Abstract:
A radar device set on an automobile includes a transmitter for transmitting forward a beam of electromagnetic waves, a receiver for receiving reflected waves of the transmitted beam from a vehicle traveling in front, a measuring device for measuring a distance to the vehicle in front based on outputs from the receiver and a command outputting device for outputting a specified command signal when the distance measured by the measuring device is decreasing and reaches a threshold distance below which the measuring device becomes incapable of measuring the distance from the outputs from the receiver, and a beam adjusting device for changing either the elevation angle of the beam or its angular range of vision in response to the command signal.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a radar device to be carried on an automobile for measuring on real time the distance to a front-running vehicle. 
     The radar device carried on an automobile is a kind of so-called pulsed radar device for transmitting a pulsed beam of electromagnetic waves forward and measuring the distance to an object in front inclusive of a vehicle which may be accelerating, decelerating or even stationary (hereinafter referred to as the front-running vehicle) or its relative speed on the basis of the time it takes to receive its reflection. It now goes without saying that ordinary visible light and x-rays are examples of electromagnetic waves. 
     FIG. 10A is a conceptual diagram showing the principle of a prior art radar device  1  carried on an automobile. A pulsed beam  3  of electromagnetic waves transmitted from its signal transmitter (TX)  2  is reflected by a body surface  4  (or any reflective surface such as a back reflector) of a front-running vehicle and received by its signal receiver (RX)  5 . If the time between the transmission of the beam and the reception of the reflected beam is T as shown in FIG. 10B, the distance L to the front-running vehicle is given by cT/2 where c is the speed of light. The relative speed between the front-running vehicle and one&#39;s own vehicle carrying the radar device  1  can be calculated from the time-rate of change in the distance L between the two vehicles, or as the slope of the curve on the graph of L plotted against the time. If the change in L along the time-axis is zero, for example, this means that the relative speed is zero, or that the front-running vehicle is running at the same speed as one&#39;s own vehicle. If L is increasing with time, this means that the front-running vehicle is accelerating with respect to one&#39;s own vehicle, and if L is decreasing with time, this means that the front-running vehicle is decelerating with respect to one&#39;s own vehicle. 
     Since the radiative energy of transmitted electromagnetic waves generally decreases inversely proportional to the fourth power of distance, it must be sufficiently large in order to obtain a sufficiently intense reflected beam. Since the capability of the signal transmitter  2  is limited, the beam  3  is generally patterned in a narrowed form, say, with the angle θ of the range of vision equal to about 4°. Such a narrowed pattern is preferred also for the purpose of improving the directional resolution. FIG. 11A shows an example of narrowed beam pattern in the horizontal direction. FIG. 11B is an example of narrowed beam pattern in the vertical direction. FIG. 11C is an example of the cross-sectional shape of such a narrowed beam. Although an example of a narrowed beam with a nearly circular cross-sectional shape is illustrated, the angle θ of the range of vision need not be equal in the horizontal and vertical directions. Such a narrowed beam is generally transmitted at a specified elevation angle (the elevation angle shown in FIG. 11B being zero) while scanning in the horizontal direction within a specified range, as shown in FIG.  11 A. The range of the scanning may be determined such that the entire width of an automobile at a sufficiently large distance from one&#39;s own vehicle can be covered, that is, about 2.5 m-3 m at 10 m. 
     With a prior art radar device  1  thus structured, a beam  6  is transmitted from one&#39;s own vehicle  7  as shown in FIG.  12 A and if its reflection from the front-running vehicle  8  indicates that the distance between the two vehicles  7  and  8  is sufficiently large, one&#39;s own vehicle  7  may be accelerated to reduce the distance. If the distance in between is found to be too small to be safe, such as shown in FIG. 12B, one&#39;s own vehicle  7  may be braked so as to avoid a collision. In this manner, a so-called stop-and-go system for creeping forward in a traffic jam while maintaining a constant distance from the front-running vehicle may be realized. 
     With a prior art radar device  1  as described above, a front-running vehicle at a certain distance can be reliably kept visible because a narrowed beam  6  is made used of. There is a problem of suddenly losing sight of the front-running vehicle, however, when the distance between the vehicles is very short. FIGS. 13A and 13B show an example of such a situation where one&#39;s own vehicle  7  may be a sports car and is relatively low while the front-running vehicle may be a large freight truck having a back reflector attached at a relatively high position. Since the radar beam  6  is usually transmitted with an elevation angle of about zero degree and a range of vision of about 4°, the front-running vehicle  8  is safely visible as long as it is at a sufficiently large distance, as shown in FIG.  13 A. When the distance between the two vehicles  7  and  8  is small as shown in FIG. 13B, however, the beam  6  goes under the body of the front-running vehicle  8  without reaching its reflector at the back. 
     FIGS. 14A and 14B show another example of such a situation where one&#39;s own vehicle  7  may be a large freight truck while the front-running car  8  may be a sports car and is relatively low. When the distance between the two vehicles  7  and  8  is sufficiently large, the front-running vehicle  8  remains within the spreading range of vision of the beam  6 , as shown in FIG.  14 A. When the distance between the two vehicles  7  and  8  is small as shown in FIG. 14B, however, the beam  6  may pass above the highest reflective part of the front-running vehicle  8 , thereby inconveniently losing sight of it. 
     These two examples show that necessary data can be reliably obtained as long as the front-running vehicle  8  is sufficiently far away because its presence can be monitored by the radar device  1  on one&#39;s own vehicle  7  but the front-running vehicle  8  may be “lost” if the distance between the vehicles  7  and  8  suddenly decreases, say, because the front-running vehicle  8  has suddenly decelerated. From the point of view of safety requirement on such a device, the aforementioned problem is one that must be solved. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide an improved radar device to be carried on an automobile with which the problem of suddenly losing sight of the front-running vehicle can be solved. 
     A radar device embodying this invention, with which the above and other objects can be accomplished, may be characterized not only as comprising a transmitter for transmitting forward a beam of electromagnetic waves having a specified vertical angular range of vision at a specified elevation angle (the “specified initial elevation angle”), a receiver for receiving reflected waves of the transmitted beam from a vehicle traveling in front, and a measuring device for measuring a distance to the vehicle in front based on outputs from the receiver, but also as including a command outputting means for outputting a command signal when the distance measured by the measuring device is decreasing and reaches a certain threshold distance below which the measuring device becomes incapable of measuring the distance based on the outputs from the receiver and a beam adjusting means for changing either the elevation angle or the angular range of vision of the transmitted beam in response to this command signal. 
     In the above, the threshold distance is the distance at which the reflecting portions such as reflectors at the back of the vehicle in front come to be at a dead angle, not reachable by the beam which is directional, usually having a very small angular range of vision. This can happen most frequently where the difference in height between the transmitter of the beam and the reflector on the vehicle in front is great, and this threshold distance can be calculated from this height difference and the angular range of vision of the transmitted beam. 
     When the distance to the vehicle in front measured by the measuring device is decreasing and reaches this threshold distance, the radar device concludes that there is a high possibility that the vehicle in front which has been sending back the reflected beam is still in front although the reflected beam may cease to be received and outputs a specified command signal. The means for outputting this command signal is hereinafter referred to as the “command outputting means.” In response to this outputted command signal, the radar device causes a change in the emitted beam of radiation either by changing its elevation angle or its angular range of vision such that the reflected waves from the vehicle in front will continue to be received by the receiver and the measuring device will continue to calculate the distance to the vehicle in front. The mechanism for thus modifying the emitted beam is hereinafter referred to as the “beam adjusting means.” 
     If the beam adjusting means is for changing the angular range of vision of the beam to be emitted, the angular range of vision is changed so as to be increased such that the front-running vehicles can continue to reflect back the emitted waves. If the beam adjusting means is of the type for changing the elevation angle of the emitted beam, the elevation angle may be changed either upward or downward. The radar device includes a height setting unit for changing (say, manually) the height of the point of emission of the laser beam. If the emission point is at a higher of the settable positions, the beam adjusting means will function to change the elevation angle downward. If the emission point is at a lower of the settable positions, the beam adjusting means will function to change the elevation angle upward. In this manner, the beam direction can be shifted and the laser beam can go after the front-running vehicle which may have escaped into one of the blind angle regions. 
     It is preferable to also provide a means (hereinafter referred to as the “return signal outputting means”) for outputting a signal (hereinafter referred to as the “return signal”) after the command signal is outputted when the distance measured by the measuring means becomes greater than the aforementioned threshold distance (or when the outputs from the receiver increases and change from lower to higher than a threshold level below which the measurement device becomes incapable of measuring the distance therefrom to the front-running vehicle), and a returning means for causing the shifted elevation angle to return back to the specified initial angle or the vertical angular range of vision to return back to the specified angular range in response to the return signal. Thus, the elevation angle or the angular range of vision of the emitted beam of waves is returned to the original state when the distance to the front-running vehicle is restored to a safe range greater than the threshold distance, or when the receiver begins to receive the reflected waves from the front-running vehicle. 
     It is preferable, furthermore, that the aforementioned command outputting means will function to check whether the front-running vehicle is traveling in the same traffic lane as the automobile on which the radar device is installed and output the command signal only if it is ascertained that they are in the same lane. With such a command outputting means, the radar device can be useful even while the automobile is traveling on a multi-lane highway. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a conceptual diagram for showing the structure of a radar device embodying this invention. 
     FIG. 2 is a diagram for showing the structure of a horizontal scanner using a polygonal mirror instead of an ordinary reflecting mirror. 
     FIG. 3A is a block diagram of the driver circuit and FIG. 3B is a block diagram of the control unit of the radar device of FIG.  1 . 
     FIG. 4 is a flowchart of a control program which may be carried out by the CPU of the control unit of FIG.  3 B. 
     FIGS. 5A and 5B, together referred to as FIG. 5, are diagrams for showing the concept of the lost-sight distance. 
     FIGS. 6A and 6B are modified portions of the flowchart of FIG.  4 . 
     FIGS. 7A,  7 B and  7 C, together referred to as FIG. 7, are conceptual diagrams showing respectively the laser beam being made incident on a higher front-running vehicle, losing sight of it as the distance of separation becomes too short and having its elevation angle shifted upward. 
     FIGS. 8A,  8 B and  8 C, together referred to as FIG. 8, are conceptual diagrams showing respectively the laser beam being made incident on a lower front-running vehicle, losing sight of it as the distance of separation becomes too short and having its elevation angle shifted downward. 
     FIGS. 9A and 9B, together referred to as FIG. 9, are conceptual diagrams showing the angular range of vision of the emitted laser beam before and after it is expanded according to this invention. 
     FIGS. 10A and 10B are conceptual diagrams for explaining the principles of a prior art radar device carried on an automobile. 
     FIGS. 11A,  11 B and  11 C are respectively a schematic plan view, a side view and a sectional view of a beam of transmitted electromagnetic waves for showing its horizontal pattern, vertical pattern and its sectional shape. 
     FIGS. 12A and 12B are schematic conceptual side views of a radar beam catching sight of a front-running vehicle. 
     FIGS. 13A and 13B are conceptual diagrams for showing a situation where a prior art radar device loses sight of a front-running vehicle. 
     FIGS. 14A and 14B are conceptual diagrams for showing another situation where a prior art radar device loses sight of a front-running vehicle. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described next by way of an example. It is to be emphasized, however, that this example is intended to be illustrative and not as limiting the scope of the invention. Those of the components having a known structure or functioning in a known manner are not described in detail but this is only for the purpose of simplifying the description and not with the intention of excluding them from the invention. 
     FIG. 1 shows conceptually the structure of a radar device  10  embodying this invention which may be adapted to function as an independent device to be carried on an automobile or may be combined with another system such as a traffic jam control system and installed on any automobile or a truck. For the convenience of description, the radar device  10  will be explained below as being adapted to be used with a traffic control system (not shown) such that the distance and/or relative speed between vehicles measured by this radar device  10  are/is adapted to be used by the control system for controlling the speed of one&#39;s own vehicle while maintaining a specified distance from the front-running vehicle within an allowed speed limit. 
     As shown in FIG. 1, the radar device  10  comprises a signal light transmitter  20  for transmitting a laser beam BM TX  in a forward direction, a signal light receiver  30  for receiving a reflected laser beam BM RX  from a target object (such as a front-running vehicle) in front and a control unit  40  for controlling the operation of the radar device  10  as a whole. 
     Although FIG. 1 shows the transmitter  20 , the receiver  30  and the control unit  40  as separately contained units, this is not intended to limit the scope of the invention. They may all be contained within one common unit, or the transmitter  20  and the receiver  30  may be unitized and placed at a front part of the vehicle such as within its bumper or front grill while the control unit  40  may be placed somewhere else such as inside the engine room. 
     The transmitter  20  includes a light emitter  21 , a horizontal scanner  22  and a vertical scanner  23 . The light emitter  21  is provided with a laser diode  21   a , an optical system  21   b  and a case  21   c  for containing them. The laser diode  21   a  is, for example, for emitting an infrared laser beam. The optical system  21   b  is for collecting the laser light outputted from this laser diode  21   a  (usually with a spread angle of about 30°) and converting it into a laser beam BM TX  having a spread angle of about 0.1° in the horizontal direction and about 5° in the vertical direction. The horizontal scanner  22  is comprised of a reflecting mirror  22   a  for reflecting the laser beam BM TX  into the forward direction of the vehicle and a driving mechanism (in horizontal direction)  22   b  for driving this mirror  22   a  so as to cause it to undergo a swinging horizontal motion. The vertical scanner  23  is provided with another driving mechanism (in vertical direction)  23   b  connected to the horizontal driving mechanism  22   b  through a shaft  23   a  such that the sloping angle of the reflecting mirror  22   a  can be varied in the vertical direction by transmitting the swinging motion of this vertical driving mechanism  23   b  to the horizontal driving mechanism  22   b  through the shaft  23   a  and hence that the elevation angle EL of the laser beam BM TX  can be freely adjusted. 
     The optical system  21   b  need not be formed with a single lens. It may be formed with a lens group with a combination of a plurality of lenses. If the angular spread (or range of vision) of the laser light beam outputted from the laser diode  21   a  is sufficiently narrow, the optical system  21   b  may be dispensed with. Although FIG. 1 shows an example with the horizontal scanner  22  disposed behind the optical system  21   b , their positions may be interchanged, that is, the optical system  21   b  may be disposed behind the horizontal scanner  22 . 
     Although the reflecting mirror  22   a  of the horizontal scanner  22  is shown in FIG. 1 as one which may be produced by using a lustrous material such as aluminum or forming an aluminum membrane over a base material such as a plastic material and mirror-polishing its surface, use as a reflector may be made equally well of a polygonal mirror. FIG. 2 shows another example of the horizontal scanner  22  making use of a polygonal mirror  22   c  instead of the reflecting mirror  22   a  shown in FIG.  1 . This polygonal mirror  22   c  has an aluminum membrane formed on each of the six surfaces of a hexagonal columnar body with their surfaces mirror-polished. This polygonal mirror  22   c  is adapted to be driven also so as to swing both in the horizontal direction by means of the horizontal driving mechanism  22   b  and in the vertical direction by means of the vertical driving mechanism  23   b . For the convenience of description, the horizontal scanner  22  is hereinafter assumed to be structured with a reflecting mirror as shown in FIG.  1 . 
     The horizontal and vertical driving mechanisms  22   b  and  23   b  may be formed with a so-called galvano-motor which is an actuator comprised of a rotor of a magnetic material having S and N poles on opposite ends of its axial line and a plurality of excitation coils disposed around this rotor, containing a Hall element inside for detecting the rotary position. It can realize a swing motion with a maximum angular amplitude of about 70° with a superior linear characteristic if the currents for the excitation coils are controlled to feed back the angular position of the rotor. 
     The receiver  30  is comprised of a light receiving element  31  such as a photoelectric converter serving to receive the reflected laser beam BM RX  and to output a light reception signal S RX  with a magnitude corresponding to the received quantity of light, an optical system (not shown) for causing the reflected beam BM RX  to be efficiently made incident onto this light receiving element and a case  32  for containing them. 
     The control unit  40  is comprised of a driver circuit  41 , a control circuit  42 , a signal processor  43  and a height setting unit  44 . As shown in FIG. 3A, the driver circuit  41  includes a light emission driver  41   a , a horizontal driver  41   b , a vertical driver  41   c , a horizontal scan position signal input circuit  41   d  and a vertical scan position signal input circuit  41   e . The light emission driver  41   a  is for causing the laser diode  21   a  to emit light by supplying power PWRa according to a light emission control signal CONTa from the control circuit  42 . The horizontal driver  41   b  is for driving the horizontal driving mechanism  22   b  (galvano-motor) by supplying power PWRb according to a horizontal driving control signal CONTb from the control circuit  42 . The vertical driver  41   c  is for driving the vertical driving mechanism  23   b  (galvano-motor) by supplying power PWRc according to a vertical driving control signal CONTc from the control circuit  42 . The horizontal scan position signal input circuit  41   d  is for carrying out a conversion, if necessary, on a horizontal scan position detection signal S HOR  received from the angular position detector (or the aforementioned Hall element) of the horizontal driving mechanism  22   b  and inputting it into the control unit  42 . The vertical scan position signal input circuit  41   e  is for carrying out a conversion, if necessary, on a vertical scan position detection signal S VER  received from the angular position detector (or the aforementioned Hall element) of the vertical driving mechanism  23   b  and inputting it into the control circuit  42 . 
     Although not illustrated, the signal processor  43  includes an input circuit for removing noise components from the output signal from the light receiving element  31  (or the light reception signal S RX ), carrying out other necessary signal processing to convert it into a digital signal and inputting it into the control circuit  42 . The height setting unit  44  may be comprised of a dip switch with a plurality of bits such that the height of the position for the emission of the laser beam BM TX  of the radar device  10  (or the radar height H RDR ) by manually setting the on-off combination of the bits of this dip switch. 
     As shown in FIG. 3B, the control circuit  42  is comprised of an input/output (I/O) interface  42   a , a microcomputer unit (CPU)  42   b , a volatile data memory device (RAM)  42   c  and a read-only non-volatile data memory device (ROM)  42   d.    
     The radar height H RDR  from the height setting unit  44 , the light reception signal S RX  from the signal processor  43 , the horizontal and vertical scan position detection signals S HOR  and S VER  from the driver circuit  41 , as well as a speed signal S SP  from a speed sensor (not shown) are inputted to the I/O interface  42   a . The light emission control signal CONTa, the horizontal and vertical driving control signals CONTb and CONTc are outputted from the I/O interface  42   a  to the driver circuit  41 . A speed control signal CONT SP  and an alarm signal CONT ALARM  are also outputted from the I/O interface  42   a  to a trailing system or an alarm device (not shown). 
     The CPU  42   b  serves to load a control program preliminarily stored in the ROM  42   d  onto the RAM  42   c  and carry it out so as to take in necessary data such as H RDR , S RX , S HOR , S VER  and S SP  from the I/O interface  42   a  while carrying out necessary calculations for the control of the entire operations of the radar device  10 , outputting various control data such as CONTa, CONTb, CONTc, CONT SP  and CONT ALARM  which are obtained by such calculations to the driver circuit  41  and other circuits such as the trailing system and the alarm device through the I/O interface  42   a.    
     The control circuit  42  is an element for carrying out a specified control processing function by an intimate combination of hardware resources such as the CPU  42   b  and software resources such as the control program stored in the ROM  42   d . When a trail command to follow a front-running vehicle is received, the control circuit  42  drives the laser diode  21   a  to cause the laser beam BM TX  to be emitted while the vertical driving mechanism  23   b  is operated to set the elevation angle EL for the beam BM TX  to a specified angle such as 0° such that the laser beam BM TX  can reach a sufficiently distant target object over the road surface. Next, the control circuit  42  operates the horizontal driving mechanism  22   b  to swing the reflecting mirror  22   a  horizontally and to thereby scan the laser beam BM TX  in a horizontal direction. The range of this swinging motion should be such that the entire width of a vehicle sufficiently far from one&#39;s own vehicle can be covered such as a width of 2.5 m-3 m at 10 m. 
     Next, data on the reflected laser beam BM RX  received from the receiver  30  such as the presence or absence of reflected waves, the intensity of the reflected waves, the time interval between the transmission and reception of the beam and the directions of the beams (or the scan directions) are sequentially collected. Noise components are removed and a so-called grouping process is carried out to detect the presence or absence of a target object, its kind (or size) and its position and speed data (such as a speed of 70 km/h at 50 m). When the target object in front is an automobile, the reflectors on its back surface are likely to be observed as separate target objects. The grouping process is a process whereby these are treated together as signals associated with a single automobile. If it is determined that there is another vehicle traveling in front of one&#39;s own vehicle at a distance less than a specified length and at a speed which is slower than that of one&#39;s own vehicle, a control signal may be outputted to the control unit of the engine control system to brake one&#39;s own vehicle. If it is determined that there is another vehicle traveling in front at a distance greater than a specified length and at a speed greater than a specified rate or that there is no front-running vehicle at any distance less than a specified length, another signal may be outputted to the control unit to accelerate one&#39;s own vehicle within the limit of not exceeding a specified maximum speed, possibly followed by a control to run one&#39;s own vehicle at a specified constant speed thereafter. 
     From the point of view of safety and also for preventing the shortening of the useful lifetime of the device and in particular that of the laser diode  21   a , it is preferable to control the device such the trailing control will not be effected when one&#39;s vehicle is remaining stationary although a trailing command is then received. 
     When one&#39;s own vehicle is trailing a front-running vehicle as explained above by keeping the front-running vehicle in sight by means of the radar device  10 , the aforementioned problem of losing sight of the front-running vehicle may occur if the distance between the two vehicles becomes too short. When such a problem of lost sight takes place, the trailing system may conclude erroneously that there is no front-running vehicle in front and may effect a control whereby the speed of one&#39;s own vehicle is dangerously increased. It now goes without saying that such a control should be avoided because the control should be in the direction of increasing safety and one&#39;s own vehicle should never be accelerated whenever a front-running vehicle disappears from the sight. 
     FIG. 4 shows a flowchart of a control program to be carried out by the CPU  42   b  of the control circuit  42  according to the present invention in order to properly handle a situation of a disappearing front-running vehicle. 
     To start, the elevation angle of the laser beam BM TX  is set (Step S 11 ), say to 0°, such that the beam BM TX  can reach a sufficiently distant object, as explained above. Next, the starting angular position for the horizontal scanning of the laser beam BM TX  is set (say, at the center of the scanning range) (Step S 12 ). After two flags (Flag  1  and Flag  2  to be described in detail below) are reset (Steps S 13  and S 14 ), the emission of the laser beam BM TX  is started (Step S 15 ). 
     Next, it is determined whether a reflected laser beam BM RX  is received from a target object in front such as a front-running vehicle (Step S 16 ). If it is determined that there is a reflected laser beam BM RX  (YES in Step S 16 ), the distance to the front-running vehicle and its speed are calculated on the basis of the light reception signal S RX  from the signal processor  43  (Step S 17 ) and it is determined whether or not this distance is less than a minimum distance of safety (hereinafter also referred to as the “lost-sight distance”), or such a short distance that may cause the loss of sight of the front-running vehicle because one&#39;s own vehicle has approached it too closely (Step S 19 ). If it is determined in Step S 16  that the light reception signal S RX  is not received from the signal processor  43  (NO in Step S 16 ), this means that the level of the reflected laser beam BM RX  or the signal level of the light reception signal S RX  from the signal processor  43  has approached, reached or become below a lowest level required for calculating the distance to the front-running vehicle or its speed. 
     The theoretical lost-sight distance D may be broadly defined as the distance between one&#39;s own vehicle and a front-running vehicle at which, as this distance is being shortened, the reflecting part at the back of the front-running vehicle moves out of the range of vision of the laser beam from one&#39;s own vehicle. If the radar height is H RDR , the vertical angle of spread (or that of the range of vision) of the laser beam BM TX  is θ and the height of the reflector at the back of the front-running vehicle is H TGT , as shown in FIGS. 5A and 5B, the theoretical lost-sight distance D is given by Formula (1) shown below if the reflector disappears from the upper edge of the beam as shown in FIG.  5 A and by Formula (2) shown below if the reflector disappears from the lower edge of the beam as shown in FIG.  5 B: 
     
       
           D =( H   TGT   −H   RDR )/tan θ +   Formula (1)  
       
     
     
       
           D =( H   TGT   −H   RDR )/tan θ −   Formula (2)  
       
     
     where θ +  and θ −  are respectively the portion of the angle of spread θ above and blow the horizontal direction, as shown in FIGS. 5A and 5B. If H RDR =0.5 m, H TGT =1.4 m and θ + =θ − =2°, as a practical example, Formulas (1) and (2) both give D=25.8 m. According to this invention, a margin α is added to the aforementioned theoretical lost-sight distance D to define a “distance with a possibility of losing sight,” taking the change in the orientation of the target object into account. 
     If it is determined in Step S 19  that the distance between the two vehicles calculated in Step S 17  is not less than D+α (NO in Step S 19 ), the radar device  10  concludes that the front-running vehicle is safely in sight of the laser beam BM TX  and the emission of the laser beam BM TX  is continued as before, by returning to Step S 15  after resetting Flag  1  (Step S 20 ) and setting the elevation angle equal to the initially set value (Step S 21 ). 
     As the distance between the two vehicles becomes shorter and it is finally determined that the distance of separation has become less than D+α (YES in Step S 119 ), Flag  1  is set (Step S 22 ) to indicate that there is a possibility for the laser beam BM TX  to be losing sight of the front-running vehicle. The laser beam BM TX  continues to be emitted in the same manner (Step S 15 ) but when it is thereafter determined that there is no reflected laser beam BM RX  (NO in Step S 16 ), it is concluded that the situation is either as shown in FIG. 5A or  5 B, not that it was because there is no vehicle in front. 
     Explained more in detail, it is checked whether Flag  1  is set or reset (Step S 23 ) when the response in Step S 16  is NO. Since Flag  1  is in the set condition (YES in Step S 23 ) at this moment, Flag  2  is set (Step S 24 ), the value of radar height H RAD  set by the height setting unit  44  is retrieved and it is examined whether the situation is as depicted in FIG. 5A or in FIG. 5B, that is, whether the reflected beam BM RX  ceased to be detected because the emitted beam BM TX  was too high or too low (Step S 25 ). 
     If it is determined that the situation was as shown in FIG. 5B, or that the radar height H RAD  was too high (YES in Step S 25 ), the elevation angle EL of the laser beam BM TX  is lowered (Step S 26 ). If it is determined that the situation was as shown in FIG. 5A, or that the radar height was too low (NO in Step S 25 ), the elevation angle EL of the laser beam BM TX  is raised (Step S 27 ). Thereafter, the processes from Step S 15  are repeated. According to the program shown by the flowchart of FIG. 4, therefore, the elevation angle of the laser beam BM TX  is shifted upward or downward vertically when the reflected laser beam BM RX  ceases to be received when the distance between the two vehicles becomes less than the lost-sight distance, or D+α. 
     There are situations where the front-running vehicle may be changing lanes. In this situation, too, the reflected laser beam BM RX  may cease to be received by the receiver, but this situation should be distinguished from the aforementioned situation because the front-running vehicle changing its lanes means that there is no longer this obstacle in front of one&#39;s own vehicle. Thus, the program shown in FIG. 4 may include the step of determining whether or not the front-running vehicle is traveling in the same traffic lane as one&#39;s own vehicle (Step S 18 ). If it is determined that the front-running vehicle is in the same lane as one&#39;s own vehicle (YES in Step S 18 ), the program directly proceeds to aforementioned Step S 19 . If it is determined that the front-running vehicle is not in the same lane (NO in Step S 18 ), Flag  1  is reset (Step S 20 ) and the elevation angle of the laser beam BM TX  is set equal to the initial value (Step S 21 ). 
     The technology for determining whether or not a front-running vehicle is traveling in the same lane with one&#39;s own vehicle is well known. This may be done, for example, from the time rate of change in the distance between the two vehicles and the direction of the front-running vehicle with respect to the direction of motion of one&#39;s own vehicle. Such a technology may be directly utilized in Step S 18 . When the road is curving, technologies for detecting its radius of curvature are also known. This may be done, for example, from the steering of one&#39;s own vehicle. If these technologies are combined in Step S 18 , a front-running vehicle apparently moving sideways as it comes to a curving road will not be considered erroneously as moving over to a different lane. 
     With reference still to the flowchart of FIG. 4, the process for bringing the elevation angle of the laser beam BM TX  back to its initially set value will be explained more in detail. When the laser beam BM TX  is shifted either upward or downward from the initially set direction, both Flag  1  and Flag  2  are in a set condition. If the distance to the front-running vehicle keeps increasing while the flags remain set and becomes greater than the lost-sight distance D+α, the response in Step S 119  becomes NO and this causes Flag  1  to be reset (Step S 20 ) and the elevation angle of the laser beam BM TX  to be returned to its initial value (Step S 21 ). The processes subsequent to Step S 15  are then repeated. During this repetition, since the front-running vehicle is farther in front than by D+α, the reflected laser beam BM RX  is reliably received and the loop of Steps S 15 , S 16 , S 17 , S 18 , S 19  and S 22  is repeated while the front-running vehicle remains in sight. If the front-running vehicle moved to another lane (NO in Step S 18 ), the program goes off this loop. 
     A portion of the flowchart of FIG. 4 may be modified as shown in FIG.  6 A. According to this modified program, when both Flag  1  and Flag  2  are in a set condition (that is, when the elevation angle has been shifted upward or downward), if the reflected laser beam BM RX  cannot be received, it is NO in Step S 16  and YES in Step S 23  but it is examined thereafter whether Flag  2  is set or reset (Step S 30 ) and, if it is found to be set (YES in Step S 30 ), Flag  2  is reset (Step S 28 ) and the elevation angle of the laser beam BM TM  is returned to its original value (Step S 29 ) before repeating the processes after Step S 15 . 
     In this repetition cycle, the elevation angle is returned to its initial value and Steps S 15  and S 16  are carried out. If the reflected laser beam BM RX  is received (YES in Step S 16 ), the loop of Steps S 17 , S 18 , S 19  and S 22  is repeated. If the reflected laser beam BM RX  is not received (NO in Step S 16 ), Steps S 23 , S 30 , S 24 , S 25  and S 26  (or S 27 ) can be carried out. Thus, it is possible to wait until the front-running vehicle is recaptured (or the reflected laser beam BM RX  is received) while returning the elevation angle to its initial value or shifting it in the upward or downward direction. 
     A portion of the flowchart of FIG. 4 may also be modified as shown in FIG.  6 B. This modified program assumes a timer which monitors the time elapsed after the elevation angle is shifted. If it is YES in Step S 23  of this program, this timer is check to ascertain whether or not a certain preliminarily set time has elapsed since the elevation angle was shifted (Step S 31 ). If the set time has not elapsed yet (NO in Step S 31 ), Flag  2  is set (Step S 24 ) as in the program shown in FIG.  4 . If the set time is found to have elapsed (YES in Step S 31 ), Flag  1  and Flag  2  are reset (Steps S 32  and S 33 ), the elevation angle is returned to its initial direction (Step S 34 ) and the process for the emission of laser beam BM TX  (Step S 15 ) is continued. 
     With a device thus programmed, after the elevation angle of the laser beam BM TX  is shifted upward or downward because the front-running vehicle has disappeared from the range of vision because the distance between the two vehicles has become too short, the elevation angle is immediately and automatically returned to the initial direction if the reflected beam BM RX  is not received after the elapse of the set time interval. 
     As explained above, the present invention has many advantages, in addition to being capable of overcoming the problem of losing sight of the front-running vehicle when the distance of separation becomes too short. Since it uses Formula (1) or (2) to calculate the lost-sight distance D and concludes that the problem has occurred after ascertaining that the front-running vehicle is in the same traffic lane as one&#39;s own vehicle, it can reliably determine whether the absence of the reflected laser beam BM RX  is due to the short distance between the two vehicles or because the front-running vehicle has moved to another lane. Since the radar height H RDR  can be adjusted by the height setting unit  44  of the control unit  40 , the elevation angle can be shifted in a proper direction independent of the type of vehicle on which the radar device  10  is installed. In the case of a smaller vehicle such as a sports car, as shown in FIG. 7, the elevation angle of the laser beam BM TX  is shifted upward as the distance to a front-running taller vehicle such as a freight truck becomes shorter than the lost-sight distance such that the problem of blind angles above the laser beam BM TX  as depicted in FIG. 5A can be obviated. In the case of a taller vehicle, as shown in FIG. 8, the elevation angle of its laser beam BM TX  is shifted downward as the distance to a front-running smaller vehicle becomes shorter than the lost-sight distance such that the problem of blind angles below the laser beam BM TX  as depicted in FIG. 5B can be obviated. 
     The basic idea of the present invention is not limited to the shifting of the elevation angle of the emitted laser beam BM TX . As shown in FIG. 9, the angle θ of the range of vision of (or the angle subtended by) the laser beam BM TX  may be adapted to be expanded. FIG. 9A shows the laser beam BM TX  under a normal condition when the distance to the front-running vehicle is greater than the lost-sight distance D. Its elevation angle is shown as being 0° and the angle θ is kept at its initial value, say 4°, as shown in FIG.  9 A. When the distance of separation between the two vehicles becomes less than the lost-sight distance D, the angle θ is expanded to a specified larger angle, say, about 10°, as shown in FIG.  9 B. This embodiment is advantageous in that the regions of dead angles both above and below the laser beam BM TX  as shown in FIGS. 5A and 5B can be eliminated simultaneously and hence in that Step  25  in the flowchart of FIG. 4 becomes unnecessary. The angle θ can be made variable by improvising the optical system  21   b  of FIG. 1 in a manner familiar to a person skilled in the art, for example, by making the focal length of the lens of the optical system  21   b  variable.