Abstract:
A radar system which is used for determining optical range includes a frequency analyzer for the analysis of differential frequency signals, which originate from a mixture of the transmitted signal with the received signals, for example reflected from a fog bank. The frequency spectrum images the distance profile of the fog bank.

Description:
BACKGROUND INFORMATION 
     A radar system is described in German Patent Application No. 196 32 889, in which, however, the range finding is limited to the measurement of distances to solid bodies. 
     SUMMARY OF THE INVENTION 
     In contrast to this, the radar system according to the present invention has the advantage of being capable of creating distance profiles to diffuse objects, in particular fog banks. Thus, for example, quantitative information concerning visual range can be obtained in a motor vehicle independently of the driver. This functionality can be integrated here advantageously via simple additional technical measures in a device which is already used for distance sensing, for example for parking or for automatic vehicle control systems. Also advantageous is the simple feasibility of digital signal processing as a result of the relatively low-frequency difference signals to be used, for which inexpensive digital signal processors are available. Particularly advantageous is the provision of automatically operating compensation means, which permit realistic imaging of diffuse objects, in particular of fog banks. In particular, the driver can detect the actual extent and the actual spatial density distribution of such areas and thus realistically evaluate the traffic situation/obstacle to visibility. 
     If light which is frequency-modulated in its intensity is used as a transmitted signal, a real image of areas of possible obstacles to visibility is provided by the type of the measurement itself, since the transmitted signal is of the same physical nature as the light beams which are detected by the human eye and produce the optical image in the eye. Moreover, in the case of using light as an information carrier, as opposed to measurements in the radio frequency range, it is not necessary to pay attention to spectral purity or to a band limitation, so that frequency modulation taking account of harmonics can also be performed in a rectangular form without impairing the measurement result. In the case of systems in the radio frequency range, on the other hand, strict adherence to the assigned frequency bands (for the most part, moreover, very narrow-band ranges, cf. Industrial Scientific Medical bands) is necessary (key words: wireless service, electromagnetic compatibility, etc.); therefore a radar signal used in the radio frequency range cannot be simply a rectangular sequence, since the harmonic waves transmitted in this case would cause a “contamination” far from the assigned band. In addition, high-performance laser diode drivers, which are offered for optical communication and are designed for pulse modulation, may be used for the process of visual range determination. 
     Furthermore, the transmitted signal is emitted divergently in an advantageous way, in order to obtain a good averaging over the density of any existing fog layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  shows a first embodiment. 
     FIG. 1 b  shows a detail of a second embodiment. 
     FIG. 2 a  shows a fog bank. 
     FIG. 2 b  shows a first distance profile corresponding to the fog bank. 
     FIG. 2 c  shows a second distance profile corresponding to the fog bank. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 a  shows a radar system with a control unit  10 . Control unit  10  has an analog multiplexer (MUX)  20 , which is supplied with a positive direct voltage V+ and a negative direct voltage V−. The multiplexer is connected with an integrator (INT)  30 . Control unit  10  further has a digital signal processor (DSP)  60 , which controls multiplexer  20 . Signal processor  60  contains a fast Fourier transformer  50  (abbreviated “FFT”), and an analog-digital converter (ADC)  40 . DSP  60  is connected to a display unit  70 . Integrator  30  controls a frequency-modulatable oscillator (VCO)  80 , which in turn operates a laser diode  95  using a control voltage. Laser diode  95  is part of transmitting unit  90 , which further includes an emitting element  96  in the form of a dispersion lens. In addition, the radar system possesses a receiving element  106  (lens) for collecting optical radiation. A photo diode  105  is located behind the receiving element. The output of the photo diode is connected to a mixer  110 , which mixes the photo diode signal with the signal of oscillator  80 . The mixed signal passes through a low-pass filter (LPF)  120  and then a compensator (COMP)  130 . The compensator in turn is connected to the input of analog-digital converter  40  of DSP  60 . 
     DSP  60  controls analog multiplexer  20  for generating a rectangular signal, which oscillates between the voltage values V+ and V−. Integrator  30  generates from this rectangular signal a triangular signal, which is used for controlling oscillator  80 . Oscillator  80  produces a high-frequency oscillation in the gigaherz range, modulated with the triangular signal. The laser diode is controlled with this high-frequency oscillation and thus emits intensity-modulated light, the frequency of the modulation varying linearly according to the triangular signal. Thus frequency modulation of the light intensity takes place in stages corresponding to a slope (that is, linear increase and decrease of the modulation frequency). Light scattered back from solid or diffuse objects is collected by receiving element  106  and converted into an electrical signal by photo diode  105 . Mixer  110  generates sum and differential frequency signals. Low-pass filter  120  is adjusted so that it lets through only differential frequency components of the mixed signal of interest. This occurs, for example, in the case of a limiting frequency of 500 MHz of the filter, when the maximum frequency deviation of oscillator  80  is of this order of magnitude. 
     Compensator  130  is given by the edge of a high-pass filter, or band pass filter, acting as a fourth-order differentiating element. This filter produces an intensity correction, by passing components of higher differential frequencies better. In this way the intensity decrease of the optical radiation is compensated in the case of a far-away scattering object. The differentiating element here is of the fourth order, since for objects in the remote field of the transmitter the received intensity is damped both by the path of the transmitted signal as well as by the path of the scattered signal, so that the inverse quadratic proportionality of the intensity to the distance is to be taken into account two times, and the differential frequency signal of each scattering object, passed by the low-pass filter, has a power which is inversely proportional to the fourth power of the differential frequency. This is exactly what is compensated by fourth-order differentiating element  130 . 
     Analog-digital converter  40  digitizes the mixed signal of the differential frequency components obtained and corrected in their intensity; FFT  50  provides a spectral analysis of the mixed signal capable of being evaluated by the DSP. The relationship between spectral intensity and differential frequency here is a direct reflection, for example, of the spatial density distribution of a fog bank representing the back-scattering object. The DSP transmits the spectral analysis directly to the display unit, so that the driver of the vehicle learns the distance at which a fog bank is located, and how far the latter extends in the direction of travel. Alternatively, in the case of recognizably short visual range (for example when it is less than a specific distance to the fog bank), to warn the driver accordingly via the display unit or via separate lights or warning sounds, or to switch on the fog lights automatically. 
     As is known in this case from German Patent Application No. 196 32 889, a differential frequency fdiff corresponds to a specific distance L, at which the scattering object (either a solid body or a particle-containing volume of air, for example a volume element of a fog bank) is located. In this case L is given by 
     
       
           L=fdiff×c /(2 ×df/dt ), 
       
     
     c representing the speed of light and df/dt the constant rise, respectively fall, of the modulation frequency in the frequency slopes induced by the triangular signal. For example, if a frequency deviation of 500 MHz corresponding to the maximum frequency deviation of the VCO is chosen, and a measuring time of 1 second is set for this (which then corresponds to a frequency of the triangular signal of 0.5 Hz), df/dt=500 MHz/1 s. A frequency resolution of the FFT of 1 Hz results because of the measuring time of 1 second. If the FFT has 512 interpolation points for the analysis of the mixed signal, the highest resolvable differential frequency amounts to 256 Hz. Because of the above-mentioned relationship between L and fdiff, this corresponds to a distance of 76.8 m, and 1 Hz corresponds to 30 cm. Thus, in this example (diffuse) objects can be detected up to a distance of 76.8 meters with a local resolution of 30 cm. The measuring times can be shortened in the case of a given frequency deviation within the framework of the steerability (input time constants of the VCO), in order to reduce the undesirable averaging influence of the motion of travel. The differential frequency components of the signal mixture generated by mixer  110  are accordingly of higher frequency. 
     FIG. 1 b  shows an alternative transmitting unit  91 , which may be used instead of transmitting unit  90 . Transmitting unit  91  includes a threshold switch  97 , whose output signal controls laser diode  95 , which emits its light via emitting element  96  in the way already described. 
     Threshold switch  97  converts the sinusoidal oscillation of oscillator  80  into a rectangular signal, so that it is possible to use laser diodes of higher power than laser diode  95 , available for optical communication, which are designed for pulse modulation. The rectangular signal still includes the original sine wave as the fundamental wave. Furthermore, mixing in the multiplier takes place with the sine signal. The harmonic waves of the rectangular oscillation remain without influence on the signal mixture to be evaluated, as long as the limiting frequency of the low-pass filter is chosen suitably. The arrangement composed of mixer and low-pass filter in principle represents a correlator (multiplication of two signals and integration of the product), in the case of which a correlate that is variable in time is obtained because of the short time integration (low-pass filter), that is, actually a “slowly” variable continuous component is evaluated. Since the harmonic waves of the rectangle are orthogonal to the fundamental wave, the system also functions as described, namely with a rectangle and the sine of the fundamental wave. The limit frequency of the low-pass filter in this case must lie below the lowest used instantaneous frequency of the VCO. This specific embodiment makes use of the fact that if light is used as a carrier, it is not necessary to pay attention to spectral purity or band limitation, since presently there are no specifications for modulated light with respect to the frequencies usable for modulation. At the present time this is also not necessary, since light is transported for communication purposes anyway, except that it is transported enclosed, preferably in glass fibers. 
     FIG. 2 a  shows a schematic representation of a fog bank  210 , which is located at a distance range of 30 to 70 meters in front of a vehicle, the x coordinate of which is at the origin. Area  220 , covered by emitting elements  90  and  91 , is marked with dotted lines. The receiving area of receiving element  106  is marked with solid lines and with reference symbol  230 . 
     The radar system provides display unit  70  with first curve  240  shown in FIG. 2 b,  for example, starting from the object position shown in FIG. 2 a.  It represents the uncompensated (that is, obtained without the use of compensator  130 ) Fourier spectrum I of the differential frequency signal mixture, the differential frequencies corresponding to the spatial extent of the fog bank in the x-direction detecting a range between 100 and 233 Hz. If, however, the radar system is equipped with compensator  130 , the normalized Fourier spectrum N (second curve  250 ), which represents an image of the relative spatial distribution of the fog density, is shown. Comparative values, which were determined from comparison measurements on a test fog area of variable density (particles per cubic meter) in transmitted light and with which the display is calibrated, may be stored for representing the absolute fog density. If individual “solid” or larger elongated objects are found in the area investigated, the detected spectrum will contain discrete “surges” or pronounced peaks. The latter may be detected and masked out with an appropriate filter mask (not described further here).