Abstract:
Embodiments of the invention provide apparatus ( 100 ) for detecting an erroneous measurement of a range of a target object  20  from a subject object ( 5 ) comprising: wireless transmission means ( 111 ) for transmitting a signal ( 111 S) having a first frequency from the subject object ( 5 ) to the target object ( 20 ); detector means ( 113 ) for detecting a portion of the signal ( 113 S) reflected from the target object ( 20 ) back to the subject object ( 5 ); range determination means ( 131 ) for determining the range ( 202 ) of the target object ( 20 ) from the subject object ( 5 ) by reference to a time of flight of said portion of the signal ( 111 S), ( 113 S) from the transmission means ( 111 ) to the detector means ( 113 ); and rate determination means ( 131 ) for determining the rate of change of the range ( 203 ) by reference to a difference between the first frequency and an apparent frequency of the reflected portion of the signal ( 113 S) detected by the detector means ( 113 ), the apparatus ( 100 ) being arranged to provide an indication ( 206 ) that an erroneous measurement of range has been made if the range ( 202 ) determined by the range determination means ( 131 ) increases whilst the rate of change of range ( 203 ) determined by the rate determination means ( 131 ) is negative.

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates to apparatus and a method for determining a range of one object from another object. In particular but not exclusively the disclosure relates to a method and apparatus capable of detecting a correct value of the range of one object from the other object in circumstances where known devices may provide an erroneous range value. 
       BACKGROUND 
       [0002]    It is known to provide apparatus for determining the range of one object such as a motor vehicle from another object such as another motor vehicle or a fixed object such as a wall of a building. One form of known apparatus employs ultrasonic radiation to determine the range whilst another form of apparatus employs electromagnetic radiation using a technique known as RADAR (RAdio Detection And Ranging). 
         [0003]    In one arrangement a source of ultrasonic radiation is mounted to a bumper (or fender) of a vehicle together with a corresponding detector. The source is arranged to transmit an ultrasonic radiation signal away from the vehicle, the detector being arranged to detect a portion of the ultrasonic radiation signal reflected by an object back towards the vehicle. A controller is arranged to determine a length of time between transmission of the signal and detection of the reflected signal. 
         [0004]    It is to be understood that knowledge of the speed at which the signal travels between the vehicle and the reflecting object may be used to determine a distance of the vehicle from the object. In some arrangements a controller may be configured to sound an alarm or apply a brake of the vehicle in the event that a distance of the vehicle from the object becomes less than a prescribed value. 
         [0005]    The present inventors have recognized that a problem exists with known apparatus in that erroneous readings of range can be given in certain situations. This is because the position on the object from which the transmitted signal is reflected can change as the vehicle approaches the object. 
         [0006]      FIG. 1  illustrates a scenario in which such a change in the position of reflection occurs. In  FIG. 1(   a ) a motor car  5  is shown approaching a lorry  20 . A transducer device  10  arranged to transmit an ultrasonic radiation signal  11  ahead of the car  5  is installed in a front bumper  7  of the car  5 . The device  10  is also arranged to detect a reflected portion  13  of the transmitted signal  11 . 
         [0007]    It can be seen from  FIG. 1(   a ) that when the car  5  is relatively far from the lorry  20  the signal is reflected from a rear door surface  22  of the lorry  20 . However, as shown in  FIG. 1(   b ), when the car  5  is relatively close to the lorry  20  the signal is no longer reflected from the rear surface  22  but from a rear axle  24  of the lorry  20 . The rear axle  24  is further from the car  5  than the rear door surface  22  by a distance d, introducing an error into the measured distance of the car  5  from the lorry  20  corresponding to the distance d. Thus, in the situation illustrated in  FIG. 1(   b ) a controller of the car  5  may fail to warn a driver of the car  5  that a distance between the car  5  and the lorry  20  is dangerously low and that a risk of collision exists. 
         [0008]    It is an aim of embodiments of the present invention to at least partially mitigate the disadvantages of known range determination apparatus. Other aims and advantages of the invention will become apparent from the following description, claims and drawings. 
       SUMMARY 
       [0009]    According to an example embodiment, there is provided an apparatus for detecting an erroneous measurement of a range of a target object from a subject object comprising:
       wireless transmission means for transmitting a signal having a first frequency from the subject object to the target object;   detector means for detecting a portion of the signal reflected from the target object back to the subject object;   range determination means for determining the range of the target object from the subject object by reference to a time of flight of said portion of the signal from the transmission means to the detector means; and   rate determination means for determining the rate of change of the range by reference to a difference between the first frequency and an apparent frequency of the reflected portion of the signal detected by the detector means,   the apparatus being arranged to estimate that an erroneous measurement of range has been made in by means of a comparison between the range determined by the range determination means and the rate of change of range determined by the rate determination means.       
 
         [0015]    In an embodiment, the range determination means may be arranged to determine the range of the target object from the subject object by reference to a time of flight of the portion of the signal from the transmission means to the detector means. 
         [0016]    In an embodiment, the rate determination means may be arranged to determine the rate of change of the range by reference to a difference between the first frequency and an apparent frequency of the reflected portion of the signal detected by the detector means. 
         [0017]    In an embodiment, the apparatus is arranged to provide an indication that an erroneous measurement of range has been made if the range determined by the range determination means increases whilst the rate of change of range determined by the rate determination means is negative. 
         [0018]    It is to be understood that the apparent change in frequency of the reflected radiation signal relative to the transmitted signal as determined by the apparatus is due to the well-known Doppler effect. The change in frequency is independent of the position on the object from which the transmitted signal is reflected and may therefore serve as a reliable indicator that an erroneous range measurement is being made. 
         [0019]    The apparatus may be arranged to provide an indication that an erroneous determination of range has been made only if the further requirement is met that the range determination means determines that the measured range of the target object represents an increase over a preceding measurement each time a measurement of range has been made for the previous n measurements where n is an integer greater than or equal to 1. 
         [0020]    The preceding measurement may be an immediately preceding measurement. 
         [0021]    The apparatus may be further arranged to provide an indication that an erroneous determination of range has been made only if further requirement is met that the rate determination means determines that the rate of change of range of the target object has been negative each time a measurement of range has been made for the previous m measurements where m is an integer greater than or equal to 1. 
         [0022]    In one example m=n. 
         [0023]    The indication that an erroneous measurement of range has been made is preferably provided in the form of an alert to a user of the apparatus. 
         [0024]    The alert may be provided by one selected from amongst an audible alert, a visual alert and a physical alert. 
         [0025]    The apparatus may further comprise range correction means arranged to correct the value of range determined by the range determination means when an erroneous measurement of range is detected based on at least one selected from amongst the rate of change of range determined by the rate determination means and the range determined by the range determination means. 
         [0026]    The range correction means may be arranged to correct the value of range based on an integral of values of the rate of change of range determined by the rate determination means. 
         [0027]    The range correction means may be arranged to correct a last known correct value of range by adding the integral of the values of the rate of change to the last known correct value of range. 
         [0028]    The range correction means may be arranged to correct the value of range based on an integral of values of the rate of change of range determined by the rate determination means. 
         [0029]    The wireless transmission means may comprise means for transmitting electromagnetic radiation and the detector means comprises means for detecting electromagnetic radiation. 
         [0030]    The wireless transmission means may comprise a radar transmitter. 
         [0031]    Alternatively or in addition the wireless transmission means may comprise means for transmitting sonic radiation and the detector means may comprise means for detecting sonic radiation. 
         [0032]    Alternatively or in addition the wireless transmission means may comprise means for transmitting ultrasonic radiation and the detector means may comprise means for detecting ultrasonic radiation. 
         [0033]    According to an example embodiment, there is provided a motor vehicle having apparatus as claimed in any preceding claim installed therein. 
         [0034]    The apparatus may be arranged to trigger one selected from amongst an audible alert, a visual alert, a physical alert and application of a brake when the range of the target object from the subject object being the motor vehicle falls below a prescribed threshold. 
         [0035]    According to an example embodiment, there is provided a method of detecting an erroneous measurement of a range of a target object from a subject object comprising:
       transmitting a signal having a first frequency from the subject object to the target object;   detecting a portion of the signal reflected from the target object back to the subject object;   determining a range of the target object from the subject object; determining the rate of change of the range; and   providing an indication that an erroneous measurement of range has been made if the determined range increases whilst the determined rate of change of range is negative.       
 
         [0040]    In an embodiment, the method comprises:
       transmitting by means of a transmitter a signal having a first frequency from the subject object to the target object;   detecting by means of a detector a portion of the signal reflected from the target object back to the subject object;   determining by range determination means a range of the target object from the subject object, for example by reference to a time of flight of said portion of the signal from the transmission means to the detector means; and   determining by rate determination means the rate of change of the range, for example by reference to a difference between the first frequency and the apparent frequency of the reflected portion of the signal detected by the detector means; and   providing an indication that an erroneous measurement of range has been made if the determined range increases whilst the determined rate of change of range is negative.       
 
         [0046]    The step of providing an indication that an erroneous determination of range has been made may further comprise providing said indication only if the range determination means determines that the measured range of the target object represents an increase over a preceding measurement each time a measurement of range has been made for the previous n measurements where n is an integer greater than or equal to  1  whilst the determined rate of change of range is negative. 
         [0047]    The preceding measurement may be an immediately preceding measurement. 
         [0048]    The step of providing an indication that an erroneous determination of range has been made may further comprise providing said indication only if the further requirement is met that the rate determination means determines that the rate of change of range of the target object has been negative each time a measurement of range has been made for the previous m measurements where m is an integer greater than or equal to 1. 
         [0049]    The step of providing an indication that an erroneous measurement of range has been made may comprise providing an alert to a user of the apparatus. 
         [0050]    The step of providing an alert may comprise the step of providing one selected from amongst an audible alert, a visual alert and a physical alert. 
         [0051]    The method may further comprise the step of providing a corrected value of range when it is detected that an erroneous measurement of range has been made based on at least one selected from amongst the rate of change of range determined by the rate determination means and the range determined by the range determination means. 
         [0052]    The step of providing a corrected value of range may comprise the step of correcting the value of range based on an integral of values of the rate of change of range determined by the rate determination means. 
         [0053]    The step of providing a corrected value of range may comprise the step of correcting a last known correct value of range by adding the integral of the values of the rate of change of range to the last known correct value of range. 
         [0054]    The signal transmitted by the transmitter may be an electromagnetic signal. 
         [0055]    The signal transmitted by the transmitter may be a radar signal. 
         [0056]    Alternatively or in addition the signal transmitted by the transmitter may be a sonic radiation signal. 
         [0057]    Alternatively or in addition the signal transmitted by the transmitter may be an ultrasonic radiation signal. 
         [0058]    According to an example embodiment, there is provided a method of detecting an erroneous measurement of a range of a target object from a subject object comprising:
       transmitting by means of a transmitter a signal having a first frequency from the subject object to the target object;   detecting at the subject object by means of a detector a portion of the signal reflected from the target object back to the subject object;   determining a range of the target object from the subject object by reference to a time of flight of said portion of the signal from the transmission means to the detector means; and   determining the rate of change of the range by reference to a difference between the first frequency and the apparent frequency of the reflected portion of the signal detected by the detector means; and   providing an indication that an erroneous measurement of range has been made if the determined range increases whilst the determined rate of change of range is negative.       
 
         [0064]    Within the scope of this application it is envisaged that the various aspects, embodiments, examples, features and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings may be taken independently or in any combination thereof, except where there is incompatibility of features. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0065]    Example embodiments will now be described, by way of example only, with reference to the accompanying figures in which: 
           [0066]      FIG. 1  is a schematic illustration of a trajectory of a beam of ultrasonic radiation transmitted from a motor car as the car approaches a lorry showing (a) reflection of the beam from a rear door of the lorry when the car is a relatively large distance from the lorry and (b) reflection of the beam from a rear axle of the lorry when the car is a relatively short distance from the lorry; 
           [0067]      FIG. 2  is a schematic illustration of a trajectory of a beam of electromagnetic radiation transmitted from a motor car according to an embodiment of the invention showing reflection of the beam from a rear axle of the lorry when the car is a relatively short distance from the lorry; 
           [0068]      FIG. 3  illustrates the functionality of a control module of the apparatus; 
           [0069]      FIG. 4  illustrates the functionality of an Identify Problem Target function block of the control module; 
           [0070]      FIG. 5  illustrates the functionality of a Compensated Range function block of the control module; 
           [0071]      FIG. 6  is a plot of distance of a car from a particular target (a lorry) as a function of time as determined by reference to a time of flight of a radar signal travelling from the car to the lorry and back to the car (trace A); and rate of change of distance of the car from the lorry as a function of time as determined by measuring at the car a difference in frequency between the radar signal transmitted from the car and the radar signal reflected from the lorry back to the car (trace B); and 
           [0072]      FIG. 7  shows the plot of  FIG. 6  with a corrected value of range plotted thereon for a period during which the target was identified as a problem target. 
       
    
    
     DETAILED DESCRIPTION 
       [0073]    In at least one disclosed embodiment apparatus is provided that is configured to determine a range of one object from another object and to determine whether the determined value is an erroneous value. If the apparatus determines that the determined value is an erroneous value the apparatus is configured to correct the determined value. 
         [0074]      FIG. 2  shows apparatus  100  according to an embodiment installed in a motor car  5 . The apparatus has a radar transmitter  111  and a corresponding receiver  113 . The transmitter  111  is arranged to emit a radar (electromagnetic radiation) signal  111 S in a direction away from the car  5  whilst the receiver  113  is arranged to detect a portion  113 S of the signal  111 S reflected by an object  20  back towards the car  5 . In the situation illustrated in  FIG. 2  the signal  113 S has been reflected by a rear axle  24  of a lorry  20 . 
         [0075]    In some embodiments the radar signal  111 S is in the form of an intermittent (pulsed) signal. In some alternative embodiments the signal  111 S is a modulated continuous wave signal. In some embodiments the pulsed signal comprises a modulated continuous wave signal. 
         [0076]    The apparatus  100  has a controller  130  having a detector module  131  and a control module  133 . The detector module  131  is arranged to determine the range of a reflecting (target) object  24  from the apparatus  100  at a given moment in time and the rate of change of the range. 
         [0077]    The detector module  131  is configured to determine the range based on a time of flight measurement of the portion  113 S of the signal  111 S emitted by the transmitter  111  and received by the receiver  113  following reflection from the target object  24 . A reflecting target object  24  may be referred to herein as a ‘target’  24 . 
         [0078]    It is to be understood that a radar signal emitted by the transmitter  111  may be reflected by a plurality of objects  24 . The detector module  131  is arranged to identify each of the objects as discrete targets  24  and to track the positions of each of these targets  24  as a function of time. In the present embodiment the detector module  131  is arranged to assign a unique identification code to each target  24 . 
         [0079]    The detector module  131  is further configured to determine a rate of change of the range of each target  24  from the apparatus  100  by measuring a difference between the frequency of the signal  111 S emitted by the transmitter  111  and the apparent frequency of the reflected signal  113 S detected by the receiver  113 . 
         [0080]    The apparent difference in frequency between these signals  111 S,  113 S is due to the well-known Doppler effect. If range of the target  24  from the vehicle  5  is increasing the reflected signal will have an apparent frequency that is less than that of the transmitted signal  111 S. Conversely, if the range of the target  24  from the vehicle  5  is decreasing the reflected signal will have an apparent frequency that is greater than that of the transmitted signal  111 S. It is to be understood that measurement of range and rate of change of range as described above are well known in the art. 
         [0081]    The detector module  131  is arranged to output data in respect of each target  24  identified by the detector module  131  to a control module  133 . The data includes: (i) the unique identifier assigned to each target  24  by the detector module  131 , being given by a parameter ‘Target Id’  201 ; the range of each target  24  from the apparatus  100 , being given by a parameter ‘Range’  202 ; and the rate of change of the range, being given by a parameter ‘Range rate’  203 . 
         [0082]      FIG. 3  is a schematic diagram showing the functionality of the control module  133 . As shown, values of the parameters Target Id  201 , Range  202  and Range rate  203  are input to a function block  205  referred to herein as an ‘Identify Problem Target’ function block  205 . It is to be understood that this function block and other function blocks described herein may be implemented in software, firmware and/or hardware. In the present embodiment the functionality of the control module  133  is implemented in software by a microprocessor system. 
         [0083]    The Identify Problem Target function block  205  is arranged to identify a target  24  as a problem target if the value of the parameter Range  202  increases whilst the value of the parameter Range rate  203  is negative. It is to be understood that a situation in which the value of Range  202  increases whilst the value of Range rate  203  is negative is not rational. This is because if the value of the parameter Range  202  is increasing it is indicative of a situation in which the value of the parameter Range rate  203  should be positive. This is because the range of the target  24  from the apparatus  100  is increasing and not decreasing according to the detected change in the value of parameter Range  202 . 
         [0084]    In this embodiment the smallest increase in the parameter Range  202  identifiable by the Identify Target function block  205  is 0.1 m, however this can be altered by a calibrateable constant. 
         [0085]    If such a situation occurs, the Identify Problem Target function block  205  sets a ‘Problem Target’ flag  206  associated with the Target Id parameter  201  to a value of 1. Thus, the apparatus assumes that the discrepancy between the change in the value of the parameter Range  202  and the value of the parameter Range rate  203  are due to a change in position on the target object from which the radar signal is being reflected. 
         [0086]    If the above conditions are not met, and a target is not identified as a problem target, the Problem Target flag  206  remains set to a value of zero. 
         [0087]    The value of the Problem Target flag  206  is provided as an input to a Compensate Range function block  210  of the apparatus (see also  FIG. 3 ). The Compensate Range function block  210  is arranged to determine a true value of the range of the apparatus  100  from the target  24  by correcting the measured value of range based on the value of the parameters Range  202  and Range rate  203 . Thus, the values of the parameters Range  202  and Range rate  203  are also provided as inputs to the Compensate Range function block  210  as illustrated in  FIG. 3 . 
         [0088]    The functionality of the Identify Problem Target function block  205  is illustrated in  FIG. 4 . It can be seen that the value of the parameter Range  202  is input to a function ‘Sample Range’  202 S arranged to sample successive values of the parameter Range  202 . The Sample Range function  202 S is arranged to determine whether the value of the parameter Range  202  has increased each time a value has been sampled for the n most recent values sampled. 
         [0089]    If the value of the parameter Range  202  has increased each time, the Sample Range function  202 S sets a flag ‘Range Inc’  202 I to a value of 1 indicating that the value of the parameter Range  202  has increased. If the value of the parameter Range  202  has not increased each time the Sample Range function  202 S sets the flag Range Inc  202 I to zero. 
         [0090]    In the present embodiment n=3 although other values of n are also useful. For example in some embodiments n=1, whilst in some other embodiments n may be 2, 4, 5, 6 or any other integer value. 
         [0091]    The value of the parameter Range rate  203  is input to a ‘Sample Range Rate’ function  203 S arranged to sample successive values of the parameter Range rate  203 . The Sample Range rate function  203 S is arranged to determine whether the value of the parameter Range rate  203  has been negative each time a value has been sampled for the n most recent values sampled. 
         [0092]    If the value of the parameter Range rate  203  has been negative each time, the Sample Range rate function  203 S sets a flag ‘Range rate negative’  203 N to a value of 1. If the value of the parameter Range rate  203  has not been negative each time, the Sample Range rate function  203 S sets the flag Range rate negative  203 N to zero. 
         [0093]    Thus it is to be understood that, for n=1, the flag Range Inc  202 I is set to a value of 1 if the most recent value of the parameter Range  202  has increased since the previous value was sampled. The flag Range rate negative  203 N is set to 1 if the most recently sampled value of Range rate  203  is negative. 
         [0094]    In the case n=2, the flag Range Inc  202 I is set to a value of 1 if the values of each of the two most recently sampled values of the parameter Range  202  represent an increase over their respective immediately preceding values. The flag Range rate negative  203 N is set to 1 if the two most recently sampled values of Range rate  203  are negative. 
         [0095]    Table 1 shows five successive values of the parameter Range  202  and Range rate  203  up to and including the most recently sampled values, sample number s. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Sample number 
                 s − 4 
                 s − 3 
                 s − 2 
                 s − 1 
                 s 
                 s + 1 
                 s + 2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Range (m) 
                 5 
                 5.2 
                 5.1 
                 5.6 
                 5.8 
                   
                   
               
               
                 Range Rate (m/s) 
                 −10.1 
                 −10.2 
                 −10.3 
                 −10.2 
                 −10.1 
               
               
                   
               
             
          
         
       
     
         [0096]    It is to be understood that for the case n=1, the flag Range Inc  202 I is set to a value of 1 since the most recently sampled value of the parameter Range  202  (5.8 m, at time s) represents an increase over the immediately preceding value (5.6 m, at time s−1). 
         [0097]    The flag Range rate negative  203 N is also set to a value of 1 since the most recently sampled value of Range rate  203  is negative (−10.1 ms −1  at time s). 
         [0098]    For the case n=2, the flag Range Inc  202 I is also set to a value of 1 because (1) the most recently sampled value of the parameter Range  202  (5.8 m at time s) represents an increase over the immediately preceding value (5.6 m at time s−1) and (2) the second most recently sampled value (5.6 m at time s−1) also represents an increase over its immediately preceding value (5.1 m at time s−2). 
         [0099]    The flag Range rate negative  203 I is set to a value of 1 since the two most recently sampled values of Range rate  203  are both negative (−10.2 ms 1  at time s−1 and −10.1 ms at time s). 
         [0100]    For the case n=3, the flag Range Inc  202 I is set to zero because the value of the parameter Range  202  at time s−2 (5.1 m) was less than the value at time s−3 (5.2 m). Thus the value of the parameter Range  202  decreased between time s−3 and time s−2 (by 0.1 m) and the conditions for Range Inc  202 I to be set to a value of 1 are therefore not met. 
         [0101]    The flag Range rate negative  203 N is set to a value of 1 because the three most recently sampled values of Range rate  202  are each negative (−10.3 ms −1 , −10.2 ms −1 and  10.1 ms −1 ). 
         [0102]    The Control Identify Problem Target function  215  compares the values of the flags Range Inc  202 I and Range rate neg  203 N. If the Range Inc flag  202 I and the Range rate neg flag  203 N are both set to a value of 1, the Control Identify Problem Target function  215  sets the Problem Target flag  206  to a value of 1. 
         [0103]    In the event that the Problem Target flag  206  transitions from not set to set (i.e. from zero to 1), the Control Identify Problem Target function  215  latches the most recent value of the parameter Base Range  211  that is known to be a reliable (i.e. correct) value in a Latch Base Range latch  211 L. The Latch Base Range latch  211 L provides an output of the value of the Base Range parameter  211  latched therein. The latched value of the Base Range parameter  211  is used by the control module  133  as a ‘last known correct value’ of the parameter Range  202  from which corrected values of range are subsequently determined by a Compensate Range function block  210 . 
         [0104]    The last known correct value of Range  202  is taken to be the value sampled before the nth most recent value was sampled. Thus, in the example of Table 1, for n=1 the last known correct value would be that at time s−1, i.e. 5.6 m. For n=2 the last known correct value would be that at time s−2, i.e. 5.1 m. For n=3 the last known correct value would be that at time s-3, i.e. 5.2 m. 
         [0105]    The functionality of the Compensate Range function block  210  is illustrated schematically in  FIG. 5 . From  FIG. 5  it can be seen that the latched value of Base Range  211  described above is input to a Control Compensate Range function  220  together with a value of each of the parameters Range  202  and Range rate  203  and the value of Problem Target flag  206 . 
         [0106]    The Control Compensate Range function  220  calculates a true value of the range of the apparatus  100  from the target  24  and outputs this value as a parameter Compensated Range  214 . In the present embodiment the Control Compensate Range function  220  calculates the value of parameter Compensated Range  214  according to the formula: 
         [0000]      Compensated Range=Base Range+(Range rate×t)
 
         [0107]    where t is the time period between successive samples of the parameter Range rate  203 . It is to be understood that the Control Compensate Range function  220  is arranged to add the current value of (Range rate  203 ×t) to the value of Base Range  211  each time an updated value of Range rate  203  is obtained (i.e. every t seconds). Thus, the Control Compensate Range function  220  is effectively arranged to integrate the value of Range rate over time and add this value to the value of Base Range  211 . 
         [0108]    Thus, it is to be understood that in some embodiments the Control Compensate Range function  220  calculates the value of the parameter Compensated Range  214  by integrating the value of the parameter Range Rate  203  in order to obtain a change in range over this period. 
         [0109]    In this manner the Compensated Range  214  is calculated by integrating the value of the parameter Range Rate  203  over time and removing this from the Base Range  211 . 
         [0110]    In some alternative embodiments the Control Compensate Range function  220  initially calculates a parameter Offset Range by subtracting the value of the parameter Base Range  211  from the value of the parameter Range  202  when the Problem Target  206  flag is initially set. This is in order to obtain an ‘offset value’ of the new reflecting position from the previous reflecting position. 
         [0111]    The value of parameter Offset Range is then subtracted from subsequent values of Range  202  whilst the reflected signal  113 S is being reflected by the new reflecting position in order to obtain a value of the parameter Compensated Range  214 . Thus the measured value of Range  202  is corrected to compensate for the offset in reflecting position that has occurred. 
         [0112]    It is to be understood that other arrangements for determining a value of the parameter Compensated Range  214  may also be useful. 
         [0113]    It is to be understood that the functionality of  FIGS. 3 to 5  may be implemented in software, in firmware, in hardware or by a combination of two or more of these means as described above. It is further to be understood that the functionality of the detector module  131  and control module  133  may be implemented by a single module or by three or more modules. In some embodiments the functionality of the detector and control modules  131 ,  133  as described above is implemented substantially entirely in software, firmware, hardware or a combination thereof in association with required transmitter and detector hardware. 
         [0114]      FIG. 6  is a plot the values of the parameters Range  202  and Range rate  203  measured by apparatus  100  according to the embodiment of  FIG. 2  as a function of time. In the time period shown the motor car  5  approaches a lorry  20 . At time t 1  it can be seen that the value of parameter Range  202  (trace A) begins to increase whilst the value of parameter Range rate  203  remains negative (trace B). It can be seen that the value of the parameter Range  202  increases by over 2 m. 
         [0115]    This increase in value of parameter Range  202  corresponds to the moment at which the signal  111 S as detected by the receiver  113  begins to be reflected from the rear axle  24  of the lorry  20  and not the rear door  22  (see the scenarios of  FIGS. 1(   a ) and  1 ( b )). 
         [0116]    The value of Problem Target flag  206  is also plotted in  FIG. 6  (trace C), shown multiplied by a factor of 10 for ease of inspection. It can be seen that the apparatus  100  detects that a problem target  24  exists at time t 1 . This is because the value of the parameter Range  202  is increasing whilst the value of parameter Range rate  203  is negative. 
         [0117]    It is to be understood that once the flag Problem Target  206  is set the apparatus  100  begins to calculate a value of the parameter Compensated Range  214 . 
         [0118]      FIG. 7  shows a plot of the value of each of parameters Range  202  (trace A), Range Rate  203  (trace B) and Compensated Range  214  (trace A′). It can be seen that, after time t 1 , the value of parameter Compensated Range  214  is lower than that of parameter Range  202  by at least 3 m, providing an erroneous range determination that differs from actual range by up to a factor of at least 10 in the example shown. 
         [0119]    It is to be understood that the ability to provide a correct value of range of a motor vehicle  5  from an object  24  represents a considerable safety advantage. This is because in a motor vehicle  5  having apparatus  100  according to an embodiment of the invention installed therein the apparatus  100  may be arranged automatically to activate a driver alert or active braking system in order to avert a collision. The driver alert may be in the form of one or more of an audible alert, a visual alert, a physical alert (such as vibration of the driver&#39;s seat) or any other suitable alert. 
         [0120]    Other arrangements are also useful. In some embodiments the apparatus  100  is configured to employ ultrasonic radiation instead of or in addition to electromagnetic radiation to measure range and rate of change of range from a target object. In some embodiments the apparatus is configured to employ non-ultrasonic sound waves such as sonic waves, sonar waves or any other suitable waveform. The apparatus may be employed in land-based vehicles, marine vessels, aircraft, spacecraft or any other suitable vehicles or objects. 
         [0121]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
         [0122]    Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0123]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.