingly, the detection areas (radio wave transmission areas) of two of the radars from among the four radars which are adjacent to each other overlap with each other. Thus, the interference between/of the radio waves likely to occur in the overlapping areas. In view of this, the vehicle surroundings monitoring apparatus according to the present invention comprises the transmission timing control means. 
     The transmission timing control means controls transmission timings of the first diagonal radars (namely, a pair of the forward right radar disposed at the front right corner part, and the backward left radar disposed at the rear left corner part) so as to perform the operation of alternating the transmission periods and the transmission stop periods synchronously (simultaneously) at the predetermined interval (cycle). In addition, the transmission timing control means controls transmission timings of the second diagonal radars (namely, a pair of the forward left radar disposed at the front left corner part, and the backward right radar disposed at the rear right corner part) so as to perform the operation of alternating the transmission periods and the transmission stop periods synchronously (simultaneously) at the predetermined interval (cycle) which are same in length as that for/of the first diagonal radars. Further, the transmission timing control means controls the transmission timings of the first diagonal radars and the second diagonal radars (the transmission timings of the four radars) such that the transmission periods of the first diagonal radars does not overlap with the transmission periods of the second diagonal radars. Namely, the transmission timing control means controls the transmission timings of the first diagonal radars and the second diagonal radars such that that the transmission periods of the first diagonal radars (namely, the first transmission period) and the transmission periods of the second diagonal radars (namely, the second transmission period) are different from each other (namely, no overlapping periods is present between the transmission periods of the first diagonal radars and the transmission periods of the second diagonal radars). 
     Therefore, the interference of/between the radio waves associated with the two radars adjacent to each other in the lateral direction (for example, the forward right radar and the forward left radar) or in the longitudinal direction (for example, the forward right radar and the backward right radar) is avoided, whereby the deterioration (degradation) of the accuracy for detecting an object by the radars is prevented. In addition, the four radars are divided into two groups (namely, the first diagonal radars and the second diagonal radars), and the transmission periods which are mutually exclusive to each other are assigned for each of the two groups. Namely, the first transmission period and the second transmission period do not overlap with each other. Therefore, the time duration for completing the transmission of the radio waves by all of the four radars becomes shorter compared with the case where the transmission periods which are mutually exclusive to each other are assigned for each of the four radars. Consequently, the case that the time duration for detecting an object around the own vehicle by the four radars (detecting duration) becomes excessively long is avoided. 
     For example, a drive support system which assists drive operation of the driver of the own vehicle on the basis of information about an object around the own vehicle detected by the vehicle surroundings monitoring apparatus according to the present invention can quickly obtain the information about the object with high detection accuracy. Therefore, such a drive support system can perform the drive assist control appropriately. 
     In one mode of the apparatus of the present invention, 
     said transmission timing control means is configure to: 
     set said first transmission period and said first transmission stop period to be equal in length to each other; 
     set said second transmission stop period in such a manner that said second transmission stop period overlap completely with said first transmission period; and 
     set said second transmission period in such a manner that said second transmission period overlaps completely with said first transmission stop period ( FIG. 6 ). 
     In the aspect of the vehicle surroundings monitoring apparatus according to the present invention, the transmission timing control means controls the first diagonal radars such that the first diagonal radars continues transmitting the radio waves during the first transmission period and the first diagonal radars does not transmit the radio waves during the first transmission stop period which is as long (the same length in time) as the first transmission period. In addition, the transmission timing control means controls the second diagonal radars such that the second diagonal radars continues transmitting the radio waves during the second transmission period which overlaps completely with (or is synchronized with) the first transmission stop period, and the second diagonal radars does not transmit the radio waves during the second transmission stop period which overlaps completely with (or is synchronized with) the first transmission period. 
     Therefore, the first diagonal radars continue transmitting the radio waves during the period when the second diagonal radars do not transmit the radio waves, and the second diagonal radars continue transmitting the radio waves during the period when the first diagonal radars do not transmit the radio waves. Consequently, the detecting duration does not become excessively long. 
     In another aspect of the apparatus of the present invention, 
     said transmission timing control means comprises:
         forward right transmission control means ( 13 FR) for controlling radio wave transmission of said forward right radar;   forward left transmission control means ( 13 FL) for controlling radio wave transmission of said forward left radar;   backward right transmission control means ( 13 RR) for controlling radio wave transmission of said backward right radar; and   backward left transmission control means ( 13 RL) for controlling radio wave transmission of said backward left radar, and wherein,       

     one of said forward right transmission control means and said backward left transmission control means is configured to send a first synchronization signal (synchronization flag F 1 ) to said forward left transmission control means and said backward right transmission control means at a timing associated with a start timing of radio wave transmission of the radar which said one of said forward right transmission control means and said backward left transmission control means (steps S 11  to S 12  of  FIG. 5 ); 
     each of said forward left transmission control means and said backward right transmission control means is configured to determine a start timing of radio wave transmission of the radar which said each of said forward left transmission control means and said backward right transmission control means on the basis of a reception timing of said first synchronization signal (steps S 21  to S 22  and S 31  to S 32  of  FIG. 5 ); 
     one of said forward left transmission control means and said backward right transmission control means is configured to send a second synchronization signal (synchronization flag F 2 ) to said forward right transmission control means and said backward left transmission control means at a timing associated with a start timing of radio wave transmission of the radar which said one of said forward left transmission control means and said backward right transmission control means (steps S 33  to S 34  of  FIG. 5 ); and 
     each of said forward right transmission control means and said backward left transmission control means is configured to determine a the start timing of radio wave transmission of the radar which said each of said forward right transmission control means and said backward left transmission control means on the basis of a reception timing of said second synchronization signal (steps S 13  to S 14  and S 41  to S 42  of  FIG. 5 ). 
     In the aspect of the vehicle surroundings monitoring apparatus according to the present invention, the transmission timing control means comprises the forward right transmission control means, the forward left transmission control means, the backward right transmission control means, and the backward left transmission control means. The forward right transmission control means controls transmission of the radio wave from the forward right radar. The forward left transmission control means controls transmission of the radio wave from the forward left radar. The backward right transmission control means controls transmission of the radio wave from the backward right radar. The backward left transmission control means controls transmission of the radio wave from the backward left radar. 
     One of the forward right transmission control means and the backward left transmission control means sends the first synchronization signal to the forward left transmission control means and the backward right transmission control means at the timing associated with the start timing of the first transmission period. 
     Each of the forward left transmission control means and the backward right transmission control means determines the start timing of the second transmission period on the basis of the reception timing of the first synchronization signal. 
     One of the forward left transmission control means and the backward right transmission control means sends the second synchronization signal to the forward right transmission control means and the backward left transmission control means at the timing associated with the start timing of the second transmission period. 
     Each of the forward right transmission control means and the backward left transmission control means determines the start timing of the first transmission period on the basis of the reception timing of the second synchronization signal. 
     According to the present aspect, in the manner described above, it is possible to have the four radars cooperate with each other by use of the first synchronization signal and the second synchronization signal so as to control the transmission timings of the radio waves from the four radars by simple processes. 
     Notably, in the above description, in order to facilitate understanding of the present invention, the constituent elements of the invention corresponding to those of an embodiment of the invention which will be described later are accompanied by parenthesized names and/or symbols which are used in the embodiment; however, the constituent elements of the invention are not limited to those in the embodiment defined by the names and/or the symbols. Other objects, other features, and attendant advantages of the present invention will be readily appreciated from the following description of the embodiment of the invention which is made with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic system diagram of a drive support system equipped with a vehicle surroundings monitoring apparatus according to an embodiment of the present invention; 
         FIG. 2  is a top view diagram showing installation positions and detection angle ranges of four radars; 
         FIG. 3  is a top view diagram showing a relative position relationship between an own vehicle and another vehicle relevant to a forward collision warning control; 
         FIG. 4  is a top view diagram showing a relative position relationship between an own vehicle and another vehicle relevant to a lane change assist control; 
         FIG. 5  is a flowchart representing radar transmission timing control routines; 
         FIG. 6  is a timing diagram representing transmission periods, transmission stop periods, and sending timings of synchronization flags; 
         FIG. 7  is an explanatory drawing representing a state of the alternate switching of radio wave transmission areas; 
         FIG. 8  is a timing diagram representing transmission periods, transmission stop periods according to a variation; 
         FIG. 9A  is an explanatory drawing representing the interfere of radio waves; and 
         FIG. 9B  is another explanatory drawing representing the interfere of the radio waves. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A vehicle surroundings monitoring apparatus according to an embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  shows a schematic diagram of a drive support system  1  equipped with a vehicle surroundings monitoring apparatus  10  according to the present embodiment of the present invention. The drive support system  1  is installed (mounted) on a vehicle (hereinafter also referred to as an “own vehicle” in order to distinguish the own vehicle from other vehicles). 
     The drive support system  1  is equipped with a drive support ECU  20 , a meter ECU  30 , a brake ECU  40 , and an electric power steering ECU  50  as well as the vehicle surroundings monitoring apparatus  10 . 
     Each of the ECUs is an Electric Control Unit which is equipped with a microcomputer as a main component. These ECUs are connected to each other via a global bus  100  which is a communication line (data bus) of a Controller Area Network (CAN) so as to send information to the other ECUs and receive information from the other ECUs. In the present embodiment, each of the microcomputers includes a CPU, a ROM, a RAM, a non-volatile memory, interfaces (I/F), and the like. The CPU is configured to implement various functions by executing instructions (programs, routines) stored in the ROM. Some or all of those ECUs may be integrated into a (one) single ECU. 
     In addition, vehicle state sensors  60  and drive operation state sensors  70  are connected to the global bus  100 . The vehicle state sensors  60  include a plurality of sensors which detect states of the own vehicle. The drive operation state sensors  70  include a plurality of sensors which detect drive operation states of a driver of the own vehicle. Both of the vehicle state sensors  60  and the drive operation state sensors  70  are components of the drive support system  1 . For example, the vehicle state sensors  60  include a vehicle speed sensor for detecting the vehicle speed of the own vehicle, wheel speed sensors for detecting the rotational speeds of the wheels of the own vehicle, a longitudinal acceleration sensor for detecting an acceleration of the own vehicle in the longitudinal direction, a lateral acceleration sensor for detecting an acceleration of the own vehicle in the lateral direction, a yaw rate sensor for detecting a yaw rate of the own vehicle, etc. 
     The drive operation state sensors  70  include an accelerator operation amount sensor for detecting an operation amount of an accelerator pedal of the own vehicle, a brake operation amount sensor for detecting an operation amount of a brake pedal of the own vehicle, a brake switch for detecting the presence of an operation of the brake pedal, a steering angle sensor for detecting a steering angle of a steering wheel of the own vehicle, a steering torque sensor for detecting a steering torque of the steering wheel steered by the driver, a winker lever operation sensor for detecting a presence of an operation of a winker lever of the own vehicle, a shift position sensor for detecting a shift (operation) position of a shift lever of the own vehicle, etc. 
     Information detected (acquired) by the vehicle state sensors  60  and the drive operation state sensors  70  (hereinafter also referred to as “sensor information” for simplification) is sent to the global bus  100  so as to be received and be utilized by the ECUs as needed. Notably, some of the sensors included by the vehicle state sensors  60  and the drive operation state sensors  70  may be connected to a certain ECU and, in this case, information detected by the sensors is sent to the global bus  100  by that ECU as sensor information. 
     The vehicle surroundings monitoring apparatus  10  included in the drive support system  1  is an apparatus which detects a tridimensional object (a three-dimensional object) which is present around the own vehicle and which sends information associated with the detected object to the drive support ECU  20 . 
     The vehicle surroundings monitoring apparatus  10  is equipped with a front-side radar apparatus  10 F and a rear-side radar apparatus  10 R. The front-side radar apparatus  10 F detects a tridimensional object which is present in the forward right area and the forward left area of the own vehicle. The rear-side radar apparatus  10 R detects a tridimensional object which is present in the backward right area and the backward left area of the own vehicle. The front-side radar apparatus  10 F includes a forward right radar module  11 FR and a forward left radar module  11 FL. The forward right radar module  11 FR may also be referred to as the right FSR module  11 FR. The forward left radar module  11 FL may also be referred to as the left FSR module  11 FL. Notably, “FSR” stands for “Front Side Radar.” 
     The rear-side radar apparatus  10 R includes a backward right radar module  11 RR and a backward left radar module  11 RL. The backward right radar module  11 RR may also be referred to as the right BSM module  11 RR. The backward left radar module  11 RL may also be referred to as the left BSM module  11 RL. Notably, “BSM” stands for “Blind Spot Monitoring.” 
     The right FSR module  11 FR includes a forward right radar  12 FR, and a right FSR ECU  13 FR which is connected to the forward right radar  12 FR. The forward left radar module  11 FL includes a forward left radar  12 FL, and a left FSR ECU  13 FL which is connected to the forward left radar  12 FL. As shown in  FIG. 2 , the forward right radar  12 FR is fixed to the front right corner part of a body (for example, a front bumper) of the own vehicle. Meanwhile, the forward left radar  12 FL is fixed to the front left corner part of the vehicle body (for example, the front bumper). Notably, in the present embodiment, the right FSR ECU  13 FR and the left FSR ECU  13 FL are provided with the forward right radar  12 FR and the forward left radar  12 FL, respectively, in an integrated manner, but it is not essential. Namely, the right FSR ECU  13 FR may be provided separately from the forward right radar  12 FR and the left FSR ECU  13 FL may be provided separately from the forward left radar  12 FL. 
     In the present embodiment, the forward right radar  12 FR transmits a radio wave which propagates within a detection area Afr shown in  FIG. 2 . The detection area Afr expands between a direction which is approximately at 75 degrees rightward to a reference line Lfr and a direction which is approximately at 75 degrees leftward to the reference line Lfr. The reference line Lfr is a straight line extending in the diagonally frontward right direction from the front right corner part of the vehicle body. The forward right radar  12 FR receives a reflected wave which is a reflected wave generated by a tridimensional object so as to detect the tridimensional object on the basis of the received reflected wave. Namely, the forward right radar  12 FR detects the tridimensional object which is present in the detection area Afr. Similarly, the forward left radar  12 FL transmits a radio wave which propagates within a detection area Afl shown in  FIG. 2 . The detection area Afl expands between a direction which is approximately at 75 degrees rightward to a reference line Lfl and a direction which is approximately at 75 degrees leftward to the reference line Lfl. The reference line Lfl is a straight line extending in the diagonally frontward left direction from the front left corner part of the vehicle body. The forward left radar  12 FL receives a reflected wave which is a reflected wave generated by a tridimensional object so as to detect the tridimensional object on the basis of the received reflected wave. Namely, the forward left radar  12 FL detects the tridimensional object which is present in the detection area Afl. The detection area Afr and the detection area Afl are symmetrically arranged about an axis line Lc which extends in the longitudinal direction of the own vehicle and which is passes through the center of the own vehicle. The detection area Afr and the detection area Afl partially overlap with each other in a region in the center front of the own vehicle. This overlapping area is shown in  FIG. 2  as dark gray (dark pattern) area. 
     Referring back to  FIG. 1 , the right BSM module  11 RR includes a backward right radar  12 RR, and a right BSM ECU  13 RR which is connected to the backward right radar  12 RR. The backward left radar module  11 RL includes a backward left radar  12 RL, and a left BSM ECU  13 RL which is connected to the backward left radar  12 RL. As shown in  FIG. 2 , the backward right radar  12 RR is fixed to the rear right corner part of the vehicle body (for example, a rear bumper). The backward left radar  12 RL is fixed to the rear left corner part of the vehicle body (for example, the rear bumper). Notably, in the present embodiment, the right BSM ECU  13 RR and the left BSM ECU  13 RL are provided with the backward right radar  12 RR and the backward left radar  12 RL, respectively, in an integrated manner, but it is not essential. Namely, the right BSM ECU  13 RR may be provided separately from the backward right radar  12 RR and the left BSM ECU  13 RL may be provided separately from the backward left radar  12 RL. 
     In the present embodiment, the backward right radar  12 RR transmits the radio wave, which propagates within a detection area Arr shown in  FIG. 2 . The detection area Arr expand between a direction which is approximately at 75 degrees rightward to a reference line Lrr and a direction which is approximately at 75 degrees leftward to the reference line Lrr. The reference line Lrr is a straight line extending in the diagonally backward right direction from the rear right corner part of the vehicle body. The backward right radar  12 RR receives a reflected wave which is a reflected wave generated by a tridimensional object so as to detect the tridimensional object on the basis of the received reflected wave. Namely, the backward right radar  12 RR detects the tridimensional object which is present in the detection area Arr. Similarly, the backward left radar  12 RL transmits a radio wave, which propagates within a detection area Arl shown in  FIG. 2 . The detection area Arl expands between a direction which is approximately at 75 degrees rightward to a reference line Lrl and a direction which is approximately at 75 degrees leftward to the reference line Lrl. The reference line Lrl is a straight line extending in the diagonally backward left direction from the rear left corner part of the vehicle body. The backward left radar  12 RL receives a reflected wave which is a reflected wave generated by a tridimensional object so as to detect the tridimensional object on the basis of the received reflected wave. Namely, the backward left radar  12 RL detects the tridimensional object which is present in the detection area Arl. The detection area Arr and the detection area Arl are symmetrically arranged about the axis line Lc. The detection area Arr and the detection area Arl partially overlap with each other in a region in the center rear of the own vehicle. This overlapping area is also shown in  FIG. 2  as a dark gray area. 
     Further, The detection area Afr of the forward right radar  12 FR and the detection area Arr of the backward right radar  12 RR partially overlap with each other in a region in the right-center of the own vehicle. Similarly, The detection area Afl of the forward left radar  12 FL and the detection area Arl of the backward left radar  12 RL partially overlap with each other in a region in the left-center of the own vehicle. These overlapping areas are also shown in  FIG. 2  as dark gray areas. 
     It should be noted that the detection area Afr, the detection area Afl, the detection area Arr, and the detection area Arl shown in  FIG. 2  represent detection angular ranges of the forward right radar  12 FR, the forward left radar  12 FL, the backward right radar  12 RR, and the backward left radar  12 RL, respectively. Namely, these areas shown in  FIG. 2  do not represent detection distances of these radars. The detection distances are suitably designed in associated with the own vehicle, and in one instance, each of these radars may be designed in order to detect a tridimensional object which is present within a detection distance of several dozen meters. 
     Hereinafter, the forward right radar  12 FR, the forward left radar  12 FL, the backward right radar  12 RR, and the backward left radar  12 RL will be also collectively referred to as the radars  12  when they do not need to be distinguished from each other. The right FSR ECU  13 FR, the left FSR ECU  13 FL, the right BSM ECU  13 RR, and the left BSM ECU  13 RL will be also collectively referred to as the radar ECUs  13  when they do not need to be distinguished from each other. 
     In the present embodiment, each of the radars  12  adopts a Frequency Modulated Continuous Wave (FM-CW) method so as to detect a distance between the radar and a tridimensional object, a relative speed of the tridimensional object to the radar, and the like. This type of vehicle-mounted radar is well known (for example, refer to Japanese Patent Application Laid-Open (kokai) No. 2010-8365). Therefore, a brief description will be provided below. 
     Each of the radars  12  is equipped with a transmission section, a reception section (receiving section), and a processing section (none of these sections are shown in the drawings). The transmission section generates a “transmission signal” obtained by modulating a reference signal of pre-configured frequency, and transmits the transmission signal (as the radio wave) toward the detection area from a transmission antenna equipped with the transmission section. A “point in time at which transmitting the transmission signal from a certain radar among the radars  12  is started” and a “point in time at which transmitting the transmission signal from the certain radar is stopped” are controlled by an certain ECU from among the radar ECUs  13 , the certain ECU connected to the certain radar. A time period when the transmission signal continues being transmitted will be referred as a “transmission period.” Meanwhile, a time period when the transmission signal is not transmitted will be referred as a “transmission stop period.” The transmission period and the transmission stop period are alternately switched at/with a predetermined interval (cycle). Accordingly, the transmission signals are transmitted repeatedly by the transmission section at/with the predetermined interval (cycle). 
     Each of the radars  12  has common specifications. Therefore, frequencies of the transmission signals transmitted from the radars  12  are identical to each other. The frequency of the transmission signal falls within the range of a millimeter waveband or a quasi-millimeter wave band. The transmission signal which is transmitted from the transmission antenna of the transmission section is also referred to as the “radio wave” as described above. 
     The reception section includes a reception antenna and an electronic circuit which processes a signal received through the reception antenna as a “reception signal.” Therefore, when a tridimensional object is present in the detection area described above, a reflected wave which is generated by the tridimensional object reflecting the radio wave radiated to that tridimensional object is received by the reception section as the reception signal. 
     During the transmission period, the processing section sequentially obtains information about the transmission signal generated by the transmission section and the reception signal received by the reception section, and stores the information about the transmission signal and the reception signal in its memory (not shown). During the transmission stop period, the processing section generates a beat signal by means of mixing the transmission signal with the reception signal and performs a frequency analysis on the beat signal by use of a method such as fast Fourier transform (FFT). The processing section detects (acquires) a distance from each of the radars  12  to the tridimensional object and a relative speed of the tridimensional object with respect to each of the radars  12  by use of the frequency analysis, or the like. In addition, the processing section also obtains (acquires) a direction and a size (width) of the tridimensional object to each of the radars  12 . Accordingly, the processing section obtains (acquires) a relative position (the relative distance and the relative direction), the relative speed, the size, or the like of the tridimensional object. Hereinafter, information about a tridimensional object obtained (acquired) by the processing section will also be referred as “tridimensional object information.” 
     It should be noted that a variety of known detection methods may be adopted by the radars  12 . For example, the radars  12  may adopt a pulse compression FM-CW method, which is one of the FM-CW methods. Alternatively, the radars  12  may adopt a “method other than the FM-CW methods” which causes the interference of radio wave which will be discussed later in the present embodiments of the present invention. 
     The forward right radar  12 FR sends (transmits) the tridimensional object information to the right FSR ECU  13 FR. Similarly, the forward left radar  12 FL sends (transmits) the tridimensional object information to the left FSR ECU  13 FL. The backward right radar  12 RR sends (transmits) the tridimensional object information to the right BSM ECU  13 RR. The backward left radar  12 RL sends (transmits) the tridimensional object information to the left BSM ECU  13 RL. 
     On the basis of the tridimensional object information transmitted from the radars  12 , each of the radar ECUs  13  generates (obtains) information required for a “drive assist control” described below in detail. For example, the radar ECUs  13  obtain information about a position of a tridimensional object represented in a two dimensional coordinate system whose original point is adjusted to a predetermined arbitrary position on the own vehicle so as to be able to specify the position of the tridimensional object relative to the own vehicle. In addition, the radar ECUs  13  extract (select) one or more of the tridimensional objects required for the drive assist control from among tridimensional objects detected by the radars  12 , and sends (transmits) information about the extracted tridimensional objects to the drive support ECU  20 . Hereinafter, information about the tridimensional object required for the drive assist control (information about the tridimensional object sent to the ECU  20 ) will also be referred to as “target information.” The target information includes a relative position (which is represented by a relative distance and a relative direction), a relative speed, a size, and the like of a tridimensional object with respect to the own vehicle. 
     The right FSR ECU  13 FR and the left FSR ECU  13 FL are connected to each other via a local bus  14 F in a master and slave mode. In the present embodiment, the right FSR ECU  13 FR operates (functions) as the master and the left FSR ECU  13 FL operates (functions) as the slave. The right FSR ECU  13 FR, which is the master, is connected to the global bus  100 . 
     The right BSM ECU  13 RR and the left BSM ECU  13 RL are connected to each other via a local bus  14 R in a master and slave mode. In the present embodiment, the right BSM ECU  13 RR operates (functions) as the master and the left BSM ECU  13 RL operates (functions) as the slave. The right BSM ECU  13 RR, which is the master, is connected to the global bus  100 . 
     In addition, the left FSR ECU  13 FL (slave) and the left BSM ECU  13 RL (slave) are connected to each other via local bus  15 . The drive support ECU  20  is also connected to the local bus  15 . 
     As for the drive support system  1 , the right FSR ECU  13 FR sends target information generated by itself to the left FSR ECU  13 FL. The left FSR ECU  13 FL sends both target information generated by itself and the target information sent from the right FSR ECU  13 FR to the drive support ECU  20  via the local bus  15 . 
     The right BSM ECU  13 RR sends target information generated by itself to the left BSM ECU  13 RL. The left BSM ECU  13 RL sends both target information generated by itself and the target information sent from the right BSM ECU  13 RR to the drive support ECU  20  via the local bus  15 . 
     As for the drive support system  1 , the drive support ECU  20  is a core control unit. The drive support ECU  20  performs a “forward collision warning control,” a “backward collision warning control,” and a “lane change assist control,” as the drive assist control on the basis of the target information transmitted (provided) from the vehicle surroundings monitoring apparatus  10 . 
     The forward collision warning control is a control for generating (issuing) a warning to the driver in a case where an object (moving object, i.e., an other vehicle which is a vehicle other than the own vehicle) approaches the own vehicle in a manner that a predicted (extrapolated) travel route of the own vehicle extending forward from the own vehicle intersects crosswise with a predicted travel route of that other vehicle, in order to notify (alert) the driver of the presence of that other vehicle and prompt (urge) the driver to make an operation for avoiding a collision with that other vehicle. The backward collision warning control is a control is a control for generating (issuing) a warning to the driver in a case where an other vehicle approaches the own vehicle in a manner that a predicted (extrapolated) travel route of the own vehicle extending backward from the own vehicle intersects crosswise with a predicted travel route of that other vehicle, in order to notify (alert) the driver of the presence of that other vehicle and prompt (urge) the driver to make an operation for avoiding a collision with that other vehicle. 
     The lane change assist control is a control for controlling the steering angle of the own vehicle automatically so as to assist drive operation of the driver in changing lanes from an “original lane” which is a lane in which the own vehicle is traveling to a “target lane” which is a lane adjacent to the original lane. The lane change assist control is performed when a lane change assist request is accepted. 
     The forward collision warning control, the backward collision warning control, and the lane change assist control will be described later in greater detail. 
     A camera sensor  21 , an operation device  22 , and a buzzer  23  are connected to the drive support ECU  20 . 
     The camera sensor  21  is equipped with a camera and an image processing section, which are not shown. The camera takes (captures) an image of a region in front of the own vehicle to obtain data representing the image (image data). The image processing section obtains (recognizes) a pair of lane separation lines (lane markers, i.e., a right separation line and a left separation line) formed (or painted) on a road on the basis of the image data obtained by the camera. The right and left separation lines define the original lane. In addition, the image processing section figures out (obtains) a lateral position and a direction of the own vehicle relative to (with respect to) the “original lane where the own vehicle is present” defined by the pair of lane separation lines. Further, the image processing section figures out a width and a curvature of the original lane. The camera sensor  21  provides (sends) the information figured out by the image processing section to the drive support ECU  20  as “lane information.” 
     The operation device  22  is a device (instrument) to be operated by the driver so as to arrange configurations (settings) about request states and parameters of each of the drive assist controls. The operation device  22  may be composed of a plurality of devices instead of a single device. 
     The buzzer  23  operates so as to generate a warning sound when it receives a warning request from the drive support ECU  20 . When the buzzer  23  generates the warning sound, the driver will become aware of the need for caution (the driver is alerted). 
     Winker lamps  31  which comprise right turn signal lamps and left turn signal lamps, and a display device  32  are connected to the meter ECU  30 . The winker lamps  31  are also referred as to turn signal lamps. In addition, the winker lamps  31  can function as a hazard indicator (hazard lights) when both of the right turn signal lamps and the left turn signal lamps blink simultaneously. Hereinafter, the winker lamps  31  will be referred to as the hazard lamps  31  when used as the hazard indicator. 
     The meter ECU  30  is equipped with a winker drive circuit (an electronic circuit), which is not shown, so as to control the winker lamps  31 . When the meter ECU  30  receives a winker blink request which includes a turning direction (namely, right or left) via the global bus  100 , the meter ECU  30  makes “lamps among the winker lamps  31 ” corresponding to the turning direction” blink intermittently. Meanwhile, when the meter ECU  30  receives a hazard lamp blink request via the global bus  100 , the meter ECU  30  intermittently makes “all of the winker lamps  31  (namely, both right and left lamps as the hazard lamps  31 )” blink simultaneously. 
     In addition, when the winker lever is tilted clockwise or counterclockwise from its neutral position by the driver, the meter ECU  30  receives, from the winker lever operation sensor, a detection signal including a turning direction (namely, right or left) indicated (designated) by the driver&#39;s operation on the winker lever. When the meter ECU  30  receives the detection signal, the meter ECU  30  makes “lamps among the winker lamps  31 ” corresponding to the turning direction included in the detection signal” intermittently blink. In this case, when the meter ECU  30  makes the lamps of the winker lamps  31  intermittently blink, the meter ECU  30  sends a winker monitor signal which is indicative of a blinking state of the winker lamps  31  to the global bus  100 . 
     Further, when the meter ECU  30  receives an indication request via the global bus  100 , the meter ECU  30  displays contents specified by the indication request on the display device  32 . For example, when the drive support ECU  20  detects a target object to which the driver should be alerted through the forward collision warning control and/or the backward collision warning control, the drive support ECU  20  makes the buzzer  23  generate the warning sound and sends the indication request for warning to the meter ECU  30 . In this case, the meter ECU  30  displays, on the display device  32 , contents to notify the driver of information represented by the indication request for warning (for example, the direction of the target object relative to the own vehicle, i.e., right or left). 
     In addition, for example, during the performance of the lane change assist control, the drive support ECU  20  sends the indication request for lane change assist which represents a performing status of the lane change assist control, and the winker blink request, to the meter ECU  30  in proper timings. In this case, the meter ECU  30  displays, on the display device  32 , contents to notify the driver of the performing status of the lane change assist control on the basis of the indication request for lane change assist. Meanwhile, the meter ECU  30  makes lamps corresponding to the direction of the target lane intermittently blink among the winker lamps  31  on the basis of the winker blink request. 
     A brake actuator  41  is connected to the brake ECU  40 . The brake actuator  41  is provided in a hydraulic circuit extending between a master cylinder (not shown) and a frictional brake mechanism  42 . The master cylinder pressurizes hydraulic oil by using the depressing force applied to the brake pedal. The frictional brake mechanism  42  is disposed on four wheels (in the right and left and at the front and rear of the own vehicle). The frictional brake mechanism  42  is equipped with brake disks each of which is fixed on the wheel, and brake calipers each of which is fixed on the vehicle body. Each of the brake calipers is equipped with a pair of brake pads. According to a request from the brake ECU  40 , the brake actuator  41  controls the oil pressure supplied to wheel cylinders each of which is built in the brake caliper so as to operate the wheel cylinders such that the brake pads is pushed against the brake disks. As the result, frictional brake force applied to the wheels is generated. Namely, the brake ECU  40  controls frictional brake force of the own vehicle by means of controlling the brake actuator  41 . 
     For example, when the brake ECU  40  receives a brake boost request via the global bus  100 , the brake ECU  40  increases the frictional brake force compared to a case when the brake ECU  40  receives no brake boost request by means of controlling the brake actuator  41 . Namely, when the brake ECU  40  receives the brake boost request, the brake ECU  40  makes the ratio of the magnitude of the frictional brake force to the operation amount of the brake pedal higher than in the case when the brake ECU  40  receives no brake boost request. In addition, for example, when the brake ECU  40  receives an automatic brake request via the global bus  100 , the brake ECU  40  makes the frictional brake force generated as much as a predetermined magnitude by means of controlling the brake actuator  41  even when the brake pedal is not operated. 
     The electric power steering ECU  50  is a control unit for an electric power steering apparatus. Hereinafter, the electric power steering ECU  50  will also be referred to as the EPS ECU  50 . A motor driver  51  is connected to the EPS ECU  50 . The motor driver  51  is also connected to a steering motor  52 . The steering motor  52  is embedded in a steering mechanism which is not shown. 
     The EPS ECU  50  detects the steering torque of the steering wheel steered by the driver by using the steering torque sensor which is disposed at a steering shaft. According to the detected steering torque, the EPS ECU  50  drives the steering motor  52  by means of controlling power supply of the motor driver  51 . As the result of the drive of the steering motor  52 , steering torque is given to the steering mechanism, whereby the steering operation by the driver is assisted. 
     In addition, when the EPS ECU  50  receives a steering request from the drive support ECU  20  via the global bus  100 , the EPS ECU  50  makes the steering torque generated by driving of the steering motor  52  according to the amount of control specified by the steering request. The steering torque given to the steering mechanism in this case is not a steering assist torque generated in order to assist a steering operation by the driver as described above, but a torque which is caused by the steering request from the drive support ECU  20  and which is not attributable to a steering operation by the driver. 
     Next, the drive assist control performed by the drive support ECU  20  will be described. First, the forward collision warning control, which is a part of the drive assist control, will be described. As shown in  FIG. 3 , in a case where a predicted travel route of the own vehicle extending forward from the own vehicle intersects crosswise with a predicted travel route of an other vehicle (a vehicle other than the own vehicle) and that other vehicle approaches the predicted travel route of the own vehicle, the drive support ECU  20  generates (issues) the warning to the driver when a probability that the own vehicle collides with that other vehicle becomes high. In the present embodiment, a control to boost a brake force which is generated in accordance with an operation of the brake pedal by the driver is included in the forward collision warning control. 
     Hereinafter, an object which satisfies all of the following conditions (a) to (c) will be also referred to as a “crossing object.” 
     (a) The object is present around (in the vicinity of) the own vehicle. 
     (b) A predicted travel route of the own vehicle intersects crosswise with a predicted travel route of the object at a position ahead of the own vehicle. 
     (c) The object approaches the predicted travel route of the own vehicle. 
     The drive support ECU  20  determines whether or not the crossing object is present repeatedly on the basis of the target information sent from the vehicle surroundings monitoring apparatus  10  and the “own vehicle state information” such as the vehicle speed, the accelerations, the yaw rate, the steering angle, detected by the vehicle state sensors  60  and the drive operation state sensors  70 . When the drive support ECU  20  determines that the crossing object is present, the drive support ECU  20  figures out (obtains) an intersection point (position) at which the predicted travel route of the own vehicle and the predicted travel route of the crossing object intersect with each other, as a predicted crossing point PX. Thereafter, the drive support ECU  20  figures out (obtains) an object velocity Vx and a margin distance Dx, on the basis of a relative position relationship between the own vehicle and the crossing object, and the own vehicle state information. The object velocity Vx is a traveling velocity (speed) of the crossing object at the present point in time. The margin distance Dx is a distance between a position of the crossing object at the present point in time and the predicted crossing point PX. 
     The drive support ECU  20  figures out a margin time Tx which is a time from the present point in time to a point in time at which the crossing object reaches the predicted crossing point PX. The margin time Tx is figured out under the assumption that the velocity of the crossing object remains unchanged from the present point in time. Specifically, the margin time Tx is figured out by dividing the margin distance Dx by the object velocity Vx (namely, Tx=Dx/Vx). 
     The drive support ECU  20  determines whether or not the margin time Tx is equal to or shorter than a predetermined time threshold Tref. The time threshold Tref sets (determines) the timing to generate (issue) a warning to the driver and is set to a time which is long enough for the driver to make an operation to avoid the collision with the crossing object (collision avoidance operation) after the warning. When the margin time Tx is longer than the time threshold Tref, the drive support ECU  20  repeats this determining processes described above periodically. Accordingly, the drive support ECU  20  repeatedly determines whether or not the crossing object is present, and every time it is determined that the crossing object is present, the drive support ECU  20  figures out the margin time Tx for that crossing object and determines whether or not the margin time Tx is equal to or shorter than the time threshold Tref repeatedly. 
     While the drive support ECU  20  repeats those described processes, the drive support ECU  20  makes the buzzer  23  generate the warning sound and sends the indication request for warning to the meter ECU  30 , when it is determined (detected) that the margin time Tx has become equal to or shorter than the time threshold Tref. When the meter ECU  30  receives the indication request, it displays, on the display device  32 , the contents to notify the driver of information represented by the indication request for warning (for example, contents being the direction of the crossing object relative to the own vehicle, i.e. right or left). This warning enables the driver to recognize that the crossing object is approaching and to make the collision avoidance operation as needed. 
     In addition, the drive support ECU  20  sends the brake boost request to the brake ECU  40  at the same time as sending the indication request for warning to the meter ECU  30 . Accordingly, when the driver operates (press down) the brake pedal, a frictional brake force greater than a usual frictional brake force in accordance with the brake pedal operation is generated so as to assist the brake pedal operation of the driver. 
     While an ignition switch (not shown) of the own vehicle is in an ON state, the forward collision warning control is performed repeatedly. Thus far, the forward collision warning control has been described. 
     As described above, the crossing object targeted by the forward collision warning control is an object with which the own vehicle will collide when the own vehicle travels forward. In contrast, a crossing object targeted by the backward collision warning control is an object with which the own vehicle will collide when the own vehicle travels backward. Namely, the backward collision warning control is different from the forward collision warning control only in how to specify a crossing object as a target of the control. Therefore, descriptions of the backward collision warning control are omitted. 
     Next, the lane change assist control will be described. As mentioned above, the lane change assist control is a control for automatically controlling the steering (angle) of the own vehicle in such a manner that the traveling position of the own vehicle changes from the original lane to the target lane so as to assist drive operation of the driver for changing lanes. Therefore, according to the lane change assist control, a lane change can be performed without steering operation (steering wheel operation) of the driver. 
     The lane change assist control is a control relevant to the lateral position of the own vehicle with respect to a lane in which the own vehicle is traveling, and is performed when the lane change assist request is accepted. The lane change assist request is generated through a predetermined operation by the driver such as a predetermined operation of the winker lever. Alternatively, a dedicated operation device for generating the lane change assist request other than the winker lever may be provided the own vehicle. When the drive support ECU  20  receives the lane change assist request, the drive support ECU  20  determines whether or not a “lane change assist start permission condition” described below is satisfied. If the lane change assist start permission condition is satisfied, the drive support ECU  20  starts the lane change assist control. Notably, the lane change assist request includes information to specify a direction of the lane change (namely, right or left). 
     The lane change assist start permission condition contains at least the following two conditions (1) and (2). 
     (1) A lane marker (e.g., a white line) which is a boundary line between the original lane and the target lane is a broken line. 
     (2) No obstacles to the lane change are detected, and thus, it is determined that the lane change can be safely performed. 
     If the lane change assist start permission condition which contains both of the conditions (1) and (2) is satisfied, the drive support ECU  20  accepts the lane change assist request and starts the lane change assist control. 
     The condition (1) is a condition to observe a traffic regulation that a lane change is allowed in a case where the lane marker which is the boundary line is a broken line. The drive support ECU  20  determines whether or not the condition (1) is satisfied on the basis of the lane information provided from the camera sensor  21 . 
     In order to determine whether or not the condition (2) is satisfied, the drive support ECU  20  obtains a position of an object (specifically, an other vehicle) which is travelling in the target lane on the basis of the lane information provided from the camera sensor  21 , and the target information provided from the vehicle surroundings monitoring apparatus  10 . This is because, in order to perform a lane change from the original lane to the target lane, a sufficiently large empty space on the road is required to secure an appropriate distance between the own vehicle and the other vehicle traveling in the target lane. For example, as shown in  FIG. 4 , it is necessary to secure a sufficient (appropriate) distance for/as an inter-vehicular distance D 1  between the own vehicle C and the other vehicle C 1 , and to secure a sufficient (appropriate) distance for/as an inter-vehicular distance D 2  between the own vehicle C and the other vehicle C 2 . 
     In view of the above, for example, the drive support ECU  20  determines whether or not a relative distance between the own vehicle and an other vehicle traveling in the target lane is greater than a predetermined lane change acceptance distance on the basis of target information provided from the vehicle surroundings monitoring apparatus  10 . If the relative distance is greater than the lane change acceptance distance, the drive support ECU  20  determines that the condition (2) is satisfied. 
     If the lane change assist start permission condition is satisfied, the drive support ECU  20  starts the lane change assist. At this time, the drive support ECU  20  figures out a target route of the own vehicle on the basis of lane information provided from the camera sensor  21  at the present time point. The target route is a route from the present position in the original lane to the central position of the target lane in the lateral direction (width direction) in order to complete the lane change. 
     During the performance of the lane change assist control, the drive support ECU  20  figures out a target steering angle sequentially such that the own vehicle travels on the target route, and sends the steering request which specifies the target steering angle to the EPS ECU  50  every time the target steering angle is updated. The EPS ECU  50  controls the steering motor  52  such that the actual steering angle coincides with the target steering angle. As a result, the own vehicle travels on the target route so that the lane change is performed. The drive support ECU  20  ends the lane change assist control when the drive support ECU  20  determines that the own vehicle reaches the final target position (namely, the central position of the target lane in the lateral direction) on the basis of lane information provided from the camera sensor  21  at the present time point. Further, during the performance of the lane change assist control, the drive support ECU  20  sends the winker blink request so as to make lamps corresponding to the turning direction blink among the winker lamps  31 . 
     Thus far, the drive assist control which is performed by the drive support ECU  20  included in the drive support system  1  has been described. It should be noted that the above description regarding the drive assist control is a mere example and the drive support ECU  20  may be configured to perform any assist controls other than the drive assist control described above. 
     Here, a drive assist control which is performed without involving the drive support ECU  20  will be described. Each of the right BSM module  11 RR and the left BSM module  11 RL performs a BSM control independently. 
     A right indicator  16 R is connected to the right BSM ECU  13 RR. A left indicator  16 L is connected to the left BSM ECU  13 RL. For, example, the right indicator  16 R is disposed at a partial region on the mirror surface of a right side mirror (a right side door mirror) of the own vehicle. The left indicator  16 L is disposed at a partial region on the mirror surface of a left side mirror. 
     The right BSM ECU  13 RR stores information about a “right side blind area” on the right side of the own vehicle. Meanwhile, the left BSM ECU  13 RL stores information about a “left side blind area” on the left side of the own vehicle. The right side blind area is preconfigured to include a blind area in which the driver cannot see an object through the right side mirror in many cases. The left side blind area is preconfigured to include a blind area in which the driver cannot see an object through the left side mirror in many cases. For example, the right side blind area is defined as follows. The length of the right side blind area in the lateral direction is 3 meters. The distance in the lateral direction between the right end of the vehicle body and the left end of the right side blind area is 0.5 meters. The length of the right side blind area in the longitudinal direction is 5 meters. The distance in the longitudinal direction between the rear end of the vehicle body and the rear end of the right side blind area is 4 meters, and the rear end of the right side blind area is in the backward direction from the own vehicle. The left side blind area and the right side blind area are symmetrically arranged about the axis line Lc. 
     While the ignition switch is in the ON state, the right BSM ECU  13 RR and the left BSM ECU  13 RL perform the BSM control as described below. The right BSM ECU  13 RR determines whether or not an other vehicle is present in the right side blind area on the basis of tridimensional object information provided by the backward right radar  12 RR in order to detect a “caution requiring object.” Specifically, if all or a part of the body of another vehicle is contained in (overlaps) the right side blind area, the right BSM ECU  13 RR determines that an other vehicle is present in the right side blind area, and that other vehicle is the caution requiring object. In addition, the right BSM ECU  13 RR determines whether or not an other vehicle (specifically, an other vehicle approaching from behind) will enter the right side blind area on the basis of tridimensional object information provided by the backward right radar  12 RR within a predetermined time period. If an other vehicle is likely to enter the right side blind area within the time period, the right BSM ECU  13 RR determines that other vehicle is the caution requiring object. Notably, the right BSM ECU  13 RR performs this determination processing for each of other vehicles detected by the backward right radar  12 RR, in consideration of the relative distance and the relative velocity between the own vehicle and the other vehicle, and a direction of the other vehicle to the own vehicle. Meanwhile the left BSM ECU  13 RL also performs a similar determination processing so as to detect a caution requiring object on the basis of tridimensional object information provided by the backward left radar  12 RL. 
     When the right BSM ECU  13 RR detects a caution requiring object, the right BSM ECU  13 RR lights the right indicator  16 R. In this case, however, if the winker blink request for the right turn is been sending via the global bus  100  at that time, the right BSM ECU  13 RR makes the right indicator  16 R intermittently blink. Meanwhile, the left BSM ECU  13 RL detects a caution requiring object, the left BSM ECU  13 RL lights the left indicator  16 L. In this case, however, if the winker blink request for the left turn is been sending via the global bus  100  at that time, the left BSM ECU  13 RL makes the left indicator  16 L intermittently blink. Accordingly, when the driver operates the winker lever so as to turn to a direction where the caution requiring object is present, an attention arousing level to the driver is increased. 
     Thus far, the BSM control has been described. 
     Regarding the vehicle surroundings monitoring apparatus  10 , the front-side radar apparatus  10 F is utilized for the forward collision warning control, and the rear-side radar apparatus  10 R is utilized for the backward collision warning control. In addition, both the front-side radar apparatus  10 F and the rear-side radar apparatus  10 R are utilized for the lane change assist control. Further, the rear-side radar apparatus  10 R is utilized for the BSM control. 
     Generally, the forward collision warning control and the BSM control are performed perpetually while the ignition switch is in the ON state. Therefore, the front-side radar apparatus  10 F and the rear-side radar apparatus  10 R operate to monitor surroundings at the same time. Namely, the radars  12  respectively disposed at four corners of the vehicle body (specifically, the forward right radar  12 FR, the forward left radar  12 FL, the backward right radar  12 RR and the backward left radar  12 RL) detects tridimensional objects at the same time. 
     As described above, two of the detection areas (two of the “radio wave transmission areas”) of two of the radars from among the radars  12  which are adjacent to each other partially overlap with each other. That is, two of the detection areas Afr, Afl, Arr, and Arl, adjacent to each other, have an overlapping area. Since each of the radars  12  has common specifications, frequencies of the transmission signals (namely, the radio waves) transmitted from the radars  12  are equal to each other. Therefore, the radio waves interfere with each other where the radio wave transmission areas overlap with each other, and thus, the accuracy for detecting an object may degrade. 
     In view of the above, the vehicle surroundings monitoring apparatus  10  according to the present embodiment prevent the interference of the radio waves by means of controlling the transmission timing of the radio waves from the radars  12  in cooperation and synchronization with each other. In order to achieve this control, each of the radar ECUs  13  cooperates with each other. Hereinafter, a “radar transmission timing control processing” which is performed by each of the radar ECUs  13  will be described. Flowcharts shown in  FIG. 5  represent the radar transmission timing control routines executed by the radar ECUs  13 . The leftmost flowchart shown in  FIG. 5  represents a backward right radar transmission timing control routine executed by the right BSM ECU  13 RR. The second flowchart from the left end shown in  FIG. 5  represents a backward left radar transmission timing control routine executed by the left BSM ECU  13 RL. The third flowchart from the left end shown in  FIG. 5  represents a forward left radar transmission timing control routine executed by the left FSR ECU  13 FL. The rightmost flowchart shown in  FIG. 5  represents a forward right radar transmission timing control routine executed by the right FSR ECU  13 FR. Each of the radar ECUs  13  starts executing each respective radar transmission timing control routines when the ignition switch becomes the ON state, and continues executing each respective radar transmission timing control routines while the ignition switch is in the ON state (until the ignition switch becomes the OFF state). 
     First, the backward left radar transmission timing control routine executed by the left BSM ECU  13 RL will be described. When the backward left radar transmission timing control routine is stated, the left BSM ECU  13 RL proceeds to step S 11  so as to send a radio wave transmission request to the backward left radar  12 RL. When the backward left radar  12 RL receives the radio wave transmission request, the backward left radar  12 RL transmits the transmission signal from the transmission antenna equipped with the transmission section of the backward left radar  12 RL to the detection area Arl for a predetermined (fixed) transmission time Ton. Namely, the backward left radar  12 RL continues transmitting the radio wave until the transmission time Ton elapses. The transmission time Ton is a fixed time and, for example, is 20 milliseconds. The backward left radar  12 RL is configured to measure a time (time duration) from the start of the transmission of the radio wave (transmission signal) and stop transmitting the radio wave at a time point at which the measured time reaches the transmission time Ton. Alternatively, the left BSM ECU  13 RL may be configured to measure the time (time duration) from the start of the transmission of the radio wave. In this case, the left BSM ECU  13 RL sends a radio wave transmission stop request to the backward left radar  12 RL at the time point at which the measured time reaches the transmission time Ton (that is, when the transmission time Ton has passed since the radio wave transmission request was sent). 
     Subsequently, the left BSM ECU  13 RL proceeds to step S 12  so as to send a synchronization flag F 1  (i.e., a signal indicative of flag F 1 ) to the right BSM ECU  13 RR and to the left FSR ECU  13 FL. The synchronization flag F 1  is sent via the local bus  14 R and the local bus  15 . The synchronization flag F 1  is for synchronizing the operations of the radars  12 . 
     Meanwhile, when the backward right radar transmission timing control routine is started, the right BSM ECU  13 RR proceeds to step S 21  so as to determine whether or not the right BSM ECU  13 RR has received the synchronization flag F 1 . The right BSM ECU  13 RR repeats this processing until the synchronization flag F 1  is received. When the right BSM ECU  13 RR receives the synchronization flag F 1  sent from the left BSM ECU  13 RL (S 21 : Yes), the right BSM ECU  13 RR proceeds to step S 22  so as to wait until a predetermined delay time Tdelay elapses after the reception of the synchronization flag F 1 . It should be noted that a history indicating that the synchronization flag F 1  was received is deleted at step S 23 . This delay time Tdelay is as long as the transmission time Ton, and, in this example, is 20 milliseconds. Since the synchronization flag F 1  is sent at the same time when the left BSM ECU  13 RL sends the radio wave transmission request to the backward left radar  12 RL, the right BSM ECU  13 RR receives the synchronization flag F 1  at the same time when the backward left radar  12 RL starts transmitting the radio wave. 
     When a predetermined delay time Tdelay has elapsed since the right BSM ECU  13 RR received the synchronization flag F 1  (S 22 : Yes), the right BSM ECU  13 RR proceeds to step S 23  so as to send the radio wave transmission request to the backward right radar  12 RR. When the backward right radar  12 RR receives the radio wave transmission request, the backward right radar  12 RR transmits the transmission signal from the transmission antenna equipped with the transmission section of the backward right radar  12 RR to the detection area Arr for a predetermined transmission time Ton. Namely, the backward right radar  12 RR continues transmitting the radio wave until the transmission time Ton elapses. This transmission time Ton is as the same as the above-described transmission time Ton for the backward left radar  12 RL. The backward right radar  12 RR is configured to measure a time (time duration) from the start of the transmission of the radio wave (transmission signal) and stop transmitting the radio wave at a time point at which the measured time reaches the transmission time Ton. Alternatively, the right BSM ECU  13 RR may be configured to measure the time (time duration) from the start of the transmission of the radio wave. In this case, the right BSM ECU  13 RR sends a radio wave transmission stop request to the backward right radar  12 RR at the time point at which the measured time reaches the transmission time Ton (that is, when the transmission time Ton has passed since the radio wave transmission request was sent). 
     Since the delay time Tdelay is set to the same value as the value of the transmission time Ton, the backward right radar  12 RR starts transmitting the radio wave at the same time when the backward left radar  12 RL stops transmitting the radio wave. 
     The right BSM ECU  13 RR returns to step S 21  after sending the radio wave transmission request to the backward right radar  12 RR at step S 23  so as to repeat the above-described processing. 
     Meanwhile, when the forward left radar transmission timing control routine is started, the left FSR ECU  13 FL proceeds to step S 31  so as to determine whether or not the left FSR ECU  13 FL has received the synchronization flag F 1 . The left FSR ECU  13 FL repeats this processing until the synchronization flag F 1  is received. When the left FSR ECU  13 FL receives the synchronization flag F 1  sent from the left BSM ECU  13 RL (S 31 : Yes), the left FSR ECU  13 FL proceeds to step S 32  so as to wait until a predetermined delay time Tdelay elapses after the reception of the synchronization flag F 1 . It should be noted that a history indicating that the synchronization flag F 1  was received is deleted at step S 32 . This delay time Tdelay is as long as the transmission time Ton, and, in this example, is 20 milliseconds. Since the synchronization flag F 1  is sent at the same time when the left BSM ECU  13 RL sends the radio wave transmission request to the backward left radar  12 RL, the left FSR ECU  13 FL receives the synchronization flag F 1  at the same time when the backward left radar  12 RL starts sending the radio wave. 
     When a predetermined delay time Tdelay has elapsed since the left FSR ECU  13 FL received the synchronization flag F 1  (S 32 : Yes), the left FSR ECU  13 FL proceeds to step S 33  so as to send the radio wave transmission request to the forward left radar  12 FL. When the forward left radar  12 FL receives the radio wave transmission request, the forward left radar  12 FL transmits the transmission signal from the transmission antenna equipped with the transmission section of the forward left radar  12 FL to the detection area Afl for a predetermined transmission time Ton. Namely, the forward left radar  12 FL continues transmitting the radio wave until the transmission time Ton elapses. This transmission time Ton is the same as the above-described transmission time Ton for the backward left radar  12 RL and the backward right radar  12 RR. The forward left radar  12 FL is configured to measure a time (time duration) from the start of the transmission of the radio wave (transmission signal) and stop transmitting the radio wave at a time point at which the measured time reaches the transmission time Ton. Alternatively, the left FSR ECU  13 FL may be configured to measure the time (time duration) from the start of the transmission of the radio wave. In this case, the left FSR ECU  13 FL sends a radio wave transmission stop request to the forward left radar  12 FL at the time point at which the measured time reaches the transmission time Ton (that is, when the transmission time Ton has passed since the radio wave transmission request was sent). 
     Since the delay time Tdelay is set to the same value as the value of the transmission time Ton, the forward left radar  12 FL starts transmitting the radio wave at the same time when the backward left radar  12 RL stops transmitting the radio wave. 
     Subsequently, the left FSR ECU  13 FL proceeds to step S 34  so as to send synchronization flag F 2  (i.e., a signal indicative of flag F 2 ) to the left BSM ECU  13 RL and the right FSR ECU  13 FR. In common with the synchronization flag F 1 , the synchronization flag F 2  is sent via the local bus  14 F and the local bus  15 . The synchronization flag F 2  is for synchronizing the operations of the radars  12 . The left FSR ECU  13 FL returns to step S 31  after sending the synchronization flag F 2  at step S 34  so as to repeat the above-mentioned processing. 
       FIG. 6  is a timing diagram representing (depicting) the transmission periods in which the transmission signals are sent from the radars  12 , and the transmission stop periods in which the transmission signals are not sent from the radars  12 .  FIG. 6  also represents (depicts) the timings when the synchronization flag F 1  and the synchronization flag F 2  are sent. Names of the right FSR module  11 FR, the left FSR module  11 FL, the left BSM module  11 RL, and the right BSM module  11 RR are written on the leftmost side of the diagram. The transmission periods are shown in  FIG. 6  as gray areas. In addition, arrows shown in  FIG. 6  represent transmission timings, sources and destinations of the synchronization flag F 1  or the synchronization flag F 2 . Black filled circles connected to the arrows represent the sources, and end points (wedge-shaped ends) of the arrows represent the destinations. The uppermost chart in  FIG. 6  represents the transmission periods and the transmission stop periods of the forward right radar  12 FR. The second chart from the top represents the transmission periods and the transmission stop periods of the forward left radar  12 FL. The third chart from the top represents the transmission periods and the transmission stop periods of the backward left radar  12 RL. The bottommost chart represents the transmission periods and the transmission stop periods of the backward right radar  12 RR. 
     The left BSM ECU  13 RL sends the radio wave transmission request to the backward left radar  12 RL at a time point t 1  (step S 11 ). As the result, the backward left radar  12 RL starts transmitting the radio wave at the time point t 1 . At the same time (time point t 1 ), the left BSM ECU  13 RL sends the synchronization flag F 1  to the right BSM ECU  13 RR and the left FSR ECU  13 FL (step S 12 ). 
     The backward left radar  12 RL stops transmitting the radio wave at a time point t 2  when the transmission time Ton (=the delay time Tdelay) has passed since the time point t 1 . At the same time (time point t 2 ), the right BSM ECU  13 RR sends the radio wave transmission request to the backward right radar  12 RR (step S 23 ) and the left FSR ECU  13 FL sends the radio wave transmission request to the forward left radar  12 FL (step S 33 ). As a result, transmitting the radio waves is started by the backward right radar  12 RR and the forward left radar  12 FL at the time point t 2 . 
     In addition, at the time point t 2 , the left FSR ECU  13 FL sends the synchronization flag F 2  to the left BSM ECU  13 RL and the right FSR ECU  13 FR. 
     Referring back to  FIG. 5 , after the left BSM ECU  13 RL sends the synchronization flag F 1  at step S 12  of the backward left radar transmission timing control routine, the left BSM ECU  13 RL proceeds to step S 13  so as to determine whether or not the left BSM ECU  13 RL has received the synchronization flag F 2 . This synchronization flag F 2  is sent from the left FSR ECU  13 FL. 
     As described above, the synchronization flag F 2  is sent when the delay time Tdelay has elapsed since the left FSR ECU  13 FL received the synchronization flag F 1 . In addition, the delay time Tdelay is as long as the transmission time Ton. Therefore, the left BSM ECU  13 RL receives the synchronization flag F 2  at the same time when the backward left radar  12 RL stops transmitting the radio wave. 
     When the left BSM ECU  13 RL receives the synchronization flag F 2  (S 13 : Yes), the left BSM ECU  13 RL proceeds to step S 14  so as to wait until a predetermined delay time Tdelay elapses after the reception of the synchronization flag F 2 . It should be noted that a history indicating that the synchronization flag F 2  was received is deleted at step S 14 . This delay time Tdelay is also as long as the transmission time Ton (is set to the same value the value of the transmission time Ton) (e.g., 20 milliseconds). 
     When the delay time Tdelay has elapsed since the left BSM ECU  13 RL received the synchronization flag F 2  (S 14 : Yes), the left BSM ECU  13 RL returns to step S 11  so as to repeat the above-described processing. As a result, the backward left radar  12 RL starts sending the radio wave (step S 11 ). 
     For example, after the left BSM ECU  13 RL receives the synchronization flag F 2  at the time point t 2  shown in  FIG. 6 , the left BSM ECU  13 RL sends the radio wave transmission request to the backward left radar  12 RL at a time point t 3  when the delay time Tdelay has passed since the time point t 2 . As a result, the backward left radar  12 RL start transmitting the radio wave again at the time point t 3 . At the same time (time point t 3 ), the left BSM ECU  13 RL sends the synchronization flag F 1  to the right BSM ECU  13 RR and the left FSR ECU  13 FL (step S 12 ). 
     Referring back to  FIG. 5  again, when the forward right radar transmission timing control routine is started, the right FSR ECU  13 FR proceeds to step S 41  so as to determine whether or not the right FSR ECU  13 FR has received the synchronization flag F 2 . This synchronization flag F 2  is sent by the left FSR ECU  13 FL at step S 34 . The right FSR ECU  13 FR repeats this processing until the synchronization flag F 2  is received. When the right FSR ECU  13 FR receives the synchronization flag F 2  (S 41 : Yes), the right FSR ECU  13 FR proceeds to step S 42  so as to wait until a predetermined delay time Tdelay elapses after the reception of the synchronization flag F 2 . It should be noted that a history indicating that the synchronization flag F 2  was received is deleted at step S 42 . This delay time Tdelay is set to the same value as the value of the transmission time Ton. Since the synchronization flag F 2  is sent at the same time when the left FSR ECU  13 FL sends the radio wave transmission request to the forward left radar  12 FL, the right FSR ECU  13 FR receives the synchronization flag F 2  at the same time when the forward left radar  12 FL starts transmitting the radio wave. 
     When the predetermined delay time Tdelay has elapsed since the right FSR ECU  13 FR received the synchronization flag F 2  (S 42 : Yes), the right FSR ECU  13 FR proceeds to step S 43  so as to send the radio wave transmission request to the forward right radar  12 FR. When the forward right radar  12 FR receives the radio wave transmission request, the forward right radar  12 FR transmits the transmission signal from the transmission antenna equipped with the transmission section of the forward right radar  12 FR to the detection area Afr for the predetermined transmission time Ton. Namely, the forward right radar  12 FR continues transmitting the radio wave until a transmission time Ton elapses. This transmission time Ton is the same as the above-described transmission time Ton for each of the radars other than the forward right radar  12 FR among the radars  12  (specifically, the forward left radar  12 FL, the backward right radar  12 RR, and the backward left radar  12 RL). The forward right radar  12 FR is configured to measure a time (time duration) from the start of the transmission of the radio wave (transmission signal) and stop transmitting the radio wave at a time point at which the measured time reaches the transmission time Ton. Alternatively, the right FSR ECU  13 FR may be configured to measure the time (time duration) from the start of the transmission of the radio wave. In this case, the right FSR ECU  13 FR sends a radio wave transmission stop request to the forward right radar  12 FR at the time point at which the measured time reaches the transmission time Ton (that is, when the transmission time Ton has passed since the radio wave transmission request was sent). 
     The right FSR ECU  13 FR returns to step S 41  after sending the radio wave transmission request to the forward right radar  12 FR at step S 43  so as to repeat the above-described processing. 
     For example, the synchronization flag F 2  is sent from the left FSR ECU  13 FL to the right FSR ECU  13 FR at the time point t 2  shown in  FIG. 6 . The right FSR ECU  13 FR sends the radio wave transmission request to the forward right radar  12 FR at the time point t 3 , when the delay time Tdelay has passed since the time point t 2 . As a result, the forward right radar  12 FR starts transmitting the radio wave to the detection area Afr at the time point t 3 . After that, the forward right radar  12 FR stops transmitting the radio wave at a time point t 4  when the transmission time Ton has passed since the time point t 3 . 
     Each of the radar transmission timing control routines is repeatedly executed as described above by each respective one of the radar ECUs  13  in the vehicle surroundings monitoring apparatus  10 . 
     As described, according to the vehicle surroundings monitoring apparatus  10 , the forward right radar  12 FR and the backward left radar  12 RL perform the operations for alternately repeating the transmission periods and the transmission stop periods synchronously (and simultaneously) at the predetermined interval (cycle) which is equal to the sum (=Ton+Tdelay) of the transmission time Ton and the delay time Tdelay. Similarly, the forward left radar  12 FL and the backward right radar  12 RR also perform the operations for alternately repeating the transmission periods and the transmission stop periods synchronously (and simultaneously) at the predetermined interval (cycle) (=Ton+Tdelay). 
     The forward right radar  12 FR and the backward left radar  12 RL are arranged at the diagonal positions of the vehicle body of the own vehicle. A pair of the forward right radar  12 FR and the backward left radar  12 RL correspond to the first diagonal radars of the present invention. Hereinafter, the pair of the forward right radar  12 FR and the backward left radar  12 RL will be also collectively referred to as the first diagonal radars. Each of the first diagonal radars performs the operations for alternately repeating the transmission period and the transmission stop period synchronously (and simultaneously) at the predetermined interval (cycle). 
     Similarly, the forward left radar  12 FL and the backward right radar  12 RR are also arranged at the diagonal positions of the vehicle body. A pair of the forward left radar  12 FL and the backward right radar  12 RR corresponds to the second diagonal radars of the present invention. Hereinafter, the forward left radar  12 FL and the backward right radar  12 RR will be also collectively referred to as the second diagonal radars. Each of the second diagonal radars performs the operations for alternately repeating the transmission period and the transmission stop period synchronously (and simultaneously) at the predetermined interval (cycle). 
     The transmission period of the first diagonal radars and the transmission period of the second diagonal radars are alternately arranged. Specifically, when the first diagonal radars transmit the radio waves, the second diagonal radars do not transmit the radio waves. On the other hand, when the first diagonal radars do not transmit the radio waves, the second diagonal radars transmit the radio waves. Namely, the transmission period of the first diagonal radars and the transmission period of the second diagonal radars do not overlap with each other. In other words, the transmission period of the first diagonal radars and the transmission period of the second diagonal radars are mutually exclusive. 
     The transmission period of the first diagonal radars corresponds to the first transmission period of the present invention. The transmission stop period of the first diagonal radars corresponds to the first transmission stop period of the present invention. Meanwhile, the transmission period of the second diagonal radars corresponds the second transmission period of the present invention. The transmission stop period of the second diagonal radars corresponds to the second transmission stop period of the present invention. The first transmission period, the first transmission stop period, the second transmission period, and the second transmission stop period are as long as each other (or the same length time duration as each other). 
     Namely, according to the vehicle surroundings monitoring apparatus of the present embodiment, as shown in  FIG. 7 , the transmission of the radio waves by the first diagonal radars and the transmission of the radio waves by the second diagonal radars are alternately performed at the predetermined interval (cycle). Therefore, the interference between the radio waves transmitted from the two radars adjacent to each other in the lateral direction or in the longitudinal direction is avoided. As a result, the degradation (deterioration) of the accuracy for detecting an object (e.g., an other vehicle) is prevented. In addition, in the present embodiment, the radars  12  (namely, the four radars) are divided into two groups, and the transmission periods which are mutually exclusive to each other are assigned for each of the two groups. Therefore, the time duration for completing the transmission of the radio waves by all of the radars  12  becomes shorter compared with a comparable case where the transmission periods which are mutually exclusive to each other are assigned for each of the four radars. In other words, the present embodiment can shorten an interval between the consecutive transmission periods of each of the radars  12  as compared with the comparable case. Consequently, the time duration (the detecting duration) for detecting an object around the own vehicle by the radars  12  does not become excessively long. 
     Further, in the present embodiment, the first diagonal radars continue transmitting the radio waves during/over the period when the second diagonal radars do not transmit the radio waves, and the second diagonal radars continue transmitting the radio waves during/over the period when the first diagonal radars do not transmit the radio waves. Consequently, the present embodiment can more effectively shorten the interval between the consecutive transmission periods of each of the radars  12  (or the case where the detecting duration becomes excessively long can be more effectively avoided). 
     In addition, the synchronization among the radars  12  is performed accurately by simple processes using the synchronization flag F 1  and the synchronization flag F 2 . Further, neither the synchronization flag F 1  nor the synchronization flag F 2  is sent via the global bus  100 , but they are sent via the local bus  14 R, the local bus  14 F, or the local bus  15 . Therefore, an increase of traffic on the global bus  100 , via which a large amount of communication is conducted, due to sending the synchronization flag F 1  and the synchronization flag F 2  is avoided. 
     The embodiment of the vehicle surroundings monitoring apparatus according to the present invention has been described, however, the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the scope of the invention. 
     For example, in the present embodiment, the transmission period of the first diagonal radars coincides (completely matches) with the transmission stop period of the second diagonal radars, and the transmission stop period of the first diagonal radars coincides (completely matches) with the transmission period of the second diagonal radars. However, in some embodiments, there is a time period when neither the first diagonal radars nor the second diagonal radars transmit the radio waves, as long as the transmission period of the first diagonal radars does not overlap with the transmission period of the second diagonal radars. In one instance, as show in  FIG. 8 , the delay time Tdelay may be set to a value (length) which is slightly longer than the transmission time Ton to provide the time period when neither the first diagonal radars nor the second diagonal radars transmit the radio waves. 
     In addition, in the present embodiment, the left BSM ECU  13 RL sends the synchronization flag F 1  at the same time when the left BSM ECU  13 RL sends the radio wave transmission request. However, in some embodiments, the left BSM ECU  13 RL sends the synchronization flag F 1  at any time determined based on (or associated with) the timing of transmitting the radio wave by the backward left radar  12 RL, which is controlled by the left BSM ECU  13 RL. For example, in some embodiments, the left BSM ECU  13 RL sends the synchronization flag F 1  when a predetermined margin time (specifically, a predetermined (fixed) margin time which is shorter than the delay time Tdelay) has elapsed since the left BSM ECU  13 RL sent the radio wave transmission request. In this case, the delay time Tdelay may be set to a value which is shorter than the transmission time Ton by the margin time. Similarly, in some embodiments, the left FSR ECU  13 FL sends the synchronization flag F 2  at any time determined based on (or associated with) the timing of transmitting the radio wave by the forward left radar  12 FL, which is controlled by the left FSR ECU  13 FL. 
     In addition, in the present embodiment, the left BSM ECU  13 RL, which controls the timing of transmitting the radio wave from one of the first diagonal radars, sends the synchronization flag F 1  to the left FSR ECU  13 FL and the right BSM ECU  13 RR, which control the timings of transmitting the radio waves from the second diagonal radars. However, in some embodiments, the right FSR ECU  13 FR, which controls the timing of transmitting the radio wave from the other of the first diagonal radars, sends the synchronization flag F 1  to the left FSR ECU  13 FL and the right BSM ECU  13 RR. Meanwhile, in the present embodiment, the left FSR ECU  13 FL, which controls the timing of transmitting the radio wave from one of the second diagonal radars, sends the synchronization flag F 2  to the left BSM ECU  13 RL and the right FSR ECU  13 FR, which control the timings of transmitting the radio waves from the first diagonal radars. However, in some embodiments, the right BSM ECU  13 RR, which controls the timing of transmitting the radio wave from the other of the second diagonal radars, sends the synchronization flag F 2  to the left BSM ECU  13 RL and the right FSR ECU  13 FR. In these cases, preferably, the synchronization flag F 1  and the synchronization flag F 2  are sent via a local bus which connects the right FSR ECU  13 FR and the right BSM ECU  13 RR. 
     Further, the vehicle surroundings monitoring apparatus  10  comprises a front radar which is disposed at the central front end of the vehicle body (for example, at the central position in the lateral direction of the front bumper) in addition to the radars  12 . In this case, the drive support ECU  20  may perform a collision avoidance control using at least the front radar. The collision avoidance control is a control for generating a warning to the driver and for generating the frictional brake force automatically as needed so as to avoid a collision with a tridimensional object detected using at least the front radar when the probability that the own vehicle collides with the tridimensional object becomes high. In this case, preferably, the front radar is configured to transmit a radio wave whose frequency is different from those of the radars  12  (so as to prevent the interference between the radio wave from the front radar and the radio waves radars other than the frond radar).